THE PRINCETON UNIVERSITY CONFERENCE PARTIALLY IONIZED …

NASANational Aeronautics andSpace Administration

THE PRINCETON UNIVERSITY CONFERENCE

PARTIALLY IONIZED ANDURANIUM PLASMAS

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Proceedings ofThe Princeton University Conference

PARTIALLY IONIZFO PLASMASincluding the THIRD SYMPOSIUM ON URANIUM PLASMAS

Edited by

M. Krishnan „

Department of Aerospace and Mechanical SciencesPrinceton University

Papers delivered at the One Hundred Thirty Third Meetingof the Princeton University Conference en June 10-12, 1976

September 1976

Published by tNational Aeronautics and Space Administration _£

Washington, D.C. X

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PREf'-CK

The Princeton University Conference on Partially Ionized Plasmasincluding the T.iird Symposium on Uranium ??asraas has been undertaken with thesupport of the Research Division of the National Aeronautics and SpaceAdministration. It has the enthusiastic sponsorship of Mr. F. Carl Schwenk,who is Director of the Research Division and Dr. Karlheinz Thorn, Manager ofthe Plasma Program.

The Conference is he ing held ac a time of ferment in this fieldespecially regarding the possibilities for nuclear pumped lasers. Fundamentalsof both electrically and fission generated plasmas have been studied for aconsiderable time and will be reported at the Conference. Research in gaseousfuel reactors using uranium hexaflotiride vill be described and their andother parcially ionized plasma applications will be discussed.

The members of the Conference Committee are listed on the followingpage. Their willingness to serve is acknowledged and greatly appreciated.Mr. F. Carl Schwenk is thanked for his luncheon speech entitled ?UTURE POWERSYSTEMS IN SPACF. and Dr. Theodore B. Taylor, Chairman of the Board of theInternational Research and Technology Corporation is thanked for hischallenging dinner talk NUCLEAR PCWER RTSKS AND SAFEGUARDS.

The preparation of the papers for this Conference and theirpresentation are greatly appreciated and the attendance and participationof many prominent workers in the field are recognized as essential to thesuccess of the Conference.

The administrative assistance of the Princeton University Conferei 'eStaff and The Aerospace Systems Laboratory is gratefully acknowledged.

Conference Co-Directors:

September 1976

LIST OF CONFERENCE COMMITTEE MEMBERS

Conference ChairmanRobert G. Jahn, DeanSchool of Engineering 5 Applied SciencesPrinceton University

Conference Co-DirectorsJ. Preston LaytonThe Aerospace Systems LaboratoryPrinceton Uniyersity

Jerry GreyConsultant

Conference EditorM. KrishiianDepartment of Aerospace § Mechanical SciencesPrinceton University

Technical Program CommitteeKarlheinz Thorn, ChairmanResearch DivisionNASA Headquarters

Kenn ClarkPrinceton University

Herbert H. HelmickLos Alamos Scientific Laboratory

Frank HohlNASA Langley Research Center

S. H. LamPrinceton University

Thomas S. LathamUnited Technologies Research Center

Richard T. SchnsiderUniversity of Florida

Woldemar von JaskowskyPrinceton University

Herbert WeinsteinIllinois Institute of Technology

TABLE OF CONTENTS

Page

TITLE PAGE

PREFACE i

LIST 01" CONFERENCE COMMIT! i-.'E MEMBERS ii

TABLE OF CONTENTS iii

I. INTRODUCTION

Helcome, 1R. G. Jahn, Princeton University

Purpose, 2J. P.. Layton, Princeton University

Background, 5

J. Grey, Consultant

II. ELECTRICALLY GENERATED PLASMAS

1. On the Emission Mechanism in High Current Hollow Cathode Arcs,

M. Krishnan, Princeton University 82. A Scaling Law for Energy Transfer by Inelastic Electron-

Molecule Collisions in Mixturer,G. K. Bienkowski, Princeton University 24

3. Pulser-Sustainer Glow Discharge Operation in Static CarbonMonoxide Laser Mixtures,D. J. Monson and C. M. Lee, NASA Ames Research Center 32

It. Plasma Research in Electric Propulsion ,-it Colorado StateUniversity,P. J. Wilbur and H. R. Kauf-ntn, Colorado State University 37

5. An Efficient Procedure for Compuving Partidl Ionization of aGas in Numerical Fluid Dynanics,S. Garribbs and L. Quartapelle, Centro Studi Nucleari EnricoFermi, Milano, Italy 4g

III. FISSION GENERATED PT.ASMAS

6. Analysis of Nuclear Induced Plasmas,J. E. Deese and H. A. Hassan, North Carolina State University 52

7. Energy Distributions and Radianioi. Transport in Uranium Plasmas,G. Miley, C. Bathke, E. Maceda and C. Choi, University ofIllinois

8. Recent Measurements Concerning Uranium Hoxafluoride-ElectrenCollision Processes,S. Trailer, A. Chutjian, S. Srivastava and W. Williams, JetPropuision Laboratory, and D. C. Cartwright, Los AlamosScientific laboratory

9. Measurement Techniques for Analysis of Fission Fragment ExcitedGases,R. T. Schneider, E. E. Carroll, J. F. Davis, R. N. Dsvie, T. C.Maguire and R. G. Shipman, University of Florida 80

IV. NUCLEAR PUMPED LASERS

10. The Pumping Mechanism for the Neon-Nitrogen Nuclear Excited Laser,G. W. Cooper, J. T. Verdeyen, W. E. W°lls, and G. H. Miley,University of Illinois at Urbana-Champaign

11. Direct Nuclear Pumped Lasers Using the Volumetric He Reaction,R. J. De Young, Vanderbilt University and N. W. Jalufka, F. Huhland M. D. Williams, NASA Langley Research Center 95

iii

TABLE OF CONTENTS (Continued)

12. Recent Nuclear Pumped Laser Results,G. H. Miley, W. E. Wells, M. A. Al-.erman and J. H. Anderson,University of Illinois

13. Nuclear Fission Fragment Excitation of Electronic TransitionLaser Media,D. C. Lorents, M. V. McCusker and C. K. Rhodes,Stanford Research Institute

14. Development of a Higher Power Fission-Fragment-Excited CO Laser,D. A. McArthur, Sandia Laboratories

V. NUCLEAR PUMPED LASERS

15. Review of Coaxial Flow Gas Core Nuclear Rocket FluidMechanics,H. Weinstein, Illinois Institute of Technology

16. Properties of Radio-Frequency Heated Argon Confined UraniumPlasmas,W. C. Roman, United Technologies Research Center

17. The Emission Characteristics of Uranium Hexafluoride at HighTemperatures,N. L. Krascella, United Technologies Research Center

18. Uranium Plasma Emission at Gas-Core Reaction Conditions,M. D. Williams, N. W. Jalufka and F. Hohl, :MSA LsngleyResearch Center and J. H. Lee, Vanderbilt University

19. Studies on Color-Center Formation in Glass UtilizingMeasurements Made During 1 to 3 MeV Electron Irradiation,K. J. Swyler and P. W. Levy, Brookhaven National Laboratory

20. Heat Transfer Evaluation in a Plasma Core Reactor,D. E. Smith, T. M. Smith and M. L. Stoenescu, PrincetonUniversity

\... GASEOUS FUEL REACTOR RESEARCH

21. Research on Plaoma Core Reactors,G. A. Jarvis, D. M. Barton, H. H. Helmick, w. Bernard andR. H. White, University of California, Los Alamos ScientificLaboratory *

22. Parametric Analyses of Planned Flowing Urinium Hexafi.uorideCritical Experiments,R. J. Rodgers and T. S. Lathan., United Technologies ResearchCenter

23. Circulation System for Flowing Urar.ium Hexafluoride CavityReactor Experiments,J. F. Jaminet and J. S. Kendall, United Technologies ResearchCenter

24. Measurements of Uranium Mass Confined in High Density Plasr?".R. C. Stoeffler, United Technologies Research Center

VII. APPLICATIONS

25. Magnetoplasmai'ynamic Thruster Applications,E. V. Pavlik, Jet Propulsion Laboratory

26. Mini-Cavity Plasma Core Reactors for Dual-Mode Space NuclearPower/Propulsion Systems,S. Chow, Princeton University

27. Plasma Core Reactor Applications,T. S. Latham and R. J. Rodgers, United Technologies ResearchCenter

Page

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109

123

136

146

156

17C

138

197

2C5

211

217

224

iv

TABLE OF CONTENTS (Continued)

Page

28. Broadband Photoexcitaticsn of Lasers,F. Brockroan jnd R. V. Hess, NASA Langley Research Centerand A. Javan, Massachusetts Institute of Technology 239

29. Status of Photoelectrochemical ""reduction of Hydrogen andElectrical E'.ergy,C. E- Byvik and G. H. Walker, NASA Langley Research Center 244

30. UF, Breeder Reactor Powe.r Flants for Electric PowerGeneration,J. H. Rust and J. D. Clemment, Georgia Institute of Tech-nology and F. Hohl, NASA Langley Research Center 248

31. Georgia Institute of Technology Research on the Gas CoreActinide Transmutation Reactor (GCATR),J. D. Clement, J. H. Rust and A. Schne'ider, GeorgiaInstitute of Technology and F. Hohl, NASA Langley ResearchCenter 255

32. Application of Gaseous Core Reactors for Transmutation ofNuclear Waste.,B. G. Schniczler, R. R. Paternoster and R. T. Schneider,University of Florida 263

33. Gas Core Reactors for Coal Gasification,H. Weinstein, Illinois Institute of Technology 269

34. Cunaiutrdtioua ro Achieve Directionality :"cr Gamma RayLasers,S. Jha, University of Cincinnati and J. Blue, NASA LewisResearch Center 275

VIII. WORKSHOP SESSIONS

35. Plasma Cecieration and Confinement4J. S. Kendall, Chaiman 280

36. Plasma Characteristics,J. H. Lee, Chairman 281

37. Nuclear Pumped Lasers,F. Hohl, Chaiiman 283

38. Reactor Concepts.. Systems and Applications,H. Weinstein, Chairman 285

LIST OF PARTICIPANTS 287

Welcome

Robert G. JahnPrinceton University

It is my great pleasure to welcome you allhere to this conference on Partially IonizedPlasmas including the Third Symposium onUranium Plasmas. It is a particular pleasurefirst of all because I see so many close personalfriends and colleagues involved in the programand in the audience; colleagues from both otheracademic institutions and the industrial sector,and especially our very good body of friendsfrom the space agency. I also find it a particu-larly happy occasion to welcome you here becauseof the quality of the program that has beenassembled by Karl Thom and his committee. Fromwhat could have been a very disparate and polyglotspecturm of topics, they have put together ameeting of great interest and considerablecohei nee. I am looking forward to participatingin i s much as I possibly can.

Then, coo, it is a meeting which covers afield that we here at the School of Engineeringand Applied Science have identified as one ofthose relatively few areas in which we aregoing to attempt to have something to say. Thishas been the case for many years. A significantnumber of our faculty and a corresponding cadreof research staff and graduate students haveworked in the areas that are the concern of thisconference for many years, and you will hear fromsome of them during the course of the meeting.

Also, this subject we are discussing overthe next few days is a particularly appropriateone to be aired in an academic institution. Itis not a field that has enjoyed by any stretchof the imagination the priorities and the highpressure of some other research activities inthis country. We certainly do not compete withthe big programs of controlled fusion, defensesystems, etc. Notwithstanding, it is an areawhich has been ripe for good scholarship, forcareful and leisurely work, the joy of exploringside avenues and checking allowed alternativepossibilities and seeing some substance beginto grow out of this nebulous domain in which webegan many years ago. I have the sense rightnow that the field is coming ripe for somevery significant developments out of the basicscience, and I hope we see some of that comingto fruition here in the next two or three days.

In addition to satisfaction in the meetingand my hope that you will all share our interestin this program, I hope you will have some funwhile you are here and take the opportunity toenjoy our campus and our town and to see thevarious technical activities here within theschool. Thank you all for coming.

Purpose

J. P. LaytonPr:lnceton University

During the past several years a numl er of newavenues of ressarch on partially ionized plasmashave opened, particularly in the area offissioning and fission-fragment excited plasmasand in nuclear pumped lasers. Important progresshas also been made in the science and technology ofuranium hexafluoride cavity reactors. Thisconference will review past accomplishments, surveycurrent research, and examine future growth of thescience, technology and applications in theseinteresting and potentially important topic areas.Workshop discussions will concentrate on thestatus in the various areas of this field,including the identification, of new researchtasks.

As I have observed the field over some periodo£ time, fission plasmas have not received thebroad fundamental attention which they deserve.Considerable ground work has been laid, and it isto be hoped that in publishing the proceedings ofthis conference and in future papers, theses, andarticles in archive journals 'hat the topic offission plasmas arid other partially-ionizedplasmas will begin to receive the attention thatis warranted by the kinds of applications whichappear to be offered in a number of future tech-nologies.

Some fields suffer from too much physics. Iam not sure that, even with some tongue in cheek,I can offer the fusion field as suffering in thatway, but perhaps. Sometimes engineers over-react

by rushing in to developmei.t efforts involvingindustry spending of tremendous sums without anadequate technology I'ase or bor.ifide applications.So far work in fission plasmas hasn't erred ineither of those directions, and I hope that itcan be kept from such errors. As I have seenresearch here in the academic thicket at Princetonfor 25 years, a much closer working relationshipbetween practitioners in the basic sciences andthose employing the engineering approach is highlydesirable. Engineers also have to get a broaderunderstanding of their overall responsibilities inbringing new technologies into use than they havehad in the past.

I have been a missionary for systems analysis,particularly of the broad and parametric sort, fora number of years and have found there are alot more savages than there are missionaries. How-ever the way to nirvana is undoubtedly as hard onthe savages as it is on the missionaries. It is mypurpose to urge the need to pursue the overalltopic of partially ionized plasmas from both basicscience and technology standpoints, but it alsoneeds to be tied together in an analytical wayfrom the syflte-s and applications viewpoints. Itis hoped that this field will move toward itsapplications with better balance than has beenevident in many fields involving sophisticatedscience, high technology and expensive develop-ments.

I offer Figure 1 as presenting an essential

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message in how to consider an overall system. Thisis a sheet that we m\de up in The Aerospace SystemsLaboratory (TASL) here at Princeton when we wereundertaking Che parametric systems amly?.is offusion power stations. It attempts to identifyan economically and environmenLally viable stationas the whole system. The point is that the fusionreactor system at the center has to derive inputsfrom and is dependent on the concept and definitionof all the other systems that make up the station,as well as on the demand and other considerationsto provide outputs of value to prospective users.This approach needs to be applied to partiallyionized plasma systems at the same time that basicfeasibility is being resolved to assist in planningthe research and technology programs and inidentifying the economic applications.

Methodologies are available for reducing theblock diagram of Figure 1 to an overall parametricsystems analysis basis. The TASL approach to theanalysis of nuclear fusion systems is shown inFigure 2. The interdependence and interaction

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Figure 2 TASL Approach to the Analysis of NuclearFusion Systems

between the three primary analytical concerns -performance, operstions and economics are indicatedas well as the feedback to the power demand andother considerations such as the environment inthe concepting and definition of the overal?system and its i.omponent5.

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for computer analysis and relating them to realityby reference to existing point designs. Anyanalysis of future technology is essentiallyuncertain and therefore probabilistic. A methodfor handling uncertainty in parametric systemsanalysis is presented in Figure k. Although theamount of data required is substantial such ananalytical process is ultimate!;' necessary toprovide proper benefit/cost p.nd risk analysesneeded by a decision maker before proceeding withlarge scale 2nd very expensive developments.Fortunately, the entire process does not need tobe activated at the outset since much can be, andneeds to be, learned from simple and partialanalyses while the basis for the final analysis isbeing established.

I will not at this time belabor these pointsby a more detailed discussion. My purpose v;as toargue that much work lies ahead - conceptual,analytical and experimental before major systemsbased on partially ionized phasmas are realized.Tt is the purpose of this conference to examinewhere the field stands at the present time and toproject a course for the future.

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Background

Jerry GreyConsultant

This conference has two major branches ofeffort: one is in electrically-generated plasmasand one is in fission-generatad plasmas. Theseaie the subjects of. the first two sessions. Athird area, not covered at this conference, iscoisbustion-generated plasmas (seeded or not) whichhave been principally considered for MHD systems.

Although the electrically-generated andfission-generated plasma technologies have enor-mously disparate backgrounds, as Dean Jahnmentioned earlier, the modern concepts dealtwith in this conference both started about thesame time: the late '40's and early 'SO's. Thefirst paper on fission-generated plasmas waspublished by Shepherd and Cleaver in 1951,although there was considerable discussion beforethat time in the '40's. In that field, inciden-tally, thy most extensive literature coverageappeared in the first two symposia on uraniumplasmas, of which this one is the third. Thosetwo volumes, both of which came out in the earlypart of this decade, constitute an excellentbackground in the field.

In electrically-generated plasmas, theie ..san enorroous electrical engineering literature,dealing primarily with switching arcs. Classicaltexts such as Cobine were published some timeago. The modern science and technology of thesubject of this conference, partially ionizeddense plasmas, came about with the proliferationof thermal arcjets which occurred sometime inthe 'SO's. There was a major growth in thatfield in the late 'SO's and early '60's.

In each of these two primary branches therewere two najor-sub-branches, giving four almostdifferent topics to discuss. In the fission area,there are the open-cycle concept and the closed-cycle concept. In electric generation, we havesteady and pulsed generation of plasmas.

Looking first at electrically-generatedplasmas, in the steady state area, I will dealonly with relatively modern developments. Thefirst real push was in the commercial applicationsof resistance-heated gases. Although inductionheating technology existed at that time, thecompanies in the field such as Giannini, ThermalDynamics, Humphreys, and LinJe were principallyconcerned with commercializing this new scienceand technology as rapidly as possible usingresistance-heated plasma devices. There was afair amount of growth in the fields of plasmaspraying, welding, and cutting (replacing oxy-acetylene torches), and also in the developmentof an effective research tool for those whowanted to study very high teaiperature partiallyionized plasmas.

The most advanced wort: in this field wasdone principally in the testing area, and themost elaborate tools for generating and studyingsteady stats arc-heated plasmas were thosedeveloped for siiwilation of atmospheric reentry,wh.\?.h was one of the key technical problems inthe rapidly-growing ipace program. The only wayone could simulate reentry was to generate theequivalent stagnation temperatures, and thebest way to do this was with a controlledatmosphere using electrically-haated gases. Someelaborate wind tunnel facilities wore built upat KPAFB, AEDC, Ames and other centers, cul'ina-ting in the enormous 50-iuegawatt electric tunneldeveloped and built at Wright Field in the late'60's. These facilities still exist; they arestill quite actively used and continue to develop.

During this time, also, there was considerableinterest in continuous arc-jet capabilities forspace propulsion. Here AVCO was the spearhead,although some early work ijas done by Gianniniand a number of other laboratories, includingNASA and the Air Force. There is a current rs-surgence of interest in thermal arcjets forpropulsion, despite their relatively lowefficiency compared to some of the others, andwhether or not this develops is a matter forresolution in the near future.

Most of the work on these steady stateplasma generators was done in gasas. Airwas particularly useful for the simulation ofreentry, but a lot of work was done in nitrogenin the commercial arc-jets and also in windtunnels, as well as argon and helium for con-venience in basic research. For propulsion,hydrogen was the principal gas.

During this time, however, there was someinterest in developing higher conductivitiesby the use of potassium and other seeds. Therewas also a fair body of work in the alkalinemetals, lithium in particular, at Los Alamosand Langley. Toward the end of this era thedevelopment of inductively-heated plasmas,particularly useful for simulating fissionplasmas, began tc accelerate.

In the pulsed plasma area, the earlytechnology was developed by Winston Bostickat Stevens Institute of Technology in hisplasmoid experiments. A propulsion effort atRepublic Aviation formed the early engineeringphase of these devices, but by far the bulk ofthe technology and the literature was developedhere at Princeton under Dr. Jahn's direction.

In the fission-generated plasma area, allthe early work was propulsion oriented. Theconcept of a new high-thrust, high-specific

impulse rocket technology for space flight wasthe motivating factor. The open cycle wasearliest; preliminary work was directed atsome form of containment, or separation of thefissirning plasma material from the propellai.t;that is, uranium is a poor propellant and hydrogenis a good one, and so one would like tc generateenergy in the uranium and exhaust only the hydro-gen.

The earliest studies were based principallyon stabilized vortex separation; e.g., theoreticalanalyses by Kerrebrock, Meghrebiian, Rosenzweig,Easton and Johnson, and Grey, The effort beganto expand significantly, only with the introductionof the coaxial reactor concept, which Weinsteinand Ragsdale suggested in an American RocketSociety paper around 1960. The program thengrew rapidly under Frank Rom's direction atNASA's Lewis Laboratory, extending to a numberof laboratories around the country. These studieswere reviewed in a comprehensive serie: ofpapers, mainly by Rom and Ragsdale, in themid-'60's and early '70's.

The principal effort continues to remainin fluid mechanics and heat transfer: thebasic problems of containment and how to heatthe propellant fluid without burning up the wallsof the rocket engine and nozzle. Tbsse arecovered in a survey paper by Herb ijeinstein.

The early studies were mainly cold flowsimulation, which led to the spherical geometrythat has been examined in most detail. Hot-flowsimulations used induction heated plasmas, whichsimulate the active heating of a plasma whileit is simultaneously being cooled by the propell-ant, modeling the open-cycle system more effec-tively. These studies were done by the TAFAdivision of Humphreys. Ward Roman brings thiseffort up to date by describing the currentUnited Technologies Research Center program.

There were extensive and extremely capableheat transfer studies performed, particularlyin the radiation field, principally by UnitedTechnologies and Lewis researchers. They devel-oped an extensive and basically new technologyin dense plasma radiation.characteristics. Thecurrent approach is to be all-inclusive, to lookat all the possible energy contributions, as isdescribed in a paper by Smith, Smith andStoenescu.

One of the key elements in absorbing radiatedheat transfer in hydrogen in certain importantranges of the spectrum was by seeding the gaswith tungrten or graphite particles. A numberof capable studies, both theoretical and experi-mental, were done in this field, the experimental,work mainly by Chester Lonzo at Lewis in themid-'50's, and most of the theoretical work atUnited Technologies. There were also significantcontributions by Georgia Tech and the Universityof Florida..

There was fairly extensive study in thenucleohic area. Nucleonic criticality was oneof the essential elements. The early cavityreactor studies by George Safonov in tk-c early'SO's were rudimentary, but the essential con-sideration of nucleonic criticality was txtenduJrapidly by such people as Carroll Mills at LosAlamos. There was extensive experimental simula-tion of the plasma core system by Kunze andhis collaborators at Idaho in the mid to late'60's, using uranium foils distributed throughouta cavity to simulate the hydrogen plasma, whichgave some substance to the nucieonic calculations.

The first basic problem with the fissioningplasma-core system, aside from the question offundamental feasibility, was that aJ.l of theconcepts were too big. To get nuclear criticalityin a plasma core, it was necessary to ase dimen-sions of the order of several meter., as wellas very high pressures, giving rise to systemsof enormous mass.

The second basic problem was that as thevolume got bigger, so much energy was depositedin the walls and in the solid structure thatit couldn't all be absorbed by the inflowingpropellant. It was necsssary to have someexternal method of "dumping" the excess energy.A radiator was first suggested b> Meghrebiianaround 1960, and then as the systems conceptsmatured, it appeared that radiaticn of externalheat was going to be necessary.

Two relatively recent developments attemptedto cure these problems. The first was Hyland'sconcept, published in 1970. Instead of dependingtotally on the plasma core itself for criticality,Hyland placed some solid fuel elements in thereflectors which surrounded the cavity togenerate a substantial fraction of the totalreactor energy. This allowed him to shrinkhis reactor down to a dimension of somewhatless than a meter and made the system look alittle more practical. However, it aggravatedthe second problem; that is, even more heatwas now generated in tne solid material, andtherefore much more energy had to be extractedby external systems. A method for utilizingthat external energy, developed by Layton andGrey some time ago, is the dual mode conceptwhich Stanley Chow discusses in his paper.

The major efforts in plasma-core technologyterminated with the demise of the NASA nuclearprogram in 1973. The feasibility of neither theopen cycle nor the closed cycle concepts hadyet been developed; that is, the basic feasibilityof containment, and ability to operate as asystem. Tne remaining work in the field, whichis the subject of this conference, has beensupported primarily by NASA's research division.

The other basic approach to plasma-coreutilization was the closed cycle. The conceptof using a completely closed '•ontainment systemto hold the uranium plasma and separate it fromthe hydrogen was actively pursued by McLafferty's

BACKGROUND, page 3

group at United Technologies. This idea ofusing a fused silica container surrounding theplasma region and transmitting energy totally byradiation from th£ plasma region to the hydrogenwas an obvious outgrowth of the problems asso-ciated with the hydrodynamic and other containmentproblems of the coaxial flow concept. The basicproblem was radiation heating in the fused silicaand how to avoid it. One approach, stabilizationof the vortex flow necessary to produce a convec-tive separation of the hot gases from the vortex,saw a great deal oi excellent fluid dynamicsresearch, as well as the radiative heat transferwork cited earlier. Recent studies in these areasare reported by Krascella, by Blue and Roberts,and by Levy.

In addition to the two basic coaxial-flowand closed-cycle ("light-oulb") concepts, thereare also a number of cher approaches. Themagnetic and electric body-force containmentconcept was studied very early, by Gross andothers. The idea of using MHD-driven vortices wasexamined by Romero and several others. Therewas serious consideration for some time of theplasma core reactor as a source for MHD power.Principal studies were by Rosa, and more recentlyby J.R. Williams at Georgia Tech. There iscurrently a resurgence of interest in thisapplication.

This leads us into the next phase: whatis going on today and what areas are most fruitfulfor exploration? First of all, there seems tobe a renewal of interest in the whole propulsion/power application for fission plasmas and forelectric propulsion. Schwenk provides an intro-duction to and a summary of that subject.

A major recent effort has been the use offission-generated energy for stimulating lasers.There are a number of papers on this topic. Thereis also a major effort underway in uranium hexa-floride utilization as a research tool, the sub-ject of another series of papers.

There are a host of potential.new applica-tions. The energy crisis fostered the draggingout of all the old dogs to take a new look atthem and see if there are any new applications.There are some interesting new applications inspace, as discussed in detail in several papers.These will be elaborated upon in the workshopsessions, which have the purpose of settingthe stage for a whole new era of partiallyionized plasma research and applications basedupon the growing interest and the active researcheffort that is going on today in a number ofnew directions.

ADDITIONAL COMMENT BY CARL SCHWENK

Since 1973 our program has become morebasic. That has been an advantage to us. Wehave given the idea of the UF, plajma a lot more

attention because of several factors. It mightjust be-the most useful form of a fissioninggas, and it certainly is a lot easier to work

with. In addition to the experimental advantagesit gives, it may also be a very attractive energyscurce.

ON THE EMISSION MECHANISM IN HIGH CURRENT HOLLOW CATHODE ARCS*

M. KrishnanPrinceton UniversityPrinceton, New Jersey

Abstract

Large (2 cm-diameter) hollow cathodes have beenoperated in a magnetoplasmadynamic (MPD) arc overwide ranges of current (0.25 to 17 kA) a*d massflow (10 3 to 8 g/sec), with orifice current den-sities and mass fluxes encompassing those encoun-tered in low current steady-state hollow cathodearcs. Detailed cathode interior measurements ofcurrent and potential distributions show that maxi-mum current penetration into the cathode is aboutone diameter axially upstream from the t ip , withpeak inner surface current attachment up to orecatjiode diameter upstream of the t ip . The spon-taneous attachment of peak current upstream of thecathode tip is suggested as a criterion for charac-terist ic hollow cathode operation. This empiricalfiiterion is verified by experiment.

Cathode cavity conduction processes are commentedon briefly; the emission processes at the cathodesurface are examined in some detail. It is showntha*. thermionic emission cannot account for theobserved current in such MPD discharges. Fieldemission from micro-spots moving rapidly over thecathode surface is shown to be a possible primaryemission mechanism in such cathodes. Possibleenhancement of the emission due to the photoelectriceffect isalso investigated. From order-of-magnitude considerations, i t appears that a form offield-enhanced photoelectric emission can accountfor most of the observed current in the MPD hollowcathode, thus suggesting a possible novel emissionmechanism for other hollow cathodes. The emissionmodel preferred has not been experimentally veri-fied.

I. Introduction

Since the early part of this century, the precisenature of the electron emission from cathode sur-faces in arc discharges has been the subject ofconsiderable theoretical and experimental investi-gation. Despite this effort, in many types ofarcs, satisfactory theories do not exist for thecathode surface electron emission processes. Onesuch poorly understood arc is the hollow cathodearc.

Hollow cathodes were first used to advantage byPaschen*- ' as early as 1916 in spectroscopic studieswhere such cathodes were shown to be capable ofsimultaneously providing high electron number den-sity and relativelytlow temperature ions and neu-trals in an essentially field-free cathode cavity.More recently, hollow cathodes have been used aselectron emitters in advanced ion thrusters wherethey exhibit longer lifetimes than oxide coated orliquid metal cathodes.(2) As sources of dense,highly ionized plasma, hollow cathodes have alsobeen investigated at the Oak Ridge National Labora-tory and at the Massachusetts Institute of Tech-nology. These researchers point out the importantfeatures of a hollow cathode discnarge:(3)

(a) the discharge creates a very pi re externalplasma (low contamination by cathode materi-al,) dense, (neM013-10"cm-3) and highlyionized (up to 95%);

(b) the cathode enjoys a reasonably long life-time, despite high current densities and high-.athode wall temperatures (> 2500°K).

An important part of hollow cathode research dealtwith the study of the noise oscillations of the1 '•*charge(4) , since i t was interesting to evaluate

'Hties of using hollow cathode arcs asa source of quiet plasma for experimental work onwave propagation. Hollow cathodes were also stud-ied extensively in Europe in ths nineteen sixties(5,6). Large hollow cathodes are currently finding<-ppli~ations in high power laserswJ and in theproduction of high power neutral beams for plasmaheating in con'rolled thermonuclear fusion reactors.

Despite the fairly exhaustive studies conductedat these various research establishments on theinfluence of the various parameters (geometry, gasflnw rate, current, external pressure, axial magnet-ic field strength, electrode temperature, etc.) onthe performance of the hollow cathode discharge asan efficient source of dense, highly ionized plasmaand as an efficient, long lived electron emitter,the physics of processes inside the cathode cavityare s t i l l poorly understood. One major reason forthis lack of understanding is that tho typicalcathode dimensions of most of these researches areso small (0.1 to 0.3 cm. inner diameter) as to pre-clude detailed diagnostic probing of the cavityinterior. Such diagnostic probing is essential toidentify the dominant physical processes of emis-sion, ionization and conduction inside t' 'Howcathode cavity. In the few instances v largehollow cathodes have been used, the h- lergylevels of a steady-state arc discharr -- in them-selves not conducive to the use of s. a diagnos-tics.

There is therefore a strong motivation for thedetailed diagnostic study of a large diameterhollow cathode in a relatively low energy environ-ment. The MPD arc facility at Princeton (Fig. 1.)is ideally suited to such an endeavor for tworeasons:

(a] the large cavity dimensions permit the detail-ed study of the plasma by probing, photo-graphic and spectroscopic observations.

(b) the total energy content of pulsed, quasi-steady MPD discharges is low enough to per-mit the use of small diagnostic probes or"simple construction, with no cumbersome heatshielding required.

This paper describes the results of detailedmeasurements made inside the cavity of large (Z-cm-diameter) hollcw cathodes, over a wide range of

•This work supported ^v NASA Grant NGL-31-001-005. +Now at Mason Laboratory, Yale Univ., New Haven, CT.

currents and mass flows as stated above. In sec'.ionII the experimental apparatus and diagnostic tech-niques are briefly described. Section III summa;.rizes the major experimental results. Jection IVbegins an analysis of the cathcde surface emissionprocesses by examining thermioni - emission. 7nsection V field emission from rhe cathode surfaceis discussed. The third possible emission mecha-nism, photoelectric emission, is examined in sec-tion VI, followed by a summary in section VII. .

I I . Experimental Apparatus

The quasi steady MPD accelerator with a hollowcathode is shown schematically in Fig. 1. Argonpropeliant (at feed rates from 10-3 to 16 g/sec} isinjected into the discharge chamber through a fast-acting solenoid valve fed from a high pressurereservoir. The gas can enter the discharge chambereither through the hollow cathode, or through tixouter injectors set in the insulating plexiglasbackplate, or thrown both.

Current to the discharge is supplied as a fast,lisetime (< 5 psec) flit-top pulse typically of1 msec duration by .an 84 station LC-ladder network.(8) Charged typically to 4 kV, this 3S kJ network(with a total capacitance of <..2 mfd] provides,when discharged through a variable impedance^ ; inseries with the accelerator, currents from as lowas 0.2S kA up to 17 kA.

The cylindrical arc chamber is 12.7 cm in dia-meter and S.I era deep. The hollow cathode islocated on axiss screwed into an insulating circu-lar plexiglas backplate. The coaxial annularaluminum anode, displaced slightly downstream fromthe cathode t ip , has an i.d. of 10.2 cm, an outerbarrel diameter of 18.8 cm, and a thickness of 1 cm.The particular hollow cathode dimensions and j, jm-etry are noted in the subsequent text where appro-priate.

The plasma formed in the. discharge chamber duringevery (y 1 msec) firing of the discharge exhaustsinto a glass bell jar vacuum tank with an i.d. of4S cm, 76 cm long, evacuated to some 10"3 torrprior to each discharge by a mechanical pump. Aprobe carriage mounted inside the tank, controllablein two dimensions from outside the tanX, allows de-tailed probing both inside and outside the hollowcathode.

The self-induced magnetic field everywhere Withinand about the hollow cathode is measured by aninduction coil with an i.d. of 0.0S cm and an o.d.of 0.3 cm, wound from 350 turns of #44 magnet wire.The magnetic probe signal, proportional to dB/dt,is passively integrated through a 10 msec RC inte-grator and displayed'on a Tektronix 551 oscillo-scope. At very low currents, the rather low mag-netic field signal from the integrator is boostedby a Tektronix AM502 differential amplifier, withgain selection from 102-105, before being read onthe oscilloscope.

A single Langmuir probe, consisting of a cylin-drical tungsten wire 0.025 cm in diameter, imbed-ded in a quartz tube of o.d. 0.02 cm with only theend face of the tungsten exposed to the plasma,is used to measure plasisa floating potentials.

In addition to spectral photography of the dis-charge with interference f i l ters , emission spectra

from the hollow cathode discharge ivere obtainedusing a Steinheil GH glass prism spectrograph whichrecorded radiation over the spectral interval from4200 8 to 6200 8.

III . The Search for Hollow Cathode Operationjr. Large Hollow Cathodes

1. Uninsulated Hollow Cathodes

In the first phase of the experiments, the hollowcathode was uninsulated, so that current attachmentcould occur on the outer cylindrical surface, face,or inner cavity of the cathode. The purpose of theseexperiments was to determine whether hollow cathodeoperation, with preferential current attachmentinside the cathode cavity, could be obtained in largehollow cathodes in an MPD arc. For typical MPDdischarge currents of kiloamperes, the orifice dia-meter of the cathode (y 1 cm) was chosen such thaitwo arbitrary scaling pa^amaters, a mass flux m/sand a current density J/zy where s is the cathodeorifice area, were comparable to typical mass fluxesand current densities in low current, steady-statohollow c. jhode arcs. A typical uninsulated c; thodris shown in Fig. 2a. The cavity dimensions irenoted on the figure.

With this hollow cathode, when argon gas was in-jected both from inside and outside the hollowcathode to the discharge, current attachment occur-red predominantly from the fa and outer surfaceof the cathode, with no visible plasma formationinside the hollow cathcde, as shown in Fig. 3. Fig.3a shows the camera perspe;tive used in photograph-ing the 16 kA, 12 g/sec discharge through a 4880 8Ar II narrow bandwidth interference f i l ter . Fig. 3bshows clearly that the luminous regions of the dis-charge are outside the hollow cathode cavity, whilethe cavity itself appears quite dark.

Even when gas was injected soIeiy through thehollow cathode, magnetic field measurements revealedthat there was negligible current attachment insidethe cathode cavity. Fig. 4 shows enclosed cun^nlstreamlines in and p.bout the hollow cathode cavityderived from the magnetic field measurements for acurrent of 29 kA, at a mass flow of 0.3 g/sec. Thefigure shows that a mere 3% of the 29 kA dischargepenetrates the cavity under these operating condi-tions.

These early experiments indicated that merely in-stalling large hollow cathodes in an MPD arc doesnot guarantee significant current attachment insidethe cavity, accompanied by formation of a stablecavity plasma. A carefu) search was therefore re-quired to identify specific operating conditionsunder which the discharge would spontaneouslyemanate from within the cavity. To facilitate thissearch, i t was decided to insulate the face andouter surface of the hollow cathode, thereby forcingthe current to attach inside the cavity. Under con-ditions of 'forced' plasma formation within thecavity, controlled variation of three key independentvariables, cathode geometry, mass flow rate, anddischarge current, might then lead to the empiricalidentification of necessary conditions for charac-ter is t ic hollow cathode operation in MPD arcs. Thevalidity of such empirical criteria, if identified,could easily be tested by removing the insulationof the hollow cathode; to observe whether, underthese specified operating conditions, the dischargenow preferentially emanates from the cavity interior,

in spite of the available external conducting sur-face.

2. Insulated Hollow Cathodes

a. Effects of Cathode Geometry Changes. In thefirst series of experiments with fully insulatedhollow cathodes, two of the independent variables,current and mass flow, were fixed, and the effectsof cathode geometry changes on the cavity dischargewere examined. For all insulat>d cathodes, gas wasinjected only through the hollow cathode to the \discharge, with the outer injector orifices scaled.

The first question to be answered was /lethar byforcing current attachment inside the cavity, astable discharge could be maintained in t'ies<: largecathodes. Current and potential distribui iois weremeasured insids the cavity of hollow c&thcde HC"III , shown in Fig. 2e, and were found to bo quasi-steady and azimuthally symmetric. The surfacecurrent distribution,for J=7 kA, nr=4 g/sec, wasdetermined by translating the magnetic projeaxially upstream frora the cathode tip at a radiusof 0.8 cm, with the 0.3 cm-diameter probe touchingthe cathode inner surface. This surface currentdistribution, drawn normalized by the enclosedcurrent at the cathode tip, is shown as the dashedcurve in the upper half of Fig. 5. For reference,HC VIII is shown in cress-section at the top of thefigure.

The surface current density profile, deduced fromthe enclosed current profile^} is shown as theheavy line in the upper half of Fig. 5. It shouldbs noted that the peak current density (> 1 kA/cra2atthis operating condition), occurs at the cathode t ip .

The lower half of Fig. S shows floating potentialson the hollow cathode axis obtained using the Lang-muir probe. The potential profile shows that coin-cident with the region of high current densitycurrent attachment is a region of weak axial poten-t ia l gradient, < 10 V/cm.

To obtain further information on the physicalprocesses occurring in the hollow cathode, a fewnear-infrared spectrograms of the discharge wererecorded using another insulated cathode, desig-nated HCI, shown in Fig. 2b. The near infraredsj.jctrai region contains a number of strong neutralargon lines which are more readily identifiablethan those neutral lines in the visible part of thespectrum, since the latter tends to be dominated byionized.argon and impurity lines. Fig. 6 comparesthe spectral interval from 6500 A to 8600 A for theuninsulated hollow cathode in the 29 kA, 0.3 g/secdischarge with that for the fully insulated cathode,HCI, at 7 kA and 4 g/sec. Both spectrograms wererecorded looking directly upstream into the cavityalong the axis. Fig. 6a shows that the distribu-tion of radiance of the spectral lines does notreflect any of the cathode geometric features des-pite the measured 0.8 kA (see Fig. 4) attachinginside the cavity for this 29 kA, 0.3 g/sec con-dition. The AI lines are barely discernible insidethe cavity and there is no continuum radiation.

In dramatic contrast, the spectrum of the fullyinsulated cathode discharge shows a sharply definedcontinuum in the cavity with AI and All linesbrightly superimposed on i t (Fig. 6b). There isvery l i t t l e radiation recorded beyond the ca\r cyat this exposure, with a high level of radiant fluxemana'ting from the cavity.

10

Having satisfactorilv obtained a quasi-steadyazimuthally symmetric discharge inside large hollowcathodes by forcing the current to attach insidethe cavity, the effects of cathode geometry varia-tions cci Id now be examined. Some of the manycathode configurations examined are shown in Fig.2b, c, d, e, f, g and h. As shown in the figure,different tip geometries as well as different innerelectrode shapes were examined. In particular, theconical coniigurations, Fig. 2f, g, may be consider-ed as steps in the transition from a hollow cathodewith a cylindrical cavity to a solid electrode witha flat face.

Despite these many drastic changes in cathodegeometry, the measured current anci potential dis-tributions inside the cavity of these hollow cath-odes, at fixei current, 7 kA, and mass fiow, 4 g/sec,are all nearly identical to those shown for cathodeHC VIII in Fig. 5. The characteristic features ofhigh current, high mass flow hollow cathode opera-tion that emerge from these studies ar? describedelsewhereU0) in detail. They can be summarizedas follows:

(a) For all insulated configurations, the dischargecurrent attaches to the downstream portion ofthe cavity with a peak surface current densityat the cathode tip in excess of 1 kA/cm2.The region aver whJcn 80% of the current at-taches is 0.6 cm long, coincident with a weakcentral axial electric field of less than10 V/cm.

(b) measured radial profiles of floating potentialinside the cavity licate that the bulk ofthe potential drop occurs near the inner cath-ode wall, while the cavity interior is nearlyfield-free.

Due to the insensitivity of the details of thecurrent air1 potential distributions inside the hol-low cathoue cavity to changes in cavity geometryat fixed high values of current and mass flow, i tWRS decided to select one particular cathode thatwas most amenable to diagnostics, and to study thiscathode for wide variations in the other two inde-pendent variables, current and mass flow. Thecathode de=lgn selected, designated HC XII, isshown in F^g, 2h.

b. Effects of Current and Mass Flow. For thefixed cathode geometry HC XII, current and poten-tial distributions inside the cavity were measuredover as wide a range of currents anJ mass flows naspossible™' with the given experimental facility.Local measurements of magnetic field and floatingpotential inside the cavity were made for currentsfrom as low as 0.25 kA up to 17 kA, with mass flowsfrom 10"3 up to 16 g/sec.

Fig. 7 shows the surface current density profiles,obtained from magnetic field measurements(9J, atvarious mass flows from 5 x 10"3 up to 0.4 g/sec inHC XII, with the current in all cases fixed at0.25 kA. Frr>m these current density distributions,three characteristic features emerge:

(a) As the .argon mass flow is reduced from 0.4g/sec to 6.1 g/56c, the peak in the surfpee ;current density moves from 0.3 cm to 1.9 cmupstream of the cathode orifice. Over thissame range in mass flow, the length of the

cathode (measured from the end) over which80% of the input current is found to attac'lito the surface, increases from 0.7 to 2.2 *m.

(b) Correspondingly, the current attachment atthe surface becomes more diffuse, leading,for a fixed current, to a dnp in the peakcurrent density.

(c) Further reduction in mass flow from 10"' t/secto 5x10"3 g/sec causes the peak in tne cur-rent distribution to move downstream towardsthe open end of the cavity.

Fig. 8 shows surface current density profiles fora fixed, higher current cf 0.9 kA and mass flowsfrom 5x10"3 g/sec to 8 g/sec. Here, just as at0.2S kA, the current density peak moves first up-stream from the cavity end and the current attach-ment becomes more diffuse as the mass flow is re-duced, fron 8 to 6.6xlO"2 g/sec. Again, s t i l lfurther reduction in mass flow, from 6.6xlO"2 to5x10"3 g/sec, causes the peak current density to thenmove downstream towards the cavity end. However,the maximum peak penetration at the higher currentis only 0.9 cm, compared to a maximum peak pene-tration of 1.9 cm at the lower current of 0.25 kA.

Fig. 9 shows similar surface current density dis-tributions for a high current of 4.7 kA. At thishigh current, unlike at the lower currents, thecurrent density distributions are observed to bequite insensitive to changes in mass flow. For allmass flows, the peak current density occurs between0 and 0.3 cm from the cavity end, and, in all cases80% of current attachment to the surface occursover a length of 0.9 cm from the cavity end.

The data presented in the preceeding three fig-ures m?.y be cross-plotted to demonstrate the effec cof discharge current on the cavity current distri-butions as shown in Fig. 10. Here, for a fixedmass flow of 0.2 g/sec, the surface current distri-butions are plotted for three currents. Thesedistributions are shown normalized by the peakcurrent density in each case to facilitate theircomparison an a linear scale. The figure showsthat as current is increased from 0.25 to 4.7 kA,the peak current density moves nearer the cavityend, while current attachment occurs over shortercathode channel lengths. To examine whether thistrend continues to still, higher currents, the sur-face current distribution was measured for a cur-rent of 17 kA. This distribution, also sh I inFig. 10, shows that the peak current densit occurss t i l l nearer the cathode tip with a shorter cathodelength (0.8 cm from the orifice) used for 90%current attachment to the surface.

The length of the cathode cavity over which 90%of current attachment to the surface occurs isdefined as the active zone length of the cavity.Fig. 11 summarizes the surface current densitymeasurements by graphing active zone length againstmass flow rate, with discharge current as a para-meter. The surface current density distributions,characterized by an active zone length, are seento be most sensitive to changes in lbiss flow at thelowest current (0.25 kA), becoming least sensitiveat the highest current (4.7 kA). The maximumpenetration of current into the hollow cathodedecreases as the current is increased from onecathode diameter at 0.25 kA to approximately one-i.enth of the cathode diameter at 4.7 kA.

The experiments described above reveal signifi*cant changes in surface current conduction withchanges in current and mass flow. To cltain amore coraprehensive picture of the energy depositionpatterns inside the volume of the cathode cavity,these surface measurements were supplemented bycomplete map-' of magnetic field and floating poten-tial throughout the volume of the cathode cavity.

Fig. 12 shows, on a cross-sectional view ofcathode 1ICXII, contours of constant current andconstant floating potential at a fixed current of0.25 kA for mass flows of 0.4 g/sec (upper part offigure) and 0.1 g/sec (lower part of figure). Atthis current of 0.25 kA, Fig. 7 shows that thesemass flows represent extremes in the penetration ofcurrent into the cathode cavity.

For both mass flows, the equipotential lines areapproximately radial in the bulk of the cavityplasma, iiaplying a negligible radial field in thevolume of the cavity. Ir. addition, the potentialsshow a weak axial field of less than 4 V/cm. Sincethe cathode itself is the zero volt equipotentialin both cases, all the radial equipotential contoursmust bend parallel to the surface somewhere closeto it before leaving the cavity. If the potentialsbend within the debye sheath separating surfacefrom quasi-neutral plasma, high radial fields(y 10° V/cm) would exist at the surface. Theimplications of Juch fields for surface emissionprocesses are discussed later.

From the intersecting grid of current and poter-t ial lines, the power density, j / E , in any volu_,segment of the plasna can be easiTy determined.The results of this computation are graphed inFig. 13a, b which show power density as a functionof axial position within the cavity, with radiusas a parameter. For both mass flows i t is observedthat the pjwer density profiles near the surfaceare qualitatively similar to the current densityprofile measured at the same radial position.Furthermore, in both cases the power density pro-file retains i ts form well into the plasma, thuscorroborating the significant difference in cavitycurrent penetration inferred earlier from the sur-face current density measurements.

In summary, by forcing the current to attach in-side the hollow cathode cavity, the search forhollow cathode operation in large hollow cathodesin the MPD arc has revealed that the current at-taches to the surface in one of two characteristicdistributions of surface current density:

(a) At high currents, high mass flows or at verylow mass flows, the peak current densityoccurs at or very near the cathode tip andthe bulk of current attachment to the surfaceoccurs within a limited axial region upstreamof the cathode tip. This mode of currentconduction is characterized as forced currentattachment inside the cavity.

(b) For lower currents, at intermediate massflows, the peak current density occursmeasurably upstream of the cathode t ip , anda longer length of cathode surface is utilizedfor current attachment, resulting in morediffuse current conduction at the surface.The spontaneous attachment of current at apoint upstream of the cathode t ip might beconstrued as being truly characteristic of

11

hollow cathode operation, wherein physical pro-cesses peculiar to the cavity determine theparticular choice of location alonjj the innercathode surface from which most of the curranti s conducted.

The first of the abovs two hypotheses has alreadybeen tested in the early experiments with uninsulat-ed hollow cathodos. As shown in Figs 3 ar.d 4, whenthe current is high (7 kA) then for both mass flowsof 6 g/sec and 0.3 g/sec, there is negligible cur-rent attachment within the cavity, the bulk of cur-rent prefering to attach to the exposed face anilouter surface of the hollow cathode. Since therais negligibla spontaneous cavity current attachmentunder these conditions, i t is evident that uponinsulating the face and outer surface of the cathode,at similar conditions of high current (4.7 kA) andmass flows (4 g/sec and 0.4 g/sec), the current isbeing forced to attach inside the cathode cavity,with a "forced" current density distribution tsshown in Fig. 9. The test of hypothesis b) above,i s described in tho next section.

3. Characteristic Hollow Cathode Operation

An empirical criterion for characteristic hollowcathode operation in an MPD arc is that the peaksurface currsnt density spontaneously occur upstreamof the cathjdo tip along the innei cylindricalsurface, as for example, at J=0.2.'> kA, fh=0.1 g/sec,in Fig. 7. To test this criterion, the insulationon the face and outer surface of cathode HC XII wascompletely removed, and at the same operating con-dition of J=0.2S kA, HFO.1 g/sec, the surfacecurrent distribution inside the cavity for thisuninsulated cathode was measured. This distributionis shown as the heavy line in Fig. 14a. Also shownin the figure as a dashed line is the measured sur-face current distribution foi the insulated versionof the same cathode at the same current and massflow. The two distributions are normalized by thepeak current density in either case. Ihe strongsimilarity between the two distributiosis demon-strates that, when the current and mass flow insidethe cathode are properly adjusted for a givencathode geometry, the discharge does prefer to runfrom within the cavity interior in spite of theavailable external conducting surface.

To supplement the current measurements inside thecavity, photographs of the discharge were alsotaken for the uninsulated cathode at J=0.2S kA,m=0.1 g/sec. A typical photograph, taken without

any spectral filtering and time-integrated overthe, discharge duration, is •shown in Fig. 14b. Therad-j.ice outside the cavity is so low at thisexposure that the cathode is not visible at a l l .To provide a sense of perspective, the dischargewas first photographed as in Fig. 14b and then,illuminated by a spot lamp, the discharge chamberwas imaged on the same film, using different cam-era settings. A typical double exposure i s shownin Fig. 14c. Figs. 14b, c, show that the dischargeradiap.ee emanates predominantly from the cathodecavity interior, with a marked absence of radiancefrom outside the cavity. This is in sharp con-trast to the picture obtained with an uninsulatedcathode at auch higher current (16 kA) and cathodemass flow (6 g/sec), in which the cavity i t se l f isobserved to be quite dark (Fig. 3). Finally, inFig. 14b i t is observed that the downstream 0.8 cmof the cathode cavity appears darker than the regionupstream of i t . The lowering ofintensity of radi-

ance in this region is consistent with the lowersurface current density measured in this region(Fig. 14a.) and the correspondingly lower volumetricenergy deposition (Fig. 13b).

In sumnary, the spontaneous occurrence of a peakcurrent density upstream of the cathode t ip , andthe strong similarity hetween the cavity currentdistributions for the insulated and uninsulatedcathode is deeded strong evidence for the conclusionthat this type of surface current distribution, andthe values of current and mass flow at which i toccurs, are indeed characteristic of hollow -athodeconduction for the given geometry.

From the experimental results ptesented above,interesting dependencies have been reveale-i of thecathode cavity conduction patterns on both mass flowand current for a fixed cathode geometry. Thesemeasurements were not made in sufficient detail toenable the construction of a detailed analyticalmodel of the cathode conduction and emission proces-ses. Furthermore, certain physical parametersgermane to the construction of such models, such aselectron number density, temperature, state ofionization of the plasma, heavy particle temperatures,etc . , have not been directly measured in this work.The approach, therefore, is semi-empirical, butnevertheless useful insofar as i t discriminatesbetween several candidate models and thus definesspecific directions in which further research mightbe conducted.

A phenomenological conduction model, consistentwith all of the ji:easured current distributions, isdescribe! in detail elsewhere.OW In brief, themean free path, A , for escape through the cathodeorifice of energetic electrons in the distributiontai l , is identified as & characteristic conductiondimension. When A* is very short or very lo~grelative to the cavity, diameter, conduction Iselectric field dominated, occurring mostly from thecathode tip. When A* is roughly one cathode dia-meter, the resultant axial conductivity gradientopposes the field gradient, thus moving peak cur-rent attachment up to one diameter upstream of thetip.

This paper focuses on emission processes at theMPD hollow cathode surface. Thres major possibleemission mechanisms are investigated: thermionic,pure field, and photoelectric emission. We beginwith thermionic emission.

IV. Thermionic Emission

1. Uniform Thermionic Emission

The emitted thermionic current density at a metalsurface is given by the Richardson-Dushmann equation:

J th = AT2 exp(-kTJ (1)

where A - 60 A/cm2 °K* for tungsten, T is the cath-ode surface temperature and<P, the work function,is about 4.5 e.v. for pure tungsten. Fig. 15 is agraph of thermionic current den: ty versus cathodesurface temperature for the tungt n hollow cathode.Also shown on',the figure are typic, -""asuredaverage surface current densities, <j for therange of arc current from 0.25 to 4.7 kA. Fromthese current densities and the Richardson emissioncurve, i t is clear that hollow cathode surfacetemperatures of order 3200 °K or greater are required

1 2

if thermionic emission is to be the major source ol"electrons in the discharge. For currents grerterthan 4.7 kA, ^he maximum thermionic emission, cor-responding to the Dciling point of tungsten, isstill not sufficient to account for the total emit-ted current at the cathode surface. For the lowercurrents of 0.25 and 0.9 kA, the cathode, if hotenough, could provide the necessary emission cur-rent density. It is therefore instructive toestimate the temperature rise at the hollow cathodesurface due to the discharge at these operatingconditioKs.

The discharge is assumed to provide a constantflux, F , of heat radially to the cathode surface.The heat is assumed to flow purely radially insidethe cathode and the cathode wail is assumed to beinfinite in radisi thickness. The problem can thenbe treated as one-dimensional heat conduction in asemi-infinite slab with a constant flux, F , inci-dent upon it. The solution of the appropriate heat

. (-1diffusion equation is standard. ( 2) We shall savespace here by simply showing a sketch of thd pro-blem (below) and quoting the resul::

TUNGSTEN HOLLOW CATHODE(SEM1-INFNITE SLAB)

j 0| (DUE TO PLASMA)

CATHODECENTERLINE

BOUNDARY CONDITIONS: t = o

T(r=oo)=0

The temperature anywhere in the cathode is thengiven oy:(12J -

4 -(r-r ) (r-r ) (r-r

(2)

where K is the thermal conductivity of tungsten and>f is its thermal diffusivity.

The temperature at the cathode surface (r = r ) is2FQ " c

Typically, the HPD hollow cathode takes about 10"*sec to reach a quasi-steady state. For thermionicemission to be significant, the surface temperatureshould therefore rise to temperatures of order3000°K or greater in this time. For this timescale, and for characteristic values of the physicalconstants K and"){» Eq. 3 reads:

T(rc,t) = (2.2 x 10'2) FQ (°K)

where F is in units of cal/cm*-sec.

Typically the energy flux to the cathode in anarc discharge is only about 10% of the total energyin the discharge. The rest is distributed amonganode heating, gas heating, gas flow power, radia-ted power and frozen flow power. We shall, however,assume that all the power in the discharge goes intothe cathode surface and thereby obtain an estimateof the extreme upper limit to the temperature rise

at the cathode surface.

For the higher current of 0.9 kA, with a measuredterminal voltage of roughly 40 V, the total dis-charge pjwer is:

PTOTALx 10 cal/sec

Experiments have shown that the plasma inside thehollow cathode forms over a length of typically onecathode diameter. The surface area of the cathodewhich is directly heated by the plasma is therefore:

S = TTX D x D = U.3 ctiT

where D is the cathode inner diameter of 1.9 cm.The average heat flux to the surface, F , i s thus:

^TAL = 7.6 x 102 cal/cm2-sec

For this value of F , Eq. 3 gives a temperaturerise at the cathode surface of only 17°K. Also,Eq. 2 shows that, on the time scale of 10~"sei;,heat has not diffused more than 0.2 cm into thecathode. Since the cathode wall is 0.6 cm thick,our earlier assumption of the cathode as a semi-infinite slab is justifiable. At the higher cur-rent of 4.7 kA, with a terminal voltage around 120v, the surface flux, F , is a factor of 15 higherthan at 0.9 kA. Even so, Eq. 3 then gives a maxi-mum cathode surface temperature rise of only 260°K.

This simple analysis shows therefore that due tothe short duration of the MPD discharge, the cath-ode is not significantly heated by the discharge.Uniform thermionic emission in the MPD hollow cath-ode is therefore not likely as the major source ofelectrons. There is, however, the possibility thatthe cathode surface is not heated uniformly but inlocalized spots distributed over the surface. Theelectrons emitted from these local hot spots couldbe scattered rapidly to appear to cover the wholesurface; since the magnetic field measurements weremade at a distance of 0.15 cm away from the surface,due to the 0.3 cm dimension of the probe itself, theprobe could have measured only a "smoothed out" cur-rent density near the surface. However, this pos-sibility can be negated by a very simple analysisas follows in the next subsection.

2. Thermionic Emission from Localized Hot Spots

Let the number of local hot spots be N. Let thearea of each hot spot be A. The average currentdensity of each spot is therefore:

where J is the total discharge current. Now ingeneral when a current j g of electrons is emittedfrom a spot, the ion current, j^, returning to thatspot along the field lines is smaller than j .This is especially true in an arc where the sheathdrop is low (typically 10-20 v), for then the ioni-zation efficiency of the emitted electrons is quitelow thus producing a smaller ion current than theemitted electron curient.O3) This return ion cur-rent density when multiplied by a voltage dropacross the sheath gives the flux, PSPOT'

i n c i d e n t

upon each spot. In arcs in general, u the ratioCj/j ) varies from as low as 10-3 up to 1.(13,14)For our purposes here, we choose the highest pos-sible value so as to maximize the heat input tothe cathode -not. The maximum flux, FcpQT» f°

r a

typical sheath drop of 40 volts (a large estimate

13

for MPD operating conditions) is therefore:

M X T5Now the largest possible value of J/NA if theemission is thermionic is about 700 A/cm2, corres-ponding to the boiling point of tungsten '"0m Fi^.IS. ttfith this maximum value of J/NA, the flux intothe spot be-.omes:

FSPOT,max. = 7 U °With this flux, Eq. 3 yields a maximum temperaturerise at the spot of only 150°K.

It is therefore clear that whether the heat fromxhe discharge is assumed to cover the cathode sur-face uniformly or to heat any number of localisedspots distributed over the surface, the dischargeenergy is not sufficient to heat any portion o£ thecathode surface to temperatures above about 500°K,assuming thermionic emission alone is present. If,however, the maximum spot density is not limited tothe rather low thermionic values but is severalorders of magnitude higher, as for example, withfield emission^ then the incident power density tothe spot can ir fact be high enough to cause melt-ing of the individual spots. This point is dis-cussed in the next section. Here we conclude thatthermionic, emission alone is not likely to providethe measured current in the MPD hollow cathode.

V. Field Emission

On the basir, of his pioneering work oh, electro-static probe theory, LangmuirCl*) recognized thatif there is a large number of positive ions in theprobe debye sheath, it is possible to pull electronsout of the metal by lowering the potential barrier.Mackeown(lS) conducted a detailed theoretical in-vestigation of Langmuir's hypothesis. His majorcontribution to the theory was the inclusion ofthe effect of wall-emitted electrons on the sheath,giving rise to Mackeown's equation of bipolar spacecharge movement:CIS)

J^c •= 7.6 x 105 V2 {j + (J:;)2-j_}, [v2/cm2] (4)

where0C is the cathode surface electric field andV the potential drop across the sheath. In 1928Fowler and Nordheim published a classic study ofelectron emission in intense electric fields.^6)Their emission equation is:(16)

Je=j_=1.54xl0"jC2/O)exp{-6.8xl07ij>2/^c>, [A/cm

2] (5)

where i is the work function of the metal, alreadyreferred to earlier. WasserabCl?) carried out atheoretical investigation using Eqs. 4 and 5 for amercury arc. He emphasizes that if the total cur-rent density, j = j + j , is known, then j + and j_are each uniquely determined by the two equations,for a specified cathode drop V and woi!: function,\J. The result of his graphical evaluation'*'-' isshown in Fig. 16 for a rather low work function ofy> = 2v. The figure may be interpreted as follows:with small values of j there is no adequate fieldfor significant electron emission, j thereforevaries proportionally to j. At large currentdesntities, j is practically constant since, when .dealing with extreme fields, C'vlO6 V/cm), a smallalteration of j + produces a large alteration in j_,therefore j_ is proportional to j.

By considering the energy balance and the slightionization ability of the cathode emitteH electronsas mentioned above (see ComptonU3)) t Wasserabproves the condition.

If, for example, we make q = 10, then, from Fig. 16we observe that even with the small wOTk functionof 2v the field emission mechanism is possible onlytor current densities >10 A/era2. In the spirit ofan- optimistic estimate of the total current densityrequirement for uniform field emission, we mightassume that q is as large as-unity; i.e. the ionand electron currents to the cathode surface Lreeq-ial. Then, Fig, 16 shows that total surCace cur-rent densities of order lGr A/cm2 are still requiredaccording to Wasserab1s model, for pure field emis-sion at this low work function of 2v. Such currentdensities are three orders of magnitude largnr thanthe highest measured surface current densities inthe MPD hollow cathode, (see Fig. 9). The workfunction used in the above calculations must beconsidered as being very low, even taking into ac-count the eventual influence of oxide layers on thetungsten surface. Kith more realistic values of <£jC f = 3.5 to 4.5v) it has been shownC9) that still

higher current densities are required for fieldemission than those quoted above for if = 2v.

It is therefore clear that for pure field emissionfrom the cold MPD hollow cathode, high fields(> 106 V/cm) and high current densities >106 A/cm2

are required. With debye sheaths of order 10~5cmin thickness and sheath drops of order 10-20 volts,the necessary high electric fields are certainlypossible at the hollow cathode surface. However,the high current densities require that the emissionbe localized into many spots distributed over theactive surface of the cathode. As mentioned earlier,these intense local sources of electrons, if theyexisted, could not be detected by the magnetic fieldprobe due to its size and distance from the cathodesurface. Also, the uniform grayish appearance ofthe cathode actire surface could be accounted forby a rapid, random movement of the spots all overthe active surface durii.g the discharge. Such amovement could be caused, for example, by smoothingout of ths metal surface due to bombardment of ionsreturning along the field lines to the spot. Theresultant lowering of the local electric fieldmight cause the spot to wander to a region of higherelectric field, and so on. Apart from ion bombard-ment smoothing, the surface of the spot could pos-sibly be smoothed by local melting. Consider forexample the case j = j = 106 A/cm2. Such an icncurrent density incident upon the spot, fallingthrough a sheath potential of about 20v, will causethe temperature of tht spot to reach the meltingpoint of tungsten within less than 30 Usec, accord-ing to Eq. 3 above. Such local melting mig.it causethe local electric field Co drop considerably andhence cause the emission site to wander to an un-heated region where the surface is still rough.Meanwhile the molten spot nay solidify and beroughened again by random ion bombardment, thusmaintaining the chain of spot movement.

iVe conclude here that whereas conditions in theMPO hollow cathode might be appropriate tc- coldfield emission, such a mechanism does impose strin-gent requirements on the nature of the emissionsites and is practically impossible to verityexperimentally.

14

VI. Photoelectric Emission

Although photoelectric cathode emission has beenrecognized as being important in glow discharges formany years, and has been discussed extensively byLoeb(l" and others, it appears to have receivedScant attention in the literature on high pressurearc discharges. In particular, in hollow cathodedischarges, the efficient confinement of radiantenergy by the cathode cavity intuitively suggectsthe possibility of photoelectric enhancement ofthe surface emission. It is surprising, therefore,that to the best of our knowledge, no quantitativeestimates of the photoelectric emission have beenreported for hollow cathodes, particularly in situa-tions where thermionic, pure field, fohottky andother secondary emission processes have all beenshown to be inapplicable.*^) I" the following, arudimentary analysis of the rat? .;ion processesinside the MPD hollow cathode *jads to the conclu-sion that under certain conditions it is possiblefor photoelectric emission to account for more than45% of the observed cathode current. This stressesthe need for a careful experimental study of photo-emission at the surface of MPD hollow cathodes andsuggests that photoelectric emission cannot belightly dismissed from ensideration as a primaryemission mechanism in hollow cathode discharges.

1. Maximum Photoelectric Yield

The maximum possible vacuum photoelectric yieldat the MPD hollow cathode surface is estimated asfollows: First the simplifying assumption is madet'nat all of the energy in the radiation field in-side the cavity arises from a single resonant elec-tronic transition from the first excited state tothe neutral or ion ground state of argon. Thisassumption is reasonable for a near LTE argon plasmain which the resonant energy gap for the neutralatom (Arl) or singly charged ion (Aril), 10v, ismuch larger than that for the higher excited tran-sitions. Two cases are considered: a weakly ion-ized plasma at low currents^ ' in which al) theradiant energy is assumed carried by 11.6v resonantphotons of Aril, and a strr-.gly ionized plasma at.ligh currents'- ' in which 17.lv Aril resonant pho-tons are assumed to carry all the energy in theradiation field. An upper bound to the resonantquantum flux in either of these cases incidenton the cathode inner surface can be obtained byassuming that all of the energy in the cavity dis-charge is in fact in the radiation field (andneglecting end losses). If this maximum possiblequantum flux is multiplied by a maximum quantumyield at the surface (ejected electrons/incidentquantum}, one then estimates the maximum primaryphotoelectri- current at the cathode surface.

The value of the quantum yield depends on theenergy of the incident quantum, and is quite sensi-tive to the angle of incidence and the orientationof the electric field vector of the incident quan-tum. It has been shown experimentally(20) thatlight obliquely incident on a photosurface consist-ing of a liquid sodium-potassium alloy yields amuch larger photocurrent (higher quantum efficiency)when the electric vector is contained in the planeof incidence than when it is directed normal tothat plane. The ratio of photocurrents for the twopolarizations is a function of the angle of inci-dence and reaches a maximum of about 60 for 60°incidence. The magnitude of this effect for ultra-violet light incident upon a tungsten surface isnot precisely known but must be accounted for by

15

proper averaging procedures ir. any detailed analysisof photoemission in the MPD hollow cathode. For ourpurposes hare, in order to estimate the maximum pos-sible photoelectric yi.2ld, we assume the highestreported yield for tungsten in the wavelength rangeof in teres t (700-1000 A). This value, = 0.2, wasmeasured(21) from a tungsten surface contaminated byoxide films, absorbed gases, e t c . , not unlike theMPD bellow cathode surface.

Table I below, sh™s the upper bounds of the vacuumphotocurrent deruit ies for the lowest current (0.25kA) at a mass flow of O.i jj' '-*^ SI lc i f o r t h e highestcurrent (4.7 kAj at a mass i"w. of 0.1 g / s e c , respec-t ively. For comparison, th t second column of tlfetable shows the measured average surface currei.tdensity at both of these operating conditions.

Table IComparison of Calculated Photo-Current Density

with Measured Surface Current Density in Hollow Cathode

OperatingConditions

J0.25.kA

m0.1 g/sec

J4.7 kA

m0.1 g/sec

Type ofPlasms

WeaklyIonizedAll Ar I2-levelAtomicSystem

StronglyIonizedAl) Ar II2-leve'AtomicSystem

Maximum Pos-sible VacuumPhotoelectricCurrent Den-sity, A/cm2

7.6

•

387

Measured AverageSurface CurrentDensity, A/cm2

22

830

Frosi able I it is evident that if all of the powerin the cathode region of the MPD hollow cathode arcis incident upon the tungsten surface as high energyresonant quanta, then the resulting photoelectriccurrent could account for between 25 and 50% of thetotal discharge current. Thus far in the calculationwe have not considered the effect of the cathode sui--face electric field on the emission. The presenceof a strong (M06V/cm) electric field at the cathodesurface can change the photoeirission characteristics.C22] in effect, the field can, under certain cir-cumstances, cause an electron in an excited statewithin the conduction band of the metal to tunnelthrough the surface potential barrier and be emittedas a photo-electron. This has tha net effect of in-creasing.the quantum yield fiom the vacuum photo-elsctric yield generally measured experimentally andused in our calculation above.

In principle, it appears possible therefor tif a significant fraction of the power in deregion in the MPD hollow cathode is incidentthe cathode surface as high energy quanta, the.. .field enhanced photoelectric effect can account forall of the measured surface current. The task thatnow remains is to investigate the collisional andradiative processes occurring in the volume of thehollow cathode plasma ir. order to determine whatfraction of the power in the cathode.region does infact reach the cathode surface as high energy

radiation quanta. This is dons in the next twosubsections<

2. Estimate.of Wet Fraction of Cathode RegionPower Available to the Radiation T'ield •

In the MPD hollow cathode, s'.ncc most of the_cur-rent is carried by electrons, the ohmic power j«E,is invested primarily in the electrons. The elec-trons then transfer, by elastic and inelastic col-lisions, some of this invested power to the heavyparticles. The flow a^c of thecathode orifice ofheavy particles heated by electron clastic scatter-ing, of hot electrons themselves, of radiation, andalso of heavy particles in excited states ("frozen"flow power losses) represents a fraction of thecathode region power that cannot be introduced into.the surface us i-adiation. When these individual"orifice loss" components are estimated for dif-ferent hollow cathode operating conditions andadded together,(9) it is found that only 10 to 15%of the. cathode region power leaves via the orifice;the majority of the power can therefore be intro-duced, into the radiation field inside the cavity,thus justifying one of the assumptions made in theprevious subsection.

3. Integrated Line Intensity in the Hollow Cathode

We have assumed earlier that all of the power inthe cathode region is contained in a.single opticaltransition, for either the weakly ionized Ar I plas-ma or the strongly ionized Ar II plasma. It re-main"; to be shown, therefore, that the integratedline intensity from the resonant op"ical electronictransition in an Ar I or an Ar II p'.asma at speci-fied densities and temperatures can, in fact, be aslarge as the total power in the. cathode region ofthe MPD .hollow cathode plasma. This is done asfollows:

For'the two operating conditions listed in TableI, the absorption coefficient at line center, k ,is evaluated using: C "

Table II(9)

- A2

) (-£)(—•> A,, N, (6)

Here X is the wavelength at line center (in thiscase the wavelength of the resonant transition);g., and g are the statistical weights of the reso-nant and ground state,respectively; A,^ is theEinstein coefficient forthe resonant transition,and N. is the number density of atoms or ions inthe ground state. In writing k in the form of Eq.6, it has been assumed that the number density ofatoms or ions in the resonant state is much lowerthan that in the ground state. Also, other pro-cesses that could contribute to the broadening ofthe absorption line have been neglected in compari-son with Doppler broadening. When collisionalbroadening is important, the value of k can beconsiderably less than that given by Eq? 6. How-ever, even when the damping ratio is 1, that is tosay when coliisipnal broadening and Doppler broad-ening are equally important, the value of k isstill as high as 32% of its pure Doppler broadenedvalue,(23) y0I typical MPD hollow cathode condi-i.iotis the damping ratio is in fnct quite smallf^)(VL0"5). We may therefore retain the analyticallysimple form of Eq. 6 and estimate k quite accur-ately under these conditions. The values obtainedare shown in Table II, below.

Absorption Coefficient at Line Center, k

Type of Plasma

0.25 kA, 0.1 g/secAll Ar I Plasma\Q = 1067A, T o 2300-!K

4.7 kA, 0.1 g/secAll Ar II PlasmaX - 723X, T 'V 4.6 x 10 °K

k0

2 X 103

5.4 x 102

From Table II, the large values of k , for acathode cavity dimension of 1 cm, indicate that theresonance radiation is very effectively trappedwithin the cavity plasma. The spectral radiance ofthe resonant radiation at line center, ^^.l^-i, can

therefore be approximated by the blackboOy radiance,\(v )» at tne given frequency and at the tempera-ture°of the electrons, T . The total intensity ofthe resonant line radiation is then given by:

TOTx W x Av_ [W/cm] (7)

where iv_ is the Doppler half-width of the line andW is the equivalent width of the line defined as thewidth (given in Doppler half-widths) of tbdt fre-quency interval of Ihe nearby black-body spectrumnecessary to radiate the same total energy as thespectral line in question. Equation 7 may now beused to estimate the equivalent width of the reso-nant line necessary for this single line to radiate,as a black-body., a tota' power equal to the cathederegion power in the MPD hollov.' cathode. The resultsof this calculation, detailed elsewhere,(9) arepresented in Table III, where the equivalent width,W, and the quantity W x Av_, which expresses theequivalent width 111 angstrom units, are given fortwo different electron temperatures, for er^h of thetwo operating conditions already referred to above.

Table IIIEquivalent Width for Single Line to Radiate

Total Cathode Region Power(9)

Type of Plasma

0.25 kA0.1 g/secweakly ionized

4.7 kA0.1 g/secstrongly ionized

Electron TemperatureTe, e.v.

l.S2

45

W

65093

7227

X D3,80.54

1.30.53

Table III .shows that, the necessary equivalentwidths for a single line to radiate the total cath-ode region power into the cathoiksensitive to T . At T = 1.5

d &

r'ace are quiteu the 0.25 kA1 .v. for the_;,e necessarv

weakly ionized plasma, l"J and ';4.7 kA strongly ionized plasma.equivalent widths are observed t j tie 3.8 and 1.3 A,respectively. At f i r s t glance these widths seemunreasonably large. However, resonance broadeningof spectral l ines can lead to very large linewidths, par t icular ly in^high density plasraas( >Z5>Recent measurements (26J of ths half-width of the1235.8 A resonance line of Krypton behind a Mach 12shock at conditions of neutral density, N M0 1 8 cnr ' ,N M018cm-3, and T = 1 s .v . , show that the reso-

© 6

J6

nance line is 10 to 20 A wide, "hese post-shockplasma conditions are not unlike those in the MPDhollow cathode and hence the equivalent widthsrequired from Table III are quite plausible.

The equivalent width of a spectral line has alsobeen numerically evaluate to —r •-•:•_ i_r a u i i'our5 l ijif.LcunL rigures by Jansson and Korb.*- ' Theypresent a table of values of W for different valuesof damping ratio, a, and the product k £., where £is a characteristic dimension (cathode cavity radiusin our case). For an estimated value of a - 10"3

in thijJlPD hollow cathode, Die data of Jansson andKorbv" ' were used to plot a graph of W versus k It,.shown in Fig. 17. Also shoun in the figure is agraph of W versus k £ for a higher damping ratio ofunity. °

As shown in Fig. 17, the,equivalent widths of theresonance lines of'Ar I and Ar II, when the dampingratio is 10"3, for values of k^l given in Table IIare roughtly 10 to 20. From Table III, the equiva-lent widths necessary fur these resonance lines toradiate all of the cathode region power are between650 and 93 for the Ar 1 plajma, at temperatures be-tween 1,5 and 2 e.v.: for the Ar II plasma, theyare between 72 and 2 7 for temperatures between 4and 5 e.v., respectively. The agreement even tojnly a factor of 10 between the equivalent widthsobtained by the above two different approaches isclose enough to und"rscore the possible importanceof the photoelectric effect in the MPD hollow cath-ode. At higher neutrnj densities and lower heavyparticle tempeiatures than those assumed in thecalculations above, which are quite likely undersome operating conditions, the damping ratio may bequire a bit larger than 10~3. As an example of theeffect of a higher damping ratio on the equivalentwidth, the data of Ref. (27) have been plotted fora damping ratio of unity in Fig. 17. Now it isobserved that at the k i appropriate to the Ar I orthe Ar 11 plasma, equivalent widths of 102 or great-er are possible. From Table ill this would thenmean that the total cathode region power could beradiated in the resonance line at temperatureslower than those discussed above.

The significance of the above calculations isthat in the MPD hollow cathode, due to the imprison-ment of the resonance radiation, the intensity ofthe resonance lines can be built up to their maxi-mum possible equilibrium value, given by the Planckfunction, thus giving rise to a sufficiently highflux of high energy quanta to the cathode surfacewith a resulting photoelectrically emitted currentwhich is of the same order of magnitude as theobserved discharge current. Photoelectric emissionthus is a possible uniform primary emission mechan-ism in hollow cathode discnarges.

Large hollow cathodes in a variety of configura-tions have been successfully operated in high cur-rent MPD aics without the assistance of auxiliaryheating, low work function inserts, or externalkeeper electrodes. By examining current and poten-tial distributions inside the hollow cathode cavityover a wide range of currents and mass ..flows, anempirical criterion for characteristic hollow cath-ode operation has been identified. Thermionicemission has been shown to contribute negligiblyto the emitted current in these MPD hollow cathodearcs. Field emission from micro-spots on the

cathode surface, and also a form of field-enhancedphotoelectric emission, have been suggested as theprimary emission mechanisms in such cathodes.

Acknowledgement

ine author is grateful to Professor Robert u. Jahnfor valuable advice in preparing this paper. Help-ful discussions were also held with Dr. Kenn Clark,Professor S. H. Lam, Professor R, B. Miles,Mr. Michael Campbell and Mr. W. F. von Jaskorwsky.

References

1. Paschen, F., Ann, d. Physik 50, 1916, p. 901.

2. Byers, D.C., "Performance of Various Oxide Maga-zine Cathodes in Kaufman Thrusters," NASA TN D-5074, 1968.

3. Lidsky, L.M., Rothleder, S.D., Rose, D.J.,Yoshikawa, ^., Michelson, C. and Mackin, Jr., R.J., "Highly Ionized Hollow Cathode Discharge,"J. of Applied Physics, Vol. 33, No. 8, 1962.

4. Keen, B.ii. and Aldridge, R.V., "Low FrequencyWave-Mixing in a Magnetoplasma," Phys. Lett. A29 (5), 1969 p. 225.

5. Day, B.P., Fearn, D.G., and Burton, G.F., "IonEngine Development at the Royal Aircraft Estab-lishment, Farnborough," RAE Tech Rep. 71102,1971.

6. Delcroix, J.L., Minoo, H, and Trindadea A.R.,"Etude de decharges a cathode creuse en regimed'are," Rev. Roum. Phy?. 13(5), 1968, p. 401.

7. Jolly, J., "Argon Ion Laser Using Hollow Cathodeand Graphite Confining Structure," Proceedings of1st International Conference on Hoi lev-' CathodeDischarges and Their Applications, 1971, p. 20.

8. DiCapua, M.S., Ph.D. thesis, Princeton University,AMS Department 1971.

9. Krishnan, M., Ph.D. thesis, Princeton University,AMS Department, 1976.

10. Krishnan, M., vonjaskowsky, W.F., Clark, K.E. andJahn, R.G., paper #75-395 presented at AIAA XlthElectric Propulsion Conference, New Orleans,Louisiana, March 1975.

11. Krishnan, M., Jahn, R.G., vonjaskowsky, W.F., and£lark, K.E., paper #76-984 to be presented atthe AIAA Xllth Electric Propulsion Conference,Key Biscayne, Florida, Nov. 1976.

12. Carslaw, H.S. and Jaeger, J.C., Conduction ofHeat in Solids, 2nd Ed., Oxford, ClarendonPress, 1959.

13. Corapton, K.T., "On the Theory of the Mercury Arc",Phys. Rev. 37, 1931, p. 1077,

14. Langmuir, I., "Uber Feldemission", Gen. Elec.Rev. 26, 1923, p. 735,

Langmuir, I. and Mott-Smith, H. "Uber Feldemis-sion," Gen. Elec. Rev. 27, 1927, p. 449.

15. Maekeown, S.S., "The Cathode Drop in An ElectricArc," Phys. Rev. 34, 1929, p. 611.

17

16. Fowler, R.H. and Nordheim, L.W., "Electron t ' l ission in Intense Elec t r ic Fields", Proc. Roy.Soc. London AI19, 1928, p . 173.

17. Wasserab, T. , "Zur Theorie des Quecksilber-Kathodenflecks", Z. Physik 130, 195], p . 311.

18. Iceb, L.B., Basic Processes of Gaseous Elec-t ronics , Univ. of California Press, CambridgeUniversity Press, 19SS.

19. Kojariinovic, II. and Minos ii . , "hi RegionCathodique D'un Arc A Cathode Crease En RegimePulse," Laboratoire De Physique Des Plasmas,Universite de Paris - Sud, Centre D-Orscy,Rapport Internei ie , Dec. 1974.

20. Garbuny, M., Optical Physics, Academic Press,New York, 1965, pp. 395-3D7.

i i . Wheaton, J.E.G., "Photoelectric Yield of Tung-sten' ' , j . Opt. Soc. Am., 54, 1964, p . 287.

22. Williams, R., Semiconductors and Seirimetals,"•:!. 6, eds. RTR. Willardson and A. Beer,Academic Press, 1970, p., 97.

23. Mitchell, A.C.G., and Zemansky, M.W., ResonanceRadiation and Excited Atoms, Cambridge Univer-s i ty Press, 1961.

24. Suckewer, S . , Licki, J . , Mizera, B. andZelazny, P . , "Study of Various Methods forDetermination of the Plasma Temperature in theMHD Generator Duct," t ranslated from the Rus-sian in "Electr ic i ty from MKD" IAEA, Vol. IV,Vienna,' 1968, pp. 2179-2194.

25. Kulander, J .L . , "Curves of Growth for Non-Equilibrium Gases", J.Q.S.R.T., Vol. 8, 19D?>,pp. 1319-1340.

26. Carls, H.H. and Zuzak, W.W., "Measurement ofthe Vacuum Ultraviolet Radiation from a ShockHeated Krypton Plasma", J.Q.S.R.T., Vol. 11,1961 pp. 1135-1141.

27. Jansson, P.\. and Korb, C.L., "A Table ofEquivalent Widths of Isolated Lines with Com-bined Doppler and Collision Broadened Profiles",Vol. 8, 1968, pp. 1399-1409.

CAPACITOR LINEI

ARCCHAMBER

HOLLOW CATHODEWSULATOR

INJECTORS

Fig. 1 Hollow Cathode Discharge Apparatus

18

TUNGSTEN

1.28 cm

MOLYBDENUM-1

o) UNINSULATED CATHODE

t) HC I

MOLYBDENUM

TUNSSTEN

0 6 4 cm

0.48cmNYU3N BRASS-*

0.35cm

HC mr

STAINLESS STEElA

f) HC X

t2° t.9cm

TUNGSTEN - J U - 0 . 2 c m

NYLON ,BORON NITRIDE

It) HC2H

Fig. 2 Hollow Cathode Configurations

FRONT VIEW

m -12 g /sec50%HCFL0W5 0 % OUTER FLOW

a) CHAMBER t>) DISCHARGE 0 16 kA

Fig . ' 3 Discharge Through 4800 X All F i l t e r

j . 9

Fig. 4 Enclosed Current Contours; m ° 0,3 g/sec

FLO

w>W

(.Op

0.6

O.fi

0.4

0.?

\IR=0.95cm

J

m«4q/sec

'///////y.

J e n c i - ^

y

t

- ^ !

AXlflL POSITION. cm

y

AXIAL POSITION , cm

Fig. 5 Current and Potential Distribution inH.C. VIII

O.S4cm CH-1WSL

t.3cm ORIFICE

1 2 cm CATHODE 01A.

-•rua.1

i) UNINSULATED CATHODE29 W x 0.3 q/sec

b] INSULATEO CATHODE HCI7»A x 0.4 g/sec

32 2.8 2.4 2.0 1.6 12 0 8 O.< OAXIAL POSITION.cm

Fig. 7 Current Density Profiles, 0.25 kA

22 2.8 2.4 2 0 16 12 OB 0.4 0AXIAL POSITION,cm

Fig. 8 Current Density Profiles, 0.9

Fig. 6 Spectra of Hollow Cathode Discharge

20

V •FLOWI — ^

v///////////////A1

R = 0.98 cm

]

5X105 gMC -

102

4 — .

0.4

2XICT2

1

^ \

-

f

2000

1800

1600 ~

1400 <

1200 ttn

000 a

800 §

600 3

400

00

3.2 2.8 2.4 2.0 1.6 1.2 0.8 0.4 0AXIAL POSITION . cm

Fig. 11 Active Zone Length

Fig. 9 Current Density Profiles, 4.7 kA

3.2 28 2.4 20 16 12 08 0.4 0AXIAL POSITION,cm

Fig. 10 Current Density Distributions, 0.2 g/sec

FLOW » : ? :' i ;' »

t) *»0.1 g/uc

Fig. 12 Enclosed Current and Potential Profiles,0.25 kA

21

32 24 16 08AXIAL POSITION,cm

W rti » 0 : y/sec

Fig. 13 Pover Density Profiles, 0.25 kA

77777777777,UNINSULATED H.C.2I/0- ///A'//////////// 7/777/,

I

O.25WO.I g/sec

UNINSULATED-

INSULATED—^

A

3.2 ZA 1.6 0.8AXIAL POSITION, cm

a) Current Density Distributions

b) DISCHARGE RADIANCE c) OOUBLE EXPOSURE

Fig. 14 Uninsulated Hollow Cathode, J=0.25 kA,ra=O,l g/sec

:o-'

Fig. 15 Thermionic Emission, Tungsten

CURRENT DENSITY j , A / c m 2

Fig. 16 J and J as Functions of J = J + J ,Rif. (17)

LINEAREXTRAPOLATION

Fig. 17 Equivalent Width for a Voigt Contour,Ref. (27)

22

DISCUSSIONJ. L. MASON: How dependent is yo^E electrodynamicflow modelling on the fluid dynamic modelling, theturbulent flow modelling - do you have to make someassumptions regarding the velocity d is t r ibut ions ,for ejtample, and are such assumptions reallyimportant to your development of the e lec t ro-dynanic flow modelling?

M. KRISHNAN: I have not solved the analyticalproblem owing to the complex nonlinear couplingof electrodynamics with the fluid dynamics incidesuch a discharge. But Ke did do an experiment inwhich flow to the hollov cathode was cut off a l to -gether, and different ambient p re f i l l prfissi' -es ofargon (corresponding to analytically estimatedflow s t a t i c pressures for each mass £low) were setin the discharge vessel . Tlie surface currentdis t r ibut ions measured witli ambient gas in thecathode were very similar to those measured at thesame current with different mass flows through thecathode. We thus tentat ively conclude that It isin fact the s t a t i c pressure component of theflowing gas in the hollow cathode that ispredominantly responsible for establishing thecurrent conduction pat tern. Thus fluid dynamicsappear to be relat ively unimportant to theelectrodynamics in such discharges.

J . F. DAVIS: Is i t possible hat at highpressures, larPL • numbers of ar-^"" i corns arriveat the ca<-;.ode surface as moltci les ,-nd thatthereto.e s?1 f-absorption of vacrjm ul !:ra-violetphotons is hindered? \

M. KfUSHNAN: The press-ire inside the cat. odecavity varies froir as low as ^ \Q~1 _orr Jt. to^ 1 ) 0 to r r . Even at the higher pressures ,^'hecat.lode plasms .is a hot plssiia and I wi/uld '.,therefore say that there arc not a signif j.caa •.fraction of molecules in r.b is discharge. \

M. CAMMELL: The temperatures estimated Inuuch a pl^sira are aroaic? 20,000°K at whichmolecures ct^npt exist in any significant /fraction. v<.

23

A SCALING LAW FOR ENERGY TRANSFERBY INELASTIC ELECTRON-MOLECULE COLLISIONS IN MIXTURES

George K. BienkowskiPrinceton University

Princeton, N. J.

Abstract

The equation governing the electron energydistribution in the presence of a spatially uniformelectric field in a weakly ionized gas has been re-formulated into an integral equation for the loga-rithmic slope of the distribution function. Forgas fixtures in which the dominant electron energy-lass mechanism is by vibrational excitation of themolecules, this equation is suitable for approxi-maue analysis and exact numerical solution by iter-ation. Super-elastic collisions are easily in-cluded in this formulation, and do not seriouslyeffect the convergence of the numerical scheme.The approximate analytical results are only quali-tatively correct, but suggest appropriate param-eters which correlate the exact numerical resultsvery well. The distribution function as well ascertain gross properties such as net energy trans-fer into vibration, mean energy, and drift velocitydepend primarily on a single non-dimensional param-eter involving only E/N and the cross sections.This parameter can be physically interpreted as theenergy gained from the field between succ ..iveinelastic collisions divided by fhe typical inelas-tic energy loss per collision. Results for mix-tures of CO, He, and N2 are presented and shown tocorrelate most of the properties.

I. Introduction

A common and very effective way to transferenergy preferentially into vibrational states is byelectron impact in a gas discharge. Many practicalinfra-red lasers, such as CO and C02 depend criti-cal iy on this process. The analysis of the per-formance of such a device depends critically onthese electron impact excitation rates. Nighan1

has shown that using a Boltzmann distribution basedon the mean electron energy can lead to erroneousresults. The determination of the correct electronenergy distribution function is therefore criticalto the analysis an; design of electrically excitedmolecular lasers.

In most laser codes the electron distributioneffects the results directly through the excitationrates:

S i j ( e ) f e ( e ) e d E (1)

where N} is the number density of the molecule ini'th state, ne the electron number density, Sjj(e)the inelastic cross section for the i to j tran-sition by electron impact at energy e, and fe(e)is the electron distribution function. When quasi-steady results are desired the significant param-eter becomes the net power into vibration which is:

Pvib * N ne fwhere

„(£)«!£ (2)

(3)

and N is the gas number density, while £jj is theenergy exchange in the i ->• j collision. In eithercase the results are sensitive to the behavior ofthe distribution function, because the most rele-vant cross sections tend to be relatively sharplypeaked at precisely the energy where the distribu-tion function is varying most rapidly. The vari-ation of the distribution function in turn isdominated by the behavior of precisely those crosssections.

2 3In most previous work ' the electron distri-

bution function is computed for a specific electr-icfield (E/N) and gas mixture under the assumptiontha. only the excitations from the ground state areimportant in determining this distribution. Forlasing situations where a large number of highervibrational states may be appreciably populatedthis assumption may need to be relaxed. This hasbeen recognized before and some calculations esti-mating the effect of higher vibrational level popu-lations and super-elastic collisions have been pub-lished. '>* The techniques used have generallyrelied on modification of techniques developed forzero vibrational temperature and become progres-sively more difficult to apply as the vibrationaltemperature becomes large. In addition roost of the•"•evious calculations have been made, for specificm..;tures and presented as examples with no scalinglaws proposed. Judd^ showed that the mean electronenergy IT acts as a normalization parameter inde-pendent of mixture ratio for the prediction ofrates in CO2-N2-He mixtures. Unfortunately to findthis parameter for a specific mixture ratio onemust essentially do the entire calculation or atleast determine a cross plot of u versus the mix-ture ratio for the specific gases considered.

This paper re-formulates the equation for theelectron energy distribution function in a way thatsuggests natural scaling parameters that can becomputed a priori, and is amenable to numericalcomputation over a very wide range of vibrationaltemperatures.

II. Basic Assumptions and Formulation

Formulation .

A standard spherical expansion (up to secondterm) for the electron distribution function, in aspatially homogeneous weakly ionized gas leads to

the equation for the spherically symmetric part f0as Dresented bv Niahanl;

s i j

.-+*•$ •

where

uW =-•£ 6 s

s T

(4)

(5)

The symbols previously defined which depend onspecies have been superscripted with the letter s.Qms(e) is the electron-molecule momentum exchangecross section for s.pecies s, 6f is the ratio Nf/Nwhile 6s = Ns/N = * Nf/N. For convenience fo(e)can be normalized in such a way that

/fo(E) (6)

The second term f](e) is directly related to thederivative of f0 and leads to the drift velocity.f] must remain small with respect to f0 in orderfor the entire procedure to be justified.

Equation (4) can be changed to a pure integralequation by considering the negative of the loga-rithmic slope of f0 as the new variable

d in f(e)(7)

The incorporation of detailed balance on the molec-ular level (with no degeneracies)

leads to the equation

u ( e ) B ( e ) = J J Ss i j > i

- «• exp

(8)

B(n)dn)

(9)

jThis equation is suitable both for some approximateanalysis and for numerical integral iteration. Theequation is essentially exact within the standardassumptions of slight ionization level, two termspherical expansion, and small mass ratio of elec-trons to neutrals. Any successful analysis ofEq. (9) is going to require either complete know-ledge of all the cross sections and populationlevels, or additional approximations and simplifi-cations. Since the former is not available we pro-pose certain simplifications that will retain theessential features of the problem and are notdrastically inconsistent with the expected crosssection behavior.

Additional Approximations

No experimental and only limited theoreticalresults'" for electron-impact cross sections withvibrationally excited molecules exist. Since in alaser related situation some estimate of the effect

of these collisions is better than complete neglect,the following simple assumption is used. The exci-tation process from level i to i+n is in all re-spects equivalent to the excitation process fromlevel 0 to level n; i.e., the inelastic c/oss sec-tions and their associated energy losses depend onthe level difference n, but not the identity of thelevels:

(10)Eol

The assumption on the energy losses involves essen-tially the neglect of anharmonicity and its directeffect on Equation {9) is not expected to be ser-ious. The assumption about the cross sections ismuch more difficult to justify and is undoubtedlynot true in detail, the general level of the crosssections is, however, consistent with Chen's re-sults** for N2 even though the energy dependence isnot correct.

Incorporating the assumptions (10) with Eq.(9) reduces the double sum (over i and j) into asingle one over n = j-i. The resulting equation is:

/• i+ T leol/ fB(n)dn

where

S _a -n

AS/XS

j=n

(ID

(12)

The effect of the vibrational (and electronic)level populations now enters the problem solelythrougn the parameter ajj. In a situation wherevibrational excitation is dominant and the lowestlevels can be approximated by a Boltzmann distribu-tion function at an equivalent vibrational temper-ature T v o based on the 0 to 1 population ratio, of)becomes exp(-ne|-]/kTV0). For typical lasing situ-ations in CO where the "plateau" region of thevibrational level distribution is 10-2 to 10"J ofthe ground state vibrational population this ap-proximation is a very good one. Within theseassumptions Equation (11) can serve as the startingpoint for deducing effects of mixtures through theparameters 6s and Sp, and the effect of elevatedvibrational energy through 0s, as determined by theparameter T v o.

III. Approximate Solutions

Vibrational Collisions

In the region of energies where vibrational-excitation collisions are dominant, the character-istic range of integration on the right-hand sideof Eq. (11) is E 0 a typical vibrational energyspacing. Now the logarithmic slope B is assumedto be approximately constant over an energy range£Q and al s o large enough so that integrals frome + E 0 to E + 2e0 can be neglected compared tointegration from E to e + Eg. These assumptionsincorporated into Eq. (11) lead to an approximate

25

equation for g = e0B:

where

and

s a n d sv (15)

while e is an energy value between 0 and e 0 whichdefines an average value of 3y. Formally £ isdefined by the relation:

= S V ( E + E ) E / e'exdx

The parameter R(g,eo/kT ) can again be formallydefined as vo

T )vo

(£+eox)e dx

•v. . ' '&

sSv(e+E)/ e dx (17)

R(e. Eo/kTVp) ^ ST (e+e)/S»(e+e) iMhen e « eo/kTV0

and the whole term is negligible for eo/kTvo » T.When 3 is near VkTvo> R(S. EoAT vo) approachesunity and clearly represents a detailed balancebetween vibrationally exciting and supsr-elasticcollisions. Equation (13) can be solved for anynumber of approximate representation of K(3,E 0/kTvo) but the number of assumptions already incor-porated in its derivation do not warrant anythingbut the simplest solution. The term involvingR(S, E0/kTv) is assumed to be negligible except-when 0 % E0/.kTV0 at which point $ = £0/kTv is theappropriate solution. The results of thi£ simpleapproximation are shown in Figure 1 with e = £ 0/2.The approximate solution for S varies linearlywith the parameter M(e) when M(e) « 1 and as^ f T when M(E) >-• 1.

Electronic Collisions

The approximate solution above did not explic-itly depend on the fact that the dominant inelas-tic processes were vibrationa't excitation. Theassumption that the logarithmic slope is approxi-mately constant over the characteristic energy,range e 0 (the typical energy toss), however, makesthe approximation inappropriate in the energyrange where electronic collisions are dominant.A reasonable approximation can be obtained pro-vided the cross sections in this energy range canbe approximated by a simple step function followedby a slow 1/e decay.

Sj,(e)

Substitution of Eq. (18) into Eq. (11) together withthe assumption that % $ ( % ) » 1 leads to the fol-lowing approximate solution for 6e =

EmB (e):

em

1-e(19)

l-e

where

The function Ei(x) is the standard exponential inte-gral

Ei(x) -•'—CO

(21)

ng\ The result for eg » 1 (a reasonable approximationwithin the expected range of validity) is shown in

(18)

Figure 1.

Basic Parameters

Aside from the obvious parameter eo/kTvo whichmeasures the importance of super-elastic vibrationalcollisions, the electric field E/N appears in a non-dimensional grouping of the form involving a charac-teristic inelastic energy loss and the square rootof the product of the momentum and inelastic crosssections. In the region of dominant vibrationalcollisions this modified E/N parameter appears to be1/«/M(E). while for electronic ekcitation region,the parameter is 1/Sm. In either regime the param-eter can be viewed as the ratio of energy gainedfrom the field by electrons of energy e between twosuccessive inelastic collisions to the character-istic energy loss for the relevant inelastic process.The generalized parameter is therefore E £(E)/e?where ec is obviously chosen either as £p for yibra-tional collisions or s,,, tt; the electronic excita-tion process. The only pa. rietar requiring expla-nation is the length scale He). Under the assump-tion of a random walk of the electrons with manyelastic energy-conserving collisions between succes-sive inelastic collisions £ ( E ) ^ -JHc(e) Am(e) whereNc is the number of elastic collisions between twoinelastic collisions and m the momentum mean freepath is the characteristic distance between elasticcollisions. Nc in turn can be considered to be\/\n- Substitution of simple formulas for the meanfre<3 paths in terms of the cross sections leads tothe result:

for vibrational excitation

for electronic excitation (22)

Indeed while certain aspects of the approximate solu-tions shown in Figure 1 are certainly of doubtfulaccuracy the relevance of the energy ratio E£/e

c isphysically obvious and should be the principal param-eter determining the local behavior of the distribu-tion function. Figure 2 in fact shows that whilethe approximate solutions are not qualitativelycorrect, the parameters mentioned above do a very

26

good job of correlating a very large number ofexact numerical solution for many mixtures of CO,N2, and He (see Section V).

V. Numerical Solution

Basic Procedure

Equation (11) is amenable to straightforwarditeration as a numerical scheme of solution. Con-sidering Eq. (11) in the form

B(e) = K(B(e),e)

where K is an integral operator, an iterationscheme of the form

(23)

B(n)(e) = [K(Bn"1(e).e) + xB(rM)J/(l+x) (24)

is used. Iteration is continued until

< a B(n-l) (25)

where a is a specified accuracy.

The procedure is insensitive to tht choice of x aslong as B never approaches l/kTv0 and generallytakes 5 to 10 iterations. This remains true aslong as the parameter nv = NkTv0 -J (3QmSv)max/E< "x. .9.

For low fields and hign vibrational tempera-ture nv > .9 and the super-elastic collisions prac-tically balance the vibrationally exciting col-lision in the energy range where the vibrationalcross sections are high. The operator K becomesvery sensitive to small errors and the convergenceof the iteration process given by Eq. (24) becomesstrongly dependent on the value of x. Since themajor part of the problem comes from that regionof energy space where the local B(e) is very closeto l/kTV0, an additional stabilization scheme isintroduced to dampen large fluctuations in thisenergy range. The scheme is operative only whensuccessive iterants on B(e) lie on opposite sidesof a specified thin strip about l/kTV0 (l/kTVo ± Y)-The final approach to the converged solution istherefore unaffected by this procedure. Addition-ally, tests have been performed where x is reducedto zero once the solution has approached within arequired accuracy, and further iteration has notindicated any instability.

Solutions have been obtained for many caseswhere nv > .9 with values of x varying betweenabout 2 and 5. The number of iterations to con-vergence is higher than for the fly < .9 situationbut generally 12 to 15 are sufficient.

Calculations

All the calculations reported in Section VIwere performed with the accuracy, a, set at 10-3,on an IBM 360-91 computer. Each iteration takesfrom .1 to .1 seconds depending on the number ofspecies, cross sections, and energy points used.A check on the validity of the results is pro-vided by an overall energy balance. This wasfound to be always satisfied to better than 15!.

A series of calculations were performed forseveral mixture ratios of CO-He, C0-N2, and C0-H2-He,for a number of (F./N)'s. The energy range was gener-ally from E£=0 to eu

=19» with spacing Ae=.O5 up toe=5.0 and AE=.25 for e=5.0 to eu=19.0. For therange of E/N's used the upper limit on the energy £udid not influence the solution in the relevant ener- 'gy region as long as eu > 10. The vibrational crosssections for CO were taken from Schulz' and Bonessand Schulz8. A single electronic-excitation crosssection with a threshold of 6 volts and magnitude of10" 1 6 cm2, as used by Nighan', was used for CO. Thenitrogen vibrational cross sections were taken fromSchulz? andBonessand Schulz" while electronic andionization cross sections were taken from Engelhardt,Phelps, and Risk9. The momentum cross sections weretaken from Hake and Pheips10 for CO, from Engelhardt,et al.9 for N2, and from Frost and Phelps11 for He.

VI. Results and Conclusions

Scaling Parameter

As suggested bj ? approximate solution thelocal logarithmic slope should be primarily depen-dent upon the parameter M(e) within the range wherevibrational collisions are dominant. The resultsof all of the runs were thus plotted as 8 = -e9d In f/de versus M(e) with e0 chosen as the moiefraction averaged vibrationa] spacing.

(26)

A summary of the results is shown in Figure 2 to-gether with the approximate solution. As might beexpected the correlation is best at high values ofthe slope B and poorest at low values. In additionthere is noticeable separation between the resultswhere the cross section is rising with increasingenergy from those where it is falling. The corre-lation is, however, in general better than is thequantitative agreement with the approximate solu-tion. The results suggest that scaling of thelocal behavior of the distribution function from onemixture to another and one E/N to another can beaccomplished by examining the M(e) parameter, with-in the energy range where vibrational collisions aredominant.

Carbon Monoxide-Helium Mixtures

The desired results such as f(e) and its vari-ous moments must of course depend on the shape ofthe cross sections as well as some characteristicvalue of M(e}. In order to test the correlatingvalue of the parameter M(e) without too much "in-fluence of different cross section shapes, a seriesof calculations for mixtures of CO and Helium hereperformed for identical values of the parameter

(27)

but different mixture ratios (from 100% CO tc 52 CO)and E/N's. The results for the distribution func-tion are shown in Figure 3 for two different a's.While there is some spread of the results the param-eter appears to be a very good measure of the ef-fectiveness of the electric field. If the resultshad been plotted for common values of E/N ratherthan a no apparent correlation would exist JS E/N

27

for 100% CO has to be about 8 times larger thah for5% CO in order to achieve similar behavior. Thefraction of the input powor going into vibration isshowiin Figure 4 as a function of the parameter a.The correlation is extremely good and indicatesthat a ^ 1 is a good indicator where energy beginsto go into electronic states. All of fha resultsfor T v o < !500 are virtually indistinguishable fromthe results without super-elastic collisions, butbegin to deviate significantly for higher vibra-tiona] temperature. Is can be seen from the curvefor T v o = 5000°K, high vibrational temperature pro-duces a reduction of energy into vibration at afixed a. A more detailed examination of the frac-tional power into each process involving (Av = 1,2, 3, ... etc.) change in vibrationaT energy alsoshows a remarkably good correlation with the param-eter a.

The equivalent electron temperature or 2/3 themean electron uurgy has generally been considereda useful indicator of the state of the electron gasin a discharge and has been previously used as acorrelation parameter for CO2 mixtures.5 Figure 5shows this effective electron energy versus theparameter a for iwo different vibraCionaJ temper-atures and different mixtures of CO-He. The cor-relation is again remarkably good and suggest thatthe mean energy would also correlats the results asin CQ2. The parameter a, however, is a much moreuseful scaling parameter as it can be computedsimply from given quantities such as E/N, crosssections, and species concentrations. It can beused in preliminary design of experiments by pre-dicting the required change in the electric fieldto maintain similarity of electron-vibrationalpumping under changes of mixture ratios of the gasconstituents.

The parameter a appears to be an adequate cor-relating parameter for CO-He mixtures since theshape of the cross section product Qffl(e)Sv(e+e0/2)is only very mildly dependent on the mixture. Theproduct (Qm Sv),,,^ is therefore an adequate measureof the local slope B over the entire relevant energyrange rather than simply at the point of maximumslope. If this parameter is to be useful over awid'ir range of mixture gases including other vibra-tional ly active species, the effect of cross sec-tion sh«pe must be included.

Carbon Monoxide, Nitrogen, and Helium Mixtures

In ordsr to assess the importance of the crosssection shape in.the correlation, a series of cal-culations for CO-N2 and C0-N2-He mixtures wereperformed. The effect of cross section shape canbe primarily represented by the location of thepeak en and a half-width 6, as can be seen fromsoma sample plots of the product of the cross sec-tions in Figure 6. Note that in spite of the fargreater structure in the vibrationat cross sec-tions for N2, the product (Qm Sv) is not very dif-ferent in gross behavior from CO-He results, andthe major features, can be fitted fairly well by theexpression

V v ' (Vv>max (28)

Since the distribution function f(e) involves anintegral over the slope B(a(e) ), any correlationfor f is going to necessarily involve the crosssection shape parameters ep and 6. The

normalization of f further complicates the problem.A relatively simple correlation can be jbtained, how-ever, by noting that for a large part of the rangeof parameters

H(e) -v. X- e1 ' or

(29)

Since the slope normalized by 6 is more convenientfor integration, a new parair.Pter

E/N (30)

'max

becomes a more useful measure of the effect of theelectric field, and f can be shown to be

F(a*,x) (31)

where x = (e-ep)/6. fn can be estimated a number ofdifferent ways, but the most convenient is

(32)

where x0 is the location of the peak of \e f{e) andcan be either estimated using the correlation for 8or more simply tafcen as a constant.

Figure 7 shows f/fn versus x for two differentvalues of a* for several mixtures. The correlationwithin the region of vibrational excitation is re-markable. The curves separate substantially athigher values of x where electronic excitation isimportant. This is not surprising as the electroniccross sectigns will certainly not scale with theparameter a , and are cleariy different for N2 aridCO. Figur" ' ~*-iws the fraction of the input powergoing into ..t'jn:! excitation versus the param-eter a*. While t. .-e is a noticeable differencewhen Ng is included, a ± 10% correlation can stillbe achieved over a significant range of a*. Similarplots versus E/N for different mixtures would showno apparent correlation. Figure 8 can be used di-rectly in preliminary design of discharges for laserapplications. Scaling from one mixture to anotherby changing E/N to keep a* constant will preservesimilarity of the power input into vibration.

Figure 9 shows the mean electron energy e nor-malized by cD, and the drift velocity Vn normalizedby V

(33)

versus the parameter a . Both quantities correlatevery well for a* < 1. While the drift velocity iscorrelated to within about ± 10% over the range ofa* shown, the mean egergy shows a substantial sys-tematic spread for a > 1. While a complete expla-nation of this effect has not yet been obtained, themuch greater dependence of the mean energy en thehigh energy portion of the distribution functionsuggests that the difference in electronic cresssections is primarily responsible, fork is in pro-gress to incorporate the electronic cross sectionsinto an additional non-dimensional parameter thatwill correlate the behavior at high a*.

28

Summary of Conclusions

Reformulation of the equation for the electrondistribution function into an integral equation forthe logarithmic slope has led to both approximateanalytic solutions and numerical results by itera-'tion with super-elastic collisions included. Themajor results of this approach have been the follow-ing:

1) Within the region of dominant vibra-tional excitation the local logarithmicslope is primarily dependent upon the localproduct of the momentum and total vibra-tional cross sections (Qm Sv) multipliedby a characteristic energy loss squared(£0

2) and divided by (E/N) . This sug-gests a scaling law for mixtures and withrespect to the field E/N.

2) Super-elastic collisions play littlerole in the major results until Eo/kTvo < 2.Foi' sufficiently low electric field andhigh kT V 0 the super-elastics produce alocal equilibrium with vibrational statesin the energy range where vibrationalcross sections are high. This reducesthe net energy going intc vibration andproduces the previously observed raisingof the "tai1" of the distribution function.

3) For mixtures involving only one vi-brational ly active species correlation ofthe distribution function and its impor-tant moments can be achieved by the param-eter a (Eq. 27) based on the peak of thecross section product alone. This ispossible because the cross section shapeis little changed with mixture ratio.

4) For more varied mixtures such asCO-Nj-He the parameter can be conveni-ently modified to a* (Eq. 30) and theresults have to be appropriately normal-ized by parameters involving the crosssection shape parameters E and &. Thecorrelation for the distribution function,vibrational pumping and drift velocity isquite good, while for the mean electronenergy the correlation fails for a* > 1.This is probably a direct result of themuch greater dependence of the meanenergy on the high energy portion of thedistribution function where electroniccollisions are dominant.

VII. Acknowledgements

This work was performed under Air Force Officeof Scientific Research Contract No. F44G20-73-C-0059.

References

lw. L. Nighan, Phys. Rev. A2, 1989 (1970).2W. L. Nighan, Appl. Phys. Lett. 20, 96 (1972).3H. L. Hamilton, J. J. Kennedy, and T. J. Falk,U.S.A.F. Weapons Lab. Report AWL-TR-73-158, (Sept.1973).

4F. T. Wu, W. H. Yu, and E. A. Lundstrom, Appl.Phys. Lett. 25, 463 (1974).

50. P. Judd, VIII International Quantum ElectronicsConference (Abstracts) p. 76, June 1974. Also inLos Alamos Informal Report LA-5562-HS, April 1974.

6J. C. Y. Chen, J. Chem. Phys. 40, 3513 (1P64).

7G. J . Schu lz , Phys. Rev. 135, A988 (1974).8M. J . \ii. Boness and G. J . S c h u l z , Phys. Rev. AS,2883 (1973).

9A. G. Engelhardt, A. V. Phelps, and C. G. Risk,Phys. Rev. 135, A1566 (1964).

I C R . C Hake and A. V. Phelps, Phys. Rev. 158, 70(1967).

llL. S. Frost ar:d A. V. Phelps.. Phys. Rev. 1_?7, 1621(1962).

001 10.0

Figure 1. Approximate Local Solutions for theLogarithmic Slope.

•

•

, , 1

3 = ^7 /

--

1.0

0

O.I

0.01

Figure 2.

0.1 1.0 10.0

Correlation of Numerical Results for theLogarithmic Slope, a. In the energyrange where vibrational cross section isdecreasing witn energy, b. In the energyrange where vibrational cross section isincreasing with energy, d. In ths energyrange where electronic cross section hasdominant effect. —Analytic solution fromFigure 1.

29

10

•(ENERGY) 5 6 7

Figure 3. Numerical Results for the DistributionFunction in CO-He Mixtures atTvo = 1500°K. a. 5% CO-95% He mixture,b. CW CO-50% He mixture, c. )00% COgas.

i.o

II o.a

0.6

0.2

: 1

\

\

T v 0 «

\\

^ V1

\ \SOO0*K

rvo= < <so

Figure 4. Fraction of Power going into VibrationalStates in CO-He Mixtures. S 5% CO-95%He mixture. A 5(K CO-50% He mixture.© Pure CO gas.

2.0

1.6

1.2

0.6

0.4

i

A'

•

Figure 5. Tne Effective El'ctron Temperature(2/3 e) in CO-He Mixtures. El 5% C0-95XHe mixture. A 50% CO-50% He mixture.© Pure CO gas.

Figure 6. The Product 01" Elastic and InelasticCross Sections versus Energy in CO-Nj-He Mixtures.

30

-4

Figure 7. The D i s t r i bu t i on Function i n C0-N2-HeMixtures a t Two Values o f a . © 55! CO-95% He. & 10% CO-90% N2- Q 5% CO-5%N2-90X He.

2.0

1.6

0.8

0 4

0.4

0.2

Figure 9. Mean Energy and D r i f t Veloc i ty inCO-fto-He Mixtures. 0 10% CO-90% N2.A i>% CO-5% N2-90% He. x 5% CO-r.95% He.

i.O

| O.S

= 0.4S

0.2

V|I

•

Figure 8. Fract ion o f Input Power going in toVibrat iona l Exc i ta t i on . • 5% CO-55SN2

90% He and 10% C0-90%N2. O 5% CO-95% He.

DISCUSSION

D. A. McARTHUR: Hov much does you/ method dependon the cross-section displaying very narrow width?

G. K. BIENKOWSKI: I have primarily been motivatedto do this for vibrarional processes. For laserapplications, one is interested '*n narrow widthsbecause that is how one it tram, iltting the energyinto vibrations preferentially I mentionedbriefly an approximate solution. I also looked inthe energy range $her' electronic transitions aretaking place. It •- can go to the other limitwhere the cross-section -3 relat'vely ilat at thethreshold and is relatively flat over other regions,one can use a son.jhat 1 ifferent approximation andalso come up with a similar result, and ther?would now be also a correlation f~:. a relativcs;yflat cross-secti. 1 wherein the correlation wcuiddepend somehrw on the levei of »-he cross-secto.-,and presu..dbly the energy would have something todo with the threshold energy. The specificcorrelation I discussed here does depend on thefact that one has a relatively high cross-sectionover a relativel> narrow rang<- of energy. There-fore almost the en*ixe process locally is deter-mined by what is happening locally and the slope isvery high. So one <"oes not have to go very faraway from that region to produce essentially themain part of wnere the distribution function isvarying.

31

Pli ',£R-SUSTAINER GLOW DISCHARGE OPERATION IN STATIC CARBON MONOXIDE LASER MIXTURES

Daryl J. Monson and Craig M. LeeAmes Research Center, NASA, Moffett Field, California 94035

Abstract

Theoretical studies"show that efficient opera-tion of CO electric-discharge supersonic lasersrequires discharge methods that allow separate con-trol of electron density, ne, and reduced electricfield, JS/N (or electron energy). To date, externalionization by an electron beam has been successfullydemonstrated in these lasers. Other promising (butunpraven) possibilities include ultraviolet photo-ionliation, ionization by nuclear reaction products,and a high repetition race series of controlledavalanche ionization pulses (i.e., pulser-sustainer).We wish to report here successful operation of theVulser-sustalner concept in large-volume staticroom temperature CO lassr mixes. (The concept haspreviously been demonstrated in small-volume staticCC2 Laser mixes.) High voltage pulses and ultra-violet sparks at 33 kHz combined with a separatelow voltage sustainer electric field produce spa-tialty uniform plasmas in a 3.0 liter volume. Com-pletely independent control of ne and E/N is demon-

. scrated. The tests are preliminary to testing ofthe concept in supersonic flow. Results includefor se\'ei .il pure gases and mixtures the followinginea:!urements: catbode-fall voltage; self-sustainingE/N; recombination rate coefficient; average plasmane; average discharge input power; and the maximumdischarge energy loading without arcing. Modifica-tions being made to the system to allow higherpressures and energy loadings without arcing, andto a31ow operation in supersonic flow ara discussed.

I. Introduction

The CO electric-discharge supersonic lasershowt great promise for both high efficiency andpover. To achieve this, however, will requiredischarge methods whicli uncouple the plasma produc-tion from the process of adding energy to the gas.

- In particular, desirable discharge methods will-••\"ibi.tie an external ionization source with a low-volc&ge sustainer electric field which "tunes" theelectrons to an energy sufficiently high for effi-cient vibrational excitation of CO but not highenough for electronic excitation or additionalionization. (Self-sustaining discharges operateat high electric fields which produce all threeefiects.) To date, external ionization by an elec-tron beam has been successfully demons: rated inthis laser.2 Other promising, though unproveu,possibilities include ultraviolet photoionizatlon,3

ionization by nuclear reaction products,*• and thepulser-sustainez: method.5'6

jk'e wish to report in this paper operation ofthe pulser-sustainer method in large-volume staticroom temperature CO laser mixes. The tests arepreliminary to testing of the concept in supersonicflow. Basically, the method applies two dischargesto t-he gas. The first fast high-voltage pulsecreates a uniform plasma between the electrodesusing t only a otoall amount of energy. Then, thesecond 2iw-volCage sustainer discharge adds energyto the gas during the recombination period follow-ing thi> ionizJug pulse. Reilly5 first applied themethod in slowly-flowing C02 laser mixes using asingle ionizing pulse. Then Hill6 extended the

method potentially to continuous operation by usinga string of ionizing pulses at a high-repetitionrate such that only a small amount of recombinationoccurred between pulses. He demonstrated themethod in 0.3-2-volume static C02 laser mixes for aperiod extending to one traversal time through asubsonic flow device (i.e., 1 msec).

The test setup used here for CO laser mixes issimilar to that used by Hill and wij 1 be describedin Section II. The results of the tests will bedescribed in Section III, and the conclusions ofthis study will be given in Section IV.

II. Experimental Apparatus

The electrode geometry used in this study issketched in Fig. 1. The electrodes are locatedtransversely across a supersonic channel, althoughfor purposes of the present tests the channel wasblanked off and filled with static CO laser mixesat room temperature. The electrodes are separatedby 5.5 cm with a discharge volume of 3.0 P.. (Thisis an order of magnitude larger than for Hill'sinitial tests.) The electrodes are aluminum withsand-blasted surface^ and edges that are roundedbut do not conform to a Rogowski profile. Tworows of ten UV spark pins each are located 10 cmrespectively up and downstream of the main elec-trodes. (Hill located spark pins behind a porouscathode.) A high-repe ition-rate high-voltagepulse forming network7 is connected to both thespark pins and the main anode. The pulser containsa 0.002-iiF capacitor charged to 23 kV. A fast-rise-time quenching spark gap switches the charged capac-itor into the gas at a repetition rate ol 33 kHz.Because the spark pins are pointed and the down-stream ones are closely spaced from ground, theybreak down a fraction of a microsecond before themain electrodes. This produces a background oflow-level preionization for uniform plasma produc-tion in the discharge volume. (Each pin is coupledthrough a 0.00005-yF capacitor so that most of thepulse energy goes into the main discharge.)Finally, a charged 15-uF sustainer capacitor isused as a constant-voltage pumping source duringrecombination. A large inductor is placed inseries with the sustainer capacitor to isolate itfrom the ionizing pulses.

uv SPARK PINS

ELECTRODE

DIMENSIONSCHANNEL HEIGHT: S.5 cmCHANNEL WIDTH: 6 0 cm

DISCHARGE VOLUME: 3.0 lileis

Fig. 1 Sketch of the CO pulser-sustainer dischargelaser.

32

The blnnked-off test channel was outgassed andpumped out to 5 u pressure before commercial gradegases were loaded into it for testing. The impu-rity level for the tests is estimated at about200 pptn, with about half coming from the gas bottleand h-ilf from the residual outgassing in the plexi-glas channel. Sustainer voltages were measuredwith a Tektronix high-voltage probe and sustainercurrents with a Pearson current probe, and bothwere recorded with an oscilloscope camera.

III. Results and Discussion

Results will first be presented of cathode-fall voltage measurements made in CO, N2, He, andAr alone and in various combinations of interest forCO electric-discharge lasers. Then measurementsof self-sustaining discharge reduced electricfield, E/N, will be presented for these same gases.Finally, measurements of the important operatingcharacteristics of a pulser-sustainer glow dischargein these gases will be presented and discussed.

The total voltage across a glow discharge con-sists of the drop across the thin cathode-falllayer plus the drop across the positive column.Only the positive column is usable for exciting alaser gas, with the cathode fall serving only toliberate electrons from the cathode to keep thedischarge operating. For normal laser geometries,the cathode fall voltage is a small fraction ofthe positive column voltage and can safely beignored. However, for the closely-spaced trans- .verse electrode geometry and ^ubatmospheric pres-sures of the present tests, the cathode-fall volt-age is a significant fraction of the total voltageand must therefore be accurately known if the posi-tive column voltage is to be computed. Unfortu-nately, published values8 of cathode-fall voltagesfor pure gases are sparse and for mixtures do notexist. Accordingly, we measured cathode-fall volt-ages in our test setup for several gases and mix-tures. This was performed by the traditionalmethod of striking a DC glow discharge across theelectrodes, and then reducing the pressure to a lowvalue (less than 1 t.orr) until the positive columnvoltage was negligible.and the measured voltagereached a minimum.

The cathode-fall voltage measurements arepresented in Fig. 2. They are plotted against themole fraction of CO in the indicated mixtures. Thevalues range between 200-400 V. Notice that forCO in N2 or Ar, the voltage gradually decreaseswith the fraction of CO from the high value for COto the lower values for the other two gases. ForCO in He, however, the voltage stays up close tothe value for CO even for. very small fractions ofCO. Notice, also, that our measured results forpure gases are considerably higher than the pub-lished values in Cobine.8 This is not surprising,however, since Cobine points out that the resultsone obtains are highly dependent on gas purity andcathode surface condition. We established that ourresults for pure Ke were sensitive to gas purity byobserving that the cathode-fall voltage increasedfrcm its initial value after several minutes in theplexlglas channel. (This it'- consistent with thepreviously discussed results for small fractions of •CO in He.) T'.»e same test with the other gasesshowed no voltage change with time. Except for He,we feel the main reason for our higher cathode-fallvoltages is the oxide layer on our electrodes com-pared to the pure aluminum surfaces reported in

Cobine. The condition of our cathode surface iscloser to current laser practice, however.

We had some doubts that the cathode-fall volt-age results measured in a DC discharge at low pres-sure would apply to a pulser-sustainer dischargeat much higher pressure. Accordingly, we performeda test for most of the gases in Fig. 2 where weoperated a short burst of the pulser with the sus-tainer voltage turned down to a low value. Althoughwe had some problem reading the voltages accuratelyat low values due to interaction between the pulserand the sustainer, we observed that at sustainervoltages above the values given in Fig. 2 the plasmaremained conducting, whereas below those values thecurrent was zero. We are thus confident that thevalues gii'en in Fig. 2 are accurate cathode-fallvoltages for our pulser-sustainer tests.

SYMBOL KEYi COo CO/NjD CO/H6O CO/Ar> CO/N2/HC 1/2/5• CO/N2/He, 1/4/3< co/Na/ar, 1/2/5< CO/N2/A , 1/4/3

2.

0 100.

• He• Ar

COBINE, REF.8

0 02 0.4 0.6 0.8 1.0mole FRACTION CO

Fig. 2 Catnode-fall voltages for CO gas mixes andaluminum cathode.

Having estfilished 'the cathode-fall voltages,we proceeded to measure the self-sustaining E/N formany of the same gases and mixtures. We did thisto establish the upper limits on sustainer voltagefor pulser-sustainer operation, and also to estab-lish at what average electron energies self-sustaining discharges operate for these mixtures.The tests were performed by applying single high-voltage pulses to the gas to create a plasma, andthen raising the voltage on a small 1-uF sustainercapacitor to the point where the measured peakvoltage across the gas reached an asymptotic value.The cathode-fall voltages in Fig. 2 were thensubtracted from the measured maximum voltages.

The results for self-sustaining E/N are shownin Fig. 3. They are plotted against the mole frac-tion of CO in the indicated mixtures. We feel ourresults may be somewhat low since we experienced

(l.l)(1.0)

g

& 0(1.2-1.4

0(0.9-1.0)

B«.2>

)

0(1.1 -I.4J

SYMBOL KEY0 COv N 2

• Hz, DENES, REF. 3a CO/N2

• co/He0 CO/Ar> C0/N2/He, 1/2/5• C0/N2/He, I/4/S< CO/Mj/Ar, 1/2/5« CO/N2/Ar, 1/4/3hOT£ - u, leV) IN PAREN

0 02 0.4 0.6 0.8 1.0mote FRACTION CO

Fig. 3 Self-sustaining E/N for CO gas mixtures.

33

premature corona breakdown from the anode to ourspark pins at the highest sustalner voltages. Forcases where two points are given, they representmeasurements at two widely different pressures inorder to verify the pressure independence of theresults. The only measured published value wecould find to compare with was from Denes and LowUe9

for pure N2. Our value is slightly higher andagrees with Denes and Lowke's value when they delib-erately added a small amount of H2O to their gas toincrease the electron dissociative attachment rate.Thus our fairly high impurity level (about 200 ppm)may account for our high N2 result. For a few ofthe mixes, the estimated avsrage electron energy isshown in parentheses beside the data point. Thesewere obtained from computer solutions of the elec-tron distribution function.10'11 The results con-sistently indicate self-sustaining discharge elec-tron energies slightly above 1 eV. Since efficientvibrational excitation of CO occurs at an energyof 0.5-0.7 eV,J0 this verifies the statement thatwe made in the introduction that self-sustainingdischarges operate at voltages too high for effi-cient operation of CO lasers.

Following measurement of the cathode-fall volt-ages and self-sustaining electric fields, we per-formed pulser-sustainer discharge tests in many ofthe same gases at the test conditions previouslystated in Section II. The procedure we istd was toturn on a burst.of the pulser lasting 150 usec, endthen raise the sustainer voltage to a point justbelow the arc-formation limit. This representsabout 6 pulses at the stated repetition rate andapproximately simulates one flow traversal timethrough the discharge when the laser is operatedwith supersonic flow. A typical oscilloscope volt-age and current trace from such a test is shown inFig. 4, For this example, about 90 kW of dischargepower is being added to the gas. Notice that thecurrent trace takes about two pulses to build up toa full value because of the inductor in series withthe suetainer capacitor. The inductor also causesthe slow rise of current after each re7io.iizingpulse and the corresponding drop in the dischargevoltage. A smaller inductor will speed up l-herecovery of the voltage and curre .;, but it cannotbe made so small that the pulser current is shuntedinto the sustainer capacitor. The voltage droopobserved during the burst is caused ty depletion ofthe 15-uF sustainer capacitor energy. (Future testswill use a 1200-uF capacitor bank to allow testingto 1 msec without a voltage droop.)

VOLTAGE, 10001volts [

100SUSTAINERCURRENT, 50

amps0 16080 120

TIME, /isec

Fig. 4 Current and voltage traces for power loadingin 50 torr 1/2/5, CO/N2/He.

A summary of the pulser-sustainer test resultsfor the various gas mixtures is given in Table 1.Results are quoted for each mixture ; ' :he pressurethat gave the maximum energy loading. Also, mix-tures are listed In order of increasing energy load-ing. The results shown for positive columndischarge power are computed by subtracting the

cathode-fall voltages in Fig. 2 from the measuredvoltages.

Consider the results for discharge power andenergy leading for CO in He listed in Table 1. Theresults clearly show a trend of both Increasingpower and energy loading as the fraction of CO isdecreased. The maximum power of 115 kW and energyloading of 0.14 eV/CO molecule both occur for 5%CO. Large fractions of He seem to effectivelysuppress arcing and allow discharge operation atconsiderably higher pressures and powers. Alsobecause the fraction of CO is lower, the energyloading per CO molecule also goes up. Computerstudies1 show that an energy loading above0.1 eV/CO molecule is generally required for theselasers to "turn on," and 0.5 eV/CO molecule isrequired for them to reach their maximum efficiency.The present results show that we have exceededthreshold for mixtures with less than 10% CO, butthat we are still considerably below the desiredenergy loading.

Replacing part of the CO in a mixture by N2 isseen to have a desirable effect 'Jrom Table 1. Thepresence of N2 allows operation at considerablyhigher pressure and power without arcing. Ourhighest measured power of 171 kW was achieved inthe 2.5%,CO, 2.5% N2, 952 He mix. Notice that theenergy loading wiHi N2 is only up slightly, however.

Compare the sustainer E/N and correspondingelectron energies listed in Table 1 with the self-sustaining values given in Fig. 3. The presentvalues are observed to be much lower, thus verifyingthat we have achieved independent control of ne andE/N. The values achieved for electron energy areactually slightly lowev than the optimum range of0.5-0.7 eV desired.10

The average electron densities achieved werecomputed from the measured currents and computeddrift velocities :<1^ for some of the mixtures.They are listed ir. Table 1. The values range fi nn2-4x10*! cm"3 with the highest values being formixes with low fractions of CO. This compares with2xl0n cm"3 achieved by Hill6 in his pulser-sustainertests.

Finally, electron-ion recombination rate coef-ficients were computed for many of the mixes fromthe measured decay"rate of the sustainer currentafter the last pulse. The values listed in Table 1all fall slightly less than 10~7 cm3 sec"1. Thesevalues agree with measurements by Douglas-Hamilton13

In CO where the impurity level was noj carefullycontrolled (as in our case), but our v. lues areabout a factor of 5 lower than those measured byCenter111 where the impurities were very carefullycontrolled. We see a weak trend of increasing ratecoefficient with decreasing fraction of CO, but thismay be entirely due to the increasing pressurelevel.1!*

The results of this paper show that we haveachieved the energy loading "threshold" for one gasexchange time in the CO supersonic laser but thatwe are still well short of the maximum desired. Heare presently making several design changes thatshould allow us to operate at higher pressures,voltages and energy loadings without arcing. Themost important of these is that we are making newelectrodes with edges that conform to a Rogowsklprofile. (We have noticed in our present tests

34

that when we arc, breakdown invariably occurs atChe electrode edge.) Also, we are increasing thenumber and decreasing the diameter of our sparkpins to provide more UV light for pre-ioriiiration.Finally, we plan on operating our pulser at a highervoltage, a higher repetition rate, and cutting downits circuit inductance in order to achieve higherpeak currents (and therefore higher electron den-sities) in each pulse.

Beyond the above changes, the only way for us *to get higher energy loadings with supersonic flowis to increase the gas residence time in the •discharge. This c<in be done by making the dischargelonger in the flow direction or replacing He by Arto slow down the gas. To check out this laat idea,we replaced He by Ar for a few of the mixtureslisted in Table 1. The result was that we had arcformation at all useful pressures and energy load-ings. Thus, Ar does not appear desirable for COpulser-sustainer discharges. This is in contrastLO e-beam CO supersonic lasers which operate bestwith Ar.2

Although the present results are only torstatic gases, we plan next to try the pulser-sustainer concept in supersonic flow. There weexpect our biggest problem will be "blowing out" ofthe discharge downstream. In order to solve thisexpected problem, we plan on trying two solutions.First, we are going to recess ovr electrodes behinddielectric grids similar to those successfully usedby Tulip and Seguin15 in a flowing C02 laser. Thisdoes two things for us. It presents a smooth sur-face to the flow while we retain shaped electrodes,and it traps a static low-density plasma behind thegrids above the electrodes to hopefully help keepthe discharge stationary. (We have tried out gridsover our electrodes in a static gas and they don'tseem to affect the glrw stability.) Second, weplan on trying a resistive anode^ to try _o keepthe discharge spread out over the electrodes.

IV. Concljsions

We have demonstrated operation of a pulser-sustainer glow discharge in static 3.0-i-volume COlaser mixtures for times simulating one gas exchangethrough a supersonic laser. Completely independentcontrol of ne and E/N is achieved. Electron den-sities in the range of 2-4x10" cm"3 and averageelectron energies in the range of 0.4-0.5 eV areattained. This is slightly below the range of elec-tron energies that efficiently excites the CO laser.A maximum discharge power of 171 kW is loaded intothe gas at a pressure of 221 torr. This representsan energy loading of 0.16 eV/diatomic molecule forone gas exchange time and exceeds the "turn on"value of 0.1 eV/CO molecule for this laser. It isstill short of the desired value of 0.5 eV/CO mole-cule, however. Best discharge operation is shownto occur for low factions of CO in He. Replacingpart of the CO with N2 allows higher pressure andhigher power operation without arcing. It isobserved that Rogowski-shaped electrodes will benecessary to achieve higher pressures and energyloadings without arcing. Mixtures with Ar ratherthan He fail to produce a good discharge at anyuseful condition. Cathode-fall voltages between200-400 V are measured, depending on the mixture.These are slightly higher than classical values foran aluminum cathode. Electron-ion recombinationcoefficients in the range 0.4-1.0x10"7 cm3 sec"'are measured. These compare favorably with pastmeasurements in gases with similar impurity levels.

Finally, self-sustaining discharges are shown tooperate at electron energies exceeding 1 eV, whichexceeds the range for efficient vibrational excita-tion of CO. This establishes the need for dischargemethods such as the pulser-sust3iner which canoperate at a lower electron energy.

References

'Monson, D. J., "Potential Open- and Closed-Cycle System Efficiencies for the CO, Supersonic,Electric-Discharge Laser," NASA TM X-62,438, 1975.

2Jones, T. G., Byron, S. R., Hoffman, A. L.,O'Brien, B. B., ind Lacina, W. B., "Electron-Beam-Stabilized CW Electric Discharge Laser in Super-sonically Cooled CO/N2/Ar Mixtures," AIAA PaperNo. 74-562. Presented at the AIAA 7th Fluid andPlasma Dynamics Conference, Palo Alto, Calif.,June 1974.

3Lind, R. C , Wada, J. Y., Dunning, G. J. andClark, Jr., W. M., "A Long-Pulse High-Energy C02

Laser Pumped by an Ultraviolet-Sustained ElectricDischarge," IEES Journal of Quantum Elsctronias,Vol. QE-10, No. 10, Oct. 1974, pp. 818-821.

•*McArthur, D. A., and Tollefsrud, P. B.,"Measurement of Optical Gain in CO Gas F cited Onlyby Fission Fragments." TEEE Journal of QuantumElectronics, Vol. QE-12, No. 4, April 1.976,pp. 244-253.

5Reilly, ;. P., "Puls'ir-Sustainer Electric-Discharge Laser," -T-/wmal of Applied Physics,Vol. 43, No. 8, Aug. 1972, pp. 3411-3416.

6Hill, A. E., "Continuous Uniform Excitationof Medium-Pressure C02 Laser Plasmas by Means ofControlled Avalanche lonization," Applied PhysiosLetters, Vol. 22, No. 12, June 1973, pp. 670-673.

7Austin, W. E., "Test and Development of aHigh Repetition Rate Pulse forcing Network," NASACR-13/,638, 1975.

sCobiue, J. D., Caseous Conductors, DoverPublications, Inc., New York, NY, 1953.

9Denes, L. J., and Lowke, J. J., "V-I Charac-teristics of Pulsed C02 Laser Discharges," AppliedPhysics Letters, Vol. 23, No. 3, Aug. 1973,pp. 130-132.

10Niyhan, W. A., "Electron Energy Distributionsand Collision Rates in Electrically Excited N2, CO,and CO2," Physics Review A, Vol. 2, No. 5, Nov.1970, pp. 1989-2000.

"Proctor, W. A., Private Communication, May1974.

12Long, Jr., W. H., Bailey, W. F., andGarscadden, A., "Electron Drift Velocities ix>Molecular-Has-Rare-Gas Mixtures," Physics Reviea A,Vol. 13, No. 1, Jan. 1976, pp. 471 475.

13Douglas-Hamilton, D. H.,- Reeord of the 11thSymposium on Electron, Ion, and Laser Beam Tech-n°l°9H, edited by R. F. M. Thornley (San FranciscoPress", San Francisco, Calif., 1971). p. 591. Sejalso, Avco Everett Research Laboratory ResearchReport No. 343, 1972 (unpublished).

35

1''Center, R. 6'., "Electron-Ion RecombinationMeasurement* in CO at High' Pressures,"'Jouxval ofApplied Physio*, Vol. 44, Ho. 8, Aug. 1973,pp. 3538-3542." ' [

lsTulip, J., and Segnin, H. J. J.,' "High-Presaure' Clow^Discharge Using a Differentially •Pumped CatWle," Applied Physios Litters, Vol. 27,Ho. 1, Juix I S " , pp. 15-17.

. . 16Bt1anger,'P, A., Traeiblay, »., Boivin, 3.,and Ocla, G., "Atmospheric Pressure C0 2 Pulaad Laservlth Semiconducting Plastic Eltctrodta," CanadianJournal of Phyiiot, Vol. 50, Ho. 22. 1*72,pp. 2753-2755.

*fabje 1 Summary of, ulasma characteristics, for' pulser-sustaine,r glow diacharge operation Inatatlc CO laaer mixture* >

**Mb!

CO/N2/He

' 1/0/0.0/1/01 1/0/1• 1/2/5

1 1/0/41/0/9

' 1/1/181/0/19

_ l/l/ST

-

OptimumPr'eaaurk(torr)

'22 .3036506079102160 •221

E/N(10-J6J? cm»)

2.64.01.71.30.90.90.9

; 0.6.0.5' "

DISCUSSION

ElectronEnergy(eV)

% 0.5

> 0.70.4-0.50.4--•

r *'

Electron1 Density(1011 cm"3)

2.23.2 ,2.3-3.43.4

3.8

-

Recombination• Rate

Coefficient

(10"''cm3 aec"1)

0.40.50.7> -t

0.60.8-1.0

<

Disch. go Power(kW>

Total,

7297828890

JU3'' 152

138' 200

•

,PositiveColumn

52 ''7461 ,.66 '6486123115 .171

•"Energy* Loading(eV perdiatomicmolecule)

0.030.030.030.0410.050.110.13.0.140.16

J; LAUDENSUGER: <I havt a .couple of question* onyour UV pre-ionization. It.ia experimentallyevident that things likt that work in helium butnot in argon. Do you feel that the mechanism'forthis pre-.ibnii»tion ia the ionization in the volum*of thil gaa 6t photo excitation of the electron* atth* surface of the electrodes?

D. H. MONSON: We haye operated thia device vith-dielectric grids over races*ed electrodes w h e nthey are shielded from the UV,-and we get the saneperformance then as v« do when the electrodes, arcesyond to UV.

J.the COT

Do you feel that you are ionising

D...H. HONSOH:. I think we are ibniiing theiapurities^ "We feel that most of our impuritiestn the static teata arise fro* the plexiglasschamber which is pumped down to about 5 nicrons,aad therefore continue* butgassing. So weinevitably have hydrocarbons.,. We catlaate about100 parta per million of Impurities. ' So these-tssts do have a*aeed gaa in the mixture.

J. LADDEHSLAGEk: Do you hav*. any feeling for whythe argon system doein's work: i t v i i becausa the .'photon energy ia lower or »erh«ps the de-iohixationtlme'of argon is much greater?

D. M. MOHSOM: Perhaps we. are not photo-ionizingenough in the argon to create the' init ial glow.He can.opcrate «t.»ery low densities in argon,"but whir, wa e«\to useful pressure rangaa, arcingacts In right "»jay. < .

1 ' • . %

36

PLASMA RESEARCH IN ELECTRIC PROPULSION ATCOLORADO STATE UNIVERSITY

Paul J. Wilbur and Harold R. KaufmanDepartments of Mechanical Engineering and Physics

Colorado State UniversityFcrt Collins, Colorado 80523

Abstract

The effect of electron bombardment ion thrust-er magnetic field configurations on the uniformityof the plasma density and the ion beam currentdensity are discussed. The optimum configurationis a right circular cylinder which has significantfields at its outer radii and one end but is nearlyfield free within the cylinder and at the extrac-tion grid end. The production and loss of thedoubly charged ions which effect sputtering damagewithin thrusters are modeled and the model is ver-ified for the mercury prcpellant case. Electronbombardment of singly charged ions is found to bethe dominant double ion production mechanism. Thelow density plasma H O 6 elec/cm3} which exists inthe region outside of the beam of thrust producingions which are drawn from the discharge chamber isdiscussed. This plasma is modeled by assuming theions contained in it are generated by a cliarge ex-change process in the ion beam itself. The theo-retical predictions of this model are shown toagree with experimental measurements.

Introduction

Ion thnsters hold a promising position in thespace propulsion program because of their high exhaust velocity capability. Applications for thesedevices which are of current interest range fromlow total impulse satellite stationkeeping missionsthrough the higher total impulse comet rendezvousand out-of-ecliptic primary propulsion missions.

The basic cylindrical thruster is shownschematically in Figure 1. Its operation can be

ANODE POLEPIECE

CATHODEPOLEPIECE

Fig. 1 Divergent Magnetic FieldThruster Schematic

understood by recognizing that electrons are pro-'duced within the region bounded by the cathode polepiece and baffle and that they are drawn from thisregion toward the anode through the aperture ad-jacent to the baffle. The •vSOV potential differ-ence across the baffle aperture, which is maintain-ed by the anode, accelerates these electrons tosufficiently high energies that they can effectionization of propellant which has been fed intothe chamber by the mechanism of electron bombard-ment. The magnets shown in Figure 1 establish amagnetic field of the order of SO gauss between theanode and cathode pole pieces which has a typicalline of force like the critical field line shown.The bulk of the high energy electrons (primaryelectrons) coming through the baffle aperture re-side within the region defined by the surface ofrevolution of this critical field line and thescreen grid and this is therefore the region withinthe discharge plasma where the bulk of the ions areproduced. As the primary electrons have collisionsthey lose energy and they, together with the elec-trons knocked from the atoms, combine to form anelectron group having a Maxwellian distribution.The low energy Maxwellian electrons are then col-lected by the anode. Ions are free to drift aroundthe discharge chamber under the influence of minorelectric field and diffusive forces until they hita conducting surface and are neutralized or untilthey drift into the vicinity of the grids and areextracted from the discharge chamber at a highvelocity thereby producing thrust. Ion extractionis accomplished by a large electrostatic fieldestablished between the two grids and the velocityof the thrust producing ions can therefore be con-trolled by simply adjusting the r potential dif-ference through which the ion fairs in passingfrom the discharge chamber to the ion beam poten-tial (typically of order 1000V).

The typical mercury discharge chamber plasmahas an electron density of 1 0 u cm"3 and a neutraldensity of 1O1Z cm"3. The electrons have the twogroup composition mentioned with primary electronsconstituting of the order of 10% of the total.Primary electrons have an energy near 3OeV whileMaxwellian electrons generally have a temperaturenear 5eV.

One additional component not show;, in Figure 1which is essential to ion thruster operation is theneutralizer. This device which is located down-stream of the grids simply emits electrons whichare drawn into the ion beam to effect charge andcurrent neutrality of the beam plasma. Thrustersare generally classified by the diameter of thebeam they produce '0) and thrusters which producebeams from 5 cm to 1.5 m in diameter have beentested. The testing discussed here has howeverbeen conducted on 15 cm diameter thrusters only.The dishing of the accelerating grids shown inFigure 1 is employed to insure the grids will de-flect in a predictable manner and hence not short

37

unciar the influence of thermal loads from the dis-charge chamber plasma.

The low-thrust nature of ion thrusters neces-sitates long operating times to achieve missionobjectives and this necessitates long componentlifetimes. The downstream (accelerator) grid is acrucial component subject to life-limiting erosionas a result of charge-exchange ion sputtering^.This erosion process occurs when a high velocitypropellant ion exiting the grids captures an elec-tron from a nearby low velocity neutral propel]antatom. The result of the process is a fast movingneutral and a slow ion. The ion is frequentlydrawn back into the accelerator grid because itsvelocity is insufficient to enable it to escape theadverse electrir field and the resulting impact onthe accelerator grid causes the sputtering damage.The rate of sputter damage is directly proportionalto the ion current density from the thruster andbecause it is generally highest on the center!ine,grid srosion is also greatest on the centerline.Since the thrust of the device is directly propor-tional to ion current the longest lifetime will beachieved in the thruster opsrating at a giventhrust level which has the most uniform radial beamion current density profile. A thruster having auniform ion current density beam should thenoperate and produce thrust as the accel grid erodesuniformly.

Cusped Magnetic Field Thruster

The flatness of an ion beam current densityprofile is frequently defined as the ratio ofaverage-to-peak current density—a quantity re-ferred to as the flatness parameter (F). Divergentmagnetic field ion thrusters like the one shown inFigure 1 have flatness parameters near 0.5 while avalue of unity would be ideal. It has been arguedthat this quantity is low for the divergent thrust-er because of neutral propellant atom proceedingfrom the upstream to downstream ends of the thrust-er is much wore likely to be ionize if it is nearthe thruster centerline than it is if it is nearthe outer radius. This condition exists becauseatom; passing near the centerline are exposed toiodizing plectron region defined by the criticalfiald line all the way from the baffle to the gridswhile those with trajectories near the outer radiiwould be exposed over the distance "L" ifiedon Figure 1.

The cusped magnetic field thruster of Figure 2is designed to increase the path length of pro-pellant atoms through the ionizing electron regionat the outer radii thereby increasing the ir • beamflatness parameter and hence the lifetime of ..hegrids. As indicated by Figure 2, this is accom-plished through the addition of a center pole pieceand the installation of magnets to produce a north-south-north pole configuration as one proceeds fromthe cathode to center to anode po!e pieces. Thisproduces magnetic lines of force like those sug-gested by the dotted lines joining the pole piecesand suggests the increase in the length "L" sug-gested by Figure 2. A second (rear) anode hasalso been added to this thruster to facilitate uni-form electron collection—this rear anode is de-signed so it can be moved axially during thrusteroperation to vary the characteristics of the dis-charge chamber plasma.

Iron filings maps showing lines of force for

-FRONT ANODE

-FRONT ELECTROMAGNETSVREAR ELECTROMAGNETS

Fig. 2 Cusped Magnetic Field ThrusterSchematic

the cusped and divergent magnetic f i e l d configura-tions are shown in Figure 3 for the 15 cm diameter

f |-—FRONT__ J POLE PIEC f ~ POLE PIECE J\

tr-X'•''•'.-. ' '.-:' - . " • ' •' \i CATHODE y \'• ^-- -":': ^'~''1. '"

h<gr.; • V-Vff~--' : ; - . , ' POLE PIECE k c : - ; ; : ^ . ; ;

A CUSfVD FlFL1- GEOMETRY B. DIVERGENT FIEuD GEGMETRY

Fig. 3 Iron Filings Mapa

thrusters used in this test. Actually the diver-gent field map shown was obtained using the cuspedfield thruster operating in the north-nortn-southpole configuration, but the field shape is essen-tially the same as that observed in a thruste*1 nothaving the center pole piece.

Langmuir probe measurements were made through-out the discharge chambers of the cusped and di-vergent field thrusters when mercury propellant wasbeing used. These data were then analyzed usingthe two electron group theory of Strickvaden andGeiler4. Using Haxwellian electron temperatures:.primary electron energies and densities of thesespecies the frequency of occurrence of ionizingcollisions (v) was calculated at each probing lo-cation. Figures 4 and E present constant col-lision frequency contour lines obtained from suchan analysis for the cusped and divergent fieldgeometries respectively. The contour lines have

38

NORMALIZED COLUSIC'n FREQUENCY CONTOUR

ION PRODUCTION REGION BOUNDAPY

Fig. 4 Collision Frequency Contour Ma?(Cusped Field Thruster)

, =0 .2

NORMALIZED COLL.SiON FREQUENCY CONTOUR

ION PRODUCTION REGION BOUNDARY

Fig. 5 Collision Frequency Contour Map(Divergent Field Thruster)

beon normalized with respect to the maximum colli-sion frequency calculated for each configuration andthe innermost lines represent the contours on whichthe collision frequency is 90% of the maximum. Theoutermost lines are contours corresponding to 20%of the maximum. The dotted lines define the regionin which theory suggests the bulk of the ionizingreactions (^95%) occur.

Both the iron filings maps of Figure 3 and thecollision frequency contours of Figures 4 and 5 sug-gest the change to the cusped magnetic field con-figuration should result in a more uniform iondensity profile in the thruster because it facili-tates more uniform radial profiles of the ion pro-duction rate. Figure 6 shows normalized ion beam

NORMALIZED CURRENT DENSITY ~ )/fave. THRUSTE3 _F

O VO

Oo

O SERT n 0.49A CMF L/D = 0.53 0.64O CMF L/D = 0.30 0.68O CMF L/D = 0.23 0.70

- IDEAL PROFILE

NONDIMENSIONAL RADIUS ~ rfromtt

Fig. 6 Current Density Profiles

current density profiles measured using a Faradayprobe. The profiles are for a divergent fieldthruster (designated SERT II for Solar ElectricRocket Test II) and three cusped magnetic fieldthrusters (CMF) each having different lengths (L).The flatness parameter (F) and the current densitycurves themselves both show that the -rusped fieldthruster produces much flatter profiles. The varia-tion in the length of the cusped field thruster isobserveJ to have a small effect of the ion beamflatness, but other performance considerations suclias the energy cost of producing the ions (discharge1 ss) suggest the cusped field thruster having the'.<.-'Tth-to-diameter ratio of 0.3 is preferred.

Re-exami ation of Figures 3fl and 4 suggest thatthe ion beam profile could be flattened more in thecusped field thruster if the penetration of themagnetic field into the discharge chamber could bereduced to effect a more uniform ionizing collisionfrequency over the chamber. This reduction in mag-netic field penetration is the objective of themultipole magnetic field configuration.

39

Multipole Magnetic Field Thruster

A sketch of the 15-cm diameter multipolethruster is shown in Figure 7. The magnetic polar-ity of the pole pieces alternates, giving the over-all field shape shov-. in Figure 8. (Differentnumbers of pole pieces were used to investigate the

Eiec'rjmagneTs

Fig.

-Stainless steel

7 Mtipole Magnetic Field ThrusterSchematic

effects of discharge chamber length. Figure 7shows 4 side or cylindrical pole pieces, whileFigure 8 shows 7.) Anodes are located between polepieces so that the energetic electrons must passthrough the fringe magnetic field to reach theanodes. Because these fringe fields extend only ashort distance into the discharge chamber, most ofthe chamber has a very low field strength. Thislow field strength, of course, results in a uni-form distribution of ionizing electrons, hence auniform plasma density. The refractory wire loop'near the center of the chamber serves as the sourceof ionizing electrons in this thruster which ispresently not equipped with the baffle/cathode polepiece arrangement shown for the divergent fieldthruster of Figure 1. -

The multipole thruster can be considered alogical extension of the cusped field design tomore pole pieces, inasmuch as the pole pieces arefabricated in a similar manner from sheets of softion, with electromagnets used between adjacentpole pieces. But this multipole design is alsorelated to that of Hooreb and Ramsey6, primarilyin the large number of pole pieces used and thegeneral discharge chamber shape. Moore and Ramsey,however, used permanent magnets as pole pieces with

Fig. 8 Iron Filings Map(Multipole Thruster)

the magnetization direction either towards or awayfrom the center of the discharge chamber. If per-manent nagnets were used in the multipole thrusterstudied herein, they would replace the electromag-nets between pole pieces, rather than become thepole pieces.

Performance of a b.l cm long discharge chamberoperating on argon is shown in Figure 9 in terms of

1200c-

Chamber Length = 5.1cm

1000-

•a 8oo •

s eoo -

400-

200,0 2 4 6

Maximum Magnetic Induction,)8KI0-»

Fig. 9 Effect of Magnetic Field Intensityon Discharge Loss

40

beam ion energy cost (discharge loss) as a functionof magnetic induction measured at a location nearthe pole pieces. Varc is the voltage differencebetween the emitter wire and the anodes and Iarc isthe cathode emission current. The propel 1 ant flowrate (m) is expressed in rnlliamps in a^jrdancewith the convenient convention of assigning oneelectronic charge to each neutral atom entering thechamber. The use of eight electromagnets resultedin curves in Figure 9 that did not quite matchthose obtained with four at the same magnetic in-duction. The trend of decreased discharge losswith increased magnetic field strength, though, isclear. The higher magnetic field is presumablysnore efficient because it is more effective in con-taining energetic electrons. A simplified model ofelectron containment is indicated in Figure 10(a),

Uniform

Strength

Field

(a) Simplified Deflection Configuration

rewritten as

2rcB = 6.74 x 10"3/E^" ,

where 2rc is depth of the fringe field above theanode and B is the magnetic field in this region.The produce 2rcB can be thought of as the number offlux lines per unit anode length, sM is plottedagainst electron energy in Figure lu(b). The dis-tribution of these flux lines with distance fromthe anode is not important. For example, half thefield strength extending twice as far from theanode would have the same effectiveness in deflec-ting electrons. This conclusion is also valid forthe more realistic case of field strength varyingwith distance from the anode, which can be shownfor increments of deflection angle instead of cir-cular electron orbits. For a varying magnatic-field strength, the inteyral of magnetic fieldstrength

B=0/ B x d ianode

is the proper quantity to use in place of ZrJl. Inthis case the integration is carried out to thefirst point of negligible field strength (B=0).

The fringe field integral was evaluated nu-merically and was found to vary with location inthe discharge chamber. The largest departures frommean values were found at the two corner anodes.As shown in Figure 11 for the eight magnet config-uration at 10 amperes magnet current, the magnetic

(b) Required Flux per Unit Anode Length as a

Function of Electron Energy

Fig. 10 Electron Interaction with FringeMagnetic Field above Anode

with the fringe field represented by a region withuniform field strength above the anode. Picking adirection of motion for the electron such that theelectron has the deepest penetration of this fringefield, it is evident from Figure 10(a) that thispenetration corresponds to two electron cyclotronradii. The electron cyclotron radius rc (in tn) isgiven by

rc = 3.37 x 10-5/E^/B,

where Ee is the electron energy in eV and B i« themagnetic field in Tesla. This equation can be

/ / m i i'Plane o* Measurement

8x10'

Pole - pieceLocation

O SideD Corner

Distance from Pole-pieceCenterline , cm

Fig. 11 Magnetic Field Between Pole Pieces

field drops to near zero field strength much fasterin the corner than in the side location (typical ?fthe rest of the chamber). This more rapid drop isdue to the interference of the two corner fields.

41

In fact, integration from a location flush with theinside edges of the pole pieces yielded 88 x 1CT6Tesla-meters (88. Gauss-cm) for the side location,and only 42 x 10"6 Teila-meters for the cornerlocation.

The variation of discharge losses with magnetcurrent is shown in Figure 12 for the aight-magnet

7 0 0

600

-500

5400

3 0 0 -

200,

O Corner onlyD Corner outA All Anodes

4 6 8Magnet Current, amps

10

Fig. 12 Variation of Discharge Ion withMagnet Current for Different AnodeConfigurations (Argon Propellant)

configurati .1 with different anode configurations.Operation with corner anodes alone gave the poorestperformance at low magnet currents, while operationwas not possible with this configuration at highercurrents. The exclusion of the corner anodes gavethe best performance at low nagnet currents, butresulted in little difference at higher currents.

The data of Figure 12 are consistent w'thelectron loss to corner anodes being the majorlimiting factor in multipole chamber performance.Further, the fringe field above the corner anodes

. at 10 amperes is sufficient to give good contain-ment of ionizing electrons. From Figure 11, thefringe field integral at this location was foundto be 42 x ]0-6 Tesla-meters at 10 amperes. For a50 volt discharge (and about 50 eV primary elec-trons), Figure 10(b) indicates an integral of48 x 10-6 Tesla-meters should be adequate. The ex-perimental and theoretical values of fringe fieldintegral are thus, in good agreement. There areother factors involved in discharge losses, however.Additional tests have shown a slow decrease inlosses occurs for additional field strength in-creases until the integral is about twice the theo-retical value given in Figure 10(b). Design forfringe field integrals somewhat above the theoret-ical value is therefore recommended.

In the tests described above, all anodes wereflush witlt the inside edges of the pole pieces(see Figure 7). The containment of electrons canbe equalized for all anodes by changing the field

strength of corner pole pieces. A simpler approach,though, is to use the same magnetic field design forall pole pieces and to recess the corner anodes.The electrons are then required to go a short dis-tance beyond the inside edges of the pole pieces toreach the anodes. It was found in additional teststhat an anode -ecess equal to 10 percent of thepole-piece spacing gave a fringe field integral atthe corner anodes that was about equal to the sameintegral elsewhere in the chamber. A dischargechamber with corner anodes recessed in this mannerhas been tested and found to perform well. Testswith argon, xenon and mercury propel 1 ants arepresently being conducted to determine the ion beamflatness parameter of this thruster; preliminarydata suggest flatter ion beams than those observedin the cusped field thrustar.

Discharge Chamber Model

The need for long thruster lifetimes dictatesthe need for low erosion rates on surfaces exposedto the thruster discharge plasma as well as n theaccelerator grid. The plasma contituent which ap-pears to cause the bulk of the erosion on these in-terior surfaces is the doubly charged ion which isfrequently present in significant numbers and whichobtains twice as much energy as a singly charged ionpassing through the same sheath potential to a col-lision with a surface. The objective of this modelwas the prediction of doubly charged ion densitiesas a function of thruster design and operatingparameters. The analysis and verification have beencarried out for a mercury plasma.

In order to develop a model for determining thedouble ion density in the discharge chamber onlythose ionic and atomic species which wert consid-ered significant in determining the double ion den-sity were Included.' The significant species wereselected as those which have substantial electronimpact cross sections of formation over the elec-tron energy range of interest so that large numbersof these excited atoms or ions will bs produced.These states also have sufficiently long effectivelifetimes so that they can pari'cipats in productionprocesses before they decay. Only those reactionswhich lead directly or indirectly to the productionof double ions were included.

Figure 13 is a discharge chamber reaction

Fig. 13 Discharge Chamber ReactionSchematic

42

schematic showing these dominant species and thereactions in which each specie can participate.The symbols used represent the following species:

Hg° - neutral ground state mercuryHg - metastable neutral mercury {63Po and

63P2 states)Hgr - resonance state neutral mercury

(63P! and 6^, states)Hg - singly ionized ground state mercuryHg - singly ionized metastable mercury

(62D3^2 and 62D5/2 states)

Hg - doubly ionized ground state mercury

The arrows in Figure 13 indicate the variousinteraction routes considered in the analysis.Three different types of reactions are indicated inthis figure. The first type of reaction occurswhen an electron interacts with an atom or ion pro-ducing a more highly excited specie. This reactionis indicated in Figure 13 by an arrow going fromone specie to another more highly excited specie(e.g. the production of double ions from singlycharged ground state ions). The production of amore highly excited specie also represents a lossmechanism for the less excited specie. The reversereaction in which, for example, an ion captures anelectron is neglected because of its low probabil-ity.

The second type of process considered is thatof an atom or ion going to a plasma boundary. Sucha boundary could be either the discharge chamberwall on which the atom or ion would be de-excitedor it could be a grid aperture in which case theatom or ion would be extracted from the dischargeregion and replaced by an atom from the propellantfeed system. In either c s e this represents a lossrate for any of the excited states, lhese lossesto the boundary are indicated in Figurp 13 by thedotted lines to the wall of the chamber. Thelarge arrow back to the neutral ground ;tate repre-sents the resupply of neutral ground state atomseither from the walls or from the propellant supplysystem.

A third type of reaction shown in Figure 13 isrelevant only to the two resonance states. Theresonance states differ from metastable states inthat they have a very short lifetime before ieyde-excite spontaneously by emitting a photon oflight. However, the energy of this photon is suchthat it is readily absorbed by a nearby neutralground state atom producing another resonance stateatom. Since the transport time of the photon issmall compared to the excited state lifetime theexcited state can be considered to exist contin-uously. Eventually the photon can diffuse to aboundary where it will be lost; this is equivalentto the loss of a resonance state atom.

By equating the production and loss rates ofeach specie on the basis that equilibrium existsone can solve for the density of each specie ofFigure 13 in terms of the plasma properties, thevolume and surface area of the region in whichionization takes place and the ionization and ex-itat ion cross sections for the reactions. Themodel was sec up so this could be accomplished onthe digital computer using a plasma described interms of average values of primary electron energy

and density and average values of Haxwellian elec-tron temperature and density. The details of themodel are described in References 7 and 8.

In order to verify the model, several thrust-ers of different sizes and plasma characteristicswere operated while simultaneous measurements ofdischarge plasma properties and the double andsingle ioii composition of the thrust beam drawnfrom the thruster were made. Typical plasma prop-erty information obtained in a 15 cm dia divergentfield thruster are shown in Figure 14. In order to

Fig. 14 Typical Plasma Properties

orient these plots, an ion extraction grid (screen)and the critical fiftld line are identified. Plasmaproperty data like that of Figure 14 were averagedusing weighting factors determined to be appropri-ate for this analysis and double ion concentrationswere calculated from these input data. Figure 15shows a typical comparison of the measured and cal-culated double-to-single ion density ratio. Thevariable on the abscissa in this plot, propellantutilization, is the ratio of propellant that isdvawn from the truster as ion current to the totalpropellant input to the device. The two plotsshown were based on data from thrusters which usedtwo different types of ion accelerating gridsystems. T'» theoretical data of Figure 15 showgood agreement with the theory and therefore con-firm the model's validity.

Detailed examination of the model shows thatsingle ion production via the intermediate meta-stable and resonance states is appreciable butthat double ions are produced primarily by elec-tron bombardment of singly charged ground stateions at normal thruster operating conditions. Onthe basis of this latter observation one can de-velop a simpler model considering only the singlycharged ionic ground state to doubly charged ionicroute for double ion production. The doublycharged ion density (n++—nr3) is then given by thefollowing equation for a region of uniform plasmaproperties:

43

0.06

0..04

0.02

THEORETICAL

EXPERIMENTAL

SERT n GRIDS

80 100

i Q 0.06

0.C4

0.Q2

EXPERIMENTAL

HIGH PERVEANCE DISHED 6RIDS

6 * 6 0 8 0 100PROPELLANT UTILIZATION IX)

Fig. 15 Double-to-Single Ion Density —Comparison of Theory andExperiment

where ne =n + nmx

— D ) " 3

is the total electron density

n is the primary electron density—m"3

n is the Maxwellian electron density—nr3

V is the volume of the ionizing electronregion—m3

A is the surface area of the ionizingelectron region—m2

s r) is the velocity of primary electron

having an energy ?„ -m/seco+ (E) is the cross section for the singly-

to-doubly ionized transition at anenergy E - m 2

v m ( E ) is the velocity of electrons at energymx E - tn/sec

(dn^/n^) is the Maxwellian distributionfunction

lm is the Maxwell ian electron temperature-eVe is the electronic charge constant -

1.6 x 10-19 coulm,. is the mass of an ion - kgand Reference 8 contains the required crosssections and a plot of the solution of theintegral appearing in this equation as afunction of Maxwellian electron temperature.

This equation suggests that the double ion

density can be reduced most readily by reducingthe electron density. In order to maintain otherthruster performance parameters this reductionshould b<? accompanied by an equal increase in thevolume-to-surface area ratio. Because electrondensity is a squared term and volume-to-surfacearea is a linear one the net effect is a decreasein double ion density.

Charge Exchange Plasma

Depending upon the point at which the chargeexchange process occurs between ions and neutralsin the beam, the slew ion which is produced may beable to leave the beam plasma with an energy of afew eV rather than bijing drawn back into the grids.These then constitute the charge-exchange plasmathat surrounds the ion beam and thruster. A typ-ical plot of plasma density against radial dis-tance (downstream of the thruster) is shown inFigure 16. The Debye distance is typically less

20 30Rodiol Distance, cm

Fig. 16 Separation of Plasma into Ion-beamand Charge-Exchange Regions

than 1 mm in the ion beam, so the electron and iondensities are essentially erjjal in Figure 16. Thecentral hump represents the beam of high energy(•x/IOOO eV) ions, while the more uniform regionfarther out is dominated by low velocity charge-exchange ions. Because the charge-exchange iondensity decreases slowly with increasing distance,a significant plasma density can exist near muchof an electrically propelled spacecraft. This canpresent a serious problem for the solar array fromwhich operating power is obtained in that the trendis towards higher array voltages for higher elec-trical efficiency, and this increases the tendencyfor the array to interact adversely with thecharge-exchange plasma.

A typical survey of plasma density near athruster resulted if the map of Figure 17. Thedashed line shows tne approximate boundary betweenthe ion beam and the surrounding charge-exchangeplasma, with the location of this boundary deter-mined by plots similar to Figure 15. The electrondensity («e) within the ion beam where the plasmapotential is V obeys the "barometric" equation

"e = "e.ref exp[-eV/kTe],

44

1.49 x (l-nu)

where To and T- are the neutral and electron tem-peratures in °K and A is the atomic weight of thepropellant atoms. With the further substitutions" -of 500°K for To> 58,000 °K (5 eV) for Te, 200.6 forthe A of mercury, and 6 x 10-19m2 for o c e ofmercury at 1000 eV, tne plasma density becomes

3.3 x

ce

Fig. 17 Map of Electron/Ion DensityNear Thruster

which was introduced by Sellen et aj.^ and verifiedby Ogawa et al.'°' ' . ne ref in this equation isthe electron density at a location where the po-tential is defined as zero and Te is the electrontemperature. Agreement with this equation meansthat contours of constant ion density are alsoequipotential contours. Further, the electricfield direction is such that charge-exchange ionsare accelerated toward regions of lower plasmadensity.

The total production rate of charge-exchangeions Nce can ke calculated from the efflux of ionsand neutrals, together with the charge-exchangecross section. Assuming for simplicity that allthe ions leave along the axis of the acceleratorsystem, free molecular flow for neutrals give: atotal production rate of

M _ b J ce

where Jf, is the ion beam current in amperes, nu isthe propellant utilization, o c e is the charge-exchange cross section in m2, rb is the radius ofthe accelerator system (ion beam) in m, e is themagnitude of the electronic charge in coulombs, andv0 is the average neutral velocity (/8kT0/m0) inm/sec.

A simple isotropic model can be obtained byassuming that the charge-exchange ions are dis-tributed uniformly in all directions from an effec-tive source downstream of the thruster. This ef-fective source is assumed to be one beam radiusdownstream of the accelerator system, although theexact location will not be important at the usualradial distance of a solar array. To simplify themodel for plasma density, the minimum ion velocityfor a stable plasma sheath,'2

is used in place of the more complicated self-consistent solution for charge-exchange ion veloc-ity. (The charge- xchange ion velocity outside ofthe ion beam depends on the local plasma potential,but the plasma potential is determined by ion den-sity, which depends on ion velocity.) The pla-..iadensity n c e at a radius R from the effectivesource then becomes

The densities measured along a radius normalto the ion beam direction should agree best withthe isotropic model since charge-exchange ions havefree access to that direction and yet it is wellaway from the beam ion trajectories. The theoret-ical variations of plasma density from the preced-ing equation are shown in Figure 18 together with

IOO

50

| 10

O

N

Jh = 0.38 amp

V. =0.51

TheoryExperiment

c lOOrxlO6 \5 I \

a 50

UJ

20

10,

Jj,=0.63 amp

77=0.85

10 20 30Radius, cm

40 50

Fig. 18 Comparison of Theoretical andExperimental Electron/Ion DensitiesNormal to Ion-beam. Axial Position7.5 cm Downstream of IE cm Thruster.

experimental data. The agreement between the twois good for a plasma process. The simplifyingassumptions were conservative in nature (that is,tending to give higher plasma densities), so thetheory was expected to give higher densities thanthe experimental data.

The charge-exchange plasma is not distributed

45

isotropi'.-ally at a given radius. No ions arejniti.11.7 directed in the upstream hemisphere.

"Only the electric fields surrounding the ion beamserve to deflect charge-exchange trajectories intothis upstream direction. Experimental plasma po-tential Measurements showed that the electric fieldcutside *tie beam was nearly antiparallel to thebaam direction. Using this observation, togetherwith .the additional observation that the electrontemperature'outside of the beam is about half thatinside tlie beam, the plasma density at an angle (e)between 90 and 180 degrees from the beam directioncan be derived as

' nce = nc6s 9o'exp £-ctn2e],

where nct! OQ is the plasma density obtained fromthe isotropic model for the same radius R. Thistheoretical angular variation is compared inFigure )'i with experimental data for the same ra-dial distance. The theoretical density is shown as

100 r

50

6 10

>.| 5

°IOOC

o

o 50UJ

20

10

mp

TheoryExperiment

r 1106

140 liiO (00 60 SOAngle from Beom Direction, degrees

40

Fici. 19 Comparison of Theoretical and. Experimental Electron/Ion Densitiesat 34 cm Radius and 7.5 cm Down-. stream of 15 cm Thruster.

uniform for angles less than 90 degrees inFigure 19. This is because no deflection of ini-tial trajectory direction is necessary to reachthese directions. The experimental data shouldbe larger than theoretical density at a smallenough ingle with beam direction because beam ionswhich wjll be included in the experimental measure-ments are not considered in the model. This fail-ing of the model is not a problem because the.up-stream direction is the one of most interest forinteractions of thrusters with solar arrays andother spacecraft components.

The charge-exchange plasma Model describedabove is sufficiently accurate for a qualitativeestimate of interaction effects. More exact esti-mates,, though, will require tests of the specificspacecraft configuration. Examples of the 1 imita-tions of this model were obtained recently usingcones downstream of the thruster. It was hopedthat these cones would collect most of the charge-exchange ions and thereby greatly decrease theplasma density upstream of the thruster. The ex-perimental reduction was much less than expected.It appeared that the charge-exchange ions negoti-ated trajectories around the ends of the cones fareasier than would be expected fro.,i the angle vari-ation predicted by the model presented above.

References

1. Wilbur, P. J., "Hollow Cathode Restartable15 cm Dia.ueter Ion Thruster," NASA CR-134532,December 1973, pp. 9-11.

2. Kerslake, W. R., "Charge-exchange Effects onthe Accelerator Impingement of an Electron-Bombardment Ion Rocket," NASA TN D-1657,May 1963.

3. Beattie, 0. R. and P. J. Wilbur, "15 cm CuspedMagnetic Field Mercury Ion Thruster Research,"AIAA Paper 75-429, March 1975.

4. Strickfaden, W. B. and K. L. Geiler, "ProbeMeasurements of the Discharge in an OperatingElectron Bombardment Engine," AIAA Journal,Vol. 1, No. 8, pp. 1815-1823, August 1963.

5. Moore, R. D., "Magneto-ElectrostaticallyContained Plasma Ion Thruster," AIAA PaperNo. 69-620 (1969).

6. Ramsey, W. D., "12-cm Mayneto-ElectrostaticContainment Mercury Ion Thruster Development,"J. Spacecraft and Rockets, Vol. 9, pp. 318-321,(1972). ~

7. Peters, R. R. and P. J. Wilbur, "Double IonProduction in Mercury Thrusters," AIAA Paper75-398, March 1975.

8. Peters, R. R. "Double Ion Production in Mer-cury Thruster," NASA CR-1350I9, April 1976.

9. Sellen, J. M., Jr., Bernstein, W., and Kemp,R. F., Rev. Sci. Instr., Vol. 36, pp. 316-322,(1965).

10. Ogawa, H. S., Cole, R. K., and Sellen, J. M.,Jr., AIAA Paper Wo. 69-263, (1969).

11. Ogawa, H. S., Ccle* R. K., and Sellen, J. M.,Jr., AIAA Paper No. 70-1142, (1970).

12. Bohm, D., i£ "The Characteristics of Electri-cal Discharges in Magnetic Fields," (A.Guthrie and R. K. Wakerling, eds.), pp.77-86, McGraw-Hill Book Co., 1949.

DISCOSSIOM

J. P. LAYTON: Wb e does the lifetime situationstand now with respect to the kind of applicationsthat are starting to look attractive?

46

P. J. WILBUR: For thruster applications, life-times of "critical components like the cathode, inexcels of 20,000 hours have been demonstrated inthe laboratory. Thrustets have been tested over10,000 hou'rs, and tests are in progress now whichextend bevond that. For other applications, wehave a broad-beam ion source here which we use forother purposes like ion milling and machiningfunctions and sputtering functions. The lifetimesthere cart be far in excess of those associated .withspacecraft because we can use carbon grids andthey have very long lifetimes associated with them.Lifetimes are adequate for ground-based appli-cations, and I feel that they are adequate formost missions as weH.

J. P. LAYTON: Is enough now known about theseto scale them to any size or power level?

P. J. WILBUR: Tests have been run on thrusterswhich range in size from 5 cm in diameter to1% meters in diameter. There is a wealth ofknowledge available on scaling. While analyticalmodelling has been moderately successful, theempirical approach to designing these thruste.sis quite well developed.

J. P. LAYTON: How are the efficiencies of thesethrusters at the lower effective jet velocities?

P. J. WILBUR: The efficiencies are*still good.Typically the potentials that we are acceleratingthe ions through are of order 1000 volts now andwe can get efficiencies of around 70%.

W. F. von JASKOWSKY: I was intrigued by yourstatement in the abstract that your model canpredict the density of doubly charged ionsaccurately. Do you use for that predictionempirical propellant utilization data?

P. J. WILBUR: In that mode'!, you input to themodel the plasma properties, the electron temper-atures and densities that exist in ^he disch rj;echamber. You also iput to it the propellar.tflow rate. It calculates the loss rate for ionsand the loss rate for neutrals, assuming freemolecular flow for the neutrals and bohm velocitylosses associated with the foTis. It then solvesusing experimental cross-section for trie most part,for the densities of resonance, metastable, singly-ionized and doubly-ionized species that are presentin the discharge chamber. We 'nave measured thedouble-ion densities coming out In the beam usinga mass spectrometer and we have measured the plasmaproperties in the thruster simultaneously, and wehave put the plas^t properties into the computerprogram and it calculates within the 10% thedouble-ion density that we measured coming out ofthe beam. So the verification has been on doubleior.s.

M. KRISHNAI): Have you observed any change in theionization efficiency in the discharge regionwhen going from the divergent to the cuspedmagnetic field configuration?

P. J. WILBUR: No, the cost of producing ions is not.influenced significantly by the change in magneticfield configuration so far as we have learned.

47

AN EFFICIENT PROCEDURE FOR COMPUTING PARTIALIONIZATION OF A GAS IN NUMERICAL FLUID DYNAMICS

S. Sarrdbba and L»Quartapelle *

"CBSNEF, Nucl.Engng.Dept.i Politecnico di MilanoMilano, Italy 20133

Abstract.

A numju^ical procedure is described thatfor the first time allows efficient solu-tion for the three nonlinear and implicitequations of state relating the equilibriumproperties of a partia.lly (and singly) io-nized neutral gas. Energy-, pressure and Sabe equations are encountered when solving•_ne conservation l&\»~z of fluid dynamics.

In the computation, the values for theintensive variables, that is pressure,temperature and ionizetion coefficient,are iteratively obtained with a given ac~curacy starting from assigned values forenergy anc volume per unit mass. Iterativecomputation is performed only when the value for the ionization coefficient lieswithin the lower and.the upper bounds fix-ed by the required number of significantdigits; otherwise the two explicit equations of -state for nonionized or for fully. ionized gases are employed.

This procedure has been applied to the. fcudy of partial, ionization produced in o-ne—dimensional propagation of strong shockwaves in hydrogen gas. The peaked shape ofthe density ratio, vs. shock strength is . -•clearly shown. The same procedure wouldalso be easily applicable to steady or un-steady multidimensional flows where non-uniform ionization plays a substantial role. .

. I. Introduction

P7.1oblems occur in gas dynamics wherepartial ionization of a monatomic gas has• a relevant influence on the fD.ow characteristics. Cases are offered by •f;he domain ofstrong shock waves propagation both in pre_•sence and without magnetic .field '• Or bythe stellar atmospnercs with the influen-ce of partial icaization on the pulsation

phenomena . The theoretical and numericalstudy of this class of problems requiresthe solution of the coupled system of constitutive equations and fluid dynamicconservation equations. Notable difficul-ties are encountered when dealing wit;, e—quilibrium partial ionization as describedby Sana equation. Indec? the nonlinear andimplicit dependence jf temperature and partial ionization on conserved quantitiesprevents any direct solution.

An efficient procedure is therefore presen ted for solving the vliree equations ofstate of a partially ionized g&s as requi-red by the commonly used finite differenceschemes of numerical fii'i.d dynamics.The -?anp,e of applicability coversany singly ionizable monatoj.: c gasin situations of thermody.amic equilibriumwhen radiation energy contribution; can beneglected.

IT.'The equations of state of apartially ionized .gas

First, the general foiro of the equationsof state fir a partially ionizable perfect. gas must be established. When radiation ef-fects are omitted and theimodynamic equili-brium is. taken for granted, it is

=£N (1+00 JtT+MttkT . +N ( ,

OC2

2h

(I)

(-•)

3/2 T..) exp(-~). (3)

+ This research was supported by EURATOMunder contract 109-73 PIPGI

* Also EURATOK JHRC, T o ^ f Italy 21020

Here N is the ntmsber of atoms per unitra^iss; P,T,CC, E and V are the thermodynamicvariables) that is pressure, temperature,ionization coefficient, energy and volumeper unit mass, respectively. '-. is the io—nization temperature, u •> s the electronicpartition function, w is the electronic ex-citation energy. Subscript o stays for theneutral atom while subscript 1 distinguishesthe ion « Explicit forms for u and w

' 48

are given by Zel'dovich and Raizer .

The method for computation which is de-veloped applies within the domain of firstiorization of any atomic gas. For sake ofdefiniteness, however, our treatment focu-ses to the sole case of atomic hydrogen.Now for a generic gas we will assume thatthe excit3d• states of atoms ajid ions'havea negligible influence on the themtodynamicequilibrium. Therefore U Q = 2 , u,»1ar.d w =w =0. Particularly, for hydrogengas it is always u =1 and w =0, and

the preceding assumption amounts only todisregard the electronic excitation spe£trum of the neutral atom.

Then, by the use of dimeris? on less va-riables equations (1-3) become

e E= — (1 + a ) t-i-B. j

p = (i+a) t/v ,

-2 ' JAl1-a

vt J /*exp(-

U)\5)

(b,

definitions hold as follows a s/E , v = v/V , p = P/p , c a T/T\ F , VQ=N(U /2u ){iid/Zm M^*' ,/

where definitions hold as follows a sO£,e a E/E , v = v/V , p = P/p , c a T/T. andE =P°=E / . .o o o

Dealing with hydrogen gas numerical valuer foi *"he intrinsic constants areT, =1.56 .105 «K , kN = 8.3(7 .10? e.t\g/°Kgarid 2u /u =1. So that E =1.30 .1013 erg/g,VQ=4.O3 cm3/g and PQ= 3?22-1O12 dyrc/an2l

Equations (4-oexpress the relationsthat the five tbexmodyi.amic variables i ,e ,v,p and t obey at equilibrium. In numeri-cal computation "5 of fluid dynamics the f i -nite dii ."erer.ee sch'jnes' vroeu'Je at anynesh point and at .uy time step the valuesof energy and .volume per unit mass. Theseschemes then require for the next time steptne kno-ledge of pressure and, possibly,temperature and ionization coefficient.

1 In -her words, i t is necessary to solvethe three equations (4—6) with respectto the three unknowns p,t and a startingfrom assigned values of e and v. A demandwhich results into a highly nonlinear andimplicit problem.

I I I . The method of solution

ta effective numerical procedure for the OTlution of this problem should meet two mainrequirements. Fxrst, the computation ofthe comolicated set (4-6) mu t be avoided,as far as the gas i s in a r.unionized orfully ionized state, for under these circumstances the condition a=0 or a=1 holdsand use can be made of the simple andexplicit relations that result froru (4)

and (5). Second, the solution must producenumerical values of prescrioed and, desira-bly, variable accuracy. In this respect, weproceeded according with a line which seemSjso to say, to kill the two birds with onestone.

Let us indeed assume that we need resultswith n significant digits. This n-eans thatthe gas can be considered, as nonionizedwhen a<10~n= a. ,, whereas it can be re-garded as fully'ionized 'When a > 1-10*"n=a

i l 'known in advance. We can determine 'froraequations (4-6) for a l l v two func 4onse , t f (v ) and e (v), which hold f.-i ;j.etwo"cases a =a. . and a- = a , respect i -vely. The construction of e. ^(v) and

to be made only afz i.he begin—of computations. It i s performed by

evaluating the coefficients of a suitablepower expansion. Obviously, different

Now suppose that the rari£e v

e (v)lin

.

es oC r lead to different functions e. Av), n d e ( v )

After this preliminary construction ,t;ivena pair (e, v),the computation prooeecs aifol.Tows. The values e. f•'••') and e; (v)arecoraputed ana compared with e. If e §c;. ,,(v)for e^e (v)), the simple equation.-,1"?state for the nonionized (or for the fullyionized) perfect gas allow to deteimire thepressure p and the temperature t, directly.On the contrary, if e. „ < e < e , the twounknowns p and a are eliminated from (4—6)to obtain a si i.gle'equation in x t

fit) = a(t) -(?-a(t)) vt3/2exp(-i/-)=O, (7)

where a(+:) = (2/3 e-t)/(2/3+t). Then, giventhe pair (e,v) the equation is solved fort ard the root is utilized to compute aand p through (4) and (5).

To solve (7) we have resorted to searchalgorithms, for the efficient iterationfunction methods lack the initial approxi-mation which may guarantee convergence.Instead, it is always possible to generatea temperature interval [*•,>*!,] such thatf(t1)f(th)<0 and the root t

ftf [t .tj .In other words, it is possible to con-struct an interval [tn ,t J -uitable forstarting any search algorithm.

Since for a fixed v, a is an ever—increasing function of t, the extremes can beset at t± = (e-1)/3 and t =2./3e; thesetwo values correspond with the two cases:f complete lonization and.null ioni^ation,respectively. The 3ower extreme t i how-ever, may become negative and cause com—putationPl difficulties. Therefore, forall practical purposes reference has beenmade to the value tn = max{(e-i)/3j C.O1|.Finally,it is worthy to say tnat, after •

49

the evaluation of the various possiblesearch algorithms, we found that binaryjiearch behaves Well when the requirementis for one or two significant digits. Inall other cases, appreciably higher effi~cienr^es can be obtained by adopting thefalse posit j in metiicc

IV. A test c a a on gas dynamic ionia-ing~ shock wave

As a test case for the numerical pro-cedure we have'considered the problem ofpartial ionization produced by a planesnocft wave5. The one-dime.nsiop.al flowhas been simulated by solving rh3 equa-tions of gas dyna-nics in a finite diffe-rence fjMi/. TI:»i ir.clusi-r- nf \ho nrt-i °2.e.i-al viscosity teim allows smoothingof shock discontinuities.

Values of the thermodynamic quantitieshave been computed in the downstream re-gion for various upstream conditions andcompared with the exact values. These vai-lues have been obtained 'oy .ising Hugoniotrelation for the partially ionized gas.Let us refer by subscripts u and d to theupstream and downstream regions, respsc—tively. Figure 1 and 2 display downstreamionization coefficient a as a runction ofvpstream Mach number M = Uu/W;,. here I) isgas velocity and W=(dp/3p) s ^ p / Q / ^stays for cound velocity.Equation <• '4 - 6) allow to obtainexplicit expressions for dependences ofc , c and (Sp/c>p)T upcn t and a . Figures1 and 2 show six different profiles per-taining to specific volumes ' v =10', 10","1O3, 10*-, io5, IQ6. ijigner values for vu

correspond with increasingly higher valuesfor a<j. Similarly, Figures 3 and 4 givethe dependence of the density ratio R=v /v.for the same families of upstream condi-tions. We note m passing that m Figure 3-curves merge towards the left end whereionization is practically absent. Thisfact is due to the well known scaling ofthe type K = f(tj,t .) valid for a perfectgas without ionization. The values obtain-ed in numerical gas- dynamics simulationare represented in the figuresby means ofdots. The agreement among theoretical (computed l values and simulated values is ex-cellent.

For the computation- of exact values ofa and K, the Hugoniot relation,

(4-6), to arrive -it the form

ed"eu (Pd+Pu> V V W

has been applied to the gas defined by

K2-2 [?cd( 1+ad)«d-ztu( 1«u)-au

-tud+au)=0. (9)

Given this, the density ratio k must bsobtained by iteration. Indeed; assigned cheupstream values (t , a ), equations (9)and(6) express how K depends on one of the twovariables t, or a . Let us fix, for instan-ce, t^ and start from a temptative value forK. Then relations (6) provide a value rora which may be introduced in (9) to givea closer approximation for K. Computationis rer eated until satisfactorily convergentresults are obtained.

V. Conclusion

The numerical procedure has been appliedwith considerable success to the propagationof a gas dynamic ionizing shock wave.Its domain of applicability, however, extends to any steady or unsteady multidimensio-nal flow where nonuniform ionization has arole. Particularly, the procedure is tai-lored for situations wheT>e the exs.cc amountof partial ionization has sensible effecton other physical properties.Examples are given by the dynamics of eptically chin radiative gases with radiationlosses O'di2) and by magnetogasdyncimicj, whenelectrical conductivity depends on a varia—ble electron density.

References

1. C.K. Chu, and R.A. Gross, Shock Wavesin Plasma Physics; in Advances in Pla-sma Physics, Vol. 2, A. Simon ana W.B,Thompson, eds.; Interscience Publishers,Hew Yo"lc, N.Y. (1969).

2. R.F. Christy, Astrophyt. J., _144_,108(1966).

3. Ya. B. Zel'dovich, and Yu.P. RaizerfPhysics of Shock waves and High-Temperature Hydrodynamic Phenomena, Vol. 1,tpansl.; Academic Press, New York, B.Y.(1966).

4. P.J. P.oache, Computational Fluid Dyna-mics; Hennosa Publishers, Albuquerque,N.M. (1972).

5. R.T. Taussig, Phys. Fluids, _8,166 (1965);Phys. Fluids, 9, 421 (1966).

6. R.D. Richtmysr, and K.W.Morton, Siffere;;cs Methods for Initial Values Problems,"2:id ed.; Interscience Puolishers, HewYork, H.Y. (1967).

50

1.0 1.5 2.0 2.5MACH NUMBER (>101)

Figure ",. Dependence of ionization coel_ficient a, ui: Mach number M_for t..=C.O1

0.5

u

1.0 1.5 2.0 2.5MACH NUMBER (xiO1)

3.0

Figure 2. Dependence of ionization coef_ficientfor t .

on Mach number M5.03, ' u

0.80^

0.60 \-

1CE

0.40K

0.20^

0.5 10 1.5 2.0MACH NUMBER (

2.5 3.0 1.0 1.S 2.0 2.5MACH NUMBER W01)

3.0

Figure 3. ijependence of density ratio KGT Mach number M for tu=0.0l.

Figure k» Dependence cf density ratio Kon Mach numVier M for t =0.03.

51

ANALYSIS OF NUCLEAR INDUCED PLASMAST

* **Jerry E. Deese and H. A. Hassan

Mechanical and V'-ospace Engineering DepartmentNorth ' < 1a State UniversityRaleigh, :h Carolina 27607

Abstract

A kinetic model is developed for a plasma gerer-\ted by fission fragments and the results areemployed to study He plasma generated in a tubecoated with fissionable material. Because both theheavy particles and electrons play important rolesin creating the plasma, their effects are consider-ed sltmltaneously. The calculations are carriedout for a range of neutron fluxes and pressures.In general, the predictions of the theory are ingood agreement with available intensity measure-ments. Moreover, the theory predicts the experi-mentally measured inversions. However, the calcu-lated galu coefficients are such that lasing is notexpected to take place in a helium plasma generatedby fission fragments.

Introduction

Increased interest in gas core reactors1 arithe recent demonstration ot direct nuclear pump-ing2'1* focused attention on plasmas generated bythe high eneTgy fission fragments. Such systemsare rather complex and the plassi* generated in themis, in general, not in thermal equilibrium. There-fore, before one can predict their performancecharacteristics, one needs to develop a detailedand self-consistent kinetic model capable of pre-dicting the behavior of the plaomas generated inthese devices.

Several authors have analyzed the space-det indent volumetric production of ions by fissionfragments passing through a background gas. BothLeffert, et al.5 and Nguyen and Grossman6 der'vedexpressions for the spatial distribution of fissionfragment production. In both of these analyses anenergy Independent empirical value W is assumed forChe amount of energy required to produce an Ionpair. The former utilized both linear and squareenergy-loss models for the heavy particles, whileNguyen and Grossman used the Bohr stopping equationfor fission fragments. Rather than rely on am em-pirical constant Mlley and Thiess7'6 derived an ex-pression for the lonization and excitation rateswhich takes into account the effect of the energydistribution of the Incident particles. Such acalculation requires estimation of the energy de-pendent cross section for ex<*'_ation and lonizationby heavy charged particles. . 3ethe-Born type re-presentation was employed for the case of heliumexcitation by alpha particles and fission frag-ments7'6, and later Guyot, et al.9 employedGryzinski cross sections1 for helium ionization byalpha particles and lithium ions.

The calculation of the electron energy distri-bution function in electric discharges and in theabsence of a high energy volumetric source is astandard procedure. However, these are only a fewanalyses of distributions resulting from a flux ofhigh energy particles where there is a high energy

t Supported, lev part, by NASA Grant NSG 1058.* Research Assistant.**Professor.

volumetric electron source. Calculations using aMonte Carlo method were carried out by Wang andMiley11. Later, Lo12 and Lo ami Miley13 used asimplified version of the Bolcztnann equation to de-termine the electron energy distribution in a he-lium plasma produced by a mono-energetic electronsource. More recently, Hassan aid Deese1* pre-sented a more elaborate Eoltzmaun equation formula-tion which took into consideration the primaryelectron spectrum.

In general, theoretical studies of excitedstate densities have assumed the electron distribu-tion function to be a Maxwellian at some character-istic temperature. Russell15 used such an assump-tion in calculating excited state densities inargon. Leffert, et al.5 and Rees, et al.16 alsostudied nobls gas plasmas assuming the electrondistribution function to be Maxwellian. More re-cently Maceda and Miley17 calculated the numberdensities of the helium excited states using thenon-Maxwellian dist outions of Lo and Miley13;their results indicate a number of possible inver-sions.

In addition to the above analyses, there existsa number of experimental investigations dealingwith nuclear pumped lasers and laser enhancement.Work carried out before 1972 is summarized in Hef.18 while a summary of the nuclear laser effort atthe University of Illinois along with an exhaustivelist of references on virtually every aspect ofradiation produced noble gas plasmas is included inthe work of Thiess19. Of particular interest hereare experiments studying individual atomic transi-tions at various pressures and additive concentra-tion under fission fragment excitation. The earli-est study of fission fragment excited spectra isthat of Morse, et al.20 who examined the effectsof fission fragment radiation on He, Ar, N2, andair. Guyot21 measured the production of heliummetastables by BI0(n,a) fitision fragments, whileWalters22 measured the relative intensities of thevarious transitions in both helium and argon.

As a result of the numerous research effortsoutlined above, actual nuclear pumped lasing hasjust recently been reported by several authors.McArthur and Tollefsrud reported lasing action incarbon monoxide as a result of nuclear excitationonly. Helmick, et al.3 demonstrated directnuclear pumping In He-Xe gas mixtures. A thirdcase of direct nuclear pumped lasing is that ofDeifoung* in a neon-nitrogen mixture. Obviouslythe nuclear pumped gas laser research effort isstill in its early stages with the validity of theconcept having been demonstrated oaly recently.Historically experimental research work has demon-strated the feasibility of a particular laser con-cept, with subsequent theoretical developmentdirected towards understanding the actual process-es Involved and op.inization of operating condi-tions .

The object of this investigation 16 the de-velopment of a theoretical model for a plasuagenerated by high energy fissioa transients.

52

The kinetic model treats particles In differentquautum states as different species and uses themultifluid conservation equations oir mass, momentumand energy to describe the resulting system. Ittakes into consideration the following kinetic pro-cesses: ionization, excitation and deexcitation,radiative recombination, spontaneous emission, as-sociative lonlzation and dissociative and colli-sional recombination. Because both the heavy par-ticles and electrons play Important roles in cieat-lng the plasma, their effects are considered simul-taneously. The rates of reactions involving elec-trons were calculated using electron distributionfunctions obtained from a solution of a Boltzmannequation appropriate for plasmas generated by fis-sion fragments11*.

T -

where T. is the initial gas temperature.

(5)

The properties of the electrons are determinedfrom an electron Boltzmann equation. The equationemployed is that developed in Ref. 14. For thehigh pressures of interest the plasmas generated bythe fission fragments are slightly ionized. There-fore, using the Lorentz gas approximation, the re-sulting Boltzmann equation for a quasi-steady plas-ma can be written as1"

3fo e £ i a f o _ eEi 3 r 2. vjfomT "W} 3mv2 3v

tv -" v dx±

The above model is employed to study a heliumplasca generated by fission fragments. Helium waschosen because of the availability of experimental-ly measured cross sections and rates and because ofthe availability of in-reactor measurements22. Ingeneral, the results show good agreement with ex-periment. Moreover, they Indicate a number of pos-sible laser transitions; all of them,, however, arein the IR region.

Analytical Formulation

The systems to be modeled here are those appro-priate for nuclear pumped lasers. Typically, theyconsist of tubes coated with fissionable materialand filled with gas at some given pressure and tem-perature. The tube is placed In the high neutronflux region of a reactor. Under neutron bpnbard-ment fission fragments emerge from the coating andenter the gas. The ensuing energy transfer resultsin ionization and excitation of the background gas.A schematic of the slab geometry employed in thisanalysis is shown In Fig. ]L.

Treating particles in different excited statesas different species one can utilize the multifluidconservation equations to describe the plasma gen-erated by the fission fragnents. For the condi-tions under consideration, the steady state approxi-mation is appropriate. In this approximation, theeffects of gradients are assumed negligible. Thus,the conservation of species equations,

3ns

IF+ V I + R

reduces, as a result of this approximation, to

I + R » 0 .

(1)

(2)

In the above equations, s is a charged particle orany quantum state of the background gas and, forspecies s, n s is the number density, Us is thevelocity and la and Rs are the production rates perunit volume resulting from nuclear and kineticsources, respectively.

For very low Mach numbers the momentum equationreduces to

p = const. (3)

where p is the pressure, while the energy equationtakes the form

where

Qs(v')fo' - v* Qs(v)fo]

= 0 ,

Imv^Imv2.!,

(6)

(7)

v is the velocity, e is the electric charge, m Isthe electronic mass, M is the mass of the heavyparticles, H Is the gas number density, V is thecollision frequency, 1/2 ravs^ is the excitationenergy, Qs is the excitation cross section and(3fo/3t)c is the source term resulting from primaryand secondary ionization and recombination. An ex-plicit expression for (3fo/3t)c together with themethod of solution of Eq. (6) are given in Ref. 14.

The quantities Is and R8 must be determined be-fore the above system of equations can be solvedfor the various species present. To determine Isand Rs, one needs to specify the Important kineticprocesses in the system. The major reactions in-cluded here, which are appropriate for noble gases,are

1. Fission fragment excitation

ff + X •»• X. + ff

2. Fission fragment ionization

f f + X + X + + e + f f

3. Spontaneous emission

X± + X. + h

4. lonization and recombination

Ij + e * X + + e + e

5. electron excitation and de-exclta-lion

X± + e t X. + e

6. Radiative recombination

X + + e + X + h

7. Excitation transfer

X. + X t. X. + X3 *

8. Dissociative recombination

xt + e t X. + X

where P Is the power Input and Q is the conductionand radiation losses. For optically thin highlyconducting gas, Eq. (4) reduces to

9. Collisional recombination

53

10. Associative ionization

X + X., -* X* + e

11. Electron stabilized recombination

12. Neutral stabilized recombination

13. Metastable lonization

X* + X* ->• X+ + X + e

14. Charge Transfer

X+ + M * M+ + X

15. Penning ionization

X* + M •+ M+ + X + e (8)

whore M represents a substance other than the back-ground gas or an impurity.

The tetm Is Is a result of reactions of type 1and 2 while Rs is obtained from reactions of type3-15. In general, for a reaction of the type

a i A i a s As + a t At

the contribution to the production rate of speciess is

a a,as k V nj <10)

where a^ (S. = i, j, s, t) denotes a stochiometriccoefficient. The quantity k is the forward ratecoefficient and is given by

k dVj (11)

where, for^species i, f. Is the energy distributionfunction, V^ is the velocity, gy = jv^ - Vj| andcry is the collision cross section. If. 1 repre-sents the stationary background gas and j a fissionfragment or an electron, then Eq. (11) reduces to

(12)

The rate coefficients for reactions involvingthe background gas (or gases) are usually obtainedfrom experiment. On the other hand reactions in-volving the fission fragments and electrons can, inprinciple, be calculated according to Eq. (12) fromcollision cross sections determined from experimentor theory and appropriate distribution functions.In this work the electron distribution function Iscalculated from Eq. (6). Unfortunately, the situa-tion with regards to fission fragments is not wellunderstood; this is because the fission fragmentsare characterized by initial energies ranging from50 to 115 Hev, initial charges from 16c to 24e andmasses from 70 to 160 atomic mass.units. In addi-tion no data is available an cross sections forlonization and excitation by fission fragments.Because of these uncertainties, the contribution ofthe fission fragments to excitation and ionizationwas estimated using two different approximatemethods. In the first, the procedure outlined inRef. 6 was used to-estimate the average energy de-posited in the gas per unit volume per unit time,Ef. The average number of ions produced per unitvolume per unit time is given by dividing Ef by W,the energy expended per ion pair produced. Simi-larly, the total number of excited states produced

is determined by dividing Ef by Ws_, the energy ex-pended per excited state produced. Rees '•t al.'°determined that the total excited particl-. produc-tion rate from fission fragment is .53 times theion production rate, thus

W/.53 . (13)

This procedure determines only the total number ofexcited states produced by fission tragments andsome model for the distribution of these statesmust be adopted. Because of the absence of a gen-erally accepted procedure for the distribution ofexcited states a number of models have been employ-ed here and these are discussed under Results andDiscussions.

The other approach uses the procedure of Thiessand Mlley8 which is based on a heavy particle dis-tribution function derived from a sesiempiricalslowing law together with ionization and excitationcross sections based on the Born or Gryzineki ap-proximations. To utilize this procedure one needsto assume that the fission fragments fall into twogroups: a light group with an average mass numberof 96 and an average charge of 20e and a heavygroup with an average mass number of 140 and anaverage charge of 22e.

Results and Discussion

The above model is applied to a study of Heplasma generated by fiss •.on fragments and the re-sults are compared with the measurements ofWalters22 who employed a tube of radius 1.85 cmcoated with UjOg. To conform to the conditions ofthe experiment, the calculations allow for the pre-sence of a nitrogen impurity In tiie system. For agiven pressure, temperature, neutron flux and tubedimensions, the above model is capable of predict-ing the number densities of the helium excitedstates, the atomic and molecular ions and the elec-trons. From this, one can calculate the relativeintensities and gain coefficients. For the calcula-tions presented here, the gas temperature is as-sumed constant at 300°K, while the pressure rangedfrom 100-760 Torr, the neutron flux from 3.8 x1 0 " - 10!6 neutrons/cm2.sec. and the nitrogen con-centration from .001 to 50 parts per million.

All helium excited states with a principalquantum number of 5 or less are included In thecalculations. The rates or cross sections for thereactions of types (3) through (15) indicated inEq. (8) were obtained from Eefs. 23-54 whileEinstein coefficients for spontaneous emission weretaken from Refs. 55 and 56. Because the experi-mental data is Incomplete the products or the ratesof some reactions had to be estimated and these arediscussed next.

At the high pressures of interest here the re-combination process in ncble gases is complicatedby the formation of molecular ions.' For pressuresgreater than 5 Torr the reaction

He + 2 He HeJ + He (14)

quickly converts atomic ions into molecular ions2 3.The molecular Ion recombination is governed bythree reactions2*•2s

54

e + Het •+ He + He

e + e + He* •* He + He + e

e + He + He* -* 2 He + He (15)

where He denotes an excited state. The distribu-tion of excited states produced by these reactionsis not well known. However, recent studies25~27

indicate that at least 70% of the excited statesproduced are atomic metastables in the pressurerange of interest here. Although potential energycurves indicate other states such as the 23P statesare produced in molecular ion recombination57, itis assumed that He in the reactions indicated inEq. (15) are atomic metastables.

Rates for associative ionization are availablefor atomic states of principal quantum number 3 andtriplet states of principal quantum number 4, Refs.26, 28-30. Cioss sections for the excitation trans-fer reactions are available for n = 2 and n = 3 butonly for the triplet states when n = 4, Ref. 26.Because! there processes are important in the deter-mination of the final distribution of excitedstates some estimate of the unavailable rates isrequired. It is assumed here that the associativeionizacion rates for n = 5 states are the same asthe corresponding n = 4 states while the associa-tive ionization cross sections for the n*F statesare assumed equal to those for the n*D states.Moreover, three calculations were carried out inwhich the cross sections for associative ionizationand excitation transfer reactions of the tripletn = 4 states were assumed to be one-third of, equalto, and three times the corresponding singlet re-action cross seciions. Comparison of calculatedand measured excited states showed that betteragreement with experiment Is obtained when Q(4'x) £Q(43X). Therefore, unle.-.s Indicated otherwise, allcaic-jle.tions reported here employ this assumption.

As indicated earlier, when a 'W value is usedto estimate the total number of excited statesfiv.ther assumptions are needed to indicate whichstates are excited. From a study of the spectra ofnoble ^ases generated by the impact of alpha par-ticles,, Bennett56 suggested that most of the ex-cited, states in He are n'P states with the majorityof them in the 2lP state. His conclusion was basedon the fact that calculated excitation cross sec-tions, based on the Born approximation, are highestfor the n'P states. This assumption is contrastedwith that of Guyot2 where he assumed that most ofthe excited atoms resulting from the impact ofalpha particles are in the 2SS state and these areconverted to the 23S states by electron impact.Calculations using both of the above models werecarried out; for these calculations a yalue of

Comparison of the results with the experimentsof Walters22 requires that the effect of a nitro-gen contaminant be taken into considrration. Theyas used in the experiment contained a nitrogencontaminant of approximately 50 parts per million.T!ie affect of she pressure or. the number densitiesof the electrons and tht two metastable states isjiiven in ?ig. 2 while Fig. 3 shows the effect ofpressure on several of the higher states with andwithout a 50 parts per million nitrogen contaminant.For these figures, the neutron flux is 3.8 x 1 0 n

neutrons/cm .sec. Considering pure helium first,it is seen that the electron number density de-creases as the pressure increases. This is becausethe dominant helium recombination process Is theneutral stabilized recombination of the molecularIon and this process becomes more efficient as thepressure increases. The dominant processes govern-ing the excited states number densities are elec-tron excitation from the ground state and the meta-stable state 23S, ex-.itation transfer, associativeionizatijn and spontaneous emission. As the pres-sure increases, the decrease in the electron den-sity coupled with the Increased effectiveness ofthe associative ionization and excitation transferresults in the decrease of the excited states shownin Fig. 3. Only the 23S and 2]P states show an In-crease as the pressure increases; the rapid recom-bination resulting from the last reaction In Eq.(15) accounts for the 23S behavior.

As Is seen froit Figs. 2 and 3, L!;3 effect ofnitrogen on the various species is quite signifi-cant. Both the helium molecular ion and the meta-stables are quickly converted to nitrogen ionswhose dominant recombination process is dissocia-tive recombination. This not only loxrers the con-centrations of the He ions and metastables butchanges the electron number density variation withpressure as well. When nitrogen is present thedominant recombination process is the f~vo-bodydissociative recombination. This process does notbecome more efficient as pressure increases and asa result the electron number density Increases withan increase In pressure. Thus, the net effect ofnitrogen Is two-fold: it lowers the number densityof electrons and metascables making electron exci-tation from metastable states less important and,it changes the number density dependence on pres-sure. As a result of this, one would expect asignificant effect on the higher excited states aswell. Because the electron number density in-creases with pressure the electron excitation ratesincrease with pressure. Thus, as is seen In Fig.3, the marked decrease in excited state populationwith pressure in the case of pure helium is not aspronounced when nitrogen is present. The figurealso shows that the magnitudes of the number den-sities decrease with pressure.

The results indicated in Fig. 3 do not exhibitthe peak around 200 Torr shown in Figs. 4-9 of Ref.17. Both Walters22 and Thiess19 suggest that theassociative ionization process tnay be a three-bodyprocess of the form

He + 2 He -+ He* + He O ')

rather than the two-body process

He + He •+ He* (17)

assumed here. If this process is indeed three-bodyin. nature Its effectiveness will increase withpressure.

A study of the variation of the number densi-ties with neutron flux for f j case of a pure foli-um gas at 100 Torr has also been carried out. . aebehavior of the electrons and jietastables is givenin Fig. 4 and that of sc 5 higher states is shownin Fig. 5. The number densities of the higherstates increase linearly with the flux at lowerneutron flux levels. As the electron number den-sity increases, the electron stabilized three-bodyrecombination becomes important. Thus, population

55

densities highly dependent on electron excitation,*rates do not increase as rapidly at higher flux•levels. • . .

Because the nitrogen contaminant has a signifi-cant effect on the number densities, a determina-tion of the concentration range where its effectbecomes insignificant is of interest. Figure 6shows a plot of the electron and metastable numberdensities vs. nitrogen concentration at 100 Torrand a neutron flux of 3.8 x 1011 neutrons/cm2.sec.As is seen from the figure, the effect of nitrogenis negligible below a concentration of 10~8.

All of the above results assume the 2'P stateto be the only excited state produced by fissionfragments. Calculations using the procedure ofRef. C, the TM model, have also been carried out.Boltzmann plots showing log XI/gj A = log[hcn^/4irg^](where X is the wave length, I is the intensity, Ais Einsteir's transition probability, gj is the de-generacy, h is Planck' c. constant and c is the speedof light, vs. eu, the energy of the upper level,for pressures of 100 and 760 Torr are given in Figs.7 and 8 for the 2]P model and in Figs. 9 and 10 forthe TM model. Error bars in these graphs indicatethe spread in Walters' experimental data. Usingthe TM model, the triplet states are considerablyunderpopulated relative to the singlet states. Be-cause Walters found the triplet states to be ofthe same order of magnitude for states with n = 3,it appears that the 2*P model is more appropriate.

As is seen from Fig. 7, the 2*P model givesgood agreement with experiment at 100 Torr; the onlydiscrepancy being the n3D states which are over-populated compared with the experimental measure-ments. At 760 Torr the agreement of the 2!P modelwith experiment is not as good as at 100 Torr. Alltriplet states except Lhe 33S are predicted to havea population greater than experimentally measuredvalues. This could be due to an improper pressuredependence for associative ionization as discussedearlier. In general, singlet states have much

higher emission coefficients and as a result, cas-cade losses play a larger role in the singlet sys-tem. The triplet system on the other hand is pre-dominantly governed by associative ionization andexcitation transfer processes and these have agreater dependence on pressure. Thus, it is ex-pected that triplet states number densities willdecrease more rapidly with pressure. Calculationshave also been carried out assuming fission frag-ments produce only 2lS slates. However, for thiscase the 2JS nunber density is higher than the 23Snumber density and this is not in agreement withavailable experiments21.

When studying pure helium, two population in-versions have been found throughout the entirerange of pressures, neutron fluxes and fissionfragment excitation models examined on the 3!P -3lD (95.76U) and 4>P - 4:D (216u) lines"'". Inaddition, for pressures less than 200 Torr andneutron fluxes less than lO1* neutrons/cm2.sec. theTM model prgdicts an inversion of the 3lS - 2'Pline (7281 A ) . The expression for the gain coef-ficient is given in the Appendix. The gain coef-ficients together with other inversions, operatingconditions and fission fragment models employedare summarized in Table 1. The addition of thenitrogen did not change the above results. How-ever, the gain coefficients &re slightly decreasedbecause of greater depopulation of excited levels.

Figures 7-10 show only states of principalquantum number four or less; these states were theonly states considered in the determination of theInversions present. Excitation transfer cross sec-tions are not available for the n = S states an-! asa result these states are overpopulated. The im-portance of these reactions can be seen by perform-ing calculations where these reactions are neglect-ed for all states. The Boltzmann plot for such acase is shown in Fig. 21. Comparing Figs. 7 and 11,it is clear that excitation transfer plays an im-portant role in determining the relative populationof excited states and consequently, the presence of

Table.1 Rain Coefficients

Transition

3lp

31 P

3lp

3XP

3XP

41?

41?

AhA **

A P

43D

3*5

-lh- 3u

- 3HD

- 2XD

-3h-Ah

1

-4 1!)

-4*1)

-A- 4 3 P

- 2 ^

Wavelength

95.76 v

95.76 u

95.76 y

95.76 V

. 95.76 i!

216 u

216 u

216 u

216 \i

216 v'

21b v

7281 A

Excitation• Model

2lp

2lp

2lp

2-F

TM

2XP

21?

2 ^

2-P

TM

21?

TM

PressureTorr

100

760

100

760

760

100

760

100

760

760

100

100

HeutronFlux

3.8 x 10 1 1

3.8 x 1011

1.0 x 1016

3.8 x 1011

3.8 x 10 1 1

3.8 x 1O11

3.8 x 10 1 1

1.0 x 3.O16

3.8 x 1O11

3.8 x 1011

1.0 xlO 1 6

3.8 x 1 O U

0.0

0.0

0.0

5 x 10"5

0.0

0.0

0.0

0.0

5 x 10"5

0.0

0.0

0.0

GainCoefficient

0.28 x 10~9

0.76 x 10"11

0.14 x 10~5

0.42 x 10"11

0.15 x 10"9

0.12 x 1O"9

0.24 x 1O"11

0.16 x 10"6

0.14 x lO"11

0.11 x 10"10

0.20 x 10"5

0.24 x 10"8

56

Inversion.sumes

Similar conclusions hold when one as-

3 Q(43X) (18)

as is seen from comparing Figs. 6 and 11.

Concluding Remarks

Considering the incompleteness of availablecross sections, the predictions of the model are ingood agreement with available relative intensitymeasurements and measured inversions. However,more complete rate data is needed for the accuratepredictions of other possible inversions involvingthe higher states. The calculated gain coefficientsare so small that, because of cavity losses, lasingis not expected to occur in helium. Thus, heliumis not a good candidate for a nuclear pumped laser.

Appendix: Gain Coefficient

The possible jsing transitions in atomic heli-um involve electronic state transitions. The gaincoefficient for transition from upper level m tolower level n is

Y(v) • A &-h G(V)

n(2,2) ( T ) j ^ g ^ + 0.52487 exp [-0.7732 T] +

2.16178 exp [-2.43787 T] -

* ,,„ •,*-* J>-14874 . ,18.03236.43? x 10 T sin ( p 7 6g 3 -

. 7.27371) .

The required parameters for helium are60'6"

d - 2.576 x 10"8 cm

(A8)

e » 10.22 "K

Ig » 24.586 ev .

References

1. Schneider, R. T., Thorn, K. and Helmlck, H. H.,"lasers from Fission", XXVIth InternationalAstronautical Congress, Lisbon, 1975.

2. McArthur, D. A. and Tollefsrud, P. B., "Obser-vation of Laser Action in CO Gas Excited Onlyby Fission Fragments", Applied Physics Letters,

(Al) Vol. 26, Feb. 1975, pp. 187-190.

where G(v) is the shape factor, N,,, and Nn the num-ber densities of states n and n, gm and gg aretheir degeneracies, Amil is the Einstein coefficientof spontaneous emission from level m to level n,and V is the frequency of the transition.

The shape factor is affected by two processes,Doppler and pressure broadening. In general, forpressures greater than 30 Torr, pressure broadeningis dominant. Thus, Doppler broadening is neglectedin this analysis. For pressure broadening theshape factor G(v) is given by

G(v) =

where

V-t1/2

(A2)

(A3)

In Eq. (A3), s represents the lasing gas and t re-presents other gases present in the system. Whensumming on t in Eq. (A2), t can be equal to s.

The quantities Zgt can be calculated from theLennard-Jones potential parameters of the collidingmolecules60"62

Z,, - d 2 S5(2'2)(T*J (A4)

st

T/est

8t

(A5)

(A6)

(A7)

(2 2")The quantity Si ' (T) Is obtained from the empiri-cal curve fit63

3. Helmick, H. H., Fuller, J. L. and Schneider,R. T., "Direct Nuclear Pumping of a Helium-Xenon Laser", Applied Physics Letters, Vol.26, March 1975, pp. 327-328.

4. DeYoung, R. J., "A Direct Nuclear Pumped Neon-Nitrogen Laser", Ph.D. Dissertation, Univer-sity of Illinois, 1975.

5. Leffert, C. B., Rees, D. B. and Jamerson,F. E., "Noble Gas Plasmas Produced by FissionFragments", Journal of Applied Physics, Vol.37, Jan. 1966, pp. 133-142.

6. Nguyen, D. H. and Grossman, L. M., "lonizationby Fission Fragments Escaping from a SourceMedium", Nuclear Science and Engineering, Vol.30, Nov. 1967, pp. 233-241.

7. Miley, G. H. and Thiess, P. E., "A Unified Ap-proach to Two-Region lonization-ExcitatlonDensity Calculations", Nuclear Applications,Vol. 6, May 1969, pp. 434-451.

8. Thiess, P. E. and Miley, G. H., "Calculationsof Ionization-Excitation Source Rates in Gas-eous Media Irradiated by Fission Fragmentsand Alpha Particles", NASA SP-236, 1971, pp.369-396.

9. Guyot, J. C , Miley, G. H. and Verdeyen, J. T.,"Application of a Two Region Heavy ChargedParticle Model to Noble-Gas Plasmas Induced byNuclear Radiations", Nuclear Science and En-gineering, Vol. 48, Aug. li'72, pp. 372-386.

10. Gryzinski, M., "Classical Theory of AtomicCollisions. I. Theory of Inelastic Colli-sions", Physical Review, Vol. 138, April 1965,pp.. A336-A358.

11. Wang, B. S. and Miley, G. H., "Monte CarloSimulation of Radiation-Induced Plasma",Nuclear Science and Engineering, Vol. 52,Sept. 1973, pp. 130-141.

57

12. Lo, R. H., "Energy Distributions of Electronsin Radiation Induced-Helium Plasmas". Ph.D.Dissertation, University of Illinois, 1972.

13. Lo, R. H. and Miley, G. H., "Electron EnergyDistribution in a Helium Plasma Generated byNuclear Radiation", IEEE Transactions on Plas-ma Science, Vol. PS-2, Dec. 19/4 pp. 198-205.

14. Hassan, H. A. and Deese, Jerry E., "The Elec-tron Boltzmann Equation in a Plasma Generatedby Fission Products", to appear as a 3ow-iiumbered NASA CR, 1976.

15. Russell, G. R., "Feasibility of a NuclearLaser Excited by Fission Fragments Produced ina Pulsed Nuclear Reactor", NASA SP-236, 1971,pp. 53-62.

16. Rees, D. B.; Leffert, C. B. and Rose, D. J.,"Electron Density in Mixed Gas Plasmas Gener-ated by Fission Fragments", Journal of AppliedPhysics, Vol. 40, March 1969, pp. 1884-1896.

17. Thorn, K. and Schneider, R. T., "Nuclear PumpedGas Lasers", AIAA Journal, Vol. 10, April1972, pp. 400-406.

18. Haceda, E. L. and Miley, G. H., "Non-Maxwellian Electron Excitation in Helium"*Bulletin of the Amp^fcan Physical Society, Vol.20, Feb. 1975, p. 255.

19. Thiess, P. E., "Optical Emission aud Kineticsof High Pressure Radiation-Produced Noble GasPlasmas", Ph.D. Dissertation, University ofIllinois, 1975.

20. Morse, F., Harteick, P. and. Dondes, S, "Ex-cited Species of Gases Produced in the Nuclear'eactor", Radiation Research, Vol. 29, Nov..1966, pp. 317-328.

21. Guyot, J. C , "Measurement of Atomic Meta-stable Densities in Noble Gas Plasmas Createdby Nuclear Irradiation", Ph.D. Dissertation,University of Illinois, 1971.

22. Halters, R. A. "Excitation and Ionization ofGases by Fission Fragments", Ph.D. Disserta-tion, University of Florida, 1973.

23. Beaty, 2. C. and Patterson, P. L., "Mobilitiesand Reaction Rates of Ions in Helium", Physi-cal Review, Vol. 137, Jan. 1965, pp. A346-A357.

24. Berlande, J., Cheretj M., Deloche, R.,Gonfalone, A., and Manus, C., "Pressure andElectron Density Dependence of the Electron-Ion Recombination Coefficient in He", PhysicalReview A, Vol. 1, March 1970, pp. 887-896.

25. Deloche, R., Morchicourt, P., Cheret, M. andLambert, F., "High-Pressure Helium Afterglowat Room Temperature", Physical Review A, Vol.13, March 1976, pp. 1140-1176.

26. Cohen, J. S., "Multistate Curve-Crossing Modelfor Scattering: Associative Ionization andExcitation Transfer in Helium", Physical Re-view A, Vol. 13, Jan. 1976, pp. 99-114.

58

?7. Johnson, A. W. and Gerardo, J. B., "IonizingCollisions of Two Metastable Helium Atoms(23S)", Physical Review A, Vol. 7, March 1973,pp. 925A-928A.

28. Teter, M. P., Niles, F. E. and Robertson,W. W., "Hornbeck-Molnar Cross Sections for then = 3 States of Helium", The Journal of Chemi-cal Physics, Vol. 44, April 1 66.

29. Wellenstein, H. F. and Robertson, W. W., "Col-lisional Relaxation Processes for the n = 3States of Helium. II. Associative Ioniza-txon", The Journal of Chemical Physics, Vol.56, Feb. 1972, pp. 1077-1082.

30. Wellenstein, H. F. and Robertson, W. W., "Col-llsional Relaxation Processes for the n = 3States of Helium. III. Total Loss Rates forNormal Atom Collision", The Journal of Chemi-cal Physics, Vol. 56, Feb. 1972, pp. 1411-1412.

31. Rapp, D. and Englander-Golden, P., "TotalCross Sections for Ionization and Attachmentin Gases by Electron Impact", The Journal ofChemical Physics, Vol. 43, Sept. 1965, pp.1464-1479.

32. Dugan, J. L. G., Richards, H. L. and Muschlitz,E. G., "Excitation of the Metastable States ofHelium by Electron Impact", The Journal ofChemical Physics, Vol. 46, Jan. 1967, pp. 346-351.

33. Holt, H. F. and Krotkov, R., "Excitation ofn = 2 States in Helium by Electron Bombard-ment", Physical Review, Vol. 144, April 1966,pp. 82-93.

34. Cermak, V., "Individual Efficiency Curves forthe Excitation of 23S and 2*S States of Heli-um by Electron Impact", Journal of ChemicalPhysics, Vol. 44, May 1966, pp. 3774-3780.

35. Ferende.-.i, A. M., "Electron E.v.itation CrossSection of the He 2JS State from DiffractionLosses in a V.a - Ne Gas Laser", Physical Re-view A, Vol. 11, May 1975, pp. 1576-1582.

36. St. John, R. M., Miller, F. L. and Lin, C. C ,"Absolute Excitation Cross-Sections of Heli-um", Physical Review, Vol. 134, May 1964, pp.A888-A897.

37. Showalter, J. G. and K-iy, R. B., "AbsoluteMeasurement of Total Electron-Impact CrossSections to Singlet and Triplet Levels in He-lium", Physical Review A, Vol. 11, June 1975,pp. 1899-1910.

38. Jobe, J. D. and St. John, R. M,, "AbscluceMeasurements of the 2'P and 23P Electron Ex-citation Cross-Sections of Helium Atoms",Physical Review A, Vol. 164, Dec. 1967, pp.117-121.

39. Kieffer, L. J., "Low-Energy Electron-CollisionCross-Section Data. Part II: Electron-Excitation Cross Sections", Atomic Data, Vol.1, Nov. 1969, pp. 121-187.

40. Moiseiwitsch, B. L. and Smith, S. J., "Elec-tron Impact Excitation of Atoms", Reviews ofModern Physics, Vol. 40, April 1S68, pp. 124-

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45. Lowry, J. F., Tomboulian, D. H. and Ederer,D. L., "Photoionization Cross Section of Heli-um in the 100- to 250- k Region", Physical Re-view A, Vol. 137, Feb. 1965, pp. 1054A-1057A.

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50. Hurt, W. B., "Cross Section for the ReactionHe(2SS) + He(23S)n, Journal of Chemical Phys-ics, Vol. 45, Oct. 1966, pp. 2713-2714.

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53. SchEiilrekopf, A. L. and Fehsenfeld, F. C ,"De-excJ tation Rate Constants for Helium Meta-stable Atoms with Several Atoms and Molecules",Journal of Chemical Physics, Vol. 53, Oct.1970, pp. 3173-3177.

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63. Neufeld, P. D., Janzen, A. R. and Aziz, R. A.,"Empirical Equations to Calculate l^of theTransport Collision Integrals SJv1* ' for theLennard-Jones (12-6) Potential", The Journalof Chemical Physics, Vol. 57, Aug. 1972, pp.1100-1102.

64. Kieffer, L. J., "A Compilation of ElectronCollision Cross Section Data for Modeling GasDischarge leasers", JILA Information CenterReport 13, Sept. 1973, Information Center,Joint Institute for Laboratory Astrophysics,University of Colorado, Boulder, Colorado.

59

FUEL REGION

BACKGROUND GAS

Fig. 1 Slab approximation of cylindrical geonetry.

! 0 1 4

IO1'

KP

10'

SjO10

z ~ •

K39 -

I09 = . .

( 0 7 -

6

PURE HELIUM

4, =3.8x 10 NEUTRONS/CMZ SEC

^ s

,:>o

Fig.. 2

200 300 400 500 600 700 800PRESSURE, TORR

Influence of pressure on electron andmetastable number densities

•? 10'o

1.0

10

-2'P

PURE HELIUM

N2 = 50PPM

<f, = 3.8x iO1' NEUTRONS/CM2 SEC

100 200 300 400 500 600 700 800PRESSURE, TORR

Fig. 3 Influence of pressure on selected excitedstate number densities.

60

10

10" 10'

<j>, NEUTRONS/CM SEC

Fig.- 4 Effect of neutron flux on electron andmetastable number densities.

to'

\0'

10'

10°

10'

10'

10'10 10 10" 10 10" 10" 10'

>, NEUTR0iMS/CMd SEC

Fig. 5 Effect of neutron flux on the numberdensities of some excited states.

61

I 0 1 0 -

I09

I08

I01

£=3.8x io" NEUTRONS/CM2 SEC

P=IOOTORR

I I I I I I 1IO10 I O 9 I O S IO 7 IO 6 IO 5 IO4

4.0i-

3.0

2.0

o

• 3 S *

rfp.3'0

4-l<• 4'P

V D , 4*0

_ £=3.8xl0MNEUTR0NS/CM2SEC• 43S

34 P

4 PP= 760 TORRNz=50PPM

• THEORYX EXPERIMENT

I I ! I I I I I I 122 23 24

UPPER STATE ENERGY, ev

Fig. 8 Helium excited s ta tes a t 760 Tor-, 2 Pmodel.

Fig. 6 Effect of a nitrogen contaminant on elec-trons and metastabJes.

3.0

*3 'S

« 2.0o_i

1.0

•330

5»t4'P

*4'P

14'D t

U'D I, 4 V

£=3.8x10" NEUTRONS/CM* SECP=IOOTORR

I I I I

•4 30

• THEORYX EXPERIMENT

I I I I22 23

UPPER STATE ENERGY, ev24

Fig. 7 Heliua excited states at 100 Torr, Z1?modal.

3.0

1.0

3 3S.

• 33P

_ ^=3.8 X10" NEUTRONS/CM2 SECP=IOO TORRN2=5OPPM

- • THEORYX EXPERIMENT

I I I I I I 1 I I.4 3 S

22 23


Fig. 9 Hglium excited states at 100 Torr, IMmodel.

3.0

o

1.0

33S1: f3'S'3'S

$33P, 33P

<p=3.ex!0l!NEUTRONS/CM* SECP=760TCRR

- • THEORY$ EXPERIMENT

J _ I I I

J4V

J I22 23


Fig. 10 Helium excited states at 760 Torr, TMmodel.

4X)

3.0

o

• 33D I4 . 'P*4'P

1.0

343S»

_ <£«3.8xl0"NEUTR0NS/CM2SEC 4 »p=ioo ronnN£«5OPPM

- • THE0RYiQ(4'Sjc30.(43X)t EXPERIMENT

J 1 1 1 1 1 ) 1

-4'0

22 23UPPER STATE ENERSY, ev

24

Fig. 12 Effect of associative ionization and ex-citation transfer rates for n = 4 on ex-cited states.

3.0

2.0

1.0

_

33S»

$3'S, 3'S

53P* 3'D

P»

• 3'D3 3 D *

.33D

_ <£=3.8xl0" NEUTRONS/CM4

P=100 TORRN2 =. 50 PPM

- • THEORYt EXPERIMENT

1 1 1 1 1 1

4 l S .

4 3 S -

SEC 4 3

ir430

\

i r

|4'D

J«4'PP*43P

22 23UPPER STATE ENERGY, ev

24

Fig. 11 Effect of excitation transfer reactionson excited s ta tes .

6 3

ENERGY DISTRIBUTIONS AND RADIATION TRANSPORT IN URA: IjJM PLASMAS

G. Miley, C. Eathke*. E. Meceda and C. ChoiNuclear engineering ProgramUniversity of IllinoisUrbana, Illinois 61801

Abstract

Electron energy distribution functions have

• been calculated in p U" -plasma at 1 atmospherefor various plasma temperatures. (5000 to 8000°K)

and.neutron fluxes (2 x 10 " to 2 x 10 neutrons/

(cm -sec}), Two.sources of energetic electronsare included; namely fission-fragment and electron-impact ionization, resulting.in a high-energy tailsuperimposed or the theraali. ec electron distribu-tion. Consequential derivations from equilibriumcollision rates are of iiiteret; relative to directpumping of lasers and radiation emission. Resultssuggest that non-equilibTium excitation can bestbe achieved with an additive gas such ss helium orin loweT temperature plasmas requiring UFg.

An approximate analytic model, based oncontinuous electron slowing, has been used forsurvey calculations. Where more accuracy is re-quired, a Monte Carlo technique is used whichcombines an analytic representation of Coulombiccollisions with a random-walk treatment of in-elastic collisions. The calculated electron dis-tributions have beon incorporated into anothercode that evaluates both the excited atomic statedensities within the plasma and the radiativeflux emitted from the plasma. Only preliminaryresults are available from the latter code atthis time, however.

Introduction

Knowledge of distribution functions is essen-tial to understanding radiation-induced plasmasand their applications. Prior to the presenturanium plasma calculations, workers at Illinoisconsidered various noble gases, both as potentialnuclear-pumped laser candidates and to gain ex-perience with analytic techniques with simpler

cases. G. Miley*• ' ' developed techniques tocalculate the energy distribution of the primary

ions. Subsequently, P. Thiess and G. Mileyreported early calculations for helium at the 2nd

(41Uranium Conference; R. Lo and G. Miley1 firstreported the calculation of a full distribution ofelectron energy in a helium plasma created by•nuclear radiation in 1974; and B. Wang and G.

Mi ley *• ' developed a Monte Carlo simulation modelto evaluate the distribution function more pre-cisely for cases both with and without an appliedelectric field. While consuming more computertime, the Monte Carlo method is viewed as a bench-mark for testing analytic techniques which must,of necessity, contain many approximations.

Evaluation of the electron energy distribu-tion and the resulting excited state densities inuranium-UF, plasmas are necessary to accurately

determine the ladiation omitted from a gaseous-coreuranium reactor such as if currently under experi-mental study at Los Alamos Scientific Labora-tory. ' A gaseous-core uranium reactor is a po-tentially important source of radiation for thevarious applications such as chemical processing,isotope separation, and direct pumuing of gas

lasers.™

While early experiment! will use UF,, our

calculations to date have concentrated on uraniumplasma? for two reasons: First, an opportunity *odevelop the appropriate cross-sections and calca-lational techniques is afforded while working witha simpler, one-component case, and secondly, theuranium plasma is of ultimate interest for high-grade power reactors.

Plasma temperatures ranging from SOOO^K to8000°K at a pressure of 1 atm (the boiling pointof uranium is 4407°K at 1 atm) hith neutron fluxes

from 2 x 10 to 2 x 10 neutrons/(cm -sec) areconsidered. The uranium plasma under study isunique in that it is a fissioning plasma. Theneutron flux within the plasma causes fission-fragments to be formed. They in turn produce anon-Maxwellian electron flux which excites theatoms in the gas, c mating radiation emission.Two important questions result: Are there anyinverted energy levels that could lead to lasing?Does the line structure tha: is superimposed onthe emission continuum caust any unique featuresrelative to other uses?

The study' has two distinct aspects: First,the effect of fission-fragments on the electronenergy-distribution function in the plasma is

evaluated. ' Parametric studies of the neutron-flux (a measure of the fission-fragment density)and the temperature dependence of the distributionfunction ire included as part of this work.Second, the excited-state densities within the

(9)plasma are studied. ' Possible inversions aresought and the radiative transport of photons, i.e.the plasma emission spectrum, is considered.

In determining the rate of occurrence of fis-sion reactions, the urai.iura is assumed to consist

entirely of the U isrtope. Furthermore, theneutrons are assumed to be in thermal equilibriumwith the plasma so that the fission cross-sectioncan be averaged over the neutron flux distributionusing an effective neutron temperature. Someallowance for spectrum hardening is made in themore strongly absorbing cases.

Theoretical Models

In subsequent sections we concentrate on theelectron energy distribution created by fission

This work is supported by the U.S. NASA Grant_KSG-1063.

Present Address: Plasma Physics Laboratory, Princeton University, Princeton, NJ 08540

64

fragments. Over the density range of interest,electron-impact excitation-ionization dominatesover tne contribution by direct fission-fragmentcollisions. Consequently, it is important tohave the correct energy-spectrum for primaryelectrons produced by fission-fragment impact, andthis is calculated by techniques developed by Guyot

For the plasma parameters mentioned above, thedensities of the various plasma constituents can bopredicted by the Saha equations, and the densitiesare illustrated in this Fig. 1. A first urdci-approximation of the perturbation to these den-sities caused by the production of fission-frag-menr generated electrons is only of the order of1/10% for cases of interest here. Therefore, i.further correction for radiation tffeccs isgenerally negligible.

Despite their small number, the high energyelectrons in the tail of the distribution make asignificant contribution to excitation reactions,hence, th tail is calculated explicitly. Anapproximate, but useful, analytic formulation forthe distribution function is obtained with theassumption of continuous slowing down of electrons.In an infinite, isbtropic medium and in the absenceof external forces or fields, the steady-statesolution for the distribution function at highenergies {i.e. for energies where recombination is

negligible) is found to be1-11'12^1

1 S(E') dE'

dErdt E

E > ET

The denominator, jj-jg, of :-;q. (11 gives the

energy loss rate of an electron at energy E due toCoulombic collisions and inelastic collisions(ionization and excitation of neutral and singlelyionized uranium). It is composed of a sum of en-ergy loss rices Tor each type of collision, i.e.,

dE,dt|ionization

collision?

dF _ JEIdt ~ dt|coulombic

collisions

dt|excitationcollisions

Numerous treatments for

(2)

dE,existICoulorabic

collisionsin the literature, such as the t'okker-Planck

model.^ ' However, the electron density in theuranium plasma is unique in that it does not satis-fy the conditions necessary for Dsbye shielding,e.g. in a typical case less than one electron iscontained per Debye sphere. Consequently, themore general energy loss rate of T. Kihara and

(14)0. Aono1 'interactions.

is used, which incorporates collective

In the case of ionization and excitation, theenergy loss rate may be obtained by

dt|E = <E>loss £3)

where v is the speed of a vest electron relative tothermal electrons and E is the macroscopic inelas-tic cross section. The average energy loss percollision, <E> is defined as

loss

loss" JasjFiwdo(E.E')— d£T-~

<J(E)

where E is Che energy of a test particls and E1

is the energy lost by the test particle as a re-sult of a collision. The microscopic cross sec-tion, o(E) and the energy transfer differential

(4)

cross section, are calculated from a

Gryzinski modfc!

e t , l . ^

using the data of Parks,

for uranium atomic states.

While there are many uncertainties in thesecross section estimates, this approach is the onlyone feasible in view of the scarcity of experi-mental data or detailed calculations. Whileabsolute magnitudes for the resulting distribu-tions will reflect this uncertainty, hopefullyrelativs comparisons among the various calcula-tions will be more accurate.

I.i the numerator of Eq. (1), the quantityS(E') is the electron production rate from allsources per unit spatial volume per unit energy.The electron sources are of two types, categorizedaccording to their origin; namely, nascent elec-trons and secondary electrons. The former elec-trons are the result of fission-fragment-inducedionization of uranium while the latter are theresult of electron-induced ionization of uranium.Since the secondary electron source is dependentupon the nascent electron source and the manner inwhich they thermalize, an expression for the totalsource, S(E) cannot be known a priori. Conse-quently, S(E) and the distribution function f(E)must be calculated in an iterative manner.

Comyutatlonal Techniques

The starting point in the iterative scheme isthe calculation of the nascent electron source S

owhich requires a knowledge of the fission-fragmentenergy distribution. A simple estimate of thisdistribution can be obtained using the equivalenceof Eq. (1). The source of fission-fragments is sonarrow in energy that it can be considered as adelta function; but two distinct fission-fragmentsresult from a single fission event at averageenergies of 67 MeV and 98 MeV and masses of 140 amuand 96 amu, respectively. Then, the fission-frag-ment distribution is

S<dEtdt E

(5)

6 5

3« 10

NEUTRALURANIUMDENSITY

2xl016

5000 6000 . 7000

TEMPERATURE, °K

8000

Fig. l. The density of the uranium plasma con-stituents plotted versus teraperat reassuming a pressure of one atmosphere.

where S' represents the number of fission events/

(cm -sec) and j-L is the energy loss rate of a

fission-fragment at energy E. The quantity Sp isdetermined by the choice of the neutron flux andthe temperature which indirectly yields the uran-ium density and an averaged fission cross section

characterized by the plasma temperature.^17^

The fission-fragments experience electroncapture over their entire track such that1 ~ % v/vQ where qQ and vQ represent the initial

charge (~16e) and velocity, respectively. Conse-

quently, the energy loss — is a minimum at the

beginning of their track. A semi-empirical for-

mula'' ' for the energy loss of a fission-frag-

ment at energy E is given by

dx = Xup"

-1/2

(6)

where EQ is the birth energy of the fission frag-ment and \ is its range (the latter is obtainedfrom Eq. 3.50 of Ref. 18). Then, the fission-fragment distribution is

(7)

where M is the mass of the fission-fragment andthe relationship dE/dt = v dE/dx = /2E/M • dE/dxhas been utilized. The nascent electron sourceterm SQ appearing in Fig. 2 is then obtained byaveraging the fission-fragment distribution ov«r aGryzinski energy-transfer cross-section for ic.n-ization events, generalized for heavy, multi-charged ions. C15'10)

The nascent electrons S relax into a primary

electron distribution f (E) according to Eq. (1).

During the thermalization process, the primaryelectrons further ionize the background uraniumgenerating a source of secondary ilectrons S^ '")

(as with S , S, is obtained by averaging the pri-mary electron distribution over a Gryzinskienergy-transfer cross-section for ionization).This first generation of secondary electrons dis-tribute themselves in energy as prescribed byEq. (I), i.e. insertion of S,(E) in the equation

yields f.(E), producing a second generation of

secondary electrons S,(E). This process is con-

tinued until the sum of S.(E)'s converges to S(E)

and likewise the sum of the f. (E)'s converges to

f(E) (see Fig. 2). Convergence is readily ob-

ENERGY, eV

Fig. 2. The nascent source (....) versus energyas well as the distribution functionplotted versus energy. The distributionconsists of a Maxwellian (-) and theconverged solution of the high-energytail (-•-). The initial tern in theseries representation of f(E is alsoplotted (--).

66

tainable within a few iterations, in agreement withearlier observations of a similar process by Fano

and Spencer. '

Crucial to the evaluation of the electronenergy distribution function is the accurate repre-sentation of the inelastic cross sections. Sincethey have not been measured experimentally, theywere computed from a hydrogen model of

Gryzinski, ' modified for uranium by utilization

s)f the atomic state daca of Parks. '

As seen from Fig. 3, the uranium corss sectionsexhibit an abrupt rise at the threshold energy.The appearance of discontinuities in the slopes ofthe cross sections are indicative of one electronicstate's participation in an event being over-shadowed by another state; e.g. the discontinuity

in the slope of the U ionization cross section at18 eV represents the appearance of an inner-electronionization process which, at high energies, over-shadows the ionization of the outer-most electron.At very large energies, the fine structure of theenergy levels gives way to an 1/E energy dependence.

Iff1

o io"'8

a) 10

oI02

.-23

-kxC.TATION OF^ U•J lu

fU°IONIZATION OF< U*

BY ELECTRONIMPACT

10° 10'ENERGY, eV

F.'g. 3. The ionization and excitation cross

sections for U, U+, and U++ versus energy.

The excitation cross sections are found to belargsr in magnitude than the corresponding ioniza-tion cross sections. An analogous trend is ob-

servei for cesium (see L. J. Kieffer's work" ')which is quite similar in electronic structure.These trends can be attributed to the fact thatsuch c-oss sections are in general inversely pro-portional to the energy transferred. Then, thesmallei energy transfer afforded by excitationcollisions results in the increased probability oftheir o:currence over ionization collisions.

Results, Distribution Calculations

Upon accumulation of the necessary crosssections, energy loss rates, source terms, andspecies' densities, the distribution function maythien be evaluated as previously desi .-ibed. Results

of such calculations were illustrated earlier inFig. 2. It is observed that the distributionfunction is essentially composed of two parts: athermalized bulk electron population plus a high-energy tail. While the present technique assumesthat, based on_earlier arguments, the lower energydistribution (< 15 eV) follows a Maxwellian shape,two points should be stressed: First, the relativeamplitude and temperature of this distribution havebeen determined in a self-consistent manner rela-tive to the' tail electrons (recall the balancespreviously cited). Second, fcr present purposes themain region of interest is the high-energy tailwhich can cause non-Maxwellian excitation andionization. The bulk of the low energy elect' usare energetically unable to cause excitations.Their only role is as a s.lowing-down agent forhigh energy electrons, and, to a first approxima-tion, this is not too sensitive to the details ofthe low energy distribution.

At lower energies, near the region where thatail of the distribution and the Maxwellian join,the assumption of continuous slowing down is poorsince the energy loss per collision can be com-parable to the total electron energy. Since thisregion is important for subsequent excitation cal-culations, a more precise Monte Carlo modt;l was

employed for select cases. ' ^ Tiis method em-ploys a governing Boltzmann equation vhich relaxesthe assumption of continuous slowing for inelasticcollisions. The solution employs a-i analytic simu-lation of Coulombic collisions while ionization andexcitation events are simulated in a tradit -ialMonte Carlo, i.e. random walk, fashion. The secon-dary electron avalanche is included explicitly. Acomparison of tne Monte Carlo and analytic results

is shown in Fig. 4 for a neutron flux of 2 x 10

neutrons/(cm -sec).

The Monte Carlo results consistently fall be-low the analytic model, and this is attributed tothe error introduced by the continuous slowing ap-proximation in the latter. Fortunately, the erroris not so great, however, as to completely invali-date the analytic model which it is thought canstill provide insight into trends>

Parametric studies indicate that the amplitudeof the high-energy tail is roughly linearly propor-tional to the neutron-flux level and inversely pro-portional to the temperature (see Figs. S, 6 and 7).In both cases, the trends observed essentially fol-low the electron source term S(E) of Eq. (1). In-creasing the neutron flux increases the nascentelectron source term while decreasing the tempera-ture results in an increase in the uranium density

[(n - (kT/p)"1] which inelectron source term.

turn increases the nascent

The absolute magnitude of the high-energy tailis also dependent upon the fraction of the energyloss rate of Eq. (1) contributed by Coulombic dragrelative to inelastic collisions. As notedearlier, in cases where Coulombic collisions areimportant, the unified slowing model has been em-ployed to account for collective behavior. Therelative importance of this is illustrated in Fig.8 for a case where Coulombic collisions are thedominant energy loss mechanism. It appears thatthe correction for collective effects is of order

67

ENERGY, eV

Fig. 4: A comparison of Monte Carlo results (A's)and analytic solution (dahsed line] forvarious temperatures at a neutron flux of

12 22x10 neutrons/(cm -sec). A Maxwelliandistribution = solid line. A verticallydisplaced analytic solution is also pre-sented (-.-).

of 20%. This is not too serious, then, when itis considered that many of the cross-sections in-volved contain much larger uncertainties. Still,the unified model has been retained in the calcu-lation since it is not difficult to do so andsince this avoids unnecessary inaccuracies.

The tail of the electron distribution is muchlower in magnitude relative to the thermalizedMaxwellian in uranium than was the case found

earlier in noble gases such as He.^4>S^ This occursbecause of the large contribution from thermalionization at the elevated temperatures involved.Consequently, the contribution to excitation fromtail electrons is not so important in the uraniumcases. Still, since, as shown in Fig. 5, theenergy levels of interest fall in the region ofthe tail, the latter electrons cannot be ignored.For example, for the'plasma conditions of 8000"K

and 2 x 1 0 ° neutrons/(cm2-sec), the calculated

distribution causes 6 x 10 more excitation

events/fern -sec) than would a Maxwellian distribu-tion.

Since the non-Maxwellian region is of interestfor pumping laser lines, these results suggestthat pure uranium may not be .as interesting for

10

>Ofi

nBo<io'

BTjV

to

•£go I05

u.zo1-

OQ

* 10°

o

1 r

^ \

\

\

Range of Ionization(—-) Energies of Ii

ii

$, neutrons/(cm2-sec)

2x!01 4v..

— , .

8000°K

(-) and Excitationterest in Uranium

F iiiiiiri

10' 10'

ENERGY, eV

Fig. 5. The effect of varying the neutron fluxupon the distribution function for a con-stant temperature of 8000°K.

I01

ENERGY, 3V10'

Fig. 6. The distribution function versus energy

at a constant neutron flux of 2 x 1016

neutrons/cm -sec for temperatures of 8000°K, 7000°K, 6 0 0 0 % and 5000°K.

68

^-2xlOl''neutrons/(cm -sec)

10' io2

ENERGY, eV

Fig. 7. The distribution function for a lower

flux C? x 10 1 4 n/cm2-sec versus 2 x 1016

n/cin -sec in Fig. 6].

t5 10'°

Ie«>„- 10s

QI- 10°

' neutrons/(cm-sec^

- UNIFIEDSLOWINGDOWNTHEORY

FOKKER-PLANCK-

10'' 10° I01 IOZ I03 I04

ENERGY, eV

Fig. 8. Comparison of the effect of using dif-ferent Coulombic slowing treatments.

this purpose as hoped. [This arg'iient assumescoilisional excitation; a recombination lasermight be very attractive.] There are two ways toimprove the situation, however. First, the ther-malized electron population will lie at lower en-ergies in low-temperature UF& plasmas, and this

should lead to a dominance of tail-excitations.

Second, as suggested by recent calculations,mixtures such as He-U, Ar-U, etc. should be con-sidered. Then, tail electrons can be used to ex-cite the added gases which are selected to haveenergy levels that fall in the higher energy re-gion (e.g. -30 eV for He).

Excited State Densities and Radiation Transport

Techniques and data necessary to evaluateradiation transport in an uranium plasma havebeen developed over the past year. This work isstill in process, but a brief description of thework to date will be presented.

To determine the excited state densities weconsider the processes illustrated i.i Fig. 9. Theelectron distribution discussed above generatesthe excited state densities within the plasma.Radiation emitted from the excited states is ampli-fied or absorbed within the plasma, possibly al-tering the excited state densities. Consequently,the radiation and state densities are coupled with-in the plasma.

NEUTRON FLUXCAUSES FISSIONFRAGMENTS TO BEPRODUCED

FISSION FRAGMENTSPRODUCE A NON-MAXWELLIAM ELECTRONFLUX

CGENERATES EXCITEDSTATE DENSITIESWITHIN THE PLASMA

7EXCITED ATOMS ANDIONS EMIT RADIATIONWITHIN THE PLASMA

ABSORPTION ANDAMPLIFICATION OFRADIATION WITHINTHE PLASMA ALTEREXCITED STATEDENSITIES

Fig. 9. Schematic representation of physicalprocesses within a uranium plasma.

69

Before such calculations are possible, a model

of uranium energy structure is necessary. J Thestate energy levels and inverse lifetimes were ob-tained using data from NBS ( 2^ and LASL.(25'26)

Using the intensity data in these reports, energylevels and inverse lifetimes were computed and

normalized to the lifetime data of (Close.1- •'

An example of the energy levels and the rela-tive inverse lifetimes derived in this fashion isgiven in Table I. These results are obtained onthe basis of a j-j coupling scheme which was se-lected to be consistent with the original analysisof measured radiation intensities. It is to benoted, for example, that a spontaneous emission of

radiation from an initial state, 14644 cm , to the

final state, 0 cm" , is at least twice more prob-able than a transition to a higher final state;

620 cm" , about four times more than to the final

state 6249 c m , etc. Further discussion of theseresults is given in Ref. 23.

The time dependen' rate equation for excitedstate q is

dN

P(V) Nv

g(v)

(E) dE

Dq V*

- N (E) dE

- \j ,(E) dE

dEi dEth (8)

In our model we assume a steady-state solu-tion, hence the time derivative is set equal tozero. Since the system is large [e.g. about 1-mlong with a 1-m diameter based on an experimental

plasma design at Los Alamos^ •"], we assume an in-finite medium system wb^re diffusion losses arenegligible. With these two assumptions, Eq. (8)reduces to a matrix equation of the form

B Npq q V

As a test of the validity of this model, ini-

tial calculations were done for helium. Some re-

sults from this study are reproduced in Table II .There,

experimental values^ ' for fission-fragment ir-radiation of He are compared with the theoreticalcomputations for a single step (Case I) anddouble step (Case II) excitation. ("Doublestep" refers to excitation collisions originatingfrom metastable states whereas "single step"processes start from the ground state.) Dis-crepancies arise due to the significant contri-bution of Maxwel liars distribution, expeciallyfor the more realistic conditions of Case II.However, it is important to notice that there are

inversions predicted for the 3 P-3 D and 4 P-4 Dstates. These results are described in more de-tail in Ref. 9. Comparisons with other data andcalculations, to the extent possible, showed goodagreement, lending confidence to the model.

Radiation from the uranium plasma will con-sist of a continuum black-body type radiation witha discrete line structure superimposed. The con-tinuum will be calculated using conventional tech-niques while perturbations due to line structure

Table I. Relative inverse lifetimes for some states of UI based on Ref. 24,. Ref. 25

and Ref. 26. Energies are in cm" . * indicates data is from Ref. 24.

FINAL STATE

15721

15638

15632

14644

13463

.001455*

.00143*

.00457*

.00642*

.00178*

.00127*

.004*

.00281*

.00094*

.00203*

.00101*

.00075S

.00047

. 00266

.00168

.00213

.001S9

.00096 .00014

.00316

.OO25S

.00296

.001S7

.00239

.00137

.00348 .00103

.00356

.00237

.00166

.00319

.00370

.00047

.00152

70

Table II. Fission-Fragmsnt Excitation of He.

°3'P

93'pn3'0

SHIPMAN

1.7

2.83

CASE I

1.189

1.982

CASE H

0.693

1.155

n4.p

"I'D

g4'o nt'p

94'Pn4'D

1.8

3.00

0.855

1.425

0.65

1.083

will be obtained from the excited-state density-calculation described above. A code to do thishas been devised that divides the plasma into dis-crete regions to account for temperature anddensity variations. An iteration technique is em-ployed to account for the radiation and state-density coupling, but this connection should onlybe important for a few strong lines.

Conclusion

Cross sections, lifetime data, etc. andmethods have been developed for the calculationof the electron energy distribution in a fission-ing uranium plasma. This is the first time suchresults are available. The thermalized componenttends to dominate in high-temperature, pure uran-ium. It is suggested, however, that high-energyelectrons in the tail of the distribution may bevery effective in producing non-equilibrium exci-tation in mixtures where the additive gas has arelatively high threshold energy for excitation.The noble gases are examples of potentially at-tractive additives.

These methods of analysis are now being ap-plied to lower temperature UFfe which is of prime

interest for near-term experimental endeavors. Inaddition, radiation emission spectra from the uran-ium plasma are under study.

References

1. G. H. Miley, "Fission-Fragment Transport Ef-fects as Related to Fission-Electric CellEfficiencies," Sucl. Sci. Eng., 24, 322 [196f.).

2. G. H. Miley, Direct Conversion of NuclearRadictior. Energy, Am. Nucl. Sac., Hinsdale,IL (1970).

3. P. Thiess and G. Miley, "Spectroscopic andProbe Measurements of Excited State and Elec-tron Densities in Radiation Induced Plasmas,"2nd Symposium on Uranium Plasmas: Researchand Appliaation, p. 90 (1971).

4. R. Lo and G. Miley, "Electron Energy Distribu-tion in a Helium Plasma Created by NuclearRadiations," IEEE Trans, on Plasma Sci.., PS-Z,198 (19743.

5. B. Wang and G. Miley, "Monte Carlo Simulationof Radiation-Induced Plasmas," Ifual. Sci.Engr., S2, 130 (1973).

6. H. H. Helmick, et al., "Preliminary Study ofPlasma Nuclear Reactor Feasibility," LA-5679(1974); K. Thorn, "High Grade Power from Fis-sioning Gases," NASA Report (.1976).

7. K. Thorn, "Gaseous Fuel Reactor Research,"3rd IEEE Int. Conf. on Plasma Sai., Austin,TX (1976).

8. C. G. Bathke and G. H. Miley, "Electron Dis-tribution Functions in Uranium Plasmas," Trans.Am. Mucl. Soc., 19, 64 (1974).

9. E. L. Maceda and G. H. Miley, "Non-MaxwellianElectron Excitation in Helium," Prac. 27thAnnual Gaseous Electronics Conf., Houston,TX, p. 118 (1974).

10. J, C. Guyot, G. H. Miley and J. T. Verdeyen,"Application of a Two-Region Heavy ChargedParticle Model to Noble-Gas Plasmas Iriduced byNuclear Radiations," Nual. Sci. Engr., 48,373 (1972).

11. L. V. Spencer and U. Fano, "Energy SpectrumResulting from Electron Slowing Down," Phya.Rev., 93, 1172 (1954).

12. G. Safonov, R. Witte,.S. Altschuier andW. Simmons, "An Experiment for Pursuit of theFission Laser," unpublished internal report,TRW Corporation, 11 Segundo, CA, July 1971.

13. D. V. Sivukhin, "Coulomb Collisions in a FullyIonized Plasma," Reviews of Plasma Physics,Vol. 4, Consultants Bureau, New York (1966).

14. T. Kihara and 0. Aono, "Unified Theory of Re-laxations in Plasmas, I. Basic Theorem,"J. Phys. Soc. Japan, 18, No. 6 (1963).

15. A. Burgess and I. C. Percival, "ClassicalTheory of Atomic Scattering," Adv. Atom. Molec.Phys., 4, p. 109-141 (1968).

16. D. E. Parks, et al., "Optical Constants ofUranium Plasmas," NASA CP-72348, N.T.I.S.,Springfield, VA (1968).

17. R. W. Bussard and R. D. DeLauer, NuclearDocket Propulsion, McGraw-Hill Book Co., Inc.,New York, p. 160 (1958).

18. See, for example, p. 108 of Ref. 2.

19. U. Fano and L. V. Spencer, "Quasi-Scaling ofElectron Degradation Spectra," Int. J. Rod.Phys. and Chem., 7, 63 (1975).

20. L. J. Kieffer, "A Compilation of Electron Col-lision Cross-Section Data for Modeling Gas Dis-charge Lasers," JIUL Information Center Report

71

#13, Univ. of Colorado, Boulder, CO, Sept.1973.

21. C. Bathke, "Calculations of the Electron Ener-gy Distribution Function in a Uranium Plasmaby Analytic and Monte Carlo Techniques,"rh.D. thesis, Nuclear Engineering Program,Univ. of Illinois, Urbana, IL (1976).

22. M. Makowski, C. Choi and G. Miley, "ElectronEnergy Distribution in Radiation-InducedU-He Plasmas," 3rd IEEE Int. Conf. on PlasmaSei., Austin, TX (1976).

23. E. L. Maceda, C. G. Bathke and G. H. Miley,"Uranium Excited-State Parameters," Trans. Am.Sucl. Soe., 22, 153 (1975).

24. K. t. Meggers, C. H. Corliss and B. F. Scrib-ner, Tibles of Spsatval Line Intensities^ NBSMonograph 32, pt. 1, Washington (1961).

25. D. W. Steinhaus, L. J. Radzienski, R. D.Cowan, et al., "Present Status of the Analysisof the First and Second Spectra of Uranium(UI and UII) as Derived from Measurements ofOptical Spectra," LA-4501 (1971).

26. D. W. Steinhaus, M. V. Phillips, J. B. Moody,et al., "The Emission Spectrum of Uranium Be-tween 19,080 cm"1 and 30,261 cm"1," LA-4944(1972).

27. T. Z. Klose, "Mean Life of the 27887 cm"1

Level in UI," Phys. Rev. A, II, 1640 (1975).

28. G. R. Shipman, R. A. Walters, and R. T.Schneider, "Population Inversions in FissionFragment Excited Helium," Trans. Am. Nuel.Soo., 17, 3 (1973).

?2

RECENT MEASUREMENTS CONCERNING URANIUMHEXAFLUORIDE-ELECTRON COLLISION PROCESSESt

S. Trajmar, A. Chutjian, S. Srivastava,and W. Williams

Jet Propulsion Laboratory,California Institute of Technology

Pasadena, California 91103

and

D. C. CartwrightLos Alamos Scientific LaboratoryLos Alamos, New Mexico 87544

Abstract

Scattering of electrons by UFs molecule uasstudied at impact energies ranging from 5 to 100 eVand momentum transfer, elastic and inelasticscattering cross sections were determined. Themeasurements also yielded spectroscopic informa-tion which made possible to extend the opticalabsorption cross sections from 2000A to 435A. Itwas found that UFS is a very strong absorber inthe vacuum UV region. No transitions were foundto lie below the onset of the optically detected3.0 eV feature.

I. Introduction

Considerable need has developed recently forspectroscopic information and electron collisioncross section data for UFS in connection withthe UF6 gas core reactor, isotope separationschemes and nuclear pumped lasers. Theoreticalmethods are not expected-to yield reliablepredictions for such a complex molecule and elec-tron impact T.aasuremints have not been reportedin the literature.

He summarize here the results of our investi-gations concerning electron scattering from UFs .Some of these results have been published orsubmitted for publication very recently*"^ andsome of them are reported for the first time.

The photoabsorption spectrum of UF6 has beenstudied by many workers. Recently, DePoorter andRofer-DeFoorter obtained detailed absorptionspectrum and absolute absorption cross sectionsin the 2000 to 4200A region. Rabideau5 also madeabsolute photoabsorption cross section measure-ments in the 1950 to 2500A spectral region. Aninvestigation of the absorption spectiMm of UF6at low temperatures, in both solid and vaporphase, was carried out by Lewis et al between2000 and 4300A. No such studies are availablefor wavelengths less than 2000A.

The techniques of electron impact spectro-scopy can be utilized (at high impact energiesand low scattering angles) to give opticalabsorption cross sections. 9 On this basisHuebner et al10 have reported apparent oscil-lator strength distributions for a number of

molecules. Generally their data agree well withoptically measured values even for incidentelectron energies as low as 100 eV. The sametechnique is utilized in the present investi-gation to extend the photoabsorbtion crosssections from 2000A to 435A.

At low incident energies and high scatteringangles the electron impact energy-loss spectraare dominated by optically-forbidden transitionsand yield information which is not availableby the means of conventional optical spectro-scopy.ll This method was utilized here to searchfor optically-forbidden states.

The electron scattering intensities in bothenergy regions have been normalized to yielddifferential and integral cross sections andsome of these results are reported here.Evaluation of the laboratory measurements arestill in progress and will be reported later.

II. Apparatus and Method

Details of the experimental procedures havebeen published elsewhere.1~^»12,13 Briefly, theapparatus consists of an electron gun which pro-duces a colligated, energy-selected beam of elec-trons of impact energy Eo. The electron bejracrosses the target UFa beam at 90°. This targetbeam is generated by flowing the UFS through acapillary array. Electrons scattered through asolid angle dfi (~ 10"3sr) at .-.n angle 8 with re-spect to the incident electron beam are detectedand energy analyzed. An energy-loss spectrum isobtained by using pulse counting and multichannelscaling techniques. The energy resolution wasabout 80 meV (FWHM). The incident energy scalewas calibrated against the He 19.35 eV reso-nance and the true zero scattering angle wasdetermined from the symmetry of the scatteringintensity around the nominal zero scatteringangle. The critical features in the presentexperiments were that a. the gun, electronoptics, and detector were differentiallypumped relative to the scattering chamber., andb. the target UF6 beam was condensed on a liq-uid nitrogen cold trap placed immediatelyabove the scattering region. Even with theseprecautions, the electron optics became poisoned by the UF6 after two weeks of operation.

^Supported in part by the National Aeronautics and Space Administration under ContractNo. NAS7-100 to the Jet Propulsion Laboratory and in part by ERDA Order Ho. LS-76-5.

73

III. Elastic Scattering IV. Photoabsorption Spectrum arid Cross Sections

The elastic ittering intensity (whichincludes rotational and vibrational contributionswithin the instrumental resolution) was measuredas a function of scattering angle at fixed impactenergies ranging from 5 to 75 eV. Under identicalconditions the same procedure was applied to Heand the ratio of the UFe scattering intensity toHe scattering intensity was determined. Fromthese intensity ratios absolute elastic scatteringcross sections for UFe were obtained by a pro-cedure utilizing He elastic scattering crosssections as secondary standards. The details oftht: procedure are described in Ref. 1. Thedifferential and integral elastic cross sectionsas well as the momentum transfer cross sectionsare summarized in Table I.

The electron impact energy-loss spectrumobtained at 100 eV impact energy and 5° scatter-ing angle should be very closely the same asthe optical absorption spectrum. This spectrum(Fig. 1) was utilized to get the generalizedoscillator strength distribution or the relativeoptical absorption cross sections in the 4000 to435A region. The relative values were normalizedto the absolute scale with the help of the crosssections of DePoorter and Rofer-DePoorter at2255A (5.5 eV). For detailed discussion seeRef. Z. The results are given in Table II andshown in Figs. 2 and 3. There is an excellentagreement between-the optical and the electror.impact results in the overlapping region exceptat around 3-4.5 eV energy losses where the cross

TABLE I. UF6 Differential Elastic Cross Sections CT(8), (10"aom3/sr). The

percentages contributed by the extrapolations of o(6) between 0and 20° and between 135° and 180° to the elastic integral ajic".momentum transfer cross sections are shown in parentheses. Thedata reported here for the 5 aV electron impact energy have beenobtained by extrapojating a(6) values at other energies.

Eo (eV)

6(deg)

20253035404550556065707580859095100105110115120125130135

INTEGRAL

(io-aV)

Mom. Transfer

(io-2°ma)

5

1.601.301.201.100.880.700.520.340.250.160.160.160.200.180.180.180.18'0.150.130.110.120.110.110.12

4.3(23%)

2.4(27%)

10

3.752.90?.502.151.751.351.000.680.500.400.370.380.400.430.440.^40.43f .410.400.400.410.410.430.45

9.6(25%)

5.9(32%)

15

7.005.604.303.302.251.601.150.900.840.981.251.451.401.381.251.150.95O.9C0,820.780.821.001.301.85

25(397.)

24(55%)

20

12.50,8.004.502.601.601.151-091.151.251.351.351.351.281.100.950.860.620.580.570.600.660.780.901.15

27(43%)

19(46%)

30

5.804.002.751.801.451.351.251.100.900.740.600.480.420.360.340.390.470.500.520.560.640.801.001.35

17(43%)

13(54%)

40

8.203.101.501.451.451.301.150.860.680.660.650.540.450.360.310.300.300,32 .0.350.400.580.821.101.40

19(55%)

13(58%)

50

9.403.502.151.951.951.401.000.630.520.530.530.460.380.2/0.190,200.210.230.310.500.741.101.401.70

22(59%)

13(55%>

60

6.002.101.951.801.450.980.640.460.410.500.380.280.210.170.160.170.180.220.320.53/.801.101.351.55

18(60%)

75

3.202.501.951.401.05 •

0.740.540.420.J5-0.290.250.230.220.230.230.240.280.330.410.510.600.740.860.94

9,5(38%)

?.O47X)

74

F i g . 1

6 8 10 12 14 16 18 20 22 24 26 28 30ENERGY LOSS, eV

Energy-loss spectra of UFe at 5°scattering angle for 75 eV and 100 eVelectron impact energies.

sections are small and the electron impact curvecould have some non-optical contributions.

A quantity of interest for some applicationsis the integral cross section OjOPt for aparticular band between energy losses Ej and E3 .This can be obtained by:

opt (1)

We have calculated the integrated photoabscrptioncross section using Eq. (1) with Ei =2.5 eV andEfe = E where E is the energy of any portion of thespectrum. The results are shown in Fig. 4 and inTable III. From tht.se data the integrated crosssection for any arbitrary energy loss intervalcan be easily obtained.

From Fig. 3 we find that the spectrum of UFeat wavelength less than 2000A exhibits consider-able structure and the spectral features compris-ing this structure have stronger photoabsorpfioncross section than those observed above 2000A.

TABLE II

Photoabsorption cross section per electron volt, da/dE, for selected valuesof th^ electron energy loss E. The wavelength X equivalent to each energy loss E isalso shown. rhe da/dE values are the smoothed values obtained from Fig. 3.

E(oV)

4.04 .55.05.56.0'6.57.07.58.08.59.09.5

10.010.511.011.512.012.513.013.514.014.515.015.516.0

•(nm)

310.0275.5248.0=525.5206.6190.8177.1165.3155.0145.9137.8130.5124.0118.1112.7Iu7.8103. j99.295.491.988.685.582.730.0.'7.5

(10"' ' '

000110124•1

320011277542322

-/dEcm3 eV"1)

.014

.086

.15

.15

.27

.99

.15

.49

.03

.65

.11

.72

.92

.88

.76

.57

.30

.66

.17

.29

.22

.99

.41

.87

.99

E(eV)

16.517.017.518.018.519.019.520.020.521.021.522.022.523.023.524.024.525.025.526.026.527.027.528.028.5

X(nm)

75.272.970.868.967.065.363.662.060.559.057.756.455.1'53.952.851.750.649.648.647.746.845.945.144.343,5

da/dE(10 1 ? cmE eV"1)

3.063.263.343.343.64i.763.763.833.763.643.643.453.263.373.143.073.453.453.263.262.882.382.882.792.68

75

WAVELENGTH, nm354.3 275.5 225.5 190.8

2 0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0ENERGY LOSS, eV

Fig. 2 A comparison of the present results(Curve A) with the photoabsorption spectraof DePoorter and Rofer-bePoorter have been•normalized to the data of DePoorter andRofer-DePoorter at 5.5 eV. The correspond-ing wavelengths (X) in nanometers areindicated on top of the figure.

Fig. 3 Photoabsorption cross section of UFgderived from the electron impactenergy loss data of Fig. 1 andnormalized as indicated in Fig. 2.

TABLE H I

Integral photoabsorption cross section, CTj between energy losses 2.5 eV and EThe wavelength X equivalent to each energy loss E is also chown.

2(eV)

4.04 .55.05.56 .06.57.07 .58.08 .59 .09 .5

10.010.511.011.512.012.513.013.514.014.515.015.516.0

Mnm)

310.0275.5248.0225.5206.6190.8177.1165.3155.0145.9137.8130.5124, C118.1112.7107.8103.399.295.491.988.685.582.780.077.5

529311224578991111122222 .2 .

.66

.83

.48

.43

.06

.55

.10

.82,65.99.06.63.49.8906

.12224473072643597486

(cms)

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

10 "2°10"1B

10"l«

lO" 1 7

10" 1 7

lO" 1 7

io~17

10~17

I O - 1 7

i o - "lO"17

10"17

io-1 7

lO" 1 6

io-1 6

10" i e

10"1B

10" l s

10"16

10"16

io-1 6

10'1S

lO"16

lO"16

E(ev)

16.517.017.518.018.519.019.520.020.521.021.522.022.523.023.524.024.525.025.526.026.527.027.528.028.5

Mnm)

75.272.970.868.967.065.363.662.060.559.057.756.455.153.952.851.750.649. G48.647.746.845.945.144.343.5

333333344444455555555666.6.

°T

.01

.14

.27

.42

.56

.74

.89

.05

.22

.3955

.71

.87001527405467799305172939

(en3)

x 10"1S

x 10"16

* W~ltx 10"16

x 10"1S

x 10~ ie

x 10"1E

x 10-16

x 10"16

x 10" i e

x 10"1G

x 10-16

x lO"16

x 10-16

x 10-1B

x 10-16

x 10-16

x 10-16

x 10"1S

x lO"16

x 10-16

x 10-1 6

x 10" l e

x lO"16

x 10"16

76

310.0 155.0WAVELENGTH, nm

103.3 77.5 62.0 51.7 44.3

12 16 20ENERGY LOSS, eV

28

Fig. 4 Integral absorption cross sectionsbetween energy losses 2.5 and E eV asobtained from Eq. (1).

V. Electron Scattering at Low andIntermediate Impact Energies

In Fig. 5 are shown energy-loss (AE) spectraat an incident electron energy (Eo) of 20 eV,and at scattering angles (9) between 20° and135°. The elastic scattering peak (AE = 0 eV)is shown for each spectrum. Also shown abovethe top spectrum is the optical absorptionspectrum of gaseous 'JF6.6>7 The locations ofall peaks, in eV, are also indicated.

No transitions are found to lie below theonset of the first-detected feature at 3.0 eV.This is also confirmed in spectra (not shown)taken at Eo = 10 eV, even closer to threshold.The feature in the J.0-3.6 eV region mustcorrespond to the lowest triplet and singletcharge-transfer excitations from a tiuo orbitalcentered on the fluorine atoms to !:he azuuranium 5f orbital." In general, the variationwith 9 of peak intensities in an energy-lossspectrum is known to give important informationabout the spin multiplicity of electronictransitions involved. For example, spin- anddipole-allowed transitions are strongly forwardpeaked in 6, while spin-forbidden transitionsinvolving shorter-range exchange forces tend tobe nearly isotropic.il The fact that there isno substantial change in Fig. 5 of the lowest-energy feature with 9 is very likely due to thestrong spin-orbit mixing in these states,and/or to contributions from overlapping singletand triplet vibronic excitations (including hotbands)." One also sees in Fig. 1 that the ratioIn the optical spectrum of the (optically-allowed) 5.8 eV to the 3.3 eV (lowest—nergy)feature is ~ 10a, while in the 20° spectrumit is ~ 50. This fact points up the overalloptically-forbidden character of the 3.3 eVfeature.

Fig. 5 Energy-loss spectra of UFS at 20 eV and atthe indicated angles. The elastic peakis shown at AE = 0. The optical absorp-tion spectrum of DePoorter and Rofer-DePoorter is shown for comparison. Thepeak energies in eV for both theoptical and the electron impact spectraare indicated.

Other differences between the optical andelectron scattering spectra exist in theintensities of features in the 3.6-5.0 eVregion relative to the 5.8 eV excitation, first,the 4.2 eV excitation is greatly enhancedrelative to the 5.8 eV feature in the electron-scattering spectra, and is clearly separatedfrom the optically-allowed transition for 8>20 .This fact indicates that the 4.2 eV excitationis optically forbidden. Second, the fairlystrong optical absorption at 4.8 eV is observedto "fill in" at 9 = 20° but is practicallyabsent at higher angles. This is indicative ofa weak dipole-allowed transition. This featurehas indeed been interpreted as a vibrationically-allowed transition.6 We also note that thefeatures observed above AE = 6.2 eV have notpreviously been reported.

An estimate of the absolute differential crosssections for the intlastic excitations can beobtained by measuring inelastic-to-elastic inten-sity ratios directly from the spectra (using theappropriate scale-change factor) and multiplyingthese ratios by the normalized absolute elasticDCS listed in Table I.

The interpretation of the UFs spectra or thedetermination of optical and electron impactcross sections in terms of individual transitionsis quite difficult. Work is in progress along

77

tKis line. An example of the unfolding of the10 eV, 60 spectrum is shown in Fig. 6. At the

Fig. 6

i 5ENERGY LOSS leV>

Decomposition o£ UFg energy-lossspectrum.

present time, the results can be given only aselectron impact excitation cross sections perunit energy-loss rnnge at various energy lossesor as the value of the cross section associatedwith a broad spectral feature without specifyingthe e:xact nature of the underlying transition(s).An example of this latter procedure is given inFig. 7 and Table IV. The differential andintegral cross sections for the broad featurespeaking at 3.3, 4..2 and 5.6 eV energy losses at10 eV electron impact energy are given. Theseresulrs have been obtained by determining thescattering intensity ratios at the specifiedpeaks with respect to elastic scattering,correcting this ratio for the difference in thewidth of the inelastic and elastic features,then utilizing the absolute elastic crosssections of Ref. 1. Due to the uncertaintiesassociated with the shape of the inelasticfeatures, with the interpolations and extra-polations of the experimental measurements(which were carried out only at 20, 60, 90 and135° scattering angles) and with the normali-zation procedures, th^se cross sections areconsidered to be accurate to within about afactor of two.

M 80 100SCATTERING ANGLE <OEG)

Fig. 7 Differential cro=s sections at 10 eVimpact energy for elastic scattering andexcitation of the features at 3.3, 4.2and 5.6 eV energy-losses.

Table IV. Differential and integral electronimpact excitation cross sections forthe broad features located at 3.3,4.2 and 5.6 eV energy losses.(10~19 eir3/sr units)

6(deE)

0102030405060708090100110120130140

3.3 eV

804121139.27.05.85.04.54.44.65.36.58.714

Cross Section

4.2 eV

14010070513828211614131417212940

5.6 eV

3,0001,250510280170120887058515053607181

119.2 367.3 1,895

78

References

1. S. K. Srivastava, S. Trajmar, A. Chutjian andW. Williams, J. Chem. Phys. 64, 2767 (1976).

2. S. K. Srivastava, D. C. Cartwright, S. Trajmar,A. Chutjian and W. Williams, J. Chem. Phys. toappear (1976).

3. A. Chutjian, S. K. Srivastava, S. Trajmar, W.Williams and D. C. Cartwright, J. Chem. Phys'.to appear (1976).

4. G. L. DePoorter and C. K. Rofer-DePoorter,Spectroscopy Letters ji, 521 (1975).

5. S. W. Rabideau (private communication'.

6. W. B. Lewis, L. B. Asprey, L. H. Jones, R. S.McDowell, S. W. Rabideau, A. H. Zellmann andR. T. Paine, J. Chem. Phys. to appear (1976).

7. H. A. Bethe, Ann. Phys. J>, 325 (1930).

8. E. N. Lasse>:tre and S. A. Francis. J. Chem.Phys. 40, 1208 (1964)

9. C. E. Kuyatt and J. A. Simpson in AtomicCollision Processes, M. R. C. McDowell, Ed.(North-Holland Publ. Co., Amsterdam, 1964),p. 141. "

10. R. H. Huebner, R. J. Celotta, S. R. Mielczarekand C. E. Kuyatt, .1. Chem. Phys. 59, 5334(1973) and 63, 241 (1975).

11. S. Trajmar, J. K. Rice and A. Kuppermann,Advances in Chemical Physics, Vol. 18, Inter-science Publishers (1970).

12. S. K. Srivastava, A. Chutjian and S. Trajmar,J. Chem. Phys. 63, 2659 (1975).

13. A. Chutjian, J. Chem. Phys. 6_1, 4279 (1974)

14. W. B. Lewis (private communication) estimatesthat the two states in the 3.3 eV feature areapproximately equal mixtures of singlet andtriplet character.

79

MEASUREMENT TECHNIQUES FOR ANALYSIS OFFISSION FRAGMENT EXCITED GASES

R. T. Schneider, E. E. Carroll, J. F. Davis, R. N. Davie, T. C. Mapuire,and R. G. Shipman

University of Florida

Abstract

In pursuit of nuclear pumped lasers,experimental efforts have been directedtowards the measurement of population in-versions in gases and determination of thefission fragment interaction processesinvolved. Spectroscopic analysis of fis-sion fragment excited He, Ar, Xe, No, Ne,Ar-N2, and Ne-N2 have been conducted.Boltzmann plot analysis of He, Ar and Xehave indicated a nonequilibrium, recombin-ing plasma,and population inversions havebeen found in these gases. The observedradiating species in helium have beenadequately described by a simple kineticmodel. A more extensive model for argon,nitrogen and Ar-N2 mixtures was developedwhich adequately describes the energy flowin the system and compares favorably withexperimental measurements. The kimitieprocesses involved in these systems arediscussed. These models have indicated aneed for the measurement of the kineticprocesses in fission fragment excitedgases. A Cf252 source was used with aphoton counting system to measure thelife-times and collisional rate coeffi-cients for gases excited by fissionfragments. Experimental measurements ofthe energy deposition properties of fis-sion fragments emanating from uraniumoxide coatings were also performed. Todevelop efficient nuclear pumped lasers,the energy density requirements are high.These problems are discussed and a fis-sioning uranium plasma energy source isindicated. Spectral emission from Ar-N2-UFg mixtures have shown that the additionof UF6 has adverse affects on the emittingspecies. Further research is clearly in-dicated to understand the properties offission fragment excited UF6 and othergases which is necessary for the develop-ment of high-powered, efficient, nuclearpumped laser systems.

I. Introduction

The nieasurement techniques describedin the following are intended for genera-tion of design information for the eventualconstruction of an efficient nuclearpumped laser.

The desireO information, therefore,deals with the detection of populationinversions existing in a fission fragmentexcited gas — for the sake 'of achievinglaser action — and with the explanationof the excitation mechanism itself — forthe sake of obtaining a laser which has anefficiency large enough to be of practicalinterest.

The usual method of generating fissionfragment excited gases for research purposesis to line a test cell with a uranium oxidecoating. The test cell is Chen filled withthe gas of interest and exposed to a neutronflux, usually in a nuclear reactor.

The usual method of exciting a nuclearpumped laser is depicted in Figure 1. The

SS Suppoi" Ro; NoCI Window^ rAI Window

HV Connect OuaMz Tube Poly«lftyi»ne Al Plates

Figure 1. Section Drawing of Laser Assembly

device contains two stages, an electricallyexcited stage and the nuclear stage. BoLhstages are within the same cavity. Theelectrical stage serves for alignment pur-poses only and is not energized duringneutron irradiation. The nuclear sjage issurrounded by a polyethelene moderator, ifa fast burst reactor is used as a neutronsource.

The approach for a practical nuclearpumped laser is twofold, namely surface orvolume excitation.

Surface excitation would surmise theuse of a large surface area of solid fissilematerial in form of thin coatings, whilethe concept of volume excitation would em-ploy a fissioning gas.

Both approaches show advantages anddisadvantages. Since the main advantage ofnuclear pumping seems to be the potentiallyvery high input power density, volume exci-tation seems to have an advantage for largedevices just on account of geometry. Inprincipal, at best 50X of all fission frag-ments generated will ever enter the lasergas in the case of surface excitation, alimitation which does not hold for volumeexcitation. However, the particularitiesof the structure of the UFg molecule mayprevent the buildup of laser action in anadded laser gas. For this reason, furtherresearch in both fields, surface and volumeexcitation, is necessary.

II. Fission Fragment Excitation

One of the purposes of the researchdecr-ribed in this chapter is to investigate

80

processes involved in the fission fragmentexcitation of gases. In addition, theresearch is aimed at the detection of pop-ulation inversions created by fissionfragments.

Fission fragment interaction withgases can be distinguished from othercharged partioles due to their higher mass(-97 for the light fragments and ~138 forheavy fragments) and their higher initialcharge (20e and 22e, respectively). Thefragments are bom with average energiesof 95 and 67 MeV . It should be notedthat due to fission fragment's higherinitial charge, many of the primary ionsproduced upon interaction with a gas : willhave charges exceeding unity. These doubly,triply or even higher ionized particleswill quickly react with surrounding un-charged particles, forming ions of lowercharge. Therefore,, ,the-distribution ofions, excited- -iut-s arici jjxcited neutralsproduced by fitsi'on-')ir.agments is conceive-ably different" that/ orid generated by otherforms of ionizing 'radiation which displaysless energetic interactions.

To study fission fragment excitation,the spectroscopic analysis of the radia-tion produced in a gas of interest by theinteraction of charged particles wasundertaken. Generally the experimentalapparatus consists of a cylindrical testchamber lined with a U23502 or U2j508coating. This test chamber was pumped down to10 "^ torr before being filled with thetest gas. The fission fragment excitationof the test gas is achieved by insertingthe test chamber into the University ofFlorida Training Reactor. Upor; irradia-tlc~. with neutrons, the uranium oxidecoating emits a fiss'<•"•> fragment flux intothe test gas.

It the gas being excited by any meansis in local thermodynamic equilibrium andoptically thin, the observed radiationintensity can be described by:

I - no £ e-E^kT !f A.o u A

This relation can be rearranged as:

where c

In

nohc

U

= In c - .4=

x constant.

(D

(2)

partial pressure of radiating species

statistical weight

partition function

excitation energy

temperature

transition probability

Planck's constant

Boltzmann's constant

velocity of light

wavelength

For a Maxwell-Bolanarm distribution of ex-

cited states, a plot of ln(—r-)vs. excitation

energy should result in a straight line,

having the slope of - ,4p Any deviation

from straightn^ss indicates over or underpopulation.

A typical example is shown in Figure 2

4.0

1.0

BOLTZMANN PLOT

Species - Argon IIPressure = 450 torr.Temperotare = 82262.7 Peg. K

9 20 21 22 23

'Jpper Stole Energy- EV

25

Figure 2. Nuclear Energy Produced Non-'_quilibrumof Excited .Ions.

for states of Aril, excited by fissionfragments.2 Since the gas was close toroom temperature, it is obvious that theslope of the line does not describe aphysically realized temperature. The ob-served states are nowhere near a Maxwell-Boltzmann distribution within the energyinterval observed.

A measurement of the relative popula-

tions ( - to the factor i -) depends on the

accuracy of the intensity measurement and onthe accuracy of the knowledge of the tran-sition probability. While the first is noparticular problem, the latter certainly is.As a rule, only a few gases (e.g., H andHe) are available for which most transitionprobabilities are known with satisfactoryaccuracy. Based on this method, the popu-lation inversion compiled in Table I havebeen detected.3' h

Measurements of the intensity ofspectrum lines as a function of pressuregive additional information on the excita-tion mechanism.3 The points in Figure 3indicate the measured intensity vs. pressurefor several He lines excited by fissionfragment. The population and depopulation

81

GAS

Hel

Hel

XellXellXell

TABLE I . Experimentally Observed

UPPER LEVEL

4 lP3 'P

(3p) 18 1 / 2

(3p)6d*P3/2

(3p)7s4Pl /2

LOWER LSV".L

" 4 'D3 'D

(3p)6p4P^/2

(3p)6p*P3/2

(3P)6p4p ! /2

Population Inversions

TRANSITION

?.16)J

95y

436 9A

4180A

4296A

INVERSION RATIO

3.5 (max)1.5 (max)5.358.61

21.00

id

He I

^ 4 — I

. Hel

He I

He I

6678. It 23.07ev

i

728I.3S 22.92ev

5015.71 23.09ev

4921.9* 23.74e»

i

•

A

0 100 ZOO 300 40C 500 609 700 800

Pressure Oorr.)

Figure 3. Pressure Dependence of FissionFragment Excited Helium Lines

reactions of the two levels between whichthe transition under observation occurs,have ro be described by a rate equationapproach. The solid lines in Figure 3were obtained with the following very sim-ple model. The mechanisms accounted forare:

1) direct formation of an excited state bythe reactions

ff + He •* He* (n,£) + ff (3)

e" + He ->• He* (n,£) + e" (4)

2) excitation transfer between levels

He*(n,-£) + He -<• He + He*(n,£') (5)

and by means of trapped resonanceradiation.

3) radiative decay by spontaneous emission

4) depopulation by collision losses as inthe reaction

He* + 2He •+ He2+ + e" + He (6)

and various other reactions with im-purities.

The rate equation for this model is:

(7)

dt(8)

where f is the formation rate; a-jj , aj^,the transfer rates i-.j an-i j^i, respec-tively; c, deexcitation by collision; and A,transition probability. The rates f, a, andc are per unit pressure.

In this simple model, the energy stor-age capability of the metastable states of_the noble gases was neglected. The specificenergy available in these excited states ofthe rare gases has led to the production ofmany well-known laser systems (e.g., He-Ne,He-Xe, Ar-N2). The collisions betweenmetastable argon atoms and nitrogen causeexcitation of the N2(C3nu) state which .gives rise to the second positive groupemissions.

Experiments were undertaken wherebyvarious quantities of N2 were added tc ar-gon and the spectral output of the argonand nitrogen w«re monitored.5 Figure 4

10' iOz 10' I C 10*Nitrogen Concentration (ppm)

10"

Figure 4. Meosurtd Populations of An 2p Leveland N2C State vs 'k Concentrationfor Fission Fragment Exciforion.

82

indicates the populations of the 2p levelsof argon (Paschen notation) arH he N2(C3iIu)state (designated N2C) versus nitrogen con-centration for a total pressure of oneatmosphere. The N2 second positive groupis a prominent emission in all of thespectra containing N2- The red Arl linesoriginating from the 2p levels of argon areall strongly quenched by the nitrogen,whereas the Aril lines (e.g., 4545A) arenot radically affected. This suggests thaithe Aril excitation may be direct while,for the Arl lines, it may not.

To evaluate this data, a more sophis-ticated rate equation model than that usedfor helium was developed. For this study,the excited levels of argon were dividedinto six groups as indicated in Table 2.Each group was treated as a single excitedlevel and assigned a transition probabili-ty that was an average over the gA valuesof the levels of that group. As indicatedearlier, the energy deposition by fissionfragments in a gas is primarily in theproduction of delta rays, and it is thedelta rays and their secondary electronsthat are primarily responsible for theexcitation of the gas.

A resonably complete set of discreteionization cross sections for argon exci-tation by electrons is given by Petersonand Allen.6 They used combinations ofdata and theoretical extrapolations of the

generalized oscillator strengths. Theircalculations result in the final popula-tion for each excited state as a functionof incident electron energy. These popu-lations are a result of the degradationof the primary electrons and all genera-tions of secondary electrons and ispresented as an efficiency where:

E = W t N/E, (9)

where, W ^ = threshold of the level i;N = number of excitations;g = energy of the incident elec-

tron.

The efficiency versus incident electronenergy is relatively constant for inci-dent energies above approximately 80 eV.In Table 2 are tha results of Petersonand Allen that correspond to our sixgroups of the atom and for the ion forincident electron energies of 100 eV.

From Platzman,7 the average energyloss per ion pair, W, can be related tothe equation:

where, the energy necessary to producean ion;

TABLE 2. PROPERTIES OF THE SIX GROUPS OF ARGON

GROUP .

AR(1)

AR(2)

AR(3)

AR(4)

AR(5)

AR(6)

AR*

AR #

(ADJUSTED)

LEVEL REPRESENTED[PASCHEN NOTATIOH)

is

2P

2s,3s',3d

3P

'id

HIGHER LEVELS

2 P 5 2 P 3 / 2

AVERAGEENERGY(EV)

11.68

13.21

14.1

14.59

14.77

15.2

15.715.7

AVERAGEGA (X 10"8

-

1.3

0.5

0.055

0.1

0.021

—

£.08

0.04

0.04

0.0C5

0.008

0.019

0.560.594

# EXCITED/IOOEVABSORBED

.685

.303

.284

.034

.054

.125

3.443.79

83

E e x = the energy for the excited atomin state g;

F s e = the energy of the subexcitationelectrons;'

N e x/N. = the ratio of number of excitedatoms to the number of ions.

For the noble gases, the quantity E~iexceeds the ionization energy I because ofthe energy used in producing excited ionsand multiply charged ions; The ratio ofEi/I for the noble gases is approximately1.06. This results in the adjusted valuein Table 2 for the e of Ar+. Using Eq.(10)and the values in in Table 2, approximately60% of the deposited energy is used in theformation of ions, 20% in the formation ofexcited states and 20% of the energy isleft in subexcitation electrons. Thus,since a large portion of the depositedenergy is used in the formation of ions, theresultant spectrum will, at least in part,be determined by recombination processes.

For pressures above a few torr, recom-bination in argon will proceed predominatelythrough the molecular ion ArJ and not theion Ar+. This is due to the rapidity withwhich the ion is converted into the molecu-lar ion by the process:

(11)Ar"1" + Ar + Ar •* Art + Ar.

The recombination of the molecular ion hasa large rate coefficient and proceeds bythe process of dissociative recombination.

Arz(v) + e t (Ar|)unstable - Ar* + Ar + K.E. (12)

where, v indicates different levels of ex-citation of the Art. The spectrum emittedin argon afterglows has indicated that thekind and number of excited species producedby dissociative recombination is dependentupon the initial energy of the Ar|. Figure5 shows a single stable potential curveof the molecular ion and single repulsivebranch of the unstable excited molecule.9

Higher vibrations levels of the molecularion will have smaller recombination coef-ficients and dissociate upon recombinationinto higher excited states of the atom.

To develop the model for the Ar-N2system, the interaction of fission frag-ments with a gas was divided into threecategories. First is'the excitation of theprimary gas, Ar, by the incident fragmentand secondary electrons. For the presentanalysis the argon ion, Ar+, and six groupsof excited levels of the atom, Ar(l)-Ar(6)was considered as discussed above. Thesecond category to be considered was ef-fect:* taking place in the argon after theprimary excitation has been produced. Anexcited level of the atom may be furtherpopulated by cascading of higher layingpopulations into this level, by collisionalexcitation caused by electrons and atoms,and by dissociative recombination intothis level. Deexcitation takes place via

INTERNUCLEAS SEPARATION

FIGURE 5: DISSOCIATIVE RECOMBINATION PROCESS Hi ARGOH

collisions with electrons or atoms, spon-taneous emission and collisional processesleading to formation of molecules (dimers).

To account for recombination effects,the molecular ion was divided into two

groups, ArJ' and Ar^". The Ar 2' comprises

the lowest excited levels of the molecular

ion which dissociate upon recombination in-

to Ar(l), Ar(2) and Ar(3). The ArJ" will

comprise all higher states of the molecular

ion and will be assumed to dissociate upon

recombination into Ar(2), Ar(3), Ar(4),

Ar(5), and Ar(6) and will also have colli-

sional losses to form ArJ' upon collisions

with argon or impurity gases.

Two levels of the molecular argon wereconsidered. First, the 3p levels of argon,Ar(4), will rapidly undergo 3-body colli-sions to produce a molecule Ar2", the upper

level of the 2250A continuum observed inthe experimental studies.1" Secondly, theargon metastable, Ar(l) , will, also be con-verted by 3-body collisions to form anargon eximer, Ar2'. Transitions from thesedimers to the repulsive ground state will

give rise to che 1250A continuum.

The third category of interactions isdue to the additional events caused by theaddition of a second gas, in this casenitrogen. The addition of nitrogen willprovide a path for collisional transfer ofexcitation from the argon to the nitrogen.The nitrogen will have two additional ef-fects on the kinetics. First, it will

84

change the electron energy distribution,and secondly, it will collisionally relaxthe vibrational levels of the molecularion. Both these latter two effects willundoubtedly change the excited speciesproduced upon dissociative recombination.

Clearly to describe the system re-quires the knowledge of a large number ofreactions. Considered in the presentmodel were 89 reactions for 20 species.The flow chart in Figure 6 summarizes thereactions considered in the Ar-N2 system.

r

measured populations of the argon 2p leveland the N2C state from Figure 4. Themodel solutions are within a factor of tenof the measured populations (within exper-imental error) and adequately predict therelative change in magnitude of the mea-sured populations versus nitrogen concen-tration.

An experiment by DeYoung" was recentlysuccessful in lasing an argon 3d-2p transi-tion by using the 3He(n,p)T reaction toexcite a 3He-Ar gas mixture. Lasing was

FIGURE 6. KINETIC MODEL FOR ARGON-NITROGEN MIXTURES EXCITED BY FISSION FRAGMENTSThe population of the species present

can be found by solving a system of non-linear simultaneous equations. Initially,each species equations was formed as adifferential equation in time using allthe source and sink reactions for thatspecies. Then the differential equationis set to zero for the steady state case.The equations were solved using a Newton-Raphson iterative procedure for nonlinearsimultaneous equations.

Computer solutions of the excitedstate population densities were obtainedfor a number of cases upon varying thenitrogen concentration, pressure, andenergy deposition form. Figure 7 indicatesthe computer solution for the population ofthe species of interest versus nitrogenconcentration at one atmosphere total pres-sure using an energy deposition rategermane to the experimental results. Theheavy dark lines in Figure 7 indicate the

achieved at 1.79 microns in atomic argonat a total pressure ranging from 200 to700 torr with 10% argon. The model pre-dicts this population inversion. Figure 8indicates the populations predicted by themodel for various argon species based on2 ppm nitrogen. Note that the Ar(3) grouppopulation (3d levels) exceeds the popula-tion of the Ar(2) group (2p levels). Theenergy deposition rate used assumes auranium coated tube of 1.0 cm radius and afission density of 4xlO12 fissions/cm3-secin the coating.

Thus, it has been shown that the modeldescribes the experimental results of fis-sion fragment excitation of Ar-N2 systemwith a reasonable accuracy. In addition,the model has indicated its usefulness bypredicting a physically realizable nuclearpumped laser.

85

1010*

Nitrogen Concentration ippm)

10*

Figure 7. Population of Exited An and N2

Species vs. N2 Concentration Far75Otorr Total Pressure

10

10'

10

-.10"

I010

100 200 300 400 500

Pressure (torr)

600 700

Figure 6- Population of Argon Excited Species«s. Pressure for 2 pom Nz.

Ill. Lifetime Measurements

One shortcoming of the methods de-scribed so far is the questionable accuracyin the knowledge of transition probabilities(or radiative life-times of the states) andcolllsional life-times. This problem canbe alleviated somewhat if radiative life-times of the states are measured at thesame experiment, where the relative lineintensities are measured.

This can be done using a very moderatefission fragment flux. In the experimentsdescribed in the following," a Cf252source,located inside a small vacuum chamber, em-ploying sapphire windows, was used. Cf"2undergoes spontaneous fission, and thesource used emitted about 6 x 103 fissionfragments per second. Figure 9 shows theexperimental setup.

TO GAS HANDLING ANDVACUUM SYSTEM

j OELAY I

STOP TIME TOPULSE HEIGHTCONVERTER

START

MULTICHANNELANALYSER

FIGURE 9. SYSTEM FOR LIFETIMEMEASUREMENT

The birth of a fission fragment issignaled by a burst of y-emission, which isdetected by the plastic scintillator andattached photomultiplier. The resultingpulse starts the time to pulse height con-verter. The fission fragment traversesthe vacuum chamber in a time small com-pared to the radiative lifetime of theatomic or molecular state under observa-tion. Therefore, when the transition fromthe excited state to a lower state isfinally made, a light pulse is emitted andobserved by photomultiplier number 2 (PM2).

The time to pulse height converterassigns this pulse a certain height, de-pending on the time passed after the startpulse. The multichannel analyzer willthen, store this pulse according to itsheight in the appropriate channel. Theresulting distribution of observed pulsesis an exponential decay as a function ofchannel numbers. The decay constant givesthe collisional lifetime of the state.

86

An example is given in Figure 10 which

N 2 800 Ion-

Figure 10-

Time tnono sec)

Decay Curves for Nitrogen C*Hi State

shows the decay curves for 800 torr N2 and100 torr N2- The time resolution of thesystem is 2.5 nsec. The radiative lifetime(or transition probability) can be obtainedby measuring the collisional lifetime as afunction of pressure and extrapolating backto zero pressure. Figure 11 shows this forthe upper level of the 3371 N2 line. Themeasured radiation lifetime is 45 nsec,which compares favorably with the litera-ture values as indicated on the figure.

In order to apply this technique formeasurement of population inversions, the

Ar/N2 system was chosen as a test case. TheC^nu->-B3n transition has been lased usingan electron beam by other research groups." A computer model predicting the populationsof the two levels involved was published inreference 14. The so predicted populationsare reproduced in Figure 12.

From this figure, the lifetime ofN2(B) is 131 nsec and of N2(C) is 18 nsec.The result of the measurement of theselifetimes using our system is shown inFigure 13. Our result is 300 nsec (ascompared to 131 nsec) and 40 nsec (as com-pared to 18 nsec). However, our gaspressure was .1 atm as compared to 3 atm.A correction for the pressure differencewould bring the two sets of data to abetter agreement.

3371 N2

Measured Lifetime 4? nsecPublished Values

25 50 100 200 400 600

P (torr.)

Figure I I . Measurement of The Radiative Lifetimes

CLIO,

0.020 40 60 100 120 140 160 180 200

Figure 12. Population Densities of N2(B) and N2(C) vs.

Time Due To Excitation of 3 Atm. A R » 5 % N 2

By A Z5^ft ,20NS2 Beom. (Atter Ret 14)

The advantage of the method describedabove is that it allows for search of pop-ulation inversions caused by fission frag-ment excitation, without having to use areactor, and the resulting complexity ofthe measurement procedures. The questionwhich has to be addressed next is the ex-trapolation of the results obtained withthese low fission fragment fluxes tohigher fluxes which are typical for nuclearpumped lasers. The one test case describedabove seems to support the scaleabilitytowards higher f luxes;. However, furtherresearch is needed.

IV. Measurement of Fission Fluxes

Emanating from Coatings

As indicated above, it is of interestto know: (a) How much energy is depositedin the laser gas, (b) Optimum coatingthicknesses to maximize energy depositionand minimize cooling requirements, (c) Theangular distribution of fission fragmentsemitted from coatings of different thick-nesses, (d) The possibility of increasingenergy deposition by increasing the surfacearea of the coating through surface cor-rugation.

67

800 TortN 2 - 1st Positive

. » • 3 0 0 nsec

10° 10' Id2

Tims ( nonosec)10*

Figure 13- Lifetimes of Excited Nitrogen LevelsIn 95% A* ond 5% Nt Mix turfAf 800 Torr.

Initial measurements use the chambershown in Figure 14. An aluminum chamber

Silicon Surfact Barrl»r D«!«ctor

Lno.ll Changa*

Figure 14. Fission Fragment Range and EnergyDetector (FFRED)

was constructed which could be evacuated orfilled with gases at various pressures. Thefission fragment source-was a U-235 U3O8coating about 0.5cm x 2cm long made by thelacquer painting and firing technique. Thecoating can be rotated for angular distri-bution measurements. A silicon surfacebarrier detector about 2 cm in diameter de-tected the emitted alpha particles andfission fragments. The detector could beplaced at various distances from the sourceto vary the geometrical efficiency or an-gular resolution. The entire assembly wasplaced in a polyethylene moderator with a10 Ci Pu-Be neutron source to provide theneutron' flux.

Coatings of various thicknesses of 93%enriched U-235 were made and the thickness

assayed by alpha counting in a 27r-gas flowcounter and by ••' servation of the 133 keVgamma rays fro_i :.he U-235 decay chain usinga large Ge(Li) detector. (Host of the alphaparticles actually were produced by 0-234decays.)

Pulses from the surface barrier de-tector were analyzed in a multichannelanalyzer and typical spectra for two dif-ferent thicknesses are shown in Figure 15.

i1900

s

1140

f 380

Counting Time * 60 min.Coating Thiclinass:

— D - 4.5u

^ 0 32 64 96 128 160 192 224% Channel No. (Energy- 1

Figure 15. Typical Spectra af Fission Fragments Worn TwoDifferent Source Thicknesses

The 4.5 micron coating produced more fis-sion fragments, but mainly at lower energieswhere they are far less efficient at de-posting their energy in gases.

F5.gure 16 shows the total number of

One Hour Counts

/

OS I *(# of ALPHASl/250,000

Figure 16. Fission Fragments Observed vs. CoalingThickness

observed fission fragments versus thenumber of alpha particles counted. Theexperimental points deviate from a strai itline for the larger count rates where thecoatings are thick enough to allow thealphas to penetrate. Some thick coatingswere made on corrugated surfaces (groovesmilled in the stainless steel backingplates) and the results fell on top of therther data indicating no improvement inthe fission fragment fluxes for thosegeometries and coating thicknesses.

Angular distribution measurements onthe fragments are continuing and measure-ment of the absolute energies deposited indifferent gases at different pressures arebeing made.

V. A SpectroscopicStudy of a Fissioning Gas

One advantage of nuclear pumping overconventional laser excitation methods isthe potential to obtain higher energy den-sities in a nuclear pumped laser. Theachievement of high energy deposition intothe lasing gas is thus of key interest.

The alternative approach is to employa volume source of fission fragments. Sources"made of solid fissionable'materials, asc'.escribed in the previous chapter, arelimited by th^ fact that source thicknessescannot exceed the range of fission frafments in the material. As pointed out, in-creased energy deposition can be obtainedonly by increasing the source surface area(i.e., many sources) or the neutron flux.Bemuse VFf, is the only known uranium bear-ing as at physical conditions of interest,it is the prime candidate to serve as afission fragment source in a nuclear pumpedlaser. Information is thus required on thesuitability of UF6 as a laser gas itselfand the compatibility of laser gas mixtureswith UFg. Therefore, a spectroscopic in-vestigation of light emitted by fissionfragment excited UF6 and UF6-Ar-N2 mixtureswas conducted. I5

The fission fragment flux supplied bythe fissioning UF6 was augmented by a fluxemitted from a 3 micron thick, 93% enrichedU02 planar source mounted along the insidewall of the gas cell. Arl, Aril, N2 and

N^ emissions from the gas were spectro-scopically monitored i-ver the 2200-8000Arange as a function of UFg concentration ina 10:1 Ar-N2 gas mixture at a total gaspressure of 760 torr.

The results indicated in Figure 17show that all optical radiation from thefission fragment excitation of the

N2 and N2 molecule is severly quenched by

the addition of UFg to the gas mixture.T..e radiation coming from Arl is actuallyenhanced by small concentrations of UFgand not as sevarly quenched by higherconcentrations of UFg. Radiations coiningfrom Aril is relatively unaffected by lowconcentrations of UF5 and is moderatelyquenched at higher concentrations of UFg.Pure UFfj was found to emit no radiationdue to fission fragment excitation overthe spectral region investigated. Theresults of this study suggest that signi-ficant additional work will be requiredbefore a satisfactory fissioning laser gasmixture is identified. There is no .indi-cation however, that such a mixture cannotexist.

3577 NaCO.I) (2x 10-*)

.0.01 0,1 ' 1.0

UF<-, -Admixture (%)

Figure 17. Relative Intensity of As I, Anil , N 2 ,and Nt As A Function of UFS

Admixture. Wavelength In A

Acknowledgment s

The work was supported by the Nation-al Aeronautics and Space Administration.

References

1. Helmick, H.H., Fuller, J.L, andSchneider, R.T., "Direct Pumping ofa Helium Xenon Laser", Appl. Fhys.Lett., 26,6,327, March 1975.

2. Schneider, R.T., "On the Feasibilityof Nuclear Pumping of Gas Lasers",Laser Interaction and Related PlasmaPKihomena, Vol. 3, pp. 35-107, editedby H.J. Schwarz and H. flora, PlenumPublishing Corp., New York,(1974).

3. Shipman, G.R., Walters, R.A. andSchneider, R.T., "Population Inver-sions in Fission Fragment ExcitedHelium", Trans. Am. Nncl. Soc., 17,3(1973).

4. Davie, R.N., Davis, J.F., and SchneiderR.T., "Population Inversions Observedin Fission Fragment Excited Xenon",ANS Annual Meeting, 13-18, June 1976.

5. Davis, J.F. , Dissertation, Univ. ofFlorida, Gainesville, Florida, to bepublished (1976).

6. Peterson, L.R., and Allen, J.E.,"Electron Impact Cross Sections forArgon", Journal of Chem. Phy., 5b,No. 12, 6068 (HJ72).

89

7. Flatzman, R.L. , "Total lon iza t ion inGases by High-Energy P a r t i c l e s : AnAppraisal of Our Understanding",Journa*. of App. Rad. and Iosotopes,IG, 116-127 (1961J.

8. Loeb, L.B. , Basic Processes of GaseousElec t ron ics , Chapter-6, Univers i ty of"Cal i fornia Press ( J . 9 6 1 ) .

9, Biondi, M.A. and Hols te in , T. , PhysicsRev., 82, 963 (1951).

10. Birot A, Brunet, H., Galy, J . , Mi l l e t ,P . , Tsyss ier , J . L . , "Continuous Emis-sions o,f Argon and Krypton in the NearUltraviolet ' .1 , Jour, of Chem. Phys. , 63,No. k, 1469 (1975).

11. DeYoung, R . J . , Ja luska , N.W., Williams,M.D. and Hchi, F . , "Direct NuclearPumped Lasers Using the VolumetricHelium-Three Reaction", The PrincetonUniversi ty Conference, Meeting No. 133,June 10-12, 1976.

12. Shiptnan, G.R. Ph.D. Di s se r t a t i on ,Universi ty of F lor ida , Gainesv i l le ,F lo r ida , to be published (1976).

1 3 . Hill, R.M., Gutcheck, R.A., Huestis,D.L, Mukherjee and Lorents, D.C.,"Studies of E-Beam Pumped MolecularLasers", Stanford Research Institute,(1974); also, Searles, S.K. and Hart,G.A, Appl. Phys. Letters, 25, 79

74T14, Ault, E.R., Bradford, R.S. and Bhaumlk,

M.L., "UV Gas Laser Investigations,Semiannual Technical Report", NorthropCorporation, May 1975.

15. Davie, R.H., Albrecht, G., Carroll,E.E. and Schneider, R T., "OpticalRadiation of Fission Fragment ExcitedUFg and Ar-No Mixtures", ANS AnnualMeeting, 13-18, Tounto, June 1976

DISCUSSION

D. C. LORENTS: I would l ike to comment about yourquenching studies of UFj. There are two poss i -b i l i t i e s . One i s that It i s just pure opticalabsorption by the UF6; the othei- Is that i t i squenching of some excited s tate in the energy flowchain. Did you look at the re lat ive in tens i t i e sof the 1-0 vs the 0-0, in order to i so late that?Right in that region, there i s a sharr dip in theabsorption cross-sect ion of UF,.

6

J. P DAVIS: He considered absorption because thesails we were using were three feet leu 3 and theactive volume was only about 11 inches of that, sowe feel that a large percentage of quenching wasby absorption.

D. C. LO5ENTS: As far as I can see, the model youhave is exactly the same model we use in theelectron-beam excitation, but you said that therewere some differences.

J. P. DAvis: I have tried to take into accountexcitation of the upper state by higher es.cite<3stsces of argon collisionally exciting the nitrogen.!>y doing that, I could find agreement with theobserved intensity of argon versus nitrogenconcentration and also more adequately jescribe thepopulation of the nitrogen. I was also then ableto describe the relative trer.ds of the argoncontinuum at 2250 8. Even though a smallpercentage of the energy was flowii.g through theseupper excited states of argon, it w;is the purposeof the study to try to describe thostf states too.,Host of the ennrgy is traveling very quicklythrough the metostable states, et a pressure ofone atmosphere.

G. H. MILEY: Do yju account for a chang? inelectron energy -listrlbucion in the calculation?

J. F. DAVIS: No, I didn't take into accourt anychange of the Boltanann electron energy distri-bution with additional nitrogen.

G. H. MILEY: So this mixture will last withelectron beams interacting with any gas or gasmixture.

J. F. DAVIS: When a fission fragment interactswith a gas, it is like a small e-beam; and we havea lot of small low-energy "beams correspondingto each fission fragment. So I feel e-beamexcitation and fission-fragment excitation aresimilar in that respect.

G. H. MILEY: How many groups of e-beams dr> youneed to duplicate fission fragment excition?

J. F. DAVIS: With f .ssion-fragment excitation,you have localized elfctron ionization, leadingto polymer recombination effects, but F youproceed to a high enough fission fragment flux,such localized effects will begin to average out.We feel that Be have accomplished this in thepresent case.

M. KRISUNAW: In your model of dissociativerecombination, you say that molecular ions fromdifferent vibrationally excited levels result indissociated atoms in different electronic con-figurations. Since electronic excitationsinvolved energies of •— 1 e.v. and vibrationalexcitations involve much lower energies ~ 0.1 e.v.,it appears that the bulk of the energy forelectronic excii_ation upon recombination arisesfrom the dissociation energy of the molecule.If this is so, vibrational excitations shouldno play a major role in this recombination model.

90

THE PUMPING MECHANISM FOR THE NEON-NITROGENNUCLEAR EXCITED LASER

G.W. Cooper, J.T. Verdeyen, W.E. Wells, G.H. MlleyNuclear Engineering Program

and 'Gaseous Electronics Laboratory

University of Illinois at Urbana-ChampaignUrbana,- Illinois 61801

Abstract

The neon-nitrogen laser has generated consider-able interest having been pumped directly by nuclearradiation. Lasing was observed on two transitionsof atomic N, the 3p P3 •*• 3s^ P 3 (8629.24 A) and the

3p2 D° + 3 S2 P 3 (9392.79 A ) . lie Ne-N2 system also

lases In an afterglow of an electrical discharge,which has similar characteristics to a nuclear radi-ation generated'plasma. In order to determine thephysical proce ses for pumping this laser, a de-tailed study of the afterglow system has been per-formed. The pumping mechanism has been found to becollisional-radiative electron-ion recombination.Microwave quenching of both the laser and spon-taneous afterglow light have shown cohclusivelythat a recombination process directly -iroduces anitrogen atom in either the upper laser level or,more likely, in a higher lying energy level whichrapidly de-excites to the upper laser level.Studies of the temperature dependence of the recom-bination coefficient Indicates a colllsignal-radi-ative process allowing the recombinlng ion to betentatively identified as N • Since this processis highly compatible with the reactor producedplasma, it is not unreasonable to assume that thisrecombination process is also the pumping mechanismin the nuclear excited case.

I. Introduction

The electrically-eKcited neon-nitrogen laserwas discovered in 1964 and several subsequentpapers have appeared. Lasing occurs on severaltransitions of atomic nitrogen during the afterglowof an electrical discharge and will hereafter bereferred to ns the afterglow neon-nitrogen laser.Lust, year Interest was greatly revived in the neon-'iirogen laser when DeYoung et al, succeeded inpumping this laser directly with nuclear radiation.They observed lasing on two transitions of atomicnitrogen: 3p V5/2 •* 3sz P 3/ 2 (9392.79 A) and

3p 2 P°./2 + 3s2 P 3 / 2 (8629.24 A ) .

The results of a detailed Investigation of theafterglow neon-nitrogen laser's pumping mechanismare presented here. The research was undertaken inorder to provide a better understanding of thenuclear pumped laser which is necessary to fullyevaluate the laser's ultimate applicability and tohelp identify possible analogous systems for nucle-ar pumping. The afterglow system was chosen forstudy because of the difficulty of doing diagnos-tics in the reactor environment. Since the charac-teristics of both the afterglow and nuclear-radia-tion-generated plasmas are dominated by their lowelectron temperatures, it is quite probable thatthe pumping mechanism in both systems would be thesame.

ENERGY LEVEL DIOCRAM OF N,

\ N(4S°).Nt3P>

Sp^^-flQ&a rci.69eV \ \ 10.69 P

0.4 08 1.2 lfi 2.0 H4 2B 32 56 4 0 4.4 4.8

Fig. 1. Partial potential energy diagram of the S2molecule also showing soae levels of W*",atomic nitrogen and excited neon species.

The problem of the pumping mechanism is bestdescribed by considering the potential energydiagram of Nj shown in Fig. 1. Also shown in thisfigure are the lasing levels of atomic nitrogen.As can be seen, it requires a minimum of 21.76 eVof energy to simultaneously dissociate N, and ex-cite atomic nitrogen to the upper laser level.Since in neon-nitrogen lasers the neon density istypically 103 to 10 times greater than the nitro-gen's, most of the input energy will go into theneon. Thus, the energy to pump the laser must comeprimarily from collisions of nitrogen with excitedneon species. However, even Ne+ haF insufficientenergy to excite the laser in one step. Therefore,the pumping process requires a minimum of twosteps.

The fact that the neon-nitrogen laser operatesin the afterglow strongly suggested that recom-bination was playing an integral role in the pump-ing mechanism since it is a major process in after-glows. Since two recombination processes, dis-sociative and collisional-radiative are rapid

9 1

/-ATTENUATOR

/IICROWAVESOURCE

V

^WAVEGUIDE

YILASERJJOUTPUT

SMALL HOLEWAVEGUIDE

ADJUSTABLE SHORT

LASER TUBE

Fig. 2'.f Schematic of the microwave quenchingexperiments.

enough to be of importance and are both inverselyp ortional to electron temperature, the tech-nique of microwave quenching was employed. Inthis technique the electrons are selectively heatedby a microwave pulse, and, as a result, the after-glow light which is primarily produced by recom-bining electron-ion pairs is "quenched." A dia-gram of the experimental set-up is shown in Fig-2. The effects of electron heating on both thelaser light and the spontaneous sidelight weremonitored.

The microwave quenching experiments, to bedescribed in part A. of the next section, haveshown that the neon-nitrogen laser is pumpeddirectly by a recombination event. Further micro-wave quenching experiments described in part Bof the next section, have shown that: die processis most likely collisional-radiative recombina-

II. Results and Discussion

A. Microwave Quenching Experiments

The effects of microwave heating of the elec-trons on both the laser signal and the spontaneousemission of atomic nitrogen and neon have been in-vestigated. The laser was found to be pumpeddirectly by recombination. This conclusion isbased on the following data.

Figure i .hows the effect of microwave heatingon the laser Itself. The top trace shows the laser-Ing in the afterglow in the absence of microwaves.The second trace shows the laser output as it wasperturbed by the microwave pulse shown in thebottom trace. The laser intensity has obviouslybeen quenched, indicating the inhibition oi. someprocess by microwave heating. Obviously, the elec-trons are not "cooled" by the microwaves, hencedirect excitation from a lower state can be ruledout; it must be pumped from above. Thus, recom-bination must be playing an integral role in the. pumplr,d, process.

Now consider Fig. 4 where the effect of micro-wave heating on Che spontaneous sidelight of anatu;nlc nitrogen laser transition (93,93 A) is com-pared to that of a neon line (8654 A). Note thatthe time responses of the neon and nitrogen after-glow light to the microwave pulse are virtually

Fix. 3. Effect of microwave heating of tj}e elec-trons on the laser output (9393 A);(a) laser output in absence of microwaves,(b) laser output perturbed by microwaves,(c) current, (d) microwave pulse.

Fig. 4. Effect of microwave heating of electronson both nitrogen's and neon's spon-taneous sidelight;, (a) nitrogen (9393 A)(b) neon (8654 A), (c) current, (d) micro-wave pulse.

identical. This implies that the role of recom-bination cannot be an indirect process, but must bedirectly pumping the laser. To elaborate, Atkinsonand Sanders* proposed an indirect pumping scheme:

Ne* + e Ne (M) + Ne

Ne (M) + N 2 * Ne + N +

(1)

(2)

Here a neon metastable, Ne (M), collides with anitrogen molecule, N , that is sufficiently excitedto allow the moleculg to be simultaneously dis-sociated and leave a nitrogen atom, N]IT, in theupper lasing level. Recombination enters thepumping process as a major production mechanism ofneon metastables. This or similar pumping schemescan account for quenching of the laser pulse bymicrowave heating since the neon metastables willcease to be produced. However, it cannot accountfor the near identical time responses of the neon

92

and nitrogen afterglow light. While it is truethat electron heating inhibits p-oduction of neoi.mats;tables, it leaves those already produced un-affected. Therefore, since the process in Eq. (2)takes time, the response of the nitrogen afterglowlight to the microwaves would be expected to be muchslower than neon's response. Since it was not, theupper laser level must be being populated directlyby a recombination event. That is, a process of theform

+ e => H,, (3)

Here XN represents the ion which upon recombin:'.ngyields atomic nitrogen either directly in the upperlaaiug level or in a more highly excited state thatrapidly de-excites to the upper lasing level. Thisrecombination can be either dissociative or col-lisional-radiative. There were several candidatesfor the recombining ion: _J,+*. Netfi N andN+. Note that N 2

+ and N|*'i must be highly ex-cited, matastable-like states in order to dis-sociate to. the upper laser level upon recombining.Although buch states are possible, they have nocbeen observed and have a low probability of exis-tence. Similarly NeN"^ while energetically pos-sible, has not as yet been observed. The experi-ments whi allowed the ion to be identified as If1*are desci ed in the next section.

B. Sensitivity of the Recombination to ElectronTemperature

+* -f-Of the possible recombining ions, N , NeN

and Nj all must recombine dissociatively to yieldatomic nitrogen while N + recombination must becollisional-radiative. The theoretical collisional-radiative recombination rate" varies T- T~"/2while dissociative recombination9 variese •>. Ig" '

2.Thus, H"1* could be Either inferred or eliminated asthe recombining ion by measurement of the electrontemperature dependence of the nitrogen recombination.Measurement of the electron temperature dependence,using microwave quenching techniques, indicatedthat the recombination was collisional-radiative,suggesting that the recombining ion was fT*. ThusEq. (3) could be written more explicitly as:

N + +• 2e N** + e

hvw(5)

Here the double-barred arrows imply net rather thandirect processes and N * represents any energy levelabove the upper lasing level, N .

ULIn studying this recombination process, the

absolute temperature was not determined. Instead,microwave quenching of the neon-nitrogen mixturewas used to compare the temperature dependences ofthe neon and nitrogen recombination coefficients.Since the temperature dependence of neon recom-bination is well known^ 11,12 such a comparisoncould provide information about the recombinationprocess in nitrogen.

In Fig. 5, the ratio of quenched to unquenchedlight was plotted as a function of the attenuationof microwave power for both neon and nitrogen. Theratio of quenched to unquenched light provides ameasure of the temperature-dependent recombinationcoefficient. Although the absolute electron tem-perature corresponding to each microwave attenuationwas not known, the electron temperature was commonto both neon and nitrogen recombination events.

Si-I

3

a

N 9386.8 AHe 8654A

40 torr Ne12.8 mtorr N,20 A64 i±s into ofterglow

24

ATTENUATION OF MICROWAVES !N db

Fig. 5. Comparison of the electron temperaturedependence of the recombination coef-ficients of neon and nitrogen at 40 tontotal neon pressure.

The data shown in Fig. 5 were obtained at atotal neon pressure of 40 torr, a pressure at whichthe dominant recombination process in neon is dij-sociative.13,14,15 Consideration of the data inFig. 5 shows that the nitrogen spontaneous lightwas much more sensitive to the electron temperaturethan that of neon. This difference implied thatthe recombination process producing excited atomicnitrogen was different from the dissociativeprocess known to be dominant for neon under theseconditions. Thus, these results seemed to ruleout a dissociative recombination process fornitrogen, indicating that the meohanisiu must becolli«<onal-radiaf<"c. Given the probable avail-ability of If1" as tr_ recombining ion, tUc suggestionof a collisional-radiative mechanism for nitrogenrecombination seemed i reasonable one.

The sensitivity of neon and nitrogen recom-bination to electron temperature was also measuredat total neon pressures of iU t.orr and 6 torr. Theresults are shown in Figs. 6 ar.d 7, respectively,which can be compared with the 40 L O T data inFig. 5. The figures show that a total i.ecm pressureof 20 torr, the difference between the temperaturedependence curves for neon and nitrogen is not asgreat as the difference a'- 40 torr. At a totalneon pressure of 6 torr, the curves for neon andnitrogen are nearly identical.

This change in the temperature dependence forneon, indicates a change in the recombinationmechanism with decreasing pressure. This is asexpected since the dissociative recombinationprocess, which dominates for neon at 40 torr, islimited by the formation rate of Hfc2 •

Hnicll> i n

turn varies15 as P*« Thus at low pressures theformation rate of

pressuis too slow, and the

93

Xis

auiXoz

oUlXUIDo

5 2o

°= 15

Ne

• N 93£S.8A« Ne 865 4 A

20 torr MeIZ.8 mtorr N,20 a64 ^s -into afterglow

1 1 1 1 1

24 20 16 12 8 4 0

ATTENUATION OF MICROWAVES IN do

collisional-radiatlve mechanism dominates for tothneon and nitrogen.

C. Additional Considerations

The proposed pumping mechanism can be fartherconsidered An terms of its ability to account furthe lasing observed. A first consideration iswhether the collisional-radiative ' .uanism wouldbe fast enough to produce the oba<=rve<-' laser power.A recombination coefficient was estimated using thepeak laser power (1 mW.N, and the electron density(1013 cm"3) inferred from earlier microwave data.The estimated value of 5 x 10 cm3 sec~ - seemedreasonable in the light of the theoretical valuesfor collisional-radiative recombination coef-ficients given by Bates at al. Thus the esti-mated recombination coefficient for the neon-nitrogen laser seemed compatible witli a collisional-radiative mechanism.

An additional consideration is whether suf-ficient N* can be produced to support a collisional-radiative laser mechanism. Figure 8 shows a numberof possible pathways for production of N . Sincemost of the cross-sections involved are unknown, anexact calculation of the rate of If*" formation isnot possible. Estimates of N* production by themore likely pathways do not rule out the formationof sufficient If*" to support lasing, although thekinetics are made somewhat unfavorable by the

Fig. 6. Comparison of the electron temperaturedependence of the recombination coef-ficients of aeon and nitrogen at 20 toirtotal neon pressure.

I

Q

U

U

O

o

oo5rr

20 16 12 8 4 0

ATTENUATION 0< MICROWAVES IN do

g. 7. Comparison of the electron temperaturedependence of the recombination coef-ficients of neon and nitrogen at 6 torrtotal neon pressure.

T ~..~ ~ ~ ' 11 rI

ii

I

ii

i

11

iii

N* c |1

1

! £i

^* IN111<N

' 1

iii

_ _L

DISCH*e

t "!M)|

r i—-

- - -

» N

M

l £ l N,- N *

Me

Me*ACTIVEDISCHARGE T

Me*

AFTERGLOW,),

N,

We

*le*N,

N;\

e

N - - 2N -

* 1c

M—* LASES

I

FLOW CHART FOR PRODUCTION OF N*

Fig. 8. Flow chaxt showing possible pathways forproduction of N*.

9 4

requirement for two collisions with excited neon.In this-regard, the kinetics of formation of anyother proposed recombining ion would suffer fromeven more serious problems due to the same require-ment for two collisions with excited neon. How-ever, given that sufficient nitrogen atoms arepresent from the active discharge (and prior ones),the Penning reaction of neon metastable withnitrogen atoms is estimated to be quite adequateto produce the required IT1" ions.

The helium-nitrogen and argon-nitrogen systemswere also briefly investigated. The helium-nitrogen systeir has previously been reported tohave lased on the same transitions as does theneon-nitrogen laser. Microwave quenchiag experi-ments on the helium-nitrogen laser showed that thissystem was also being pumped directly by a recom-bination event. The argon-nitrogen system, however,could not be made to lase. These results tend toeliminate NelT" as the possible recombining ionsince the existence of the analogous ion, HeN isextremely improbable and the known ion1'' ArN* doesnot yield a laser. Also if the Penning reaction isa major production mechanism of N as suggestedabove, the helium metastable could undergo the samereaction but the argon metastastable has insuffi-cient energy to ionize ground state atomic nitro-gen. Thus the helium-nitrogen and argon-nitrogenresults are compatible with the pumping mechanismbeing collisional-radiative recombination of {&•

III. Summary

Microwave quenching experiments have shown thatthe neon-nitrogen afterglow laser is pumped direct-ly by a recombination event. Measurements of theelectron temperature dependence of the recom-bination coefficient have implied that this processis collisional-radiative recombination. Thisallows the recombining ion to be identified as if*".The process of collisional-radiative recombinationis highly compatible with the nuclear-radiation-generated plasma which has high electron densitiesat low temperatures. Therefore, it is not unreason-able to infer that the pumping mechanism for thenuclear-pumped neon-nitrogen laser is the same asfor the afterglow laser. It should be pointed out,however, that the steps leading to the formationof H* could be quite different in the two lasers.Indeed, initial indications are that these stepsare different and research is being pursued inthis area.

References ,

[1] M. Shimazu and Y. Suzaki, Japan J. Appl. Phvs.,3, 561, (1964).

[7] G. M. Janney, IEEE J. Quant. Electr., QE-3,133, (1967).

[3] L. N. Tuaitskii and E. M. Cherksav, Sov. Phys.- Technical Physics, 13, 1696, (1969).

[4] J. P. Atkinson and J.'H, Sanders, J> Phys, B(Proc. Phvs.. j>oc.), 1, 1171, (1968).

[5] R. J. DeYoung, W. E. Wells, G. H. Miley andJ. T. Verdeyen, Appl. Phys. Lett. 28. 519,(1976).

16] A. K. Bhattacharya, J. T. Verdeyen, F. T.Adler, and L. Goldstein, J. Appl. Phys., ,38,527, (1967).

[7] L. Goldstein, J. M. Anderson and G. L. Clark,Phys. Rev., 90, 486, (1953).

[8] E. Hinnov and J. G. Hirshberg, Phys. Rev..125, 795, (1962).

[9] D. R. Bates and A. Dalgarno, in Atomic andMolecular Processes, ed. by D. R. Bates,(Academic Press, Inc., New York, 1962).

[10] C. L, Chen. C. G. Leiby and L. Goldstein,Phys. Rev., 121, 1391, (1961).

[11] L. Frommholil, M. A. Biondi, auu F. J. Mehr,Phvs. Rev., 165, 44, (1968).

'12] J. Phllbrick, F. J. ttehr, and M. A. Biondi,Ph,ys. Rev., 181, 271, (1969).

[13] T. R. Connor and M. A. Biondi, Phys. Rev.,i_., A778, (I960).

[14] h. Fronmhold and M. A. Biondi, Phys. Rev..184, 244, (1969).

[IS] A. P. Vituls and H. J. Oskum, Phys. Rev. A.,1, 2618 (1972).

[16] D. R. Bates, A. E. Kingston, and R. W. P.McWbirter, Proc. R. j>oc., A267. 297, (1962).

[17] M. S. B. Munson, F. H. Field, and J. LiFranklin, J. Chem. Phys., 37, 1790 (1962).

95

DIRECT NUCLEAR PUMPED LASERS USING THE VOLUMETRIC 3He REACTION*

R. J. De YoungVanderbilt University, Nashville, TN

N. W. Jaluffca, F Hohl, and M. D. WilliamsNASA, Langley Research Center, Hampton, VA

Abstract

Lasers pumped by energetic charged particlescreated directly by nuclear reactions have comeinto existence through the use ox either Boron-10or Uranium-235 coatings. A new class of nuclear-pumped lasers using the He reaction has beendeveloped at the Langley Research Center. Sincehelium is a major constituent of many gas lasersof interest, it is convenient to replace the usual•*He with the 3He isotope. In a thermal neutronflux, the 3He(n,p)3H reaction deposits energynearly uniformly throughout the laser volume. Thecharged particles created (proton of 0.57 MeV and3H oi 0.19 MeV) ionize and excite the gas media.By this method direct nuclear pumping of a 3He-Ar(10% Ar) laser has been achieved. Results ofreactor experiments are presented which show thescaling of laser output at 1.79u (Ar I) withneutron flux and tota1. 3He-Ar pressure. Otherlaser systems presently under study at th.2 LaneleyResearch Center include the %e-Ne-C>2 (8446 A 01)system. Spectra of nuclear pumped 3He-Ne-O2taken at die Aberdeen Army Pulse Radiation Facilitywill be presented. The above systems aie consid-ered to be "proof of principle" systems and futuremore practical nuclear-pumped lasers, usingexcimer species, will be discussed.

I. Introduction

Attempts at pumping gas lasers by chargedparticles created from nuclear reactions have beenmade for many years.1'2 Figure 1 shows the Majornuclear reactions used to pu>-.p the lauer medium.Recently, however, rapid progress has been accom-plished in direct nuclear excitation with thedevelopment of nu:lear-pumped lasers using eitheruranium-2353>4>5 or boron-105 coatings on theinternal walls of the laser cell. Coatings arebasically inefficient for nuclear pumping sincefissions occur primarily inside the coating andmost of the charged-particle energy (80%) is lostin the coating. Since all the energetic charged-particles are created in the wall coating and thentravel inward toward the laser cell centerline,homogeneous volume pumping cannot be realized.This effect results in such problems as the "gas-focusing effect" zX. higher pressures.

A much more efficient means of laser pumpingincorporates the 3H3(n.,p)3H reaction. The 3Hereaction is initiated by a thermal neutron whichis absorbed by the 3He nucleus. The nucleusthen disintegrates, producing a proton of 0.57 MeVand a triton of 0.19 MeV. Both these chargedparticles ionize and excite the gas medium

homogeneously. Thus, homogeneous pumping can beachieved even at high pressures assuming no degrada-tion of the neutron flux. Since He is a majorconstituent of many gas-laser systems, it is con-venient (although expensive) to replace 4He withthe He isotope. The 3He reaction has previous-ly been investigated with encouraging results butno proof of lasing.^'^ We describe here the achiev-ing, for the first time, of a nuclear-pumped laserusing the volumetric 3He reaction. This is thesecond most fundamental advancement in nuclear-pumped laser research; the first advancement was thedemonstration of nuclear lasing by use of wall coat-ings. These advancements are expected to lead toan eventual self-critical nuciear-pumped lasersystem using UFg.

I. B10(n,(x)Li or U235tn,f)FF

Y 8 NEUTRON REACTORFLUX

ALUMINUM TUBE

LASING GAS'' U^or ~B"> COATING

H«3f LASING GAS

IE. V FLUX_ V FLUXREACTOR

Figures 1.

LASING CAS

Nuclear reactions for direct nuclearexcitation of lisers.

* This research has been carried out under partial support from NASA Grant NSG-1232.

96

II. Direct Nuclear-Pumped Volumetric3He-Ar Laser

Argon has many laser transitions which operatewith high gain in the infrared region of the spec-trum. Thus, it is a good candidate for volumetricnuclear pumping. In a recent experiment at theArmy Pulse Radiation Facility, a mixture of3He-Ar was found to lase when exposed to the neu-trons from a fast burst reactor.

-i-EA3 SHIELDING

OUTPUT

BEAM SPLITTER

BORON SHIELD

Figure 2. Experimental setup for nuclear-pumpedlaser experiments at the Army Pulse RadiationFacility.

The reactor experiment setup is shown inFig. 2. The laser cell was constructed from quartztubing, 81 cm long and 2 cm diameter, with each endcut at the Brewster angle. A dielectric-coatedflat back mirror (99.5% reflective at 1.7p) and a2-meter radius of curvature output mirror (1%transmission at 1.7u) were used to form the opticalcavity. A 61 cm long by 15.5 cm diameter poly-ethylene moderator was placed around the laser cellto thermalize the fast neutrons generated by theArmy Pulse Radiation Facility fast-burst reactorat the Aberdeen Proving Ground. A vacuum and gas-handling system was attached to the laser cell.The cell was baked-out prior to filling withresearch grade 3He-Ar and base pressures on theorder of lir6 Torr were achieved. Electrodes werealso attached to the laser cell which allowedelectrically pulsed lasing at 1.79u at low pres-sures (50 Torr}. Using the electrically pulseddischarge the cavity mirrors and detection systemwere aligned. The detection system consisted of avariable filter wheel and an InAs (at 300°K)detector placed in a shielded cavity about 20meters from the reactoi-. No radiation, gamma, orneutron noise was observed from the shielded InAsdetector during any neutron pulse. The detectorwas calibrated for power output by using a c.w.3.39y He-Ne laser.

Figure 3 is a photograph of the laser outputsignal (upper trace) and the moderated neutronpulse (lower trace]. Note the sharp laser thresh-old during the rise of the neutron pulse which istypical of nuclear-pumped lasers. The shape of thelaser signal shows that energy is being depositedin the gas as long as a neutron flux is availableand terminates only when the neutron flux ends.The laser produces an essentially steady-stateoutput since the atomic and molecular processes

Figure 3. The upper trace shows the total laseroutput. The sharp threshold at ^2.5 * 1016

n/cm^-sec indicates lasing action. Total pres-sure is 200 Torr 3He-Ar, 10% Ar. The averagemoderated neutron flux across the length of thelaser cell is 8,4 * 1016 n/cm2-sec. The modera-ted neutron pulse shape is shown in the lowertrace.

operate on a much shorter time scale than the laserpulse shown in the photograph. The duration of thelasing pulse is simply dependent on the duration o£the neutron flux available.

When the back cavity mirror was blocked, nolasing signal was observed. Al.so, when ^He wassubstituted for 3He during a neutron pulse, nodetectable output was observed, Thus, the 3Hereaction did indeed pump the laser medium producingthe first volumetric nuclear-puraj/ed laser.

Figure 4 shows the scaling of the laser outputat 1.79u with the average moderator neutron fluxover the length of tae polyethylene moderator. Adefinite lasing threshold was noted at 1.4 * 1016

n/cm^-sec. Nott that with most high-gain c.w.

50 r—

30

3 20

12x10"

Average thermal neutron flux, n/tm -sec

Figure 4. Laser output power (mW) at 1.79ii isshown vs. average moderated neutron flux alongthe length of the quartz laser cell. The totalpressure was held constant at 400 Torr 3He-Ar,10% Ar. The laser threshold for low yield neu-tron pulses was 1.4 * 10*6 n/cm^-sec.

97

noble gas lasers, saturation effects appear soonafter threshold is reached. Nevertheless, the out-put power of the nuclear-pumped laser continues toincrease with neutron flux over the range investi-gated. The highest power output at 1.79M was 50mW with approximately 10sW deposited in the gasvolume.

Figure 5 shows the scaling of laser outputpower at 1.79ti with total pressure. , Here the Arconcentration was held constant at 10%. Lasingtook place from 200 to 700 Torr He-Ar. Furtherstudies are needed to determine the minimum andmaximum pressures for laser operation. Also, thegas mixture of 10% Ar in 3He used in the experi-ments may not be optimum for nuclear pumping. Itis important to note that lasing took place at 700Torr 3He-Ar. This is the highest pressurereactor-driven nuclear-pumped laser to date, anddemonstrates that a homogeneous discharge can beproduced at high pressure with volumetricexcitation.

300 400

Total He-Ar pressure, Torr

Figure 5. Laser output power (mlV) at 1.79p isshown vs. total ^He-Ar pressure with the Arconcentration held constant at 10%. The averagemoderated neutron flux was held constant at7.6 x 1016 n/cm2-sec.

III. Electrically Pulsed He-Ar Laser

Using a fast-burst reactor to pump a laseimedium is a slow process at best. On the average,only five reactor pulses per day can be achieved.Thus, it is desirable to use other means to studyand optimize the laser parameters before nuclearpumping is attempted. With the He-Ar laser ithas been found that a high-pressure (100 Torr)electrically-pulsed laser could be used to opti-mize the laser parameters.

Electrical lasing occurs, in the laboratory,at three wavelengths: 1.79M, 2.31jj, and 1.27M.

Only the 1.79M transition lases at high pressures(> 50 Torr He-Ar) and in the afterglow of thepulsed discharge. Thus, the 1.79M transition wasthought to.be the best candidate for.nuclearpumping for two reasons: First, high-pressure-electrical lasing was achieved which indicatedthat pressure broadening of the transition didnot adversely affect lasing behavior; and, second,electricai lasing too): place in the afterglow of

the discharge which indicated that processes suchas collisional radiative recombination were import-ant and lasing did not depend on a very sharp rise-time excitation pulse. The 1,27M transition wasfound to be selfterminating; lasing stopped insidethe voltage pulse and the transition was ruled outfor nuclear pumping. Further research is needed atthe reactor to definitely show that nuclear-pumpedlasing is not occurring at 1.27M and 2.313p.

T0IA1 f HSSUBE (H.-Art,I0«!

Figure 6. Electrically pulsed laser output at1.79M VS. total pressure of He-Ar in Torrfor varying concentrations of Ar. Electricalpumping (E/P) was held constant.

Energy level Diagram ol He-Ar Laser

Figure 7. Energy level diagram of the He-Arlaser system showing the He atomic metastablespecies (far left), He2 molecular species andthe argon laser transitions of interest.

98

632.8 nm (reference laser*

640.2 nm We)

844.6 nmelectricallyexcited laser

667.8-nm (He) i

706.5 nmlHel j

0.001 torr-O^ 1.6torr-Ne, lOOtorr- He

0.04torr-O2, lOtorr-Ne, 600turr- He

Nuclear Excitation 0.02torr-O2, 5torr-Ne, 300torr- He

Nuclear Excitation a008torr-O2, 2torr-Ne, 133torr- He

427.8 nm (N2 I

xl

X2

xlO

850

WAVELENGTH, nm

Figure 8. Comparison of nuclear and electrically e?;cited^He-Ne-02 spectra. The nuclear spectra at 600 Torris 10 times as intense as the spectra at 133 Tor?. Theaverage thermal neutron flux r s held constant at5 x 1015 n/cm2-sec.

Figure 6 displays the results of a concentra-tion study of lasing at 1.79u versus total pres-sure of He-Ar. From the figure it appears that10% Ar is very near the optimum concentration ofAr in He and, thus, this was the concentrationused in the reactor experiments described earlier.Again, further research is needed to confirm chatthe electrically pulsed afterglow optimum concen-tration of 10% is the same optimum concentrationunder reactor excitation. It is hoped that elec-trically pulsed afterglow studies will help pre-dict other more powerful and efficient nuclear-pumped lasers.

IV. Laser Excitation Mechanisms

Figure 7 is an energy level diagram of theHe-Ar laser system. Note that there is no directmatching of energy levels between the helium andargon atoms. In the electrically pulsed laserexperiments, lasing at 1.79ii was found in theafterglow, thus, direct electron input excitationof the upper laser level should be ruled out. Theonly other process that can adequately account forlaser pumping is collisional radiative recombi-natior of electrons with argon ions. The argon

ions can be produced either by Penning ionization(ty He 23 S metastable) or by direct electronimpact ionization of argon. Collisional radiationrecombination then occurs, eventually populatingthe upper laser level, the 3d [1/2]° state."(The excess energy of recombination can be carriedby either an electron, neutral atom, or molecule.)The 1.79u transition probably has a higher gainthan the 2.313u transition and thus effectivelyquenches lasing on 2.315u at higher presjures.

The addition of chlorine to the He-Ar laserwas found to produce a substantial improvement inJaser output^'1^ (factors of 2 to 6). This hasbeen attributed to the depopulation of the argon 4Slevels by resonant transfer to chlorine. Thus,back excitation from the 4S to the 4P[3/2Jj lowerlaser level is reduced and output power at 1.79uincreases. This same process will be attempted inthe nuclear-pumped laser experiments.

99

V. He-Ne-O^ Spectra by Nuclear Excitation

The spectra shown in Fig. 8 is a comparisonof He-Ne-02 spectra taken under nuclear and

; electrical pumping. The top trace is the: He-Ne-02 spectra with lasing occurring at 8446 21 in' 01 under electrically pulsed excitation. Thelower three traces show ^He-Ne-02 spectra takenwith nuclear excitation using the same laser

; cavity and cell used for the electrical excitationspectra. Note that: nuclear excited lasing at8446 8 is conspicuously missing from the nuclear

• induced spectra. Lasing has not been achieved in, 3He-Ne-02 for total pressures from 30 to 600 Torrand average thermal neutron fluxes from 8 * 10^5to 5 x 1016 n/cm2-sec. Although lasing did notoccur, the spectra of the nuclear and electricallypulsed discharges are very similar. The broaden-

' ing of the nuclear spectra at high pressures is

most probably due to the high-intensity light•spilling over into adjacent channels of the opti-cal multichannel analyzer used for recording thespectra.

These results indicate that the electricallypulsed discharge at high pressures (>50 Torr).nay have the same dominant process as the nuclear-induced discharge. This should help.inraiensely in

prc 'icting and optimizing future nuclear-pumpedlasers. ,•

VI. Summary

Listed in Table 1 is a sumn* >r r of all nuclear-pumped lasers as of May 1976. Th. Jast fournuclear lasers have been pumped with reactors andare the most important for practical applications.Very rapid progress is now being made in this newfield. The two major means of energy deposition,coating and volumetric sources, have been demon-strated, with the goal of a self-critical nuclearlaser still being agressively sought. Afterexamination of Table 1, it is encouraging to notethat operating pressures are rapidly increasing.This is important if high-power nuclear lasers areto become a reality. In the future more attentionmust be paid to higher pressure excimer lasersystems. Such systems are ideally suited fornuclear pumping since they operate at high pressuresand have high saturation intensities. The lasingwavelengths are approaching the visible region ofthe spectrum which is of major interest in laserfusion and isofope separation. The duration of thelasar output is also increasing and has reachedessentially steady-state opeiation. This is ofmajor importance since one of the major advantagesof the nuclear-pumped laser is that it has thepotential of being the only practical steady-statehigh-power laser system. Laser power outputs andefficiencies have been quite small (except for theCO lasei) Lmt this has been due to the high-gain

NUCLEAR PUM-.DLASERS

(May 1976)

Y-PumpedKF laserLos Alamos

y-PumpedXe Amplified

spontaneousemission

Livermore

Fission FragmentCO Laser

Sandia Labs

Fission FragmentHe-Xe Laser

Los Alamos

B10(n,cx) L i 7

Ne-N2 Laser

Univ. of I l l ino is

He3(n,p)H?He-Ar LaserNASA-langley

WAVELENGTH

- 1700 S

5.1-5.6 vm

3.0-4.2 urn

8529 Iand

9393 8

1.79 v

PRESSURE(TORR)

1.3

5,2007,800

10,50013,00015,700

100

200

75-400

200-700

THERMALFLUX

THRESHOLD

(n/cm2 -sec

- 5 x l 0 1 6

3 x l 0 1 5

i x l O 1 5

1.4xlO16

ENERGYOEPOSITE

(J/DTIME

PERIOD

= 92312 nsec

5x10^ to15xl0 J

17nsec

200150 usec

50150 usec

313 t:>1000

10 msec

24 to 270

150 us

LENGTH OFLASER

OUTPUT

12 nsec

31-10 nsecA.S.E.

50 usec

235 usec

6 msec

550 psec

PEAK POWERDEPOSITED

(W/H)

8 x 1 0 1 0

2.7 X 10 1 1

to ,-., x 10n

f.3 x 106

3.3 x 105

3 x 10*to 5

1 x 10a

.6 x 105

t o K.8 x 10 6

PEAK LASEROUTPUT(WATTS)

(J/ i )

S x 10*92.3

Z - 60.?

> .01

2.8 x 10'5

1.5 x 10'3

2.4 x 10"5

0.05

Table 1. A table of all published nuclear-pumped laser. results as of May 1S76. The Y-pumped results were obtainedfrom underground nuclear explosions; the other lasers arereactor-driven.

100

laser systems studied thus far. Future researchmust move in the direction of low gain, high-saturation-intensity laser systems. In this re-gard, the excimer system appears very encouraging.

The assistance of the staff of the AberdeenArmy Pulse Radiation Facility and of Mr. J. Fryeris gratefully acknowledged.

References

1. Schneider, R. T.; and Thorn, K.: Nuclear Tech.27, 34 (1975).

2. Thorn, K.; and Schneider, R. T.: AIAA J. 10,400 (1972).

3. McArthur, D. A.; and Tollefsrud, P. B.: Appl.Phys. Lett. 26, 187 (1975).

4. Helmick, H. H.; Fuller, J.; and Schneider,R. T.: Appl. Phys. Lett. 20, 327 (1975).

5. De Young, R. J.; Wells, W. E.; Miley, G. H.;and Verdeyen, J. T.: Appl. Phys. Lett. 28_,519 (1976).

6. De Young. R. J.; Wells, W. E.; and Miley,G. H.: "Appl. Phys. Lett. 28, 194 (1976).

7. Andriakhin, J. M., et al.: ZHETF, Pis. Red.12, 83 (19,70).

8. Pressley, R. J., ed.: Handbook of Lasers,(The Chemical Rubber Co., Cleveland, Ohio,1971) Chap. 6, p. 210.

9. Dauger, A. B.; and Stafsudd, 0. M.: Appl.Optics 1£> 2690 (1971).

10. Dauger, A. B.; and Stafsudd, p.- M.: IEEE Jour,of Quantum Electronics, QE-6, 572 (1970).

DISCUSSION

ANON: Did you test lasing by detuning thecavity?

R. J. De YOUNG: No, we didn't detune because ofthe experimental difficulty involved. However, wedid, under conditions ideal for lasing, replacethe ^He with ^He in the cavity. We observed nolasing, thus concluding that it is quite definitelythe 3He reaction that is pumping the laser and notany stray gammas or fast neutrons.

101

. , RECENT NUCLEAR PUMPED LASER RESULTS

G. H. Miley, «. E. Weljs, M. A. Akerman and 3. H. AndersonNuclear Engineering ProgramUniversity of IllinoisUrbana, Illinois 61801

Abstract

Recent direct nuclear pumped laser researchat the University of Illinois has concentrated onexperiments with three gas mixtures (Ne-N,, Ke-Ne-

Oj, and He-Hg). One mixture has been made to lase

and gain has been-achieved with the other two. Allthree of these mixtures are discussed with partic-ular attention paid to He-Hg. Of interest for DNP

work is the 6150-A ion transition in Hg . Theupper state of this transition is formed directlyby charge transfer and by Penning ionization.

Introduction

Research on radiation-induced plasmas hasbeen pursued at the University of Illinois since

1963. In the last decade, work has concen-trated on plasmas suitable for nuclear la-sers. " ' Previous reviews of this effort werepresented in Refs. 14-18. Other experiments havedealt with nuclear enhancement of electrically

pumped gas lasers.C19'23) Also, more recently thepossible use of nuclear pumping for laser fusion

applications has been proposed. '

Recent Direct tJuclear jumped (DNP) laser re-search at the University of Illinois has centeredon three gas mixtures. The first is neon-nirrogenwhich has been made to lase in the U. of I. TRIGA

reactor. *• ' The second is helium-neon-oxygen,

for which gain has been measured. *• •* The mostrecent work has concentrated on helium-mercury,in which gain also has been achieved. Work on thefirst two mixtures will be briefly reviewed whileHe-Hg results will be described in detail.

The Ne-N2 DNP Laser

The neon-nitrogen laser is important forseveral reasons. It has the bhortest wavelengthand lowest neutron flux threshold of any DNP laserdemonstrated to date. Also the pumping mechanismappears to lead the way to an important class ofrecombination driven DNP lasers.

The electrical pumping mechanism of the neon-nitrogen laser has been shown to be recombination.

Recent studies by G. Cooper, et al.'- ' indicate

that collisional-radiative recombination of N in-to the upper laser level is the most probablepumping mechanism. The basic question unanswered,

however, is how N+ is so efficiently formed in theradiation induced plasma. As can be seen from theenergy-level diagram of Fig. 1, this would require

5 0 -

40

10

IMPORTAf; r MECHANISMS

IN TH£ Ne-N2 LASER

IM-E9.6 eV

0 N -

Fig. 1. Energy level diagram of *-he Ne-N, DNPL.

a two-stsp process if direct neon-nitrogen col-lisions are involved because the energy levels of

Ne and lower excited states are too low for one-step excitation. Further, there is strong evidencethat absorption of nitrogen on the laser tube wallplays a role in the process. This has been demon-strated by the dependence on the previous historyof nitrogen in the laser cell. We are currentlypursuing research to understand this very compli-cated process.

Since the bulk of electrons in a DNP plasmaare low energy, recombination can be more effi-cient than in electric field sustained plasmas

where the electron temperature is high. '

In present experiments the laser is placednext to the reactor core and external optics arealigned to focus laser light on the slits of twomonochromators and an S-l photomultiplier. Thisis pictured in Fig. 2. A fan, driven by a highspeed electric motor, alternately unblocks andblocks the back mirror so that gain and/or lasingcan be positively identified.

This work was supported by the Physical Science Division of ERIH.

102

REACTOR EXPERIMENTAL SETUP

NEUTRON PULSE ANDTRIGGER LASER OUTPUT RECORDED

.TRIGSER

FWII8 S - l -, BEAM <*v -f

'.HEATHEU-700

1 RCA"31034

JARRELL-ASH B2-0000 5 METER EBERT

Fig. 2. Reactor experimental setup.

While lasing has been achieved with uranium

coated tubes in other experiments, *• " a

0.4 mg/cm coating of B is normally used on theinside surface of an aluminum tube which forms thelaser discharge region. Excitation is then pro-vided by the reaction:

D io 2.3 MeV

Figure 3 shows a typical Ne-N2 laser output

with the neutron flux profile superimposed. The

top trace shows the 8629-A laser line which tendsto cut off near the peak of.the neutron pulse.

The 9393-A transition shown on the trace lasesabout two Billiseconds past the peak. This dif-ference ir. timing is attributed to competition forexcitation between the upper states.

Particular attention should be paid to thechopping of the signal which is the result of afan inside the cavity (see Fig. 2). This providesan easy, but conclusive, proof of lasing eventhough the laser is behind 11 feet of concreteshielding.- The saire technique provides a sensi-tive method to measure gain and this will be dis-cussed later with regard to optimizing He-Ne-02

and He-Hg mixtures. In addition to these detailsof the Ne-N2 laser, a peculiarity with respect to

the tine histoty that may hold the key to the Nproduction mechanism in the reactor should bementioned. When a small amount of N2 is added to

the normal mixture, lasing almost stops during thenext pulse. Subsequent pulses where no N^ is

added yield much J-arger laser signals. The cycleis repeated when another small amount of nitrogenis added. This behavior, diagramed in Fig. 4, isinterpreted as resulting from nitrogen being ab-sorbed on the wall such that emission during the

reactor pulse in some way contributes to N pro-duction and subsequent lasing by recombination asdiscussed earlier.

He-Ne-02 Optical Gain

Early work with a Ne-02 laser utilizing B

as the source of excitation showed promise.However, results were erratic and it was difficultto control the small 02 concentration required due

to absorption on the tube wall. The discoverythat the addition -<f large amounts of helium madehigh pressure electrical lasing possible was a

breakthrough/34' It also made possible the use

of the He (n,P)T reaction to produce excitation inthe reactor, effectively controlled the oxygenabsorption and provided effective pumping by col-

N e - N ? DIRECT NUCLEAR PUMPED LASER

9393ALASER OUTPUT

8629ALASER OUTPUT

PEAK NEUTRONPULSE

TOTAL LASER ANDSPONTANEOUSOUTPUT(S-l PHOTOMULTIPLtER)

5 msec/DIV.

Fig. 3. Oscilloscope trace of Ne-N2 DNPL. Shows

laser output vs . time.

| 1 1 1 r

iOJAL_PE.A^LASER.OUTPyT_.ys,.

2.5»10'5n/cm2sec PEAK FLUX150 TORR NEON

° \\

\\\1

/

1

\ N2

\ ADDED It

\

\\

\ i

j 1 1

PULSE

\

\

VX

\* <.,\ ™2\ ADDED

\ !\ i

ri

/

REACTOR PULSE NUMBER

Fig. 4. Variation of tota! output vs. number ofreactor pulse in a series of 10 pulses.

103

lision transfer. While actual lasing was notachieved, extensive chopper experiments (usingthe fan as previously described to alternatelyunblock and block the cavity) conclusively demon-strated gain on the 8446-A line (the same lineused in-the electrical He-Ne-O2 laser). This is

illustrated in Fig. 5 where the chopped output from

8446 A is shown along with two other lines, 777S A

and 8780 A. Kith helium pressures of 700 torr anda 0.2% (>2 concentration, gain was observed. The

unblocked to blocked ratios of the two other lines,though within the high reflectivity bandwidth ofthe cavity mirrors, are roughly two, i.e.' near thevalue expected for spontaneous emission. Sinceneither line is expected to invert, this demon-strates that the technique is working when com-

pared to the 8446 A output where ratios approaching5 are observed. Such measurements indicate a gainof ~0.9%, has been achieved with a mixture of 0.2%02 and 0.52% Ne at pressures up ta 1 attn. Optimal

mixtures are discussed in some detail in Ref. 28,and it is thought that higher neutron fluxes andrise time could lead to sufficient gain to make anattractive laser. If successful this could replaceNe-N2 as the shortest wavelength to date.

He-Hg Optical-Gain

This mixture differs from the first two mix-tures in that energy transfers directly fromhelium ions and metasr.ables to the upper laserlevel via thermal-energy charge-exchange and Pen-ning ionization of metastable states, respectively,(see Fig. 6) This pumping mechanism is compatiblewith radiation-induced plasmas. The potential use

of He to achieve a volume source of excitation at

UNBLOCKED-BLOCKbD RATIOS FOR

TRANSITIONS IN OXYGEN a NEON

3OoVi-

CAVITY'TUNED

B446A-0

77754-0

8780A-N.

TIME (lOmsec/cm)

Fig. 5. Unblocked-blocked ratios vs. tine. Showscomparison between 8446 A oxygen tran-sition exhibiting gain, and two othersthat do not.

HELIUM-MERCURY LASERENERGY LEVEL DIAGRAM

20eV

lOeV -

OeV<—He

Fig. 6. Energy level diagram for the helium-mercury system. The 6150 A-transition

from'the 'P5,2 level is shown.

high pressures is an important feature of He "<»Finally, when it lases, it will provide an tin the visible range.

The 61S0-A transition was first men cd asa possible Direct Nuclear Pumped Laser by

Andriakhin who reported a large light outputin the visible region of the spectrum when a He-Hglaser was operated in a pulsed reactor. At theUniversity of Illinois a hollow cathode laser de-sign was adapted to operate next to the reac-tor core. The hollow cathode was a 60-cm lengthof aluminum cylinder 2.5 cm in diameter with an

internal coating of B . It seives a dual purposeof being a hollow cathode for the electrical laserand providing the nuclear reactions for the DNPlaser. The electrodes were contained in a pyrextube that was wrap7id with three sections ofheater tapes. By heating the tube,the density ofmercury within can be controlled. Three chromel-alumel thermocouples are used to monitor tempera-tures along the laser. Mercury is contained intwo reservoirs which are maintained at a tempera-ture slightly lower than that of the main tube.

104

Again, a chopping fan in the laser cavityprovides unblocked to blocked measurements (seeFig. / ) . The laser is mounted on an aluminum car-riage and placed next to the core of the reactor.

HELIUM-MERCURY LASERHOLLOW CATHODE DISCHARGE

Fig. 7. He-Hg laser and carriage used in theseexperiments.

The placement of the laser is similar to theNe-N2 experimental arrangement shown in Fig. 2.

The laser is evacuated with a forepump, and thenfilled to the desired helium pressure When thereactor is pulsed, signals such as those shown inFig. 8 are observed. The top trace shows the nodu-

lated 6150-X signal produced by the chopper fan.The middle trace displays the neutron pulse whilethe bottom trace shows the modulated signal with

the monochromator set at 6560 A. The light thus

collected could be from the 6560-A He-II transi-

tion, or the 6563-A H-I transition due to a gas

impurity. As with the Ne-N, experiments shown

earlier, this transition should not be inverted.It lies within the high reflectivity range of thecavity, hence, serves as a reference to demon-strate that no extraneous effects are occurring.As expected, the unblocked-blocked ratio for it is

o

~2, whereas the 6150-A ratio is -5 providing aclear demonstration of gain.

Unblocked to blocked ratio data from Fig. 8are shown as a function of neutron flux in Fig. 9.

oThe 6560 A ratios remain at ~2 within the accuracy

oof the measurement. The 6150-A ratios increase

from ~2 at 4 x 1014 n/cra2-sec to 4.8 at 3 x 1015

2 14n/cm -sec, sugg.sting a threshold for gain <10n/cm -sec. In later experiments, the mirroralignment and mercury pressure were optimizei' andeven higher ratios, approaching 8 were obtained.This is also shown in Fig. 9.

RATIO COMPARISON

600T0RR HELIUM6mT0RR Hg

3.3xio'5n-cm2-sec'

Fig. 8. Ratio comparison showing 6150-A Hg-IItransition with gain, and another tran-sition for which gain is not expected.The helium and mercury pressures were 600Torr and 6 mTorr respectively.

UNBLOCKED/BLOCKED RATIOS

VS

NEUTRON FLUX

6 6 0 A

CAVITY

OPTMIZEO

6560 A

600 TCRR HELIUM

6 mTORR MERCURY

0 1 2 3 4

NEUTRON FLUX (X's |O15]

Fig. 9. Bottom two curves: Variation of un-blocked-blocked ratios vs. neutron fluxfor Fig. 8. Top curve: The 6150-Aratios vs. neutron flux for more optimalconditions.

The gain can be predicted from our unblockedto blocked ratios using the approximate ray-tracing theory oS Ref. 28. In the present case,gains up to 1.5%/m are estimated. Losses for thepresent cavity are >5%, but if these losses canbe reduced somewhat, the gain may be sufficientfor lasing. If not, use of a higher neutron flux,such as can be obtained with a fast burst reactor,may be required.

Another important variable is the gas pres-sure. Preliminary attempts to optimize this areshown in Fig. 10 in which the unblocked to blocked

105

10

8

o§ 6a:

4

2

0

UNBLOCKED / BLOCKED RATIO

AT 6150 A

MERCURY PRESSURE

4 mTORR

?ig. 10. Ratios vs. helium pressure, showing amaximum value around 600 Torr.

ratio is shown as a function of helium pressurefor a fixed mercury pressure (~4 mTorr). A broadpeak is observed near 600 Torr helium pressure.A similar curve with mercury pressure as thevariable shows ~4 mTorr to be the optimum mercurypressure.

He-Hg Spectra

.* A camera compatible with the CCA McPhersonEU-700 monochromator was constructed to photo-graph the spectra generated in reac-or tests ofDNP lasers. Figure 11 shows a side view cutawayof the reactor thruport, in which the laser plasmatube is placed, while the nonochromator and cameraare at the exit. Poloroid 57 film was usedto photograph the spectra, though the camera iscompatible with any 4"x5" sheet film.

Figure 12 is a photograph of the helium-

mercury spectra between 5450 A and 6400 A. ^'

The left strip shows the spectra created during a

TRIGA REACTORCORE

Fig. '11. Reactor arrangeaent for making photo-graphic spectra measurements (r.ot toscale).

HELIUM-MERCURY SPECTRA

I-5461-1

-2848-

O 100 200 300 400 500 600 700 800

HELIUM PRESSURE, TORR

-6150-•

i100 TORR !00 TORR 5 TORR

1 . t 1NUCLEAR ELECTRICAL

EXCITATION EXCITATION

Fig. 12. Spectra for three different conditions:

On the le-.'t the 6150-1 line is present at100 Torr with nuclear excitation, whiletwo electrical excitation spectra areshown at right for comparison.

neutron pulse at 100 Torr helium pressure. Theright strip shows the same tvelength range at 5and 100 Torr, but with electrical excitation only.

It is observed that the 6150-A line is presentin the 5-Torr electrical case but disappears at100-Torr. In the nuclear case it remains clearlyvisible at l^Q-Torr, emphasizing that there is adifference i.i pumping mechanisms whereby highpressure operation with nuclear pumping may beuniquely possible. The excitation mechanisms arenot well understood yet, however, so the detailsof the mechanisms involved must await furtherstudy.

There ar^ other differences between the twospectra that do not show up in this reproduction

of the spectra. Two Angstrom resolution has beenachieved with the camera-monochromator combination

and an integrated neutron flux of 6.6 x 1014 n/cm2.

Conclusion

Several schemes for producing DNP lasers hav^been described along with the experimental workthat has been accomplished to date. A nuclearpumped laser operating on two wavelengths in thenear IR and two other systems that have nuclear-induced population inversions have been discussed.One of these, the He-Hg system, potentially offersa visible laser. In addition, a simple means ofidentifying gain using a modulated cavity has beendescribed. Finally, a means of photographing spec-tra and some results with He-Hg have been de-scribed.

DNP laser research at the University ofIllinois has centered not only on a search for

106

high pressure, high power nuclear lasers, but alsoon a search for the unique mechanisms that willallow suitable systems for future applications tobe predicted.

Figure 13 lists a summary of nuclear lasersproduced as of May 1976,C27,30-32,35,36) thatutilize research reactors and have been describedin archival literature. Referring to the Illinoisresults, the Ne-N2 laser exhibits the lowest ther-mal neutron flux threshold, shortest wavelengths,-and longest duration of any of the DNP lasers.Further, it appears to employ a recombinationpumping scheme, that could be the precursor ofother more efficient and powerful lasers. TheHe-Ne-O, and the He-Hg systems are both promisingin that gain has been observed experimentally.They represent a search for suitable lasers that

utilize He as a volume source, i.e., they car;operate at high pressure, and also have wave-lengths in or near the visible.

NUCLEARPUMPEDLASERSMAV1976

3 H e - H gMOSCOWSTATE

COSANDIALABS

He-XeLOS ALAMOS

UNIV. OFFLORID*

Ne-N2

UNIV. OFILLINOIS

3 He-ArNASA

LANGLEY

PUMPINGREACTION

3He(n,p)T

235U(n,F)FF

235Uln,FJFF

235U(n,F)FFond

°B(n,o<l7LJ

3He(n,p)T

WAVELENGTH THERMALFLUX

THRESHOLD(n/cm2-sec

LASING IMPLIEDBUT UNVERIFIED

5.1-56/1

3.5/1

8629Aand „

9?93A

l.79,i

. ~5«IO16

3XI0 1 5

i»,o1 5

I.W6

LENGTHOF

LASER[OUTPUT

1 msec

SO/isec

!50/isec

6 msec

365(1 sec

PEAKLASERPOWER

7

E-6W

>.OIW

l.5mW

50mW

Fig. 13. Summary of DNPL experiments performedwith research nuclear reactors.

References

2.

G. H. Miley and P. Thiess (eds.), "Proceedingsof the Illinois 1963 Summer Institute onDirect Conversion," Vol. I, II, and III.Sponsored by AEC-ASEE; published, N.E. Program,University of Illinois (1964).

G. H. Miley, "Fission Fragment Energy SpectraEffects in Fission-Electric Cell EfficiencyCalculations," Trans. Am. tiuol. Society, 7,No. 2 (1964).

3. A. K. Bhattacharya, L. Goldstein, F. T. Adler,J. T. Verdeyen and E. P. Bialecks, "MicrowaveMeasurement of Dynamic Reactor Response,"

Appl. Phys. Letters, 5, 242 (1964).

4. G. H. Miley, "Fission-Fragment Effects as Re-lated to Fission-Electric-Celi Efficiencies,"lluc. Sai. andEng.,.24, 322-331 (1966).

5. G. H. Miley, Direct Conversion of NuclearRadiation Energy, American Nuclear Society,Hinsdale, IL, pp. 7-12 (1970).

6. J. C. Guyot, G. H. Miley and J. T. Verdeyen,"Metastable Densities in Noble-Gas PlasmasCreated by Nuclear Radiations," J. of Appl.Pkye., 42, 5379 (1971).

7. J. C. Guyot, G. H. Miley and J. T. Verdeyen,"Application of a Two-Region Heavy ChargedParticle Model to Noble-Gas Plasmas Inducedby Nuclear Radiations," Nue. Sai. and Eng.,48, 373-386 (1972).

8. P. E. Thiess and G. H. Miley, "Optical Emis-sion from Noble-Gas Plasmas Created by AlphaParticles," Trans. ASS, 16, 134 (1971).

9. P. E. Thiess and G. H. Miley, "Optical Spectraof High-Pressure Helium Irradiated by AlphaParticles," Trans. ANS, IS, 706 (1972).

10. P. E. Thiess and G. H. Miley, "New Near-Infrared and Ultraviolet Gas-ProportionalScintillation Counters," IEEE Trans. Hue.Sci., NS-21, p. 125 (1974).

11. P. E. Thiess and G. H. Miley, "Optical SpectraProduced in Neon and Argon Mixtures byIonizing Radiation," Trans. Am. Nuel. Soo.,19, 64 (1974).

12. P. E. Thiess and G. H. Miley, "Optical Spectraof High-Pressure Helium Mixtures Irradiated byAlpha"particles," Trans. Am. Nucl. Soc., 16,56 (1971).

13. J. C. Guyot, G. H. Miley and J. T. Verdeyen,"Calculated and Measured Atomic MetastableDensities in Noble-Gas Plasmas Induced byNuclear Radiations," Trans. Am. Nucl. Soc,16, 135 (1971).

14. J. C. Guyot, G. H. Miley, J. T. Verdeyen andT. Ganley, "On Gas Laser Pumping Via NuclearRadiations," Trans, of the Symp.. of Researchon Uranium Plasmas and their TechnologicalApplication, NASA SP-236 (1970).

15. G. H. Miley, "Nuclear Radiation Effects onGa Lasers," in Laser Interactions, (Schwarzand Hora, eds.). Plenum Press, pp. 43-57(1972).

j,,. G. H. Miley, J. T. Verdeyen, T. Ganley, J.Guyot and P. Thiess, "Pumping and Enhancementof Gas Lasers via Ion Beams," 11th Symp. onElectron, Ion and Laser Beam Technology(R.F.M. Thomley, ed.), San Francisco Press,Inc. (1971).

17. G. H. Miley, J. T. Verdeyen and W. E. Wells,"Direct Nuclear Pumped Lasers," Proc. 28thGaseous Electronics Conf., BB-1, Univ. of MOat Rolla (197S).

18. G. H. Miley and W. E. Wells, "Direct NuclearPumped (DNP) Laser," IXth Int. Conf. onQuantum Electronics, Amsterdam, The Nether-lands, B-S (1976).

107

19. T. Ganley, J. T. Verdeyen and G. H. Miley,"Enhancement of CO2 Laser Power and Efficiency

by Neutron Irradiation," Appl. Phys. Letters,18, no. 12, p. S68 (June 1971).

20. R. J. Dejoung, W. E. Wells and G. H. Miley,"Enhanced Output from He-Ne Laser by NuclearPreixmization," 1974 Int. Electron DevicesMeeting, Wa£,"iin£ton, DC (Dec. 1974).

21. R. ,?. DeYoung, E. Seckinger, W. E. Wells andG. H. Miley, "Studies of Nuclear RadiationEnhancement' and Pumping of Noble Gas Lasers,"Paper 2A10, 1974 Int. IEEE Conf. on PlasmaScience, Univ. of TN, Knoxoille, TN (1974).

22. R. J. DeYoung, M. A. Akerman, W. E. Wells andG. H. Miley, "Studies of Radiation-InilucedLaser Plasmas," 1975 Int. Conf. on Plasma,3D2, Univ. of MI, Ann Arbor, MI, 75CH0987-8-NPS (1975).

23. M. A. Akerman, M. Konya, W. E. Wells andG. H. Miley, "Nuclear Radiation Enhancementof a Helium-Mercury Laser," Trans. Am. Dual.Soo., 22, 133 (1975).

24. G. H. Miley, Fusion Energy Conversion,Published by ths American Nuclear Society(1976).

25. G. H. Miley, "Direct Pumping of.Lasers byFusion Reactors," Trans. Am. Nual. Soo., 15,£•33 (1972).

20. H. E. Wells, "Laser-Pellet Fusion by EnergyFeedback to a Direct Nuclear Pumped AuxiliaryLaser," Proc. 797c IEEE Conf. Plasma Science,3D4, Univ. of MI, Ann Arbor, MI (1975).

27. R. DeYoung, W, E. Wells, G. H. Miley andJ. T. Verdeyen, "Direct Nuclear Pumping of anNe-N2 Laser," Pppl. PhyB. Letts., 28, 519

(Ma/ 1976).

28. R. .1. DeYoung, W. E. Wells and G. H. Miley,"Optical Gain in a Neutron-Induced 3He-Ne-0-,Plasma," Appl. Phys. Letts., 23, 194(Feb. 1976).

29. G. Cooper, J. T. Verdeyen, I1,. WeZls and G. H.Miley, "The Pumping Mechanism for the NeonNitrogen fiuclear Excited Laser," The ThirdConf. on Partially Ionized Plasmas, Prince-

• tori University (1976) '.

30. D. A. McArthur and P. B. Tollefsrud, "Obser-vation of Laser Action in CO Gas Excited Onlyby Fission Fragments," Appl. Phya. Letters,26, 181 (1974).

31. H. H. Helmick, J. L. Fuller and R. T.Schneider, "Direct Nuclear Pumping of aHelium-Xenon Laser," Appl. Phys. Letters, 26,327 (1975).

32. J. W. Eerkins, Technology Review Program forResearch and Technology Division, Air ForceSystems Command (Jan. 1967) (unpublished).

33. E. L. Seckinger, "Study of the Neon-OxygenLaser undo-.- Heavy Particle Bombardment,"M.S. thesis, Univ. of Illinois (1974)(unpublished).

34. R. J. DeYoung, W. E. Wells and G. H. Miley,"Lasing in a Ternary Mixture of He-Ne-02 at

Pressures up to 200 Torr," J. of Appl. Phys.,47, No. ? (April 1976).

35. V. H. Andriakhin, V. V. Vasil'nov, S. S.Krasil'nikov, V. D. Pis'mennyi and V. E.Khvostinov, "Radiation of Hg-He3 Gas MixtureBombarded by a Neutron Stream," JETP Lettei's,12, 2, p. S8 (July 1970). ?

36. H. Wieder, R. A. Meyers, C. I. Fisher, C. G.Powell and J. Colombo, "Fabrication of WideBore Hollow Cathode Hg+ Lasers," R.S.I., 38,No. 10, p. 1538 (Oct. 1967).

37. M. A. Akerman, «. E. Wells and G. H. Miley,"Charge Exchange Phenomena in a NuclearRadiation Produced He-Hg Plasma," 1976 IEEEInt. Conf. on Plasma Science, 3C10, Univ. ofTX at Austin, 76CH1083-S-NPS (1976).

38. N. W. Jalufka, R. J. DeYoung, F. Hohl andM. Di Williams, "A Nuclear Pumped 3He-ArLaser Excited by the 3He(n,p)3H Reaction,"To be published, Appl,Aug., 1976.

Phys. Letters,

108

NUCLEAR FISSION FKAGMDT UCITATION-..'.-. OF ELECTRONIC TRANSITION USIR MEDIA

.. D. C. Lorents, H. V. McCusker, and C. K. Rhodes. ' Molecular Physics CenterStanford,Research institute, Menlo Park, California M035

. .'• A b a t r a c t ' ; • ••' ..;•' : •' •' "

> ; . . . ',. •; . - . . : • - . . • ' • ' • - ' " ' , . ; • • • . • • ' , ',

Tha propartlaa of high -energy electronic tranii-tlon laaara axcited by fission fragments 'a're •••. 'examined. SpacUlq charactarlatica of tha aadlaincluding d*nBity,'excitationrateB, wavelength,kinatlca, 'fissile material; scale ai*e, and medium •uniformity are.assessed. Tlie use of epithermalMutrons^ homogeneously.mixed fiaalla material, and

'special high cross aactlon nuclear'isotopes to op-timise coupling of the energy to the medium are'shown- to ba important conaldarationa maximising thjscale size, energy deposition, and medium unifor-mity. , A performance limit point of :~ 1000 J/lita/.1

in ~ 100 voiee pulaaa.ia aatabiiahed for'a larga 'claaa of syatans' operating in tha near ultraviol'.taid viaible .spectral r«jion», It ia damonatrataiithat *-bea* axeltation can be uaad to aiwilatenuclear puaping conditions to facllitatta the searchfor candidate aedia. Experimental data.for <the

vkineticn of a'XeF* laser operating-in Ar/Xe/Fj/OFgmixtures are given.' These;raactor-puaped systemsaxe. suitable for scaling to voiaua on the order of

Table I

Operating characteristics of SPR III

,_.••••'•• :•..•..-:•'•"'••"•'•;- -i» ' I n t r o d u c t i o n . •-..'.' .,;;'"'

* - ? 1 ' " " • ' • ' • • • ' ' " ' " " ' " . * • • • •• " • ' ' •

Pulsed neutron generators are a compact sourceof enorabus energy in; the fora of' fast fissionfragments. The Sandia SPR III reactor, whose

1operating; paraaeters are suanarisad in Table I, ra-preaenta a modern configuration which dissipatesseveral i^gajoulea of'energy in a single pulse. 'Efforts have W e n underway for aeveral years torealise methods for producing coherent radiantenergy with a nuclear anargy source.''' The first-successful efforts.to obtain laser oscillation in

.fisaion-fragaeat-puaped media were recently report-ed in several laboratories,3 In this paper,,wewilldiscuaa the considerations necessary to efficientlyconvert fission fragment energy to coherent radia-tion. Several critical, aapecta of this couplingwill be exaained, particularly specific aediuacaaracterietics such as density,.wavelength, kine-tic*, fissile material', scale sise, energy deposi-tion, and uniformity'. We will show that officiantcoupling of a.pulsed reactor with m appropriategain medium will result in aegajoule outputa of.coherent energy at ultraviolet and viaible wave--

Neutron Flux (cavity) r '<" 1019 n-see~l ca"a

Prompt Neutron Pulse Width -~ SO fee

Average Neutron Energy ~ aoo keV

Cavity Volume • ~ IS liter

II. Keactor-Medlum Coupling'

A. Inera-y Deposition. Medium Penalty and Coapoaition

Optical flasion-fragaent-excltad laser systemsrequire efficient coupling of the fission fragment.energy to the laaer nediua. The basic fissionprocess involving uranium la

+ U 3 * zi za

v n

which released ~ 165 NeV of kinetic energy In thetwo highly, ionised fragments. These fission frag-ments transfer their energy to a host aediua byseveral processes, including charge exchange, colll-alonal stripping, direct lonlzation^ and Auger pro-cesses, all of which lead to ionlza'tioh- and excita-tion of the host material. As ,tbe considerations ofLaffert, sees, and Jamaraon* indicate,'the energyper ion pair produced by fission tragaenta in raregases, Wff, is close.to the corresponding value,W#, for electrons. This is reasbaable physicallybecause the ionization and excitation In both casesla produced largely by secondary electrons at ener-gies much lower than that of the primary particle.To a good first approximation, then, ~ 50% of theenergy depoalted in-dense'rare gases by fiasion frag-pents will produce electronic excitation, just as Inthe case of electrona.5 Therefore, we can apply theknowledge'gained recently froa energy transfer stu-dies of e-beaa-excited rare gases and rare gasmixtures to the potential fisslon-fragment-puaped .m e d i a . .. •'•• • v ' ''.'". ' v ; ' ""' '.- •'' ' "• ;-.-

In general, we desire a aediua whose propertiesara such that it minimises the kinetic lossesthrough efficient coupling to the upper laser level *while simultinoously utlllilng physical aechaalsaathat.are basically insenaitive to the kinetic teap-erature. The maiii limitatlona om the optical- gala 'Include gas heating, optical absorption by allspecies present, and medium lnaoaogaalety. The 11- 'aitatlon due to gas heating arise* froa the fact'

109

that many laser molecules dissociate at elevatedtemperatures. This effec'; establishes an upperlimit on the energy deposited in the gas (andtherefore on energy output), and we can estimatethat upper limit as follows. The deposited energy,E d, is related to the optical efficiency -7), themedium density p, and the maximum permissibletemperature rise of the medium ATg, by the speci-fic heat equation

3pkAT,B

2(1-11) (2)

For example, if we restrict ATg to £ lOOC^K andassume T| = 0.10 and a host material consisting ofargon at a density p^r =" 10 cm"*5 (~ SO amagats),these conditions permit an energy deposition E d of2 x 10 joules/liter and provide (at the statedefficiency) ~ 2 x 103 J/liter of laser energy.

At the high gas density indicated, the fissionfragment range will be low Ci'jj < 1 nra1).4 Thisimposes the condition that the fissile material behomogeneously incorporated into the laser mediumif we are to achieve uniform deposition over dimen-sions R ^> Rff• Homogeneously mixed fissile mater-ial also insures efficient deposition of the fissionfragment energy in contrast to configurations in-volving foils of fissile material located at theboundary of the medium, for which only ~ 20% of theenergy is deposited in the gas. On the other hand,it requires the existence of a fissile material(such as UFg) that is volatile at moderate tempera-tures.

The density of fissile material required to"-generate a given energy deposition can be calcula-ted from the relation

Ed = PfTsfTsf (3)

where j n is the neutron flux, < O"j > is the fissioncross section averaged over the neutron energyspectrum, T is the neutron pulse duration, and e^is the energy released per fission. The cavity .neutron flux typical of current pulsed reactors(SPR III) is approximately 10 1 9 n/cm2-sec in an~ SO \xsec pulse as indicated in Table I. In orderto maximize the fission cross section of the fis-sile material, the neutrons must be moderated tolow energies. We assume that this moderation pro-cess can reduce iialf the neutrons to E n ^ 0.4 eVwithin a pulse length of ~ 10 see" (passage of'the neutrons through 2.5 cm of polyethylene willproduce this partial moderation). Fission crosssections 7'8 for U ,34 and Am242"1 as a function ofneutron energy are presented in Figure 1; thesecurves show the importance of moderating the neu-tron energies.

For these conditions, fission-generated energydeposition is shown in Figure 2 as a function ofII235 and Am m. These estimates were made assum-ing a flat energy distribution for neutrons overthe range 0 < E n

s 0.4 eV, We see that ~ 100 torr2 3 5 leads to a deposition of 6 kj/4, while the

r! I I I I I I I I I I I I I I I

0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2

E n ieV)

Fig. 1 Fission cross sections Cj as a function of .neutron kinetic energy E n for U

23^ and Am 2 4 2 m.

same energy density can be achieved with approxi-mately one tenth the fissile material density forAm becausa of its larger fission cross sectionat these neutron energies. Reducing the neutronenergies to thermal values would in turn reduce theamount of u""7 required for a given energy deposi-tion, but it would also increase t.ne pulse lengthand reduce the power density. On the other hand,americiuni has a high fission cross section for neu-tron energies as high as 1 eV. Other transplutonicmaterials such as Cm24^ which has a fission crosssection ^>1(' and other properties intermediate1*between those of u 2 3 5 and Am 2 4 2 m will be discussedin section E. These densities of fissile materialare low enough that they may not interfere with thebasic kinetics of the excitation process; we ad-dress this point later.

The ability to deposit the maximum permissible' energy density in a laser gas mixture can beachieved with e-beams as well as with this proposedfission-fragment-pumping technique. A aajor advan-tage of the latter technique is that the pumpedvolume is limited only by the range of low energyneutrons, which is on the order of meters, wherease-beam systems are limited by the range of highenergy electrons, which is on the order of centi-meters. Furthermore, the weight and volume of areactor pump is expected to be substantially smal-ler than that of an e-beam pump of comparableenergy.

Be conclude on the basis of the above considera-tions that the laser medium for a high energyfission-fragment-pumped laser will consist of a

110

i i r j i i i i i i i i r i r

NEUTRON fLUX a 2.5 x 10"tan2

En < 0.4 .V

PULSE LENGTH K* at

I I I I I0 10 20 30 40 50 GO 70 W 90 100 110 120 130 140 150

FISSION MATERIAL DENSITY (ton)

Figure 2 Deposited energy Ed as a function offissile material density pf for U

2 3 5 and Am242m

assuming the performance characteristics of theSPR III pulsed reactor as noted in the text. Thevapor pressure of UFg (111.9 torr) at 25°c isindicated.

large volume, high density gas (e.g., argon) con-taining a homogeneous mixture of ~ 10 to 100 torrof a volatile fissile material. The ai.ual lasermolecule could be any of several candidates alreadydemonstrated with e-beam-excitation, e.g., Ar-N2,Ar-J-2, XeO, ArF, KrF, or XeF. It is essential thatthe laser kinetics allov/ essentially cw operationin order to utilize all the excitation from therelatively long pulse of neutrons, but this isapparently the case in the systems suggested.

B. Scale Dimensi -ng and Medium Uniformity

The maximum volume that can be pumped by neutron-induced fussion is set by the range of neutrons inthe medium, which is set in turn by the scatteringand absorption lengths for the neutrons. Thescattering length will be governed by the primaryconstituent, say Ar, which has a total scattering „cross section for thermal neutrons of ~ 10~24 cm2

ponds to a scattering length of ~ 1C meters. Theabsorption will be due to the volatile fissilespecie; total neutron capture cross sections are 1.5to 2 times the fission cross sections shown in Fig-ure 1. The absorption length will in fact be con-trolled by the energy deposition by the relation

JnT ef (4)

for .the conditions described above, this is on theorder of 3 meters. Thus, the maximum dimensions ofthe pumped volume will be governed by absorption ofneutrons. It is this ability to uniformly pump suchlarge volumes that provides the major scaling advan-tage of taia proposed techDique.

The range of the neutrons is sufficiently largethat the transit time of moderated aeutrons throughthe gas can exceed the neutron pulse length. Infact, for 0.2 eV neutrons (vn ~ 6 x 10

5 cm/sec), thedistance travelled during a 100 t*sec pulse is ~ 60cm. For very large volumes, the neutron pulse willbe essentially a travelling excitation wave. Thisshould not be a problem if appropriate output coup-ling of the laser energy can be achieved.

It ijj important that the neutron velocity be largecompared to acoustic velocities, vs, so that pulse-generated disturbances do not disturb the opticalhomogeneity of the flow.12 Since vs =5 5 x 10

4 cm/sec, this is always true. However, acoustic dis-turbances could arise near the boundaries at the endof a long pulse, since R di S t u r b a n c e ~ vs-T ~ 5 cmfor T = 10"^ ser Use of'fissile materials withlarge fission cross sections for epithermnl neutronsallows minimal pulse lengths, which also m.nimizesthe effects of acoustic disturbances.

C. Kinetic Properties

Fission-fragment excitation of dense atomic gasesoccurs, as noted earlier, by the sai.te basic mecha-nisms as excitation by energetic electrons. Thisenables direct utilization of the considerable kine-tic knowledge acquired in studies of electron-beam-excited material. In particular, electron beamsexcite the rare gases and produce metastable atomsand excimers with an energy efficiency5 of ~ S0%.In addition, several e-beam-pumped laser media havebeen demonstrated in the visible to near uv rangethat utilize energy transfer from excited rare gasdonor species to either atomic or molecular accept-ors. Among these are Ar/l^, Ar/^, XeO, raie-gas-halides and He2

+/H2'

(Ar40). For Ar densities of 102VcmJ, this corres-

It follows that these previously demonstratedmedia will function properly under excitation byfission fragments provided that (a) at the densitiesrequired the fissile additives do not interfereappreciably with'the kinetic processes, (b) tr.cneutron transport, apart from fission, is not ad-versely affected, (c) any additional optical lossesarising from the fissible additive are sufficiently

l i l

small', and (d) the laser medium can operate quasi-cw. Points (a) and 'c) will establish an upperlimit on the fissile additive, and thereby theachievable energy deposition as shown in Fig. 2.Point (b) is not a limiting factor for the mater-ials, densities, and scale dimensions under consi-deration. Issue (c) is examined in section D be-low.

It is important to note that the kinetic studiesthat will be required to evaluate candidates forfission-fragment excitation can be 'alidly perfor-med on media excited with electron beams over mostof the desired parameter space* The comparisonbetween e-beam and neutron-induced fission excita-tion is illustrated in Figure 3. In principle,electron beam machines can be used to simulatenuclear excitation and demonstrate lasing charac-teristics within the entire space below the limita-tion established by the foil, which includes allbut the highest energy operating points in Fig. 3,Roactors such as SPR III can be used tc evaluatethe operational behavior at the limits of the highenergy performance of media whose kinetic and opti-cal characteristics had been obtained by simplermethods.

) M I I I ] TTI

Figure 3 Excitation rate (P/V) in watts/cm3 as afunction of excitation time T for high pressurerare gas materials by conventional pulsed electronbeam devices and fission fragment concepts coupledto a pulsed reactor. The electron beam foil limi-tation is indicated.

From the kinetic viewpoint, we desire a minimumrequired fissile additive density pf. This favorsthe use of high fission cross section materialsand high neutron fluerice reactor concepts [c.f.

Eq. 3J. Although many volatile compounds involvingfissile material are known, UF£> because of itshigh vapor pressure at room temperature and itslow radioactivity, is an obvious initial candidate.

To test the feasibility of these suggestions, wehave made e-beam excitation studies of the fluores-cence of XeF from mixtures of Ar/Xe/F2 to whichdepleted UFg v/as added. Measurements of the fluor-escence spectrum, fluorescence intensity, andfluorescence decay time r" Jxl- ~ 3511 A wfire madeon the gas mixtures' excited with a Febetron 706and the results are shown in Table II. We notefirst that the XeF is not formed in material con-taining only UFg. In light of the quenching datanoted below, we conclude that the reaction

k(5)

is relatively slow, with a rate k s 10 cm /sec.

Table II

XeF fluorescence measurementsin Ar/Xe/F2/UFg mixtures

(pressures in torr)

Xe* + UF5 -» XeF* +

Ar

1500

1500

1500

1500

760

760

760

760

Xe

10

10

10

10

40

40

40

40

F2

4

4

0

o

4

4

4

4

UF6

0

4

4

100

0

4

10

50

XeF SpectraObserved

normal

normal

none

none

Emission

Intensity

1

1

0.6

0.36

Xenon fluoride emissions are easily seen with 4torr of F2 and no UFg; when 4 torr of UFg is addedto this mixture no change is observed in the spec-trum intensity, or decay rate of the XeF . Formixtures containing 50 torr of UFg and only 4 torrof F2, the XeF* is approximately one-third ad in-tense as with no UF-, indicating that X^ quenchingby UFg is of the order of 0.1 thai of F^. Theseresults indicate that UFg is quit-' inert it> reac-tion with electronic excited states, and may beadded in quantities at least equel to the laseradditive. These observations ha''e extremely impor-tant and positive implications for fission fragmentpumping using UF6-

D. Medium Optical Characteristics

Optical losses due to the fissile additive may•prevent laser action, even when no kinetic inter-

112

ference is present. For illustration, we show inFigure 4 the optical absorption cross section ofUFg as a function of wavelength in the range 200-400 nm according to the recent data of DePoorterand Rofer-DePoorter.16 Naturally, the best regionfor oscillation in a medium containing UFg is atwavelengths from 400 nm to the infrared where theabsorptions are known to be very small. Never-theless, it may be possible to operate successfullyin the region near the deep absorption minimum at~ 340 nm, which very closely matches the I2* s&dXeP transitions. Indeed, it may be that tha blueband of XeF ~ 42OoA is a better candidate sinceUFg absorption in that region is extremely low.Again, the use of lower densities of high fission-cross section material would undoubtedly mitigatethis problem.

•- 233.7 K DJ7 ton

* 263.1 K 7.26 ton

• 272.5 K 16.32 urt

> 296.1 K 91 .SO ton

3D0O

WAVELENGTH (Al

Figure 4 Optical absorption cross section of UFgin the range 200-400 nm according to the data ofDePoorter and Rofer-DePoorter. The positions ofthe XeF* (351 nm) and I2* (342 nm) laser transitionsare also indicated.

E. Special Nuclear Materials

The previous discussion has shown that nuclearmaterials with high fission cross sections, parti-

cularly in the epithermal region, reduce many of thelimitations which arise in the consideration of ex-citation of laser media by fission fragments. Sev-eral transplutonic materi?4ls represent an improve-ment over u 2 3 5 by up to roughly a factor of ten.An example of this comparison is illustrated inFigure 2, which compares the performance of U23*>with Am242111. Other materials such as Cm245 andCf 2 4 9 also have very attractive properties. Thomain parameters of interest are the nuclear life-times, the resonance integrals for fission (Inf)and neutron capture (Inv)> the availability, andthe existence of f. suitably volatile compound as acarrier.

Table III contains the lifetimes and resonanceintegrals for several nuclear materials. ' " Weobserve that Cm245 has a lifetime roughly one thirdthat of Pu239, decays solely by a emission and hasa large ratio of fission to capture resonance inte-grals. Although some reasonably volatile metal-organic compounds of the ^ransplutonics such asAm and Cm are known to exist,18 insufficient dataare currently available i'or a complete evaluation.

Table III

Properties of transplutonic materials

Isotope

U235

PU239

Am242"1

Cm245

Cf249

2

8

TJ (yr)

7 x 108

.4 x 104

152

.5 x 103

350

lit (»)

275

301

1570

750

1610*

1 Iv <b>144

200

(200)

101

625

*Dominated by a resonance at Bn = 0.7 eV with anarea of 8000 eV-barns.

0.5dE

III. Conclusions

Estimates of the performance limits of fissionfragment excited laser systems radiating in the nearultraviolet and visible regions indicate that ex-tremely high energy outputs appear feasible usingpulsed reactor neutron generators. Transversescale dimensions of greater than oae meter appearpractical. Scale size and medium density considera-tions show that high density gas phase media con-taining homogeneously integrated fissile materialare superior to foil configurations. The use offissile materials with large fission cross sections,particularly in the epithermal range, increase themedium scale size and relax kinetic and opticalconstraints. Simulation of the conditions producedby fission fragment excitation can be achieved overmost of the relevant parameter space by studies of

113

cand .date media with excited electron beams. Sys-tems -jf this nature may have application as phofco-1/ti . drivers for the high peak power systemsr-squ red for laser fusion.

IV. Acknowledgements

Tie expert technical assistance of H. Nakano isacknowledged. In addition, We thank Richard K.Prjar :on for his generous loan of UF. material. Weacknowledge several /ery informative >nversations0:1 t;.-ansplutonic materials with J. Browne, includinghis unpublished data on fission cross sections. Wealso thank E, K. Hulet for information regardingt:?an:;plutonic compounds. Special thanks for «Us-CU5S ..ons related to the preparation of the manu-script go to D. J. Eckstrom. Finally, we acknow-ledge informative discussions on transplutonicmate*ials with R. Heitz and L. Wood.

References

1. -. L. Bonzon, J. A. Reuscher, T. R. Schmidt,Siindia Pulsed Reactor III (SPR III): PredictedAirforraance Characteristics, SLA-73-O812,Jovembe? 1973.

•i. i. Thorn and R. T. Schneider, J. AIAA 10, 400(la72).

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4. ••"...B. Leffert, D. B, Rees, and F. E. Jamerson,

J. Appl. Phys. 37_, 133 (1966).

•). C. Lorents, Physica 82C, 19 (1976). .

S. McArthur; private communication.

5.

6.

7.

S.

9.

10.

•J. R. Stebn, et al., Neutron Cross SectionsVol III, Z=83-98, BNL 325, 2nd ed., February1.1965.

2. D. Bowman, G. F, Auchampaugh, S. C. Fultz,p. W. Hoff, Phys. Rev. 166, 1219 (1968).

J. C. Browne, private communication.

R. W. Benjamin, Survey of Experimentally Peter-mined Neutron Cross Sections of the Actinides,Savannah River Laboratory Report DP-1324 (1973),

1.1.! 3. Raman in Nuclear Cresa Sections and Technolo-,|y, Vol 1, edited by R. A. Sc.hrack and C. D.5owman (U.S. Dept. of Commerce, Washington, D.C.1975).,

}..'!. Walter N. Podney, Harold P. Smith, Jr., and A.•'•«. Oppenheim, Phys. Fluids 9, 2S57 (1966).

13.'! 1. M. Hill, D; L. Huestis, and C. K. Hhodes in1 Laser Induced Fusion and X-Ray Lag=~r Studies,•dited by S. Jacobs, M. Sargent III, and C.

•Oantrell (Addison-Wesley, Heading, Mass., 1976);1 C. K. Rhodes, IEEE J. Quantum Electron, QE-10,

153 (1974).

14. S. K. Searlss acd G. A. Hart, Appl. Phys. Lett.25, 79 (1974); 11. KcCusker et al., Appl. Phys.Lett. 27, 363 (1975); J. J. Ewing and C. A.Brau, Appl. Phys. Lett. 27, 557 (1975); H. T.Powell, J. U. Hurray, C. K. Rhodes, Appl. Phys.Lett. 25, 730 (1974); J. J. Ewing and C. A.Brau, Appl. Phys. Lett. 2£, 350 (1975); E. R.Ault, K. S. Bradford, M. L. Bhaumik, Appl.Phys. Lett. 27, 413 (1975); C. B. Collins andA. J. Cunningham, Appl. Phys. Lett. 27, 137(1975).

15. D. Brown, Halides of the Lanthanides and Acti-

nides (Wiley-Interscience, Hew Ycrk, 1968).

16. G. U. DePoorter and C. K. Rofer-DePoorfcer,LA-UR-75-792, Los Alamos Scientific Laboratory,Los Alamos, New Mexico (1975).

17. E. K. Hulet, private communication.

18. T. J. Marks, J. Organometal. Chem. 95, 301 (1975VE. Cernia and A. Mazzei, Inorganica. ChimicaActa 10, 323 (1974)- K. G. Hayes and J. L.Thomas, Organometa Chew. Rev. A7, 1 (1971).

R. V.- HESS:mixtures ?

DISCUSSION

Did you pump with noble gas-alkali

D. C. LORENTS: No, we have not looked at noblegas-alkali systems in our laboratory. We havelooked at Zenon-Mercury. That system seems tobe rather complicated, and I would not recommendit.

114

DEVELOPMENT OF A HIGHER POWER FISSION-FRAGMENT-EXCITEDCO LASER*

D. A-. McArthurSandia Laboratories

Albuquerque, New Mexico 87115

Abstract

Since our initial observation of lasingat \ ^ 5p in cooled, reactor-excited CO,we have continued to develop this laser ina small experimental program. The earlygain measurements and laser feasibilityexperiments did not use parameters typ-ical of efficient electrical CO lasers,but did show that the CO laser is prom-ising as a potentially efficient reactor-excited laser. In our recent experiments,we have observed that moderate dilutionof the CO with Ar lowers the reactor ex-citation threshold for lasing. We havealso developed very smooth and ruggedfission coatings on ceramic substrates,to minimize fouling of laser mirrors.Finally, a new laser apparatus has beenconstructed which more closely resembleslarge electrically-excited CO lasers.In initial experiments this new laser hasproduced a laser energy ^ 15 mJ (or 100W peak power), which represents a factorof 50 increase in laser energy and a mod-erate increase in efficiency. Measure-ments of the energy emerging from thefoils indicate that excitation of the gasis still below optimum values. Laser ac-tion at room temperature has also beenobserved. Modifications to the new ap-paratus are being made to permit system-atic variation of parameters, whichshould lead to further efficiency im-provements .

I. Introduction •'

When the Sandia nuclear pumping pro-gram began in late 1972, there was noclearly demonstrated case of laser actionusing only a laboratory nuclear reactor.Because of the relatively high excitationrates available from our SPR-II reactor,we decided to survey a number of possiblelaser media that had been studied by ear-lier workers, in an effort to observe atleast one clear-cut case of lasiny. Noevidence of lasing was seen in CO2, DF,HF, He-Ne, and Ar , although intenselight emission was often seen. However,the emission either decreased when lasermirrors were added, or was independent oflaser mirror orientation.

Because of the relatively low power in-put rate available with nuclear pumping,

it appeared that the a, 5y wavelengthlaser operating on the vibrational bandsof CO might be very suitable for reactorpumping. This laser is a very efficient,electrically-excited laser, operating be-tween vibrational levels which have verylong lifetimes even at high pressures.Finally, selective excitation is not re-quired to obtain an inversion, since astrongly non-equilibrium population dis-tribution spontaneously develops iri CO ifthe gas is highly excited vibrationallyyet remains translationally cold. Thusthe most important uncertainties were howmuch gas heating would be caused by fis-sion fragment excitation, and what frac-tion of the fission fragment energywould go into vibrational excitation ofthe CO.

Early in 1974 optical gain was ob-served in pure CO gas cooled initiallyto 77 K. After making initial studiesof the gain as a function of wavelengthand excitation rate, a laser apparatuswas constructed, and clear evidence oflasing was obtained. This evidence con-sisted of a >103-fold increase in thesignal seen by a distant detector, onlywhen highly-reflecting mirrors wereplaced in the apparatus and were properlyaligned. A small laser energy was alsomeasured, from which an estimated laserefficiency ^0.1% was obtained. Theseresults, the first clear demonstrationof lasing in a medium excited only byfission fragments, were described in aninternal report in July, 1974,' and pub-lished in the literature after a patentapplication was filed.2

II. Potential Efficiency of

Reactor-Excited CO

The initial series of gain measure-ments in pure CO showed a factor of tensmaller gain amplitude, compared to thatobserved in electrically-excited COlasers.3 Otherwise, the gain hehaviorwas similar to pulsed electrically-excited" CO lasers in overall time depend-ence and in its dependence on the vibra-tional band and the rotational quantumnumber within each band. Fig. 1 showsmeasured peak gain in pure CO for a reac-tor temperature rise AT_ of 320°C, with

is

*This work supported by U. S. Energy Research and Development Administration

115

2a"a1S 500

, t Tg • 80°K( T • 1000"K

| T g - 1 1 5 ° Kt ' 1 m EMfK

I Tg • 150°K

5.1 5.2 5.3

PROBE LASER WAVELENGTH k 1^1

Fig. 1 Measured Peak Gain in CO andCalculated WavelengthDependence of Gain.

calculated gain curves for various gastemperature assumptions. A thresholdenergy deposition was also required to ob-serve gain: Fig. 2 shows that the gain onthe X = 5.27 y line goes to zero well be-fore the gas energy deposition (which isproportional' to ATR) approaches zero,The smaller gain magnitude could well becaused by the low excitation per CO mole-cule associated with the undiluted CO gasused. Intensity modulation of 'the probelaser beam and a pressure rise in thegain cell upon excitation both indicatedthe presence of some gas heating by thefission fragments. The widths of the gainpulses increased slightly for the higher-v bands, which may also indicate gasheating.

Fitting the gain measurements with anapproximate steady-state vibrational dis-tribution, assuming a transient gas tem-perature ranging between 80 K and 150 K,implies that 25 to 70 percent of thedeposited fission fragment energy is ap-. pearing as vibrational excitation of pureCO.4 Demonstrated physical mechanismswhich might contribute significantly tosuch high vibrational excitation efficien-cies are direct vibrational excitation bylow-energy secondary electrons, and

50

40

POLY. ( A 1.6 cmTHICKNESS j o 2.0 cm

100 200 300 400 600 800 1000

REACTOR dT (°CI

Fig. 2 Dependence of Gain on EnergyDeposited in Pure CO.

conversion of electronic excitation of COmolecules into vibrational excitation.Other possible mechanisms are direct vi-1-rational excitation by recoil ions, andv^brational excitation resulting from re-combination of complex ions such as(CO)2

+."

The Sandia program has continued toconcentrate on th" development of the COreactor-excited laser, because our initialmeasured efficiency with CO is still muchhigher than those of the reactor-excitedHe/Xe or Ne/N2 lasers,5 and because ofthe high vibrational pumping implied byour initial gain measurements. Prelim-inary systems and reactor design studiesalso showed that very large lasers It.,J 1 «J) could be built if ei high-efficiency laser medium could be found.6

These large lasers would use arrays ofthin foils to excite the laser medium,and could potentially achieve overall ef-ficiencies t 10% with an efficient lasermedium, in spite of the inherent foillosses. Since electrically-excited COlasers have achieved efficiencies > 60%,and since the CO wavelength has potentialapplications to laser pellet fusion, thelaser-heated-solenoid fusion concept, andweaponry, further development of the reac-tor-excited CO laser was undertaken.

III. Experimental Development Steps

A. Lasing in CO Diluted with Ar

One of the first development steps wasthe demonstration that CO/Ar mixturescould be made to lase with direct nuclearpumping, and the qualitative demonstration

116

that CO/Ar laser mixtures have a lowerthreshold than pure CO.6 Jr or Sj areoften used as diluents in electrical COlasers, because they allow a fixed amountof excitation energy to be concentratedin the vibrational modes of fewer CO mole-cules . This concentration greatly in-creases the gain and efficiency of thelaser. The lower threshold observed withnuclear-pumped 50/50 CO/Ar mixtures indi-cates that this process is also occurringto some extent in nuclear pumping.

B. Foil Coating Developments

In our nuclear-pumped laser researchwe have chosen to excite the gas with athin fission foil coating, on a substratewith high thermal conductivity and spe-cific heat (alumina). Compared to homog-eneous excitation with He3 or UF6, exci-tation with a foil ailows much greaterflexibility in choice of laser gas mix-ture and laser wavelength. We have alsoused uranium oxide coatings rather thanmetallic uranium foils, because the thinoxide coatings are chemically stable athigh excitation densities even in thepresence of reactive laser gases, and be-cause the substrate acts as a heat sinkwhich reduces the danger of destroying orwarping the exciter foil at high excita-tion densities. The reliability of thelaser apparatus and the amount off excita-tion of the laser gas are determined bythe foil coating quality. Therefore, wehave put considerable effort recently intonew foil construction methods and intomethods for measuring the energy output ofthe foil, which are described in this sec-tion.

In our previous experiments the energyemerging from the fission foil coatings'and therefore the excitation energy ofthe gas) was calculated, by using a three-dimensional neutron transport coc!e to cal-culate the fission density in the foilcoating,7 and then assuming a perfect,93%-enriched 235U3Oaor

23SUO2 coating tocalculate the fraction of the fissionfragment energy emerging from the coat-ing.8 The neutron transport calculationswere checked by measuring the thermalneutron fluence at the alumina cylindersfor a known geometry.1 No independentcheck was mad 2 of the parameters which de-termine the foil energy output and ef-ficiency (foil ,thickness, chemical compo-sition, and isotopic enrichment). How-ever, there were qualitative indications(size of laser pulse energy, size of pres-sure rise in the excited gas) that theearly foils were emitting the expectedamount of energy. A severe disadvantageof these earlv foils was that at high ex-citation densities they release X flakes ofcoating material, which fouled the lasermirrors and prevented laser action unlessthe mirrors were cleaned or replaced

after each reactor burst.

To improve the bonding of the foilcoatings to the substrate, and to improvetheir mechanical quality and uniformity,an improved foil coating process was dev-eloped,9 which has resultedwin verysmooth, durable UaOe foils on Al20aand BeOsubstrates. These foils cannot bescratched off the substrates, and the foilsurface replicates the initial smoothnessof the ceramic substrates, so that highoptical reflectivity is obtained for highangles of incidence. Thus we no longerhave any problems with foil flaking ordamage by the neutron irradiation ratesroutinely used.

However, it appears that these newfoils are generating significantly lessenergy than expected from the measuredneutron fluence which irradiates them.For example, lower laser energies andlower pressure rises in the excited gasthan expected have been observed in ex-periments using these recently-constructedfoils. Therefore, measurements have beenmade of both the total energy created inthe foil (which is related to the foilcoating enrichment, the total mass ofuranium, and the neutron fluence which ir-radiates the foil) and the fission frag-ment energy emerging from the foils(which is related to the foi] structureand enrichment). We believe that thesecalorimetric measurements are the first ofthis type for nuclear-pumped lasers, andan improved laser efficiency measurementshould result from data of this kind.

Figure 3 shows a special calorimeter(the Fission Fragment Calorimeter), de-signed to measure directly the energyemerging from the fission foil coating.10

The calorimeter element consisi;s_of a 1.9cm diameter cylinder of 3.6 x 10 3 cmthick aluminum, suspended by four 1:J x10 2 cm diameter tantalum support wires.The element is surrounded by a thick alum-inum collimator and support structurewhich restricts the length of exposedelement to about 2.2 cm. The calorimeterassembly fits into the 2.7 cm diameteralumina fission foil with a small clear-ance. The element temperature rise ismeftjured by a thermopile consisting offour chromel-constantan thermocouples ce-mented to the upp«ir end of tt s element,with their reference junctions heat-sinked to the calorimeter"support struc-ture .

To measure the total energy producedin the foil coating, the alumina cylinderis used as a calorimeter, its temperaturerise being measured with a ribbon-typecopper-constantan thermocouple bonded tothe outside surface of the cylinder atits axial center. The reference junctionsin this case were protected from direct

117

-F ISSION FRAGMENTCALORIMETERASSEMBLY •

-ALUMINUMCALORIMETERELEMENT

-ENRICHEDU3°8COATING

SUBSTRATE

SUPPORT ANDCOLLIMATORSTRUCTURE

Fig.. 3 Diagram of FissionFragment Calorimeter.

irradiation by fission fragments, but notheat-sinked to any structure. The entirealumina cylinder was isolated from thestainless steel walls of the vacuum cham-ber by small felt pads and a thin plasticsupport.

Figure 4A shows a typical alumina tem-perature rise signal, which has a smallnegative background component (which isprobably caused by differential radiationheating of the reference junctions and themeasuring junction). The fission foiltemperature signal is delayed by thermalconduction through the wall of the aluminacylinder, and reaches nearly its final

• value in = 0.3 sec for an evacuated laserchamber. The alumina temperature risemeasures the average fission energy/cm2

retained by the entire cylinder.

Figure 4B shows a typical FissionFragment Calorimeter signal, which in anevacuated laser chamber measures the en-ergy per cm2 emerging from the fissioncoating over an axial region .A 2.2 cm long(after an t 30% correction is made for thegeometrical collection efficiency of thecalorimeter element). The backgroundsignal of the "Fission Fragment Calorimeter

KEASUREHENT. OF FISSION FOIL EFFICIENCY

A. FOIL SUBSTRATE TEMPERATURE RISE

1.25 DEG. C/DIV.0.5 SEC/DIV.

B, FISSION FRAGMENT CALORIMETER

0.11 J/CM2 - DIV.

0.5 SEC/DIV.

Fig. 4 Typical Alun>".naTemperature Rise and Fission Fragment

Calorimeter Signals.

was measured to be £ 5% at the peak of thecalorimeter signal, by covering the sens-itive area of the calorimeter at the col-limator surface with thin sheets of al-uminum or plastic.

The axial variation of the fissionfragment flux was also measured in twoways (Figure 5): (1) The fission frag-ments emerging from the coating were col-lected on a strip of thin aluminum, andthe axial variation of the fragment 6activity was measured;:* and (2) the den-sity of fission fragment tracks in aLexan target was measured as a functionof axial position.12

The fission fragment activity data ofFigure 5 were used to calculate the fis-sion energy created per unit area of the

1.2

1.0

0.8P2

= 0,6

e- a4

0.2 -

0.0

5 o o To

0

$° 0o o 0

0

o FISSION FRAGMENT ACTIVITY0 FISSION TRACK DENSITY

5 10 :5AXIAL POSITION ALONG CYLINDER (cm)

20

Fig. 5 Measured AxialVariation of Energy Output of

Fission Foils.

118

foil at the same axial position as theFission Fragment Calorimeter. The ratioof these signals yielded a measured foilefficiency of 12 + 2%, which is in goodagreement with the calculated efficiency(12%) of a perfect U3Oacoating having themeasured thickness of 5JJ.S However, themagnitudes of both energy signals arelower (by a factor of ^ 3.5) than would beexpected from the measured thermal neu-tron fluence and the assumption of 93% en-richment. The simplest resolution of thisdiscrepancy would be to assume that thefoils are actually about 30% enriched.Samples of i_he coating solution have beensent for enrichment analysis, but the re-sults are not yet available.

One of the recently-constructed aluminacylinders was also sectioned, polished,and observed under an optical microscope(Figure 6). Identification of the size ofthe foil coating is not very precise, be-cause of surface distortions produced bythe polishing process, and because someslight discoloration extends into thealumina substrate. A very thin blackregion of thickness "v< 3—5vi appears in thesections, but it may be a crackk insteadof a coating. Because these octical ob-servations are not very precise.. samplesare now being prepared for electronmicroprobe analysis, to make a qvanti'-.a-tive measuTF,i!K!nt of the uranium distri-bution . \

SECTION THf-.t)iJGH FISSION TOIL \A, IN REFLECTED LIGHT -\

•*-Fon.FPCUY

B. IN POLARIZED LIGHT

AL 20 3

If laser gas is added to the chamber,a different time dependence is obtainedfor the alumina substrate temperaturerise. For an evacuated chamber with thefission fragment calorimeter removed,most of the emitted fragment energy willbe deposited in the opposite surface of along alumina cylinder, and the tempera-ture rise will be characteristic of allthe fission energy created in the foil.However, with a high-preSsure gas fill,all the emitted fission fragment energywould be deposited in the gas, and couldcontribute to heating of the alumina onlyby thermal transport back to the foilsurface through the gas. The calculatedtime constants for thermal conduction tothe foil surface range from -\< 0.15 sec at100 Torr CO to -v. 0.6 sec at 400 Torr CO,for example.'3 The presence of the gasalso increases the heat conduction be-tween tfc' alumina cylinder and the wallsof the laser chamber, but calculations ofthis heat loss show that it is not afunction of pressure in the range from100 to 400 Torr,13 and is very slow com-pared to the time behavior of the aluminatemperature. Figure 7 shows the aluminatemperature rise for 104 Torr and 400Torr of a 50/50 CO/ftr mixture, and thetemperature difference between the twocases. It appears that the higher-pressure gas is absorbing from 10-15% ofthe energy from the foil (as expectedfrom the foil efficiency measurement),and is returning almost all of it to thefoil over about a 5 sec- time period,which is much longer than the calculatedthermal relaxation time of heated CO orAr gat '3 These data then raise the pos-sibility that much of the energy depositedin the gas is being stored (perhaps asvibrational energy), converted slowly tothermal energy, and then conducted backto the relatively cool foil. Furtherexperiments with gases which quench vi-brational excitation, and efforts to re-duce the background signal, are plannedto test this hypothesis.

i

SUBS

1

Fig. 6 Optical Observation ofFoil Coating Region

TIME (sec)

Fig. 7 Effect of Gas Pressure onFoil Substrate Temperature Rise.

1X9

C. Folded-Patn Laser

To obtain a valid laser efficiencymeasurement, the laser must have a hightotal gain and be operating well abovethreshold. The SPR-II reactor produces ahigh neutron flux only over a limitedregion of space, so to obtain a high totalgain it is necessary to fold the excitedoptical path back and forth in this smallregion. The segments of the optical pathshould also be approximately horizontalso that gas decomposition products do nottend to collect on the folding mirrors orthe Brewster windows. Finally, the mirrormounts should be designed so that opticalalignment can be performed after the laserexcitation chamber is in thermal equilib-rium at 77 K. An apparatus with thesefeatures, the Sandia Folded-Path Laser, isshown in Fig. 8. Excitation is providedby an array of cylindrical fission foilssurrounding the folded optical path.This apparatus scales up the total gainby about a factor of 12 (neglecting lossesin the folding mirrors and differences inthe average excitation over the largervolume).

Preliminary data have been taken withthis apparatus at relatively low excita-tion levels, because of neutron absorptionin the complex apparatus and the low en-ergy output of the new foils. Weak laserpulses have been observed as late as 0.6to 1.2 msec after the neutron excitationpulse, which indicates that energy can bestored in the laser medium. To obtain re-liable lasing it was necessary to align

the laser mirrors aftor the apparatus•hadapproximately reached thermal equilibrium.The measured laser energy has been in-creased by a factor of ^ 50 to 15 mj(corresponding to ^ 100 watts power),without optimizing any parameters such asgas mixture or output mirror coupling.Taking into account the measured lower en-ergy output of the fission foils, the(non-optimized) laser efficiency is % 1%at 77°K.

Experimental problems in the apparatushave .nade systematic parameter variationsdifficult: vacuum leaks develop when thetemperature is cycled, beam distortionresults from cooling the Brewster windows.,and the thermal equilibration time of themassive apparatus is > 5 hours. Modifi-cations are presently being made to cor-rect some of these problems. We expectfurther improvements in efficiency andenergy output as a result of optimizinglaser parameters.

IV. Observation of Lasing atRoom Temperature

Many experimental problems cau beavoided, and useful parameter studiesmade, if laser action could be obtainedat room temperature with this new, longerlaser. Therefore searches for lasingwere made in CO/N2 and CO/Ar mixtures ata gas temperature ^ 300 K. Laser actionwas not observed in 50/50 or 25/75 CO/N2mixtures, bur several laser pulses wereobserved in 50/50 CO/Ar (the only mixturetried). Figure 9 shows a typical laser

u/ui w w uu uu wFig. 8 Drawing oi! Sandia Folded Path Laser, Showing Gas Excitation

•Cylinders and Level of Liquid Nitrogen Coolant.

120

OBSERVATION GF USING INCO/AR AT ROOM TEMPERATURE

LASER PULSE AND FAST NEUTRON PULSE

5 VOLTS/DIV

100 fiSEC/DIV

Fig. 9 Observation of LaserAction in CO/Ar at Room

Temperature.

pulse (solid curve), % 50 ysec wide andbeginning slightly after the peak of thefast neutron pulse from the reactor(dashed curve). Data were taken for twodifferent thicknesses of polyethylenemoderator, and with and without Brewster'sAngle Windows (B.A.W.) in the laser cav-ity. Figure 10 shows that increasing theneutron moderation and removing the B.A.W.reduce the laser threshold by = 30%. Thelaser energy was not measurable with a :laser calorimeter, but was obtained by in-tegrating the Ge:Au detector signal traces.

• Using reasonable values for the lossesin the laser cavity mirrors, B.A.W., andthe pathfolding mirrors, the gain coef-ficient at T = 300 K is o > .04 percent/cm,or about a factor of 10 lower than meas-ured previously at 77 K.

The observation of lasing at 300 K isalso indirect evidence that an appre-ciable fraction of the fission fragmentenergy is converted to vibrational exci-tation of the CO gas. The gain measure-ments at T i 77 K discussed in Ref. 4 andSection II imply high vibrational excita-tion efficiency, if vibration-vibration(V-V) relaxation is sufficiently fast atthese low temperatures. However, it hasbeen proposed instead that the observedgain might result from a small amount ofdirect excitation to levels with v ^ 12,followed by very slow V-V relaxation outof.these levels.1" Since the V-V rates^ary rapidly with gas temperature, theserates are as much as 500 times faster atT = 300 K than at T ^ 100 K. if slow V-vrelaxation were responsible for most ofthe gain observed, it would be expectedthat the gain would drop by a factor100 in going from a gas temperature of100 K tz> 300 K. Since lasing is observedat 300 K, the gain appears to decreaseby only about a factor of 10 over this

_ 500

400

300

200

100

OO

50/50 CO/ArT - 300°KP - 100 Torr2.0 cm POLY3.3 cm POLYNO B.A.W.

y

00 100 200 300 400 500

REACTOR TEMPERATURE RISE, 4 T R (deg C)

Fig. 10 Dependence of LaserEnergy on Reactor Energy Deposition.

temperature range, which implies that slowV-V relaxation is not the dominant causeof the gain observed at I i 100 K.

V. Conclusions

The physics of the CO excitation pro-cesses will determine whether a very ef-ficient (<\. 50%) CO reactor-excited lasercan be made, and also determine exactlyhow the laser must be constructed. Themost important physics uncertainties are:(1) What fraction of the input energygoes directly into gas heating, and bywhat paths? (2) What fraction goesrapidly into vibrational excitation, andby what paths? and (3) How does the ex-citation change with changes in the gasmixture?

Several experimental approaches arenow being pursued at Sandia to answerthese questions. Tn the limited gain dataavailable at present there are no strongindications of an unusual vibrationaldistribution. However, if more accurategain measurements over a wider wave-length range were made for various gasmixtures, a much better picture of the ex-citation process might emerge. Laser en-ergy and spectrum measurements mightyield insight into the excitation pro-cesses, if the focusing effects caused bynon-uniform gas excitation do not disruptthe lasing process too much. An accuratemethod for measuring the gas temperatureas a function of time during the excita-tion pulse is also being developed.

As an example of how the answers tothese questions may affect the laser

121

construction, if the gain drops primarilybecause of gas heating, it may be pos-sible to re-cool the gas in an expansionand extract the available vibrational en-ergy as CO laser light.

References

1. D. A. McArthur and P. B. Tollefsrud,Sandia Laboratories Report SAND-74-0106, (July 1974) (unpublished).

2. D. A. McArthur. and P. B. Tollefsrud,Appl. Phys. Lett. 26, 187 (15 Feb.1975).

3. D. A. McArthur and P. B. Tollefsrud,Trans. Am. Nucl. Soc, 1£, 356 (1974).

4. D. A. McArthur and P. B. Tollefsrud,IEEE J- Quantum Electr., QE-12, 244(April 1976).

5. H. H. Helmick, T. L. Fuller andR. T. Schneider, Appl. Phys. Lett.,2£, 327 (15 March 1975); R. J. De-Young, W. E. Wells, J. T. Verdeyen,and 6. H. Miley, IEEE J. QuantumElectr., QE-11, No. 9, p.. 97D (1975).

6. D. A. McArthur, T. R. Schmidt, P. B.Tollefsrud and J. S. Philbin, Trans.Amer. Nucl. Soc., 2JL, 21 (1975).

7. J. Kimlinger and E. F. Plechaty, Law-rence Radiation Laboratory, Livermore,CA, Rep. UCRL-50532, (Oct. 1968) .

8. E. E. Lewis and R. Pfeffer, Nucl. Sci.and Eng., Vol. 21_, PP 581-585 (1967).

9. T. M. Kerley and D. J. Sasmor, SandiaLaboratories, unpublished work.

10. Designed by G. H. Miller, G. J. Lock-wood, and L. E. Ruggles, Sandia Lab-oratories, Albuquerque, N.M. 87115.

11. b. L. O'Neal, Sandia Laboratories,Albuquerque, N.M. 87115.

12.' R. I. Ewing, Sandia Laboratories,Albuquerque, N.M. 87115.

13. E. U. Condon and H. Odishaw, Eds.,Handbook of Physics, McGraw-Hill, 1958,New York, Part 5, Chapter 4.

14. C. P. Holmes, Air Force Weapons Lab-oratory, Kirtland AFB, N.M., privatecommunication.

122

REVIEW OF COAXIAL FLOW GAS CORE NtTCLEAR ROCKET FLUID MECHANICS*

By

Herbert WelnsteinIllinois Institute of Technology

Chicago, IL 60616

Abstract

In a prematurely aborted attempt to demonstratethe feasibility of using a gas core nuclear reactoras a rocket engine, NASA initiated a number ofstudies on the relevant fluid mechanics probl.~ns.These studies were carried out at NASA laboratories,universities and industrial research laboratories.

- Because of the relatively sudden termination ofmost of this work, a unified overview was neverpresented which demonstrated the accomplishmentsof the program and pointed out the areas whereadditional work was required for a full under-standing of the cavity flow. This review attemptsto fulfill a part of this need in two importantareas.

Introduction

The concept of a gas core nuclear reactorappears to have first been discussed in the openliterature about 20 years ago. The context inwhich it was discussed was as a rocket engine. Theobvious advantages of the proposed engine was thatthe propellant could be heated to very high tem-pera Lures, resulting in high specific impulse andthat- high propellant flow rates could be achievedresulting in high thrust levels. No other ad-vanced concept proposed at that time promised thishighly desirable combination of the requirements ofmanned, interplanetary travel.

The coaxial flow gas core.reactor concept asfirst proposed, was deceptively simple.1 A lowvelocity inner stream of fissioning fuel gas wassurrounded by a very fast-moving propellant stream.The hot inner gas transferred energy to the outerstream by thermal radiation, which was absorbed bythe propellant because it was seeded with absorbingparticles. In theory, at least, enough fuel couldbe fed into the cavity to maintain a critical mass.However, there were some economic limirat ions onthe rate of fuel loss. The fluid mechanics studieswhich were undertaken to determine the rate atwhich fuel had to be fed into the reactor cavity tomake up for ths loss are the subject of this re-view.

The early fluid mechanics studies were begun be-fore extensive nuclear studies had been made. Inthe earliest studies, it had not been apparent thatthe fuel had to be present in a large fraction ofthe cavity diameter and that the cavity length-to-diameter ratio had to be of the order of one.These conditions were necessary in order to keepthe critical mass requirements within reason. Fromthese early studies and the simultaneous nuclearstudies, the idea of a reactor cavity with samllL/D and large fue^ volume fraction emerged.

The fluid mechanics studies were redesigned totake into account the new picture of the cavity

* Work performed under NASA Grant NSG7039

geometry. As further work progressed, it was seenthat the flow was actually a turbulent, entranceregion flow, almost totally dependent on the in-let configuration and conditions. The flow wastransitional and unique, and the usual assumptionsof similarity and self-preservation were not validfor analytical studies, making necessary a largeexperimental program. Furthermore, in the actualreactor cavity, extremely largo temperature anddensity variations would exist and a fundamentalunderstanding of the c; -ity flow was seriouslylacking.

A program was set up t>, ASA to develop thebasic understanding for the coaxial flow GCR fluidmechanics so tuai. concept feasibility could even-tually be determined The program involved severallaboratories after the '.nitial concept formulationat Lewis Research Center. These were Lewis, UnitedAircraft Research Laboratories, Princeton Univer-sity and the Illinois Institute of Technology.Later, Aerojet Nuclear Company and the TAFA Divi-sion of the Humphreys Corporation were included.The experiments and analyses were carried out atthe installation most suited for the type of workinvolved.

In January 1973, policy and budgetary factorsforced the termination of much of NASA's spacenuclear program. Almost all of the fluid mechanicsresearch in Che coaxial flow GCR program endedabruptly. A unified overview was never presentedwhich demonstrates the accomplishments of the pro-gram and points out areas where additional work isrequired for a full understanding of the cavityflow. This review is an attempt to fulfill a partof this need.

The sp.ace limitations on this paper make itimpossible to discuss all of the work done withinche program. The two areas where discussion andcomparison of work is most useful are in thea) the factors which influence containment in coldflow studies; and b) the effects of heat generationon containment. The work in these areas have cotreceived any critical review in the past. There-fore, this paper will be limited to discussion ofthese two areas. The papers discussed are limitedto those from which the author feels a significantpoint can be made. Some important work has beenneglected because of the space limit. The reviewis structured in such a way as to compare and con-trast' the related work of the program, rather thanto preserve the chronological orjer of the work.Unfortunately, in such a structuring it oftenhappens that a single work is discussed in severalparts without ever giving the whole. However, itis felt that the significant results of the programwill be made more clearly visible with thisapproach and apologies are tendered to the authorswhose works have been so treated.

123

Factors Influencing Cold Ylc<- Containment'

In order to discusa the factors which Influencecontainment of Inner, stream fluid in a confinedcoaxial flow, it is necessary to describe thevarious sub-regions of flc« that exist along withtheir dominant characteristics. Figure 1 is aschematic of confined coaxial flow with the sub-regions labeled. Starting at the upstream centeris the inner stream potential core. This region isdominated by inertia! forces and pressure ratherthan shear forces. The shear forces do, however,control its size and for some cases the inner po-tential core nay not.exist. Surrounding the po-tential core is an annular raising region whichseparates the inner and outer stream potentialflows. Xt is a transitional shear layer with arapidly growing turbulence level (for the typicalturbulent case). It is dominated by the velocitydifference between potential floss and the shapesof the inner and outer stream velocity profiles atthe tin of the separating duct. The inner andouter stream turbulence levels and scales alsohave a moderate effect on the aixing region. Theouter fluid potential flow is also dominated byinertial forces and pressure as well as by con-tinuity effects due to the growing boundary layeron the confining duct. The annular mixing regionthickens downstream to the point where it a s ^ awith itself and the inner stream potential coreends. At this point it can be said that the momen-tum defect of the trailing edge wake disappears.The flow downstream of this point is made up of atmost, three regions. They are the jetlike or wake-like (depending on velocity ratio) "free" shearflow region, the outerstretim potential flow and theduct boundary layer. For enough downstream, the"free" shear flow region and boundary layer willmerge and eventually fully-developed pipe flow willbe approached in the duct.

The regions of interest in this paper are choseupstream of the merger of the duct wall boundarylayer and the "free" shear flow as contained bythe dashed line in Figure 1. The main parameterfor determining the nature of the flow is thevelocity ratio of the streams. The Reynolds numberhas a second order effect and only on certainregions of the flow. The variation of velocityratios can be broken up into three ranges for pur-poses of discussion.

When the velocity ratio is close to one, thepredominant feature of the flow is the annularmixing region separating the potential flows. Thismixing region is the wake from the trailing edge ofthe separating duct. Its spreading rate isaffected by the velocity profiles In both streamsat the trailing edge of the separating duct, thevelocity difference across the region and by theturbulence levels In both streams. The effects ofthe initial turbulence becomes more pronounced asthe scales become of the order of the mixing regionthickness or greater, and, possibly, as the fluc-tuating velocity magnitude becomes of the order ofthe velocity difference or greater.

These effects are brought out in photographs ofbromine-air coaxial flow experiments performed atLewis Research Center.Z'3> The experiments wereperformed for a range of velocity ratios and innerand outer stream Reynolds numbers. At each set ofvalues of the parameters, runs were made with

honeycomb inserts in: a) neither duct; b) the innerstream duct; c) the outer stream duct, d) bothducts. For the outer-stream-to-inner-streamvelocity ratios of about 0.2 to 2, the mixingregion appeared laminar for inner jet Re of 2400or less. With a honeycomb section inserted inthe inner stream duct only, the laminar appearanceremained for almost all inner jet Re achieved, upto about 4000. There are two resulting effects ofthis homeycomb which can affect the mixing regiontransition. First, the velocity profile will bevery flat and the total momentum defect will bedecreased substantially. When the inner stream hasthe higher initial velocity, this decrease inmom. ntum defect may be the most stabilizing featureof the honeycomb. There will also be a decrease inthe scale of turbulence due to the honeycomb sincethe scale will be bounded by the width of thehoneycomb passages rather than the duct diameter.The authors Imply that this is probably the pri-mary stabilizing effect. When the honeycomb wasalso added to the outer stream inlet, the stabiliz-ing effect was diminished but did not disappear.When the honeycomb was present in the outer streamonly, the mixing region waj turbulent over a widerrange of parameters than with no honeycomb at all.Of the conclusions to be reached from all thisdata, the most clearly visible is that the outerstream velocity profile has a great effect on themixing region *or velocity ratios close to one.The thicker it is, the more stable the region is.This fact Is borne out by the suffer region studieswhich are discussed subsequently. .The outer streamturbulence appears to have a lesser effect ontransition. Another conclusion from the above isthat inner stream turbulence has some effect anddecreasing its scale is stabilizing. However thechanges in turbulence scale have not been separatedfrom changes of the inner stream boundary layerthickness and the annular wake momentum defect inthese s*- ' • It also apprars that decreasingthe mome deCec. while only narrowing the wakea small auou::i iy stabilizing. When both honey-combs were present and the wake was very narrowthe result was destabilizing even though themomentum defect was smallest as were the turbulentscales. The stability of the annular mixing regionis probably governed by the level of vortlcity inthe region more than any other factor. The turbu-lence in both the inner and outer potential flowsis decaying and instabilities in the mixing regionare growing. The local vorticity will affect therate at which the instabilities grow.

Ths annular mixing region becomes of decreasingimportance as the velocity ratio deviates from one.As the velocity ratio becomes very small comparedto one, the flow becomes that of a primary jetentraining a surrounding, secondary stream. Re-circulation zones can be set up if the secondarystream can not meet the entrainment requirementsof the primary Jet. This recirculation zone is inthe form of a ring pressed against the outer ductwall a few diameters downstream of the initialface.-' This range of velocity ratios is not theone of interest here, and will not be discussedfurther.

The range of velocity ratios of interest forthe coaxial flow GCR contains alI those above avalue of about 2 or 3. The mosv pertinent featureof this range is the solid cylindrical wake of theinner duct rather than the wake of Its thin wall.

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The wake is partially filled by the inner streamfluid and for lower values of the velocity ratiothe wake effect is partially mitigated. This canbe thought of in terms of there being enough innerstream fluid to satisfy the entrainment require-ments of the outer stream flow. However, as thevelocity ratio becomes large enough, there is notenough inner stream fluid to satisfy the entrain-ment requirements of the outer stream and some-thing similar to separation takes place with thedevelopment of radial pressure gradients and re-,circulating eddies behind the blunt cylinder end.Figure 2 shows the development of the recirculationeddie as velocity ratio is increased. As the innerstream is entrained or accelerated along its outeredge, the streamlines move outward as they comecloser together. Streamlines close to the center-line must diverge as a result of the decelerationof the fluid in order to satisfy the continuitycondition. Thus, a region of positive pressure isgenerated on the centerline. If the pressure riseexceeds the dynamic head at the centerline, rever-sal of the flow takes place with the formation ofa recirculation eddy. In this case, it is saidthat the inner stream cannot Beet the entrainmentrequirements of the outer stream. If, however, themomentum of the inner stream is high enough, theaccelerated outer region will reach the centerlinebefore a complete reversal can take place. Here,the entrainment requirements are met.

The velocity is not the only factor in deter-mining if recirculation will take place in a givencoaxial flow. As has been mentioned before, theouterstream velocity profile is also very important.Its effect has been illustrated in terms of theflow induced upstream along the centerline by acylindrical vortex sheet downstream of, and at theradius of, the inner duct.^ This vortex sheetrepresents the boundary layers of the inner andouter streams. The thinner the dominant cuter-stream boundary layer is, the more "concentrated"its vorticity and the greater the strength of thevortex sheet. When tae induced velocity, whichincreases with vortex.sheet strength, reaches themagnitude of the inner stream velocity, recircula-tion is incipient.

The containment of inner stream fluid can now bediscussed in terms of the regimes and types of flowpossible. It Is obvious that recirculation willstrongly enhance mixing and result in poor contain-ment because the inner stream will be mixed inter-nally with recirculated fluid and rapidly acceler-ated by and mixed with outer stream fluid, usuallyin a highly turbulent process. If recirculation isprevented from occurring, then the mixing takesplace in the annular mixing region followed by thecylindrical wake. The containment will be affectedby the magnitude of the velocity gradients and thelevel of the turbulence. Suppressing the turbu-lence and maintaining small gradients will increasecontainment and these two conditions can usually beaccomplished simultaneously.

It was recognized rather early in the prograTnthat large velocity gradients in th£ initial plane(narrow outer stream boundary layer on the trailingedge) would greatly affect mixing and an analyticalinvestigation was performed which demonstrated thJs.The investigation was on the effects of a momentumbuffer region on the mixing and containment. Itwas found that there were optimum values of bufferregion velocity ratio and thickness. The initial

profiles of ail three streams were taken as plugwithout singularities at the ends of the separatingducts. The total outer stream flow remained con-stant while the amount in the buffer region wasvaried. This latter condition caused the outerstream to inner stream velocity ratio to increaseas the buffer region was made thicker. However,the general conclusion that decreasing the velocitygradients or vorticity increased containment wasdemonstrated for the restricted range of conditionsstudied in this work.

Q

In an experimental investigation done at IIT inorder to study the effects of free stream turbu-lence on the mixing, velocity and concentrationprofiles were measured for initial velocity pro-files which were different from those in a previousinvestigation. These profiles did not differmarkedly but it rtill can be seen from the resultsthat when the initial, outer stream boundary layersin the second investigation were thicker, highercontainment was the result. In this investigation,a screen was placed In ;:he outer stream upstream ofthe initial face. This resulted in lower turbu-lence intensities in the outer stream, and a changeIn outer stream initial velocity profile due totripping of the boundary layer on the outside ofthe inner duct. Results vith the screen were com-pared to similar results taken with a boundarylayer trip device whicr gave an outer stream pro-file similar to that with the screen but withhigher turbulence intensities. Little differenceIn mixing and containment was seen due to the dif-fering turbulence intensities. However, thevelocity ratio was quite high in most of the casesof this investigation and the solid cylindricalwake effect was the dominant one. For the caseswhere velocity ratios were high, the existence ofa recirculation eddy was shown by moving a threadof cotton along the centerline and observing itbeing pulled upstream near the exit of the innerduct.

A series of experiments performed at UARL ' '12 was conducted to determine how containment couldbe improved by tailoring initial velocity profilesand turbulence levels. In a coaxial flow experi-ment, Scott industrial foam was placed across theentire duct at the inlet plane. It was shown thatrecirculation was suppressed up to very highvelocity ratios. The effect of the foam is clearlyseen in Figure 3. In frames a and b, no foam waspresent and the inner stream, visualized withiodine, shows large scale turbulence on the edgeand rapid mixing. In frames c and d with foampresent, it is seen that the inner stream has asmooth edge and maintains its integrity to a muchhigher velocity ratio. In a parallel investigationat IIT, shadowgraphs of a similar experiment wereobtained and are shown in Figure 4.1-> Scott foamwas placed at the initial face of the same appara-tus used in reference as shown in the first frame.Pictures were taken at several velocity ratios withand without a cone that produced, in a crude way,a buffer region between inner and outer streams.The inner stream was freon 12 and the outer streamwas air. It can be seen here a]so, that the innerstream maintains its integrity and shows smallerturbulence scales with the foam present and thatthe cone provides for a wider Inner stream and asmaller scale nf turbulence. What is most impor-tant In these two figures is that the innc . Teamdecreases in width going through the foam.

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The radial pressure gradient which is set up bythe entrainment of inner strnam fiaid and resultsin.a high pressure region at the upstream end ofthe recirculation eddy when foam is not presentcauses instead a radial inflow Inside the foam.The initial profile at the downstream end of thefoam is much smoother and monotonically increasingfrom the center]ine to the beginning of the outerduct boundary layer. Because this new initial pro-file has no minimum surrounded by inflectionpoints, the shear layer generated turbulence, is ofmuch smaller scale and lower intensity. The effectof the foam on the free stream turbulence is prob-ably of' no importance at all. The major effect ofthe foam then is on the initial velocity profileand the elimination of the radial pressure gradient.The work at UARL also included detailed measure-ments on a coaxial flow with a buffer layer and theScott foam and it was shown that containment couldbe increased dramatically over a fcwo-sr.rearn systemwithout foam.

The investigation that initiated work on thespherical geometry with coaxial flow was carriedout at Lewis Research Center.•*•* It was noted thatbecause of pressure vessel design and wall coolingrequirements, a spherical cavity shape would bemore likely than a cylindrical one. The investiga-tion was to determine if the spherical shape couldbe used advantageously fr.t increasing cortainment.In the proposed design, propellant would be intro-duced all along the cavity wall to satisfy the wallcooling requirements and also to induce the innerstream to occupy a greater fraction of the cavityvolume. The flow experiment was carried out in atwo-dimensional cavicy with flat top and bottomand curved walls. The inner stream was smokey airintroduced through a "shower-head" nozzle. Outerstream air entered all along the porous curved sidewalls. Pictures taken through the flat top wereused to obtain concentration measurements and forflow visualization. The propellant entered thecavity with a radially inward directed velocity •which was adjuoted to be fairly uniform over the

whole length of curved wall. The results were veryencouraging. The flow appeared to be steady witha large volume of dense, smokey air in the middleof the cavity. Containment was calculated, basedon the measured density of smokey air at theshowerhead exit to be very large at a flow rateratio of 25. These results were understood to beof a very preliminary nature because it was a two-dimensional mockup of a three-dimensional flow andthe Reynolds number of 1100 was quite low. How-ever, they indicated that further work was warran-ted.

A series of experiments were then performed atUARL to determine containment levels in a sphericalcavity.15,16 I n a n exploratory study, varioustypes of fuel or inner stream injection configura-tions were studied separately and then the bestdesign was used in an exploration for the optimalouter stream injection configuration. The spher-ical configuration for the "best" case containmentstudies is shown in Figure 5. The inner stream isinjected through a porous sphere. The outer streamis injected at the top and bottom of the cavitywith a tangential velocity component but the cen-tral section gives essentially, only an axial com-ponent of velocity to the propellant. The reasonsfor this design being the "best" for sphericalgeometry is that it comes closest to the

cylindrical case in design. Containment measuredin the spherical geometries at UARL was alwayslower than that measured in cylindrical designs.The basic problem of the spherical design is inthe radial pressure gradients which exist withcurving streamlines. The tangential component ofthe ourer stream at the top of the cavity sets upa rad. \ pressure gradient with a pressure minimumat the enterline near the fuel injection point.This pressure gradient apparently works against theentrainment of inner stream fluid by the outerstream. However, at the downstream end of thecavity, the inward curving streamlines must beginto curve back out so that '•he fluid can exit thechamber through the nozzle This reverse curva-ture causes a high pressure area at the centerlinenear the cavity exhaust. This high and low pres-sure system is very favorable for the formation oflarge recirculation eddies. These large eddies arevisible in many of the flow visualization studieswith spherical geometries. Furthermore, thepressure field set up around the fuel injectionlocation is very complex because of the pressuregradients and may tend to influence the innerstream fluid velocity profile at the injector sur-faces while the entrainment necessary to "pull"the inner stream out to large radius against thepressure gradient probably also results in in-creased mixing of streams.

The conclusion that can be drawn from thesecontainment studies at this time is that contain-ment in cold flow studies is greatest when theradial pressure gradients are minimized and theinitial velocity profiles are smooth, monotonic andhave small velocity gradients. This combination ofvirtues nas best been accomplished in the cylindri-cal coaxial flow experiments conducted at UARL.

Effects of He.it Generation on Containment

Most of the studies dose on the containment offissionable material or fuel in the cavity con-sidered only the cold flow of gases. The effectsof the very large heat generation rate in the fuelgas were neglected because of the difficultiesencountered in treating them both analytically andexperimentally. The few studies that have betsnperformed on "hot flows" do not readily yieldvalues for containment but rather indicate how theheat release will affect the containment as it isdetermined from cold flow studies.

The experimental work done on hot flows includea series of investigations by Grey and coworkers atPrinceton University on a flowing argon plasma andcold coa>..'.cl stream of several different gases. 17«18,19 Cold flow data was also taken for comparison.Their work also included an analysis of coaxialflow mixing. Both laminar and turbulent flowswere investigated. The earlier work centered onthe laminar case, but results showed that alaminar flow could not be maintained in their ap-paratus. In the turbulent flow studies, maps ofconcentration, velocity and temperature were ob-tained with a cooled probe technique. The flow inthis system was dominated by a wake effect whichthe investigators attempted.to quantify, and highinlet turbulence level. Schlieren pictures of theflow field showed that the coaxial field fluidwhich issued from a section, of Cujes maintained itsindividual jet stratification for at least oneinner jet diameter downstream, inhibiting mixing to

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some extent. However, the wake effect was, ingeneral, so strong that mixing occurred veryrapidly. The important conclusions that were drawnfrom this work are that the effect of core tem-perature on the concentration profiles are notlarge and that containment of argon within a cylin-der defined by the argon inlet duct diameter isessentially complete. Furthermore, the spreadingrate of the argon appears to be less with the hotflow cases than in similar cold flov; caies. Theseconclusions are stated here because they are indisagreement with published results from a laterseries of arcjet studies.

This later series of hot flow studies was doneat the TAFA division of the Humphries Corp.20»21«22>" TAFA's objective was to develop an inductionheated plasma simulation of a gas core reactor.Data consisting of concentration, temperature andstagnation pressure measurements'" were taken in acoaxial flow of argon plasma and air or hydrogen.The plasma was induced by a radio-frequency fieldafter the arc was started up with a D.C. discharge.The air/argon mass flow ratios used were 0.6, 5.55and 19.7 and the hydrogen/argon mass flow ratiowas limited to jne value of 1.67. Both hot flowand cold flow runs were performed and it was con-cluded from a comparison that "the plasma elimin-ates all turbulent recirculation present in thecold flow and appears to behave as if it had adefinite skim or boundary similar to a gas/liquidinterface." Some of the hot and colS flow con-centration data is reproduced in Figures*""**,Figure 6 shows concentration measurements for coldlaminar, axial flow of argon and air. Both streamshave equal volumetric flow rates of 100 SCFH. Theparabolic lines of constant mole fraction are citedas evidence of purely laminar flow. However, thedata apparently invalidate the concentrationmeasuring scheme tmployed. The mean mixed molefraction of air for this case is 0.50. Yet at twodiameters downstream, the parabolic flow averagedmole fraction of air is at least 0.70. Further-more, at this axial station the velocity profileis probably closer to plug than parabolic, and a .plug profile averaging would give a mean, mixedmole fraction of air of about 0.8. This indicatesthat concentration measurements are accurate to,at best, about + 50%. Other data is presented forcold and hot flows successively for different massflow rate ratios. In these cases there is atangential or swirl component of the velocity ofthe argon to stabilize the arc.

The discrepancy between the measured concen-trations and the mean mixed mole fraction existsfor the cold flow" case of each of the mass flowratios investigated. The equal velocity case,Walr/ W a r g o n « 0.67, is shown in Figures 7 and 8for the cold flow and hot flow, respectively. Inthe cold flow case, at 2" downstream the air molefraction is shown as 0. r. at the centerline. Theregion off the centerline must have considerablyhigher mole fractions of air yet the mean mixedmole fraction of air is 0.451 according to a massbalance. Another peculiar feature of the TAFAresults is visible in Figure 8 for the hot flow.At the initial plane of the mixing region theargon concentration ranges as high of 0.50 in theentering air stream. This peculiarity exists inmnst of the remaining data for both hot and coldflow.

One possible explanation for the disagreement

can be based on the measuring device employed. Asample of fluid was removed from the flow fieldwith an aspirating probe. If the probe sucked influid at a rate higher than the local flow rate inthe field, it would perturb the flow field con-siderably and result in the measurement of a con-centration averaged over a volume quite large com-pared to the probe tip*

Another observation that can be made from thesedata is that the inner stream spreads rapidlyradially outward past the radius of 'the argon duct.This is In direct disagreement with essentially allother cold coaxial flow experiments with the outsrstream to inner stream velocity ratio substantiallygreater than one. ' The Princeton hot flowstudies which also had a swirl component tostabilize the arc, show a decrease in spreadingrate from the cold flow to hot flow experiments asdo the TAFA experiments, but the Princeton measure-ments also conclude that the argon does not spreadbeyond the inner duct radius. Stagnation pressuretraverses for both hot and cold flow cases werealso made at TAFA. The probe used was the samp-ling probe. However, no attempt to orient theprobe in the streamline direction was made so thatpressures reported may not be true stagnationpressures. These apparent inconsistencies and dis-agreement with previous work make it premature tobase conclusions of high containment o,. the TAFAwork.

A second conclusion drawn from the TAFA work isthat the plasma has a "hard skin." Tl.-s is basedon high speed photographs showing 250 microntungsten particles "bcun^ing" off the plasma fire-ball". It is not clear that this conclusion hasbeen confirmed by reference tc obsarvations made byother investigators working wi .h arcs wh.'-.h are notin Be fields. jin alternate possible explanation ofthe bouncing relates to the Leidenfrost phenomenonwhich explains why a drop of water will bounce offa hot f; -ing pan. The tungsten particle wouldbegin to vaporize as it entered the very hot plasmaregion. The high rate of vapor generation wouldthen drive the particle back out of the arc.

It is also true thar electrostatic tj.ei.ds existin the plasma due to the RF field. Calculationsof the electrostatic force required to balance thegravitational force on the paryide indicate .•charge equivalent tc 103 to 104 electrons on thesurface of the tungsten is required. It ispossible that a char? . of this magnitude could beaccumulated by the tungsten as it passes throughthe outside fringe of the plasma since tungstenwould tend to "poison" the at . These alternativeexplanations for the bouncing particles a.e alsonot conclusive. However, the existence of a "hardplasma skin" should be left open to question untilit is confirmed in a plasma without an RF field.

In a later report covering tests "'hic.i werecarried out by TAF\ on a curved -, ermeable wall in-duction torch, the plasma was maintained by vapor-izing solid material which ionized. 2 Aftei start-up, the plasma was fed by the vaporized solid withno through-put of ionizing gas. The solid to cool-ant gas mass flow ratio w?s bi tween 1/IjOO to . .1/5000. Because of the very different processestaking place, the relationship to containment re-quirements in a cavity reactoi are not clearlyestablished.

127

In this report, experiments were also carriedout with a flowing argon plasma and with a cooluntgas flow all of nhich entered the flow fieldradially through a porous wall. In these tests, asin the previous ones, the hot flow conditions weresaid to prevent' all .recirculation due to wakeeffects. This could be explained as being duerather to the rapid expansion of the argon streamwhich In turn is due to its rapid temperature rise.In tac~, a gas experiencing a temperature changefrom 300°K to 9000°K would'expand in volume fcy afactor of 30. Since the confining walls preventradial; expansion of the argon, the axial velocityof the argon increases by a factor on the order of30. lit all the tests performed at IAFA, except one,the initial (cold) velocity ratio was about 34 orless, ;ind the exceptional case was about 45. Ifthese ratios are divided by 30, It Is seen thatactual velocity ratios in the plasma region areorder one or less, so that the strong wake effectwhich causes recirculation is just not present inthe hot. flow cases. This axial expansion of theinner tti-eam is also apparent in the Princetonwork where velocity traverses are presented.However, the incomplete data sets make it difficultto demonstrate it quantitatively. Figure 9 showsa case with initial velocity ratio of about 1 whereinitial Velocities are based on initial planeaverage conditions. However, the centerline tem-pera tute at the initial plane of tae mixing regionis60C0°R, arid the centerline velocity is almost 3

. times the coolant gas velocity. The eenterlinetemperature decays to 3000°R in two diameters andthe' centerlins velocity falls to about half of theinitial value, indicating expansion and contractiontake pl.<ce almost entirely in the axial direction,for these experiments where the outer fluid was notconfined.

An analytical study of the effects of heat gen-eration on an entrance region coaxial flow wasperformed at IIT. In this study, the equationset was' formulated to represent the gross behaviorof the if low field and to avoid the complexity of amore- detailed analysis. The field was modeled asa. laminar, confined coaxial flow for which theboundary layer assumptions are valid. Only radialradiative transport of energy vas considered andthe two fluids were assumed to be lsmiscible. Theresults for one typical case are shown in Figure10. Here z is axial distance made dimensionlesson outer duct radius times Reynold's number, anda ia the location of the interface between the twoimmiscible streams. Subscript a is the value oftha Inner stream property at the interface. Thecenfcerline velocity chauges from 1/10 the outer

. stream value at the initial plane, to 1.4 times the'initial value of that of the outer stream. Thecenterlii.a temperature increasfis very rapidly andthen actually falls off slightly as the thermalradiation becomes dominant. It is also seen thatthe interface location falls rapidly from 0.7 toabout 0.3 as the centerline velocity increases bya factor of 14 and centerline temperature increase,,by a factor of. only 3. If the velocity increasewas due only to thermal expansion axially alongthe centerline, the velocity change' there would beonly by,about a factor of 3. However, there isexpansion of all the inner stream due to heatgeneration and the energy radiated to the outerstream causes expansion of the outer stream also,and all this fluid expands radially inward to someextent, because of the confining wall. This ef-fect, coupled with the buildup of the wall boundary

layer accounts for the additional increase incenterline velocity. This type of behavior isindicated in the TAFA results for their hot flowcases (Figure 8). In all their cases, the lines ofhigh constant mole fraction of argon stretch out toa considerable extent in a narrow region along thecenterlihe. This is consistent with the idea ofa radially shrinking, axially expanding core ofargon.

It is difficult to draw conclusions from therelatively small amount of --ark done on how theinternal heat generation affects containment. Inthe discussion of the previous section, it was seenthat cylindrical coaxial flow gave the best cold-flow containment found to date. However, thecylindrical boundary causes large axial accelera-tion when internal heat generation is present. Itis possible then, that a spherical cavity outerboundary may be desirable for containment with heatgeneration. The key to success would be to use theinduced radial pressure gradients to force thethermal expansion to take place radially instead ofaxially while mixing and entrainment are minimizedby inducing axial streamlines downstream of theexpansion.

It would appear at this time that the most im-portant work still to be done to evaluate the co-axial flow GCR concept feasibility is in this areaof coupling the heat generation to the fluidmechanics. Both experimental and analyticalstudies are necessary to determine optimal cavityshape and inlet flow details. The untimely ter-mination of the major part of the GCR programended several efforts in this area before theyobtained the definitive results necessary to makea realistic evaluation of concept feasibility.

References

1. Weinstein, H. and Ragsdale, R. G., "A CoaxialFlow Reactor - A Gaseous Nuclear Rocket Con-cept," American Rocket Society, Preprint1518-60, 1960.

2. Ragsdale, R. G. and Edwards, 0. J., "Data Com-parisons and Photographic Observations of Co-axial Mixing of Dissimilar Gases at NearlyEqual Stream Velocities," HASA TN D-3131, 1965.

3. Taylor, M. F. and Masser, C. C., "PhotographicStudy of a Bromine Jet in a Coaxial Airstream,"NASA TH D-466C, 1968.

4. Masser, C. C,, "A Photographic Study of aBromine Jet i.n a Coacial Airstream with Honey-comb at the Injection Plane," NASA TM-X-1900,October, 1969.

5. Barchilon, M., and Curtet, R., "Some Details ofthe Structure of an Axisymmetric Confined Jetwith Backflow," J. of Basic Eng., 86, P777(1964).

6. Rozenman, T., and Weinstein, H. "RecirculationPatterns in the Initial Region of Coaxial Jets"NASA CR-1595, May 1970.

7. Ragsdale, R. G., "Effects of a Momentum BufferRegion on the Coaxial Flow of Dissimilar Gases"NASA TN D-3138.

8. Kulik, R. A., led them, J. J. and Weinstein, H.,"Effect of Tree Stream Turbulence on CoaxialMixing^" NASA CR-1336, May, 1969.

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9. Zawacki, 1. S-, and Welnstein, B. "Experimen-tal Investigation of Turbulence In the MixingRegion Between Coaxial Streams," NASA CR-959.

10. Johnson, B. V. and Clark, J. W., "ExperimentalStudy of Multi-Component Coaxial-Flow Jets inShort Chambers," NASA CR-1190, April, 1968.

11. Johnson, B. V., "Exploratory Experimental Studyof the Effects of Inlet Conditions on the Flowand Containment Characteristics of CoaxialFlows," United Aircraft Corp. NASA CR-107051,1969.

12. Bennett, J. C., and Johnson, B. V., "Experi-mental Study of One-and Two-Component Low-Turbulence Confined Coaxial Flows," NASACR-1851, June 1971.

13. Kulifc., R. A., and Weinstein, H. PrivateCommunication, 1971.

14. Lanzo, C. D., "A Flow Experiment In a Curved-Porous-Wall Gas-Core Reactor Geometry," NASATM-X-1852, August, 1969.

15. Johnson, B. V., "Exploratory Study of theEffects of Injection Configurations and InletFlow Conditions on the Characteristics of Flowin Spherical Chambers," NASA CR-196S,January, 1972.

16. Johnson, B. V., "Experimental and AnalyticalStudy of One and Two Component Flows inSpherical Chambers," NASA CR-2282, August 1973.

17. Grey, J., William, P. M., and Fradkin, D. B.,"Mixing and Heat Transfer of an'Argon Arcjetwith a Coaxial Flow of Cold Helium," NASA.CR-54438, October 1964.

18. Grey, J., Sherman, M. P., Williams, P. M.,"Laminar Arcjet Mixing and Heat Transfer:Theory and Experiments," AIAA J., 4^ p.986,(1966).

19. Williams, P.M. and Grey, J., "Simulation of• Gaseous Core Nuclear Rocket Mixing Character-istics Using Cold and Arc Heated Flows,"NASA CR-690.

20. Dundas, P. H., "Induction Plasma Heating:Measurement of Gas Concentrations, Temperatures,and Stagnation Heads in a Binary PlasmaSystem,", NASA CR-1527, February, 1970.

21. Vogel, C. E., "Experimental Plasma StudiesSimulating a Gas-Core Nuclear Rocket," AIAAPaper No. 70-691, June, 1970.

22. Vogel, C. E., "Curved Permeable Wall InductionTorch Tests," NASA CR-1764, March, 1971.

23. Poole, J. W. and Vogel, C. E., "InductionSimulation of Gas Core Nuclear Engine," NASACR-2155, February, 1973.

24. Bobba, G. K. M., "Laminar Confined Coaxial Flowwith Heat Generation," Ph.D. Thesis, IllinoisInstitute of Technology, December 1974.

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OUTER DUCT

OUTERISTREAM POTENTIAL FLOW

OUTER DUCT BOUNDARY LAYER /

FIGURE 1. Schematic of Flow Regimes in Ducted Coaxial Flow

LOW VELOCITY RATIO

HIGH VELOCITY RATIO

Tigure 2. Effect of Velocity Ratio on theStreamlines in the Initial Region of theCoaxial J e t W

INTERMEDIATE VELOCITY RATIO

AIR OUTER STREAM

a ) - Cuaxial flow at a mass /low ratio o) 30; no foaramaterial.

b ) - Coaxial flow at a mass flow ratio of 50; no foam

nv.erial.

c ) - Coaxial flow at a mass flow ratio of 30; with foammaterial present.

d ) - Coaxial flow at a mass flow ratio of 130; with foammaterial present.

Figure 3. Coaxial Flow With and With-out Foam Covered inlet(1°)

INNERSTREAMFREON-12'

CONE

a) Schematic of Configuration

b) Foam Without Cone, vo/v^ = 25.

c) Foam With Cone, vo/vi = 25.

d) Foam Without Cone, vo/v. = 75.

e) Foam With Cone, v,,/\>^ = 75.

Figure 4. Shadow graphs of a Freon 12Air Coaxial Jet System Downstream ofa Foam Covered Inlet.(13)

131

Figure 5. Cross-sectional Sketch of Spherical Cavity (16)

F igu re 6. LAMINAR MIXING PATTERN FOR ISOTHERMAL COi'DITIONS

132

11 i SCFH Air

iO Mole fraction air

mean nixedstole fractionof air 0.451

.5 1 .s 2 . 5 3.5 inches downstream

Figure' 7. MKff'G MAP WITHOUl PLASMA.AIR/ARGON MASS RATIO-0 .6T™

111 s c m Air

I I I Li..5 1 .5 ' 2.5

_13.5 inches downstream

Figure 8. MKING MAP WITH PLASMA .AIR/ARGON MASS RATIO=0 . 67 ( Z O '

133

TEMPERATUREVELOCITY, FT/SEC

ro'a c3 O H

O (t3 H Jw

0) RTJ ft H-(D C B>h M P

Oa in a3 . roH- <:rt i i a>

< n> ooaiai-1 in 3c e CBa H 3O (0 (tno om u mu on> "O n

a ft

a Hd>

oO B(U 3rt D.H-O H3So roHi H

HiH 0)» ort H-(D P

O <o roID Ort o

1.0 2,0

NITROGEN MOLE FRACTION

3 6 9PRESS DROP

0 0,2INTERFACE LOCATION

DISCUSSION

J. L. MASON: Isn't the analytical modelling ofthis kind of flow extremely difficult becauseof the need of fitting flow that requiresdifferent models together with complex boundaryconditions in between?

H. WEINSTEIN: The analytical modelling is verydifficult. I tried to brin; that out when I saidthat this was an entrance region flow and you getnone of the benefits of similarity or self-preservation. You can't characterize theturbulence. The turbulences in the mixingregions are different. He have free-sheargenerated turbulence in the annular mixing region;we have boundary layer turbulence decaying in thatregion. He have free-stream turbulence which hasonly minor interaction with the flow. We haveturbulence which is building in the outer streamboundary layer. Nobody really knows how to treatthe Interaction of the different kinds of growingturbulence.

J. L. MASON: You mentioned that there was acapability of maintaining laminar flow under onescheme of interest.

H. WEINSTEIN: It Is not laminar flow. I tried tosay that the integrity of the st-o* is preserved.I didn't want to say laminar flow. These areunstable flow situations. We can delay transitionbut we can't prevent it. These are transitionalflows, and we are going to get turbulent transition.We would like to delay transleion somewhat, becausethe L/t ratio is very small, and if we can delaytransition a little it would be helpful.

135

PROEERTIES OF RADIO-FREQUENCY HEATED ARGON CONFINED URANIUM PLAS11AS*

Ward C. RomanFluid Dynamics Laboratory

United Technologies Research CenterEast Hartford, Connecticut

An experimental investigation was performed toaid in determining the characteristics of uraniumplasma core reactors. Pure uranium hexafluoride(UFg) was injected into an argon-confined, steady-state, rf-heated plasma within a fused-silicaperipheral wall test chamber. Exploratory testsconducted using an 80 kW rf facility and differenttest chamber flow configurations permitted selectionof the configuration demonstrating the best con-finement characteristics and minimum uranium com-pound wall coating. The test chamber 'selected was10-om-long and 5.7-cm-inside diameter; operatingpressures were up to 12 atm. A UFg handling andfeeder system to provide a controlled and steadyflow of heated UFg at temperatures up to, 500 K andmass flow rates up to 0.21 g/s was employed.Follow-on tests were conducted using the UTRC 1.2W r°f induction heater facility at rf power levelsup to 85 kW and test times up to kl.5 minutes. Topermit estimating the total uranium atom numberdensity radial profile and confined uranium massin the rf uranium plasma, combined plasma emis-sion and dye laser absorption measurement tech-niques were employed. A cw single-frequencytunable dye laser and optical scanning systemwere adapted to operation at the A = 591.51* nmuranium (UI-neutral atom) line (laser line half-width = 10"^ nm). The results indicated the -taluranium atom number density reached a maximum . -'

• approximately- ICy atoms/cm^ at the centerline ofthe plasma (T^ = 98OO K). To permit a detailedpost-test analysis of the milligram quantities ofresidue deposited on the different•components ofthe test chamber, profilometer, IR spectrophoto-meter, scanning electron microprobe, x-ray diffrac-tometer, electron microscope, and Ion ScatteringSpectrometer instruments were employed. Uranylfluoride (UQ2F2) was the principal compound foundon the test chamber peripheral wall. The overalltest results demonstrated applicable flow schemesand associated diagnostic techniques have beendeveloped for the fluid-mechanical confinement andcharacterization of uranium within an rf plasmadischarge when pure UFg is injected for long testtimes into an argon-confined, high-temperature, 'high-pressure, rf-heated' plasma.

Introduction

Fissioning uranium plasma core reactors (KIR)based on the utilization of fissile, nuclear fuel inthe gaseous state could be used as a prime energysource for many spase and terrestrial applications.In addition to aerospace propulsion applications,several new space power and/or terrestrial options

are being considered; these Include:(l) Direct pumping of lasers by fission

fragment energy deposition in UF, andlas ing gas mixtures.

{2) Optical pumping of laserr by thermaland/or nonequilibriurn electromagneticradiation from fissioning UFg gas and/orfissioning uranium plasmas.

(3) Hiotochemical or thermochemical processessuoh as dissociation of hydrogenousmaterials to produce hydrogen.

CO MHD power conversion systems for gener-ating electricity.

(5) Advanced closed-cycle gas turbine drivenelectrical generators.

Reference 1 discusses the overall status ofplasma core reactor technology, the vaiious energyconversion concepts, conceptual designs of variousconfigurations, and a summary of all currentresearch directed toward demonstrating the feasi-bility of fissioning uranium FCR's.

A principal technology required to establishthe feasibility of fissioning uranium plasma corereactors is the fluid mechanical confinement of thehot fissioning uranium plasma with sufficient con-tainment to both sustain nuclear criticality andminimize deposition of uranium or uranium compoundson the confinement chamber peripheral walls.Figure 1 is a sketch of one unit cell configurationof a plasma core reactor. The reactor would con-sist of one or more such cells imbedded inberyllium oxide and/or heavy-water moderator andsurrounded by a pressure vessel. In the centralplasma fuel,zone, gaseous uranium (injected in theform of UFg o r other uranium compound) is confinedby argon buffer gas injected tangentially at theperiphery of the cell. In applications in which itis desired to couple thermal radiation from thefissioning uranium plasma to a separate workingfluid, the thermal radiation is transmitted throughthe argon buffer gas layer and subsequently throughinternally-cooled transparent walls to a workingfluid channel such as shown in Fig. 1. Thechannels would contain particles, graphite fins,or opaque gases to absorb the radiation. The mix-ture of nuclear fuel and argon buffer gas is with-drawn from one or both endwalls at the axialcenterline. For other applications in vhich it isdesired to extract power in the form of nonequi-librium, fission-fragment-induced short wavelengthradiation emissions, the transparent wall would beremoved and a medium such as lasing gas mixtureswould be mixed with either the fissioning uranium

•Research sponsored by HASA Langley Research Center, Contract HAS1-13291, Mod. 2.

136

fuel or buffer gas. The reason for this distinctionis that most transparent wall materials haveintrinsic radiation absorption characteristics att:-,e 3hort wavelengths expected to be emitted fromthe plasma in the form of fissirn-fragment-inducednonequllibrium electromagnetic radiation.

A long-range program plan for establishing thefeasibility of fissioning gaseous UFg and uraniumplasma reactors has been formulated by HASA.Reference 2 summarizes the plan which comprises theperformance of a series of experiments withreflector-moderator cavity reactors. A review ofthe past work is contained in Eef. 3. Los AlamosScientific laboratory (lASL) is currently perform-ing these cavity reactor experiments.

Integrated into this plan is the relppmentof equipment and techniques to demonstrate operationof an argon/UFg injection, separation, and recircu-lation system to efficiently separate UFg fromargon in a form adaptable to subsequent recyclingin the rf-heated uranium plasma experiment. Alsoincluded will be continued experiments to demon-strate techniques for minimizing the deposition ofuranium compounds in the exhaust duct systems*

The uranium plasma experiments describedherein comprise part of the development of the longlead time technologies required to proceed with theuranium plasma core reactor experiments.

Other previously reported experiments'^) havebeen conducted on the confinement of argon rfplasmas. These tests were directed primarilytoward development of a high-intensity, high-power-density plasma energy source (equivalent black-bodyradiating temperatures ^ 6000 K). Some of theseinitial exploratory tests included direct injectionof very dilute mixtures of UFg (typically, 1% UFgin an argon carrier-gas) into the argon rf plasma.Coating of the fused-silica peripheral walloccurred but short-time sustained plasma operationwas demonstrated.

The study described herein presents somerecent results of the uranium plasma confinementtests with pure UF6 injection and the associateddiagnostics and measurement techniques as applica-ble to plasma core reactors.

Description of Principal Equipment

The experiments reported herein were crnductedusing the UTEC 1.2 fW rf induction heater systemoperating at approximately 5.h MHz. The rf outputwas supplied by two power amplifier tubes (kkO kWoutput each) which drive a resonant tank circuit;the output of the two power amplifiers was

, resonated by a push-pull resonator. The entireresonator section consisted of two arrays of tenvacuum capacitors located within a 1.7-in-diacylindrical aluminum test tank. The rf power was .deposited into the plasma by a pair of single-turn9-cm-dia water-cooled work coils. The front of

the test tank was a removable dome containing five10-cm-dia windows for observation and/or diagnosticequipment access.

In support of the rf plasma•experiments, a UFghandling and feeder system to provide a controlledand stegdy flow of heated UFg at temperatures up to500 K was employed. Figure 2 is a schematic dia- >gram of the UF, handling and feeder system. Thesystem was designed to provide UFg mass flow ratesup to about 5 g/s and subsequent possible injectionof UFg into test chambers operating at pressures upto approximately 20 atm.

The principal components of the system werethe UFg'boiler, the boiler heat supply system, andthe UFg condenser (exhaust) system. The boiler wasa 21 Monel cylinder rated at 200 atm workingpressure. Monel was selected because of its res:* -tance to chemical attack by hot, pressurized UFg.A beat exchanger to provide internal heating waslocated in the bottom of the bciler. The thermo-couple walls, heat exchangers, and UFg flow lineswere all fabricated from g.'t-ami-Or Monel tubing.As shown in Fig. 2, electrical resistance heatertape was wrapped around the majority of con-ponents.In the tests reported herein, use of the electricalheater assembly surrounding the UFg boiler wassufficient to provide the required flow rates; infuture tests employing higher Wf, flow rates forlonger periods, electrically-heated gaseous Hg willbe supplied to the UFg boiler heat exchanger.Based on the results of exploratory UFg handling andflow metering tests, the need for elimination of asmuch contamination as possible from the UFg supplyand flow handling system became apparent.

A Matheson UFg calibrated linear mass flowmeter (Monel transducer) was located in the UFgtransfer line to provide on-line determination ofthe UFg mass flow rate prior to entering the UFginjector within the test chamber. The exhaustfroa the test chamber, craprisei. of argon, UFgj andother volatile uranium compounds, was collected inthe UFg condenser system located downstream of thetest chamber. A neutralizing trap (NaOH+H^O) waslocated downstream of the condenser system toremove any residual uranium or uranium compoundswhich passed through the flow trap shown in Fig. 2.

Figure 3 is a sketch showing a cross section 'of the basic test chamber configuration employedin the 1.2 MJ rf induction heater tests with pureUFg injection. For simplicity, only half thesymmetric chamber is shown. This test chamberconfiguratl T. was selected based on the results ofexploratory rf plasma tests using the UTRC 80 KWrf "induction heater facility''). "Jhese exploratorytests were conducted at an rf frequency of 13.56MHz and with the test chamber at atmosphericpressure and discharge power levels on the orderof 10 kW. Four different test chamber flow config-urations were tested to permit selection of theconfiguration demonstrating the best uranium vaporconfinement characteristics and minimum wall

137

coating. The test chamber configuration show:i inFig. 3 incorporated s.evsral features including theability to change: the axial location of the on-axis UFg injector, the injection area,of the argonvortex injectors, and the ability to vary the dis-tribution of exhaust gas flow and cooling water,tothe different components. The left endwall (notshown in Fig. 3) had provision for removing theexhaust gas through an axial bypass annulus locatedon the periphery of the endwall. Figure k containsa photograph showing details of this test chamberconfiguration with a uranium rf plasma present asviewed through the center view port of the testtank. The argon vortex was driven from the rightendwall by a set of eight equally-spaced stainless -steel vortex injectors located tangent to theperiphery of the endwall. In addition to the axialbypass provision in the left er.dwall, both endwallshad the option for removing varying amounts of theexhaust gas through the on-axis thru-flow ducts.As shown in Fig. 3, the UFg injector, located on-axis and concentrically within the right endwell,was fabricated from a 50-cm-long three concentriccopper tube assembly and in the majority of testswas located with the injector tip extending 2 cminto the test chamber. High pressure water (20atm) heated to approximately 350 K via a steam heatexchanger flowed at O.lU V s between the concentrictubes, of the UFg injector. This provided coolingrelative to the hot plasms environment while stillmaintaining a high enough temperature in the injec-tor to permit flowing the pressurized gaseous UFgwithout solidification in the UFg transfer line/injector. A vacuum start technique was used in alltests; the rf plasma was ignited at approximately10 mm Hg using breakdown of the argon gas at aresonator voltage o± about 4 ktf.

The total power deposited into the uranium rfplasma was obtained from an overall test chamberenergy balance by summing the power lost by radia-tion, power deposited into the annular coolant ofthe peripheral wall, power deposited into the end-wall assemblies, power conveeted out the exhaustducts, as well as, power deposited into the heatexchanger and UFg injector assembly.

, The power radiated from the uranium rf plasmawas measured using a specially constructed radio-meter and chopper wheel assembly. The total powerradiated from the plasma and in selected wavelengthbands was calculated assuming, isotropic radiationincluding allowance for blockage due to the rf workcoils. Still pictures (using neutral densityfilters) taken through the various view ports wereused for estimating the discharge diameter andshape.

Figure 5 is a schematic diagram of the' opticaldiagnostic system used for the absorption andemission measurements in the rf plasma tests withpure UFg injection. A uranium lamp (hollow cathodetype) was used as the reference standard. A con-focal Fabry-Perot spectrum analyzer with a freeopectral range of 8 GHz ( «10"z nm) was used to

define and calibrate the tuned wavelength spectrum.A power meter (together with a beam splitter) wasused to permit continuous monitoring of the dyelaser output power at the particular wavelength lineselected. A phase-sensitive-detector and lock-inamplifier and a dual set of chopper/signal generatorassemblies (capable of multiple frequency operation)were ur.ed in the system, as shown in Fig. 5. Apair of fixed front surface mirrors were used todirect the approximately 1.5-mm-dia laser beam intothe aluminum test tank (parallel to the major axisof the discharge). A beam expander (1.5-cm-diaaperture) and collimator was located inside the testtank. Another fixed front surface mirror was loca-ted in the test tank approximately 6 cm behind theaxial aidplane of the fused-silica tube test chamberat an angle of 1*5 deg to the discharge major axis.The centerline of the expanded and collimated laserbeam traversed the concentric set of fused-silicatubes which form the test chamber, 0.75-cm-.. .'-axis,as shown in Fig. 3. The laser beam exited from thealuminum test tank through an aperture and thecentral view port. A 10-cm-dia aerial lens (f/l.8,focal length = 3^.3 cm) was located inside the testtank to aid in focussing the laser beam onto theslit of the monochromator and to correct for thebeam divergence through the test chamber. The finaloptical tracking system used for making the chordalscans of the plasma emission and absorption, asshown in Fig. 5, evolved after several preliminarydesignss ray tracing analyses, and bench testexperiments. The optical system design had toaccount for a certain amount of refractio.. throughthe test chamber geometry employed. The effect ofthe concentric fosed-silica tubes was twofold.The lafractive effects of concentric, right circu-lar cylinders filled alternately with gas, quartz,water, quartz, and air were calculated and areeasily predictable. The second" effect (surfaceimperfections), however, was not smoothly varyingand cannot be compensated for in the lens andoptical system design. This effect had to becalibrated out for each test (i.e., once therotational orientation of the two concentricfused-sillca tubes was selected, they were main-tained in that orientation until completion of thatparticular test). The remainder of the opticaldiagnostic system included a fixed front surfacemirror, a 10-cm-dia, 20-cm-focal length lens, arotating front surface mirror, and a 0.25 (f/3.5optics) or 0.5 m (f/8.6 optics) monochromator sys-tem. The output signal from the photomultiplier(S-20 response) was connected to a signal processorand displayed on a strip-chart recorder. Aspecially constructed rf shielded signal generatorand controller system was used for operating themirror motor drive. The mirror drive assembly hadan independent frequency, amplitude, and off-setcontrol. The system was set-up to image a sourceof cross-sectional radius 1.5 cm without vignettingwithin the monochromator system. Since the sourceand slit exist in conjugate planes at a magnifica-tion ratio of 1.57, the 25 Mm square sjit

138

arrangement used in the monochromator representeda scanning aperture 39 P<" square at the plane of theplasma discharge centerline (both emission andabsorption). For emission measurements, the dyelaser beam was blocked and the chopper assembly(85 Hz) located adjacent to the central view port ofthe test tank was used. For absorption measure-ments, the chopper assembly adjacent to the dyelaser head was used (650 Hz). Calibration checkswere included to "erify that the dye laser outputbeam ( A= 591,5U nm) power remained essentiallyconstant for relatively long-time periods. Finetuning was accomplished during the actual UPg rfplasma tests by tuning the central frequency con-trol of the dye laser system to maximum absorption.The. A= 591-51* in uranium line was selected becauseit is a relatively strong uranium I (neutral atom)line and appears well-defined in the spectralemission scans. The lower state for this line isthe uranium I ground state. The half-width of thelaser line at this wavelength was approximatelyAA= 10-1* nm.

The signal generator u,^ controller for the.scanning mirror was set up to provide a differentscanning rate (approximate factor of 2X) betweenthe plasma discharge centerline to tb*e 1.5 cm off-axis location and the return scan from the off-axislocation back to the discharge centerline.TVpically 10 s was required for a scan. The opt'caltracking system was checked out first with only anargon rf plasma present. This permitted a verifi-cation of tracking reprod'ucibility and isolation ofthe instrumentation from extraneous vibration and/or rf interference effects.

To permit a detailed analysis of the samplesof residue collected from the various components ofthe test chamber after rf plasma tests with pureUFg injection, the following UTRC instrumentationwas employed. Of these, several have uniquefeatures.

1. Rrofilometer — permitted quantitativedetermination of the magnitude of surface coating/etehing/iirosio4 that may take place on the surfaceof the futjed-silica tube.

2. IR Spectrophotometer — permitted com-pound identification using IK spectrophotometricabsorption measurements (2.5-1+0 MM wavelengthrange) of reference material, standard compoundsand small samples of residue removed from testchamber components.

3. Scanning Electron Microprobe —permitted determination of the elements present inthe residue samples and also scanning electronmicrographs. Showed topographical and composi-tional variations and permitted x-ray mapping ofthe individual elements present.

h. X-ray Diffractometer — permittedidentification of compounds present in the residuesamples by subjecting them to copper Via radiation

and comparing the resulting x-ray diffractionpatterns with reference standards.

5. Electron Mi 3roscope using selected areadiffraction (SAD) — permitted identification ofcompounds present in the residue samples by subjec-ting them to electron bombardment. Also providedindication of relative crystallinity of thematerial.

6. Ion Scattering Spectrometer (ISS)/Secondary (or sputtered) Ion Mass Spectrometer(SIMS) — permitted identification of the.variouselements and compounds present in the residuesamples. The unique features of this instrumentthat distinguish it. from the other instrumentsdescribed above are (1) it is sensitive to just theouter monolayer of the surface; (2) its sensitivityis as high as one part of a monolayer per million;(3) it gives positive identification of the com-pounds; and, (k) it can detect hydrogen and dis-tinguish isotopes of uranium.

Discussion of Test Results

TABLE I is an example of the typical operatingconditions obtained in the emission and absorptionmeasurement tests conducted in the 1.2 Ml rf plasmatests vith pure UFg injection using the test con-figuration shown in Fig. 3. For this particularcase, the mass flow rate of the injected UFg was3.2 x 10"2 g/s. In other tests, chamber pressuresup to 12 atm and rf plasma power levels up to 85 kWwere employed.

To establish the range of dye laser uraniumline absorption (i/lo) levels through the plasma,several preliminary tests were conducted forvarious mass flow rates of UFg. The dye laser linewas set at the peak of the \= 591.51* nm line(Uranium I-neutral atom) and the 1.5-ram-dia beeio(typically 50 mW) traversed the plasma discharge onthe centerline axis at the axial midplane location(see Fig. 3). UFg mass flow rates employed rangedfrom 1.3 x 10-2 g/ s to 8.2 x 10"

2 g/s. For refer-ence, at the UFg mass flov rate of 8.2 x 10"^ g/sapproximately 9°>?o of c'ne incident intensity of the591-51* mn dye laser line was absorbed in a singlepass through the uranium rf plasma. Afteroperating for approximately 1.5 minutes at thisflow rate, the UFg injector valve was rapid]/ shutoff. The signal did not immediately return to theinitial value because of outgassing of residualhot UF'k in the transfer lines and injector assembly.Trace amounts of UFg were still observed enteringthe plasma after approximately 10 minutes. Afterwaiting for all the residual UFg to outgas, thesignal approached the initial value, indicating avery small amount of wall coating had occurred.Post-test inspection of the inner fused-silicatube verified' this condition.

The introduction of pure UFg directly intothe plasma resulted in a significant increase inthe total radiation emitted from the plasma (220

139

to 1300 nm range) as measured with the radiometersystem; a significant amount of this radiationincrease occurred in the visible and near-UV wave-length bands.

Figure 6 shows an example of the radialvajiation in the uranium rf plasma temperature andtransmission as determined from the single wave-length ( \= 591.54 nm) chordal scan measurements,atthe axial 'midplane,of the emissicn and absorptionusing the instrumentation shown in Fig. 5. Thetnordal scan measurements of thf emission and'absorption by the uranium plasma served as inputdata to a computer program used to calculate theemission and absorption coefficients. Previouslydeveloped analytical techniques fur absorption/emission coefficient determination for optically-thick plasmas were nvlapted to the UTRC dataacquisition system^,7J. Prior measurements con-ducted with similar geometries indicated vortex-confined rf plasma di .charges of this type to berotationally.symmetric. Rotational symmetry of theplasma facilitates reduction of the measuredchordal emission and absorption data to eorre-?sponding radial data by means of the Abel inversionprocedure. Under conditions of local thermodynamicequilibrium (LIE) and for plasma transmissionbetween approximately 0.2 and 0.8 (i.e., 0.2Vl/los0.8), analytical techniques have been developed fordetermining the radial spectral absorption andemission coefficients. Since the absorption andemission coefficients are related via Kirchhoff'slaw, the radial temperature profile can be deter-mined without knowledge of particle densities ortransition probabilities. This is particularlyimportant for uranium where much data are stillunknown. Hie sketch in the lower portion of Fig. 6is a simplified cross section of the test chamberand indicates the location used for the chordalscan measurements. Also shown for reference inFig. 6 is a temperature profile determined from anargon-only rf plasma test under similar test condi-tions, but with no injection of UFg. In additionto the centerline maximum temperature peak of about98OO K, e distinct off-axis temperature peak ofabout 8700 K is also evident for the uraniumplasma case. The introduction of pure UFg into thsrf plasma resulted in a suppressed temperatureprofile at tlie off-axis position between a re.dialdistance of approximately 0.2-0.8 cm. The off-axispeaks can be attributed primarily to the mechanismof depositing rf power into an annular region.

To complement these measurements and verifyv.he validity of the results, separate Uranium I( X= 591-54 nm) absorption line width measurementswere made using the dye laser system shown in Fig.5. This information was used to calculate anabsorption coefficient from which comparisons weremacu- between the neutral (Ul) uranium number den-sity l^dial distribution as determined fromemission calculations and from absorption calcula-tions; reasonable agreement was obtained.

To aid in the determination of the totaluranium atom number density distribution within therf plasma discharge, UFO and/or UFg/Ar equilibriumcalculations were made for the current range oftest parameters utilizing a computer code based onthe procedure described in Ref. 8 (ptrtial pressuresfrom 10"^ to 1 atm and temperatures from 300 to10,000 K). The following species were included: UFg,UF5, UF^, UF3, F, F", F2, U, U

+, U++, U+++, and e\Figure 7 is an example of these calculations forthe equilibrium composition of UFg at a partialpressure of 10"3 atm. For all practical purposes,at pressures of approximately 1 atm, completethermal decomposition of UFg has occurred at temp-eratures of about 5000 K. Figure 7 illustrates themole fraction of the various constituents as afunction of temperature. Note the very steepslope of the mole fractions of the various fluoridecompounds of uranium in the 2000 K regime andthat St approximately 7000 K, the concentration ofsingly-ionized uranium (UII) increases; separatemeasurements of the rf plasma characteristicsindicate that the rf plasma with argon vortexbuffer gas injection only has average temperaturesin excess of 7000 K for the type of operatingconditions reported herein. In all the calcula-tions, local thermodynamic equilibrium was assumedand lowering of the ionization potential was notincluded. Similar calculations of the variation ofthe total number density of uranium atoms and ionsas a function of UFg injection partial pressureswith equilibrium temperature as the variableparameter have also been completed for UFg/Ar mix-tures at various total pressures.

It was anticipated that operating the rfplasma with pure UFg injection at relatively highpower and pressure levels may have resulted insignificant pressure broadening to the width of theuranium, X = 591.51* nm, emission and absorptionline, lo better estimate the absorption 1-ne half-width, several nieasuretaents were made at differentpressures corresponding to approximately the sametest conditions. Calculations of the doppler widthfor the uranium A= 591-5^ nm line (no pressurebroadening) at 1000 K yields a half-width ofapproximately 3.75 GHz. The doppler line widthvaries as the square root of temperature. There-fore, at a temperature of approximately 10,000 K,the doppler width would be approximately 2.3*+ GHz.This compared favorably with the 2.5 GHz. experi-mentally measured using the dye laser system andspectrum analyzer. At a chamber pressure of 2 atm,the measurements indicated a line width vat one-half maximum intensity) of approximately 3.5 GHz.For reference, 8 GHz corresponds to 10"^ nm.

Additional calculations were completed usingthese data, the partition functions for Uranium Iand II as a function of temperature^"', the radialdistributions of temperature and transmissionshown in Fig. 6, and the variation of uraniumneutral atom number density with UFg partialpressure to calculate the neutral uranium ntomnumber density as a function of rsdjus.

140

Figure 8 shows the results obtained for Cheradial variation of total uranium atom (UI+UII+UIII)number density as a function of radial disH neefrom the centerline of the rf plasma with pure UFO

injection corresponding to the test conditions'givenin TABLE I and Fig. 6. The numbers in parenthesesare corresponding estimated values of the totaluranium species partial pressure in atm. In thisparticular case, the total uranium atom number,density reached-a maximum of approximately 10atoms/cnP at the centerline of the plasma. Based onthis plot, an estimate was made of the total con-fined uranium masssbased on these measurements ofthe neutral uranium species; this value was calcu-• lated to be 0.03 mg of uranium. The calculationsalso indicated that a relatively large fraction ofthe total confined uranium atoms existed at theouter portion of the plasma discharge.

These results were coppared to the uraniumtotal number density obtained from independent x-ray absorption measurements.'-^' At comparable testconditions to those shown in TABLE I and assuminga 23"cm-diagonal path length and a 9-5-cm-diagonalpath length, the corresponding uranium total numberdensities were 2.8 x l O ^ and 6.4 x 1016 atoms/cm^,respectively. The agteement is reasonable when oneconsiders that the x-ray measuremem.s included con-tributions due to all species (including cold UF6etc. in the boundary). Other x-ray measurementsat higher UFg injection mass flow rates (up to 9.3x 10~2 g/s) resulted in uranium densities up toapproximately k x 10^7 atoms/cm3.

The photograph shown in Fig. 9 illustrates thedegree of wall coating incurred on various fused-siliqa tube peripheral walls after tests with pureUFg injection. Eart of the tests included incor-poration of different modifications to both thetest chamber and flow control scheme to furtherimprove the confinement characteristics of theuranium plasma while at the same time aid in min-imizing the wall coating by uranium compounds.These fused-silica tubes were subssquently analyzedduring post-test inspections. The degree of coatingextended from a relatively heavy to a fairly lightcoating as can be seen in the photograph in Fig. 9.The majority of the coating occurred in the centralregion of the fused-silica tubes. Some residuewas also observed at both end regions adjacent tothe 0-ring seal location. A representative exampleof the IF spectrophotometric absorption measure-ments taken of some of the residue collected fromthe inside surface of the fused-silica tube afterthe plasma tests with pure IFg injection is shownin Fig. 10. In all cases a thin wafer KBr matrixwas used with the Perkin-Elmer IR spectrophoto-meter to obtain the IR absorbance trace as a func-tion of wavelength. In tl majority of the teststhe IE spectrophotometric measurements indicateda combination of uranyl fluoride and uranium oxidepresent on the inside surface of the fused-silicatube; traces of SiOg, silicon grease and watervapor were also noted.

A follov-on series of long run time uranium rfplasma testa indicated that the addition of siliconegrease to the 0-rings and use of as-received UFg(i.e., no NaF filter) resulted in a significantcontribution to the residue coating on the fused-silica tubes. I&rt way into the test series itbecame obvious that the use of silicon 0-rings, andunflltered argon from the labosatory bottle farm,also resulted in an increase in the residue depos-ited on the fused-silica tubes as a function oftest time. To aid in reducing this residue, the0-rings were coated with Hel-F oil. This particularlubricant, arrived at after trying other materialssuch as hydrocarbons, fluorosilicone, and halo-carbons, provided th? seal necessary between thefused-silica tube and the endwall while eliminatingthe silica gel present in the silitfone grease whichapparently reacted on the surface and causedadditional deposits. With the 0-ring lubricantchange, filtering the as-received UFg through a NaFtrap to remove the hydrogen fluoride contamination,and filtering the argon through a zeolite trapresulted in a reduction in the residue wall coatingby approximately a factor of 2. In a test employingall these modifications, the uranium rf plasma wasoperated continuously for Ul.5 minutes at a UFgflow rate of 2.2 x 10"2 g/s; 30.k mg of totalresidue was deposited on the peripheral wall.Approximately 10 mg of this was attributed to thelubricant initially applied to the 0-ring sealwhich remained on the fused-silica tube.

In other tests, aimed at determining the rangeof mass flow rates of injected UFg possible,greater than an order of magnitude increase in flowrates were achieved (21 x 10~2 g/j) while stillmaintaining the rf plasma in a confined-steady-statemode.

Results of measurements using the profilometerindicated the maximum residue wall coating on theID of the fused-silica tube to be approximately1 urn in thickness; detailed analysis of the surfaceindicated a combination of residue deposition andsurface etching had occurred. The electron photo-micrographs and x-ray mappings of the sampleresidue indicated the predominance of uranium andsilicon as compared to fluorine and oxygen. Theresults from the x-ray and electron diffractionanalyses indicated traces of several oxides ofuranium. This was also verified by the IRspectrophotometry absorption measurements shown inFig. 10. Another check on the predominance ofUOgFg was obtained froa measurements using thesecondary ion mass spectrometer (SIMS). Based onthe Atomic Mass Units fflMU) between 225 to 300, thepresence of various uranium oxide compounds wasverified.

Additional analyses were also conducted of theresiine coating on the endwall surfaces, UFg injec-tor, and exhaust ducts. In general, the followingtrends were observed'. UF4, UO2, and UO2F2 wereidentified as residue constituents on the endwallsurfaces. The same constituents were identified as

141

•being present on the UFg Injector; in addition,/ oiV^ wa.. also detected. The electron mioroprobe

x-ray cupB al>o showed the distinct presence of Cu3ti quantities comparable to the V. The exhaufrtducxfi contained traces of U02F2, 'J Og, and UF^.*fce electron photomicrographs of the various samples<jf .resi-tae removed from the different components

.each possessed a different crystalline structure.Recall tha; UO2P2) V02, SiOa (and a trace of "VUOj)vere identified as residue constituents on thefused-silica tube peripheral wall.

Thp analysis techniques developed and appliedluring this portion of the research program will beapplicable to future experiments which include

'- fipvelopmem of techniques for reconstituting thegaseous ccipounds in the plasma exhaust mixture,minimizing the wall coating, and conditioning theexhaus+ pas such that it is in a form suitable forrein.i' f-.icn :nto the plasma (recycled-closed-loopoperation).

', A mmary and Conclusions

An expr: ineital investigation was conducted•using an srgpn vertex confined uranium rf plasmadischarge to aid in developing the technologynecessary for designing a self-critical fissioning•iranium plasma core reactor. Pure UIV was in.jeeted,usi.:<? a specially developed Iffg transfer and injec-tion s/stem, into a 5.7-cm-ID by 10-cm-long testchamber obtaining the plasma and comprised of awater-coo].-d fused-silica tube peripheral wall and

• copper eniwails. Complementary diagnostic instru-mentation and measurement techniques were developed-<fcich permitted characterizing the uranium plasmaand traces of tht uranium compound residuedeposited on the t?at chamber walls. Included wasan optical scanning method for uranium plasmaemission end absorpt'on measurements using a ewsingle-frequency tunable dye laser system. Thesemeasurements provided input to a computer program(optieallj thick case) w.hich calculated thespectral emission and absorption coefficients andthe corresponding radial temperature profile usingKirchhoff's law. Ifce measurements indicated, atchamber pr :• .sures of abcut 2 atm, rf dischargepawr levels of about 60 kW, and injected UFg massflow rater of approximately 3 x. 10"2 g/s, thaturanium plasma temperatures of 9^00 K occurred atthe plasma centerline loca+ion with a distinctoff-axis peak (=8700 K) occurring at an r/R = 0.6.Edge ..of plasma temperatures were approximately6(X ~> K. I'rom these data and additional experi-mental measurements of the uranium neutral atom(iJl) absorption line width ( = 3.5 GHZ at A= 591.9*mi), the radial variation of total uranium atom(UI+UII+UIII). number density within the rf plasmawas determined. (Approximately 101" atoms/cm3 atthe axial tnidplane center line of the rf plasmadischarge1;) These atom number densities werefound to fcs comparable by two independent measure-ment techniques (x-ray absorption measuretner.ts andU plasma emission/absorption measurements).

Modifications made to the test chamber .configuration and flow control scheme and evaluatedby the various analysis techniques resulted in areduction in the residue material deposited on thecomponents of the test chamber. At UFg aiass flowrates of 2.2 x 10"S g/s, test times up to lt-1.5minutes were achieved with less than about 20 mg ofactual uranium compound residue wall coating. Inseparate tests to maximize the UFg mass injectionflow rate, up to an order of magnitude increase wasachieved (2.1 x 10"1 g/s) while still maintainingthe plasma in a confined mode of "operation. Sixdifferent complementary analysis techniques wereemployed and uranium compound reference standardsdeveloped at UTRC to permit post-test characteriza-tion and identification of the residue coatingsdeposited on the peripheral wall of the testchamber. The "mist-like" residue found on theinside diameter of the fused-silica tube duringpost-test analysis was identified as primarilyuranyl fluoride

The overall test results have demonstratedthat applicable flow schemes and associateddiagnostic techniques have been developed fvr thefluid-mechanical confinement and characterizationof uranium vapor within a plasma discharge whenpure UFg, is injected steady-ste+° for long testtimes into an argon-confined, high-temperature,high-pressure, rf-heated plasma.

1.

2.

3.

h.

5.

6.

References

Latham, T. S., et al.: Applications of PlasmaCore Reactors to Terrestrial Energy Systems.AIAA Paper 74-107*+, AIAA/SAE 10th PropulsionConference, San Diego, CA., October 21-23,

Rodgers, B. J., et al.: Preliminary Designand Analyses of Planned Flowing UraniumHexafluoride Cavity Reactor Experiments.United Technologies Research Center ReportR76-912137-1, March 1976.

Helmick, H. H., et al.: Preliminary Study ofPlasma Nuclear Reactor Feasibility. LosAlamos Scientific Laboratory Report LA-5679,August I97U.

Roman, W. C. and J. P. Jaminet: Developmentof RF Plasma Simulations of In-Reactor Testsof Small Models of the Nuclear Light Bulb FuelRegion. United Aircraft -Research LaboratoriesReport L-910909-12, September 1972.

Roman, W. C.: Plasma Core Reactor SimulationsUsing RF Uranium Seeded Argon Discbxages.AIAA 8th Fluid and Plasma Dynamics Conference,Paper *TD. 75-861, Hartford, CT., June 1975.

Freeman, M. P. ~.nd S. Katz: Determination ofthe Radial Dii> uribution of Brightness in aCylindrical Luminous Medium With Self-Absorp-tion. J. Opt. Soc. Amer., 5.0, 826, i960.

142

7. Elder, D. W., et a l . : Determination of theRadial Profile of Absorption and EmissionCoefficients and Temperatures in CylindricallySymmetric Sources with Self Absorpition. Appl.

• Opt., Vol. 4, No. 5, May 1965.

8. Gordon, S. and B. J. McBride: Computer Programfor Calculations of Complex ChemicalEquilibrium Compositions, Rockst Performance,-Incident and Reflected Shocks, and Chapman-Jouquet Detonations. NASA SP-273, 1971.

9. luascella^ K."L.: Theoretical Investigationof the Composition and Line EmissionCharacteristics of Argon-Tungsten and Argon-Uranium Plasmas. United Aircraft ResearchLaboratories Report G-9IOO92-IO, Spetember1968.

10. Roman, W. C : Laboratory-Scale Uranium PlasmaConfinement Experiments. United TechnologiesResearch Center Report R76-912205-1 (to bepublished).

List of Symbols

AMU

f

Io

1A0

logloC

I

raAR

rA0)

Atomic mats unit, dimensionle

RF operating frequency, MHz

Incident or source intensity,arbitrary units

Transmission, dimensionless

Absorbanee, dimensionless

Liters

Argon mass flow rate, g/s

HUII

"UIII

UFg mass flow rate, g/s

Number density of neutral uraniumatoms, atoms-cm~3

Number density of singly-ionizeduraniua atoms, atoms-cm~3

Number density of doubly-ionizeduranium atoms, atoms-cm"3

Total number density of uraniumatoms, atoms-cm"3

Pressure, atm> ran Hg, or toir

Chamber pressure, atts

Total rf dishcarge power, kW

Radius of plasma, cm

Radial distance, cm

Temperature, deg K

Wavelength, nm or microns

Mole fraction of species i,dimensionless

Wavelength-band, nm

<x,T Designation of differentcrystalline forms of uraniumcompounds :

Table I

Summary of Operating Conditions for Chordal ScanMeasurements of Test V Conducted in 1.2 MW RFInduction Heater With Bf Plasma and Pure UFg

Injection

See Fig. 3 for Sketch of Test Chamber ConfigurationSee Fig. 5 for Schematic of Diagnostic System Used

for Chordal Scan Measurements.

Argon Injection Mass Flow Rate 2.58 g/s

Argon Injection Velocity 21.6 m/s

Percent Axial Bypass Flow !<8

UF& Injection Mass Flow Rate 3.2xlO"2g/s

UFg Injection Velocity 0.7 m/s

Mass Ratio 0.012

Chamber Pressure 1.95 atm

RF Discharge Power 58 kW

RF Operating Frequency 5.1<21O MHz

Power Radiated (220-1300 nm) 19.5 kW

Fraction of Total dischargePower Radiated. . 0.31*

Discharge Diameter at Axial MMplane* • 2.8 cm

Test Time 2 rain

Residue Deposition 8.1 rag

Working fluidchannels containing(.articles, graphitefins or opaque gsses

internally cooledtransparent wall

(-0.50 M-{

Fig. 1 Details of Plasma Core Raa.rtor Unit Cell.

143

UFg Condenser system

1$ Control vaivt

@ Thermocouple

JPTJ Pressure transduce*.

IT) Flownister

;- i > ' U . .. .UFg Bailer System

Fig. 2 Schematic Diagram of UFg Transfer System.

. Spectrumanalyzer -^y—•

Pov">'

Test chamber

Fig. 5 Schematic of Optical Diagnostic System Usedfor Absorption and Emission Measurements inRF Plasma Tests With1 Pure UFg Injection.

,-Frobing dye laserbeam cenfe.rllne

«injector

Secllon AA '—A d siliccf tubes

Yig, 3 Sketch of Test Chamber Configuration Usedin Tests in 1.? IVW RF Induction HeaterWith Pure UP^ Injection.

F=2.4atm Qi=52kw f=5.4Z78 MHz d =2.9 cm

mAr=2.2.G/S myp =0.0S G/S

Fig. k Thotograph of RF Argon Plasma in 1.2RF Induction Heater Test Cuamber WithPure UFg Injection.

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4i -cm

Fig. 6 Example of Results of Radial Variation inTemp rature and 'Xransmission Obtained WithPure UFg Injection Into RF Argon Plasma.

0.05 -

6000

T- PEG K

8000 10.000

Fig. 7 Equilibrium Composition of UFg at siteetial Pressure of 0.001 atm.

Fig. 8 Radial Variation of Total Uranium Atou(U3J,UII,UIII) Number Dearity for UFg iffPlasma Test Case V.

Fig. 9 Photograph of lused-Silica Tubes 'Jsed inRF Plasma Tests Viith. Pure UFg :.njectione.:id Incorporating Modifications to TestChamber/Flcv Scheme:.

MOO

H2O

DISCUSSION

D. H. DOUGLAS-HAMILTON: What kind of radiationintensities do you expecf to propagate throughyour quartz window?

W. C. ROMAN:centimeter.

About 27 kilowatts per square

T. S. LATHAM: Another point we are discoveringwith UFg is thnt it is a very strong UV absorberwhich means that the radiating i,pectruE is verymuch like a black body at che edge of the fuelcloud, and we have less UV content with uraniumhexaflouride as a nuclear tue! ciian we used tocalculate for pure uranium. So the UV loadingon the fused silica walls wid. LT^ fuel shouHbe substantially reduced over what it was wit[tpure- fuel.

1600 1400 1000 600 200

H2O SiO2 UO2F2 UO2

Wavenumbot, A - cirr 1

Fig. 10 Example of IR Spectrophotometry AbsorptionMeasurements Obtained IYom Post-TestAnalysis of Residue "Wall Coating.

145

THE EMISSION CHARACTERISTICS OF URANIUM HEXAFLUORIDE*AT HIGH TEMPiSRATORES

H. L. Krascel la! United Technologies Research Center

' East Har t ford , Ct.

•j .•*•- . . Abstmjt

An experimental study was conducted to ascer-tain the spectral characteristics of uraniumhexafluoride (UFg) and'possible UF thermal decom-position products as. a function of temperatureand pressure. ,• . .

Relative emission mc-asurements were made forUFg/Argon mixtures heated in a plasma torch overa range A temperatures from 800 to about 36OO Kover a vavelength rangR from 80 to 600 nm. Totalpressures were varied -Torn ,1 to approximately1.7 »tm. Similarly absorption measurements werecarr'pd out In the visible region from h20 to •580 nil over a temperature range from about 1000to 1800 K. Total pressure for these measurementswas 1,0 atnu

The emission results exhibited relatively noemission at wavelengths below 2*0 nm over therange of temperatures investigated. At tempera-tures in excess of 1800 K an additional emis-sion oand centered at 310 nm appears and becmesmore well defined at higher temperatures. Essen-tially no pressure effect was observed with re-spect to emission at pressures up to 1.7 arm.

"Effective" spectral absorption cross-sectionswere determined at, five temperatures '-etween1000 and 1800 K and in the wavr]ength regionbetween )'2O and 580 nm. The cross-sect? TIS rangedin value from about 9 x 10"20 to 2 x 10-19 cm2.

I . Introduction

Extensive experimental and theoretical researchhas been conducte'd with respect to variousaspects of fissioning gaseous plasmas over the

. past two decaces. These studies have been general-ly .directed toward high performance nuclear .space projulsion systems^. Iu addition tc spaceapplications, the Plasma Core Reactor [TC3)concept nan beer, recognized as a possible candi-date energy source for various terrestrialapplications2. The very high operating tempera-tures generated by fissioning plasmas tend toenhance efficiencies of various thermodynamieoy:\es -ompared to conventional power generatingsystems or nuclear reactors with solid fuel

"Research sponsored by HASA Langley ResearchCenter, Contract HASI-I329I Mod. 2.

elements. Furthermore, the high operating tem-peratures associated with gaseous fissioning reac-tors provide a source of very intense electro-magnetic radiation with a number of possible appli-cations involving direct coupling of energy byradiative processes.

Although most of the applications of the PCRrequire extensive basic research and technolo-gical development, the potential benefits fron theuse of these devices uarra.it continued investi-gation of the concept. Possible PCR space and/orterrestrial applications are: *

(1) High-thrust, high-specific-impolse spacepropulsion systems.

(2) Advanced high-temperature, closed-cycle'gas turbine electrical power generation.

(3) MM) conversion systems fcr electricalpewer generation.

(h) Photochemical and/or thermoehemical^vocessing.

(5'< Mreot pucplr.g 01 lasers by deposition 0/llssion fragment, pnergy in UFg or other gas mix-tures .

(6) Optical pumping of lasers by thermaland/or nonequillbrium radiation emitted by agaseous fissioning 'JFg or uranium plasma.

A typical unit cavity of a PCR device isillustrated in Figure 1. In the PCR concept ahigh-temperature, high-pressure plasma ia sustainedvia the fission process in a uranium gas injectedas UFg or othei- uranium compound. Containment ofthe plasma is accomplished fluid-mechanieally bymeans of an argon-driven vortex which also servesto thermally isolate the hot fissioning gasesfrom the surrounding wall.

For applications vhieh employ thermal radiationemitted from the plasma, an inten-aily-cooledtranbparent wall can be employed to isolate thenuclear fuel, fission fragments, and argon in aclosnd-circuit flow loop and permit transfer ofthe radiant energy from the plasma to an externalworking fluid. For applications which employfission fragment induced short wavelengtU non-equilibriUD. radiation emitted from the plasma,

146

the working fluid such as lag ing gases can beeither mixed'with fissioning gas or injected intothe peripheral buffer gas region such that thereis no blockage of radiation due to the intrinsicabsorption characteristics of transparentmaterial wavelengths.

Three fundamental area? of research are re-quired to demonstrate the feasibility of the PCRconcept: (1) nuclear criticality; (2l f'.uidmechanical confinement; and, (3) transfer ofenergy by radiation processes. Various aspectsof these areas of technology are currently beinginvestigated at United Technologies Research Center(VTRC). In addition, cavity reactor experimentswhich employ gaseous OTV are currently being per-formed at Los Alamos Scientific laboratory (IASL)as part of the planned NASA'program to determinethe feasibility of plasma core reactors.

The present report summarizes recent results ofemission and absorption measurements in hot UFg/argon mixtures at various wavelengths, Temperaturesand pressures. These data are required to providebasic absorption data for radiation transportcalculations and reictive emission data fJr sub-sequent cvaparison cf theoretical calculations withexperimental results.

II.

The equipment used in tlie experimental evalua-tion of the spectral properties of UF5 und possibleUFg decomposition products consisted of three majorcomponents. Iheae were the plasma torch-opticalplenum assembly, the monochromator, and the UF£transfer system'. A schematic of the overall systemis shown in Figure 2; detai ls of these componentsas well as other auxil iary equipment are discussedin the following sections.

Plasma Torch-Plenum Assembly

A plasma torch faoi_Jty, designed s«nd developedby UTRC, was used to prolific hot UFg/argon mixturesfor a l l experimental deterainat i :>•". A cross-sectional schematic of the plasma t.'rch i l l u s -t ra t ing the major torch corarcneuts i i depicted inFigure 3* The principal component*, of1 th<? torchare a pin-type cathode, a liojiiw conically-shapedanode, an ar^on injector, a UF injector and twoexternal magnets. The cathode is a hemispherical-tipped 2 percent thoriati-d tungsten rod 0.1 sm indiameter and was provided with m^ans for indepen-dent water coolirg. The cathode extended par-t i a l l y into the water-cooled coniceu ly-shapedcopper anode as shown in Figure 3- Argon wasirjocted tangentially through eight equally spaced0.016-cm-diameter holes Ircated a t the base of theeathude assembly. The aerodynamic swirl impartedby the argon injection system was augmented magne-t i c a l l y b; means of two external magnets locatedconcentrically about the anode as s"iown ir.Figure 3-

A Rapid E lec t r i c , Model SRV/MAN, 200 kw dcpower supply was used to provide power to the d i s -charge. The external magnets were supplied byHypertherm, Model H-1AU dc power supplies rated a t16.2 kh (90 v, 180 A) with a 60 percent duty cycle.The large magnet was connected to -wo paral le led

- units to provide higher current (360 A) and thus ,a greater magnetic f ie ld . The smaller magnet wassupplied by one unit .

A UFg injector was located immediately adjacentto and downstream of the anode. Preheated UFgwas injected into the aigon stream heated by thetorch via two 0.0l5-cm-diameter holes. The aero-dynamicall; mixed high temperature gas stream wasthen introduced into tne cylindrical s t a in less - s t ee lplenum (see Figures 2 and 3) . The plenum wasequipped with six optical ports (3 oppositely posi- 'tioned pairs) which enabled viewing the high tem-perature gas stream spectroscopically in absorptionor emission. The optical path through the mixed gasstream was 10.2 cm in length. The hot exhaust gaseswere subsequently cooled in a heat exchanger con-s is t ing of multiple copper coils prior t o beingneutralized in a sodium bicarbonate trap system (SeeFigure 2 ) .

A high pressure (kC atm; water pumi was usedto provide cooling water for c r i t i c a l torci. compo-nents. Separate water flow loops wern used '.o coo1,the 'uthode, anode, exterral magnets .ind the OTVinjector. Water cooling was not provided for tr.eoptical plenum. Each cool.ing water flow loop con-tained a flow meter to mor.itor water volumetric flowand a thermocouple to monitor exit wat^r tempera-

te. Inle t water temperature was aino monitoredaieano of a thermoccup ?. Thus, power dissipation

in ""arious componertT couj •: be calculated- Simi-la r ly a flow meter \ras in. .*.:ied in the arson l ineto the torch to pro\ ide me'ins for quanti tat ivelymeasuring argen ma'.<3 flow in the system. Threethem- couples weve attached to the plenum to moni-to r wall temperatures in the vic ini ty of the opticalports as shown in Figure 3- A fixed thermocouplewas inserted into the hot, gas flow downstream ofthe plenv.t and immediately > ^c the heat ex-changer. In addit ion, a sca-.inlr^, thermocouple system(tungsten - 5 percent "f?eaiuni vs tungsten - 26 per-cent rhenium thermocouple) was ins ta l led such thattemperature scans could be made in the UFg/argonmixture para l le l to the optical r s th .

The plenum w?.s equipped with both a transducerand a gauge for mor'torirg votal prersure in thesystem. All thermocouples associated with monitoringva te r , component and exit-gas temp-"ature werecopper-constan-an. Temperatures were monitored bymeans of an eisht ohanne'. Sanborn recorder.

Monochromator

.. McPherson Model 235, half-meter scanning mono-chroa»*or of the Seya-Uamioks type was used for a l lspectral measurements. Iiitra;ic? anc. exit s l i t

147

systems contained three slit widths; 100, 200,and tOO nm. A 1200 G/tnm and a 300 G/nvm gratingswere available for use. The 1200 u/mm grating wasblazed at 150 nm and covered the range from 50. to about 300 nm. The 300 G/mm grating was bl- iat 550 nm and covered a wavelength range from 5l>nm to about 1200 nm. Scanning speeds could bevaried in twelve steps from 0.025 nm/min to100 nm/min for the 1200 G/mm grating and from0.1 nm/min to 400 nm/mln for the 300 G/mm grating.

The radiation detector system consisted of aMoPherson Model 650 detector assembly and aModel 790 detector electronic system. The detec-tor contains a sodium salicylate coated windowand an EMI Model 9-635B "dialkali" coated phoU-multiplier. The sodium salieylate-photomultipllersystem is sensitive from 50 to about 600 nm.The 600 nm upper wavelength limit is dictated bythe photodetector cut-off. The detector output•nas monitsreri on one channel of a Hewlet-Packarddual channel recorder.

A 2.5 cm diameter transition section approxi-mately 30 cm in length was vwecl to provide anenclosed optical path between the plenum and themonochromator. The end of the ^ransition piecein contract with the inner wall of the plenumwas fitted with a 0.6 x 1.°. cm slit to minimizeflow of UFg into the external optical path.The transition section was equipped w.'th a windowmount to pe-nit use of a lithium fluoride windowwhen scanning at wavelengths above approximately120 nm. In addition, an argon purge system wasinstalled in the transition section to provide aslight back-pressure of gas with respect to theplenum in order to prevent flow of UFg from theplenum into the monochromator or onto the win-dows. In addition, the monochrnmator housing andthe window assembly used in conjunction with thetungsten-halogen source lamp for absorption measure-ments were provided with argon purging to preventpossible contamination by UFg vapors.

UFg Transfer System

The transfer system consisted of a two IMonel supply canister rated at 200 atm withappropriate shut-off and metering valves asindicated in Figure 2. Two chro- ,1-alumel thermo-couples were installed in the a -^ter to moni-tor the temperature of the UFg liquid and gasphases. A Ifctheson linear mass flow meter wasused to determine UFg' mass flow rates duringvarious experiments. The mass flow meter outputwas, continuously monitored oo the second channelof the Hewlet-Packard dual channel recorder duringall tests involving UFg flow. The canister,valves, oass flow meter and all lines in the UFgtransfer system were electrically heated by meansof Variac controlled heater tapes. Chromel-alumel thermocouples were placied in various com-ponents to monitor temperatures at strategiclocations in the UFg flow loop.

III. Experimental Procedures

A series of preliminary experiments were con-ducted to determine the operating parameters ofthe plasma torch system and to establish coolingwater flow requirements for the facility. Thisseries of tests was conducted with argon flowonly; no UFg was used nor were spectral scansmade. During these tests water flow and tem-peratures were monitored as well as temperaturesfor various system components. Similarly, gasexit teaiporatures were measured. In addition,these tf:sts indicated that run tir -_• decreasedmarkedly (about 3 minutes) at operating condi-tions which yielded temperatures above approxi-mately 2000 K. The limiting factor was primarilyattributable to thermal overloading of the exter-nal magnet power supplies.

It should be noted that, the flow rate of argondetermines the fas-mixture tempeiature in tneplenum to a very large degree. Thus, variationof the argon flow to obtain various UFg-to-argon mass ratios was precluded. Changes in UFg-to-argon mass flow were accomplished by varyingthe quantity of UFg inu_cted into the gas stream.

Subsequently, spectral studies with flowingUFg were undertaken. Normal procedure involvedinitiating the discharge on argon injected at alow flow rate to facilitate start-up. The argonflow and discharge current were then increasedto predetermined values and the system allowedto equilibrate. Spectral scans were then con-ducted to determine the emission from argcn withoutUFg flow. Prior to starting, the UFg transfersystem was preheated to a temperature of approxi-mately t30 K. UFg was not injected until theinjector temperature was greater than 350 K toprevent condensation of UFg in the injectorports with resultant clogging of the UFg injectorsystem. After the UFg injector reached operatingtemperature, UFg flow war initiated and moni-tored by means of the lineai mass flow meter untilconstant flow was obtained.• Constant UFg flowusually was achieved between 30 to 60 secondsafter flow was Initiated. Spectral scans ineither emission or absorption were commenced

Temperature scans were taken immediately afterthe spectral scans were completed. The tempera-ture and spectral scans were not conducted simul-taneously because of objectionable photodetectornoise levels generated by the scanning thermo-couple drive-motor.

Spectral data were recorded at a rate of 400nm/min when using the 300 G/mm grating and at 100nm/min when using the 1200 G/mm grating. Thehigh scan rates were required because of realtivelyshort plasma torch operating times at high tem-peratures and to minimize exposure of the equip-ment to hot corrosive UFg. Monochromstor slit

• 1 4 8

width was constant at 100 pm for all emission

and absorption measurements.

Initial spectral results were of poor qualitybecause of high noise levels due to mechanicalvibrations. Subsequently, the monochromatortable was equipped with "isopads" and additionalmounting support provided for the plasma torch-plenum assembly. In addition, a metallic bellowswas installed in the transition section betweenthe plenum port and the monochromator entranceslit to minimize vibrational coupling between thetwo systems. With these changes the vibrationwas reduced to negligible levels.

A tungsten-helogen lamp (quartz envelope)was used as the radiation source for the absorp-tion measurements. Absorption measurements wereconducted as follows. First, a spectral scanwas made at a given argon mass flow rate and gastemperature to determine the emission due to hotargon. In all cases investigated no argon emis-sion could be observed except under very highamplifier gain as compared to the lamp or UFgemission. Consequently, the incident radiation,Io, was measured by irradiating the hot argonwith the lamp at the specified operating condi-tions. Next, a UFg flow was established and thecombined radiation, Iujy + 1^, (from the lampand UF6 emission) was measured. This determina-tion gives the total radiation emitted by the hotUP< and the quantity of lamp radiation which istransmitted through the plasma. Finally, theemission due to UFg, i.e., IUFCJ u a E determinedwith the lamp extinguishheu. Thus the relativeintensity transmitted through the absorbingplasma was determined and is given by:

V ' (1)

Shut-down procedure involved termination ofUFg flow followed by careful purging of UFgtransfer lines and the UFg injector system withargon preheated to a temperature of approximately500 K. Failure to adequately purge the systemwith hot argon invariably resulted in condensa-tion of UFg upon cooling and complete clogging ofthe small ports in the UFg injector.

Wavelength calibration of the two gratings wasperiodically effected using a mercury dischargelamp placed at the entrance slit to the mono-chromator.

IV. Results and Discussion

Relative Spectral Emission

A series of twenty-six separate emission scanswere made for wavelengths in the region between120 to 600 nm. The 300 G/mm grating was used forthese studies along with the lithium fluoridewindow. ; The window was utilized to eliminate thepossible contamination of the monochromator com-ponents by UFg. Experimental parameters inclu-ding temperature, UFg and argon mass flow rates,the calculated partial pressure of UFg and theUFg-to-argon mass flew ratio are summarized inTable I for these twenty-six runs. The pressurein the optical plenum was maintained at 1.0 atmfor all twenty-six cases described in Table I.

It should be noted that the'maximum range fora sheathed tungsten thermocouple is approximately3000 K. Therefore, the value of 356O K (scannumber 26) represents the result of three tempera-ture estimates. These are: (1) an extrapola-tion of the thermocouple calibration curve tohigher temperatures, (2) heat balance for thesystem which defines an average gas temperatureand (3) a correlation between observed tempera-ture profiles (obtained vith the scanning thermo-couple) and an average gas temperature as deter-mined by the thermocouple in the partially cooledgas stream well downstream of the optical path.

Three representative gas-mixture (.UFg/f.rgon)temperature profiles are shown in Figure '+ as afunction of relative position along the opticalpath iu the plenum immediately adjacent to theUFg injector. The profiles are relatively flatusually exhibiting a maximum temperature variationof less than approximately 100 K. Since a rela-tively large thermocouple was used (O.32 cm-dia)the steep temperature gradient adjacent to thewalls of the optical plenum are not evi-«it inFigure h. Wall temperatures measured at the outersiirfaee of the plenum ranged from 800 to about1500 K and depended essentially upon the argonflow rate and the run duration. There was nodirect correlation between wall temperatures andgas temperature since spectral scans were init-iated as soon as possible to reduce run time andexposure to UFg. In essence, equilibrium tetweenthe gas-mixture and wall temperature was notnecessarily established prior to spectral scan-ning. The effect of this thermal nonequilibriumof the walls 01, spectral scans was negligible asnoted on repetitive spectral scans at nearly iden-tical gas temperatures but different wall tem-peratures .

149

Results of typical emission scans are illus-trated in Figure 5 as a function of wavelengthfor several temperatures between approximately1000 K and 36OO If. The relative intensities shownin Figure 5 have been corrected for various varia-ble experimental parameters such as photodetectorresponse and different scale factors. The emis-sion due to argor. was always found to be negli-gible compared to UFg (at least 1 order of magni-tude less)j thus no correction for argon emissionwas required.- In addition, the measured relativeemission intensities have been normalized bydividing the corrected intensities by the corres-ponding calculated partial pressure of UFg.Therefore these-results illustrate the emittedintensities per unit pressure (nun Hg) of UFginjected into the system.

The long wavelength limit (580 nm) was dic-tated by the photodetector response which rapidlyapproached zero between 580 and 600 nm. In allcases experimentally investigated, the emittedintensity reached negligible levels between 250and 300 nm. Ho significant emission was observedat wavelengths less than about 250 nm at all tem-peratures although wavelength scans were madedown to the lithium fluoride cut-off at approxi-mately 120 nm.

All spectral traces shown in Figure 5 exhibitsimilar characteristics particularly in the visibleregion between 380 and 580 r.m. The first appearanceof a new emission band at a wavelength of about310 nm is noted at about 1800 K and becomes morepronounced with increasing temperature. Inaddition there is an enhancement in emission atwavelengths less than about 380 nm. with an in-crease in temparature.

The spectral emission results of Figure 5were integrated between 300 and 58O nm to deter-mine the variation in relative total emission asa function of temperature. These calculated totalemission results are depicted in Figure 6 as afunction of temperature and are normalized withrespect to the integrated results at 35^0 K.Ho data below 300 nm or above 58O nm were usedbecause of the very weak signal observed. Thetotal intensity between 300 and 580 nm increasesrapidl/*uf to a temperature of about 1700 K andthen much less strongly at higher temperaturesas shown in Figure 6. The emission increasesapproximately by nine orders of magnitude between800. and 36OO K. A summary of these results aswell as integrated intensities at lower tempera-tures is shown in Table II. Equivalent inte-grated blackbody. <iata over the same temperaturerange and in the same wavelength range are alsoillustrated by the dashed line in Figure 6.

Fractional integrated intensifies per 40 nmwavelength intervals were calculated from theresults of Figure 6. These calculated resultsare shown in Figure 7 as a function of the wave-length at the center of each 40 nm interval for

four representative temperatures between 1400and 36OO K. The results in Figure 7 illustratethe enhanced emission at wavelengths below-400and 450 nm as the temperature is increased andthe corresponding decrease in emission vithincreasing temperature at longer wavelengths.Additional calculated fractional intensities notshown-in Figure T are tabulated in Table IIIfor all temperatures investigated.

The calculated equilibrium composition of aUFg/ai'gon mixture is shown in Figure 8 as afunction of temperature at a \,otal pressure ofone atmosphere and for a part'al pressure of UFg

. equal to 0.5 mm Hg. Comparison of the spectralemission results of Figure 5 with the compositiondata of Figure 8 indicate that the appearance ofthe new band (peak X ~ 310 nm) occurs at about1800 K and approximately coincides with the appea-rance of the UF5 decomposition products, UF^ andUF as well as fluorine atoms, F. Assignment ofthe band to a specific species is not possibleon the basis of these data. Further assessment.of the spectral results with specific componentsfrom the thermal decomposition of UFg are pre-cluded becauue of the complexity of the compo-sition scheme at higher temperatures.

It is known from black-body relationshipsthat only about 0.21 percent of the total emittedradiation occurs at wavelengths less than 300 nmfor a black-b^dy at a temperature of 35OO K-Therefore relatively little emission of radiationwas anticipated for UFg and/or its decompositionproducts at the same temperature or at lowertemperatures. Although the previously describedexperimental results indicated no radiation below250 nm at any temperature, a series of threeemission scans were oade to confirm these results "and to examine the spectral region below thelithium fluoride wavelength cu- -off at approxi-mately 120 nm. Thase scans were accomplishedin a windowless mode using the 1200 G/mm gratingat temperatures of 10S0, 1400 and 2280 K.Additional experimental parameters are enumeratedin Table IV. Ho radiation (line, band or continuum)was observed in confirmation of the previousresults. Therefore, additional scans in the win-dowless mode were discontinued.

A final series of six emission studies wereconducted at total plenum pressures up to 1.7atm to examine the effect of pressure on theemission from hot UFg and its thermal decompo-sition products. The various experimental para-meters for these cases are summarized in TableV. The result of these tests did not indicatean appreciable pressure effect as shown by acomparison of the relative spectral emissions ata temperature of about 2400 K for a total pressureof 1.0 and 1-7 atm (see Figure 9). Similarresults were obtained at other temperatures andpressures. The two curves depicted in Figure 9

150

are also indicative of the reproducibility of the. previous emission results at similar temperaturesobserved in the scans at a total pressure of1.0 atm.

Spectral Absorption

Finally, a series of five experiments wereperformed to ascertain the absorption character-istics of. UFg in the visible region and at ele-vated temperatures. A typical set of transmissionresults at a temperature of 1270 K are shown inFigure 10 as a function of wavelength between 420and 580 run. The figure illustrates the sourceintensity (tungsten-halogen lamp) Io, the combinedintensity from the emitting UFg plus the trans-mitted source intensity (lUFg + !•%) &s well asthe emission from the hot UFg, I

The relative spectral transmission derivedfrom the results of figure 10 and similar resultsfor other temperatures are shown in Figure 11.These data were used to determine an "effective"spectral cross-section over the wavelength ofinterest as follows:

I._£ = exp - (cr NI) (2)

where I^A is the transmission (Figure 11), N,the number density, L the path length and <J thecross-section. The quantity a is an "effective"cross-section since th° UFg may be partiallydecomposed, particularly at higher temperatures.Furthermore', the individual spectral detailsassociated with each absorbing species cannot beascertained. Therefore, the particle densityfactor serves to define an "effective" cross-section per unit UFg pressure. Calculated "effec-tive" cross-section results are graphically depic-ted in Figure 12 as a function of wavelength atthe temperatures studied.

The magnitude of the "effective" cross-sectionvaries from about 2 x 10"1? cm2 at short wave-lengths to a minimum of approximately 9 x lO"2^cm2 at the long wavelengths. Tabulated values ofthe cross-section at 10 nm intervals are summarizedin Table VII for the five temperatures investi-gated.

IV. Conclusions

The broad-band relative spectral emissi mcharacteristics of UFg and its thermal decom-position products have been experimentally examinedover a range of wavelengths (80 to 600 nm),pressures (1.0 to 1-73 atm) and temperatures(800 to 36OO K). The "effective" absorptioncross-sections of UFg and its thermal decompositionproducts have been determined over the wavelengthrange from 420 to 580 nm and at five tempera-tures between approximately 1000 and 1800 K.

The following conclusions are inferred fromthese experimental investigations:

\. The total integrated intensity between300 and 580 nm increases with temperature between800 and 3600 K. The increase in intensity between800 and 36OO K is about nine orders of magnitude.

2. There is no significant emission ofradiation at wavelengths bitween 80 and approxi-mately 250 nm for the temperature regime inves-tigated.

3. The appearance of a new emission bandcentered at approximately 310 nm is associatedwith the appearance of substantial quantities ofUFg thermal dissociation products at temperaturesin excess of 1800 K.

4. The emission of radiation from UFg

or its dissociation products does not exhibita measurable pressure dependence at total pres-sures up to 1.7 atm.

5. The "effective" absorption cross-sectionsat wavelengths between 420 and 58O nm vary from9 x 10"20 to about S x 10"!9 cm2.

References

1. Thorn, K., R. T. Schneider, and F. C. Schwenk:Physics and Potentials of Fissioning Plasmasfor Space Power and Propulsion. Paper No.74-087. Int'l. Astr.onautical Federation (IAF)XXVth Cong., Amsterdam, 30 Sept. 5 Octo. 1974.

2. Latham, T. S., F. R. Bianeardi, and

R. J. Hodgers: Applications of Plasma CoreReactors to Terrestrial Energy Systems. AIAAPaper 74-1074, AIAA/SAE 10th Propulsion Conference,San Diego, Calif.., October 21-23, 1974.

3. Roback, R.: Thermodynamic Properties ofCoolant Fluids and Particle Seeds for GaseousUuclear Rockets. UARL Report C-910092-3, Preparedunder NASA Contract NASw-b47, September, 1964.

G/mm Number of lines or grooves/mm of gratingsurface

I Relative intensity for absorptionmeasurements, see Fig. 12 - dimensior *

Io Relative source lamp intensity or Inci-dent intensity - dimensionless

It Relative transmitted intensity - dimen-sionless

I^(T) Relative spectral emission at tempera-ture T - dimensionlessRelative total or integrated emissionat temperature T, dinr -.sionlessRelative integrated spectral emissionper wavelength interval, - dimensionless

L Path length - cmm Mass flow rate - g/sec

151

-3Kumber density, cm'Pressure, atm or.mmHgRs:ative position - dimensionlessAbsolute temperature - KWavelength - nm or ftEffective crosa-section - cm2 per moleculeofUFg .

TABLE I

SUMMARY OF EXPERIMENTAL COMDinOBSFOR EMISSION STUDIES

(Lithium Fluor ide JJittdov, 120 < \ < 600 nm)

• p t o t a l = 1-0 *t™ * ' "

No.

12

3456789

IP1112131415161718192021222324

2526

Temp.K

810107010301260132013301330133013401360l4oo14001400151015101530168017301810i860206023002430274027703560*

•"UFgg / s

0.0620.0560.0160.0100.0460.0480.0350.0500.0120.0020.0650.0650.0420.0140.0110.0200.12O . U0.0120.0430.00860.00840.00560.0160.0260.013

~Arg / s

0.791.461.211.082.491.932.492.491.461.082.493.161.472.491.10.2.491.483.491.463.494,715.08fi.085.464.915.46

UFgmm Hfi

6.83-21.10.801.62 . 21.21.70.710.162.31.82.50.480.860.70

6.92.70.711.10.160.140.100.250.530.21

w i t . . I f TI\

7.9 xlO"2

3.8 xlO"2

1.3 xlO"2

9-3x10-31.8 xlO"2

2.6 xlO"2

1.4 xlO"2

2.0 xlO"2

8.2x10-31.9x10-32.6x10-31. ix lO- 2

2.9 xlO"2

5-6x10-31.0 xlO"2

8.0x10-38.1 xlO"2

3.2 xlO"2

8.2x10-31.2x10-£

1.8x10-31.7x10-31.1x10-32.9x10-35-3x10-32.4x10-3

• Estimated value - see text for discussion

TABLE II

SUMMARY OF TOIAL IHTEHSITIESINTEGRATED HETOEEK 300 AMD 580 MM

Temp.K

810

1050

1260'

1340

l4oo

1520

1710

1840

2060

2300

2430

2760

3560

Ho.*

1

2,3 ' '

4

-j thru 10

11, 12, 13

14, 15, 16

17, IS

19, 20

23

2 2

23

24, 25

26

It.W

1.63 X lO"1*

4.12 x 1 0 " 1

1.38 x 101

1.55 x 103

3-78 x 103

6.45 x 103

2.78 x 101*

4.43 x 10^

7.46 x 1011

1.11 x 105

1.62 x 105

2.43 x 105

7.21 x 105

Denotes runs which were averaged.

WavelengthInterval

ran

300-340

340-380

380-420

420-460. '

460-500

500-540

54O-5BO

356O°K

4.72

8.64

15.5

20.3

19-3

17.1

.14.5 .

2740°K

3.51'

7.15

15.7

21.8

19.9

17.9

14.8

_ SUMMARY

243O°K

4.76

8.52

14.3

21.5

20.1

17.1

13.6

OF FRACTIONAL

23OO°K

3.68

7.28

15.2

28.2

20.5

17.7

13.4

2O6O°K

. 3.47

8.56

14.8

22.9

19.2

17.0

14.2

TABLE I I I

IMTEHSIEEEAT VARIOUS

Fractional Intensities

181O°K 17OOCK 1500°K

2.12

6.83

U . 9

20.4

22.3

20.2 '

16.9

1.53

5.12

9.19

20.5

23.1

21.7

18.8

1.67

4.56

12.9

22.7

22.2

19.9

16.2

TEMPERATURES

l4oo°K

1.17

4.00

9.86

21.0

24.4

21,4

18.2

133O°K

0.44

2.09

8.27

23.2

24.4

22.8

19.7

1260°K

.490

1.73

10.41

22.4

23.5

22.1

19.5

1050°K

.094

.790

6.88

23.4

25.5

2?.O

=0.3

31O°K

.012

.223

7.46

32.1

31.3

22.8

7.09

152

TABLE IV

SUMMABt OF

CONDITIONS FOR EMISSION STUDIES

(Windowless Mode, 80 < \ < 300 nm)

ptotal

Mb.

27

28

29

Temp.K

1050

1400

2280

• 1.0 atm

Sis mm Hg &

.043 1.46 2.53 2.5 x 10-2

.044 2.49 1.52 1.8 x 10"2

.046 5.10 0.78 9.C

TABLE V

SUMMARY OF EXPERIMENTALCONDITION FOR EMISSION STUDIES AT HIGH PRESSURES

(Lithium Fluoride Window, 120 < \ <. 600 na)

Ho. K g/s g/s mm Hg ~u*'6' ~Ar atm

30 1050 .056 XM 3.29 3.8xlO"2 1.22

31 1350 .044 1.93 I.96 2.3xlO-2 1.37

'32- 188O .012 3.49 2.96 3.4x10-3 1.63

33 2030 .043 4.71 0.79 9 .1x lO" 3 1.62

34 2410 .009 5.O8 0.15 1.8 x lO" 3 1.73

TABLE VII

SUMMARY OF "EFFECTIVE" CROSJ-SECTIONSAT VARIOUS TEMPERATURES

Wavelength "•*• Cross-Section, cm » -nm T a 98O K 1050 h 1270 K l4to) K 1770 K

420

430

440

450

460

470

480

490

500

510

520

530

5iO

550

560

570

580

5.75-19 6.03-19 8.54-19 6.02-19 4.82-19

5.21 5.46 7.52 4.99 4.53

4.71 4.94 6.6k 4.06 4.21

4.24

3.85

3.51

3-18

2.91

2.71

2.55

2.44

2.4o

2.44

2.53

2.71

2.98

3.38

4.45

4.03

5.82

5.09

3.68 lt.43

3.33 3.84

3.0 3.34

2.81)

£.67

2.56

2.52

2.56

2i66

2.84

3.13

3.54

3.31

2.63

2.17

1.84

1.57

2.92 1.36

2.59 1.18

2.38 1.10

S.26 1.02

2.28

2.40

2.66

3.07

3.69

3.92

3.29

3.05

2.74

2.50

2.30

a.10

1.97

9-69-ao 1.83

e.91 1.90

9.91 1.93

8.91

8.91

2.10

2.30

TABLE VI

SUMMARY OF EXPERJMEPflAL CONDITIOMSFOR ABSORPTION STUDIES

(Li th ium F l u o r i d e Window, 420 nm < \ < 580 run)

total1..0 atm

Thermal radiation

UF5 fuel

injection

Thru-flow

Nuclear fuelWorking

Argon f l « i d

buffer gas

Fig. 1 Schematic of a unit cavityof a plasma core reactor.

153

• - Metering valvea • Shut-off valveB- Two way valve

t>- Pressure gauge

0- Thermocouplelocations

LN2 trap

scrubber

Heat exchangerMonochromator

Plenum

Argon Heaterin . .

I Flow meterArgon in

Argon

Fig. 2 Schematic of UFg plasma toreii system.

Magnetic coils

inc' T o

OpticalJ view ports

Scanninthermocouple

L.

-J

Coofing water-X wall thermocouples

3fig- 3 Schematic of plasma torch assembly.

(Data taken at optical portadjacent to UFg Injector)

2000

Fig.

5000 0.2 0.4 0.6 0.8 1.0

Relative position, rAtypical temperature profiles alongthe optical path in the plenum.

300 400 500Wavelength.A- rim

600

Fig. 5 Comparison of the relative spectralintensity of UFg at various tempera-tures between 1000 and 3600 deg. K.

IA(T) HA

't(T)= /S 8°#300

I.[3560)dA

a5>S

1 10'3

/—— Equivalent/ black-body at

' temperature, T

Fig. 6

1200 2000 2800 3600

Temperature, T - deg K

normalized integrated intensity from300 i.o 56O nm as a function of tempera-ture.

/"VUIA4A"40

• 5BDJ VnettJ1 in

•a£at

5

2

101

5

2

10°

>^A3560

'^y^a/so. '/^1810

Awoo

Fig. 7

300 350 400 4S0 5QD 550Wavelength at center of interval,

AA-nm

Fractional integrated intensity perkO nm wavelength intervals.

1S4

10

Pjjp = 0,S mmHg

Major species not shown: Argon, N-10 1 8 cm

16

<? 10o

15

10

U F 6N

jAr5

. \ v

iF4

h"F"

Ul

\

\

Fig. 8

12

1000 2000 3000 4000

Temperature, T - deg K

Composition of UFg/argon mixture asa function of temperature.

e in

ten

sit

ssu

re U

Fe

Rel

ativ

un

it p

re

1.0

0.8

0.4

0.2

n

- 2430K,241 OK,

1.0 atm1.73 atttij*"* *^w^

V250 300 350 400 450 500 550

Wavelength, A - nm

Fig. 9 Comparison of relative spectral emis-sion at total pressures of 1.0 and1.73 atm'.

= 1270K

400 420 440 460 480 500 520 540 560 580

Wavelength,A-nmFig. 10 Typical relative intensity results

for absorption measurements.

•0

0

o

A

NO.

A

B

C

D

E

. T

K

9 8 0

1050127014401770

M M * flow O/MC

" f .

0.081

0.0650.1070.1020.009

At

0.730.791.461.461.46

*UF6mnHg

fl.B

10-06.36.05.8

400

Fig. 11

450 500 550Wavelength,A -nm

Relative spectral transmission atvarious temperatures.

600

No.

A

B

C

0

E

T

K

9H0

10S0127014401770

Massflo

U F 6

0.0910.0950.1070 1020 099

n g/sec

A t

0.790.791.461.461.46

ranHa9 0

10.06.3

6.0

s.a

10x10

b

I

400 450 500 550 600Wavelength, A-nm

Fig. 12 "Effective" spectral cross-sectionat various temperatures.

155

URANIUM PLASMA EMISSION AT GAS-CORE REACTION CONDITIONS

M. D. Williams, N. K. Ja'.ufka, and F. HohlNational Aeronautics and Space Administration, Langley Research Center

Hampton, Va.

and

J. H. LeeVanderbilt UniversityNashville, Tenn.

Abstract IMA

The results of uranium plasma emission producedb)' two methods are reported. For the first methoda ruby laser was irocused on the surface of a pureU 2 3 8 sample to create a plasma plume with a peakplasma density of about 10 2 0 cm"3 and a tempera-ture of about 28,600 K. The absolute intensity ofthe emitted radiation, covering the range from SCOto 7000 S was msasured. For the second method,the uranium plasma was produced in a 20 kilovolt,25 tilojoule piisma-focus device. The 2.5 MeVneutrons from tie D-D reaction in the plasma focusate moderated by polytbvlene and induce fissionsin thfc U 2 3 S. Spectra •: f both uranium plasmas wereobtained over the range from 30 to aOOO 8.Because of the low fission yield the energy inputdue to fissions is very small compared to thetotal energy in the plasma.

I_. Introduction

Electromagnetic radiation is expected to be theprime medium for obtaining high- gr^e power fromplasma-core reactors. The same radiation *il!play an important role in the internal transfer ofenergy and stead'-state operation of the reactors.Effective development of these reactors requires aknowledge of their spectral characteristics atexpected operating conditions. Two experiments atthe Langley Research Center produce uraniumplasmas that approximate gas-core reactor condi-tions and have b»«n used to record segments of thespectrum from th3 x-ray region to'the near infra-red region.

II. .'lasroa- Focus Experiment

The plasma-fosus apparatus is a coaxial plasmagun connected to a capacitor bank. Figure 1 is across-sectional view of the gun's coaxial cylindri-cal geometry. The center electrode is 5 cm ii»diameter and t.ie inside diameter of the outerelectrode is ".0 cm. Inner and outer electrodesare,connected to lower and upper collector plates,respectively, which are, in turn, connected to acapacitor bark through spark-gap switches. Thecapacitor bank stores 25 kJ of energy at 20 kV.When ;discharj!ed, a current sheath forms over thesurface of tiie insulator at the base of the center

'electrode.' fhe J x B force drives the plasma up; the annular region between ths electrodes. Just

•'/ past the end of the center electrode the plasma is'forced toward the gun's axis by the strong self--; induced magnetic field. The plasm reaches the: focus condition just as the capacitor bank reaches. -a iaximum cirrent of one neganpere in three micro-

seconds. Deuterium (at -5 Torr pressure beforethe discharge) is swept before the plasma sheath

URANIUM

Figure 1. Plasma Focus Electrode Arrangement

e focal position to form a very hotio- i0 uciisc i>10-°/tm-') plasma which

to the(SO x io£ iO uciiao i>lGls/cm-} plasma which emitsintense bursts of neutrons, gammas, x-rays,•and practically the entire EM spectrum. Simul-taneously, the end of the center electrode beginseroding at its centerline. A solid uranium sampleplaced at this position is partially evaporated byimpingement of energetic electron and ion beamsemitted from the plasma-focus region. The uraniumplasma rises about 2 cm above the electrode sur-face in less than 2 microseconds. Neutrons (-10y)produced by the plasma focus are reflected back

TO 2M VACUUM UVSPECTROGRAPH

10 li ENTRANCE SU1URANIUM SAMPLE

URANIUM PLASMAPLUME

500 m LASER

Figure 2. Experimental Arrangement for theLaser Produced Uranium Plasma Experiment

156

into the uranium plasma by a polyethylene moderatorwhich surrounds the focus and is about IS cm away.The temperature and pressure of the uranium plasmahave been estimated at 1 to 5 eV and 160 to 800atmospheres, respectively.

Thus, the plasma-focus experiment produces auranium plasma which approximates expected gas-core reactor conditions, including some fissionaction of the uranium 23S.

III. Laser Plasma Experiment

The laser plasma has been produced by focusingthe radiation of a Q-spoiled ruby laser onto theflat surface of a solid ifi3® sample. Four to sixjoules of energy were contained in the laser pulse.The pulse duration r.t half-maximum was 20 nano-seconds. The uranium sample was contained in avacuum chamber at the focal point of a quartz lensnear the entrance slit of a spectrograph. Figure2 shows the general experimental arrangement.Plasma created by the laser's rapid heating of thesample expanded perpendicular to the sample sur-face. Placement of th' "sample relative to thespectrograph entrance slit made possible the obser-vation of different cross sections of the plume.Each cross section exhibited different plasmaconditions. A cross section near the surface was.found to a2'proximate gas-core reactor conditionsof temperature (-38,600 K) and pressure(-500 atm.).

5000WAVELENGTH. A

Figure 3. Spectra of Uranium Plasma and Carbon Arc(a) Calibrated and time-resolved spectrum ofuranium plasma 0.25 mm from sample surface;(b) Phototube data scaled in volts;(c) Unsealed carbon arc spectrum. •

V 125

I 75

j§ 50

25

3000 4000 6000SHOO

WAVELENGTH. A

Figure 4. Comparison of Uranium Plasma andBlackbody Emission, (a) Calibrated uraniumspectrum 0.5 mm from sample surface;(b) Spectrum of 38,600 K blackbody.

IV. S-pectra

Visible spectra from 3S00 K. to 7000 8 and ultra-violet spectra from 300 A to 1600 A have bean ob-served in the laser plasma experiment.

The visible spectra were recorded with a half-meter monochromitor equipped with a 1200 line/mmgrating biased at 5000 8. Phitomultiplier tubeswere used to observe 8 A portions of the spectrumand the resulting electrical pulses were recorded onoscillograms. A carbon arc was used to calibratethe absolute intensity of the spectrum. The result-ing data were computer processed aiid are shown infigure 3. Figure 4 is a comparison of these datawith the curve for a 38,600 K blackbody.

TheJJV spectrum wus recorded in a similar experi-ment. A 2-meter vacuum spectrograph/monochroaiatorwas used with sodium salicylate-coated photomulti-plier tubes U, record the spectra at 25 X intervals.The data are shown in figure 5. An absolute inten-sity calibration of this spectrum was not made dueto the lack of a good UV standard. Instead, inten-sity sstimates were calculated by using the bestavailable data on the optical components involved.The corrected spectia shows a cont'iiuum peak at750 A. This corresponds to a blackbody temperature

800

WAVELENGTH, A

1600

Figure 5. Time-Resolved Ultraviolet Spectrum ofUranium Plasma Generated Near the Edge of theSample Surface, (a) Blackbody curve (38,600 K)normalized to the peak of the corrected spectrum;(b) Corrected spectrum; (c) Phototube data.

400 1630 2000

Figure 6.

100 1200

WAVELENGTH, A

Density Trace of Spectrogram of UraniumPlasma

157

°f SS.jjOO k. For the estimated particle densityof 102U cm"3, the plasma pressure was about 500atmosphere?. A spectrogram (Fig. 6). was made toprovide better wavelength resolution. It shows themagnitude of line emission and other structure incomparison to the continuum emission, ihe maximuminstantaneous power of the observed plasma was ofthe order of 1 0 n W/mz.

EXPERIMENTAL DATA

SPECTRUM OF U 2 5 3

SPECTRUM OF U 2 3 5

7000

WAVELENGTH, A

Figure 7. Absolute Intensity of UraniunPlasma Emission

Visible and ultraviolet spectra were also ob-served from the plasma-focus experiment. Visiblemeasurements ranged from 3500 8 to 10,000 8 in thenear IR. The experimental apparatus was similarto that used in the visible laser plasma measure-ments, including a caxbon arc calibration source.Both U2ia and U " b were used to generate the ob-served plasma in an effort to detect any spectraldifferences caused by fission action. Figure 7shows the resultant measurements. Each data pointis the average of many shots due to experimentalnonreproducibilities. However, within experimentalerror, no differences could be detected between thespectra of U 2 3 5 and I)238. Also, no" nonthermalradiation peaks could be detected.

The search for a spectral difference betweenplasmas of the two isotopes was extended into theextreme' ultraviolet spectrum. A one-meter Seya-Namioka spectrograph/monochromator viewed thespsctrum from approximately 100 8 to about 600 8.A 1-meter grazing incidence sgectrograph coveredthe spectral segment from 30 A - 400 X. Bot;hspectrographs were connected to the experimentthrough a special differentially-pumped vacuumsystem. First, the Seya-Namioka spectrograph wasused. Its spectrogram (Fig. S) revealed the possi-bility of a spectral difference between the twoisotopes at wavelengths less than about 200 8. Sothe Seya-Naroioka spectrograph was replaced by thegrazing incidence spectrograph to provide lowerwavelength coverage with greater spectral resolu-tion. The grazing incidence spectrogram (Fig. 9)revealed a radiation peak at about 93 8 whichvaried greatly in intensity from shot to shot.There appeared to be a larger peak, for the U23S

results. However, because of the large intensityvariation from shot to shot, no definite differencein the spectrum from the two uranium isotopes

could be establishes. The intensity variation ofthe peak at 93 h could net be corrected with anyexperimental parameter sa^h as neutron yield and i;probably due to nonreproducibilities An the observ-ed plasma. Both spectrographs used a plat.'nom-coated grating ruled with 600 1 xjies/mra' at a 1°blaze angle. At ah 85° angle of incidence theblaze wavelength of Lhe grazing incidence spectro-graph was about 93 A. This contributed to theamplitude of the observed peak. No calculations or

U 2 3 8 ,n .

100 300

A . Angstrom

500 600

Figure 8. Comparison of Plasma Focus ProducedU-238 and U-235 Plasma in the Region From100 to 600 Angstroms

s£

i\1 \

U - 235focus SVPressur.? 6Torr

U -J38Focus I IVPressure M Torr

150 „Wavelength, A

210 270

Figure 9. Comparison of Plasma from ProducedU-235 and U-238 Plasma in the Region From30 to 270 Angstroms

comparison to a UV source were made to establishabsolute intensity values. Three factors are be-lieved to be the major contributors to the apparentcontinuum of the spectrograms: (1) Light diffract-ed from the central image; (2] Light scattered b.the ruled edges of the grating and a very lightfilm on the grating surface; (3) the compacting ofmany line transitions at some wavelengths. Hence,the spectrogram is primarily due to line transi-tions of uranium and copper (eroded from

158

electrodes). The copper lines were verified byreplacing the uranium sample at tht end of theplasma-focus electrode b/ a. copper sample. Figure10 shows that indeed son,e of the lines observed inthe .uranium plasma were cojxer lines. It alsoshows that most of the radiation peak at 93 A wasreal (not due to the blaze effect) and was causedby the uranium plasma.

150 210Wavelength, A

i,L..hJ270 33ti 390

Figure 10. Comparison of Plasma Focus ProducedCopper and Uranium Spectra

Ibl COPPtR PLASMA

L_

V. Conclusions

Spectral segments from the near infrared to thesoft x-ray region have been recorded using two ex-periments which produce uranium plasmas that simu-late expected gas-core reactor conditions. ThesespectTa illustrate qualitatively and, in thevisible region, quantitatively the radiationcharacteristics that may be expected from gas-corereactors operating at several hundred atmospherespressure and several eV temperature. Line emissionand other structure were insignificant compared tocontinuum radiation from the infrared region toseveral hundred angstroms. Intensities approachedthose of a blackbody of the same temperature. Nolarge nonthermal radiation features could be foundat tae high plasma temperatures and pressuresinvestigated. At wavelengths shorter than thosewhere blackbody radiation is significant, lineradiation was prevalent, especially at approxi-mately 93 8. X-ray spectra indicate that.lineemission will make an important contribution to thetotal x-ray energy. No radiation effects fromfission action was detected. However, this wasproiably due to the low fission yield available inthe experiment. Much higher fission yields may yetprove to be the cause of nonthermal features in theradiation spectrum.

References

1. Lee, J.I!.; McFarland, D.R.; Hohl, F.; andKim, K.H.: Production of Fissioning UraniumPlasma to Approximate Gas-Core ReactorConditions. Nuclear 1'echnology, vol. 22,June 1974.

2. Williams, M.D.; and Jalufka, N.W.; VisibleSpectral Power Emitted from a Laser-PioducedUranium Plasma. NASA TO X-72626.

3. Williams, M.D.: Ultraviolet Spectrum Emittedfrom a Laser-Produced Uranium Plasma. Journalof Opt. Soc. Am. vol. 63, p. 383, March 1973.

4. Lee, J.H. et al.: Investigation of High EnergyRadiation from a Plasma Focus. Final Report,NASA Grant NGR 43-002-033, August 1975.

6.0 " 8.0 HI.)

WAVELENGTH. Jt

Figure 11. Comparison of Soft X-Ray Spectrumfrom Copper and Uranium Plasmas

A segment of the soft x-ray-spectrum (3^ - 98;emitted by uranium and copper plasmas in the plasmafocus are represented in figure 11. These spectrawere obtained with a bent crystal spectrograph andrecorded on x-ray film. The uranium spectrumshows a significant increase in white x-ray emis-sion which varies with the si[uare of ttie atomicnumber. It also shows characteristic lines at4.7, -1.9, S.6, 8.7, and 8.8 X. Especially stronglines below the 4 A wavelength may be due to non-thermal electron beams (E > 5 KeV) from the •plasma focus. The expected large density of highlyenergetic electrons associated with fission frag-ments in a gas-core reactor could produce similarhard radiation with deleterious effects on thefirst reactor wall.

159

To be published In the Proceedings of the3rd Conference on Partially Ionized and UraniumPlasmas, Princeton, New Jersey, June 10-12, 1976

BNi. 21531

Co• STUDIES ON COLOR-CSmER FORMATION IN GLASS UTILIZING MEASUREMENTS

MADE DURING 1 to 3 KeV ELECTRON IRRADIATION*

K. J. Swyler and P. W. LevyBrpokhaven National Laboratory

Upton, New York 11973

Abstract .

•"• The coloring of NBS 710- glass has been studiedusing a new facility for making optical sbzorptionmeasurements during and after electron.irradiation.The induced absorption contains three Gaussianshaped bands. The color center growth curves con-tain two saturating exponential and one linearcomponents, After irradiation the coloring decaysand can be described by three decreasing exponen-tials. At room temperature both the coloringcurve plateau and coloring rate increase with in-creasing dose* rate. Coloring measurements made ata fixed dose rate but at Increasing temperatureIndicate: 1) The coloring curve plateau decreaseswith Increasing temperature and coloring is barelymeasurable near 400 C. 2) The plateau is reachedmore rapidly as the temperature increases. 3) Thedecay occurring after Irradiation cannot be de-scribed by Arrhenius kinetics. At each temperaturethe coloring can be explained by simple kinetics.The temperature dependence of the decay can beexplained if it is assumed that the thermal untrap-ping is controlled by a distribution of activationenergies.

I. Introduction

The search for materials which will remaintransparent to light while subjected to intensenuclear irradiation requires both engineering andbasic radiation damage data quite different fromthat generally available from completed studies.As is well known, almost every normally transparentsubstance becomes colored by exposure to radiation.Some materials, e.g. photochromlc glasses, becomecolored in the process of absorbing rimpletelynegligible amounts of energy. In contrast, sub-stances like very pure fused silica or crystallineAI2O3 do not become colored in the vlaible untilthe radiation has imparted hundreds of joules.The coloring of these substances is clearly attrib-utable to color-center formation. Unfortunatelyalmost all of the available information on radia-' tion induced color-center formation is not appro-priate for evaluating materials which must remaintransparent during energetic particle irradiation.In the past almost all studies on the coloring ofglasses and crystals were lade by irradiating thesamples in an x-ray or gamm-ray source or in areactor and then taking the samples to a spectro-photometer for measurement at a later time.Clearly, if the absorption changes appreciablybetween irradiation and measurement such measure-ments do not provide data that is applicable dur-ing irradiation. In particular, if the coloringcontains a large decaying component, measurements

made in this way will seriously undeiestimate theabsorption during irradiation.

In addition to the uncertainties introduced bymaking measurements sometime after Irradiation,very little data is available to evaluate effectsdependent on tvo other important practical param-eters, sample temperature and irradiation dose rateIt is well known that the F-center formation in thealkali-halides does not have a simple dependent ontemperatures. Also, at a given temperature boththe coloring rate and the maximum coloring increasewith Increasing dose rate. Roughly the same be-havior is to be expected from fused silica andother glasses. However, there is very little dataavailable to test this supposition.

To study the basic physics of radiation in-duced electronic and atomic displacement processes

REFERENCE BEAMDETECTOR

COMPUTER CONTROLLEDM0NOCHR0MATOR No.1 LIGHT

SOURCE

S r N C - SIGNALPICKUP .

BEAMSPLITTER

COMPUTER CONTROLLEDMONOCHROMATOR N0.2ITUHABLE OPTICAL

OPASS FILTER)

SAMPLE BCAMOETECTCR

COMPlTEFc CONTROLLEDM0WCHROMATOR No. 3

I TUNABLE OPTICAL.•JANDPASS FILTER I

VERT.'CALlELECTRONACCELERATOR/BEAM BENDIfMAGNET

Figure 1. The experimental facility for measuringclerical absorption and luminescence during andafter 0.5 to 3.0 HeV electron irradiation.

^Research supported by the U.S. Energy Researchand Development Administration.

160

Wavelength - I to 256 pointsIntegration t in* (No. of crcteilot roch paint

Record! (at eoch point)

Absorption end luminescence data

Sample beom:

' I Ligftf front monoehromo'or le» light

absorbed in tomple; IjjmlnetcenceTrprn

2) Luminescence 1i

Reference bevm:

3( L.'fllif tram iron

mpl*,*ifi(3ow.oir,etc

from windows,oir,etc.I 4) Lumincitenet from windon.oir, ek .

Enperimeni limaBVORI cofreniSompte t«mperolurfl

Figure 2. The control and data acquisitionsyetec for the double-buam spectrophotometershown In Figure 1.

and to obtain engineering data on radiation effactsin nonmetals two f. lities have been constructedat Brookhaven National Laboratory to make opticalmeasurements during irradiation. Inasmuch asmeasurements can be made during and after irradia-tion they are not subject to the inadequaciesdescribed above. One of these special facilitiesis used to study the optical properties of samplesduring 60Co Irradiation. It has provided data onthe coloring of KC11 and NaCl;! at room temperature,KC1 at 85K, the effect of strain applied duringirradiation on KC1," the coloring of barium alumno-borate glasses at room temperature,5 and the color-ing, radioluminescence and thermoluminescence ofnatural and synthetic quartz between 80 and 300K.6

The barium alumnoborate glass sturty provided manyresults which will be described in a later section.

The other facility, which will be describedbriefly In the next section, is used to simultane-ously measure the optical absorption and lumines-cence of substances during irradiation with elec-trons at any energy between 0.5,and. 3 MeV. Thisequipment provides data OIL the absorption changesoccurring after irradiation and, in addition, onany phosphorescence occurring after irradiation.Most importantly, the absorption and luminescencemeasurements are not restricted to a single wave-length. An entire 256 point absorption spectrumand a 256 point luminescence spectrum can be re-corded every 40 seconds'. At the moment, thesemeasurements cover the range 200 tn 4QQ nra or bVOto 800 nm. However, the range can be extended tc11 microns cr more by installing suitable lightsources and detectors.

A small number of radiation Induced coloringstudies utilizing measurements made during garama-ray irradiation were cited above. Even fewerstudies using measurements made during electronirradiation can be found in the literature. In-cluded in these is a study of the electron bombard-ment induced coloring of fused silica,7 particular-ly the absorption at 215 nm with the sample atvarious temperatures between 100 and 500 C. Asurvey of the radiation induced absorption and theradioluminescence of optical glasses, AI2O3, crys-tal quartz and fused silica produced by 2.5 MeVelectron irr liation has been reported recently.8

In addition, there are a number of studies onradiation effects in fibre optics that contain a

large amount of information on the radiation in-duced coloring of glasses.9"12 Lastly, there areprevious reports on the radiation induced coloringof the K3S 710 glass described in this paper.13~1?

For convenience. Information in the previous re-ports which is particularly pertinent for this dis-cussicn is included here. Consequently, the roomtemperature jf.udies at different dose rates and theresolution of the growth and decay curves Into com-ponents are described in reference 13. Additionalstudies describing the coloring and decay at aconstant dose rate and at different temperatureswill be described in this paper.

II. The Facility for Making Optical Measure-ments During Electron Irradiation

The equipment used to make optical measure-ments before, during and after electron irradiationis illustrated in Fig. 1. Electrons at any energybetween 0.5 and 3.0 MeV are produced In a verticalelectron accelerator. After emerging from theaccelerator they are magnetically deflected 90degrees into a horizontal tube that transmits thebeara into the irradiation chamber. This tube isequipped with a variety of focusing coils, severalFaraday cups, and a thin gold scatterer. TheFaraday cups and scatterer can be inserted or re-moved from the beam at will. The thin gold scat-terer Insures that £he sample is uniformly irradia-ted. The beam parameters can be adjusted with thesample In place but not exposed to radiation. Wilendesired, the irradiation is Initiated, or termi-nated, by a fast acting cup which can be moved inor out of the beam within 0.5 seconds. Duringirradiation the been traversing the sample ismonitored by a Faraday cup located downstream fromthe sample. The incident electron energy is alwaysadjusted to insure that the particles pass throughthe sample. With the arrangement described aboveit is possible to obtain reproducible irradiationsat known beam currents and avoid all of the diffi-culties associated with "turning-on" the acceler-ator.

400 600 800

TIME, sec .600 800

Figure 3. F-center growth and radioluminescencefrom single crystal LiF during Irradiation. Afterirradiation this sample did not exhibit detectablephosphorescence and the radiation induced coloringchanged very little.

NBS 710 GLASS - .1BS0RPTI0N MEASURED DURING AND AFTER 1.5 MeV ELECTRON IRRADIATION

I I I I • i

Figure 4. A "3-D" plot of the s.bsorption induced in NBS 710 glass by 1.5-MeV electron irradiation andthe decay of the absorption afttx the irradiation has been terminated. Spectra were recorded at 40 secor'or longer intervals. Every otheir spectrum has been omitted.

The simultaneous optical absorption and lum-inescence measurements are made with the computercontrolled double-beam spectropliotometer shown inFig. 1. Monochromator Ho. 1 provides a monochro-matic beam for absorption measurements. The beam -is chopped, split into sample and reference beams,and focused by the optical system into sample andreference images in the target chamber. Both beamsare subjected to equal changes in window absorptionand luminescence, air glow and any other perturba-tions occurring during and after irradiation.

After passing through the target area thebeams are focused on monochroEtators Nos. 2 and 3.Monochromator Ho. 2 serves both as a tunable opti-cal bandpass filter for absorption measurements andas an analyzing monochromater ::or luminescencespectrum measurements. When operating in the band-pass mode the monochromators prevent all lumines-cence from reaching the photot jbes except that inspecified wavelength regious. In the presence ofluminescence from the sample tills increases thesignal to noise ratio for absorption measurementsby orders of magnitude. Actually, for all measure-ments' described in this paper, a bandpass monochro-mator, No. 2, was used only in the sample beam.The reference beam was focused directly on thephotomultiplier. This arrangement is demonstrablyquite adequate for the glass studies describedbelow since they did not exhibit strong lumines-cence.

The monochrotnator Is entirely computer con-trolled. As many as 256 preselected points may bemeasured, during each scan. If all 256 points arerecorded the scans may be repeated every 40 sec.or at longer intervals. Scans with fewer pointsrequire less time and at a single wavelength datapoints may be recorded at millisecond intervals.The computer specified wavelengths, time Intervalbetween scans, number of signal, averaging cycles,etc. At each wavelength four measurements aremade to determine the optical absorption and lum-inescence. They and other functions of the controland data acquisition system are outlined in Fig. 2.la the present configuration the sample to refer-ence beam intensity ratio is determined beforeeach irradiation measurement and often checkedafterwards. Tests over long periods of time andday-to-day experience Indicate that this ratioremains so stable, there is not necessity to installdevices to measure this ratio <?jring measurements.

After each scan the absorption and luminescencedata and other information such as sample current,beam current, etc., is transferred to magnetic tape.In turn, the .recorded data is processed on a largecomputer. An absorption and luminescence spectrumis obtained for each scan. These spectra are fullycorrected for absorption and luminescence in thetarget windows, air glow surrounding the irradiationchamber or attributable to exchange gas in thesample furnace, etc. It is particularly noteworthythat the luminescence spectra may be fully correctedfor radiation Induced self absorption in the sample.Thus one can determine unequivocally if a change laluminescence is real or is caused by self absorption.

A typical data set for a sample which exhibitsstrong radioluminescence is shown in Fig. 3. Thisplot shows both the radioluminescence and the F-center absorption exhibited by single crystal LiFduring electron irradiation. Spectra are shown at40 second intervals. After 450 seconds the elec-tron beam was "turned-off" by interposing the rapid •acting Faraday cup. During irradiacion the lumin-escence rises rapidly to a constant value, remainsat that level until the irradiation is terminated,and then drops abruptly to a negligible level.Concomitantly, the LiF F-center absorption increasesmonotonically and when the beam is interrupted theabsorption drops negligibly or not at all. LiF isthe only substance measured to date which, at roomtemperature, does not show large absorption changeswhen the irradiation is terminated.

The absorption spectra obtained from NBS 710glass recorded during and after Irradiation isehown in Fig. 4. This plot was made from separatemeasurements in the 200 to 400 and 400 to 800 runregions and "joined" during computer processing.It shows both growth and coloring during irradia-tion and the decay occurring after the irradiationwas terminated. This glass does not, for allpractical purposes, eahibit luminescence duringIrradiation nor phosphorescence after irradiation.This data will be used to illustrate many of theradiation induced color center growth and decaykinetics illustrated in the next section.

162

III. The Kinetics of Radiation Induced ColorCenter Growth

To begin this discussion of color-center for-mation during irradiation and the growth or decayof centers after the irradiation, consider the pro-cess that applies to a single center, or, equiva-lent ly, a single absorption band. This may beassumed to be formed by trapping of an electron onan electron trap. However, with straightforwardchanges in charge (sign) the discussion appliesequally well to centers formed by hole trapping.First, electron traps occur in crystals andglasses at points where the normal lattice isperturbed. This can occur at lattice sites con-taining substitutional impurities, at vacanciesor divacancies, or where interstitial atoms occurbetween the atoms on normal lattice sites. Thesedefects may, or may not, be in valence or chargestates Jifferent from that associated with thenormal lattice at the same point. For example,if a negative atom is missing from the lattice, toform a negative ion vacancy, the lattice will belacking an electron, i.e. there will be a localcharge unbalance. Next, if a radiation-inducedionization electron comes close enough to thenegative-ion vacancy it will be trapped. In fact,an electron trapped on a aegative-ion vacancy isthe well-known F-center. Such centers have manyof the properties of atoms and molecules and im-part color to che host crystal by undergoingoptional absorption transitions. Similarly, twoside-by-side vacancies (a divacancy) may trap twoelectrons to become an M-center. Likewise a Ca**ion on a normal Na+ lattice site has a high prob-ability of being an electron trap since localcharge neutrality may be established by trappinga charge of this sign. A very large number ofdifferent kinds of defects can be electron traps.Even neutral substitutional atoms such as a K* ionc a Na+, site may be traps. In certain crystals,at 'aw temperatures, charges may be "self-trapped"on normal lattice sites. An example is the self-trapped hole which forms V^ centers in alkalihaj'ides. However, for this discussion it isnecessary to consider only the more common defectand impurity related traps.

3.0PHOTON ENERGY {eV>

Figure 5. A typical radiation induced absorptionspectrum foi NBS 710 glass. The spectrum has beenresolved iuco three Gaussian shaped bands.

Each type of trap undergoes a number of dif-ferent radiation related processes. The importantprocesses will be described below, in turn. LetNo be the concentration of traps prior to anyirradiation. Typical values of No are in the 10

11*to lO'Vem3 ranges. Consider first the processesthat occur during irradiation. Let tj> be the doserate. It can be expressed in any unit but it isconvenient to think of it in teems o£ ion pairsper unit time created by the incident radiation.Then if N(t) is the concentration of color centerswhich have already been formed by irradiation attime t, the concentration of uncolored centers orempty traps is (No-N). The probability that theseempty traps are converted to centers at time t isproportional to the product of (No-N) and the ion-ization electron concentration <(i or fiJ>(N0-N) wherefifi is simply the fraction of uncolored centera con-verted to centers per unit time.

Up to this point it has been assumed that thenumber of traps is fixed, a contention that ismost appropriate for impurity related traps. How-ever, in many cases defect related traps may beintroduced during irradiation. In this case theconcentration of traps, both filled and unfilled,could be a complicated function of irradiationtime. In fact, the numerous different colorcenter vs. dose, or growth, curves obtained forthe barium alumnoborate glasses, described inreference 5, can all be "explained" by differenttrap vs. dose curves. Only the simples possiblecurve will be considered here. That is, It will beassumed that the trap concentration can be de-scribed by the equation P = Bo + kijit where P is thetotal number of traps at time t. This is equiva-lent to assuming that the trap concentrationinitially has the value Ho and increases linearlywith irradiation time or dose. In this situationthe rate of color-center formation is glvan byf<p(No + k<(iT - a).

2 4 .6 8 10TIME, IN UNITS OF 10* SEC

Figure 6. Growth curve of the absorption at 4.0 eVvs. irradiation time in NBS 710 glass.- The doserate was 2.2 Mrad/h. The solid line through thedata points was computed from the equations in thetext and contains two saturating exponential andone linear components.

163

In addition tu color-center formation bycharge trapping there are two important processesthat reduce the rate of color-center formationduring irradiation. The first or these is elec-tron-hole recombination. The probability that atrapped electron will encounter a mobile ioniza-tion produced hole is proportional to the productof the trapped electron concentration, N, and themobile or free hole concentration. The latter isproportional to the dose rate; more specificallyit is i)>Th, where T^ is the hole lifetime. Thus,at any time t, when the trapped electron concen-tration is N, the probability of recombination isgiven by <j>rN. The quantity r includes T[, andother factors such ai3 the cross-seccion for re-combination.

The other important process which tends toreduce the trapped electron concentration isthermal untrapping. This is well known and isusually called an Arrhenius-type process. Theprobability that a trapped charge can escape fromthe trap by thermal motion is proportional tos exp(-E/kT) where s is the "attempt-to-escape"frequency, E is the activation energy for untrap-ping, and k and T have the usual meaning. Thusif at time t there are N trapped charges thenumber escaping per unit time is just uN whereu - s exp(-E/kT).

So far only thermal untrapping from the colorcenters thermselves has been described. The infor-mation available now indicates that more than oneprocess may contribute to the observed decay of agiven center. For example, part (one componentof)•of the decay can be attributed to untrappingfrom the center and other parts (components) toelectron-hole recombination with holes thermallyreleased by hole traps. Also, as discussed indetail below, it would appear that the color centerdecay processes may be more complicated than thesimple Arrhenius process.

3 4 5 6

TIME, IN UNITS OF 10** SEC

Figure 7. Decay of the absorption at 4.0 eV inthe SBS 710 glass after irradiation. The solidline through the data points was computed fromthe equations in the text anu cottains threeexponential components.

10 15 20

TIME,IN UNITS OF I03 SEC

25

Figure 8. Growth of the radiation induced coloringat 2.6 eV in NBS 710 glass at different dose rates.

The simplest equation which might be expectedto realistically describe the growth of colorcenters during and after irradiation can beobtained by combining the coloring, recombinationand untrapping processes described above. Namely,

f(f>(N0 + k* - N) - rijiN - uN . (1)

If the sample is uncolored when the irradiationis started, i.e. if N=0 at t=0, the solution ofthis is

This can be written

-atN = A1(l-e ) + aLt

where

(f+r)(Hu

(2)

(3)

(4)

(5)

This simple treatment predicts that thegrowth curve will, ia general, contain a lineai:and saturating exponential component. Also, itpredicts that the constants in the growth currehave explicit, dependencies on the dose rate. Forexample equation (5) indicates that the exponen -tial constant aj is a linear function of the loserate with a non-zero intercept if u j6 0. Thisaccurately describes the growth of F-centers inKC1 at 85K3 and the coloring of one preparation

164

of barium alumnoborste glass.5 However, coloringcurves obtained from a large fraction of the sub-stances studied to date contain 2, 3, or occasion-ally 4, saturating components. There are a largenumber ofdifferent ways of extending the treat-ment given above to Include more than one saturat-ing exponential component. These range from simpleones, e.g. there could be two types of percursorsfor a given center; or complicated situationswhich cannot be described mathematically other thanby coupled differential equations. s » l 8 « 1 7 In anycase, it appears that appreciable physical Insightabout the radiation induced coloring process canbe obtained by analyzing data in terms of the(admittedly) simple treatment described above.

IV. The Decay of Color Centers After Irradiation

As mentioned above, there are very few sub-stances that do not show color-center decay afterbeing irradiated at room temperature. In fact,the only example in this category that we can citeis the F-center coloring of LiF. In terms of themodel developed above the decay behavior should beparticularly simple, namely, if D o color centersare present at the termination of irradiation theconcentration D at a later time t is D»Doexp(-ut).Obviously, a plot of In D vs. t should be linear.

Most Importantly, u»s exp(-E/kT) and a plotof In u vs. 1/T is the well-known Arrhenius plotfrom which one obtains the activation energy Efrom the slope and a quantity proportional to 8from the intercept. Decay <jata at different well-controlled temperatures is needed to determine anaccurate value of E.

The currently available data, most of whichis confined to room temperature, indicates thatthe radiation induced color centers do decay ex-ponentially at room temperature. However, withfew exceptions, the observed decay consists oftwo or more components. One class of explanationsfor multiple components is discussed In ref. S.Here it is sufficient to point ou': that a center

1.0 i-

I 2 3 4 5 6 7 8

TIME, IN UNITS OF I04 SEC

Figure 9. Decay of the absorption in NBS 710glass after the samples were colored at differentdose rates.

2.0

DOSE RATE , Mrod/h

Figure 10. The exponential growth constant a^,obtained from the data shown in Fig. 8. The growthkinetics described in the text predicts that thisquantity will be a linear function of dose rate.

may decay by thermal untrapplng of the trappedcharge and by recombination with opposite signtrapped charges released by other traps. In thisway any number of decay components may occur.

There is very little, if any, data availableto determine if the temperature dependence pre-dicted by this model is obeyed. One of the primeobjectives of the study described in this paper into determine if this model applies to radiationinduced color centers in glass. As will be shown,it apparently does not.

V. Radiation Induced Color Center Formationin MBS 710 Glass

Many of the initial measurements on thisglass, made with the equipment described above,are given in reference 15. A number of these willbe described here, but only those that are neces-sary Co provide reasonable understanding.

Experimental Details: All measurementsdescribed below were made on NBS 710 glass whichconsists of 70.5% S102, 8.7X Na20, 7.72 K2O, and11.6% CaO. Samples were roughly 1 by 2 cm and1 mm thick. The largest surfaces were polished andaccurately parallel. The electron beam energy was1.5 KeV which is more than sufficient to insurethat the incident particles pass entirely throughthe sample. This is necessary to prevent chargebuildup In the glass. The samples were irradiatedin either the "room-temperature" irradiator or Ina (stainless steel furnace. The latter consists ofa solid stainless block with two fused silicaoptical windows and two thin "havar" windows totransmit the electron beam into the furnace,through the sample and into a Faraday cup. Elec-trical heaters are contained in the block. Thesample contains helium at a pressure sufficient toinsure that the sample remains at the furnacetemperature. Measurements indicate that the glasstemperature is known to about one degree and aworst case calculation Indicates that duringIrradiation the maximum uncertainty is 3, or atmost 4, degrees. The furnace is electronicallycontrolled and remains within 0.1C of the Indicatedtemperature.

165

Optical measurements w«re made befn *° - duringand after irradiation. Usually, one measurement Ismade, at: a given temperature and beam current, inthe 200 to 400 nm range and another in the 400 to800 ins range. During processing the data is"joined" to produce numerous absorption spectrasuch as is illustrated in Fd.g. 4.

The dose imparted to the sample, i.e. the doserate, is computed from the measured beam*current,the sample thickness, and published dE/dx tables.In the 1 to 3-MeV range dE/dx is almos : independentof E which means that energy degradation in thegold scatterer, havar windows, etc. does not intro-duce uncertainties. At the moment the absolutedose rates given below may be in error by as muchas 40 or 50 percent'. However, the error in therelative dose rates is only a few percent.

Absorption Spectra: An absorption spectrarecorded at room temperature is shown in Fig. 5.Also, this figure shows the resolution of thespectrum into Gaussian shaped absorption bands.Although it is not illustrated, all spectra record-ed at higher temperatures can be resolved into thesame set of bands. This is ,-iso true for spectrarecorded after the irradiation is terminated.

Growth and Decay at Room Temperature andDifferent Dose Rates: Absorption spectra wererecorded at room temperature and at dose ratesranging from 0.81 to 3.72 Mrad/h. In every casethe decay was also studied. A typical growth curveis shown in Fig. 6. This and all other growthcurves are accurately represented by the equation

a(t) (6)1=1

The solid line through the data points (most easilyseen in the 0 to 103 sec region) was computed fromequation (6) after the constants had been deter-mined with a computerized best fit procedure. Moreexplicitly, the. growth curves are very accuratelydescribed by equation (6).

35-NBS 710 GLASS, 4.0«V

0 I 2 3 4 5 6 7 8 9 10 II 12IRRADIATION TIME IN OMITS OF 10* SEC.

Figure 11. The growth of the absorption at 4.0 eVin WoS 710 glass at different temperatures and ata constant dose rate of approx. 1 Mrad/hour.

o

JO-

2S-

H 20-

8 16 24 32 40

OECAV TIME, IN UNITS OF 10* SEC48

Figure 12. The decay, following irradiation, ofthe absorption at 4.0 eV for each of the growthcurves shown in Fig. 11.

A typical decay curve, obtained after the dataused for Fig. 6 had been recorded, is shown inFig. 7. Although it is hard to discern, thisfigure contains a curve through the data pointscomputed from the equation

a(t) exp(-d.t')i

(7)

where t' is the time after the irradiation is term-inated and the constants were obtained by a best-fit procedure. In other words, the decay occurringafter irradiation is very accurately representedby the sum of three exponential components.

Growth curves obtained at four different doserates are shown in Fig. 8. Three of the corres-ponding decay curves are contained in Fig. 9.After these curves have been fitted to equations(6) or (7), data is available to evaluate some ofthe predictions obtained from the "simple kinetictreatment" described above. For example, equation(4) predicts that the exponential constants, thea^, should be linear functions of the dose rate.That this is observed is shown by Fig. 10. Fromsuch results one concludes that the simple kinetictreatment adequately describes the coloring of NBS710 glass at room temperature.

Growth and Decay at a Constant Dose Rate andat Different Temperature: The growth of the radia-tive induced coloring of the NBS 710 glass at diff-erent temperatures and at a fixed dose rate ofapprox. 1 Mrad/hour is shown in Fig. 11. It isapparent that there are two marked dependencies ontemperature. First, as the temperature increasesthe steady-state or equilibrium level of the color-ing decreases. However, in contrast, the rate ofapproach to the equilibrium level increases withincreasing temperature. Also, in the temperature,dose rate and irradiation time regimes covered bythis data there are not any indications that thecoloring curves contain a maximum. In other words,the ct'vs do not increase to a maximum and thendecrease as the irradiation continues. A detailedanalysis of the growth characteristics will bepublished elsewhere.

166

10

io3

S _ooUl

IEO

o

if H

io6-.0026 .0028 .0030 .0032 .0034

INVERSE TEMPERATURE,(DEGREES K.)"

Figure 13. An Arrhenius plot computed from decaydata, for the NBS 710 glass, such as is shown inFig. 12. The activation energies obtained fromthis plot are much too snail to attribute thetemperature dependence shown in Fig. 12 to simpleArrhenius kinetics.

tzCDft:

>-

V)

UlOQ. -

TRA

-

KA4- A

* \+ \

V+ 2SC *t

^ +

•+ +

+ 4 67C+

4 + • / "

* / /

l i l t

SslOK SEC'1

V; \

,+ 112 C \

I •

.7 .8 .9 1.0 II \2ACTIVATION ENERGY

Figure 14. Part of a trap density vs. activationenergy distribution curve obtained by applying thePrimak-Holmes analysis to decay data such as isshown In Fig. 11. This plot supports the conten-tion that the observed dependence of the decaydata on temperature can be attributed to a distri-bution of thermal untrapping energies.

The decay of the coloring after irradiation,recorded with the samples maintained at the irradi-ation temperature, is shown in Fig. 12. Althoughnot illustrated by plots, all of thase curves canbe accurately resolved into exponential decay com-ponents. This is in accord vith the room tempera-ture measurements and the kinetic theory describedabove. An Arrhenius plot constructed from thedecay data is shown in Fig. 1.3. This plot is rotin accord with the kinetic theory In two respects.First, the points are not linear and, second, whenit is assumed that the data can be approximated bystraight lines the activation energies obtained areappreciably too low to account for the observeddecay at the different temperatures. Thus one mustconclude that the decay is not attributable to asimple Arrhenius process. More specifically, onemust conclude that the decay cannot be describedby the u = s exp(-E/kT) equation.

Distribution of Activation Energies: Thereare a number of physical processes which couldaccount for the color-center decay measurementsmade after irradiations at different temperature.These include tunneling, combinations of tunnelingand Arrhenius processes, temperature assistedtunneling, recombination of so-called geminatepairs, and processes involving distributions ofactivation energies. This last mentioned situa-tion arises whan the thermal untrapping process ischaracterized by a distribution of activationenergies, not by a single energy.8'17

In probability terms, the usual single valued acti-vation energy Is characterized by a delta-functiondistribution whereas the non-single-valued case ischaracterized by a probability distribution in Espace. A good example is a Gaussian distributionspecifying the density of traps with activationEnergy E. In this case, traps characterized bythe entire distribution of activation energies arefilled when the sample is irradiated at low temper-ature. At an intermediate temperature the shallowor low activation energy traps are emptied as soonas they trap charges. Finally, If a sample Is ir-radiated at a very high temperature all traps areunstable and no trapped charges ere retained.

Because of the random nature of glass latticesit Is likely that they will exhibit distributionsof activation energies and other parameters. Forexample, a given defect in crystal quartz will havethe same local surroundings wherever it occurs.In contrast, in fused silica the same defect willbe subjected to local surroundings which vary frompoint to point. Thus the thermal untrappingenergy, E, will be the same for each trap in crys-tal quartz. However, in glassy quartz the energywill be spread out in a distribution. The largerthe local fluctuations the wider the distribution.

167

A method for determining distributions ofthermal untrapplng energies has been worked out byboth Primak18 and Holmes.19 The details are toolengthy to reproduce here and can be obtained fromthe original references. However, the data shownIn Fig. 12 Is quite suitable for this purpose.Figure 14 shows the distribution obtained when themethod is -applied to the 26, 67, and 112C curvesand it is assumed that s » 1012. At such tempera-tures, the structure in the data to the left of thepeak could represent the actual distribution, orit might be the result of approximations includedIn the analysis, or a combination of these. Thecurve to the right of the peak should be indicativeof the distribution in the energy region that isunstable at the indicated temperature. The exten-sion of each data set to higher activation energiesrequires decay data for impractically long times.In any case, the data is sufficient to indicatethat the observed dependence on temperature, i.e.data in Figs. 11 and 12, can be explained byassuming that the activation energy for thermaluntrapplng is not single-valued but can be de-scribed as a distribution of trapping levels and arange of activation energies. Perhaps it need notbe stated explicitly, but this conclusion does notrule out the possibility that this data representsstill another process; it indicates that the ob-served dependencies are compatible with theassumption that the activation energies are de-scribed by a distribution;

VI. Summary

Using recently completed equipment for makingoptical absorption during electron irradiation, theradiation induced coloring of NBS 710 glass hasbeen studied at room temperature at various doserates, and at different temperatures using a fixeddose rate. At room temperature, the coloringcurves recorded during irradiation and the decayof the coloring after irradiation are in accordwith a simple kinetic theory based on three postu-lates: 1) the color-centers are formed by chargetrapping on defects that exist in the sample beforeirradiation and increase linearly during irradia-tion, 2) the color-center concentrations arediminished during irradiation by electron hole-recombination and by thermal untrapping, and 3)the decay occurring after irradiation results fromthermal decay. The measurements made at tempera-tures between room temperature and 400C indicatethat the coloring during irradiation is in agree-ment with the simple kinetic theory but the de-crease occurring after irradiation is not in accordwith the postulated Arrhenius type thermal decay.These are a number of different physical processesthat could account for this discrepancy. The dataobtained to date indicates that the observed decayis consistent with the assumption that there is adistribution of activation energies for thermaluntrapping from the observed color-centers, incontrast to the single-valued energy usuallyobserved..

This data, as well as measurements made onalkali halloas,1"* indicates that studies onirradiation induced coloring are best made by mak-ing measurements during irradiation. In fact, itwould appear that this is the only reliable way todetermine if materials to be used during irradia-tion will maintain the required characteristics.

References

1. P. W. Levy, P. L. Mattern and K. Lengueiler.Phys. Rev. Letters 24, 13 (1970).

2. P. L. Mattern, K. Lengweiler and P. W. Levy.Solid State Comm. ±, 935 (1971).

3. K. Lengweiler, P. L. Mattern and P. W. Levy.Phys Rev. Letters 2£, 1375 (1971).

4. P. W. Levy, P. 1. Mattern, K. Lengweiler andM. Goldberg. Solid State Comm. j>, 1907 (1971).

5. P. W. Levy, P. L. Mattern, K. Lengweiler andA. M. Bishay. J. Am. Ceramic Soc. 5_7, 176(1974).

6. P. L. Mattern, K. Lengweiler and P. W. Levy.Radiation Effects 26, 237 (1975).

7. G. E. Palma and R. M. Gagosz. J. Phys. Chem.Solids 33, 177 (1972).

8. M. J. Treadaway, B. C. Passenheim and B. D.Kitterer. IEEE Trans, on Nuclear ScienceNS-22, 2253 (1975).

9. G. H. Sigel, Jr., B. D. Evans, R. J. Gintner,E. J. Friebele, D. L. Griscom and J. Babiskin.Naval Research Laooratory Report 2934 (1974).

10. B. D. Evans and G. E. Sigel, Jr. IEEE Trans,on Nuclear Science NS-21, 113 (1974); NS-222462 (1975).

11. P. L. Mattern, L. M. Watkins, C. D. Skoog, J.R. Brandon and E. H. Barsis. IEEE Trans, onNuclear Science NS-21, 81 (1974).

12. P. L. Mattern, L. M. Watkins, C. D. Skoog andE. H. Barsis. IEEE Trans on Nuclear ScienceNS-22, 2462 (1975).

13. K. J. Swyler, H. H. Hardy, II, and P. W. Levv.Bull. Am. Phys. Soc. 2,°.. 431 (1975).

14. K. J. Swyler and F. W. Levy. Bull. Am. Phys.Soc. 21, 345 (1976).

15. K. J. Swyler, W. H. Hardy, II, and P. W. Levy.IEEE Trans, on Nuclear Science NS-22, 2256(1975).

16. P. W. Levy. J. Am. Ceramic Soc. 43, 389(1960).

17. J. H. Mackey, H. L. Smith and J. Nahum. J.Fhys. Chem. Solids 22, 1773 (1966).

18. W. Primak. Phys. Rev. 100, 1677 (1955).

19. D. K. Holmes. Proc. of the 1960 Prague Symp.on Chem. Effects of Nuclear Transformations,IAEA, Vienna, 499 (1961).

168

DISCUSSION

J. BLUE: In your basic assumption that theoptical effects are oniy due to trapped electrons,is it possible there that the conduction electrondensity gets high enough that there is an inter-action between the light and electrons in theconduction band?

P. W. LEVY: At some level the effects that I havebeen talking about will perhaps be low, and thenbe comparable to such things as the conductionband, or rather the free-carrier absorption in thesematerials. Under the very high do.se rates that areapplicable to this discussion, there is a possi-bility that there is a free atom contribution tothe absorption. The data to confirm that is notavailable. I have snid everything in terms ofelectrons. You can apply this same idea forwhole color centers and mixtures.

169

HEAT TRANSFER EVALUATION TN A PIASMA CO«E REACTOR

Donald E. Smith, Timqthy M. Smith andMaria L. stoenescu

Plasma Physics Laboratory, Princeton UniversityPrinceton, New Jersey 08540

Abstract

Numerical evaluations of heat transfer in afissioning uranium plasma core reactor cavity,operating with seeded hydrogen propellant, havebeen performed. A two-dimensional analysis isbased on an assumed flow pattern and cavity wallheat exchange rate. Various iterative schemes were 'required by the nature of the radiative field andby the solid seed vaporization. Approximate for-mulations of the radiative heat flux are generallyused, diie to the complexity of the solution of arigoro sly formulated problem. The present workanalyzes the sensitivity of the results with re-spect to approximations of the radiative field,geometry, seed vaporization coefficients and flowpattern. The results present temperature, heatTlux, density and optical depth distributions inthe reactor cavity, acceptable simplifying assump-tions, and iterative schemes. The present calcula-tions, performed in cartesian and sphericalcoordinates, are applicable to any most generalheat transfer problem.

Problem Definition

The complexity of rigorous calculations fora general heat transfer analysis in highly radia-tive media makes the use of approximate formula-tions particularly attractive. The most appropriatecompromise between simplicity and accuracy has tobe determined finally by the purpose of the evalua-tions.

The energy conservation for a unit volume ofa one-component gas in steady state is given byEquation (1). The. solution of this equation forthe spatial distribution of the enthalpy requiresadditional information on the flow, the shearstress tensor and on conductive and radiati ve heatfluxes.

Both for the implications it has on the com-putational procedures' and for tYj self-consistencyof equation (1) it is important to know whether theheat fluxes are completely described by the para-meter temperature.

Three cases are differentiated on Table 1for the correlation between the radiative heat flux •and the temperature: One, in which the radiativeheat flux in a given point depends on the localtemperature, valid, as seen further, for a gas inlocal thermodynamic equilibrium (LTE), grey andoptically thick; the second, in which the radiativeheat flux depends on the temperatures of all theregions in the gas, valid for LTE, and the third,in which the radiative heat flux canr.ot be describedby the temperature, and data on bound electron andatomic velocity distribution functions is necessary.

Various expressions of the radiative heatflux are shown in relations (2) - (8). The generalexpression of the radiative heat flux is presentedin relations '.2) - (2b) and is illustrated inFigure 1. The radiative heat flux calculation

requires the knowledge of all local emissionabsorption and scattering coefficients, and inte-gration over directions, at each point in space,for every frequency interval. The spectral sourcefunction, including the scattering source is re-presented in relation (2c), and may depend ontemperature for LTE or on atomic distributionfunctions for non-equilibrium (NE) . The magnitudeof the radiative intensity resulting from relation(2) may differ considerably from an equilibriumblack body intensity at the local temperature. It"the medium would have uniform temperature, includ-ing the boundaries, the calculated intensity wouldbe equal to the source function of the nearestelement, therefore equal to the black body intensity.For a nonunifonn temperature medium, the radiativehea*". flux is different from zero and contains, inLTE, information on temperatures of various regionsof, the gas.

Fig. 1. Variables in radiative heattransfer equations.

Relation (3) is a contracted form ofrelation (2), valid for cases in which the sourcefunction, absorption coefficient, and boundaryvalue of the spectral intensity have a planarsymmetry (thus the surfaces A and B are parallelplanes), and the scattering part of the sourcefunction is negligible. (An equivalent of rela-tion (3) for a spherically symmetric spatialdistribution is described in a future publication).

For LTE conditions, the source functions inrelation (3) depend exclusively upon the localtemperature, as expressed in relation (4), or inrelation (5) wnich represents the analogue of re-lation (4) for the total radiative flux, if the

170

absorption coefficient is independent of frequency("grey gas").

Further simplifications of relation (5) maybe obtained for specific distributions of theproperties of the medium. Relation (6) is validfor slowly varying T with respect to the opticaldepth; relation (7) is a variant of relation (6) •for large optical depth between the oo.rnt at whichthe heat flux is calculated and one of the bound-aries; and relation (8), known as the "diffusionapproximation", represents a similar variant forlarge optical depth throughout the medium.

The conductive heat flux for quasi-equilibriura situations and in the absence ofdiffusion currents, is represented in relation (9)and is therefore a function of the local tempera-ture and pressure.

It results that for a system in LTE, equa-tion (1), completed with the appropriate relationfroid (2) - (8) for the radiative heat flux, andv?i th relation (9) for conductive heat flux, andwith assumptions concerning the flow pattern andshear stress tensor, constitutes an apparentlyself-consistent.nonlinear, second order differen-tial equation for the unknown temperature.(Complete self-consistency would exist for a flowpattern independent of temperature.) For a systemin NE additional equations or experimental datajftre needed for the optical coefficients.

In the present study the shear-stress tensoris considered negligible, ar.d variant (10) of theenergy conservation equation is solved numericallyin two dimensions, in polar and cartesian coordi-nates, for a nuclear gas core reactor cavity.Equation (11) represents the expansion of equation(10) in spherical coordinates. As the heat trans-fer in the present reactor concept is mostlyradial, part of the results are given for a one-dimensional solution of equation (11), along theradius, for assumed non-radial transfer.

Computational Procedure

Three separate procedures have been develop-ed to integrate equation (10) and therefore deter-mine the energy distribution and transfer in a GasCore Nuclear Reactor.

The first procedure is a radial one-dimensional solution of equation (11) completedwith any of the expressions (6), (7) or (8) forthe radiative heat flux and with (9) for the con-ductive heat flux. Assumptions are made for thedistrioution of the polar temperature gradientand, consistent with the mass continuity equation,for the distribution of mass flow rate. Equation(11) is itegrated numerically propagating solu-tions for temperature and heat flux from the cavitywall to the cavity center, for guessed boundaryvalues of the temperature and heat flux at thewall. Due to the nature of relations (7) and (8)which determine the heat fluxes at given radiusas function of temperature and mass flow ratevalues at larger radii previously considered inthe amputation (the velocity profile is necessaryto determine the stage in the seed vaporizationprocess which contributes to the local opacity),Uie solution is completely determined by theboundary and does not require iterations on

temperature profiles. The requirement of zero fluxat the cavity center is set to determine iterativelyeither the pressure or the fuel radius. Ifrelation (6) is used for the radiative heat fluxinstead of (7) or (8), the solution propagationfrom wall to center becomes iterative on the temp-erature profile itself which greatly complicatesthe procedure indicated on Table 2 but only slightlyalters the solu'-ion. For this reason all resultsshown for the one-dimensional radial analysis arebased on expression (7) for the radiative heat flux.

The second procedure is similar to the first,but here the radiative heat flux is calculateddirectly from relation (2) through a numericalintegration of the radiative intensities. Itera-tions on entire temperature profiles are involved.The convergence has been tested on a trial case,however only results from the first iteration arepresented here.

The thiv<3 procedure is developed to modelthe two-dimensional behavior of the physicalparameters in the propellant region. The methodconsists of an iterative search for a consistenttwo-dimensional temperature profile. The iterationproceeds by improving the values.of certain func-tions and their derivatives with respect to thenon-radial direction. The method employs a two-dimensional version of equation 7, 9 and 10, andhas proven to be rapidly convergent.

Description of Results

Figures 2, 3, and 4 show cavity profilesfor variations in propellant mass flow rate, cavitypressure, and seed vaporization rate, respectively.In each of these figures^ the solid-line profilerepresents a result for a nominal case, character-ized by the following parameters:

Cavity wall temperature

Total heat flux incident oncavity wall

Cavity pressure

Cavity radius

Propellant seed mass fraction

Propellant mass flow rate

Uranium fuel pressure fraction

Normalized seed vaporization rate

V = 1.6 x 10 Thermal neutron flux at the fuelneutron/cm s edge

Values of these parameters for eanh figure(2) - (4) are nominal unless otherwise indicated.In each case treated, the fuel radius is adjustedso that boundary conditions are satisfied.

In Firure 5, the total heat flux for thenominal conditions is shown together with itsradiative and conductive components.

Figure fia shows, for the propellant region,the nominal radiative heat 1 lux, which was calcu-lated using relation (7), and also the radiative

TCO

q,,

P

Rc

M

P

Mf

RSV

= 600°K

= 1.72 X 10erg/cm -s

= 200 atm

= 30 cm

= 0.25

= 46 g./s.

= 0.68

= 1.0

171

heat flux found by .a numerical integration of ra-diative intensities using relations (2) and (2b) 'and the nominal temperature and absorption co-efficient profiles. Equilibrium values were takenfor emission coefficients. Relation (7) predictssignificantly larger values than those predictedby (2) and (2b) in the optically thin region ofthe propellant. Figure 6b makes the same com-parison in the fuel region as does Figure'6a inthe propellant region, except that, due to largevalues of absorption coefficient in the fuel, fargreater accuracy was required for the numericalintegration of intensities. The nominal radiativeheat flux (relation (7)) has the smallest values inFigure 6b; successive degress of accuracy in in-tegration of intensities (relations (2). (2b))produce successive profiles which approach thenominal profile from above.

Conclusions

The computational procedures developed havethe capability of analyzing a general heat trans-fer problem in one or two spatial dimensions. Theyare able to take into consideration data on LTE ornon-equilibrium optical coefficients, ojticalproperties of the boundaries and data on vapori-zation kinetics of solid particle additives. Theresults provide information on energy transfer anddistribution in a Plasma Core Fissioning Reactorfor given boundary conditions and negligible shear-stress tensor, and constitute a partial analysisof a coupled heat-transfer C > fluid-dynamicstudy.

The heat transfer analysis is performedrigorously, though the presented-results are basedon local thermodynamic equilibrium emission co-efficients. The diffusiof. approximation of theheat flux was proven inappropriate to describe thepropellant region, particularly the transparentdomain containing vaporized seed.

The conductive heat flux is treated assumingnegligible diffusion of various comFonets at theuranium-plasma-propellant interface and negligibleturbulent mixing, as if an ideal perfectly flexibleand transparent wall would separate the two media.Both assumptions are clearly unrealistic. Thoughthe radiative heat flux dominates the heat transfer,and is independent of the type of flow for similarmass distributions, the conductive and radiativeheat fluxes have the same order of magnitude inregions of high temperature gradients, and errorsin the conductive heat flux evaluation influencethe overall results.

An effort continuation in the study of heattransfer and fluid dynamics of highly radiativegases is estimated necessary, before reliablepredictions of advanced concepts performancecould be made.

Fig. 2a. Variation of Temperature withPropellant Mass Flow Kate.

10 IS 20RADIUS (cm)

acknowledgement

To Stanley Chow, who compiled information onphysical properties of the propellant and fuel,and performed initial computations in propellantheat transfer.

Fig. 2b. Variation of Total Heat Flux withPropellant Mass Flow Rate.

172

10 15 20 25

RADIUS (cm)

(0 15 20RADIUS (cm)

Fig. 2c. Variation of Radiative Heat Fluxwith Propellant Mass Flow Rate

Fig. 2e. Variation of Optical Depth withPropellant Mass Flow Kate - Fuel Region.

22 24 26RADIUS (cm)

Fig. 2d. Variation of Optical Depth withPropellant Mass Flow Rate - Propellant Region.

10 IS 30RADIUS (cm)

Fig. 2f. Variation of Uranium Fuel Densitywith Propellant Mass Flow Rate.

173

10 15 20 25 30RADIUS (cm)

22 24 26RADIUS (cm)

28 30

Fig. 3a. Variation of Temperature withPressure •

Fig. 4. Variation of Optical Depth withNormalized Seed Vaporization Rate - PropellantRegion

-T 1 1 r~

10 15 20 a 30

RADIUS (cm)

} 101E

Total

Rodioiivt

Conducliw

10 15 20 25

RADIUS (cm)

Fig. 3b. Variation of Total Heat Flux withPressure

Fig. 5. Total Heat Flux and ComponentsNominal Case

174

• 10 15 20

RADIUS [cm)

Fig. 6a. Comparison of two Physical Modelsof Radiative Heat Transfer - Propellant Region

Table 1.

Correlation between Radiative Heat Flux

•"rad

Srad

(r)

<r) i f

r ad

(?) B f

(r) = f

and Temperature

•+ T(r)

T(r) ,pj LTE,grey,thick

T(all r),p,boundary radiation! LTE

bound electrons and atomsenergy distribution funot..ions I

Table 2.

One-Dimensional Heat Transfer Flow Chart

Input SystemParameters

Input lower and upper bounds for R

yes

Guess new R using bisectionmethod

Propagate values for temperature,total heat flux, radiative heatflux, conductive heat flux, andoptical depth from cavity wall tocavity center.

Use the following relations forpropagation

#10 energy balance# 9 conductive heat flux

either #6, #7, #8 radiative heatflux

is Qtot(r=o) = 0?

Redefine bounds for R,_

printerrormessage

STOP

yesoutputsolution

None: Physical properties ofplasma {p r a , etc.) aredependent on Known localproperties (T, P, etc.)of the plasma.

10 15 20 25

RADIUS (cm)

Fig. 6b. Sensitivity of IntensityIntegration to step size - Fuel Region

175

Total energy conservation equation, stea3y state: Spectral radiative heat flux for planar symmetry:

p v grad — + div(v'T) + div q

+ div q = £ S - " srad p m

H v p v—• s e + •* * + *— = — •p atomic 2 p p

(1, % ""Jo ^ °

(la)

I . e

Bvo vocos8

cosevoTVOcos 9

H - total enthalpy per unit volume

e . - internal atomic energy plus kineticatomic a t o m i c energy

p - mass density

p - scalar pressure

v - average flow velocity

T - shear stress tensor

q - conductive heat flux

q - radiative heat flux

IS - sun of energies from all sources

S - mass source

s - index ranging over component species ofgas

Spectral radiative heat flu*:

cos6

cos6 sin8 d8 =

B E, (TB - T )vo 3 vp vo

B

i \IO * * *

S (T ) E^(T - T ) dTv v 2 vo vo vo

fTvo *- J/ Sv'V (3)

(3a)

Jo J,r/2(Ty)cos8 sine d8 dp

cos6 sin8 d6 dp (2)

Spectral radiative heat flux for planar

symmetry and LTE:

q. = 2irIB E (T B - T )T). VO 3 VO VO

" VSv e

I (T ) = Iv v v

dT

S = — - + — Iv a 4TT I Ko'l*

(2a)

(2b)

(2c)

'

B

ir f T % ) °J Tvo

2 vo

Suhv3

V

8irhv

I

hv/

*dT

vo

3

1

1

c3 e h V / M * - 1

X E-(T - T ) dT2 vc vo vo

(4)

176

Radiative heat flux for planar symmetry,

LTE, and 'grey' gas:

q „Trad 2irS B AI E, (T - T ) - 2ir I E, (r )o 3 o o o 3 o

Conductive heat flux

q = - k grad Tc c

(9)

Energy conservation, scalar pressure, steady stafce**- *

-20 E 2 ( T - / , dx* (5)

Equation (10) in spherical coordinates

3T 1 3 , 2 , 2 ,

Radiative heat flux for planar symmetry,

LTE, 'grey' qas/ ', _: ' =0

~ TS- (sine sine (11)

dx I

- to)j

TB . T , {TB _o o 3 o o

T o E 3 ( T o ' + E 4 t T o - V + V V " I I <6>

R-idiative heat flux for planar symmetry.

32T4 BLTE, ' g r e y ' g a s , —r— = 0, T - T >> 1:

2 ° °

q r a d E 3 ( T o J

dT f

-2" sr K * B . " . ' -IIi f TB - T >> 1

o o (7)

Radiative heat flux for LTE, 'grey' gas.B0, T - T » 1, T » 1:

1 7 7

RESEARCH OK PLASMA CORE REACTORS

G. A. Jarvis, D. M. Barton, H. H. Helmick, William Bernard, and R. H. WhiteUniversity of California, Los Alamos Scientific Laboratory

Los Alamos, New .Mexico 875*15

Abstract

Experiments and theoretical studiesare being conducted for NASA on criticalassemblies with one-rmeter diameter by one-meter long low-density cores surrounded bya thick beryllium re"lector. These assem-blies, make extensive use of existing nu-clear propulsion reactor components, facil-ities, and instrumentation. Due to ex-ccssive porosity in the reflector, theinitial critical mass was 19 kg U(93.2).Addition of a 17-cm-thielt by 89-cm-diametorberyllium flux trap in the cavity reducedthe critical mass to 7 kg when all theuranium was in the zone just outside theflux trap. A mockup aluminum UFg containerwas placed inside the flux trap and fueledw:'.th uranium-graphite elements. Fissiondistributions and reactivity worths offuel and structural materials were measured.Finally, an 85,J00 crr.3 aluminum canisterin the central regioft was fueled with UP6gas and fission density distributions de-termined. These results //ill be used toguide -..he cesien Df a prototype plasmacore reactcr which will test energy removalby optical radiation.

T. 3. La-ha- ir.d collaborators1 atthe United rechr.zLo%ies Research Center(iJTRC) have recently suggested severalattractive applications cf cavity reactcrsystems aiiried at meeting future criticalenergy r.eeds. Uranium fuel in gaseous orplasma form permits operation at muchhigher temperatures than possible withconventional solid fueled nuclear reac-tors. Higher working fluid temperaturesin general imply higher thermodynamiecy-rle efficiencies, whicl lead to proposalsfor advanced closed-cycle gas turbinedriven electrical generators and MHD powerconversion systems for electricity produc-tion. Cycle efficiencies ranging from 50to 65 percent are calculated for thesesystems.

Further, the possibility of energyattraction in the form of electromagneticradiation allows consideration of manyphotochemical or thermochemical processessuch as dissociation of hydrogeneousm.iterials to produce hydrogen. Lasers maybe energized either by direct fissionfragment energy deposition in uraniumhexafluoride (UFg) and lasing gas mixturesor by optical pumping with thermal or non-equilibrium electromagnetic radiationflowing out of the reactor.

This work was supported by the ResearchDivision, Office of Aeronautics and SpaceTechnology, National Aeronautics and SpaceAdministration, NASA Contract No. W-13755.

The Los Alamos Scientific Laboratory(LASL) has a modest program to acquire ex-perimental and theoretical informationneeded for the design of a prototype plasmacore reactor which will test heat removalby optical radiation. Experiments are be-ing conducted on a succession of criticalassemblies with large low-density coressurrounded by a thick moderating reflector.Static assemblies, first with solid fuel,';hen with UFg in containers, will be fol-lowed by others with flowing UFg.

Plasma Core Assembly (PCA) Design

In the first quarter of 197^, a jointworking group formed by NASA from LASL andUTRC pevsonnel produced an engineering fea-sibility design study revealing the possi-bility of constructing a large berylliumreflected cavity reactor from existing nu-clear propulsion program materials and con-trol components.2

Reactor Component Availability

Substantial quantities of berylliumreflector parts Kere stored at Los Alamosand the Mevada Reactor Development Stationafter the termination of the nuclear pro-rpulsion program. Beryllium reflector as-semblies from 11 Rover/NERVA reactors, re-presenting an original cost of approxi-mately $5 million, were transferred to LosAlamos> Also, four additional berylliumreflector assemblies, including a set of18 stepping motor driven control drum actu-ators, were available at Los Alamos.

LASL Facilities

The facilities of the LASL CriticalExperiments Group include three remotelycontrolled critical assembly laboratories,called Kivas, equipped with a variety ofcritical assembly nachines of varying com-plexity. A central control building hous-es control rooms, offices, and laboratoryspace. Parts of these facilities areavailable "for the initial phases of theplasma core reactor project.

The cavity-type assemblies will be onthe "Mars" critical assembly machine(Fig. 1) located in Kiva 1. This machinewas built early in the Rover program andwas used for neutronic and core optimiza-tion studies for the- Kiwi, Phoebus, andNuclear Furnace reactors. Early berylli- 'um-reflected cavity reactor experimentswere also performed on this machine.'J1*Figure 1 shows the Mars machine with aPhoebus II reflector installed. The over-all size is similar to that of the plannedcavity assemblies. Some of the 18 controldrum actuators are visible above the

178

reflector.

The principal features of the Mars ma-chine are:

1. A framework that includes a baseplate for supporting the reflector, an upperplatform for mounting the existing control.drums, and a personnel platform,

2. Provision for removing a fueledcore from the reflector. The core is sup-ported on the platen of a hydraulic cylin-der, which is centered beneath the reflectorand can be retracted as a safety device.

3. Provision for removing the corefrom beneath the machine. The lowered coreassembly rests on a cart that may. be rolledout on guide rails from beneath the re-flector to provide easy access for altera-tions to the core.

Reflector Design

The critical assembly reflector designmakes use of beryllium from most of theRover reactor types. Figure 2 shows reflec-tor parts from several of the Rover reactorsassembled to form a cross section of thecylinderical reflector wall. Figure 3identifies major Rover beryllium componentsby location. The end plugs of Nuclear Fur-nace, Pewee, and Honeycomb reflector pieces,shown in Fig. 2 and 3, consist of berylliumthat averages 90% of normal density. Graph-ite fillers, 38-mm thick separate thesefrom 190-mm-thick NRX and Kiwi B sectors ofberyllium also at 90$ normal density. Asucceeding graphite annuli.s Hk-rnn thick isfollowed by a 203-mm-thlck Phoebus II as-sembly which contains 1*-' control drums andis beryllium at 87.5% normal density.

All of the Rover beryllium reflectorcomponents have holes In them required forcoolant flow, tie rod access, instrumenta-tion lead channels, reflector density ad-justment, and control drum containment.These lead to loss of neutrons by streamingand result in increased critical mass ifleft unfilled.' Calculations indicate a re-activity loss of about 25$ for each percentreduction in the density of the Be reflec-tor.

Figure k shows the calculated penaltyfor substituting a 50-mm annular zone ofgraphite for beryllium as a function ofthe location of the graphite in the reflec-tor. In the outer two-thirds of the re-flector, high purity graphite is a goodsubstitute for beryllium but is of littlehelp in the inner region. The reflectorencloses a 1.02-m-diam by 1.05-m-high cavityin which the various solid and gaseous ura-nium fuel cores will be studied.

Experimental Program

Core Number 1

A homogeneous uranium distribution inthe cavity, simulated by use of uranium

metal foils laid on a set of ten equallyspaced aluminum discs, Fig. 5, had a crit-ical mass ox" 19-kg U(93.2). The reactivityswing for the 18 control drums was 6.1$.Figure 6 gives the control drum calibrationcurve. Measured reactivity worths of corematerials are uranium 73tf/kg, aluminum-3<t/kg, and graphite 2.Si/kg.

Core Number 2

An early goal for the Plasma Core As-sembly was to operate with a central zoneof UFg gas surrounded by a zone of soliduranium fuel. To mock up this situationcore number 2 consisted of uranium metalfoil attached to a 0.9^-m-diam Uhin-walledaluminum cylinder, Fig. 7, centered withinthe cavity. A critical mass similar tothat for core 1 was indicated by subcrit-ical comparison.

The critical mass of about 19 kg ismore than a factor of two higher than cal-culated for uniformly distributed fuel inan idealized reflector, indicating under-moderation in the reflector and/or con-taminants having high thermal neutron ab-sorption cross sections in the berylliumand graphite. Neutron leakage through thelarge number of small straight coolantchannels in the beryllium also tends to in-crease the critical mass value.

The beryllium specification limitedthe iron content to less than 0.18? byweight. One-dimensional criticality cal-culations showed that this amount of ironwould increase the critical mass by about9%. Two-dimensjonal'criticality calcula-tions indicated that two hundred parts permillion boron containment in the graphitewould account for the excess critical massif boron alone were the cause. The largecritical mass is believed to be due to acombination of the effects discussedabove.

Core Number 3

The simplest solution to the neutronundermoderation problem was to add a zoneof beryllium to the cavity. A suitableberyllium annulus 0.55-m i,d. by 0.89-mo.d. by 1.0^-m high, Fig. 8, was former!from Pewee and Honeycomb parts. Thisstructure had a mean density 85% that fornormal beryllium and was surrounded by a0.9^-m-diam aluminum cylinder to supporturanium metal foil. The initial criticalmass with all uranium in the outer fuelzone was 6.84-kg U(93.2).

The reactivity worth of the controldrums is influenced by the amount of ura-nium in the core. Since core number 3with its added inner beryllium zone sharp-ly reduced the critical uranium loading,the control_drums were recalibrated, usingpositive period measurements. The reac-tivity swing for one control drum is 22.Hcents, giving 4.03$ for the 18 controldrums. Figure 9 gives the calibrationcurve for the entire 18 control drums.

179

The control drum worth is now only twothirds that for the empty cavity config-uration of cores one and two, but is con-sidered adequate for the planned experi-ments.

Table I lists the reactivity worths ofuranium, aluminum, and carbon for indicatedpositions in core number 3«

It should be noted that the uraniumworth values of Table I imply an apprecia-bly greater fission density in the centralzone than in the outer fuel zone. Theadditional neutron moderation in the fluxtrap results in a central zone uranium re-activity worth six times that for uraniumin the outer zone. As the fission densityin a highly thermalized system varies asthe square root of the reactivity, VFQ inthe central zone will benefit from a fis-sion density enhancement of about 2.4 overthat for uranium in the driver zone.

The effects of flux trapping by anadditional beryllium zone within a cavitysurrounded by an undermoderating reflectorwas examined using one-dimensional neutrontransport calculations on an equivalentspherical moekup of the PCA cylindricalgeometry. The calculated fission densityin the central zone was 2.3 times that forthe outer or driver fuel zone.

Core Number ft

Core number ft uses the beryllium fluxtrap of core 3 and uranium-graphite Rover

fuel elements in the outer driver zone,Pig. 10. The inner experimental zone isprovided with a mockup of an aluminum UFgcontainer to be used in later experiments.Low density uranium-graphite fuel elementsare used in this region to simulate UFgloadings.

Table'II gives the loading specifica-tions for the uranium-graphite fuel ele-ments.

Figure 11 shows the available fuelelement holes in the driver and experimen-tal zones. Figure 12 shows core No. ftpositioned to be raised into the PCA re-flector.

Material Reactivity Evaluations

Reactivities for uranium and othermaterials were measured in the central ex-perimental and driver zones. Resultsagreed with those found for core 3.

As seen in Table 1, uranium has a muchhigher reactivity worth in the inner zonethan in the outer or driver zone, so thatthe critical mass will decrease as uraniumis added to thp central region of thecore. Figure 13 shows the measured vari-ation of critical mass with the fractionof fuel in the inner zone.

Fission Density Measurements

Fission density distributions weredetermined for two different core loadings

TABLE I

REACTIVITY WORTHS OF MATERIALS IN PCA CORE NO. 3.

Mater,

U (93.5%)

U (93.5%)

Al

Al

Honeycomb C

PCA Reflector C

Material location

On axis

Near end of Al support

On axis

At Al support

On axis

On axis

Worth In cents/kg

+ 3350

+ 554

- 10.8

- 3.0

- l.ft

- 3.2

TABLE II

Element Location

Driver zone

Experimental zone

Uranium Density

too mg U/em3

100 mg U/cm3

Uranium/Element

82,,8 g

20.7 g

180

for the experimental and driver fuel zonesby beta scanning U-loaded Al wires irradi-ated in the central holes of selected fuelelements. Figure lH shows the fuel ele-ment configuration and wire locaticns.Figure 15 shows the core mid-plane radialfission distribution. The fission densityrises slowly from the center outwards.The ratio of the central element to driverelement fission density values is 2.110,in good agreement with the value of 2.4noted in section III-c which was determinedfrom uranium reactivity measurements. Fig-ure 16 gives fission distributions for twocentral fuel zone axial positions.

Core Number 5

UFg gas was used as the fuel for thiscore in the inner zone using the UTRC dou-ble walled canister and gas handling sys-tem. A Be flux trap separated the gas froma driver fuel element zone composed oflightly loaded uranium - graphite fuel rodsdescribed in the previous section. Figure17 is a photograph showing the canistermounted on a ram underneath the reflectorready for assembly. Two pie sections ofthe lower Be end plug have been removed toshow the flex lines that connect the can-ister to the gas handling system in theforeground.

Figure 18 is a schematic of the gashandling system. 235uFg in the supply bot-tle is a solid at room temperature. Thisbottle is immersed in a hot water bath andheated for one hour at about boiling tem-perature (SS^C). This converts the solidto a gas and by suitable valving, a knownamount of the gas is trapped in the trans-fer bottle of the same manifold. Verifica-tion of the amount of gas in the transferbottle is obtained by weighing the bottleand contents. The entire gas system andcanister are next heated to about 66°Cusing heat tapes and hot flowing He gas.This requires about three hours. The UFgnow remains in the gas phase for criticaloperations.

Recovery of the UFg is accomplished bypumping down the canister and gas systemthrough a glass liquid nitrogen (LN) trap.Material recovery is evaluated by weighing

the glass trap and contents. After com-pleting the recovery cycle, we find verylittle residue in the canister, as affirmedby running the reactor at delayed critical.However, weighing showed that some UFg re-mained in the transfer system. We suspectthat the majority of this material is inthe corrugated flex lines linking the gassystem to the -canister. Table III showsthe results obtained from out two transferoperations.

UFg Reactivity Evaluation

A lightly loaded Kiwi fuel elementwas placed in the region between the can-ister and the Be flux trap in order to re-evaluate the reactivity worth of uranium inthe central region. A value of 2,95cents/g U was obtained which translatesto 1.99 cents/g UFg. An Initial operation-al limit of one dollar for the gas in thecore restricts the UFg to a minimum of 50grams.

Three fission density distributionswere measured in the gas core along pathsas indicated in Figure 19. U-loaded Alwires were placed on the central axis andon a parallel line displaced 10 cm radi-ally from this axis. Another wire was ona diameter at the mid-plane of the core.The wires were irradiated for 10 minutesat a power level of about 70 watts andthen scanned with a beta counter to obtainthe data plotted in Figure 20. We foundsimilar distributions for the two coreloadings of 12 to 2<4 grams of UFg, The ra-dial traverse, not shown, is flat to with-in 0.1?, as expected for such a light ura-nium loading. An earlier experiment notedin section III-c, using 393 grams of ura-nium in a solid uranium-graphite fuel mock-up of the gas core, had a 3% rise in fis-sion density from the center to the coreedge, indicating a slight uranium selfshielding effect. The axial fission den-sity falls off about 8% from the mid-planeto the core end. This results from holesin the reflector end walls for plumbingand thinner beryllium in this region.

Concluding Remarks

A beryllium reflected critical

TABLE III

SUMMARY OF DELAYED CRITICAL RUNS ON PCA

Run

Run

1

2

UFg GasTransferred

(g)

12.0

24.6

Reactivity

Change(cents)

20

50

UFg

Recovered(g)

3.6

14.9

UFg leftin canister

(g)

2.2

2.5

UFg leftin lines

(g)

6.2

7.2

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assembly has been constructed which issuitable for performing tests on staticand blowing UFg cores and' ultimately on ad-vanced models employing hydrodynamlcal con-tainment of tne UFg by a flowing buffer gas.Dividing the core Into a driver zone con-taining a major fraction of the fuel Insolid form separated by a neutron flux trapfrom a central experimental zone for gase-ous fuel has several advantages:

1. The fission density in the gasfueled region is boosted by a factor of 2.4over that in the driver zone.

2. An idea] gas core geometry withlength about three times the diameter isavailable.

3. Initial evaluation of reactorsafety problems for the flowing UFg fuel-ed core critical.

References

1. Latham, T. S., F. R. Biancardi, andR. J. Rodgers: Applications of PlasmaCore Reactors to Terrestrial EnergySystems. AIAA Paper No. 74-1071*,AIAA/SAE 10th Propulsion Conference,San Diego, California, October 21,23,1971.

2. Helmick, H. H., G. A. Jarvis, J. S.Kendall* and T. S. Latham*: Prelim-inary Study of Plasma Nuclear ReactorFeasibility. Los Alamos ScientificLaboratory report LA-5679, ~uly 197^.*Uniteci Aircraft Research Labora-tories,

3. Jarvis, G. A. and C. C. Byers: Crit-ical Mass Measurements for VariousFuel Configurations in the LASL DjOReflected Cavity Reactor. AIAA PaperNo. 65-555, AIAA Propulsion JointSpecialist Conference, June 1965.

4. Paxton, H. C , 3. A. Jarvis, and C. C.Byers: Reflector-Moderated CriticalAssemblies. Los Alamos ScientificLaboratory report LA-5963, July 1975.

Pig. Rover beryllium reflectorcomponents used in PCA.

Fig. 1 Mars machine with Phoebus II;Rover Assembly.

182

c.

End plug: Nuclear Furnace,"Pewee, Honeycomb pieces

-Graphite filler

_NRX and Kiwi S"reflector assumbly

-Kiwi B sectors

f s— Graphite filler

1 . 0 2 m

_Phoebus IIreflectorassembly

e air> to

1.98 m dia-

Fig. 3 Plasma core assemblyreflector.

60 70

Radius-cm

Fig.4 Reactivity loss associated withsubstitution of graphite for Be.

60 120Control drum angle-degrees

180

Fig. 6 Control drum calibration curvefor core number one.

Fig. 5 Core No. 1

183

ft-

, too

Fig. 7 Cere No. 2

Aluminum cylinderfor uranium foils

Pig. 8 Core 3

0 6C 120Control drum angle-degrees

Fig. 9 Control drum calibration forcore number three.

V

180

Fig. 10 Core number four with partialload of U-graphite driver fuelrods.

184

Aluminum Spacer RingsFor Outer Fu.it Pods11.04 in O.a it O.9O I.D.1

-Aluminum Spocer DIsktFor Inner fuel Rods(0.55 m Otamettr)

Pip., 11 Aluminum support structure for' supporting U-graphite rods forcore number four.

6 7Critical mass kg U(93-2)

Pig. 13 Critical mass dependence onfraction of V in the center fuelzone.

O -100 ma U/cc Uranium- Graphite Rods

®-400mg U/cc Uranium - Graphite Rode

Fig. 14 Fuel element loading and wirepositions for core No. H

Fig. 12 Mars machine with corenumber four ready forassembly.

185

0 5 -

-i 1 1 r -I 1 1

- Jmer Fud Zen *' I t "- B e Flux Trap 1| [•—

Outer PelZone

1 j f I t iS 2 O 2 5 3 O 3 5 4 O 4 5 5 C 5 5

Aldan ti cm

Pig. 15 Radial fission distribution,core No. 4

Fig. 17 Core number five with aluminumcanister- for UFg gas.

I T 7 T I I T I I r I I i

1 .1 ' .J» ' .k • 4 '.1 ' .J» •a Ditpbjannt tttxn On Corfu tori •

Fig. 16 Axial fissiondistribution, core No.

Fig. 18 Schematic of UFg gas.... ' handling system;

186

Uranium GraphiteFuef Elements-*.

o On axis

* JO cm off-axis

10 0 io 20Axiol Distance From Center fcml

Pig. 30 Axial fissiondistributions, core No. 5

DISCUSSION

ANON: Why did you use argon as yourbuffer gas?

T s LATHAM. A r g o n l s a h l g h e r density buffergas, giving a higher density per mole. Thus ata given pressure, more UFg can be -confined withargon rather than helium.

To UFt

Supply

Fig. 19 Location of fission wires incore number five.

187

PARAMETRIC ANALYSES OF PLANNED FLOWING UEANIUM HEXAFLITORIDE CRITICAL EXEERIMEHTS*

R. J . Rodgers , Research Engineerand

T. S. Latham, Chief Project EngineerUnited Technologies Research Center

East Hartford, Connecticut

Analytical investigations were conducted todetermine preliminary design and operating charac-teristics of flowing uranium hexafluoride (UFg)gaseous nuclear reactor experiments in which ahybrid core configuration comprised of UFg gas anda region of solid fuel will be employed. Theinvestigations are part of a planned programinvolving National Aeronautics and SpaceAdministration (HASA), Los Alamos Scientific.jeboratory (LASL), United Technologies EesearchCenter (UTRC), and other organizations to performa series of experiments of increasing performance,culminating in an approximately 5 KM fissioninguranium plasma experiment.

The results, indicate that confined flow testsof a gaseous UFg-uranium fuel element hybrid core

• configuration appear feasible. A preliminary .design is described for an, argon buffer gas con-fined, UFg flow loop system for future use inflowing critical experiments. Flowing gaseous UFgwculd be confined within the cavity core canisterby tangential injection of argon buffer gas toproduce a radial inflow vortex flow pattern.Initial calculations to estimate the operatingcharacteristics of the gaseous fis-foning UFg in aconfined flow test at a pressure cr k atm, indicatetemperature increases of approximately 100 and1000 K in the UFg may be obtained for total-testpower levels of 100 kW and 1 MJ for test times of320 and 32 s, respectively.

Introduction

Extraction of power from the fission processwith the nuclear fuel in gaseous form allowsoperation at much higher temperatures than those ofconventional nuclear reactors with solid fuelelements. xgher operating temperatures in generallead to more efficient thermodynaoic cycles and, inthe case of fissioning uranium plasma core reactors,result in many possible applications employingdirect coupling of power in the form of electro-magnetic radiation.

A program plan for establishing thefeasibility of fissioning UFg gas and uraniumplasma reactors has been formulated by NASA and isdescribed in Refs. 1 and 2. Briefly, the series ofr»» . ir tests consists of gaseous nuclear reactor-•xperiisents of increasing performance, culminatingin an approximately 5 MW fissioning uranium plasmareactor experime;-^. Initial reactor experiments, will consist of low-power, self-critical cavity

reactor configurations employing undissrelated,nonionized UFg fuel at neai minimum temperaturesrequired to maintain the fuel in gaseou., form.Power level, operating temperature, and pressurewill be systematically increased in subsequentexperiments to approximately 1000 kW, 1800 K, and20 atm, respectively. The final 5 MW reactorexperiment will operate with a fissioning uraniumplasma at conditions for which the injected UFg willbe dissociated and ionized in the active raaetorcore.

Cavity reactor experiments have been conductedto measure critical masses in cavity reactors atboth LASL and at the National Reactor TestingStation (NRTS) in Idaho Falls. Critical mass mea-surements have been made on both single cavit/ andmultiple cavity jonfipurations. In general,nucleonics calculations have corresponded to withina few percent of the experimental measurements. Areview of these experiments is contained in Ref. 3.These studies provide the basis l'or selectingadditional experiments to demonstrate thefeasibility of the plasma core nuclear reactors.

Analytical investigations described hereinwere performed to formulate a preliminary designfor an argon-buffer-gas-confined UFg flow systealoop, ttis design will be used in future cavityreactor experiments in which hot flowing UFg is tobe confined within a cavity core canister bytangentially injected argon buffer gas. Calcula-tions of principal operating parameters for 100 kW• and 1 tW cavity reactor experiments with buffer-gas-confined flowing UFg were made to define the con-trol requirements for the gas flow systems and thereactor experiments.

Initial Reactor Series Tests

The initial series of planned low-powercritical cavity reactor experiments has beenconducted at LASL. These critical cavity reactorexperiments consisted of sequential tests on thefollowing core loading configurations: (l)eistri-buted uranium foil core loadings in a right circu-lar cylinder, 1-m-dia by 1-m-long cavity surroundedby a 50-cm-thick beryllium reflector-moderator, and(2) hybrid fuel loading configurations in which thecentral S't'-cmniia by 1-m-loug pottion of the corecavity volume contained An aluminum (Al) canisterfilled with a gaseous UFg-Helium (He) mixturesurrounded by a 17.2-em-thick annular berylliummoderator region surrounded in tain by a rggioncontaining solid nuclear fuel elements. Resultsof the foil loading tests are described in Ref. k.

•Research sponsored by Los Alamos Scientific laboratory, Contract XP5-5W*7l|--l.

188

The tests were conducted to determine criticalBass loadings, control drum calibrations, endoverall control drum reactivity worth, reactivityworths of the structural Al, and the reactivitypenalty due to holes required in later experimentsfor introducing increased amounts of gaseous UFgand for observing possible fission gragmeetinduced electromagnetic radiation emission.

The second series of planned cavity reactorexpc-\xents will employ flowing gaseous UFg-Hemixtures. The experiments will employ a hybridfuel loading geometry in which pproximately10 percent of the core cavity volume will containa canister similar to that used in the initialcritical experiments. In the planned experiments,gaseous UFg will be mixed with varying amounts ofHe and circulated within a closed flow loopthrough a canister located in the central regionof the core. That portion of the total flowingUFg inventory contained in the core canister willbe included in the total critical mass loading.The remaining portion of the flawing UFg inventorywill be contained in the flow lines and associatedhardware external to the cavity volume. UFg notin the canister is not neutronically part of theactive core except for that UFg contained in theflow lines within the lower section of reflector-moderator. Initially, most oi' the fuel loadingwill be comprised of solid fuel in zhe form ofdistributed uranium foil or solid fuel elementsfabricated for earlier Rover program1 reactors.The flowing UFg testa will be conducted usingthe canister and. gaseous circulation flow systemdescribed in Ref. 5. These tests will providebasic reactor physics and engineering data for de-sign of subsequent reactor tests in which flowingUFg will be fluid-meehanically confined away fromthe canister peripheral walls by argon buffer gas.

Reflector Moderator ConfigurationsThe basic beryllium reflector-moderator was

assembled from available materials from the Roverprogram'1'. The reflector-moderator configurationis shown in Fig. 1. The reflector surrounds aright circular cylindrical cavity approximately 3k- 'cm-dia x 1-m-long. The reflector-moderator config-uration contains an approximately 6.5-cm-wideannulus for locating solid fuel or distributeduranium foil between the inner Bewee Beryllium andthe outer beryllium reflectors. Critieallty Mea-surements'^) indicated that a uranium foil loadingof 6.7 kg was required for criticality with unlOformly distributed foil in both the gap between theinner Pewee beryllium and outer beryllium reflec-tors, and the central cavity. The reflector-modera-tor configuration shown in Fig. 1 is well-suitedfor flowing UFg critical experiments in which thecore canioter would be located within the approxi-mately 5U-cm-dla x 1-m-long cavity. The total outerberyllium reflector thickness is =50 em.

The reflector-moderator is mounted on thePajarito Critical Assembly Facility at LASL. Thisfacility is described in detail in Ref. 6. Thecritical assembly facility has a framework which

includes a base plate for supporting the reflector,an upper platform for nouritirjg the control drums,and a personnel platform. In addition, provisionis made for removing the fueled core from thereflector by lowering a platen on a hydraulic ramcentered beneath the reflector. Further provisionfor renoving the cure from beneath the machine isalso made. When lowered, the core assembly restson a cart which may be rolled out on guide railsfrom beneath the reflector to provide easy accessfor maintenance and modification of the coreassembly.

Analyses of Flowing 'JF6 Experiments

With Buffer Cas Confinement

As part of the cavity reactor experimentalprogram continuing at LASL> the third phase of theplanned series will involve performing reactorexperiments in which flowing UFg will be fluid-mechanically confined in a central volume of acore cavity by A or He buffer gas injectedtangentially from the cavity peripheral walls.The objective of the cavity reactor experimentprogram is directed toward demonstrating thefeasibility of fissioning uranium plasma reactors.Toward this end, a final approximately 5 Mfreactor experiment will operate with a self-critical fissioning uranium plasma confined by Abuffer gas at conditions such that the injectedUFg will be dissociated and ionized in thereactor core. In examining a near-term series ofbuffer gas confined UFg experiments, a hybridreactor (driver zone plus a fissioning UFg gaseouscore) configurations will be used to determinethe operating characteristics as well as to obtainbasic engineering data for plasma core reactors.The hybrid fuel and reflector-moderator configura-tion which is shown in Fig. 1 and which isassembled for flowing UFg and static UFg cavityreactor experiments, could also be employed in theinitial buffer gas confined UFg experiments.

Core Canister Assembly and Gas Flow Systems

The results of a preliminary design of an Abuffer gas confine; UFg canister, UFg and f flowsystem, and a UFg separation and reprocessingsystem for use in these future cavity reactorexperiments are indicated in sketches of thecore canister assembly and schematic diagram ofthe flow systems shorn ir. Figs. 2 and 3,respectively. Ths- core canister assembly is made-up of two concentric 6061-0 Al eanisteis and hasan approximately 3*t-cm-ID inner canister Theouter canister is 37-cm-dia and is used tc providea flow region for thermal control of the innercanister, as well as a safeguard against leakageof the fuel into the external environment. Theoverall length of the canister assembly isapproximately 91* cm. The canister dimensions aresuch that it occupies a volume of approximatelyO.OB n?. A continuous flow of A buffer gas- isinjected tangentially from a single slot whichextends along the entire length of the canister

189

peripheral wall. lire tangential injection of Abuffer- gas ereatss a radial inflow vortex and,therefore, a radial pressure gradient within whichthe fuel is contained. Sons of the injected Aflows directly through the buffer gas region andprevents the fuel mixture from reaching the periph-

• eral wall. This bypass flow if withdrawn through aperforated section of the peripheral wall whichextends along the entire length of the inner canis-ter and which is just upstream and adjacent to theA injectiOE slot. The bypass flow is then removedfrom the canister assembly via an axial duct loc-ated behind the perforated section of the peripheralwall and between the inner and outer canister walls.The ratio of bypass flow to thru-flow is adjustedto give the best fuel containment. Some of the Abuffer gas flows radially inward in the endwallboundary layer and is removed from the cavity viathru-flow ports located on the centerline in bothendwalls.

Continuous fuel injection into the vortex flowwill be accomplished from one endwall through dis-crete injectors usually located at a constant radiuswell away from the centerline, typically 75 to 80percent of the canister radius. Some of the injec-ted fuel is confined within the vortex centralregion. However, a portion of the fuel mixesraj ily with the A buffer gas flow in endwallboundary layers and exhausts with the A flow. Thethru-flow exhaust stream typically consists of ahigh concentration of confined fuel and A buffergau. The bypass flow consists of mostly A buffergas with small concentrations of fuel. In additionto the canister, the buffer-gas-confined UFg flowsystems sh ?ig. 3 consists of the fuelrepBoces' _ . /stem, a fuel recirculation system,and a buffer gas recirculation system.

The fuel reprocessigg system is used to treatthe effluent flow from thi canister. Its primarypurpose is to separate the UFg from the A*buffergas, This is accomplished by cold trapping in acontinuously sequenced set of heat exchangers.After a prescribed desubliming time, each heatexchanger will be evacuated to remove the regain-ing noncondensible gases, then heated to drive thepure gaseous UFg to a condenser where it will beliquefied. The minimum UFg concentration in theoutlet process gas stream is limited by its temp-erature and corresponds to the UFg vapor pressureat that temperature. The temperature of the out-let process gas stream is approximately 225 K forwhich the corresponding UFg vapor pressure isapproximately 0.001 atm. The trace UFg remainingin the process gas stream will In removed from thegas stream in a KaP absorption-desorption bed.Desorption of the WaF bad can be effected subse-quent to the reactor test or series of tests.

The fuel recirculation system Mhicn iscomposed of a pump, heater, and connecting heatedflow lines, is used to process liquid UFg and toprovide for re-injection of UFg into the oorecanister in gaseous form. The buffer gas

recirculation system which is composed of an Apump, heater and connecting flow lines, is used toprocess the A buffer gas for re-injection into thecore canister. The buffer gas has been stripped oftrace UFg quantities by the BaF chemical trap. Amore detailed description of the flow systems isgiven in Kef. 7.

UF6 Contained Masa Calculations

Extensive experiments have been conducted todetermine the containment characteristics ofradial-inflow vortexes for application to the gascore nuclear rocket programf""11). The results ofthose experiments also ha^e application in theplasma core reactor program l-»"*)} as well as thebuffer gas confined flowing UFg cavity reactorexperiments. In these investigations, a light gas,air or He, was used to drive the vortex and aheavy gas was injected separately to simulategaseous nuclear fuel to be contained within thecentral region of the vortex test chamber. Fluidmechanics experiments have indicated that a radialinflow vortex is well-suited for applications whichrequire the preferential containment of the heavygas within the light gas created vortex flow. Theradial-inflow flow pattern is characterized by anessentially laminar radial stagnation surfaceacross which there is no inward radial buffer gasflow. The radial position of this surface isnominally defined to bt uhe edge-of-fuel location;that is, the radial locfa on outwardly beyond whichthe density of fuel is assumed to be essentiallyzero. The radial stagnation surface can be locatedas far from the centerline as 80 to 90 percent ofthe total vortex radius. In a fissioning UFO fuelregion a negative temperature gradient through thebuffer gas region to the peripheral wall, insuresthat a positive density gradient exists through thebuffer gas region and this should reduce thediffusion of fuex radially outward through tfcebuffer gas.

Calculations were performed to determine themuss of UF6 which can be confined in the centralregion of an A vortex .within e core canister withthe same voluae and dimensions as shown in Fig. 1.In this configuration, the UFg is contained withira vortex flow and away from the canister wallbjiy Abuffer gas injection tangentially from the periph-eral wall of the canister. Based on containmentexperiments of isothermal two-component gasvortexes'^), a radial stagnation surface wasselected for the calculations at a fuel region tocavity radiaa ratio of 0.75, where the cavityradius is 16.92 cm. The cylindrical length of thefuel region was taken to be approximately 91 cm.Both the A and fuel mixture (UFg and He) are t^ beinjected initially into the cavity at a temperatureof approximately 400 K.

Calculations of the amount of UFg w&ich couldbe contained within the above described fuelregion vere performed with the following assump-tions : (l) the density at all locations in the

190

fuel region is constant and equal to the density ofA at the radial stagnation surface which is assumedto be at a temperature of 400 K; (2) the fuelregion gaseous constituents (UFg-He-A) are at auniform temperature equal to or greater than the. Abuffer gas tepperature of 4oO K; (3) the ratio oftotal volume-averaged partial pressure of the fuelmixture, (Wg+He) in the entire canister to thebuffer gas pressure at the peripheral wall, p/jQpis between 0.3 and 0.4. "he results of calculationsof the mass of UFg contained in the vortex for totalcavity pressures of 1, 4, and 10 atm and fortemperatures between 400 ana 2000 K sire shown inFig. 4. The UFg contained mass is relativelyinsensitive to temperature and ranges from approxi-mately 45 to 450 g as the total pressure within thecanister is varied from 1 to 10 atm. These are theapproximate ranges in the variables of pressure,temperature, and mass of UFg expected in the Abuffer gas confined flowing UFg low -power r-cavityreactor experiments. The Al-canister will be loc-ated in a cylindrical cavity approximately 54 cm indiameter and 100 cm in length. The UFg fuel con-tainment canister, hybrid fuel and associatedreflector-moderator configuration, as shown in Fig.1, will be used in this series of reactor experi-ments .

Calculations of operating characteristics ofthis reactor configuration wer^ parformed, based ona maximum total number of fissions per tests of1 x 10l8. This fission limitation has been useu toconform to present operating procedures for critic".!tests at the LASL Pajarito site. The calculatedcritical laass of a similar configuration was k.3TKg of U (0.935 U-235) as reported in Kef. 12, wasobtained using a spherical reactor model in a one-dimensional neutron transport theory computer codecalculation. The calculation also assumed a rela-tive fission power density (specified as W/g ofU-235) for fuel In the canister to that in thesurrounding solid fuel to be 2.3 as calculated andreported in Kef. 4. The UFg fission rate ratio isa function of the fission power density ratio,(<iUP6/(JS0L:I:D. between UFg and solid fuel; the UFgmass, Mypg, contained in the cani&ter; and criticalra3/5s, MQ, and is given by the expression

F Rv

where the UFg is assumed to b'e 0.935 U-335. Thevariaxicn of UF- fission ratio with containedmass of UFg is shown in Fig. 5. As the fissionrate ratio varies from 0.01 to 0.20, the corres-ponding mass of UFg contained in the canistervaries from approximately 30 to 685 g.

Operating Power Deposition Levels

The characteristicsvariation of totaloperating power with, the associated maximum timeat which the reactor experiment can bp operated atthat power level such that the total number offission is equal to 1 x 101" is shown in Fig. 6.The total power r°ng,es from 32 Ml to 8,89 kW as theti^e at these power levels varies from 10 to 3600 s,respectively.

A substantial portion of the total energyreleased is associated with the solid, static fuelregion and, therefore, an important feature of theA buffer gas confined UFg flowing experiments is todetermine the distribution of power which.can bedeposited in the UFg gaseous fuel contained in thecanister. The variations of power depssited in theUFg is shown in Fig. 7 for total operating tiiiiesbetween 1 and 360 s, The manner in which the powerdeposited in UFg varies for a given total operatingtime at power is shown as the fission rate ratio ofUFg to total fission rate varies from 0,01 to 0.20.For example, at a fission rate ratio of 0.10, thepower deposited in the UFg ranges from 3.2 MJ to8.89 W for a maximum operating time at fullpower which varier from 1 t-> 360 s, respectively.As discussed previously, these ranges of power andtime are determined by a value for total number offissions per test equal to 1 x iO^aand the calecuiated ratio of fission power deDsity of UFg tosolid fuel of 2.3.

Based on the probable values of canisterdimensions, buffer gas"injection geometry,temperature, and vortex flow parameters, a nominaldesign value for total buffer gas weight flow of200 g/s was estimated for the confined UFg flowingreactor experiment. Also, based on isothermalfluid mechanics flow experiments^"), an A bjrpassratio of O.96 has been chosen which should providegood fuel mixture1 containment in the central regionof the vortex flow. This infers that O.96 of thetotal injected A flow will be removed from thecanister through a perforated section of canisterwall via the axial duct located behind this section.Tho remaining A flow is removed from the canisterwith the fuel mixture via the on-axis thru-flowducts. The average time the A resides in the j?r -ister between injection and removal is designatedas the A residence time and is estimated as, T ^ =Mft/W , where % is the mass of the A contained inthe canister and % is the total A weight flow.

Calculations were performed in which the fuelresidence time was equal r.o both one and five timesthe buffer gas residence tine. Fuel weight flow canthen be determined by the relationship betweenmass, residence time, and weight flow. As a resultof the variation in fuel weight flow, energy willbe removed by convection from the fuel adxtuie inthe canister at different rates. This effect hasbeen included in the results shown in Fig. 8, wherethe variation in power deposited in UFg, for totalpressures of 1, 4, and 10 atm and for fuel to

19X

buffer gas residence time ratios, TJ./TA, of 1 andZ, are shown as a function of time at power to reacha given bulk-average fuel mixture temperature. Forexample, when 60 W is deposited in the UFg, thereactor could operate at power for 30 and 3U s,respectively, \s? % total canister operatingpressure of k atm and fuel-to-buffer gas residencetime ratios of 1 and 5. The variation in therequired time at power to reach a given gasbbulk-averaged temperature is shown in Fig. 9 for totalpressures of 1, k, and 10 atm,,fuel-to-buffer gasresidence tine ratios of 1 and 5 andratios of 0.3 and O.k.

The parametric calculations described abovewere performed over the ranges of pressure,temperature, power, and gaseous residence time in tthe canister, for vortex containment geometry andconditions which are expected to exist in the UFgconfined experiments. A relationship exists betweenthe temperature and containment characteristics ofthe gas in the buffer-gas-confined fuel region.The analytical constraint assumed for containmentis to require that frcm the radial stagnation sur-face inward, the density at any location be equalto the density of the A buffer gas at the radialstagnation surface. With this constraint and fora given total canister gas pressure, there existsan upper limit on the amount of UFg (which has thehighest molecular weight) that can be mixed with Aand He at a given temperature. Thus, a furtherincrease in the amount of UFg confined in the fuelregion can occur if either the total canister gaspressure increases, or if energy is added to thefuel region gas such that the fuel region tempera-ture insreases while the total canister gaspressure and density in the fuel region remainfixed. In the latter instance, constant densityis at aed by replacing some light gas, He, by anequivalent amount of heavy gas, UFg.

Summary of Results

and 5 times the buffer gas residence time, respec-tively. A considerably higher total power would berequired to bring about a 500 to 1000 K increase inUFg gas temperature. The small amountsof UFg con-tained,* hO g, and short residence time of i;hegases, at a pressure of 1 atm, do not appear to beconducive to generating a hot UFg gas. The resultsin TABLK I, for which the total canister pressure isIt atm, are considerably mere interesting. Tempera-ture increases in the UFg gas of 100 to'approxi-mately 1000 K have been calculated for a total powerlevel between 100 kW and 1 JW, respectively. Animportant effect to be cognizant of is illustratedby comparing the results for a pressure of k atm asthe total power is increased. For equivalent gasresidence times, the amount of UFg which can becontained increases as the gas temperature increasesIn order to satisfy the conditions of constant totalpressure and a constant total density in the fuelregion equal to the A buffer gas density at atemperature of U00 K. Therefore, for an initialUFg contained mass, as the total power isincreased, the power deposited in the UFg increasesproportionately with an accompanying increase inUFg gas temperature. This allows an increase in thecontained UFg mass until a new steady-state level isreached.

References

1. Helmick, H. H., G. A. Jarvis, J. S. Kendall,and T. S. Latham: Preliminary Study of PlasmaNuclear Reactor Feasibility. Los AlamosScientific Laboratory Report LA-5679, PreparedUnder NASA Contract W-13721, August 197't.

2. Thorn, K., R. T. Schneider, and F. C. Schwenk:ihysics and Potentials of Fissioning Plasmasfor Space Power and Propulsion. InternationalAstronuatics Federation, XXVth Congress,Amsterdam, Holland, September 30 - October 5,

Based on the analytical results presented in 3.Figs, h through 9, it appears feasible to conductinitial argon buffer-gas confined flowing UFg testsusing a canister geometry similar to that used inthe static UFg testsW and-planned flowing'UFgtests. Specific operating characteristics for sixdifferent tests have been selected from theparametric data in Figs. 1* through 9 and are given k.in TABLE I. 'Operating characteristic? are shownfor power levels of 100 Hf and 1 MW, and for totalcavity pressures of 1 and h atm. This pressurerange was chosen for the calculations of the buffergas confined flowing .UFg tests because of theupper limit of h atm pressure for the aluminum 5.canisters designed for the static UFg tests andplacned flowing UFg tests. Higher performancebuffer gas confined flowing UFg tests, are plannedwith canisters designed to operate ai a pressure Inthe range of 10 atm. The results indicate that fora total pressure of 1 atm and total power of 100 itW, 6.small U?g gac temperature increases of 5 and 27 Kore expucted in which tlrt fuel residence time is 1

Latham, T. S., F. R. Biancardi, andR. J. Rodgers: Applications of Plasma CoreReactors to Terrestrial Energy Systems. AIAAPaper 71t-107>t, AIAA/SAE 16*,h PropulsionConference, San Diego, CA., October 21-23;

Bernard, ¥., H. H. Helmick, G. J. Jarvis,K. A. Flassmann, and R. H. White: ResearchProgram on Plasma Core Assembly. Los AlamosScientific Laboratory Report LA-5971jMS,May 1975.

Jaminet, J. F. and J. S. Kendall: InitialTest Results of Circulation System for PlannedFlowing Uranium Hexafluoride Cavity ReactorExperiments. United Technologies ResearchCenter Report R76-91213U-I, March 1976.

Orndoff, J. D. and H. C. Paxton: OperatingProcedures for the Pa;iarito Site CriticalAssembly Facility. Lcs Alamos Scientific

Laboratory Report LA VO37-SOP, Rev. 1973-

7. Rodgers, R. J., T,, S. Latham, and

J. F. Jaminet: Prelim!nary Design and Analysesof Planned Flowing Uranium Hexafluoride CavityReactor Experiments. United TechnologiesResearch Center Report R76-912137-1, May l°7o.

8. Jaminet, J. F. and A. E. Jfensing:Experimental Investigation of Simulated-FuelContainment in RF Heated and Unheated Two-Conponent Vortexes. United Aircraft ResearchLaboratories Report J-9IO9OO-2, Prepared UnderContract SNPC-7O, September 1970.

9. ffensing, A. E. and J. F. Jaminet:Experimental Investigations of Heavy-GasContainment in R-F Heated and Unheated Two-Component Vortexes. United Aircraft ResearchLaboratories Report H-910091-20, PreparedUnder Contract NASw-847, September 1969.

10. Ker.aall, J. S.: Experimental Investigation ofHeavy-Gas Containment in Constant-TemperatureRadial-Inflow Vortexes. United AircraftResearch Laboratories Report F-910091-15,September 1967. Also issued as HASA CR-1029.

11. Mensing, A. E. and J. S. Kendall: ExperimentalInvestigation of Containment of a Heavy Gas ina Jet-Driven Light-Gas Vortex. United AircraftResearch Laboratories Report D-91OO91-lt,Murch 1965. Also issued as NASA CR-68926.

12. Rodgers, R. J., T. S. Latham, and1J. L. Kraseella: Analyses of Low-Power andPlasma Core Cavity Reactor Experiments.United Technologies Research Center ReportR75-911908-1, Prepared Under ContractXP4-51A99-1, May 1975.

List of Symbols

UF5 fission rate/total fission rate,

dimensionless

Critical mass, Kg of U

Mass of confined UFg, g

Molecular weight of UFg/ g/moleWUF6

^ - 2 3 5 Molecular weight of U-235, g/mole

Ratio of volume-averaged fuel toperipheral wall pressure, dimensionless

PTOTTotal pressure^, atm

Total power, W

Power deposited in contained UFg, B 01 iB

Gas temperature, deg K

Temperature at edgcirof-fuel, deg K

Tims of full power, s

Time at power to reach temperature,

Creek Symbols

^ Argon residence time in canister, s

ij. Fuel residence time in canister, s

Power density ratio, dimensionless

TABLE I

CALCULATED OPERATING CHARACTERISTICS FOR ARGONBUFFER-GAS COHFUJED UFg CAVITY REACTOR

EXPERIMENTS

Total F i s s ions = 1 x IO 1 8

Power Density in UFg = 2 . 3 Power Densityi n Solid Fuel

Total Pressure ,atm 1 1 It If 1* U

Total Power,100 100 100 100 1000 1000

Power Depositedin UF6, Hf 1.3 1.3 It.8 5.2 56 65

Maes of UFgConfined, g 37 38 150 162 175 205

TemperatureIncrease inUFg, K 5 27 20 100 2lfO IOUO

Test Time, s 320 320 320 320 32 33?

Fuel MixtureResidence Time,s 0.3 1.5 1.2 6.0 1.2 6.0

Argon Residence

Tine, a 0.3 0.3 1.2 1.2 1,2 1.2

193

U fuel elements-

Gap for canisterInsulation

& gas piping —

Cylindrical model

Be end plug

Be end plug

KIWI-BBere-

flector

Phoebu>

flector

-Graphite

Fig. 1 Reflector-Moderator Configuration forLow-Power Critical Experiments.

GBufferrecirc.system

Corecanister

Fuelreproc.

Fig. 3 Preliminary Schematic of FuelConfinement Flow System.

TEOF 400 K

Radius of gas volume =16.92 cmLength of gas volume=94.0 cm

(a) AXIAL CROSS SECTION

Argonrbufferyegion

Thru-flow,port

Fuelinjectors

\ Argon bypass.removal duct ~

(b) CYLINDRICAL CROSS SECTION

Aluminumy-canister

jjf walls

A>goninjection

slot

Argonflow

directioi

Thru-flowport

rgon thermal'control annulus

Fuelinjectors

Aluminuicanister walls

Argon-injection slot

Argonbypass

removalducts

Fig. 2 CRnister Injection and Exhaust,

Configuration for Buffer pas Confined

UJg Flowing Experiments*

600• P F / P T O T = ° . 3

500 1CO0 1500 2000TCL-DEG K

Fig. h Variation of UIV Contained Mass WithGas Temperature.

= 4.37 Kg of U

Fig. 5 Variation of UFg Contained Mass WithFission Rate Ratio.

194

Total fissions-1 x10"I8 - 400K

101 102 103 10

W O T " 0 - 4 0

101 102 103 104

Fig. 6 Variation-of Total Operating Power WithMaximum Time of Operation Between 10 and3600 Seconds.

Fig. 8 Variation of Power Deposited inContained UFg With Time at Power toReach Temperature.

Total fissions = 1 x 10 1 8

MC =4.37 Kg of U

0.08 0.16FRUF6

io°i—r-

(0

O i n 2

0 800 1600 2400TCL - OEG K

Fig.'7 Variation of Power Depesited in UFgWith Fission Rate Ratio for OperatingTime Between 1 and 360 Seconds.

Fig. 9 Variation of Time at Power forContained UFg to Reach Temperature.

195

DISCUSSION

J. BLUE: If the object of this, experiment is toobser-re the uranium plasma with a fairly highfission density, another way to do this withouthaving the complications of criticality would be- to take the aluminum can and have it bombardedwith 100 microamperes of 500 MEV protons. Youcould achieve the same. kind, of fission productpower densities without the complications ofcriticality an! it would allow you to do someinstrumentation that you are not able to do in areactor environment'where you have to bringdiagnostics into a high fluK region or else youhave to take your instruments so far away that theinformation obtained is diluted in value.

R. J. RODGERS: What is the deposition profile ofthe proton beam in the gas? Does it reallysimulate energy deposition in the gas?

J. BLUE: Energy deposition in the gas is theresult of the protons causing fission, so when auranium fission, there is very little differencewhether it was fissioned by a neutron or a proton.It is true there'is some difference in the energydistribution. The protons themselves also pro-duce ianization of the gas. The ionizatio.i causedby the protons is very much like the ionization:aused by gamma rays* So it would be unreasonableto expect that the protons would do anything thatwould be drastically different. The advantage withproton beams is that you can-focus them onto theregion and you can make the density higher becauseyou make the volume smaller than the volume youare talking at t here.

T. S. LATHAM: We have done years of small-volumesimulation with our RF plasma experiments. Oneof the things we are striving to do now is toget up to Cannister sizes and flow volumes that aremore like full-scale devices,

H. H. HELMICK: The reason we are doing criticalityexperiments is that in looking at all the optionsat LASL, these are tie cheapest experiments. Tomount an experiment at LAWF is a very long leadtimejob that is pretty expensive because of the largenumber of people involved in that facility; we knowthat sooner or later we need to do the checkoutswe are talking about now, and since it is easy toconduct this with merely a four-man team, it is afactor of 20 cheaper than equivalent experimentaltime at LANF.

196

/Jt

CIRCULATIG3 SYSTEM FOR FLOWING URAHIUMHEXAFLUOKIDE CAVIfY REACTOR EXPERIMENTS*

J. P. Jaminet and J. S. KendallPower Systems Technology Section, Fluid Dynamics Laboratory

United Teciinologies Research CenterEast Hartford, Connecticut

Research related to determining thefeasibility of producing continuous power fromfissile fuel in the gaseous state is presented. Inaddition to high-thrust, high-specific-impulse spacepropulsion applications, development of gaseousnuclear reactors could open new options for meetingfuture energy needs. Accomplishment of the UFg.critical cavity expeslaents, currently in progress,and planned confined flowing .UFg initial experimentsrequires development of reliable techniques forhandling heated UFg throughout extended ranges oftemperature, pressure, and flow rate. The develop-ment of three laboratory-scale fle» systems forhandling gaseous UFg at temperatures up to 500 K,pressures up to approximately 40 atra, and continuousflow rates up to approximately 50 g/s is presented.

A UFg handling system fabricated for staticcritical tests currently being conducted at LosAlamos Scientific Laboratory (LASL) is described.The system -was designed to supply UFg to a double-walled aluminum core canister assembly at tempera-tures Between 300 K and UOO K and pressures up toh atm. A second UFg handling system designed toprovide a circulating flow of up to 50 g/s of gas-eous UFg in a closed-loop through a double-walledaluminum.core canister with controlled temperatureand pressure is described. This system will beused in planned cavity reactor experiments at LASLin which UFg will flow through an active cavityreactor core region. Data from flow tests usingUFg and UFg/He mixtures with this system at flowrates up to approximately 12 g/s and pressures upto k ata are presented. A third UFg handlingsystem fabricated to provide a continuous flow ofUFg at flow rates up to 5 g/s and at pressures upto toatm for use in rf-heated, uranium plasmaconfinement experiments is described.

Introduction

Research to investigate the possibility ofproducing continuous power from fissile fuel in thegaseous state is presently being conducted. Inaddition to high-thrust, high-specific-impulsespace propulsion applications, development ofgaseous fueled nuclear reactors could open newoptions for meeting future energy needs, such as:(1) magnetohydrodynamic electric generating powersystems; (2) photochemical or theroochemicaldissociation of water to produce hydrogen; and

(3) direct nuclear pumping of lasers by fissionfragment energy deposition in UFg and lasing gasmixtures.

Extraction of power from a fission process wi';hnuclear fuel in gaseous form allows operation atmuch higher temperatures than conventional nuclearreactors, leading to more efficient thermodynamiccycles. The continuous peprocessirg of gaseousnuclear fuel leads to a low steady-state fissionproduct inventory in the reactor and. limits thebuildup of long half-life transuranium elements.

A series of experiments utilizing a berylliumreflector-moderator cavity critical assembly hasbeen initiated at LASL to establish the feasibilityof fissioning UFg gas reactors and/or uraniumplasma reactors.° The cavity critical assembly,shown in thefphotograph in Fig. 1, has provisionfor raising or lowering a fueled core from thereflector and removing the core from beneath thereflector assembly. A core Canister assembly con-taining gaseous UFg is positioned within thereflector cavity. Cavity configurations ofsystematically increasing complexity will be usedto progress from simply distributed uranium foilexperiments to the use of flowing, vortex-confined,gaseous UFg fuel. Future experiments are beingplanned in which an approximately 5 JW self-criticalfissioning uranium plasma core reactor will beoperated.

Aceomplishmen. of the static UFg criticalcavity experiments, currently in progress, andplanned confined flowing UFg initia: experimentsrequires the development of reliable techni -'uesfor handling heated UFg throughout extended rangesof temperature, pressure, and flow rate. Thispaper describes the development of threelaboratory-scale flow systems for handling gaseousUFg at temperatures up to 500 K, pressures up toapproximately kO atm, and continuous flow rates upto approximately $0 g/s.

TIFg Handling System for

Static Critical Tests'

A UFg handling system was fabricated forstatic critical tests currently being conducted atLASl£. The major components of the statichandling system are the thermally controlled hicapacity UFg supply system, the 85 Z internalvolume double-walled aluminum core canister

•Research sponsored by Los Alamos Scientific Laboratory, Contracts XP^-5W>59-1 and

197

and the Ng flow system used for core canisterthermal control. The, aluminum core canister issimilar to that used in the flowing UFg handlingsystem and is described in detail in the followingsection.

The general arrangement of the static UFghandling system is shewn in the photograph in Fig.2. The system is mounted on a movable frame and isattached through stainless steel flexible hoses tothe double -vailed aluminum core canister assembly.This allows the core canister assembly to be raisedand lowered into position within the cavity criticalassembly. Materials in the handling system were'olimited to the use of Monel,' stainless steel, andaluminum due to the extreme chemical reactivity ofUFg., Gaseous UFg Is introduced into the canisterby control of the UFg supply oven temperature. TheUFg is removed from the cavity and returned to thesupply cylinders by cryopumping and desublimation. •Also shown in the photograph are connections tovacuum and eold-brap systems for purging and fillingof the UFg handling system.

The UFg supply system is designed to supplyUFg to the core canister at temperatures between300 K and U0O K and pressures up to h atm. Thesystem is shewn schematically in Fig. 3. Thissystem consists of four 1000 ml Mbnel cylindersconnected through a manifold to the UFg supply line

'• which is in turn directly connected to the aluminumcore canister. The cylinders are located within acirculating flow oven whose temperature can becontrolled within approximately -1 C. The ovenprovides the heat required to vaporize UFg withinthe supply cylinders and produce the desired testpressure. The supply system is double-valved toreduce the possibility of leakage of UFg into theoven environment. Each cylinder is equipped with amanual Monel diaphragn shutoff valve. In addition,the manifold connectirjg the four supply cylindersis equipped with a stainless steel remitelyactuated bellows valve. A copper coil is solderedto the bottom of each cylinder through which liquidKg may be passed to cryopump the UFg out of thecanister and into the cylinders.

Thermal control of the UFg supply in the corecanister and flexible ducts connecting the carecanister to the UFg supply is provided by tlowingheated Hg through the flexible duets and corecanister assembly. The Hg flow system is elsoshown in the schematic in Fig. 2. The major compo-nents of the N2 system are the bellows pump, Ngheater, pressure control system, and Ng flow meter.A bellows pump having a 2.U- i/s capacity isutilized to provide a flow of circulating Ngthrough the, system. The pressure within the %system is automatically adjusted, using a pressurecontrol system, to a value greater than the corecanister pressure. The N2 is heated by a 2500 Wresistance heater located downstream of the bellowspump in the flow system. Also included in the N2flow system are a flow meter, pressure gages,'control valves, and associated equipment utilized

in tests to determins the flow characteristics of' the Hg loop. Static pressure and vacuma leak testswere performed on the gas flow system upon comple-tion of fabrication. The UFg system was determinedto be helium leak tight over the range of pressuresfrom approximately 5 x 10"° atm to 10 atm.

A series of initial tests using this equipmentvas conducted to verify the operational character-istics of the system. Thermal response tests wereconducted to determine the rate of heatup and cool-down of each system component. This informationwas necessary for documentation of the transientthermal characteristics of the system. The resultsof the thermal response tests indicated that norapid rate-of-change temperature fluctuations arelikely to occur in the system diiring the staticcritical tests.

UFg Handling System for

Flowing Critical Tests

A UFg handling system was designed andfabricated to provide a circulating flow of up to50 g/s of gaseous UFg or a UFg-He mixture in aclosed-loop.with controlled temperature andpressure.3 The major components of the flowing UFgsystem are similar to the static UFg handling sys-tem except for the addition of a UFg pump, flowloop, and flow and pressure instrumentation.

The UFg and Ng circulation system includingthe core canister is completely enclosed within aLucite safety enclosure to prevent any possibilityof UFg leakage to the surrounding environment. A30,000 2/min airflow is continuously exhaustedfrom the safety enclosure .,0 a scrubber system. Inthe scrubber the airflow is continuously washed bya spray of water/sodium bicarbonate solution andthen passed through an absolute HEPA filter priorto venting to the atmosphere. The HEPA filterremoves > 99-999 percent of all particles greaterthan 0.3 M m <Ha.

The UFg circulation system shown in thephotographs in Figs, k and 5 is mounted on amovable frame and is attached through flexiblelines to the double-walled aluminum core canisterassembly. The circulation system equipment isshown in the photographs in Figs. 1* and 5 priorto installation of heating tapes and insulationused during test operation. Gaseous UFg is intro-duced into the circulation system by increasing theUFg supply oven temperature, thereby creating ahigh UFg vapor pressure. The UFg is removed fromthe circulation system by desubliming the UFg from

a UFg-He mixture which is continuously pumpedthrough one of the oven supply cylinders. The,circulation system equipment is designed foroperation at test pressure levels up to h atm andtemperatures up to 400 K.

Details of the UFg circulation system areshown in the schematic in Fig. 6. The circulationsystem equipment is comprised of two gas flow

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systems, the UFg supply and f.1 ow system and -the H2flow system. The UFg supply system consists of two1.0 liter volume UFg supply oylinders which areconnected through appropriate valving and piping tothe UFg flow, or circulation, system. One of thecylinders is a single-port Monel cylinder whichprovides a convenient location for additional UFg'storage. The other cylinder, called the transfercylinder, is a stainless steel cylinder designed toallow flow through the cylinder from the UFg circu-lation system. This design provides for removal ofthe UFg from a UFg-He mixture in the circulationsystem by continuous desubliming of the UFg as theUFg-He mixture flows through the cylinder. Thecylinder has two close-coupled valves allowing thecylinder to te removed from the oven for filling andweighing. Weighing of the cylinder before and aftera UFg transfer is the method utilized to determinethe exact amount of UFg which hi either transferredinto or removed from the circulation system. Thepresent accuracy of the weighing procedure isestimated to be ^0.5 g. >JFg is transferred intoboth cylinders by eryo-pumping and removed by heat-ing to produce a high UFg vapor pressure. A coppercoil is brazed to the bottom of each cylinderthrough which liquid H2 is passed to cryo-pump ordesublime the UFg into the cylinder. »During thecryo-pumping or desubliming process the upperhalves of the cylinders are heated to approximately350 K using the supply system oven to preventplugging of tfce cylinder ifilet.tubing with UFg. Avacuum connection is provided in the cylinder flowloop to provide for removal of any air that istrapped intthe lines when the transfer cylinder isreconnected into the loop.

The cylinders are located within an air circula-ting-flow oven whose temperature can be controlledwithin approximately ±1 K. The oven provides theheat required ~o vaporize UFg within the' cylinders.and produce a desired UFg vapor pressure. Theheated UFg fron the supply cylinders is used tocharge the complete circulation system includingthe double-walled core canister assembly.

The UFg flow system consists of a 2.h Z/sbellows pump, a Monel venturi and differentialpressure transducer assembly, and appropriatetubing, flexible hoses, and valving to allow cir-culation of UFg through the double-walled corecanister assembly. The bellows pump was modifiedfor operation at temperatures up to h^O K bythermally-isolating the motor from the pump bodyand replacing the crank bearings with special high-temperature versions. The pump is located in asealed enclosure which is pressurized and heated bythe N2 flow system. This configuration greatlyincreases the efficiency of the pump at high systemstatic pressures by eliminating the differentialstatic pressure across the bellows.

All components and tubing are wrapped withheater tapes and covered with at least 1.3 cmthickness of Carborundum "Fiberfirax" insulation tomaintain the UFg In. a superheated state by reducing

thermal losses. To prevent condensation on thesolid su: faces within the circulation system,temperatures in the circulation system are main-tained at least 25 K .greater than the saturationtemperatures corresponding to the system opeisatlagpressure. Thus, at a system pressure of i atm, thetemperatures are maintained at values of approxi-mately kOO K or higher.

Pressure measurements of the UFg circulationsystem were made using three special Monel 6.8 atmmaximum full-scale absolute pressure transducers.A Monel venturi was used to measure the UFg-Hemixture flow rate in the UFg circulation system. Aspecial wet/wet-type 0.3^ atm maximum differentialpressure transducer made of 316 stainless, steel wasusdd to measure the differential pressure across theventuri assembly.

Thermal control of the UFg in the corecanister assembler and flexible ducts is provided byflowing heated Wg through the annular regions in thecore canister assembly and the coaxial flexiblemetal ducts (see Fig. 6). The heated H2 flow sys-tem consists of two 14,000 rpm centrifugal blowersin series, a 750 W resistance heater, and a heatertemperature controller. The two 1^0 elm maximumcentrifugal blowers for circulating the heated Kgwere modified for operation at temperatures up to1+50 K and pressures up to 5 atm by substitutingviton gasket material and fabricating metal casesto replace the original plastic cases. The Ng flowrate is maintained constant and is monitored bymeasurement of the pressure difference across theblowers. In initial tests of the 1^ flow systeo ata system pressure of 1 atm, tests with two differentflowmeters resulted in aa Ng flow rate of approxi-mately 225 slm with a total differential pressureacross both centrifugal blowers of 0.22 atm. Allsubsequent tests were conducted without a flowmeterin the N2 system to minimize thermal losses andwith the II2 system differential pressure across the*blowers equal to approximately 0.22 atm.

The Kg flow system is designed to be main-tained at a pressure slightly higher than the UFg .system pressure during UFg flow tests. This servesto eliminate any possibility of UFg leakage intothe H2 flow loop which would increase the diffieeulty of recovering the UFg. Any UFg remaining inthe IPg circulation system or that has escaped intothe N 2 flow system can be removed by using the N 2

purge and vacuum system shown in1Fig. 6. Thevacuum system flow is first passed through a coldtrap maintained at liquid Kg saturation temperature,78 K. The flow then passes through a HaF chemicaltrap before being exhausted from the vacuum pumpto the scrubber system.

Core Canister Assembly

The core canister assembly shown in detail inFig. 7 is nadeup of two concentric 6061-0 aluminumcanisters. Alloy 6061 was selected as the primarycanister material due to its low thermal neutron

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cross section, strength properties, and weldability.The physical dimensions were chosen such that theapproximate 85 i volume of UFg contained within theinner canister will occupy approximately UO percentof the volume enclosed by the reflector-moderatorconfiguration. The core cnniPtcr and flexible metalhose assembly is designed to be raised and loweredinto and out of Its operating position within thecavity critical assembly. The 3^-00-01. inner can-ister has provision for positioning fission wiresat two locations and also has provision for Install-ation of a 13-cm-dia fused-silica window at the topof the canister for transmission of possible fissionfragment-lnducedopptlcal radiation emission whichoay be produced during the cavity experiments.During the tests described herein, an aluminum platewith provision for pressure and temperature sensorswas installed in lieu of the fused-silica window.The 37-ea-OD outer canister is designed to enelotan tmnular flow regioa for thermal control of theInner canister. During flow tests'the inner canis-ter contains flowing UFg at a preselected pressureand temperature. The heated UF5 flows to and fromthe core canister via heated flexible ducts. Controlof the UFg temperature in the core cauister assemblyand coaxial flexible ducts is provided by flowingheated nitrogen to the annulus in the core canisterand coaxlnl flexible ducts. The canister designfor the present flowing UF6 cavity reactor experi-ments is similar to that fabricated for the staticcritical experiments. However, several changeshave been made in the flowing UFg canister design.These include provision for the continuous flow ofUFg into acd out of the canister, the addition oflarger diameter tubing for the Inlet and outletflow, and a larger diameter (13 cm) viewport at thetop of the canister.

: Final assembly leak tests were performed on thecore canister assembly both during and upon comple-tion of fabrication. The inner •. Ister was evac-uated to approximately 10*5 Torr and leak testedusing a helium leak detector. After completion ofassembly of the outer canister around the innercanister, the outer canister was also evacuated toapproximately 10"5 Torr and leak tested using ahelium leak detector. The helium leak detector wassensitive to a leak rate of approximately 3.5 x10 "^ std. cc/s. Static pressure tests up to 6 atmwere also conducted on the complete core canister• assembly. No leakage was noted in any part of theoanister core assembly during any of these tests.

Results of USg Circulation System Tests

Initial UFg circulation system tests consistedof vacuum leak tests in which the complete UFg sys-tem was sealed-off and the rate of pressure risemonitored. This test resulteC in a pressure rete-of-rise of approximately 3.7/xm/hr from a hasepressure of 10"5 Torr over a 72 hour time period.Initial UFg circulation system flow tests consistedof flow measurements with 1 atm pressure of He inthe system. . The three absolute pressure transrduoers were monitored and indicated pressure pulsesof less than 1 percent of full-scale at the outletof the bellows pump.' However, considerable

pulsation at a frequency of 28.2 Hz was noted inthe output of the differential pressure transduce*at the venturi. These pressure pulsations originnated from the bellows pump.

The output of the k pressure transducer'' wasmeasured using a Honeywell Visicoraer. All h trans-ducers were calibrated in-situ in preliminary testsusing a Wallace and Tiernan O76.8 atm maximum full-scale pressure gage connected to the flow system.During initial operation of the flow system, theproper heater control settings to provide approxi-mately uniform temperature around the flow systemwere determined. The flow system was operated atteiaper/itures up to approximate^ 36O K and the UF£supply >_ven up to ^50 K. . Using the inlet and cnt-let temperatures at the core canister, it was cal-culated that the totsl thermal loss to the atmo-

• ,,, iIom t h e c o r e canister was approximately- l8.1

„ „ tar fuel operating temperature of 338 K.

The UFg circulation system was operated asfollows. Beginning with the system initiallybilled with dry He at 1 atm, the H2 flow system wasfilled to 1 ato and all heaters, as well as the UFgbellowE pump ei.d Hg blowers, were turned on. Theheater controllers were u.Tjally set higher than thenormal operating setting initially and graduallyreduced during the system heutup to shoi-ten theheatup tims. The complete flow loop reached normaloperating temperatures in less than 1 hoar.

The initial loading of UFg into the circula-tion system was accomplished as follows. The en.11-plote UFg circulation system, with dry He at 1 atm,and the H2 flow system were brought up to thedesired operating temperature. The bellows pumpwas shutoff and the heated He evacuated from theUFg circulation system. UFg from the supp?.y ovenat a temperature of approximately 1*25 K (greeterthan 10 atm vapor pressure) was then piped to theUFg circulation system. A transfer of '' of UFgto the circulation system was then aci .shed.For the measured circulation system < . of 90-05/this quantity of UFg results in a s pressureof approximately O.kx atm at a temperature of 3^0 K.

Following the introduction of the UFg into thecirculation system, flow tests were conductedusing first pure UFg and subsequently various UFg-He mixture ratios. Additional He was then added tobring the circulation system to a preselected totalpressure. He was used in the circulation system totailor the LVg-He Mixture density to correspond tothat of pure UFg at the higher temperature whichwill exist in the planned critical fissioning UFgcavity experiments. This provides data enablingbetter simulation of the core canister flow condi-tions expected in fugure tests.

The results of initial tests with He only,UFg only, and UFg-He mixtures are shown in Fig. 8.wherein the fuel mass flow rate is plotted versusthe total pressure in the circulation system andcore canister. The three flow loop as composi-

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tions are delineated as indicated by the threetypes of symbols. The variation In mass flow rateat a given total pressure for the He and UFg-Hedata was obtained by throttling the circulationsystem flow using valve No. 2 (see Fig. 6) duringdynamic tests.

As would be expected, the mass flow rate'. j increases with increasing gas density, or systemtotal pressure and with increasing gas molecularweight. The'maxitaum mass flow rate obtained was12.5 g/s at 3.55 atm pressure. Future tests areplanned in which much greater loadings of UFg Inthe circulation system at temperatures up toapproximately 1+50 K will be conducted. It should.then be possible to obtain the approximate 50 g/s]design mass flow rate of the system.

Dynamic simulation pressure tests of thecirculation system were conducted in which theinlet and outlet valves (Nos. 2 and 3 in Fig .6)were throttled or shut off completely for shortperiods of time with the bellows pump operating.In all tests, the pressure changes were slow andeasily controlled. This Is to be >acpected sincethe core canister volume (851 ) Is large comparedto the capacity of the bellows pump (Z.k i/s).

A spec1 -1 UFg transfer test using the transferand recovery sy3tem in the s'.pply oven was conduc-ted. Using a transfer bottle containing $89 g ofUFg in the supply overi, 25 g of UFg was transferredto the evacuated and heated circulation system.Helium was then added to bring the circulation sys-tem up to 1 atm total static pressure. Recovery ofthe UFg-He mixture was then accomplished as follows.Referring to Fig. 6, Valve 1 , the air-operatedvalve, and the four hand valves in the transferbottle loop in the sapply oven were opened. Valve2 in the circulation system was throttled almostcompletely closed allowing only a slight bypassflow in the main UFg circulation system. Thesupply oven and, therefore, the upper half of thetransfer bottle was heated to approximately S50 K.Liquid K2 was simultaneously passed through thetransfer bottle cooling coils thus chilling thelower half of the bottle. After approximately onehour of operation, measurements indicated that 20±0.5 g of the 25 *0.5 g of UFg originally trans-ferred into the circulation system were recovered.This was accomplished using the original transferbottle in the supply oven which therefore stillcontained 76!+ g of UFg at the start of the recoveryoperation. Evidence from this test indicates thistransfer and recovery system will be suitable foruse in flowing UFg critical experiments.

UFg Handling System for

RF Plasma Confinement Tests

A high pressure UFg supply system for rfplasma confinement tests vas fabricated to supplyUFg to testRohambers operating at pressures up to20 atn anfl to provide UFg at mass flow rates up to5 g/s. The principal components of the system are

the UFg boiler, the boiler heat supply system, andthe UFg condenser system. A schematic diagram ofthe flowing UFg system is shown in Fig. °.

The UFg boiler was fabricated by electron beanwelding two 200-atm-rated working pressure, Itcapacity, Honel gas sampling cylinders to form asingle 2 1 capacity cylinder. A aultiturn coilheat exchanger, fabricated from 6.1>-am-dia Moneltubing, was installed in the bottom of the UFgboiler. Thermocouple wells which protrude intothe gaseous or liquid UFg are installed at the topand bottom of the boiler.

Two 1750 W semlcylindrieal electrical heaters,which surround the boiler, were used to bring theboiler to desired equilibrium temperature andpressure prior to flowing UFg from the boiler.During flow conditions, gaseous !^ heated byelectrical heaters may be supplied to the heatexchanger In the UFg boiler. The Jig flow rate andelectrical r-wer may be varied to control theamount of heat supplied to the heat exchanger inthe UFg and, thus, to control the rate of evapor-ation of UFg in the boiler. During period when noUFg is flowing, the hot N2 can be bypassed aroundthe boiler.

During operation of the system, the gaseousUFg flows from the boiler, through a meteringvalve and a i-fetheson linear u ass flow meter priorto entering the test chamber. The gaseous UFg iscollected in the UFg condenser systeti locateddownstream of the simulated experiment chamber.The condenser consists of two 500 ml capacity 120atm stainless steel cylinders immersed in a bathof liquid N2. A 6.1 £/s vacuum pump is used toevacuate the system before UFg flow tests areVnitiated. During tests the vacuum pump wasisolated from the flow system by a welded bellowsvalve. After completion of a test the trappedUFg was heated and transferred to a NaOH trap forremoval of UFg by chemical reaction as NagUO^ or

A number of initial tests were made of thehigh pressure UFg flow system. All UFg flow lines,valves, and the flow meter were netted by means ofelectric heating tape. Recorders were used tomonitor thermocouples, pressure transducers, andflow meter output. At a UFg boiler temperatureof 500 K and a pressure of approximately UO atm amaximum UFg flow rate of 5.6 g/s was achieved.

. The high pressure UFg handling system was usedextensively in rf-plasma confinement tests tosimulate flow and thermal conditions expected ininitial uranium plasma core reactor experiments.'During these tests the system operated reliablyand satisfactorily met the flow and pressurerequirements of the plasma test program. Theoperating experience gained In the tests with allthree UFg handling systems verified the importance'of maintaining uniform system temperature, minimumwater vapor content, and system cleanliness toinsure system performance i^A reliability.

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Future work on UFg handling systems involvesthe development of UFg flow systems for cavitycritical experiments in which UFg is continuouslyinjected and confined in an argon vortex flow inthe cere canister. Equipment for continuouslyseparating the UFg from the argon and reprocessingthe UFg for reuse is presently being designed.^

References

1. Helmick, H. H., G. A. Jarvis, J. S. Ksndall,fand ". S. Latham: Preliminary Study of PlasmaNuclear Reactor Feasibility. .Lc AlamosScientific Laboratory Report LA -J79, August197k.

2. Kendall, J. S. and B. C. Stoeffler: Develop-ment of Equipment for Handling UraniumHexafluoride in Cavity Reactor and PlasmaExperiments. United Technologies ResearchCenter Report R75-9U9O9-1. M*y 1975.

3. Jaminet, J. F. and J. S. Kendall: InitialTest Results of Circulation System for PlannedFlowing Uranium Hexafluoride Cavity ReactorExperiments. United Technologies ResearchCenter Report K76-91213U-1, March 1976.

h. Roman, W. C : Laboratory-Scale Uranium RFPlasma Confinement Experiments. UnitedTechnologies Research Center ReportR76-912205, May 1976.

5. Bodgers, S, J., T. S. Latham, and J. F.Jaminet: Preliminary Design and Analyses of 'Planned Flowing Uranium Hexafluoride CavityEeactor Experiments. United TechnologiesResearch Center Report R76-§12137-1,March 1976.

Fig. 2 Photograph of UFg Handling Equipment forStatic Testa.

© - TEMPER ATUSJ MEASUREMENT

(p) - I-BESSURE MEASUREMENT

ASSEMBLY"^,!

Fig. " Schematic of UFg Handling Equipment forStatic Tests.

HEATED N 2 j ) F F L O W L Q O p . .HjFLOWLOOP-

Fig. 1 LASL Critical Cavity Assembly.Fig. h Photograph of UPg and K2 Flow Loops.

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ALUMINUMCORE CANISTER

Fig. 5 Photograph of Core Canister for UFgFlow Tests.

J) SOLENOID OR AIR OPERATED VALVES

J HAND VALVES

«l£5tUHE MEASUREMENT

TEUfERATURE MEASUREMENT (GAS)

^TEMPERATURE MEASUREMENT (WALL!

Fig. 6 Schematic of UFg Handling Equipmentfor Flow Tests.

j RETURNLOWrtS

INrdULUS

Fig. 7 Sketch of Core Canister Assembly.

SYMBOL

O•A

FUEL COMPOSITION

VF6 • He

He ONLY

UF6 ONLV

5

£

10.0

s

2

1.0

FUEL TEMPERATURE RANGE: 340 K-360 K

. A O

k

Lm_m I

oo

n

o

Dft

oo

Q

•

oo

*°o

a

8 d P

a c

a D

•

TOTAL PRESSURE, PT-ATM

Fig. 8 Results of Initial UI> Flow Tests.

DESIGN tLOWRATE - 6 oh

CONTROL VALVE

©TEMPERATURE 'MEASUREMENT

© PRESSUREMEASUREMENT

Fig. 9 Schematic of UFg Handling.Equipmentfor RP Plasma Confinement Tests.

203

DISCUSSIOK

T. C. MAGUIRE: Do you have- to worry aboutdecomposition products Interacting with th<?aluminum?

T. S. LATHAM: The answer to the question is "no.The UFg does not react with water vapor that maybe trapped or other impurities. That is whatcauses the deposition on tt.e materials.

204

Measurements of Uranium M»ss Confined In High Density Plasma*

R. C. StoefflerPower Sy**ems Technology Section, Fluid Dynamics Laboratory

United Technologies Research CenterEast Hartford, Connecticut

Abstract

An x-ray absorption method for measuring theamount of uranium confined, in high-density, rf -heated uranium plasmas is described. Preliminarytests were conducted to calibrate the measurementsystem using argon, uranium hexafluoride (UFg), andmixtures of argon and tteg at room temperature. Acomparison of measured absorption of 8 keV x-rayswith absorption calculated using Beer's Lawindicated the method could be used to measureuranium densities from 3 x l6'-6 atoms/cm3 to3 x 101^ atoms/em3.

Tests were conducted to measure the density of' uranium in an rf-heated argon plasma yith UFg in-jection and with the power to maintain the dis-charge supplied by the United Technologies ResearchCenter (OTRC) 1.2-JB rf induction heater facility.The uranium density was treasured as the UFg flowrate through the test chamber was varied. A maxi-mum uranium density of 3.85 x 101? atoms/cm3 wasmeasured. •.

The aystem consisted of an x-ray generator,the test chamber for containing the test gases, andthe.x-ray detection components. Eight Kev x-raysemijtted from the target of a CA 8-S/copper x-raydiffraction tube were collimated with a 0.025-nm-wide slit and reflected at the Bragg angle from anagnesium oxide crystal monoctarooator beforepassing through the test chamber. The test chamberconsisted of a 57-mm-ID fused-silica tube withmetal endwalls having 0.025-mm-thiek berylliumwindows. The x-rays were detected using a Norelcoscintillation detector. .A Tennelec TC 216 linearamplifier and single-channel analyzer was used toprocess the output signal from the scintillationdetector and to provide an input signal into aTennelec 5^5 counter-timer and a TC 590 ratemeter.

I. Introduction

Research to investigate ihe prospects forproducing nuclear power from fissile fuel in thegaseous state has been conducted for several years.Most of this work was concentrated on the gaseousnuclear reactor technology required for high-per-formance space ;ropulsion systems. However, inaddition to high-thrust, high-specific-impulsespace propulsion applications, gaseous fuelednuclear reactors offer several new options formeeting future energy need*.

Extraction of power from the fission processwith nuclear fuel in gaseous form allows operationaVmnch higher temperatures than conventionalnuclear reactors. Higher operating temperature, in'general, leads to more efficient thermodynamiccycles and, in the case of fissioning uraniumplasma core reactors, results in applications usingdirect coupling of energy in the form of electro-magnetic radiation. The continuous reprocessing ofgaseous nuclear fuel leads to a low steady-statefission product inventory in the reactor and limitsthe buildup of long half-life transuranium elements.

Cavity reactor experiments and theoreticalanalyses perforatd over the past fifteen years havedemonstrated that available analytical techniquesfor calculating cavity reactor nuclear characterls-tics are reasonably accurate1. These studies haveprovided a basis for selecting additional experi-ments to demonstrate the feasibility of plasma corenuclear reactors.

The x-ray absorption method described in thispaper was developed to provide a method for messsuring uranium density in two-component UFg con-finement experiments which will be accomplishedprior to the planned future cavity reactor experi-ments. The x-ray method can be used to measureuranium densities of 101(> atoms/cm3 to lO1? atoms/cm3 expected to be present in these experiments.Present optical methods, are not veil suited formeasurement of uranium densities greater thanapproximately 10** atoms/cm^ in the experimentalconfiguration used in the two-component tests.

II. System for X-Ray Absorption Measurements •

The system used to make UFg and argon x-rayabsorption measurements during calibration and rf-heated uranium plasma tests is shown schematicallyin Figure 1. 'the system consists of the x-raygenerator, the test chamber for containing the testgases (argon, UFg and argon-UFg mixtures), and thex-ray detection components. The x-ray generatorconsists of a Diano Corporation CA 8-S/copper x-raydiffraction tube with an XRD-6 x-ray power supply -and controller supplied by single phase 220 V/UO A.service through a 7.5 kVA isolation transformer. ;The emitted x-rays Irom the target pass through aeollimator with a vertically oriented O.O25-an-wide slit and are reflected at the Bragg angle, for8 keV (A =» 0.15^ run) x-ray« from a magnesium oxidecrystal monochromator before passing through thetest chamber. The test chamber, consists of a

•Research sponsored by NASA Langley Research Center Uhder Contract NAJI-13291, Hod. 2.

205

f>7-mm-ID fused-silica tube with metal endwalls. Theendwalls contain ports for supplying and exhaustingtest gases'and x-ray viewing ports having windowswhich appear almost totally transparent to 8 keVx-rays. The distance between the windows is 22.4cm. The x-rays were detected using a Norelcoscintillation detector (photomultiplier tube with athallium-activateu sodium iodide crystal mounted ina lighttight enclosure at the window end) with ahigh voltage 0.9 kV regulated dc power supply. ATennelec TC 216 linear amplifier and single-channelanalyzer is used to process the output signal fromthe scintillation detector and to provide an inputsignal into a Tennelec TC 5^5 counter-timer and aTC 591-" ratemeter. The. ratemeter output signal wasrecorded using strip chart recorders. Thescintillation detector output was also displayedusing an oscilloscope. The ratemeter and oscillo-scope signals were used as visual aids duringalignment of tha components of the x-ray measure-ment system.

III. Description of Chamber-Crystal-Slit Systemand Procedure Used in Calibration Tests

Details of the test chamber and the chamber-crystal -slit arrangement used during calibrationtests are shown in Figure 2a and the photograph inFigure 3. The- test chamber consists of a 57-mm-IDfused-silica tube and two copper endvells. Thex-ray beam from the x-ray coiilimatbr (having a0.025-mm-wicle vertically-oriented slit) was reflec-ted at the Bragg angle toe 8 keV x-rays parallelto the chamber axis through 6.35-mm-dia portslocated in each endwall. The ports were sealedusing 0.125-mm-thiek nylar windows. X-rays trans-mitted through the detector viewing port aredetected, through a 0.635-mm-wide slit (verticallyoriented) using the scintillation detector. Duringthese tests, the crystal was located approximately3.8 cm from both vhe exit of the x-ray collimatorand the source viewing port window and, the- 0.634-mm-wide slit was located approximately Z.^k cm and12.7 cm from the detector viewing port window anddetector, respectively (as measured along the beampath).

Calibration tests were made, using this testchamber-crystal-slit system, with pure argon, pureUFg, and argon-UFg mixtures at 300 K in the testchamber. During tests with pure argon or pure UFg>x-ray absorption was determined by comparing thex-ray intensity, Io, obtained with the chamberevacuated to the.x-ray intensity, I, obtained vl-ththe chamber at known pressures of argon or UFg.' •Absorption measurements were made for argonpressures from 5 mm Hg (1.7 x 101? atoms/cm3) to700 mm Hg (2.4 x io!9 atoms/cm^) and UFg pressuresfrom 5 mm Hg (1.7 x lO1^ atoms/cm3) to 86 mm Hg(2.9 x 101" atoms/crn^). The following procedurewas used in calibration tests with UFg-argon mix-tures. Io was measured with the chamber evacuated.The chamber was filled with UFg to a knownpressure (UFg pressure ranged from 5 mm Hg to 52.5mm Hg). Argon'was added to the chamber at partial

pressuressin steps up to 700 mm Hg. At each stepthe intensity, I, was measured. The absorption byUFg was determined by comparing I for each UF6 andargon partial pressure with the intensity, Io,.multiplied by l/l0 for the partial pressure ofargon as calculated from Beer's law.

IV. Description of Chamber-Crystal-Slit Systemand Procedure Used in RF-Heated Plasma Tests

Details of the test chamber and chamber-crystal-slit arrangement used during the rf-heatedplasma tests are shown th Figure 2b and the photo-graph in Figure k. The rf-heated plasma test con-figuration differs slightly from that used duringthe calibration tests. The system is arranged sothat the x-ray beam traverses the plasma coreregion diagonally (path length equal to 23.0 cm)passing tnrough the chamber axis where the maximumuranium density is expected to occur. Accordingly,6.35-mm-wide by 19-mm-long x-ray viewing ports arelocated in the brass endwalls, as shown in Figure2b, to allow passage of the x-ray beam. Duringthese tests 0.025-mm-thiek beryllium windows wereused to seal the viewing ports. X-rays transmittedthrough the detector viewing port are detected bythe scintillation u 'tector through a 0.635-mm-videvertically oriented slit mounted on the face- of thedetector. The crystal was located approximately12.7 en and h.k cm from the exit of the x-raycollimator (with O.O25-mm-wide vertically orientedslit) and source viewing port window, respectively.The 0.635-mn-«ide slit mounted on the face of thedetector was located approximately 4.6 cm from thedetector viewing port.

This test chamber-crystal-slit system wascalibrated using pure argon and the proceduredescribed previously. Argon absorption measure-ments were made for argon pressures from 50 mm Hg(1.7 x IO18 atoms/cm3) to 1790 mm Hg (5.8 x 1020atoms/cm3).

The following procedure was used to determineuranium density from x-ray absorption measurementsin an rf-heated argon plasma with UFg injection.The x-ray intensity, Io, was measured with the testchamber evacuated. Argon was injected tangentially(through vortex injectors shown in Figure 2bl) intothe test chamber and exhausts to vacuum through anaxial thru-flow exhaust and axial bypass exhaustduets. The chamber pressure was maintained at 10mm Hg until the rf discharge was initiated and thenincreased to a pressure sufficient for operationwith UFg injection, approxinately 1275 mm Hg (powerto maintain the discharge was supplied by the OTEC1.2-Mtf rf induction heater facility). The varia-tion of x-ray intensity, 1^, with amount of rfpower supplied to the rf-heated argon plasma wasmeasured (rf power was varied from 30 kW to 56 kW).UFg was injected axially into the test chamber andthe variation of x-ray intensity, I. with UFg flowrate vas measured (UFg flow rate was varied from0.013 g/s to 0.134 g/s in a series of thirteentests). UFg density was determined by computing

206

the ratio l / lA = (l/lo)/(lft/lo) for comparable itpower levels) and using Beer's law, l / l o = e ^ i 1 1

where /i is the absorption cross section for uraniumin cm2/atom, ( M = 1.12 x 10^-9 cm?/atom for uraniumand 8.0 keV x-rays), % is the number density inatom/cm3, and L is the path length in cm.

V. Results From Calibration TestsWith Pore Argon

Data obtained using the calibration system andthe rf-heated plasma system and pure argon at 300 Kare presented in Figure 5. The measured variationof fraction of x-rays transmitted, I/Io, with varietation in argon pressure, PA, is coppared with thetheoretical variation calculated using Beer's lawand the perfect gas law. Data obtained using thfcalibration system are given by the circularsymbols and data obtained using the rf-heatedplasma system are given by the triangular symbols.Agreement between experiment and theory is good forintensity ratios up to approximately 10~2 (argnpressures up to approximately 900 mm Hg). Atpressures greater than 900 mm Hg the argon appears(except for one data point) less dense than thatpredicted using Beer's law. It is possible thatthe x-zuy beam is not monochromatic aijd that atlarge values of absorption, l/lo less than 10~

2,the intensity of high-energy components (possiblyharmonics) in the beam becomes important relativeto the intensity qf tne primary components(absorption decreases with increasing x-ray energy)making the measured argon density appear less thanthat given by theory. It would be possible tooperate at argon pressures greater than 900 mm Hgand intensity ratios greater than 10~2 by using anx-ray tube having a target such as molybdenum whichemits x-rays having energies (17.8 keV) greaterthan those emitted from a copper target x-ray tube.However, measurement of uranium densities nearlO1^ atoms/cin3 would be more difficult (theoreticall/l0 = 0.992 for 17.3 keV x-rays compared withO.971* for 8 keV xrrays).

VI. Results From Calibration Tests• With Pure UFg

Data obtained using the calibration system andpure UFg at 300 K are presented in Figure 6. Themeasured variation of the fraction of x-raystransmitted, i/lg, with UFg pressure, Puyg, iscompared with theory.

During these tests it was necessary toreplenish the UFg supply system with UFg. Dataobtained "before the'UFg was replenished is in goodagreement with theory. Curing initial tests con-ducted p.fter the UFg was replenished, the systemwas contaminated (p.pparertly with HF since thequartz test chamber and pressure gauge glass wereetched) and apparent uranium and fluorine, com-pounds were deposited in the system (white depositson inside of quartz tube are evident in Figure 3).During subsequent calibration' tests it appeared"that the injected UFg reacted with the contaminants

in the system resulting in a reduction of UFgdensity with time and, therefore, an increase inx-ray intensity with time. The data obtained duringthese tests have been corrected for the timewieedrift in x-ray intensity. These data are also ingood agreement with theory; however, the discrep-ancies between experiment and theory are greaterthan those which occurred before the system wascontaminated. In general, the(agreement betweendata a2!d theory is good. However, for intensityratios, l/lo, greater than approximately 10"

2 (UFgpressures greater than apprjximately 60 mm Hg) theUFg (as did argon) appears less dense than thatpredicted using Beer's law.

VII. Results From Calibration TestsWith Mixtures of Argon and UFg

Data obtained using the calibration system anclmixtures of argon and UFg are presented in Figure7. Data obtained before and after apparent con-tamination of the system (open and solid symbols,respectively) are presented for UFg partialpressures from 5 mm Hg up to 52.5 mm Hg and argonpartial pressures from 0 to 700 ram Hg. In general,the data indicate fair agreement with theoreticalintensity ratios. The date also indicate that themeasured density of UFg decreases with increasing .argon pressure and that this difference increaseswith increasing UFg pressure. _ This behavior wouldbe expected, based on results obtained for pureargon and pure UFg, when the partial pressures ofargon and UFg are such that the total fraction ofx-rays transmitted is lees than about 10 . Thedashed curve through the data in Figure 7corresponds to a total fraction of x—ays trans-mitted equal to 10"2. For data located to tb?right of the dashed curve the total fraction trans-mitted is less than 10"2 and to the left greaterthan 10~2.

VIII. Results From TestsWith RF-Heated Uranium Plasmas

X-ray absorption measurements were made in aseries of thirteen tests to determine uraniumdensity in rf-heated uranium plasmas. During thesetests 3-2 g/s of argon were injected tangentiallyaround the UFg which was injected axially at flowrates between 0.013 g/s end 0.131* g/s. Thechamber pressure was controlled to be 1275 mm Hgand the rf~power in the plasma increased from 38kW at a UFg flow rate of 0.013 b/s to 75 kW at aflow rate of 0.131* g/s. The flow was exhaustedthrough the thru-flow and axial bypass exhausts.

In some tests the rf-power deposited in t .plasma with UFg injection was greater than thatwhich could be maintained with pure argon bufferinjection. To determine uranium density by com-paring x-ray intensities obtained with and withoutUFg injection, it was necessary to isolate effectsof changes in rf-power level. The variation ofintensity ratio, I /l-,, for pure argon, with

207

amount of rf-power deposited in the plasma vasmeasured and is shown in Figure 8 (circularsymbols), A straight line approximation is used torepresent and extrapolate the data to rf-power. levels which occurred during operation with UFginjection. The measured variation of intensityratio, l/l0, for argon-UFg mixtures with rf poweris also shown in Figure 28 (square symbols with UFgflow rates in g/s given next to the symbols).

The fraction of x-rays transmitted through theUFg, l A A > vas determined using the measured frac-tion transndtted through the argon-UFg mixture,l/lOJ and the fraction transmitted through argonI A A O obtained from the straight line approximation

• shown in Figure 8, at equivalent rf-power levels.

The intensity ratio l/lo was used with Beer'slaw for uranium to calculate the uranium numberdensity. The variation of uranium number densitywith .UFg flow rate is shown in Figure 9. Thevariation is presented for assumed UFg path lengthsof 23.0 cm and 9.5 cm (circular and square symbols,respectively; with rf-power levels noted next to thesymbols). The data indicate that the uraniumnumber density increases with increases in UFg flowrate and that the rate of increase decreases forUFg flow rates greater than approximately 0.08 g/s.Maximum uranium densities of 1.65 x 10^-7 atoms/cm^(path length = 23.0 cm) and 3.85 x 101? atoms/cm(path length ='9.5 cm) were measured for a UFg flowrate equal to 0.093 g/s.

In some tests at UFg flow rates greater thanapproximately 0.093 g/s tee measured x-rayintensity decreased with time (for fixed test con-ditions ) indicating that UFg or uranium compoundswere being deposited on the windows. Furtherevidence that UFg or uranium compounds had beendeposited on the windows was obtained in subsequenttests with pure argon after the UFg injection wasstopped. X-ray intensity ratios were significantlyless than those attained witjt pure argon prior toUFg injection. The data presented ia Figure 8 wereobtained from tests in which the x~ray intensityobtained with UFg injection did not decrease withtime and in whi&i intensities obtained with pureargon subsequent to tests with UFg injection wereapproximately the same as intensities obtainedprior to UFg injection.

The results presented here indicate that thisx-ray absorption technique can be used to supple-ment the optical methods for determining the amountand distribution of uranium vapor confined in theuranium plasma for rf tests in which the atomdensity of confined uranium might be expected toexceed levels greater than approximately lO1^atoms/cm3.

DC. References

1. Helmick, H. H., G. A. Jarvis, J. S. Kendall,ana T. S. Latham: Preliminary Study ofPlasna Nuclear Reactor Feasibility. Los

Alamos Scientific Laboratory Eeport LA-5g79,August 197'+.

Roman, W. C : Laboratory-Scale Cranium RFPlasma Confinement Experiments. UnitedTechnologies Research Center Report S76-912205,May 1976.

To argon andUFg supply

MgOcrystal

VacuumDC supply

ColNmator L-B keV x-rays$f GWB.154nm)

-X-ray tube

I- ecordei

Ratemetgr

220V

Oscilloscope

Amplifier andsingle channel

Counter-timer analyzer

Fig. 1 System for UFg and Argon X-ray AbsorptionMeasurements

Fused silica tube -, ,,To vacuum

Mylar -window

Window

MgCt

_/ X -ray ports' MySar window^

(a) Calibration Test Chamber

^-Vortex injectors

Slit

gcrystal

- 8 keV x-rays

(b) KP-Heated Plasma Test Chamber

Window

Endwall

Fig. 2 Sketch of Test Chambers for UFg and ArgonX-ray Absorption Measurements.

Fig. 3 Hiotograph of System for UFg and ArgonAbsorption Measurements in CalibrationTests.

208

Fig. 1+ Photograph of System for UFg X-rayAbsorption Measurements in RF-HeatedPlasma Tests.

10°

Temperature=300 K

Symbol0

ConfigurationCalibration

RF test

.? 10-1

* 10-2

Ix

10-3

yL-23 cm

Theory for 8 keVx-rays L-22.4 cm

400 1200 2000Argon pressure -rnmHg

Fig. 5 Measured Variation of Transmission of8 KeV X-rays With Argon Pressui'e.

Calibration test configuration

10 Of

Temperature 300 K

"- 10-1om

CD

- 10'2!>>

(D

X

•

-

\

/

\

-

ro

Theory for 8 keV x-raysL = 22.4 cm

' " 0 2fi 40 60 80 100

UFg pressure-mmHg

Fig. 6 Measured Variation of Transmission of8 KeV X-Rays With UFg Pressure.

Calibration test configuration Temperature = 300 K10 °K

1 0 -

10-3

K

Syiri' Argonsr

gonssuremHg

Argpressure_mmHg_

0SO

150300500700

Theory tor Q keVx-rays L=22.4 cm

n = 1O19P/T-cm'3

O 20 40 60 80 100UFg pressure -mmHg

Fig. 7 Measured Variation of Transmission of8 KeV X-Rays With UFg Pressure forSeveral Argon Pressures.

0.10

Symbol0

D

GasArgon

UF6 + argon

0.06

ix

0.02

= 0.028g/s

I

/A0.063

0.0a93

\0.081

3—0.093

0.093

Fig.

°0 20 40 60 80Power in plasma-kw

Variation of Fraction of X-raysTransmitted for Argon and UF6-ArgonMixtures With RF Power.

Fig

0.02 0.04 0.06 0.08 0.10UF6 flow rate • g;s

9 Variation of Measured Uranium DensityWith UFg Flo* Rate.

2 0 9

DISCUSSICrt

M. D. WILLIAMS: I noticed you use a magnesiumoxide crystal for re f lec tors . What is thereflectance of that crystal? Wh.it is the sigt.al-to-noise ra t io?

R. C. STOEFFLER: Signal-to-notse ra t io was atleast 100 for the cases where «e were measuringthti Tnaxium densi t ies ,

R. X. SCHNEIDER: Why did you < ose the X-raytechnique as compared to regulai nuclear assaytechniques?

T. S. LATHAM: Me' haven't looked at nuclear arraytechniques. Ue chose the soft. X-ray because i thail a significantly different cross-section fromthe argon, and our calculations indicated icwould give us measurements in the range of uraniumto argon.

210

MAGNETOPLASMADYNAMIC THRUSTER APPLICATIONS

E. V. PawlikJet Propulsion Laboratory-

Pasadena, California

Abstract

Advance study activities within NASA indicatethat electric propulsion will be required to makecertain types of potential missions feasible. Thelarge power levels under consideration makemagnetoplasmadynamic thruster s a good candidatefor these applications since this type of electricthruster is best suited to operation at high powerlevels. This paper examines the status of the mag-netoplasmadynamic thruster and compares it tothe ion thruster which also la a candidate. The useof these two types of electric propulsion devicesfor orbit raising of a self-powered large satelliteis examined from a cost standpoint. In additionthe use of nuclear electric propulsion is describedfor use as both a near-earth space tug and for ailinterplanetary exploration vehicle. These pre-liminary examinations indicate that the magneto-plasmadynamic thruster is the lowest cost thrusterand therefore merits serious consideration forthese applications.

Introduction ,

Planning activities at NASA are presently con-cerned with using electric propulsion for variousapplications that range irom multiwatt levels forsatellite station keeping to multi-megawatt levelsfor orbit raising of a space power station. Pro-pellant weight savings over long mission lifetimescan be obtained by using the high exhaust velocitiest'aat the electric propulsion devices are capable ofachieving. These weight savings can make the useof electric propulsion economically attractive whencompared to" the use of chemical propulsion forthese missions.

The growing need for high power levels forelectric propulsion is indicated in the Outlook forSpace Studies (Ref. 1) and Ref. 2. Commencingin the mid-1980's, large payloa'ds such as com-munications networks and satellite power stations,are expected to be placed in geosynchronous earthorbit. The transportation of these large payloadafrom shuttle orbit (500 km or 270 nm altitude) togeosynchronous orbit (35, 806 km or 19, 323 nmaltitude) will require high electrical power levelsfor propulsion. In addition, the exploration of thesolar system will also require high power levelsfor high energy missions such as sample returnfrom the outer planets. Power levels in the rangeof 400 to 1000 kW are currently being examined.

Presently two thruster types can be consideredfor these primary propulsion applications. Theseare the ion thruster and the magnetoplasmadynamic(MPD) thruster. The ion thruster is a highlydeveloped device having received a high level of

continuous funding for the last 15 yea- s withseveral flight tests being conducted. The liisstadvanced ion thruster suitable for primary pro-pulsion has been developed at Hughes ResearchLaboratories (Ref. 3). This ion thruster producesa 30 cm diameter beam and uses mercury as thepropellant. It requires about 3 kW input power toits power processor and operates at a specificimpulse near 3000 sec. A near term possibleapplication for this device is a high energy solarelectric propulsion mission such as an out-of-ecliptic mission described in Ref. 4. Whereas amodular approach is visualized when using thesedevices, the present module size is much toosmall for many of the high power missions cur-rently being studied. Drawbacks for this thrusterinclude (1) low thrust density, - large thrustareas are required for large power levels result-ing in large thruster modules and a large numberof thruster s, (2) a high degree of complexity, -the operation of a thruster requires 14 powersupplies and integrate logic built both into thepower processor and into software for the digitalcomputer required to monitor thruster operation,(3) high cost, - the ion thruster is labor-intensive,containing many areas that are extremely sensitiveto changes in geometry and fabrication processes,(4) limited reliability, - life and flight tests ofthis type of thruster have been plagued with prob-lems that accent the difficulty in obtaining highreliability.

An attractive alternative to the ion thruster isthe MPD thruster. This thruster typo has neveradvanced beyond initial laboratory tests and there-fore a comparison between the two devices canonly be made in terms of projections of the anti-cipated MPD thruster that could be developedagainst a mature ion thruster technology. Whereasa long history also exists for the MPD thruster,a continuous high support level hasbeen lacking.Furthermore, a focused system activity such as aflight demonstration was never initiated. Theadvantages of the MPD thruster are the antithesisof the ion thruster drawbacks. These include(1) high thrust density, - thrust densities up to10, 000 times higher than those of the ion thrustercan be obtained resulting in a small number ofreasonable size thruster s for a given application,(2) simplicity, - thruster operation should requireonly a single power supply, (3) low cost, - simpleconstruction and insensitivity to small dimensionalchanges should result in low cost flight units,and (4) high reliability, simple construction andminimum number of different power level require-ments should result in a much more reliablethruster than the ion thruster.

Performance is an additional area of com-parison between the two thrusters. The

This paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory,California Institute of Technology, under Contract NAS 7-100, sponsored by the National Aeronauticsand Space Administration.

211

performance of the ion thruster when used withmercury propellant is well known. Relatively-high efficiency levels kavo been demonstrated.Earth orbiting applications, however, potentiallyrequire operation with argon as the propellant inorder to minimize environment effects. Althoughion thruster performance with gaseous propellantscan be projected with & high degree of accuracy,limitations exist at low levels of specific impulse.The MPD thruster is known to demonstrate its bestefficiency at high power levels of operation. Ithas also demonstrated relatively efficient operationin the low specific impulse region where the iontbruster cannot operate on low density gaseouspropellants. Achievable performance levels of adeveloped MPD thruster over a range of specificimpulse values can only be approximated by draw-ing from on-going research activities.

In this paper, areas where the MPD thrustercould find application are identified. Clear advan-tages exist over chemical propulsion. Demon-strated advantages over the ion thruster will dependupon further development of the MFD thruster.

Thruster Description

Brief descriptions of the ion and MPD thrustersare included in this section. Table I presents pro-jected performance data for each thruster type.Additional information for the ion thruster can befound in Ref. 5.

Ion Bombardment Thruster - This thruster ishighly developed to operate with mercury as a

Table I. Electric Thruster Characteristics

A. 50-cm Ion Thrusters

Exhaust Velocity(km/8) ,EfficiencyPower Input .(kWe)Thrust (N)Mass (kg)Temperature(•K)Cost ($K)

60

0.6340.5

0.856.5

900

15

SO

0.7583

1.556.5

950

15

100

0.785146.5

2.3

6.5

950

15

B. 7. 5 MWe MPDExhaust Velocity(km/a)EfficiencyPower Input(kWe)Thrutit (N)Masu (kg)Temperature(•K)Cost ($K) '

Thruster10

.0.57500

750

700

1700

100

25

0.57500

300700

1700

100

50

0.57500

150700

1700

100

propellant. Operation on other gases auch asargon, nitrogen, and xenon have also been reported.The propellant is metered into an ionization cham-ber where it is ionized by electron bombardment.The ionized propellant is accelerated electrostat-ically by two closely spaced 'iished grids of veryprecise geometry. A retarding .potential appliedto the exterior grid prevents exhaust plasma elec-trons from returning to the thruster. The elec-trons stripped from the propellant are replenishedby an ancillary electron source labeled a neutral-izer. Maximum thrust levels are directly pro-portioral to the accelerator grid area. Thrustercomplexity is increased over the simple conceptdescribed above due to multiple propellant source,electromagnetic field, heater, and keeper electroderequirements. These are required in order toachieve high performance and long lifetimes.

MPD Thruster - This thruBter usually consistssimply of a central cylindrical cathode and a con-centric axisymmetric anode. Anode cooling isnormally required and a high voltage spike isrequired to initiate a discharge. Operation athigh-current levels provides self-induced magneticfields resulting in a lighter and more efficientthruster than when electromagnets are used. Twomechanisms contribute to the plasma acceleration.These are (1) an aerodynamic force due to heatingand expansion of the propellant and (2) acting ofj x B volume forces formed by ihe discharge cur-rent and the azimuthal self-magnetic field. Aschematic of a typical thruster experimental testsetup is shown in Fig. 1.

MPD Thruster Status

The MPD thruster was the subject of a largenumber of experimental investigations from about1960 to 1968 whe** this work was severely curtaileddue primarily to a lack of identifiable applicationsfor high power level thrusters. The ion thrusterappeared at that time to be a more viable candidatefor any near term applications In addition, con-siderable uncertainties had arisen regarding theperformance obtainable with the MPD thruster.The most serious problem was entrainment c:'ambient low pressure gas into the thruster exhaustplume. An excellent review of this early phaseof the work is described in Ref. 6.

CAPACITOR LINE

T

- A N O D E

POWERSUPPLY

ARC CHAMBER

Fig. h. Typical quasi-steady MPDresearch apparatus.

212-

During the period described above, the thrusterwas ini'cially conceived as an arc jet which impartedthermal energy to a propeUant flow passingthrough an arc. Modifications were introducedearly to utilize j x B forces as the governing thrustproducing mechanism. Several thruster approacheswere explored in mainly an empirical fashiontoward increasingly higher levels of thrust effi-ciency and specific impulse levels. However,much of this early performance data is not con-sidered reliable because of the ectrainment prob-lem and the possibility of interactions between thethruster electromagnetic fields and the vacuumtank walls. Most of theue experimental thrusterswere designed to operate at a power level between20 to 100 kW. Lifetimes of 50 to 75 hours wereachieved.

Efforts to minimiee test environment inter-actions with the thruster resulted in a quasi-steady ,state technique being evolved (Ref. 7). Thisquasi-steady state concept requires the applicationof a current pulse of sufficient duration and mag-nitude to allow the arc current and mass flow toreach a steady state. Stable and diffuse currentpatterns can be established in t veral tens ofmicroseconds in a coaxial configuration (Ref. 8).These techniques resulted in more realistic valuesof thruster performance being obtained. Maxi-mum thruster efficiencies w.sre found to rangefrom 10 to 30% and specific impulse values of 3000sec for the heavy propellants to 400 Bee for thelight propellants. Pulse techniques offer the addi-tional attraction of being able to operate at thehigher power levels during the pulse while oper-ating from a low average power level. The effi-ciency of the MPD thruster, which has been foundto increase with power level (Ref. 9), can there-fore be maintained at a higher level. In addition,varying the duty cycle provides power throttlingcapabilities without any apparent sacrifices inoverall efficiency allowing the thruster output tobe matched to variations in a power supply duringa mission.

Ba3ic limitations previously appeared to exist,limiting thruster operation because of couplingbetween arc current and accelerated mass flowrate. As arc currents are increased for a givenpropellant flow rate, a point is reached abovewhich various undesirable effects occur such asinsulator and electrode ablation, mass recircula-tion, and terminal voltage fluctuations. The onsetof these effects was taken as an upper limitationon arc current for a given flow rate. Recentresults (Ref. 10) have shown that this limitation isassociated with cathode phenomena. Modificationof the cathode arcj allows these limitations to bebreached. The higher values of arc current asso-ciate with larger cathode areas implies greaterexhaust velocities based on the fundamental self-field thrust relation T a Jz.

. The projected performance of this MPD thrusteris shown in Fig. 2. The existing performance andtrend are indicated along with a performance pro-jection obtained from Ref. l i . Ion thrusterprojected performance obtained from Ref. 12 isalso included on the figure for comparison. Allcost comparison made in this paper assume theuse of a 7.5 MW thruster capable of operatingeither steady state or in a pulsed mode.

0.8

0.7

0.6

| 0 . 5

§0.3

0.2

0.1

.— MPD THRUSTERREF. 11

- I O N iHRUSTCRREF. 12

-MPDTMUS/ERREF. 7

-RF.F. 14

10 30 30 40 50 60 70EXHAUST VELOCITY, kiV

80 90 100 110

Fig. 2. electric thruster efficiencywith argon.

Space Power Platforms Applications

With the advent of an operational shuttle vastopportunities will open up for the use of space.Near earth applications will require increasingamounts of electric power for space base support,space industrialization and manufacture, andpossibly generation of power as an electric utility.At the present time it is visualized that these plat-forms will begin on a moderate scale and thatincreasingly larger space power platforms will berequired to generate this power, which will betransmitted to other satellites or to earth bylaser or microwave transmission. Power levelsunder consideration range from 100's of kilowattsin the 1985 to 1995 time period to 100's of mega-watts after 1995 and eventually growing to 1000'aof megawatts. The orbit ior these platformswould be in geosynchronous earth orbit (GEO) atnearly continuous sunlight and at a fixed positionrelative to earth.

Transportation of the building materials for thispower platform from the shuttle low earth orbit(LEO) to GEO is a major consideration in theeconomic viability of this concept. Chemical andelectrical propulsion are both being examined forthis application. The £V required for impulsivetransfer from LEO to GEO is 4300 m/sec. Theelectric propulsion option will require a higherenergy spiral orbit and therefore will require 6200m/sec with an additional 10% gravity gradienttorques. Use of electric propulsion would permitthe platform to be assembled in LEO and then drawupon its electrical power for vehicle transfer tothe higher orbit. The low thrust levels of electricpropulsion are consistent with this concept sincethe low thrust level tnrustera could be distributedover the lightweight structure, thereby minimizingfurther structural requirements during orbitraising.

A cost analysis of the various transportationoptions is described in Ref. 13. The cost for thechemical delivery of payload to GEO is estimatedat $330/kg. Costs using electric propulsion are

213

shown in Fig. 3 for a 4000 M W platform. Themass to povrer levels are consistent with flighttimes that range from 10 to 120 days from LEO toGEO (Fig. 4). The shorter flight-timeB,occur withlower exhaust velocities and with smaller mass topower ratios. In this analysis it is assumed thatthe full array power is used for propulsion duringthe orbit transfer. The cost advantage that isevident for the MPD thruster for this applicationaccrue mainly due to the lower exhaust velocity,lower power processing mass, and the high thrustdeneity advantage that the MPD thruster can pro-vide. .Thruster and power processor weights wereused to arrive at comparative costs using the samecost/kg for each. The cost curves of Fig. 3 in-crease at lower exhaust velocities due to theincreased propellant consumption. For the samepower available the heavier mass will have ahigher mass to power ratio requiring longer trip

. time and propellant resulting in increased costs.

Based on projected efficiencies and subject tothe cost estimates used, the MPD thruster appearspromising as a candidate for this type of applica-tion. Further work is required on the MPDthruster in order to obtain better performance andcost estimates.

! : i

Nuclear Electric Propulsion iApplications

Future exploration of the solar system willrequire the use of nuclear electric propulsion toaccomplish many of the planned high energy mis-sions. Work is underway at JPLand ERDA onthe design of a nuclear electric propulsion (NEP)spacecraft capable of accomplishing these mission!(Ref. 14). The spacecraft will utilize a fastnuclear reactor power source, thermionic conver-sion of heat to electric power, and electric pro-pulsion for electric power to thrust. High specificimpulse thruster operation will r-e required forthese missions (near 9000 sec). The large exhaustarea that ion thrusters require creates problemsin incorporating these thrusters into a spacecraft.Recently the operation of an MPD thruster oper-ating from a thermionic converter matrix has been

300

150

100

50

I O N THRUST SUBSYSTEM •

15 kg/MVe

\ 10 kg/kWe

MPD THRUST SUBSYSTEM

i i

200

150

i loo

50

MPD THRUSTER ION THRU3TER

25 50EXHAUST VELOCITY, km/sec

75

Fig. -4. LEO to GEO flight time - self-powered4000 MWe SEP without solar

degradation (Ref 13)

examined (Ref. 15', for a 400 kW thrust powerlevel. A system schematic is shown in Fig. 5.The system weights involved were found to becomparable with those of an ion thruster system.Power conversion efficiencies of 90% were cal-culated. The small MPD thruster size makes inte-gration into the NEP spacecraft a much less com-plex task. Higher efficiencies, specific impulselevels, and lifetimes will, however, be requiredof the MPD thruster before it can effectivelycompete with the ion thruster for this applicati i .

# Another use of the NEP vehicle could be fortransfer of cargo from LEO to GEO. A drawingof an NEP vehicle used as a cargo transfer vehicleis shown in Fig. 6. The cargo would be trans-ferred in containerized pallets. The containerwouJd be brought up to LEO in a Separate shuttle-launch with the sequence of operations being asdepicted in Fig. 7. A logistics depot would berequired to maintain separation between the shuttleand the nuclear spacecraft. Cargo transfervehicles of this type would be required to 3upplymaterials for manned GEO satellites, space manu-facturing and also if a space station were to beassembled in GEO. The economics again deter-mines the most optimum approach. The cost anal-ysis from Ref. 13 is shown for this nuclear earthtug application in Fig. 8. The co3t of this tugusing tne MPD thruster s is much lower than whenusing the ion thruster. This is because the orbit

25 50

EXHAUST VELOCITY,

75 100

Fig. 3. LEO to GEO transportation costs -self-powered 4000 MWe SEP without

solar degradation (Ref. 13)

Fig, 5. Self inductive type of circuit forcoupling a thermionic reactor and a

MPD thruster.

2X4

GUIDANCE AND CONTROL

SUPPORT BOOM

RADIATOR AND POWER PROCESSORSUPPORT AND LOW VOLTAGE

BUS BAR 7NEUTRON SHIEURADIATOR

/ /INDUCTOR

THERMIONICCONVERTERS

NUCLEARREACTOR

CONTROLACTUATORS

MPOTHRUSTER

NEUTRONSHIELDCOOLANT PLUMBING

GAMMA SHIELD ANDPROPELLANT T. .NK

In light of these potential uses and recent re-search that has indicated possible thruster improve-ments, it appears highly desirable to initiate an MPDthruster development program in the near future.

Fig. 6. Nuclear electric propulsion cargotransfer vehicle.

transfer time is less w._ the low specific impulse.In this case a clear optimum is indicated at anexhaust velocity of about 20 km/s, in the regionwhere MPD thrusters have recently been operated(Ref. 16).

Summary

On f*>e basis of :os': comparisons .-irtrt projectedperformance, ..pace applications exist for ihe MPDthruster in both t\e orbit raising of e pov.-ei plat-form and fjr a rrtclear cargo tug. Performs neeimproverr.ents over those presently projected,corthis thruster would also make the MPD tlirustey acandidate (or use on interplanetary explorationvehicles. An evaluation of the potential for SUCRJperformance improvement is currently in'progress. : \

35, 800 km(19, 323 NM)SYNEQ ORBIT

References

1. Outlook for Space Reference Volume, "AForecast of Space Technology, " prepared bya NASA Task Group, July 15, 1975.

2. Melbourne, W. G., et al, "On PerformancesComputations with Pieced Solutions of Pianetc-centric and Heliocentric Trajectories for LowThrust Missions, JPL Sps No. 37-36, Vol. IV,1965.

3. King, H. J., et al, "Low Voltage 30 cmThruster," NASA CR-120919, Feb. 1972.

4. Yen, C. L., "Optimal Orbit Transfer forOut-of-Ecliptic Missions Using a SolarElectric Spacecraft", paper 75-420 AIAA11th Electric Propulsion Conference, NewOrleans, March 1975.

5. Kaufman, H. R., "Technology of Electron-Bombardment Ion Thrusters", Advances inElectronics and Electron Physics, Vol. 36,Academic Press, 1974.

6. Nerheim, N. M., and Kelly, A. J., "ACritical Review of the Magnetoplasmadynamic(MPD) Thruster for Space Applications, ''JPL Technical Report No. 32-1196,February 15, 1968.

7. Malliaris, A. C. , .etal , "Performance ofQuasi-Steadv MPD Thrusters at High Power, "AIAA Journal, Vol. 10, No. 2, February

500 km(270 NM)ORBIT

GROUND

DOCK SPENTPAYLOAD ANDREFUEL

PAYLOADSCHANGED

•INITIAL MISSION- SUBSEQUENT MISSIONS-

Fig. 7. Baseline NEP tug geoaynchronous orbit mission profile.

215

200

^ 150

8

100

I ,5

lOWeAs

6.67We/ks

5.00 We/kg

3.33W«Ao'

Po«RETURN

UP TIME TIME

ASSUMPTIONS:O / k

3.33 370dayi 417 day J5 247doy» 291 dayl6.67 185day> 228dayi

10 123 days 165 day.

L E 5 0 / k »N E P T U G - t M M / M W o

a ' 23 kgAWe' LIFETIME = 70,000 HOURS

(FULL POWER)MPD ARC 17 = 0.5

10 15 20 25 30

EXHAUST VELOCITY, W M C

35 40 45

Fig. 8. Transportation costs to GEO for areusable NEP tub.

8. Clark, K. E . , "Quasi-Steady Plasma Acceler-ator, " Report 859, Dept. of Aerospace andMechanical Sciences, Princeton University,Princeton, N. J . , May 1969.

9. Saber, A. J. and Jahn, R. G., "Anode PowerDepositionin Quasi-Steady MPD Area, "

AIAA Paper 73-1091, AIAA 10th ElectricPropulsion Conference, October-197?.

10. Boyle, M. J . , Clark, K. E. , and Jahn, R. G.,"Flow Field Characteristics and PerformanceLimitations of QuaBi-Steady MagnetoplasnrK-dynamic Accelerators, AIAA Paper 75-414,AIAA 1 lth Electric Propulsion Conference,March 1975.

11. Clark, K., Private communication, Prince-ton, 1976.

12. Byers, D., Private Communication, NASALewie Research Center 1976.

y13. Stearns, J . , "Large- Payload Earth Orbit

Transportation with Electric Propulsion, "JPL TM to be published.

14. Estabrook, W. C., Phillips, W. M., andHsieh, T. , "System Design for a NuclearElectric Spacecraft Utilizing out-of-coreThermionic Conversion, " JPL TM to bepublished.

15. Britt, E. J. , et al, "Inductively CoupledTI-MPD Spacecraft Electric Propulsion",

H t h l E C E C . Stateline Nevada, September,1976.

16. Rudolph, L. K., Jahn, R. G. , Clark, K. E.and Vonjaskowsky, W. F . , "PerformanceCharacteristics of Quasi-Steady MPD Dis-charges, " Proposed AIAA paper for 12thElectric Propulsion Conference, November* O"7f

DISCUSSION

J . P. LAYTON: Have you identified the overallthruster system including i t s feeding, valvliig,tightening, e tc .? Some delineation of the over-a l l systen that other people could use when theyare doing enncepting for primary propulsion forthis system would be very useful.

F. V. PAWLIK: Work hasn' t proceeded that far asyet.

216

/ ^

MINI-CAVITY PLASMA CORE REACTORS FOR DUAL-MODESPACE NUCLEAR POWER/PROPULSION SYSTEMS*

Stanley ChowPrinceton UniversityPrinceton, New Jersey

Abstract

A mini-cavity plasma core reactor is investi-gated for potential use in a dual-mode space powerand propulsion system. In the propulsive node,hydrogen propelIant is injected radially inwardthrough the reactor solid regions and into thecavity. The propellant is heated by both soliddriver fuel elements surrounding the cavity anduranium plasma before it is exhausted out thenozzle. The propellant only removes a fractionof tha driver power, the remainder is transferredby a coolant fluid to a power conversion system,which incorporates a radiator for heat rejection.In the power generation mode, the plasma and pro-pellant flows are shut off, and the driver elementssupply thermal power to the power conversion sys-tem, which generates electricity for primaryelectric propulsion purposes. Neutronic feasibi-lity of dual mode operation and smaller reactorsizes than those previously investigated are shownto be possible. A heat transfer analysis of onesuch reactor shows that the dual-mode concept isapplicable when power generation mode thermalpower levels are within the same order of magni-tude as direct thrust mode thermal power levels.However, adequate uranium plasma retention stillneeds to be demonstrated and mission analyses arerequired to identify the mission capabilities ofthis concept and compare them with alternativeapproaches.

1. Introduction

In 1971, Hyland of the NASA Lewis ResearchCenter offered an alternative to the much largercoaxial plasma core reactor with his mini-cavityconcept shown in Figure 1. * By surrounding thecavity with uraniuc carbide fuel elements, a neu-tronics analysis showed that a substantial reduc-tion in size is possible from the all-plasma fuelconfiguration. Hyland has shown that the reactoris small enough to be carried in the space shuttlecargo bay and would be appropriately sized forautomated space missions.

Wark on the mini-cavity reactor concept wasdiscontinued at the Lewis Research Center follow-ing the cancellation of the United States nuclearrocket program in 1973. The present NASA effortis devoted to the lower cost uranium hexafluorideprogram which will contribute enormously to thebasic understanding of all plasma core concepts.

Should interest in the mini-cavity reactor berevived, an additional benefit of this reactorconfiguration should be considered. Hyland hasonly analyzed his reactor for propulsion purposes.By extracting thermal power from the solid fuelelements, long-term electric power is available

EXAMPLES OfDR.VER FUELjLEHENTS

Figure 1 Mini-Cavity Plasma Core Engine

via a power conversion system. The use of thereactor for both electric power generation andpropulsion purposes is called dual mode operation.

The idea of a dual-mode space nuclear reactorsystem originated with Beveridge 2 for the solidcore NERVA (Nuclear Engine for Rocket VehicleApplication.-,) and later at Los Alamos ScientificLaboratory for the SNRE (Small Nuclear RocketEngine). In each case, an electric power con-version system was tacked on to the engine byutilizating its hydrogen-cooled structural elementsCtie-tubes) as an energy source. Subsequent tothrust termination, the power system was designedto deliver long-term power: 10 kWe for the SNREand 25 kWe for NERVA.

The power generated by these two systems wasintended for use in payload operations, e.g.,spacecraft communications, attitude control bysmall electric thrusters, etc. However, the realadvantage of the dual-mode concept lies in itsability to generate sufficiently high electricpower levels (0.1 MWe or greater) for use inprimary electric propulsion. Thus, the high-thrust nuclear propulsion mode is employed forplanetocentric operations, where high thrust-to-mass ratios are desired, and the low thrust elec-tric propulsion node is utilized for heliocentricoperations, where high effective exhaust velocitiesare optimum.

It is desirable to see if the mini-cavity plasmacore reactor is capable of dual mode operation inuseful propulsion and electric power ranges. The

This research was performed for a M.S.E. thesis and1 was supported by NASA grant NGR-31-001-328.

217

dual mode system is shown in Figure 2, includingthe engine and power conversion systems. Eithera dynamic (Brayton or Rankine) or direct (ther-mionic) power conversion system may be used.The thermionic system may utilize either in-coreOT out-of-core .diodes. In addition, heat pipesmay be used to remove heat from the driver region'instead of the usual coolant fluid.

HHM-PIPE BADIftTOB

Figure 2 Nuclear Fission Plasma CoreDual-Mode System Concept

In the propulsive mode, hydrogen is injectedradially inward through the pressure shell. ThepropelIant absorbs a certain amount of driverpower as it passes through the solid regions ofthe reactor. It then undergoes further heatii.gfrom the fissioning plasma in the cavity regionbefore being exhausted through a nozzle to producethrust. Meanwhile, the excess driver thermalpower which is not absorbed by the propellant istransported away by the coolant fluid to a radia-tor where it is dissipated into space.

In the power generation mode, the hydrogen andplasma flows are shut off and the remaining gasesin the core are exhausted into space. The reac-tivity of the driver is increased to maintainreactor cvitXcality and the driver power level isadjusted as dictated by electric power require-ments. In this mode, the driver thermal power isremoved solely by the coolant fluid or by heatpipes.

Figure 3 Spherical Reactor Analytical Model

The analysis is based on the use of the generalenergy equation:

pv Vh + V• 1c + V CD

where p is the fluid density, v is^he fluidvelocity, h is the fluid enthalpy, qc is theconductive heat flux vector, qr is the radiativeheat flux vector, and S is the internal energygeneration per unit volume.

The source term S is the energy generatedin the plasma core or solid driver region bynuclear fission. In order to evaluate this term,the neutron flux through the reactor is calculated.Again, because of the first-order nature of thisanalysis, the simplest one-group diffusion methodis used.

The neutron flux has the general solutions

E cos(Br) E, sin(Br)A = _i i _±

for the fueled regions and

E4e

(2)

(3)

II. Analysis

A spherical model is adopted to describe themini-cavity reactor and is shown.in Figure 3.Only variations in the radial direction are con-sidered. The use of a one-dimensional model mayintroduce sizable errors, but is appropriate forthis preliminary investigation.

for the remaining regions. E^ are constantscomposed of region radii and material properties,B is the buckling, L is the diffusion length,and r. is the radial position.

The pressure shell is essentially transparentto thermal neutrons and therefore the neutronflux fyj vanishes for this region. Equations

218

(2) and (3) are applied to the five remainingregions along with ten boundary conditions whichrequire the flux to remain finite at the center,the flux to vanish at the extrapolated boundary,and the neutron fluxes and current densities

The source term S can be written as:

S = e (7)

(-p3 to be continuous across the interfaces.

The neutron fluxes are then expressed in termsof a constant and the driver buckling B4 .

Specifying the driver power will determinethe constant. The driver buckling oust have avalue such that the reactor is critical. This isaccomplished by changing the driver reactivityuntil the condition is met.

where <£j is the plasma neutron flux obtainedfrom the neutronic analysis.

Since the plasma is optically thick, the radia-tive heat flux can be written as:

16a _3 dTT -

The total reactor power Q is the sum ofthe plasma core power Qc and the driver powerQd . or

r 2dr

dr]

where e is the average energy released per

fission event, Z is the macroscopic fissioncross section, ijij and <j>4 are the neutronfluxes through the plasma and driver regionsrespectively, r, is the radius of the plasma,and and are the inner and outer radiiof the driver region respectively.-

Since equation (4) is temperature dependentthrough the plasma macro: opic fission crosssection, it must be solved an iterative method.An initial temperature distribution is assumedthroughout the entire reactor and then the neutronflux is calculated in each region for a criticalreactor. The neutron flux distribution is thenutilized to calculate a new temperature distribu-tion by using the heat transfer analysis describedin the following paragraphs. The new distributionis compared with the old and the entire processis repeated until the error between successivedistributions is reduced to a specified value.

The heat transfer analysis is applied to theuranium plasma, propellant, and solid regions.Starting with the stationary, sphericallysymmetric plasma, equation (1) becomes:

(5)

If 0 . is assumed to be small compared to, equation (5) can be readily integrated:

IT = K L '*2 S dr* (6)

where <3 is the Stefan-Boltzmann constant, aR

is the Rosseland mean absorption coefficient,and T is the temperature.

Since the heat fl"x vanishes at the center andthe plasma edge tern; ture is found from thepropellant heat tran. analysis, equations (6)to (8) are used to det.. ne the plasma tempera-ture distribution.

The radial component of the energy equation (I)for the propellant region is given by:

„•> in apvr 3F + dF q ) • = 0 (9)

where Vr is the radial velocity component.

Calculation of the true radial velocitydistribution would require the solution of thetwo dimensional Navier-Stokes equations. Thisis not appropriate for this preliminary analysis,and therefore a very simple model is used todescribe the radial velocity. It is assumedthat the velocity decreases linearly from thecavity wall to a value of zero at the propellant/plasma interface.

The conductive heat flux is simply

= - Kcd?

where K£ is the thermal conductivity.

The radiative heat flux is written as

qr = 2a T 24 E3(T) -20 T

4 E 3 M +

20 f r [T E3 ( T ) + E 4 C T ) " l](ID

where E = / u e du = exponential integraln 0

T = /„ a dr = optical depth

219

a = absorption coefficient of the seededgas

and T 2 = cavity wall temperature.

By specifying T2 and the inlet propellantvelocity at the wall, equations (9) to (11) aresolved for the propellant temperature distributionin the core.

It is assumed that all heat transfer in thesolid regions is by conduction, and that allpower generated in the driver region is extractedby the inflowing propellant or by the workingfluid of the power conversion system. Sincesolid-gas heat transfer coefficients are unknown,the temperature distributions in the solid regionsand in the propellant in these regions are assumedto be identical. Therefore, the Foisson equationis solved for the driver region and the Laplaceequation for the remaining solid regions (reflec-tor, moderator, and pressure shell).

(12)

(13)

for the driver region and

T = C + j-

for the remaining regions, a, b, c, and d areconstants evaluated from the boundary conditionswhich require temperatures and heat fluxes tobe continuous across interfaces and temperaturesto be specified at the cavity wall and at the outerwall of the pressure shell.

q<l is an effective power density which isevaluated in the following manner. Qj t thedriver powur, is distributed between the propellantand the porfer conversion system working fluid.The parameter K is defined as the fraction ofthe driver power delivered to the power conversionsystem. Therefore, (1-K) Qj is absorbed by thepropellant in the driver region, and the totalpropellant power (1, is given by:

Qp - Qc (14)

(l-K)<?d/[4/3ir(r4S.T3

3)].qj is then equal to

K is determined from the fuel element operatingtemperature.

The analysis used above describes the propulsivemode and provides a method to determine reactorpower allocations. Therefore, for given reactorconditions, the driver power can be varied to obtaina range of core, total propellant, and heat rejec-tion powers.

In this mode, the thermal power KQ^ extractedby the power conversion system working fluid

is rejected into space by the radiator (exceptpossibly for a small amount nee'sd for housekeepingpurposes). Therefore, the power conversion systemis essentially idle in the propulsive mode.

In the power generation mode, the fuel and pro-pellant flows are shut off and all the thermalpower produced in the driver region is deliveredto the power conversion system. The first stepin analyzing the power mode is to investigatereactor criticality. This is accomplished withthe neutronics analysis developed above, withplasma and propellant densities taken snail enoughto simulate an evacuated cavity. If a reactor iscritical in the power mode, it will also be criticalin the propulsive mode, as the jlasma contributesonly a small positive reactivity.

A heat transfer analysis was not performed forthe reactor operating in the power mode; however,conclusions drawn from the propulsive mode anlaysismay be extrapolated to determine the general char-acteristics of the dual-mode system. As in thepropulsive mode, the thermal power is assumed tobe delivered to the power conversion system viaheat pipes or a working fluid.

If the driver fuel elements were operated atthe same power level for both modes, the totalenergy delivered to the power conversion systemwould be greater during the power mode, sincethere is no propellant flow to draw off a fractionof the driver pownr. However, the driver powercan and should be operated at a lower power levelin the power mode, since lifetime requirements(e.g., several years) dictate lower materialtemperatures than in the relatively short-termpropulsive mode. Thus the maximum driver powerin the power mode should be KQJ , the same asis absorbed by the working fluid in the propulsivemode. Then the temperature constraints imposediti ?o';ul >ive mode will not be violated in thepow mode, Power levels may be further decreasedfrom KQj as necessary for lifetime needs.

The electric poweris then

generated by the system

where TL is the efficiency of the powerconversion system and Q.' is the driver powerin the power mode.

The engine mass can only be estimated in thispreliminary study. It is the sum of the reactor,turbupump, nozzle, control, and structural masses.The total system mass is the sum of the engineand power conversion systen (including radiator)masses. All except the reactor mass are calculatedby simple power-dependent scaling.

It should be noted that the radiator mass isalso dependent on radiator sink temperature aswell as the amount of heat to be rejected. Itis difficult to say whether the radiator massis greater in the power or propulsive node withouta cycle analysis. The more massive radiator shouldbe used for both modes.

220

III. Discussion of Results

The accuracy of the model used here is checkedagainst the more sophisticated model used byHyland. For a 500-atmosphere reactor with adriver power of 19.7 megawatts, a core power of4.95 megawatts is calculated as compared toHyland's value of 4.55 megawatts. It is thereforeconcluded that this model is accurate enough fora preliminary survey.

However, this configuration rejects a sizableamount of thermal power: 17.7 megawatts. It isclearly desirable to operate at lower driver powerlevels. In addition, Hyland fixed his reactordimensions to fit in the shuttle bay. Smallersize reactors should be investigated.

The first step is to estimate the minimum sizereactor which could be critical either with orwithout the core. Accurate definition of the trueminimum size reactor, which would require adetailed parametric study incorporating variablereactor region dimensions and compositions, isnot considered appropriate for this study.

Hyland's reactor is again used as the referencereactor and various parameters are perturbed.Three temperatures must be fixed in the solidregions for the analysis to proceed: themaximum solid fuel driver temperature (set at2000 K), the reactor inlet temperature (set at293 K], and the cavity wall temperature (1523 K).

A constant a is defined as the dimensii-ilessnew reactor radius (a = 1 corresponds to Hyland'sreactor dimensions). The region*sizes of the newreactor are obtained by multiplying th3 corres-ponding dimensions of the reference reactor bya . Therefore, the dimensions of the regions arealways-in-the same proportion as Hyland's reactorfor each reactor examined.

It is assumed that there is no plasma presentin the cavity and K is arbitrarily selectedas 0.5. With K and the temperature constraintsselected, a is a function only of Qj. There-fore an a is generated for each Qj selected.

However, a must also satisfy the criticalityrequirement. For the fuel loading of 20 kg of233

U in the driver region, the reactor becomessubcritical below a = 0.708. Table 1 shows thedimensions and mass of this minimum size reactoras compared with that of Hyland, indicating aconsiderably smaller mass than Hyland's, whichin turn was itself considerably lower than thatof the all-plasma fuel concepts. This suggeststhat the dual-mode plasma-core engine couldprovide excellent payload performance.

The calculations are then repeated with plasmain the cavity to determine the core power. Al-though the driver fuel provides almost all thereactor's reactivity, the plasma contributed asmall positive reactivity which can be controlled,by increasing neutron absorption with controldevices in the driver region, to prevent thereactor from becoming supercritical.

Radius of Plosna, m

Radius of Cavity, ro

Thickness of Inner Moderator,

Thickness of Outer Moderator,

Pressure Shell Thickness, m

Hcactor Mass

1. Fraction

3. Maximum dr

H

K of pouerdeliv

iver fuel element

n,

ered

and Miss

Hvland's

0-213

0.3048

0.1524

o.osos

0.0508

0.0B12

5386

to po*er gencr

temperature 2000 K,

ation syst

shell ten

MinintF

0.1506

0.2158

0.1079

0.0*60

0.0359

0.0363

U 1 S

en is 0.5.

perature

For this minimum size reactor with K = 0.5,the driver power was 2.89 megawatts and the corepower 0.206 megawatts - too low for propulsivemode operation. Therefore, it is of interestto examine other driver powers for the minimumsize reactor.

In order to satis1"/ the temperature constraints,(1-K) Qj is kept constant as Qj is varied. Theresults are presented in Figure 4, which shows Kplotted against the core/driver ratio Qc/Q<jfor various total powers (sum of the driver andcore powers), Q . This map is then used tocalculate the overall system performance.

Figure 4 Power Level and Distribution Mapfor Minimum-Size Plasma Core

Reactor

Table 2 shows the breakdown in power alloca-tions and masses for two values of 0,,/Qd . Thetotal propellant power is given by equation (14)and the electric power by equation (15), with thepower conversion system efficiency taken to be0.25. The engine mass includes reactor, turbo-pump, nozzle, control, and structure masses. Thetotal system mass is obtained by summing theengine mass and power conversion syste' iss.

221

I i

Table 2 Breakdown of Total Systca Mass forMinimus Reactor Ma»5

TotalPoner,(Q)

MWt

2.2

J.2

4.3

t . 4

10.6

IS.8

Core'Drivor

O.I

0.4

O.I

0.4

0.1

0.4

0.4

1.88

LSI

2.15

1.02

2 17

1.81

-••.22

4.45

n. ico

D M

(1.600

1.54

S.28

Z.SO

. ,

1 2B4

1 Z'J'I

1 510

1 j l u

1 319

1 548

i 502

1 525

• " « *

760

J20

1 ' 90

1 060

1 S3U

4 SfcO

8 ISO

6 ISO

11 100

9 990

• 044

1 610

snou

4 019

5 '76

5 95*

J 60S

9 88 !

7 675

11 (.56

These results are used to plot Figure 5, whichshows the variation of total system mass versuspropellant thermal power and electric power forvarious values of Q/Q

I

J

| ..„I1

1

0e - PlM

/

r thanui p

/

P

/I

1

/

' 11

y

I

/

7Pcvcr for q

Figure 5 Total Systen Mass vs Electricand Propellant Thermal Powers

Note that there are two curves f.or the propell-ant thermal.power, since this quantity is depen-dent on Qc/Q(j . However, the electric powercurve is quite insensitive to Qc/Qj so thatonly one curve is needed to represent all Q /Q,values. c ^*

From Figure 5, it can be concluded that forthe minimum-sized reactor, dual mode operationis limited to high electric power requirements(greater than 1 MWe). This limitation is imposedby the fact that if interesting levels of propul-sive power are to be utilized, the reactor must

be operated at high total power. Therefore, alarge amount of driver power must be delivered tothe power conversion system.

For example, if a dual mode system is requiredto produce 2.0 MWt of propel lant power at Qc/Q<j =0.1, Figure 5 can be used to find the electricpower and the system mass. In this case, thesystem mass is 8000 kg and the electric powerproduced in the power mode is 1.6 MWe.

It is observed from Table 2 that the powerconversion system mass dominates the total systemmass at the higher powers. It is beneficialto the performance if the power conversion systemmass can bp reduced, suggesting the use ofadvanced heat pipe, Brayton cycle, and thermionicconversion technology.

A full map of possible reactor sizes and powerlevels is not explored in the present study;clearly this needs to be done before the scale,configuration, or performance specifications forreactor models can be selected.

IV. Conclusions

The mini-cavity plasma core reactor was foundto be capable of dual-mode operation. A referencereactor was found to have excess reactivity, andas a result the reactor size was substantiallyreduced (about 30%). The resulting reactor wassmaller than any other plasma core concept pre-viously investigated. It was critical with orwithout plasma present in the cavity and thereforewas neutronically feasible for dual mode operation.

The fraction of driver power removed to thepower conversion system K was plotted againstthe ratio of core to driver power Qe/Oj forseveral total reactor powers. From this mappropellant thermal powers, electric power:;,engine mass, power conversion system mass, andtotal system mass were calculated and used togenerate a performance map of total system massversus electric or propellant thermal power forvarious Q C/QJ •

It was observed that this reactor possessescharacteristics for dual-mode applications whichrequire the driver thermal power level duringthe power mode to be of the same order of magnitudeas the nozzle exhaust power level during the prop-ulsive mode. For example, one dual mode systemproduces 2.0 MWt of propellant thermal power inthe propulsive mode, 1.6 MWe of electric power inthe power mode (utilizing a 2S'i efficient powerconversion system), and has a total system massof 8000 kg. Such an electric power output canbe used by magnetoplasmadynamic arcJets forprimary electric propulsion.

The analysis is preliminary in nature. Th«neutronics calculations should be redone usinga multigroup method (diffusion or transportcode). Bstter representation of the heat transfer.and fluid dynamics processes occurring in thecavity is needed, A two-dimensional heat transferanalysis of the driver region is necessary,particularly in the power mode. Systems analysesare needed for the power conversion system

222

(including radiatorJ and various subsystems suchas the nozzle and turbopump. Finally, a broadrange of reactor sizes and powprs should beexamined.

Aside from this analysis, two critical questionsmust be answered. The first is the feasibility •of uranium plasma retention, which should beanswered by planned reactor experiments. Thesecond concerns the usefulness of the dual modeconcept. Mission analyses are required to deter- •mine what missions are appropriate for this conceptand how it compares with alternative methodssuch as a chemical-nuclear electric rocket combina-tion.

References

1. Hyland, R.c., "Mini Gas-Core PropulsionConcept," NASA TM X.-679S8, October 1971.

2. Beveridge, J.H., "Feasibility of Using theNERVA Rocket Engine for Electrical PowerGeneration," AIAA Paper No. 71-639, June 1971.

3. Booth, L.A., and Altseimer, J.H., "The NuclearRocket Energy Center Concept," Los AlamosScientific Laboratory Report L/f-DC-72-1262,1972.

4. Chow, S., Grey, ,T., and Layton, J.P., "Prelim-inary Analysis of Mini-Cavity Plasma CoreFission Reactors for Dual-Mode Space NuclearPower and Propulsion Systems/' PrincetonAMS Report No. 1230, October 1975.

DISCUSSION

F. C. SCHWESK: What kind of /.hrust levels wereyou getting?

S. CHCW: 400 Newtons.

F. C. SCHWENK: So when you fcere using thepropulsive mode with the plasma, it was still afairly low thrus-t machine.

J. GREY: If you look way back in the early 1960's,one felt that the ideal rockec would be one thatgave high thrust to mass ratios in the neighbor-hood of a planet and very high specific Impulsewhen not near a planet. So we therefore said wehave been looking at this dual mode thing for aIcing time in the solid core configuration, and thesolid core system constraints are such that youget at least an order or two of magnitude ratiobetween the direct thrust and the amount of poweravailable for electric, because you are taking mostof the heat out on direct thrust. This gas coresystem uniquely offers an opportunity to have allthose power levels where t'ue electric power is veryhigh (the same order of magnitude) as direct thrust.It turned out there were some missions that lookedvery nice, with this, but we haven't yet comparedthem with missions which might be done the sameway with, say, a chemical rocket plus a nuclearreactor, and that is a comparison that must bemade.

223

PLASMA CORE REACTOR APPLICATIONS*

T. S. Lathan, Chief Project Engineerand

R. J. Rocigers, Research EngineerUnited Technologies Research Center

East Hartford, Connecticut

Abstract

Analytical and experimental investigations arebeing conducted to demonstrate the feasibility offissioning uranium plasma core reactors and tocharacterize space and terrestrial applications forsuch reactors. Uranium hexafluorlde (UFg) fuel isinjected into core Bvities and confined away fromthe surface by argon buffer gas injected tangen-tially from the peripheral walls. Power, in theform of thermal radiation emitted from the high-temperature nuclear fuel, is transmitted throughfused-silica transparent walls to working fluidswhich flow in axial channels embedded in ssgmentsof the aavity walls.

Radiant heat transfer calculations wereperformed for a six-cavity reactor configuration;each ccvity is approximately 1 m in diameter by4.35 m in length. Axial working fluid channels arelocated along a fraction of each cavity peripheralwall. The remainder of the cavity wall is con-structed of highly reflective aluminum whichfocuses radiant energy onto the working fluidchannels. Results of calculations for outward-directed radiant energy fluxes corresponding toradiating temperatures of 2000 to 5000 K indicatetote.l operating pressures from 80 to 650 atm,centerline temperatures from 690O to 30,000 K, and

• total radiated powers from 25 to 2500 Mil,- respec-tively.

Applications are described for this type ofreactor such as (1) high-thrust, high-specific-impulse space propulsion, (2) highly efficient sys-tems "or generation of electricity, and (3) hydro?gen or synthetic fuel production systems using theintense radiant energy fluxes.

Introduction

Since 1955, various researchers haveconsidered the prospects for utilizing nuclearenergy with fissile fuel in the gaseous state,Most of this work was concentrated on the gaseousnuclear reactor technology required for high-per-formance space propulsion systems. The currentresearch program on gaseous nuclear reactorsincludes continued consideration of high-thrust,high-specific-Impulse space propulsion applicationsand, in addition, plasma core reactor (PCH) appli-cations for meeting terrestrial energy needs.

Extraction of energy from the fission processwith the nuclear fuel in gaseous form allows oper-ation at much higher temperatures than those ofconventional nuclear reactors with solid fuelelements. Higher operating temperatures, ingeneral, lead to more efficient thermodynamiccycles and, in the case of fissioning uraniumplasma core reactors, result in many possibleapplications,employing dire?' transfer of energy inthe form of electromagnetic radiation. The appli-cations for PCR's require significant research andtechnology development, but the benefits in poten-tial increases of domestic energy resources andutilization, reductions in environmental impact,and the development of n«w highly-efficient techni-ques for extracting energy from the fission processwith nuclear fuel in the gaseous or plasma statejustify an investment to establish the feasibilityof fissioning uranium PCR's as a prime energysource,,

Possible applications for plasma core reactors

(1) High-thrust, high-specific-impulse spacepropulsion.

(2) Advanced closed-cycle gas turbine drivenelectrical generators.

(3) MHD power conversion systems for generatingelectricity.

CO Photochfemical or thermochemical processes suchaE dissociation of hydrogenous materials toproduce hydrogen.

(5) Thorium—Uranium-233 theriual breederreactor with gas turbine driven electricalgenerators.

(6) Direct nuclear pumping of lasers by fissionfragment energy deposition in lasing gasmixtures.

(7) Optical pumping of lasers by thermal and non- •equilibrium electromagnetic radiation fromfissioning UPg gas and/or fissioning uraniumplasmas.

Cavity reactor, experiments he Oeen conducted

to measure critical masses in ce .'-y reactors to

obtain data for comparison witu theoretical

^Research sponsored by HftSA Langley Research Center, Contract NAS1-13291, Mod. 2.

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calculations at both Los Alamos Scientificlaboratory (LASL) and the national Reactor TestingStation (NRTS) in Idaho Falls. Critical massmeasurements have been made on both single cavityand multiple cavity configurations. In general,nucleonics calculations have corresponded to with-in a few percent of the experimental measurements.A review of these experiments is contained inRef. 1. These studies provide the basis for selec-ting additional experiments to demonstrate thefeasibility of the plasma core nuclear reactors.

A program plan for establishing the feasibilityof fissioning UFg gas and uranium plasma reactorshas been formulated by NASA and is described inEefs. 2 and 3. Briefly, the series of reactortests consists of gaseous nuclear reactor experi-ments of increasing performance, culminating in anapproximately 5 MJ fissioning uranium plasma reac-tor experiment. Each reactor experiment in theseries will yield basic physical data on gaseousfissioning uranium and basic engineering datarequired for design of the next experiment. Initialreactor experiments will consist of low^power,self-critical cavity reactor configurations employ-ing undissociated, nonionized UFg fuel at nearminimum temperatures required to maintain the fuelin gaseous fonn. Power level, operating tempera-tures, and pressures will be systematicallyincreased in subsequent experiments to approximate-ly 100 kW, 1800 K, and 20. atm, respectively. Thefinal 5 MW reactor experiment will operate with afissioning uranium plasma at conditions for whichthe injected UFg will be dissociated and ionizedin the active reactor core. A review of theinitial UFg reactor experiments in the plannedseries"conducted at LASL is given in the Proceed-ings of this conference and in Ref. k. Discussionsof experimentally realized nuclear pumped lasersare also given in the Proceedings of thisconference (Session III) and in Kef. 5.

which indicate the overall features of the applica-tion systems and the method of integration of theprincipal components with the gaseous nuclearreactor energy source.

Plasma Core Beactor Configurations

The analytical investigations reported hereinwere performed to examine potentiality attractiveapplications for gaseous nuclear reactors fueledby UFg and its decomposition products at operatingtemperatures of 2000 to 6000 K and pressures ofapproximately 100 to 650 atm. Emphasis was placedon predictions of performance of this class of gas-eous nuclear reactors (1) as the primary energycource for high-thrust, high-specific-impulse spacepropulsion applications, (2) as the energy sourcefor highly efficient systems for generation ofelectricity, (3) as the source of high intensityphoton flux for heating seeded working fluid gasesfor applications such as hydrogen production andMHD power extraction, and (h) in a Thorium—Urani-um-233 nuclear breeding fuel cycle. Configurationsto permit the coupling of the intense radiantenergy fluxes to working fluids are presented.Energy conversion systems using the gaseous nuclearreactor as the prime energy source were analyzed todetermine system performance and thermoaynamieefficiencies. Conceptual designs are presented

In the pT.asma gorer£««e8?rc£8eB$tcoricept ahigh-temperature, high-pressure plasma is sustainedvia the fission process in a uranium gas injectedas UFg or other uranium compounds. Containment ofthe plasma is accomplished fluid-mechanically bymeans of an argon-driven vortex which also servesto thermally isolate the hot fissioning gases fromthe surrounding wall. For applications whichemploy thermal radiation emitted from the plasma,an internally-cooled transparent wall can beemployed to isolate the nuclear fuel, fission frag-ments, and argon in a closed-cycle flow loop andpermit transfer of the radiant energy from theplasma to an external working fluid. For applica-tions which employ fission-fragment-induced, shortwavelength, nonequillbrium radiation emitted fromthe plasma, the working fluid such as lasing gasescan be either mixed with fissioning gas or injectedinto t\\e peripheral buffer gas region such thatthere is no blockage of radiation due to theintrinsic absorption characteristics of transparentmaterials at short wavelengths. The FCR configura-tions discussed belo.. are for applications based onuse of intense thermal radiation transmittedthrough transparent walls to working fluids;closed-loop circulation of gaseous nuclear fuel andbuffer gas is an intrinsic feature of the config-urations.

Geometry of Unit Cells and Reactor

Concepts for coupling radiant energy from afissioning plasma to working fluids are shown inFig. 1. The unit cell shown at the top of Fig. 1is from the nuclear light bulb space propulsionconcept described in Ref. 6. Energy is transferredby thermal radiation from gaseous uranium fuelthrough an internally-cooled transparent wall toseeded hydrogen prppellant. The fuel is kept awayfrom the transparent wall by a vortex flow fieldcreated by the tangential injection of buffer' gasnear the inside surface of the transparent wall.The buffer gas and the entrained gaseous nuclearfuel pass out through ports located on the center-line of the endwall of the cavity.

An alternate unit cell configuration is shownat the lower left of Fig. 1. The fuel and buffergas zone is surrounded by a reflective aluminumliner with axial working fluid channels along-por-tions of the periphery of the. fuel cell surface.The reflective liner would be made of aluminum,for example, which has a reflectivity of approxi-mately 0.9 for the spectral distribution of .thermalradiation emitted from the nuclear fuel. The linermaterials which are highly reflective to thermalradiation reduce heating of the cavity surfacesand concentrate the thermal radiation onto the

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working fluid channels. The working fluid couldbe heated by being passed over graphite fins whichare not surrounded by fased-sllica tubes. Or, thegraphite fins could be replaced by micron-sizedparticles or opaque gases to absorb the thermal-Eradiation from the fissioning plasma.

A working fluid assembly which consists of aseries of uncooled U-tube-shaped fused-silicacoolant passages is shown in the lower right ofFig. 1. The tubes have vails sufficiently thick towithstand compressive pressure loadc should it bedesirable to operate the working fluid at apressure significantly lower than that In thefissioning uranium plasma region. Interstitialzones surrounding the U-tubes are filled with inertgas (argon or helium) at the same pressure as theplasma region. Working fluid such as helium passesthrough the fused-silica tubes and is heated byconvection from high temperature graphite fins in-side the fused-silica tubes which absorb thermalradiation emitted from the plasma. The upper limiton working fluid outlet temperature imposed by alimit on the fused silica operating temperaturewould be approximately 1200 K.

_ A sketch of a conceptual design of a plasmacore reactor for use in generating electricity isshown In fig. 2. The reactor consists of /six unitcells which are imbedded in a beryllium oxidereflector-moderator and surrounded by a pressurevessel. Each of the six unit cells ia a separatecylindrical unit consisting of a fuel regionassembly and an outer Working fluid assembly. Thevuo assemblies can be withdrawn from opposite endsof the reactor configuration for periodic main-tenance and inspection. The fuel assembly consistsof a plasma fuel zone with nuclear fuel injected inthe form of UFg. The uranium used' in the UFg canbe either highly enriched U-235 or U-233. Gaseousnuclear fuel is confined In the central region ofthe fuel zone by argon buffer gas. The mixture ofnuclear fuel and argon buffer gas is withdrawn fromone or both endwalls at the axial centerline ..forseparation and recirculatlon.

A cross section of a breeder reactor version •of a plasma core reactor is shown on the. bottom ofFig. 2. Results of calculations of breedingratios arid doubling times for a plasma core breederreactor are described in a subsequent section.

Plasma core reactors which have been discussedthus far have working fluid channels closely .coupledneutronically to the reactor. Applications forplasma core reactors are under consideration forwhich It would be more advantageous to locate theworking fluid channels at positions other thanthose adjacent to the fuel cavity, so that theneutron absorbing characteristics of material inthe working fluid channels will be unimportant.Thus, a transmission cell must be provided so thatthermal radiant energy flaxes can be transmittedfrom the nuclear fuel cavity, through the moderatorand perhaps through the pressure vessel, to the

working fluid channels.

A schematic diagram of two possible plasma corereactor configurations which make use of trans-mission cells is shown in Fig. 3. The configura-tion on the left shows an arrangement in whichtransmission cells are connected to wciking fluidchannels within the pressure vessel, but outsideof the beryllium oxide reflector-moderator, thufminimizing the neutrpnic coupling between the fuelcavity and working fluid channel. Potential appli-cations for this configuration are principallyterrestrial. The configuration shown on the rightin Fig. 3 has a single centralized working fluidchannel which receives thermal radiant energy fromeach of the surrounding six nuclear fuel cavities,and which is outside of the innar pressure vesselwall structure. This particular configuration canbe employed in providing a high temperature gaseouspropellant stream for space propulsion applicationsor for terrestrial power conversion applications.Further, radiant energy deposited in the reflectiveliner as well as neutron and gamma energy depositedin the moderator can be extracted to provide on-board power for the space vehicle.

The transmission cell' employs series offused-silica transparent walls with intermediateregions of gaseous hydrogen and/or deuterium gaato balance pressure between the fuel cavity regionand the transmission cell. The hydrogen and/ordeuterium gas provide a transparent light path forthe thermal radiation emitted from the plasma.The gas also scatters neutrons effectively and,therefore, reduces leakage of fast andtthermalneutrons from the system. Thettransmission cellsupporting structure is cylindrical in shape andthe inner cylindrical wall is lined with a highlyreflective material such as aluminum. The reflec-tive liner minimizes the loss of radiant energy tothe walls of the transmission cell as it is trans-mitted from the fuel cavity to the working fluidchannel. The fused-silica walls which are compo-nents of the transmission cells are not completely•transparent to thermal radiant energy andabsorption must be included in evaluating thetransmission cell efficiency.

A summary of '- "\nsmisslon cell performance isgiven in Table I. These results ar^ calculated foran outward-directed thermal radiant energy fluxcorresponding to a black-body temperature of UOOO K.One component of transmissivity is related to theradiant energy absorbed by the fused silica and isa function of the wall thickness. This componentvaries from 0.9^1 tc 0.85 for wall thicknesses of0.25 and 3«5 cm, respectively. A second componentof transmissivity is related to losses from theincident beam resulting from multiple wall reflec-tions within the transmission cell.along the pathto the external working fluid channel. The trans-missivity of a right circular, cylindrical trans-mission cell with aluminum reflecting walls wascalculated as a function of the cell length-to-diameter ratio. Based on the Monte Carlo

226

technique developed in Ref. 7 in1 which the trans-mission or radiation from the diffuse source wascalculated along a specular reflecting cylinder,the transmissivity at the exit of the cell variedfrom 0.92 to O.56 as the length-to-diameter ratiovaried from 0.5 to k.O. The total transmissivitycan be estimated, to a first approximation, bymultiplying the two independent transmissivitycomponents.

Fresnel reflection losses for fused silica and'gas interfaces were not considered. The trans-mission cell must have a jdnimum of two fused-silica walls and thus four potentially reflectingsurfaces. However, not all of the reflected energyis lost. Some of the energy will be reflected backinto the fuel region to be reabsorbed and, sub-sequently, re-radiated by the plasma; some will bere-reflected within the transmission cell and firdits way to the working fluid channel. Furthennjre,reflection losses occurring at the interfaces cthe fused silica can be induced within given wave-length bands by depositing anti-reflection coatingson the surfaces. Further analyses or measurementsare required to quantify the effects of interfacereflections on overall transmission cell perfor-mance.

Criticality and Radiant Heat Transfer

Calculations of critical mass were performedusing the one-dimensional neutron transport theorycomputer program, AHISK (Kef. 8), for the non-breeder reactor configuration shown in Fig. 2.The volume of each region was transformed intoequivalent-volume spherical zones. A twentygroup neutron'energy structure was used in thecalculation for which neutron cross sections wereobtained from the HRG (Hef. 9), TEMPEST-I1 (Kef. 10),and SOPHIST-I (Ref. 11) computer progra-ns. Acritical mass of 86.k kg of U-235 was calculated;Xh.h kg is in epch of the'six unit cells.

iluclear fuel will be injected into the fuelregion of plasma core reactors in the form ofgaseous UFg. Upon entering the plasma zone, theUFg will dissociate such that at high temperatures( -8000 K) the total pressure of the mixture willconsist primarily of contributions from uraniumatoms and ions, free fluorine atoms and ions, ihecorresponding electrons from ionized species, andsome argon.buffer gas which will mix into theplasma zone. The composition of UFg as a functionof teraperature and pressure were calculated usinga UTRC computer code described in Ref. 12. Thefollowing species were included in the analyses:UF6, UFj, UFh, UF3, F, F", F2, 0°, U*, V

+s, '.J+3,and electrons. A composite plot <3f the variationof the ratio of fluorine to uranium and uraniumfluoride species is shown as a function of tempera-ture: for several total pressures in Fig. •*. Theabrupt increase in fluorine partial pressure withtemperature occurs with the onset of the dissocia-tion of UFg. A plasma core reactor must by defini-tion be an ionized gas. If fueled by UFg, the six

fluorines and electron partial pressures add tothose of the uranium species, resulting in operatingpressures of several hundred atmospheres.

In calculating the temperature distributionfor the fissioning plasma region far a given radiantheat flux at the plasma edge, the containmentcharacteristics in the fuel and buffer-gas regionmust be considered. A reasonable constraint forcontainment is to require that from the edge-of-fuellocation inward, the local density at any stationbe less than or equal to the density of the buffergas at the edge-of-fuel location. With this con-straint, and for constant total pressure, there <exists an upper limit on the amount of uranium(which has a higher mass number) that can be con-fined with the fluorine anfi argon at a given local •temperature. Calculations were performed to deter-mine the ratios of uranium to fluorine and argon at4.ocal temperatures in the fuel region sunh that thetotal pressure is preserved and the total densityof uranium, fluorine, and argon is equal to or lessthan that of the argon at the edge-of-fuel location.The compositions of fluorine, argon, and uraniumas functions of temperature and pressure, includingthe effects of ionization at high temperature-,were taken from Refs. 13 and lU, and from the UFgdecomposition calculations. By using UFg decomposi-tion products and argon decomposition products inconjunction with trs estimated spectral absorptioncross sections, Rosseland mean opacities were cal-culated for the mixture of argon and UFg decomposi-tion species in the fuel region. The eEtlmates ofthe spectral characteristics of UFg and its dominantdecomposition products over the range of pressure,temperature, and wavelength important to the plasmacore reactor concept, were reported in Hef. 15.Results of current research on the experimentalmeasurement of spectral emission and absorptioncharacteristics by UFg an<5 its decomposition pro-ducts is reported in Ref. 16.

These mixture opacities were used in aradiation diffusion analysis to determine thetemperature distribution required to deliver a ne4

heat flux at radial boundaries, which are located at110 cm intervals from the centerline of the fuelzone, equal to the total energy release due to thefissioning of the nuclear fuel within each boundary(local argon, fluorine, and uranium densities andpartial pressures were calculated using the programdiscussed above). The calculation converges whenthe heat flux at the outer boundary corresponds tothe net heat flux at the edge-of-fuel location andthe contained uranium, based on the imposed densityand total pressure constraints, equals the critical

Radiant heat transfer calculations wereperformed for the unit cells in the reactor con-figuration shown in Fig. 2. Temperature distribu-tions were aaloulated for edge-of-fuel temperaturesof 2000 K, 3000 K, 1*000 K, and 5000 K, and workingchannel duet to total cavity surface area ratios of0.0, 0.1, 0.2, 0.3, and 1.0. The centerline

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temperatures and total cavity pressures which.iielude the resulting UFg and argon decompositionproduct partial pressures are given in Table II.The critical mass was held constant .for the differ-ent operating conditions, so that th; characteris-tic of the'pressure increasing as the centerlinetemperature '.nereases is indicative of the degree •of UFg dlsscoiation. ' '• ~\,-.'-

.,- The reflective aluir'num liner tends to trapphotons in the fuel region. A portion of thereflected thermal radiation is reabsorbed by thenuclear fuel in the edge-pf-fuel region whichcauses i;he local temperature to rise. Thespectral heat flux incident on the aluminum liner

. was used to calculate an aluminum spectrum-weightedaverage reflectivity which at the liner surface wasO.9O9. The effective reflectivity is dtfferent from

. the spectrum-weighted average ref'ectivity of thealuminum liner because of geometrical factors.Diffusely reflected radiation from the liner wouldhave a cosine distribution ebout the inward normal.Some of the reflected radiation, therefore, would. not intercept the fuel cloud but would pass by thecloud and reflect off another portion of the liner.

The equations which describe the effectivereflectivity for a given fuel region and reflectiveliner geometry are derived in Eef. 17. A givensteady-state outward-directed heat flux and thereflected component of the thermal radiation whichis absorbed by the fuel oloud, are related unique1/when the fuel cloud is assumed to be a.cylindrical,optically-thick radiating fuel cloud. The radiantenergy which is not reabsorbed by the fuel cloud ineither absorbed by the reflective liner or isincident on the working channel duct where it .represents energy available for an energy extrac-tion cycle. The distribution of radiant energyabsorbed by the working fluid channel and the re-flective liner as a function of ratio of the areaof the working fluid channel to the total cavitysurface area is shown in Fig. 5. These energydeposition distributions were used to determine theradiant thermal powers deposited in the line- iduct for the parametric series of calculati.Calculations were also performed to estimatt ,,*convective removal of energy from the fuel cavityby the flowing gases as Well as the energy deposi-ted in the moderator by fission neutrons and gammas.The model used for the convective energy removal issimilar to that described in Ref. 18. Using thetemperature distributions calculated for eachthermal heat flux condition and with a buffergas residence time of 30 s and a ful residence •time of 60 s, the" total convective energyremoval, QcoNVs was calculated. In addition,the power deposited ia the moderator, QMQD, was 'oaleulated to be equul to 0.125-times the totalreactor power. The resulting energy balances forthe jases calculated are given in Table III.

For the conditions at which the edge-of-fuel •temperature is l»0io K and the working channel duet-to-cavity surface area ratioi is 0.2, the calculated

radial density distributions of the fuel, fluorine,and argon within the unit cell is shown in Fig. 6.The temperature variation was assumed to be linearwith radius through the argon buffer region. Thisbuffer gas region temperature variation andcorresponding density variation was matched to thetemperature and density radial distributionsobtained from the radiation diffusion analysis ofthe fuel region. The actual temperature anddensity distributions in the buffer gas region arenot as linear as shown in Fig. 6, but this approxi-mation does indicate the expected steep densitygradient which should result In a strong stablevortex flow for containing the hot fuel gas withinan outer cool buffer gas layer.

Plasma Core Reactor Applications

The salient feature of the plasma corereactor applications investigated is the couplingof power to working fluids by radiant heat trans-fer. The applications studied include high-jthrust,high-specific-impulse, space propulsion systems,electric power generators using closed-cycle heliumgas turbines, an MHD power conversion concept, andphotochemieal/thermochemical processes for theproduction of hydrogen. In addition, Thortum-U-233breeder configurations of plasma core reactors wereanalyzed to determine possible ranges of breedingratios and doubling times,

High-Thrust. High-Specific-Impulse SpacePropulsion

Historically, research on gaseous nuclearreactors was focused on high-performance spacepropulsion systems. Research was conducted on twomajor concepts of gaseous nuclear rockets: theopen-cycle\ coaxial flow concept and the closed-cycle nuclear light bulb concept. Comprehensivesurveys of the ranges of performance of the open-cycle and naalear light bulb rocket engines arediscussed in Refs. 19 and 20, respectively. Inaddition, a version of the nuclear light bulbengine with size and critical mass reduced by useof cold beryllium reflector-moderators (less than300 K) backed by cold deuterium-compound modei-atormaterials and by, use of axial propellant channelsis described in Ref. 21. Table IV summarizes theperformance characteristics of these propulsionsystems.

The performance studies described beiow --eredone for closed-cycle gaseous nuclear rocketengines which employ gaseous UFg nuclear fuel andits decomposition products with radiating tempera-tures of 6000 K and lower and total pressures of afew hundred atmospheres. m e resulting systemsshould be considered "first generation" plasmacore reactor thrusters; the higher performancesystems which should follow with technologicalimprovements and growth having been thoroughlymodeled and analyzed in earlier studies.

22£)

Engine performance characteristics werecalculated for a derivative of the referencenuclear light bulb engine. Dimensional character-istics and component veights for this engine aregivt '. in Ref. 6. The assumptions employed todetermine the performance of the engines over arange of fuel radiating temperatures from 4000 to6000 K were as follows:

(1) Eropellant exit temperature assumed to be 80$of fuel radiating temperature.

(2) Heat loads were assumed to be the same fracr-tion of total powar as those calculated forthe reference nuclear light bulb engine inRef. 6. Operating pressure and fuel andbuffer gas heating were based on the calcula-tions described"in the. preceding section.

(3) Specific impulse was reduced to 81$ of theideal value to allow for Incomplete expansion,friction and recombination losses, nozzletranspiration coolant flow, and for the flowof tungsten seeds in the propellant to absorbthermal radiation.

(ft) The total flow passing through the nozzle exitwns increased by approximately 16$ to includetungsten seed flov, and transpiration coolantflow.

(5) Engine weight was determined by adding thereference nucleai light bdb moderator weightto the pressure vessel ---eight calculated usinga weight factor given i,y Z = Wpy/FV = 1.125kg/atm-m3.

Performance characteristics for the casesconsidered are given in Table V. For the deriva-tive of the nuclear light bulb reference enginewith radiating temperatures from U00O to 6000 K,the performance characteristics are: engine mass32,700 to U2,900 kg; operating pressure 3kO to 900atm; I8p 880 to 1220 s; thrust-to-weight, Q.Xhto 0.1*7. •

For comparison purposes, preliminary analysesbased on the techniques described in Ref. 22 wereconducted of the performance of the engines des-cribed in Tables VI and V for a mission requiring atotal V of approximately 853 m/s (i.e., low circu-lar orbit to escape velocity, applicable to supplymissions to synchronous orbit, for example). Com-parisons were made in terms of initial massrequired in earth orbit (IMEO) to a chemical rocketemploying H2/Q2 propellant with a specific impulseof 450 s, a weight of 1135 kg, and a thrust levelof 890,000 U.

The missions comprised minimum energy Hohmanntransfers from' low circular earth orbit (100nautical miles) tc -ynehronoua orbit and back usingtwo rocket burns per transfer. Due to relatively •low accelerations, gravity lots correction factorswere Included. The payload yas assumed to be four

space shuttle payloads, approximately 90,000 kg.Coojperisons were made for two plasma core reactorrocket engines! a derivative of the nu&lear lightbulb reference engine with UFg fuel "with an Isp of1220 a and a thrust-to-weight ratio of 0.1*7 (seecolumn 3 of Table V) and a small nuclear light bulbengine with an Isp of 1150 s and a thrust-to -weightratio or or.h.

For the ca^es calculated, the IMEO's for theplasma core rocket engines were about kO to 60percent of those for the chemical system. Theperformance advantage increases with increased pay-load also. These results indicate that the plasmacore reactor engine with UFg nuclear fuel with"first generation" performance characteristicscould be a desirable system for ferrying spaceshuttle payloads to selected earth orbits andpossibly for other operations in cis-lunar space.The benefits of performance extension by technologygrowth and Improvements to the high-thrust, high-specific -impulse systems described by the perfor-mance ranges quoted in Table V from earlier workare clear.

Closed-Cycle Helium Gas Turbine ElectricalGenerators

Basic performance analyses of closed-cycle gasturbine systems which could effectively utilize thehigh temperature capability of plasma core reactorswere performed. The closed-cycle system uses ahelium-driven gas turbine coupled to an electricalgenerator with a nominal output of 1000 W e . Heatfrom the reactor can be transferred directly to theclosed-cycle helium working fluid or can be trans-mitted through a secondary heat exchanger. Thecycle under consideration would employ a multistageturbine connected to compressor spools. The v,ork-ing fluid leaving the main turbine would enter asecond multistage turbine directly connectf to theelectric generator. Several heat exchangers wouldbe incorporated in the system (i.e., a regenerator,preeooler, intereooler) to increase the systemperformance. A schematic diagram of the closedcycle gas turbine system is shown in Fig. 7.Additional power generation is also possible byutilizing some of the heat from the working fluidat the regenerator and intercooler exhaust tempera-tures to operate supplementary steam or organicworking flUid power systems to increasing the sys*tem overall efficiency. These additional systemswere not investigated in this study.

The reactor configuration for electricalpower generation was described in the precedingsection. Sketches of the configuration and unitcells for power extraction are shown in Figs. 1 and2. The mechanisms for extracting heat from thereactor can be either the system employing fused-silica U-tubes with graphite fins within the tubesto absorb thermal radiation, or the system withaxial working fluid channels in which graphite fine,absorb the thermal radiation without surroundingthem with fused silica. The fused-silica U-tubes

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permit a pressure differential between theoperating pressure of the reactor and the operatingpressure of the helium gas turbine loop. Forexample, fused-silica tubes under a compres:;iveloading of 500 atm would require a ratio of 0D toID of 1.19 for a design point oompresslve stress of1.36 x 10? N/n£ (20,000 psi). Due to the tempera-ture limitation on fused silica, the helium outlettemperature for the U-tube system is limited toapproximately 1200 K. The alternate unit cell con-figuration with axial graphite fins would operateat working fluid pressures equal to those in thereactor fuel region. The working fluid would beconducted through a secondary heat exchanger acrosswhich a pressure drop could be sustained such thatthe helium gas turbine system could be operated ata desired cycle pressure. Helium outlet tempera-tures would be limited only by material limits inother system components. The latter unit cellwould use less fused silica and structure in thereactor core and would be more adaptable to breederreactor configurations where it is necessary tominimize'parasitic neutron absorbers to maintain adesirable breeding ratio.

Performance was optimized by determining thesystem's overall efficiency, electrical power out-put divided by the total reactor thermal power, fora range of pressure ratios across the helium com-pressors and for three turbine Inlet temperaturesof IO98, 1506, and 1922 K. Efficiencies of thecompressors, turbines and regenerator were assumedto be 0.9. The generator efficiency was assumed tobe O.98. Fractional pressure drops of AP/P = 0.02were assumed across all heat exchangers and throughthe reactor core. A compressor inlet temperatureof 322 K was selected to permit use of a drycooling tower for heat rejection (selection andevaluation of specific heat rejection systems wasnot includei1. in the study, however). Based on thecalculated results, a compressor pressure ratio of1.75 was selected for case3 with turbine inlettemperatures of 1089 and 1505 K and a compressorpressure ratio of 2.0 was selected for *' casewith a turbine inlet temperature of 19; Threeoperating pressures were considered, 20, ^j., and153 atm. The overa]l cycle efficiency *as found tobe relatively insensitive to cycle pressure overthe range from 20 to 1J50 atm. Thus, the principalimpact of pressure selection would be on eq ..• r.mentsize and cost.

The variation of overall cycle efficiency withturbine inlet temperature is shown in Fig. 8 »ithinlet temperatures noted for various bladematerials and turbine blade cooling schemes. Theprogression of gas turbine technology into the1980's is discussed in Eef. 23. With the use ofcooled molybdenum alloy (TZM) blades and vanes,operation with turbine inlet temperatures of about1900 K might be feasible by the mid 1980's. Thecross-hatched region on the plot at working iiuidtemperatures of the order of 2500 K is indicatedwith an MHD label. The description of possibleconcepts which could be used for.MHD system is

given in the following section.

MHD Power Conversion Concepts

The high working fluid temperatures availablefrom plasma core reactors make the use of MHDpower extraction concepts attractive options. Acomprehensive review of MHD power conversion sys-tems based on gasecus nuclear reactor technology isgiven in Ref. 2l*. Plasma core reactors with fuelregion radiating temperatures sufficiently high(1*000 to 5000 K) to heat MHD working fluids todesired temperatures of 2000 to 2500 K have oper-

. ating pressures on the order of 500 atm. Conduc-tivity of MHD working fluid with alkali metal seedsdecreases rapidly with increasing pressure (a?"sfor thermal ionization, aP"^ for nonequilibriumionization). Therefore, MHD cycle performance fora given working fluid temperature tends to favoroperation at a few atmospheres pressure. In theprocess of conceiving and evaluating configurationswhich might be used to couple MHD power extractionsystems to plasma core reactors, two principalconsiderations are dominant; (l) the MHD ductoperating pressure should be a few atmosphereswhile the reactor operating pressure is of theorder of 500 atm; and (2) space occupied by MHDduct magnets and electrodes and parasitic neutronabsorptions by the MHD duct materials should besmall to minimize their impact on reector eritica?-ity. The transmission cell concepts described inthe preceding section provide a means to satisfyboth constraints.

The configurations shown in Fig.' 3 are bothadaptable for use as MHD systems. In both cases,the MHD ducts would be located outside the nuclearfuel and moderator zones and essentially neutronic-ally isolated. The transmission cells with seriesof fused-silica ports under compressive load wouldpermit operation with the appropriate pressuredifferential between the PCR fuel region aid theMHD duct. Two mode:: of operation would bepossible. The MHD duct could be attached at theends of the working fluid channels iwn in Fig. 3.Power would be extracted after the fxuid washeated as in most MHD concepts. A second optioncould be to install the MHD ducts with magnets andelectrodes in the working fluid channels. Powerwould be extracted as the working fluid washeated, resulting in a very efficient cycle sincethe working fluid temperature anJ conductivitywould be constant in the MHD duct. The latterconstant temperature MHD power conversion systemwas first suggested in Ref. 25.

Preliminary investigations were made ofpossible MHD seeding materials. The most attrac-tive working fluid and needing materials identi-fied for a PCR-MHD system were suspensions ofthermionically emitting; particles of barium oxideor mixed oxides of barium, calcium, and strontiumin argon gas. The main theoretical advantages ofan emitter suspension over an alkali metal seededgas are its higher conductivity at temperatures up

230

to about 2000 K (especially at press ores of 10 atmadd above) and its relatively small variation inconductivity with changes in pressure (arP"0-1^).The latter property permits conceptual designs ofMHD systems with operating pressures up to 50 atm

-with the resulting savings in component size.Discussions of theoretical and experimental inves-tigations of the conductivities of gas borne sus-pensions of thermionic emitters are- given in Refa.26 and 27. Particle suspensions also tend to bebroadband absorbers of radiant energy, making themideally suited as PCR working fluids.

Related analyses of a closed-cycle nuclearMHD system using dust suspension described in Ref.28 indicate that overall cycle efficiencies up to~ 60 percent are theoretically.possible with con-

ventional nuclear realtors. Overall cycle analysesand identification of system components ihoulc1. be-performed for a reference PCR-MHD power plant toevaluate its potential as an efficient power gen-eration system.

Photodlssoclation of Halogens to Produce Hydrogen

Plasma core reactors have been_proposed as ahigh power source of radiant energy »for whichefficient use of high intensity photon fluxesemitted from the radiating ionized fuel cloud canbe employed in thermochemical and photochemicalemploying hydrogen as a fuel is attractive becauseit is nonpolluting. Hydrogen may be produced fromenergy sources such as nuclear reactors or solarradiation to the exclusion of production fromfossil fuel sources. However, fcr hydrogen tobecome a viable fuel, satisfying significant futureenergy requirements, a means of producing vastquantities in an . sonomic process must be identi-fied and demonstrated.

Studies viere conducted to evaluate methods forproducing hydrogen using the intense photon fluxesemitted from plasma core reactors. A relativelysimple concept proposed here for the photolyticdecomposition of water in which the unique radia-tion emission characteristics of the plasma corerealtor are utilized is shown in Fig; 9. Threesuccessive working fluid channels are employed inthe process. The first two channels provide areaction site for the two-step, closed-cycle photo-lytic decomposition of water and the third channelis provided to absorb the residual thermal radia-tion either by flowing particle laden gases or bygraphite finned rods which transfer the absorbedenergy to a flowing gas stream by convection. Theconcept utilizes radiation and the chemical proper-ties of halogens and hydrogen halides and the uni-que radiation characteristics w" the plasma corereactor to circumvent the problems associated withthe direct thermal or photolytic decomposition ofwater. The key to the concept depends upon theability of ha ogens to react with water to form tbecorresponding hydrogen halide and oxygen species.

Br2(G) + 2I2(G) +

• 2I2(G) + Og(G)

In the concept a series of relatively lowtemperature thermal or photolytic reactions areused to effect decomposition of water and to permiteasy separation of reaction products. The combinedoverall reaction describing the process may beexpressed as

(1)

Results of composition calculations indicatethat molecular bromine (Brg) does not appreciablyreact with water. However, similar compositioncalculations with atomic bromine (Br) and wateryield hydrogen bromide (HBr) and oxygen (O2) as theprincipal products. Thermal dissociation of thehalogens occurs to an appreciable degree (~ 5" per-cent) only at temperatures greater than 1500 K. Inthe concept, the radiant flux is used to inducephotodissociatlon of the bromine and iodine mole-cular species and thermal dissociation is used forthe hydrogen iodide and iodine monobromlde species.Gaseous molecular bromine (BK2) and water (%0) areallowed to react at approximately ^50 K in thepresence of radiation (365 = X s 535 nm) to yieldgaseous hydrogen bromide and oxygen. Since HBr ienot appreciably dissociated at temperatures belowapproximately 1500 K, HBr is then allowed to reactwith i ine (12) at U56 K in the presence of radia-tion (1*30sx^ 7kO nm) to yield gaseous hydrogeniodine (Hi) and iodine monobromlde (IBr). Coolingthe reaction mixture below the boiling point of IBrpermits separation of liquid IBr from gaseous HI andHBr. Iodine (I2) and bromine (Br^) are regeneratedfrom IBr at a temperature of about 700 K. The HI -HBr mixture is heated to about 700 K to thermallydecompose HI to hydrogen (H2) and lg. Uponquenching to a temperature below 456 K, Ig Isliquefied and separated from the Hg and HBr.Finally, the solubility of HBr in H20-is utilizedto separate Hg from HBr.

For plasma core reactors with radiatingtemperatures between 1*000 K and 6000 K, iodineabsorbs over a larger fraction of the availablespectra than bromine and overlaps portions of thespectra in which bromine also absorbs. To maximizebromine photodissoclation to atomic bromine, thebromine region is positioned to intercept theincident thermal radiation first and is of suffici-en^ 'aickness to reduce the spectral flux to lessthan one percent of its incident value. The iodineregion is locat"! behind the bromine region in thethermal radiation path and i s also of sufficientthickness to absorb all but one percent of thespectral flux in its photodissociation wavelengthrange.

The fractions of radiant energy available toinduce photodissociation of bromine and iodine are0.25, 0.353, and O.I+38 for black-body radiatingtemperatures of l>000 K, 5000 K, and 6000 K,respectively. The concept process also requires

231

two other dissociation reactions namely,(HI andIBr into H2, I2, and Br2- The dissociation of HIand IBr can be carried out thermally in a consecu-tive sequence of separation chambers. By consider-ing these photodip-oeiation and thermal dissocia-tion reactions only, and assuming the higher heat-ing value of water equal to- 68.4 K-Cal/Mole ofhydrogen consumed, a maximum thermal efficiency of0.5; -as calculated for the case where recombina-tion and other thermal losses are neglected. Theother thermal energy requirements considered werethose related to heating the initial species ofVgO (liquid), bromine (liquid), and iodine (solid)from room temperature, to the gaseous state at atemperature of !»56 K and at which the gaseousconstituents enter the wording fluid channels wherethey are exposed to the incident thermal radiantflux. Thermal efficiencies were calculated foreach spectrum and were approximately O.Ul. Thecalculated efficiencies are based on the assumptionthat there is no recombination of dissociated3pecieB and that there is no recovery either of -iheheats of reaction for the closed-cycle process orof the energy devoted to initial heating of theconstituents. Thus in an actual process, the rangeof thermal efficiency would be expected to bebetween 0.1*1 and 0.55.

Because of the- discriminatory manner by whichthe thermal radiant energy spectra are absorbed inthe production of. hydrogen for the photodlssoeiafrtion reactions of bromine and iodine, the residualenergy in the spectra may be a considerable frac-tion of the total radiant energy available. Thisenergy could be utilized for some other energy con-version system. The actual fraction of residualenergy available depends upon the radiatingtemperature of the plasma and the spectral luxdistribution emanating from the fuel region. Forradiating temperatures between UOOO K and 6000 K,approximately 25 and 1+5 percent, respectively, ofthe spectral flux is available for use in bromineand iodine photodissociation reactions. For aplasma core reactor operating over this range ofradiating temperature, this hydrogen productionconcept could be used as an auxiliary cycle to aprimary energy conversion system such as theclosed-cycle helium gas turbine system described ina previous section. The advantages of this dualenergy extraction possibility is that the reactorcan be operated at normal power during times whendemand for electric power is not high and hydrogenmay be produced. The hydrogen could be stored forsubsequent usage. It is also possible to operatein a mode such that hydrogen production and thehelium gas turbine system operate simultaneously.Here the hydrogen produced could either be con-sumed in fuel cells to aid In meeting the immediatepower demand or stored for subsequent use.

Thorium—Uranlum-g33 Breeder Reactor

Plasma core reactors have several featureswhich are of benefit in terrestrial applicationsin addition to increased thermodynamic cycle

efficiency resulting from high temperature opera-tion. One of these features involves use of theThorium--U-233 breeding cycle. Studies Here per-formed to estimate the operating characteristics ofthe plasma core reactor configuration shown on theright of Fig. 2 operating as a thermal breederwith the ThcDium—U-233 cycle.

Fuel bred in a fertile blanket can beseparated on-site and incorporated directly intothe reactor fuel cycle flowing inventory or storedfor use in other reactors. Such on-site reprorceasing of the nuclear fuel would eliminate fuelelement fabrication cost' and avoid the necessityte transport expended fuel elements to remotelocations. A thorough study of the safety ofplasma core reactors has not been conducted;however, it is expected that the consequences of

. an accident in a plasma core reactor would beminimal because fission products are separated andremoved continuously in the on-slte reprocessingfacility. The high pressure, power producing coreregion contains relatively small amounts ofradioactive materials which potentially could bereleased from the reactor in the event of anaccident. The breeder reactor concept couldprovide high temperature working fluids forefficient, closed-cycle gas turbine systems. Inthe breeder configuration shown in Fig. 2, struc-tural neutron poisons are minimized by employingheavy-water as the reflector-moderator materialand by encasing the working fluid and fuel regionzones in Zircalloy tanks. A graphite-lined,fertile Thorium breeding blanket surrounds theheavy-water moderator. The complete assembly iscontained within a pressure vessel. The reactorconfiguration consists of six 1-m-dia by 4.35-m-long cylindrical fuel cavities embedded in theheavy-water reflector-moderator. The fertileThorium blanket Is composed of a molten salt solu-tion (ThFij-BeFa-LiF with mole fractions of 0.27,0.02, and 0.71, respectively) similar to thatemployed in the molten salt breeder reactor con-cepts described in Ref. 29. The fissile fiuelearfuel-is U-233 which allows exploitation of theTh-232 and U-233 breeding cycle. The technologyrequired for removing the U-233 produced in themolten salt fertile thorium solution is well- •developed and could be employed for a plasma corebreeder reactor configuration. The unit cells ofthe breeder configuration are similar to those inthe nonbreeder plasma core reactor configuration.However, in the breeder the graphite fins couldnot be enclosed in fused-silica tubes due toneutron absorption in the fused silica.

For the breeder reactor configuration des-cribed above, the breeding ratio was obtainedfrom absorption rates obtained in one-dimensionaltransport theory critical' mass search calculationsusing twenty neutron groups. Also, detailedanalysis of the breeding ratio possibly achievablewas performed. The reactor has a calculatedcritical mass of 8U.1). kg of U-233, an operatingfuel partial pressure of U98 atm, and a maximum

232

calculated breeding ratio of 1.080, assuming thatall ^h-232 absorptions in the fertile breedingblanket are-concerted to 0-233 atoms. However, inan operating breeder reactor, the complete conver-sion and reclaimation of U-231> atoms from theTh-232 atoms which absorb neutrons is not possibleand, hence, the actual achievable breeding ratio issomewhat reduced. Included in the model of thebreeding chain, are the processes by which(l) uranium is reclaimed by fluoridation (UFl +PUFg gas) and removed from the molten fertile breed-ing blanket, and (2) protactinium ie removed via achemical process.

The particular leg of the U-233 productionchain which would maximize the breed'ng ratio is

ZJ.h d

where ^ p is equivalent to x decay constantdescribing the removal rato of Fa from the fertilebreeding blanket by chemical processing. In thepresence of a neutron flux' field, the isotopes ofTh, Pa, and U undergo transmutation to other iso-topes in the chain via (n,7) absorption reactionsor by fission reactions. The only desired neutronabsorption reaction is the transmutation of Th*3£to Th 33 V i a the (n,7) reaction. All other neutronabsorption reactions and fission reactions are losschannels which diminish the breeding ratio. A com-puter program, ISOCRU1JCH (Kef. 30), which solves thegeneral differential equation describing the pro-duction and removal of an isotope in a reaction anddecay scheme, has been modified and a procedure hasbeen implemented to solve the differential equationswhich describe the production and removal of theisotopes in the Th-U-233 breeding cycle chain. Theprocedure makes use of the twenty neutron groupflux distributions from one-dimensional sphericalcritical mass calculations to determine correspond-ing fission and absorption rates for each of theisotopes. The fission and absorption rates are usedas input to the ISOCRUNCH computer program.

Breeding ratio results for the previouslymentioned breeder reactor configuration were calcu-lated for two different continuous chemical pro-cessing and fluoridation rates. The independentchemical processing and fluoridation rates wereassumed to be equal for each of the cases. Thechemical processing and fluoridation halfKives forthe two cases are twelve hours and deven days (thehalf-lives are defined as the time required toremove half of the desired isotopes from the breed-ing blanket by the chemical or fluoridation proc-esses). The calculated breeding ratios asymptotic-ally approach 1.077 and I.067 for the chemical pro-cessing and fluoridation half-lives of twelve hoursand seven days, respectively. The concentration r?Th-232 atoms in the breeding blanket was assumed tobe constant with time for these calculations. For

continuous chemical processing and fluoridationhalf-lives of twilve hours, the breeding ratio isgreater than 1.06 after approximately seventy days.Similarly, for continuous chemical processing andfluoridation half-lives of seven days, the breedingratio is not greater than 1.06 until approximatelytwo years.

It has been calculated that a plasma corebreeder reactor operating at a thermal power levelof 2500 MJ with a constant breeding ratio of 1.06,will have a doubling time of approximately fiveyears for a total fuel inventory which includesfuel in the recirculation system equal to fourtimes the critical mass. To achieve doubling timesof approximately five years, it is necessary tohave a time-averaged breeding ratio of 1.06,indiea*._:ig that short chemical processing andfluoridation half-lives on the order of twelvehours to sevtn days are necessary to bring thesystem to secular equilibrium as quickly aspossible and to minimize U-233 losses frmm thebreeding blanket due to unwanted neutron raections.

Concluding Remarks;

The investigations described above haveindicated that several space and terrestrialapplications for plasma core reactors appearfeasible and offer significant advances in perfor-mance and therraodynamie cycle efficiency. Progressreported in the other sessions of this conferenceon cavity reactor experiments, nuclear pumped laserdemonstrations, uranium plasma experiments, anddevelopment of UFg handling technology is also veryencouraging. Clearly, as soon as plasma corereac-ors with capability to provide intenseradiant energy fluxes become available, theapplications described above and many others as 'yet not considered will be coupled to them. In ourjudgment, it is most important to press forwardwith the planned cavity reaetrr experiments and theassociated development of TJTg nandling technologyand plasma confinement experiments to demonstratethe feasibility of plasma core reactors as aviable fission energy source for the 1960's andbeyond.

References

1. Latham, T. S., F. R. Biancardi, andR. J. Rodgers: Applications of Plasma CoreReactors to Terrestrial Energy Systems. AIMPaper No. 74-1071*, AIAA/bAE 10th PropulsionConference, San Diego, CA., October 21-23,

2. Helmick, H. H., G. A. Jarvis, J. S. Kendall,and T. S. Latham: Preliminary Study of PlasmaNuclear Reactor Feasibility. Los AlamosScientific Laboratory Report LA-5679, preparedunder NASA Contract W-13721, August 1974.

233

3. Thorn, K., R. T. Schneider, and F. C. Schwenk:Physics and Potentials of Fissioning Plasmasfor Space Power and Propulsion. InternationalAstronautics Federation, XXVth Congress,Amsterdam, 30 Sept. - 5 Oct. I97h.

h. Bernard, W., H. H. Helmick, G. A. Jarvis,E. A. Plassmann, H. H. White: ResearchProgram on Plasma Core Asseniily. Los AlamosScientific Laboratory Beport LA-59U-MS,prepared under NASA Contract W-13755, May 1975.

5. Schneider, R. T., K. Thorn, and H. H. Helmick:Lase.-s From Fission (Gaseous Core Reactors andNuclear Pumped lasers for Space Power Genera-tion and Transmission). International Astro-nautics Federation, XXVIth Congress, Lisbon,21-27 Sept.. 1975.

6. Rodgers, R. J. and T. S. Latham: AnalyticalDesign and Performance Studies of the NuclearLight Bulb Engine. United Aircraft ResearchLaboratories Report number L-910900-16,prepared under NASA Contract SMPC-70, Sept-ember 1972.

7. Corlett, R. C.: Direct Monte Carlo Calculationof Radiative Heat Transfer in Vacuum. J. of

. Heat Transfer, Trans. ASMS, Series C, Vol. 88,

1966, p.* 376.'

8. Engle, W. W., Jr.: A User's Manual for AHISN,A One-Dimensional Discrete Ordinates TransportCode With Anisotropic Scattering. UnionCarbide Corporation Report K-1693, 1967.

9. Carter, J. L.: HRG3 - A Code for Calculatingthe Slowing-Down Spectrum in the PI or BlApproximations. Battelle-Northwest LaboratoryReport BNWL-1U32, June 1970.

10. Shudde, R. H. and J. Dyer: TEMEEST-II, Aneutron Thermalization Code. North AmericanAviation Report AMCD-111, September i960.

11. Canfield, E. H., R. N. Stewart, R. P. Freis,and W. H. Collins: SOPHIST-I, An IBM 709/7090Code Which Calculates Multigroup TransferCoefficients for Gaseous Moderators. Universityof California, Lawrence Radiation LaboratoryReport UCRL-5756, October 1961.

12. Roback, R.: Themodynamie Properties ofCoolant Fluids and Particle Seeds for GaseousNuclear RocXets. United Aircraft ResearchLaboratories Report C-910092-3, September196k, prepared under MSA Contract mSw-8kJ.

13. Krascella, N. L.: Theoretical Investigationof the Spectral Opacities of Hydrogen andNuclear Fuel. Air Fore? Systems CommandReport RTD-TDR-63-llOl, prepared by UnitedAircraft Research Laboratories, November 1963.

Ik. Krascella, H. L.: Spectril AbsorptionCoefficients of Argon and Silicon and SpectralReflectivity of Aluminum. United AircraftResearch Laboratories Report L-910904-3,prepared under Contract SNPC-70, September1972.

15. Bodgers, R. J., T. S. I-atham, and

N. L. Krascella: Analyses of Low-Power andPlasma Core Cavity Reactor Experiments. UnitedTechnologies Research Center ReportR75-9U908-I, prepared under Contract XPk-5^59-1, May 1975-

16. Krascella, N. L.: The Spectral Properties ofUFg and Its Thermal Decomposition Products.United Technologies Research Center ReportR76-912208, prepared under Contract HAS1-13251,Mod. 2, May 1976.

17. Rodgers, R. J., T. S. Latham, andN. L. Krascella: Investigation of Applicationsfor High-Power, Self-Critical fissioningUranium Plasma Reactors. United TechnologiesResearch Center Report R76-9122O1*, preparedunder Contract NAS1-13291, Mod. 2, May 1976.

18. Rodgers, R. J., T. S. Latham, and H. E. Bauer:Analytical Studies of Nuclear Light BulbEngine Radiant Heat Transfer and PerformanceCharacteristics. United Aircraft ResearchLaboratories Report K-910900-10, preparedunder Contract SNPC-70, September 1971.

l->. Eagsdale, R. 0. and E. A. Willis: Gas-Core

Rocket Reactors - A New Look. NASA TM

X-67823, 1971.

20. Latham, T. S.: Summary of the PerforaaneeCharacteristics of the Huclear Light BulbEngine. AIAA Paper 71-6U2, AIAA/SAE 7thPropulsion Joint Specialist Conference,June 1971.

21. Latham, T. S. and R. J. Rodgers: Small NuclearLight Bulb Engines With Cold Beryllium Reflect-ors. AIAA Paper 72-1093, AIAA/SAE 8th JointPropulsion Specialist Conference, Nov. 1972.

22. Titus, R. R.: Evaluation of NIB for SpaceMissions. United Aircraft Research. Laborator-ies Report Number J-170817-1, February 1971.

23. Biancardi, F. R., G. T. Peters, andA. M. Landeman: Advanced NonthermallyPolluting Gas Turbines in Utility Applications.United Aircraft Research laboratories ReportJ-97O978-8, March 1971. Also issued as EPAIieport 6130 DHE 0371.

234

2k. Williams, J. E.: Nuclear Power From Space.Final Report prepared under NASA GrantNDR-11-OO2-181. Georgia Institute ofTechnology, November 1971*.

25- McLafferty, G. H.: Nuclear Magnetohydro-electric Generator. U. S. Patent No.3,11*0,410, July 7, 1964.

26. Sodha, M. S., C. J. Palumbo, and J. T. Daley:.Enhancement of Gas Conductivity by DustSuspensions and Its' Application to ClosedCycle MHD Jover Generation. Proceedings,International Symposium, MHD Electrical PowerGeneration, Paris, Vol. 2, 1964.

27. Waldie, B. and I. Fells: An Experimental

Dtudy of Gas Borne Suspensions of Thermionic

Emitters as MHD Working Fluids. PhiloBophical

Transactions, Royal Society of London, Series

A, No. 261, July 1967.

28. Hooper, A. T., D. Mewby, and A. E. RusselsClosed Cycle Nuclear MHD Studies Using DustSuspensions. Proceedings of Symposium onElectricity from MHD. Salzburg, July 1966.

29. Hasten, P. R., et al.: Summary of Molten-SaltBreeder Reactor Design Studies: Thorium FuelCycle - Proceedings of 2nd InternationalThorium Fuel Cycel Symposium, Gatlinburg,Term., May 3-6, 1966, p. 1*1.

30. Friend, C. W. and J. R. Knight: ISOCRUIKH -Modifications to the CRUNCH Program for theIBM 7090. Oak Ridge National LaboratoryReport ORNL-3689, January 1965.

QTOT

R

CL

or QL Radiated thermal power to l i ne r ,Mtf/Cell

Total power, Mtf/Cell

Radius, cm

Aluminum reflectivity, dimensionless

Temperature, deg K

™ or Black-body temperature corresponding_L to outaard directed radiant flux atBB. edge-of-fuel location, deg K

Centerline temperature, deg K

Wavelength, microns or nm

Density, g/cnP

TABLE I

TRANSMISSION CELL PERFORMANCE

Spectral Weighted Al R e f l e c t i v i t y « 0.909

'BB'OUT - 4000 K

Cell L/D

0 .5

1.0

2 . 0

4 . 0

CellTransmissivityWithout SiO2

0.920.830.720.56

TotalSiO2 Thickness,

0.250.501.0

2 . 0

3 . 5

Transmiss ivityof S1O2

0.9410.9230.9000.8740.850

List of Symbols

R a t i o o f w o r k i nS = h a n n e l

total cavity surface area,dimensionless

p Pressure, atm

P F / ( P U + P U F Y ) Fluorine to uranium bearing speciespressure ratio, dimensionless

PT0T Total pressure, atm

Convective power removal by fuel andbuffer flows, HW/Cell

Power deposited in moderator,/

Radiated thermal power, MS/Cell

Radiated thermal power to duct,

MW/Cell

TABLE II

PERFORMANCE CHARACTERISTICS FOR U76-FUELED GASEOUS KOCLEAR REACTORS

Cylindrical Reactor with Six Unit Cells -- See Fig. 2Critical Mass . 86.4 kg of 0-235See Table III for Energy Balance

Black-BodyTemperature .For Outward

DirectedRadiant Flux,

*Tra'oiir" d e g . K

200020002000200020003000300030003000300040004000400040004000500050005000SOOOSOOD

Duct-To-Cavity

Area Ratio,AVCD/AC,

Dltaenaionless

0.00 . 10.20 . 31.00 .00 .10.20 .31.00 .00 .10.20 .31.00 . 00 . 10.20 . 31.0

Black-BodyTemperature

For NetRadiant Flux,

<TBB>NCT' d e 8 *

1170138515201620200017602075.2280243030002345276530403245400029303460380040555000

CenterllneTemperature

deg K

55006300690073008900

10,50012,50013,70014,60018,00014,40017,20018,80019,90024,00018,20021,20022,80024,30029,800

CavityFreasure

J¥i110

,. 95808590

190230255270330320365395420505400465505530650

*Equlv«Unt Black-Body Radiating Temperature

235

TABLE I I I

ENERGY EALAHCE FOR UFg-FUEIED GASEOUS HUCIEAR BEACTCJiS

See Table I I For A s s o c i a t e d Barometers

Black<-BodyTemperatureFor, Outward

DirectedRadiant PIox,

<*|B W D e e K

20002O0O20002000

2000

30003000300030003000

1(000

uooo1*000i*oooUooo

50005000500050005000

Duct-To-Ctvity,

Area Ratio,

DimsnsionleBS

0 . 00 . 10 . 2

0.31 . 0

0 . 00 . 10 . 2

0.31 .0

0 . 0

0 . 10 . 2

0 . 31 . 0

0 . 00 . 10 . 2

0.31 . 0

Tota lPower,

%3T'Mf/Cell

3.311*.716.51

• 8.21I6.;>lt

13 .Zk20.6027-2933-506d.7O

30.30'52.2872.8092.03

201.35

gl*.3OII6.9!.166.21212.671*79.28

RadiatedThermal

• power'To Bttct

%!ADD>W/Cell

0.001.292.513.66

10.27

0.00•3.51*

12.7218.5652.02

0.0020.671*0.7358.65

161* ,39

0.0050.1*5

• 9 8 . l l11*3.18fcOl.35

RadiatedThermal

Power.To Liner

9RADL>M?/Cell

1.211.06O.910.780.00

6.135.361*.633.9^ .0.00

19.3616.93If*. 6312.1*50.00

1*7.261*1.3235.7130.1*00.00

PowerConvected by

Fuel andBuffer F l o w s ,

QCOUV'MJ/Cel l

1.691.772.282.71*U.20

5.1*66.136.536.818.09

7.158.158.899.13

11.79

9.0010.5511.6212.5118.02

PowerDeposited in

ModeratorQMOD>

tW/Cell

0.1*10.590.8l1.032.07

1.652.573.1*11*.198.59

3.7S6.529.10

11.5025.17

e.oi*ii(.6220.7726.5859-91

•Equivalent Black-Body Radiating Temperature*

TABLE IV

PlIVntKMKZ CHMtACIERISTlv-S OP GASEWS NUCLEAR. UX3ZT EHGtBVS

C o u l i l Flow, Optn Cycle Farforaanc* from tmt. 19 'Nuclear LLKht Bulb PcrfoiHiice from ««f. 20 .G H U Huclur Lithe Bulb P«rlot»«tn:« trem U f . 21

Inglns H J » I , V»

Opmrmttag Pr«»ur», <t»

Sp«elfic Iapul««, a

Thruit*to-W*i$ht

Co«xUl FlcmOpen Cycla

40,000-210,000

490-2ODO

2500-6000

O.0S-Q.20

HUClMtLl(he lult>

2^,900-45,300

400*900 '

1100-2500

0.3-1.6

HuclurLight Buli-KcfarcncaEnglna

31,730

300

1670

1.3

S u l l HucUarLlaht Bulb

15,000-26,000

300-650

923-1550

0.14*0.29 |

FERFOnXUKI XT HUCLEA* L1GKT BULB tDCUX DBIHIVHTH UUHIUH H D U n U t t l W FOIL

Engln* F i r w u r

Optratlng Pr«uur«, « n

Uruiiua PArctal Pt«liur*t dtn

Po««r, mrroptlluc Exit T«9tncura, <J«t Kftl«ity rn^llant W«Hhc rio», Vj/tCorr«et«d 9p«clflc lapul«0» •tlmiit, aBl(ln>>u», k(nini,t-u-wi|hc ut io

Tual lUdlatlng TCBParaeurc4000 KMO

60

244

32005.21'880

4S.00032,700O.U

9000 K

720

SO

S96

4000

9.55

1030

96,300

37,900

0.26

6000 I

900

100

1236

4800

16.61

1220

199,600

42.900

0.47

Transparent wsll-Thermaf radiation—

Fuel Injection-1 f\

Thru-flow

Space propulsion

Rart*ctiv«llnar

Particles,graphiteftns or opaque

Nuclear fuel-Buffer gasSeeded propellent

Transparent tubescontaining graphite

Argon-. Hna7

Argon

r H j pi ycle helium gas turbine

Fig. 1 Concepts for coupling tfaeimal radiationto working fluids.

236

Non*br««d6.*

1.7 5 M PrtaaurarSafambl, ^ - » " « « '

-BaO

BraadarThorium-uranium 233

Doubling tima&6 yaara

ZircalloywallaD 2

Unll callFartlia-thorlumblankat

1.7SM

Fig. 2 Plasma core reactor configurations forelectric power generation.

TBB=4000K

100

=0.909

Fig. 5 Variations of thermal radie/tion powerdeposition for plasma core reactor unitcells with reflecting liners.

Unit call

Transmission^call

BeOinsmfsslon-' oAlg

auambly

Fig. 3 Transmission cells in plasma corereactors.

3000 4000 5000 6000 7000 8000 9000T-degK

Fig. k Pressure ratios of fluorine to uraniumand uranium fluoride species.

T£ - 4000 K,100

T-degK

Fig. 6 Density and temperature distributions forplasma core reactor with edge-of-fueltemperature of itOOOK.

237

TCI-3J2K

f™£2L HP«urt>he,

T-108JK

Fig. 7 Helium closed-cycle gas turbine systemwith plasma core reactor heat source.

Cycleafficltncy

0.2

0.1

\-250D K,

..1400 K, coolednickel-base alloy

-1100K, uncoolednickel-base alloy

o l • 1 1 1 i I 1 I 1 11000 1400 1B00 2200 2800 3000

Turblna inlet temperature, deg K

7ig. 8 Calculated cycle efficiency for heliumclosed-cycle gas turbine system withplasma core reactor heat source.

•TrMtpmnt

Fig. 9 Concept for photochemical/thermochemicalhydrogen production using thermal radiationfrom plasma core reactor.

238

BROADBAND PHOTOEXCITATION OF LASERS

R. V. HessNASA, Langley Research Center, Hampton, VA 2366S

A.1 JavanMassachusetts Institute of Technology

Cambridge, MA 02139

P. Br"ockmanNASA, Langley Research Center, Hampton, VA 23665

Abstract

The purpose of this study is to review exist-ing techniques and to discuss novel approaches forbroadband photoexcitation of lasers. This subjectis of interest because of the emergence of future,intense near-equilibrium and blackbody sources,such as the gas-core reactor, and electric dis-charge plasma devices, which are the key to thedevelopment of intense broadband radiation sources.The possible use of radiation from solar concen-trators in space also fits into this general area.The varied uses of photoexcitation of lasers arediscussed ranging over direct optical pumping,photodissociation, and photopreionization. Inaddition, the new use of lasers for broadbandphotoexcitation of new laser transitions andchemical reactions is shown. This is accomplishedthrough multiline operation of high-pressuretunable lasers which can use the broadband radia-tion for multiphoton excitation. Acknowledgmentis made of helpful discussions with D. H. Phillips,R. S. Rogowski, and B. D. Sidney at the LangleyResearch Center, and J. Verdeyen (Illinois Univ.),D. L. Hess (Hughes Research Labs), C. Ifittig(Southern Cal. Univ.), and T. Cool (Cornell Univ.).

I. Broadband Photoexcitationwith Blackbody Sources

Photoexcitation of lasers requires that thespectral range of the radiation source overlap theabsorption lines or bands of the laser and that thepower density of the source in this range be suffi-cient for exceeding the threshold power density forlasing. For convenient analysis of these criteriaPlanck's law for the distribution of power/area/unit wavelength of blackbody radiation over variouswavelengths is plotted in Figures 1 and 2 for6000°K, 15,000°K, and 20,000°K vs wavelength.

P ential sources for broadband photoexcita-tion i the gas-core reactor which will be capableof radiating at temperatures from 6000°K to over20,000°K,electric discharges which may even oper-ate at higher temperatures and the Sun. Use ofsolar radiation requires concentration of the1.4 x lO-iW/cm2 radiation intensity in Earthorbit. The maximum concentration of radiant energyis limited by the invariant N/n2 where N is theradiance in power per unit area per steradian andn is the local index of refraction. In freespace the maximum possible concentration of black-body radiation will yield an irradiance equal tothat found at the surface of the blackbody, whichat 6000°K would be fJ7000 Watts/cm2. However, in

a high index of refraction material this concentra-tion is increased by a factor of n-. The addi-tional concentration due to the change in index ofrefraction does not change the radiation tempera-ture, since by Planck's law

3v c02 e h v / k T - 1

where c. is the velocity of light in free space.This additional concentration has been used forenhancement in photon counters behind high nmaterial. Application to high index of refractionlaser media would require special considerations.As indicated by the large concentration ratios,use of large lasing media would require large spacestructures. Use of solar concentrators for Nd:YAGlasers is discussed in references 2 and 3.

Broadband photoexcitation requires a combina-tion of broadband absorption in the lasing mediumand sufficiently low threshold power density tomatch the power density of the blackbody source inthe spectral range of the absorption region. Forexample, both the solid-state Nd:YAG laser and theliquid dye laser absorb in the visible spectrum,but the Nd:YAG laser has orders of magnitude lowerpower density threshold. A threshold of less than20 W/cm5 is quoted in reference 4 for an Nd:YAGlaser, whereas reference 5 gives a threshold of20 kW/cm3 for a dye laser. As a result, theNd:YAG laser can be excited by 6000°K blackbodysources (Fig. 1), whereas, a dye laser would re-quire a blackbody source somewhat in excess of20,000°K (Fig. 2), to yield sufficient powerdensity in the visible spectrum. Since a 20,000°Ksource has its peak power density in the near UV,such direct excitation of the dye laser isinefficient.

Broadband photoexcitation of gas lasers offerspotential advantages over solid and liquid lasersbecause material limits are less severe, and largeuniform lasing media are available. Two majorcategories of gas lasers are'discussed: photo-dissociation lasers which have chemical revers-ibility through regeneration of the initial prod-ucts, and are suited to continuous photoexcitation;and other lasers utilizing optical pumping ofmolecular vibrational states in the near infrared,which, when operated at high pressures, permitbroadband pumping because of pressure broadeningand overlapping of laser line.-.

239

II. Broadband Photodissoeiation Lasers

. NOC2 and IBr have been used to producechemically reversible photodissociation lasers.For the NQCSL system, lasing occurs on thevibrational-rotational transitions of NO at•5.95-6.30 jJtti (Ref. 6). For the IBr photo-dissociation system electronic excitation of Broccurs with lasing at-2.714 pm (Ref. 7). Earlyflashlamp experiments indicate that the thresholdfor the N0C£ system may be low enough to allowsolar pumping, however^ farther studies of NOCS.and IBr lasers are required. The photo-dissociation lasers using organic iodides CFjIand C3F7I (Ref. 8) yield higher gain and powerfor analagpus transitions of atomic iodine, I*,at 1.315 liin. These lasers have a broad absorptionband from 2500-2900 A which matches the peakintensity region of blackbody radiation at^20,000°K and could probably be excited by a gas-core reactor operating at that temperature.

Figure 3 shows a simplified diagram of anI'lictronic-vibration (E-V) energy transferhi -CO2 laser. This type of photodissociationlaser produces electronically excited Br* fromBr2 molecules which have absorption bands from thenear UV through the Visible. T h e excited Br*atoms are, however, not used directly for lasil'g,but for near resonant excitation of vibrationalnolecular states, e.g., CO2, which lases in theinfrared (Ref. 9). This type of system may bevery inportant for pumping IR lasers with solar andothev high-temperature blackbody sources, sinceabsorption occurs near the peak of the blackbodyspectrum, (Fig. 1), Several molecules can be usedin high efficiency IR lasers. .These moleculesshould be investigated/, to determine if their powerdensity .thresholds are sufficiently low to be pumpedby these; bl'ackbody sources. Communications withC. Wittig (Southern Cal.. Univ.) suggest that lowthresholds; maybe possible'. Since .the conversionis from short-to-.long wavelengths, the energytransfer efficiency is reduced by the ratio of the

, wavelengths..' . . .. ' '

• More efficient transfer could be achieved in _lasers which radiate at wavelengt% shorter thanthe CO2 laser'. One such system has been demon-strated :for KCN las ing- at 5.85 woi, (Kef. 10). Thislaser, however, requires intense, excitation tomaintain a steady-state population inversion dueto the short lifetimes of the excited HCN states.For efficient E-V photodissociation lasers,molecular systems operating at short IR wavelengthsand with long-lived excited states, such as CO,should be investigated.

Figure 4 shows a typical excimer laser;. excitners, bound excited molecular states, form bythe combining of electxonically excited atoms withnoble gas atoms. Such states have been obtainedfor noble gas-halides, noble gas-oxygen, and noblegas-alkali systems. The repulsive ground statesencourage continuous losing and reversibility.Broadband photodiss! -tiatipn can play a role, e.g.,in the (ArO) ; laser-(Ref. 11'); however, theproduction of 0* by broadband p;.olodissociationoccurs in the far i(Vi Photoexcltation, in thevisible, of noble gas-alkali excimer lasers is be-ing studied (Refs. 12 and 13). Recent communica-tions with J. Verdeyeri (llljuiois UnivO indicate

that the absorption of tunable dye Jaser radiationby cesium in high-pressure xenon gas occurs over abroader spectral range than the width of Csabsorption lines. The lasing threshold was calcu-lated to be 3.46 kW/cm^. Communications withL. D. Hess (Hughes Research Labs.) indicate studieswith flashlamp excitation which, however, did notattain the high efficiencies (up to 10%) theoreti-cally projected for excimer lasers.

III. Broadband Optical Pumping ofVibrational Molecular LaserTransitions in the Infrared

For solar radiation the radiation intensity inthe infrared is much smaller than at peak intensity(Fig. 1). The existence of many rotational-vibrational laser lines over r. wide spectral bandin the infrared combined with the broadening andoverlapping of these lines at high pressures allowsbroadband absorption of the available radiation.

For the present preliminary study, opticalpumping of thf CO molecule was studied, becauseof it> long V-T relax .tion rates, "1.9 x 10"3/sec/Torr, which facilitate buildup of steady-statepopulation inversions even at high pressures.Since the CO molecule has a small anharmonicenergy defect, the energy may shift to highervibrational levels unless it is removed. Thisremoval can take place either by cooling or byenergy transfer to other molecules. The feasibili-ty for optical pumping of tlu 0 •* 1 CO vibra-tional-rotational band with subsequent nearresonant V-V energy transfer to r.O2, OCS, NjO,C2H2 (Fifi- 5) was demonstrated in reference 14using the second harmonic of the P(24) laser lineof the 9.6 11m C02 band, which falls close to theCO 0 •*• 1, P(i4) laser transition.

For 60f)0°X blackbody radiation M Watts rercm are available over an integrated absorptionbandwidth of -x.2000 A (P«R branch) of tho5 pm 0 •+ 1 CO transition, corresponding to '"•O.2watts per cm2 per 100 8 at S pm (Fig. 1). The lineabsorption • oefficients in this region are of the

...xirder of 10/cm, thus the total power would beabsorbed over a distance of M0"^ cm, yielding a.power density of i>40 watts/cm^.

Longer penetration depths may be achieved by 'direct optical pumping of the O •* 2 CO transitionusing 2.4 iun radiation.. The absorption coeffi-cients are *fl x 10"^/cm and at 2.4 pm concentiat-ed solar radiation provides 4 Watts/cm2/100 8(Fig. 1), and 24 Watts/cu2 across 600 A integratedabsorption bandwidth..; The power density absorbedin the lasing medium will be "V.1.7 Watts per cm3.

For transfer of the v = 2, CO vibrationalenoTgy to CO2 and other molecules, V-V transferfirst occurs to the w = 1, CO state. Calcula-tions indicate that for several torr of CO2 inatmospheric CO sufficient gain should occur in CC^•to make this a potentially useful laser. The"actual power output depends cr: the exactvibrational-roTational bandwidth available for ab-sorption, fcre detailed calculations and experi-ments to determine the efficiency of energy trans- ;fer are required.

240

IV. Use of Mul*-' -j.ne Lasers for MultiphotonPumping of Lasers and Chemical Reactions

Two-photon pumped lasers have recently beendemonstrated (Rcf. 15) where simultaneous pumpingwith both P(12) and P(14) transitions of a C02

TEA laser were used to obtain lasing in SFg at15.9 pm (Fig. 6). The desirability of bettertemporal and spatial coincidence, and increasedpower of the two laser pump lines is stated and theneed for careful tuning is noted. In an article(Ref. 16) on stimulation of chemical reactions withlaser radiation it is shown that the absorption ofa large number of low-energy quanta rather than onehigh-energy quantum facilitates considerable devia-tion of resonance mode from the average temperature- and that this is very important for controlledstimulation of chemical reactions. A laser hasbeen designed and operated by A. Javan, to achieveindependent simultaneous operation of laser linesand broadband tunability (Fig. 7). The independentlaser lines are obtained by spatially separatingtheir production in a laser medium, which operatesat'low power to achieve optimum control of fre-quency and tunability. This multiline laser radia-tion passes co-linearly through another lasingmedium which i ,, forced to operate on these injectedlines, thus maintaining the high frequency stabil-ity at considerably higher powers. Such injectionlocking has boen demonstrated in the past for oneinjected laser line (e.g., Ref. 17), but is new formultiline operation. Furthermore, fci- high-pressure operation controlled continuous tuningover pre ire broadened overlapping lines could beachieved. Rapid chirping over a wide spectralrange could approach conditions of an intensebroadband nonequilibrium source, in contrast toequilibrium blackbody sources.

V. Photopreionization for High PressureLaser Plasmas

Photopreionization of high-pressure CO2electric discharge lasers and other lasers hai. beenextensively studied in. recent years to provide analternative to electron beams in the production ofuniforni laTge volume high-pressure lasers, withhighly efficient ind stai-le operation. A noveltechnique involves the addition to the lasingmedium of a small quantity of organic seed material(e.g., tripropyiamine or trimethylamine) havinglower ionization potential than the laser medium.Since the xasing media do not absorb ai. the wa\ e-ler.gth-- which preionize the seed and, since theseeddt iity is low, an increased penerration depthis achieved. This technique was originally demon-strated by A. Javan* and J. S. Levine (Ref. 18) andadditional studies have been reported in references10 and 20. Figure 8 shows a high-pressure COTphotopreiomzed laser operating at the NASA laiigleyResearci: Center. The laser designed by A. Javanuses flashrodi with open spark gaps, which avoidcontamination,buildup by the seed material such asmight occur when using flashlamps.

References

1. Nicodemus, F. E.: Radiance. American J. ofPhysics, May 1963. pp. 368-377.

2. Young, C. G.: A Sun Pumped CW One-Watt Laser.Applied Optics, vol. 5, no. 6, June 1966,pp. 993-997.

3. Falk, Joel; Huff, L.; and Taynai, J. D.: GTEEylvania: Solar-Pumped YAG, CLEA Paper 4.6,1975.

4. Ross, D.: Lasers, Light Amplifiers, andOscillators. Academic Press, Inc., New York,N.Y., 1969.

5. Bass, M.; Deutsch, T. F.; and Weber, M. J.:Dye Lasers. Chapt. 4, p. 301 in "Lasers aSeries «f Advances." Ed. Levino, A. K.; andDe Maria, A. J., vol. 5, 1071.

6. Guilliano, C. R.; and Hess, L. D.: ChemicalReversibility and Solar Excitation Rates of theNitrosyl Chloride Photodissociative Laser. J.of Appl. Physics, vol. 3S, no. 1], OCT. 1967.

7. Puilliano, C. R.; and Hess, L. D.: Reve-r.iblePhotodissociative Laser System. J. of Appl.Physics, vol. 40, ro. 6, May 1969.

8. Hohla, K.; and Kompa, K. L.: Gigawatt Photo-chemical Iodine Laser. Appl. Phys. Lett.,vol. 22, no. 2, January 15, 1973.

9. Peter.ien, A, B.; Wittig, C ; and Leone, S. R.:Electronic-to-.Vibrational Pumped CO2 Laser^rating ax 1.3, 10.6, and 14.1 um. J- of

•*T>j. Physics, vol. 47, no. 3, March 1976.

id. Petei-sen, A. B.; Wittig, C ; and Leo.ie, S. R.:Infrared Molecular Lasers Pumped by 31ectronic-Vibratipnal Energy Tram.fer fromBr(42P1/2):CO2, NjO, KCN, and Cfa. J. of

/.ppl. Physics, vol. 47, no. 3, March 1976. .

11. Hughes, IV. M.: Experiments on 553-nm ArgonOxide Laser System. Appl. Phys. Lett., vol.28, no. 2, January IS, 1970.

12. Tam, A. C ; Moe, G.; Bulos, B. R.; and Happer,W.: Excimer Radiation from Na-Noble Gas andK-Noble Gas Molecules. Optics Com»unic.tions,vol. 16, no. 3, March 1976.

13. Cherrington, B. E.; Verdeyen, J. T.; Eden,J. G.; and Leslie: Studies of DischargeMechanisms in High Pressure ^ises - Applicationto High Efficiency High Pccer Lasers. Analysisof Excimer Systems by Combination o£ ExcitedCS(72S,52D) and Xe. Semi-Annual ProgressReport NASA Grant NCT 74-OCIS-2D0, Univ. of .Illinois, November 1, 1975.

14. Kildal, H.; and Deutsch: Optically Pumpedinfrared V-V Transfer [sseii. App]. Phys.Lett., voi; 27, no. 9, November 1, 1975'.

IE. Barch, II. E.; and Fetterman, H. R.: OpticallyPumped 15.90 um SFg Laser. Optics Communica-tion.1!, November/December 1975.

16. Basov, N. U.; Oraevsky, A. M.; and Pankratov,A. V.: Stimulation of Chemical Reactions withLaser Radiation. Chemical and BiochemicalApplications of Lasers. Ed., Moore, C. B.,vol. 1, p. 211, 1974.

This woik partially supported under NASAcontract NAS1-14289,

241

17. Buczek, C ; and Freiberg, R. J.: Hybrid In-jeccion Locking of High Power CO., Lasers.IEEE J. of Quantum Electronics, vol. QE-8,.no. 7, 1972.

18. Levine, J. S.j and Javan, A.: Observation ofLaser Oscillation in a 1 atm COj-^ He LaserPumped by an Electrically Heatea PlasmaGenerated via Photoionization. Appl. Phys.Lett., vol. 22, ho. 2, January IS, 1973.

19. Judd, 0. P.; and Warda, J. Y.*. Investiga-tions of UV Preionized Electric Discharge andC02 Laser. IEEE J. of Quantum Electronics,vol._ QE-10, no. 1, January 19 74.

20. Sequin, H. J.; Tulip, J.; and McKen, D. C :Ultraviolet Photoionization in TEA Lasers.IEFE J. of Quantum Electronics, vol. QE-10,no. 3, March 1974.

4000

3500

3000

2500

ENER3Y (cm"1) 2 0 0 0

1500

1000

500

0

3.

-

-

101)

Rlpm*

4.3(1 m

^100)

-

(0211

N _(Oil)

10.6pm ^

(020)

1010)

f

i.3pm

t-« B ;

^ (0011

E-V energy transfer laser

6000°K

Wcm?/5000A '

40000 W000

Blackbody rfidiation distribution at6000°K '

ENERGY

BOUND EXCIHD STftrt

NOBLE GAS

- LASER TRANSITIONS

HALIOFSOXYGENALKALIS

/ - REPtllSiV1. GROUND STATE

Figure 4. Typical excimer laser

• c

, 20000°K

2000 .4000 60O! 8000 10000

Figure 2. Blackbody radiation distribution up to

20,000'K

2000

1500

ENERGY (cm'1)

1000

V-l

DOUBLED

co2- PUMP

9.6um B»*;o

5 0 O - ! .

LASER .

ILASER

•ML

•iofo

00°l

oiooV

LASER .8|im

OOttfl1

LASER

to ocs VFigure S. Molecules, pumped by energy transfer

from CO

242

SPATIALLY SEPARATED BEAMS OF DIFFERING

P(12)

- P(14)10.6Lim

WAVELENGTH SELECTION

GRATING

Figure 7.

Figure 6. Two-photon pumping for SFft laserIndependently controllableraultiwavelength laser

Figure 8. High-pressure photo-preionized CO, laser

DISCUSSION

F. HOHL: What sort o£ intensities do you need topump an excliner laser?

R. V. HESS: It is very unlikely that one can pumpexcimer lasers by this inetbad. Verdeyen calculateda threshold of 3.4 kilowatt per fubic ce for noblegas-alkali pumping which is too high. That wouldneed flashlasp pumping. But some of the otherlasers we mentioned - the photo dissociationlasers, N0C1, Iodine bromide and also the brominemolecule that has an e.v. transition, and alsothe CO which has long lifetimes and therefore lowthresholds are potentials for pumping by thismethod.

K. THOM: What do you think about the possibilityof using solar pumping for such lasers?

R. V. HESS: The Nd-Yag laser is being solarpumped but that is a solid state laser, beingconsidered for communications. The best candi-date" for solar pumping seem to be N0C1, brominemolecules, IBR, and potentially CO because ci:isdisplays broadband excitation and has a longlifetime and a relatively low threshold. We are

looking more at there possibilities right now.

24J

STATUS OF PHOTOELECTROCHEMICAL PRODUCTION OF HYDROGENAND ELECTRICAL ENERGY

Charles E. Byvik and Gilbert H. WalkerNASA Langley Research Center

:V-->nton, Virginia

Abstract

The efficiency for conversion of electromag-netic energy to chemical and electrical energyutilizing semiconductor single crystals as photoai.odes in electrochemical cells has been investi-gated. Efficiencies as high as 20 percent havebeen achieved for the conversion of 330 nm radia-tion to chemical energy in the form of hydrogen bythe photoelectrolysis of water in a SrTiO3 basedcell. The SrTiO3 photoanodes have been shown tobe stable in 9.5 M NaOH solutions for periods upto 48 hours. Efficiencies of 9 percent have beenmeasured for the conversion of broadband visibleradiation to hydrogen using n-type GaAs crystalsas photoanodes. Crystals of GaAs coated with500 nm' of gold, silver, or tin for surface passi-vation showed no significant change in efficiency.By suppressing the production of hydrogen in aCdSe-based photogalvanic cell, an efficiency of9 percent has been obtained n conversion of633 nm light to electrical energy. A CdS-basedphotogalvanic cell produced s. conversion effi-ciency of 5 percent for 500 nm radiation.

I. Introduction

The utilization of broad band radiation from afissioning plasma or of narrow band laser radia-tion in space! requires transformation, into otherenergy forms. Ideally, this energy transformationshould be performed by a device that is (a) effi-cient, (b) sensitive over a broad wavelength inter-val, (c) stable both thermally and chemically, and(d) intensity efficient (especially for the case ofhigh photon energy densities characteristic oflasers and plasma reactors). Recent researchhas indicated that semiconductors used as photo-anodes in an electrochemical cell with an aqueouselectrolyte can be used to convert photon energyto both chemical energy2-8 (in the form of mole-cular oxygen ana molecular hydrogen) and electri-cal energy.g>10 . The purpose of this report is to"present tiie status of recent research in the elec-trolysis of water and the production of electricalenergy by photoelectrolytic cells and photogalvaniccells where semiconductors have been used as

. photoelectrodes.

H. Background

Electrochemical cells a. ±n wide use today asbatteries, fuel cells, catalytic reactors and manyother applications. The photoelectrolytic andphotogalvanic ce)ls with semiconductor electrodesare of particular interest at NASA-LaiigleyResearch Center because they are promising tech-niques for converting photon energy to chemicaland electrical energy.

Early work with semiconductors used as photo-electrodes, in electrochemical cells indicated thatelectrolysis of water could be achieved and elec-trical energy could bo produced at the expense ofthe semiconductor photoelectrode. V- was foundthat photon energies greater than the bandgap of

the semiconductor electrode could enhance the dis-solution of the semiconductor.

Fujishima and Honda3 reported results of atitanium dioxide-based photoelectrolytic cell whichindicated that the TK>2 photoanode might be stable.The photocell used by them is shown schematicallyin figure 1.

C T T ^ / POWER SUPPLY \ s

Fig. I Schematic of Photoelectrolytic Cell.

The overall reaction proposed to be taking place,in the cell when light having energy greater than thebandgap of TiO2 (3.0 eV) was

>—1/2O2+H2 (1)

where the photon energy is converted to chemicalenergy stored as the free energy of hydrogen(68.3 kcal/mole).

A simple extension of this photon assisted elec-trolysis of water in a semiconductor-based electro-chemical cell is to suppress the hydrogen andoxygen production by introducing into the electro-lyte a suitable redox couple (an ionic couple whichis oxidized at ons electrode and reduced at theother without changing the composition of the elec-trolyte) and utilizing the electrical energy producedby placing a load in the external circuit. This isthe principle of a photogalvanic cell.

The results reported for the TiO2-based photo-chemical cell indicated that further research wasneeded to (1) establish the nature of the photoninduced processes at the semiconductor-electrolyteinterface; (2) determine the energy conversionefficiency of the semiconductor-based electro-chemical cells; and (3) explore the use of other

244

semiconductors as photoelectrodes in both photo-galvanic and photoelectrolytic cells. The resultsof the NASA sponsored work in this area is thesubject of this paper.

m. Results

Photoelectrolysis Cells

A detailed study of the TiO2-based photoelec-trolysis cell was made by Wrighton, et al.5 Theirwork concluded that, for a cell with a single elec-trolyte and UV radiation (351 nm, 364 nm):(1) oxygen is evolved at the photoanode (TiO2) andhydrogen at the cathode (Ft) in the stochiometricratio of two to one in agreement with equation (1);(2) a minimum external voltage of 0.25 volts isnecessary for the photoelectrolysis to take place;(3) the photoelectrolysis reaction does not producedissolution of the TiO2 anode and is therefore acatalytic reaction; and (4) optical energy storageefficiency of the order of 1 percent can beachieved by the production of hydrogen.

This research was followed by studies of othersemiconductors used as photoanodes in photoelec-trolytic cells.. Table I gives a listing of the semi-conductors used as photoanodes in the work to date,as well as their efficiencies, the photon sourcesused, and the need for external power supplies.

Titanium dioxide, antamony doped tin oxide, thetentalates and strontium titariate have been shownto be photochemically stable as photoanodes inelectrochemical cells. It should be noted that thesemiconductor photoanodes TiO2, Sb-SnO2, and thetantalates need external power to electrolyse waterbut the strontium titanate- and the galliumarsenide-based cells do not. Maximum efficiencyfor the SrTiO3 -based cell is achieved when

external power is added. This maximum conver-sion efficiency is measured to be 20 percent for330 nm photons. The energy storage efficiency forthe SrTiO3 cell as well as the others cited in thissection is determined by the formulr

Efficiency = Enthalpy (H2) - Energy (Power Supply)Incident photon energy-

Efficiencies lower than 20 percent result when thepower supply is eliminated in the SrTi03-basedcell. .

Experiments with gallium arsenide crystals asphotoanodes have shown that electrolysis pioceedswith efficiencies as high as 9 percent when a broad-band light source is focused on the crystal.2 in anattempt to passivate the surface of the GaAs crys-tals, a sample with an efficiency of 6 percent tobroadband radiation was coated with a gold layerapproximately 5 nm thick. The efficiency of thecoated sample was not significantly different fromthe uncoated sample. Similar coatings of tin andsilver indicated no degradation of efficiency.

The spectral response curve for a GaAs photo-electrode in a photoelectrolytic cell is shown infigure 2. This cell has very broad responsethrough the visible and into the near infra-red. Thelong wavelength cut-off corresponds to the bandgapof GaAs (1.43 eV or 867 nm). This response canbe contrasted to the response of the other semi-conductor-based cells listed in Table I which aresensitive to light in the UV only. Gallium Arsenideis, therefore, a good candidate photoanode materialfor sources such as the sun and other visible lightsources.

TABLE I. SEMICONDUCTOR PHOTOANODES USED INPHOTOELECTROLYSIS CELLS

PHOTO-ANODE

TiO2

Sb-SnO2

KTaO3

SrTiO3

GaAs

AuGaAs-Ag

Sn

CHEMICAL STORAGEEFFICIENCY

1%

1 %

6%1 4%

20%

9%

6%

PHOTO SOURCE

ARGON IONLASER (UV)

WATER FILTEREDXe-LAMP

300 nm LIGHT

WATER FILTERtDHg-SOURCE (330 nm)

TUNGSTEN FILAMENTLAMP

TUNGSTEN FILAMENTLAMP

EXTERNAL POWER

YES

YES

YES

YESNO

NO

NO

245

POWERSUPPLY

10

.5 .6 .7 .8 .9WAVELENGTH (\xm)

Fig. 2 Spectral response of GaAs-basedphotoelectrolytic ceil.

Photogalvanic Cells

As was mentioned previously, the addition of aredox couple into the electrolyte can lead to thesuppression of the photon assisted evolution ofhydrogen and oxygen at the electrodes in the semi-conductor-based photoelectrolysis cells. In the

- ideal case, only electrical energy is producedwhich may be utilized by connecting an electricalload in the external circuit. This cell is called aphotogalvanic cell or an electrochemical photocell.

A recent paper by Gerischer9 suggested that thebandgaps of CdS and CdSe lie in an optimum range(1.1 eV to 2.1 eV) for use as electrodes in solarenergy converters. The principle limitations tothe use of these semiconductors as photoanodes isthe problem of photbdissolution. Recently, how-ever, Ellis, Kaiser, and Wrightonl° reported thatthe addition of Na2S and sulfur to a sodium hydrox-ide-aqueous electrolyte suppresses the evolution ofhydrogen at the platinum cathode and quenches thephotodecomposition of both the CdS and CdSe elec-trodes. A schematic of their cell is shown infigure 3. They report an energy conversion effi-ciency of 5 percent, for the CdS-based cell at500 nm and 9 percent for the CdSe-based cell at633 nm.

LOAD

hvh?h|/\A/V

r .-TREDOXTT: ELECTROLYTEM<r^/-

-PHOTO-ANODEFig. 3 Schematic of the photocell used in ref. 10.

IV. Conclusions

Titanium dioxide, strontium titanate, antimonydoped tin oxide, and potassium tantalate have beenshown to be chemically stable as photoanodes inphotoelectrolysis cells. Of these, only SrTiC>3 h a s

been shown to photodissociate water without theaddition of electrical energy, but 20 percent energyconversion efficiencies for 330 nm light have beenachieved with the addition of energy from a powersupply. These semiconductor photoanodes areeffective energy converters for photons in theultraviolet. Gallium arsenide photoanodes havebeen shown to have a broad spectral response inthe visible to near-IR and experiments with coat-ings of Au, Ag, and Sn suggests that the passiva-tion of the surfaces of GaAs crystals can beachieved without degrading their energy conversionefficiency. Like SrTiC-3, GaAs-based photoelec-trolysis cells dissociate water without the need ol •additional external energy.

Cadmium sulfide and cadmium selenide crystalsused as anodes in photogalvanic cells have beenshown to be photochemicaUy stable with the addi-tion of Na2S and sulfur to aqueous solution ofNaOH. Energy conversion efficiencies as high as9 percent for 633 nm light have been measured inthe CdSe-based photogalvanic cells.

A few semiconductors used as photoanodes inboth photoelectrolytic cells and photogalvanic cellshave been shown to be chemically stable and effi-cient for the conversion of electromagnetic radia-tion to chemical and electrical energy. Furtherresearch is necessary to determine the ultimateefficiencies of these two techniques using semi-conductors as photoelectrodes as well as theeffects of intensity of the incident photon energieson their performance.

246

V. References

1. Schneider, Richard T.; and Thorn, Karlheinz:Fissioning Uranium Plasmas and Nuclear-Pumped Lasers. Nuclear Technology,vol. 27, 1975, pp. 34-50.

2. Byvik, Charles E.; and Walker, Gilbert H.:Photoelectrolytic Production of HydrogenUsing Semiconductor Electrodes. NASATM X-72830, 1976.

3. Fujishima. A.: and Honda, K.: ElectrochemicalPhotolysis of Water at a SemiconductorElectrode. Nature, vol. 238, 1972, pp. 37-38.

4. Fujishima, A.; Kohayakawa, K.: and Honda, K.:Hydrogen Production Under Sunlight with anElectrochemical Photocell. J. Electrochem.Soc, Electrochem. Science and Technology,vol. 122, 1975, pp. 1487-1489.

5. Wrighton, M. S.; Ginley, D. S.; EUis, A. B.;Morse, D. L.; and Lmz,.A.: PhotoassistedElectrolysis r' Water by Irradiation of aTitanium Dioxide Electrode. Proc. Nat.Acad. Sci. U.S., vol. 72,1975, pp. 1518-1522.

6. Wrighton, M. S.; Morse, D. L.; Ellis, A. B.;Ginley, D. S.; and Abrahamson, H. B,:Photoassisted Electrolysis of Water byIrradiation of an Antimony Doped StannicOxide Electrode. J. Amer. Chem. Soc,vol. 98, 1975, pp. 44-48.

7. Ellis, A. B.; Kaiser, S. W.; and Wrighton. M.S.: SemirDnducting Potassium TantalateElectrodes: Photoassistance Agents for theEfficient Electrolysis of Water. To bepublished.

8. Wrighton, M. S.; Ellis, A. B.; Wolczanski, P.T.; Morse. D. L.; Abrahamson, H. B.; andGinley, D/S.: Strontium Titanate Photo-electrodes: Efficient Photoassisted Elec-trolysis of Water at Zero Applied Potential.J. Amer. Chem. Soc, vol. 98, 1976, pp. 0000.

9. Gerischer, H.: Electrochemical Photo andSolar Cell Principles and Some Experiments.Electroanalytical Chem. and InterfacialElectrochem., vol. 58, 1975, pp. 263-274.

10. Ellis, A. B.: Kaiser, S. W.; and Wrighton, M.Sc: Visible Light to Electrical Energy Con-version. Stable Cadmium Sulfide andCadmium Selenide Photoelectrodes in AqueousElectrolytes. J. Amer. Chem. Soc., vol. 98,1976, pp. 1635-1637.

F. C. SCHWENK: Mr. Tuck from Bell Labs who hasbeen working with Indium Phosphide, says It hasabout the same kind of response as gallumarsenide, but i t has a lot more stabi l i ty thangalium arsenide: further, he was able to processIt or heat i t to fairly high temperatures withouthaving i t break down. So i t might be an interestinglead to look into, and i t is less expensive thangalium arsenide.

J . H . LEE: You have a problem with corrosion ofthe anodes. Does the anode have to be Immersedin water or can It be external to the electrolyte?

C. E. 5YVIK: The only thing you have to have isan electrical conduction path between the anodeand cathode, whether i t be an aqueous electro-lyte or solid electrolyte, i t would work. Aslong as there i s e lectrical conductivity betweenthe two electrodes, the circuit would work.

DISCUSSION

ANON: Have you found that surface metal films areeffective over a long period of time or 3hbrt time?

C. BYVIK: This is a short duration experiment. Weare looking into a l i t t l e more detail now on thismetal film idea at Langlsy. .

F. C. SCHWENK: Have you done any analysis as towhat levels of efficiency might be expected - forsay a semiconductor like galium arsenide?

C. BYVIK: We haven't made those estimates. Theliterature points out the ultimate efficiency maybe about 28%, but we haven't checked that. Thatwas a guess made without any analysis.

247

OF. BREEDER REACTOR POWER PLANTS FOR ELECTRIC POWER GENERATIONo : •

J. H. Rust and J. D. ClementGeorgia Institute of Technology

Atlanta,. Georgia

F. HohlNational Aeronautics and Space Administration

"Langley, Virginia

Abstract

The concept of allfj fueled gas core breederreactor (GCBR) is attractive f"-r electric powergeneration. Studies indicate that UF& fueled reac-tors can be quite versatile with respect to power,pressure, operating temperatvre, and modes of powerextraction. Possible cycles include Brayton cycles,Rankine cycles, MHD generators, and thermionic di- -odes. Another potential application of the gascore reactor is its use for nuclear waste disposalby nuclear transmutation.

The reactor concept analyzed is a OTV coresurrounded by a molten salt (Ll'F, BeF2, ThF4)blanket. Nuclear survey calculations were carriedout for both spherical and cylindrical geometries.A maximum breeding ratio of 1.22 was found. Fur-ther neutronic calculations were made to assessthe effect on critical mass, breeding ratio, andspectrum of substituting a moderator, Be or C, forpart of the molten salt in the blanket.

Therraodynamic cycle calculations were performedfor a variety of Rankine. cycles. Optimization ofG Rankine cycle for a gas core breeder reactor em-ploying an intermediate heat exchanger gave a max-imum efficiency of 37%. .

A conceptual design is presented along with, asystem layout for a 1000 MW stationary power plant.The advantages of the GCBR are as follows:' (1)high efficiency, (2) simplified on-line reprocess-ing, (3j inherent safety considerations, (4) highbreeding ratio, (5) possibility of burning all ormost of the long-lived nuclear waste actinides,and (6) possibility of extrapolating the technologyto higher temperatures and MHD direct conversion.

I. Introduction

For about more than a decade, NASA has supportedresearch on gas core reactors which consisted ofca,vity reactor criticality tests, fluid mechanicstests, investigations of uranium optical emissionsspectra, radiant heat transfer, power plant stu-dies, and related theoretical investigations.1>2>3

These studies have shown that UFg fueled reactorscan be quite versatile with respect to power, pres-sure, operating temperature, modes of power extrac-tion, and the possibility of transmuting actinidewaste products. Possible power conversion systemsinclude Brayton cycles, Rankine cycles, MHD gener-ators, and thermionic diodes. Additional researchhas shown the possibility of pumping lasers by fis-sion fragment interactions with a laser gas mixture

which leads to the possibility of power extractionIn. the form of coherent light.*

Gas core reactors have many advantages when com-pared to conventional solid fuel reactors in currentuse. Table 1 lists several advantages oE gas corereactors.

Table 1 Advantages of gas core reactors

I. Small Fuel Loadings

II. Simplified 9n-Line Fuel Reprocessing

III. Greater Safety due to Small Inventory ofFission Products

Require Less Structural MaterialIV.

V.

VI.

VII.

VIII.

Higher Breeding Ratios and Shorter DoublingTimes

Potential for Higher Neutron Fluxes WhichHakes Actinide Transmutation Practical

Operates at Higher Temperature with IncreasedPower Plant Efficiencies

Possibility of Extrapolating Technology toHigher Temperefcurea and Use MHP Direct Con-version

One of the major advantages of VFc reactors forpower generation is the simplified fuel reprocessingscheme which the gaseous fuel makes possible asshown in Fig. 1.

WASTE PRODUCTS

Fig. 1 Simplified diagram of UF& breederreactor fuel cycle.

This research was supported by NASA GrantsNSG-7068 and NSG-1168.

Part of the OFg can be extracted from the core con-tinuously and sent to a fuel reprocessing facilityfor removal of waste products. The waste productremovaf can be accomplished by fractional distilla-tion' or cold trapping. After an appropriate waitingperiod, the waste products can be reprocessed forrecovering long-lived fission products and actinidesfor return back to the reactor for transmutation to

248

short-lived isotopes or fissioning or. the actinides. NaBF* and NaF and Is quite compatible with UFg.

An additional advantage of gas core reactors isthat they do not require the core structural ma-terials that are necessary for solid fuel reactors.This lack of materials which undergo parasitic neu-tron capture enables higher breeding ratios forgas core reactors in comparison to conventional re-actors. This paper reports a design study per-formed at Georgia Tech to evaluate the merits ofgas core reactors for use as breeder reactors forelectric power generation.

II. Materials

Mat trials selected for use for the gas corebreeder reactor are listed in Table 2.

Table 2 Materials for UFg gas core breederreactor

Core: UF,(U-233)o

Blanket: Molten Salt—LiF-BeF.-ThF, (71.7-16-12.3mole%)

Structure: Modified Hastelloy-N

Secondary Coolant: NaBF4(92%) NaF(87.)

Uranium hexafluoride was chosen as the fuel becauseit exists in a gaseous state at low temperatur.es.U 3 3 was selected as the fissionable Isotope of thefuel because it enables use of the uraniira-thoriumfuel cycle which results in the direct productionof U 2 3 3 from breeding. An additional advantage isthat the U233-Th fuel cycle, does not produce asgreat a buildup of actinides as fuel cycles employ-ing U235-U238 or Pu239-U238.

Several concepts were considered for the reactorblanket material. It was throught that a fluidblanket would be desirable so as to capitalize onthe continuous reprocessing which is possible withth' fluid fuel. The best material for use as theblanket is a molten salt similar to the type em-ployed by the molten salt breeder reactor. Thissalt has a composition of LiF-BeF2-ThF4 which has amelting point of 500°C, has a low vapor pressure atthe operating temperature, and is stable in theproposed range of application (540-970=C). Inorder to reduce parasitic neutron capture in lith-ium, the lithium is enriched to 99.995% in Li7.

A modified Hastelloy-N was selected for thecore liner, reactor pressure vessel, and primarypiping. This material was developed for the moltensalt breeder program and is qu.'.te compatible withthe blanket salt and UFg over operating tempera-tures less than 900°C. Modified Hastelloy-N isvery similar in composition and other relatedphysical properties to standard Ilastelloy-N; how-ever, the modified version is superior because ofits ability to resist helium embritclement underneutron irradiation.

It was thought that it would be undesirable forUFg to interface with water which will be used asthe working fluid for the power conversion cycle.Consequently, an intermediate coolant was selectedfor exchanging heat with the UFg. This intermedi-ate coolant is a molten salt which is composed of

III. Nuclear Analysis

Nuclear calculations were performed using theMACH-I one-dimensional, diffusion code5 and theTHEHMOS transport code. MACH-I employs 26 energygroups with the thermal neutron energy being 0.025eV. Because of the high temperatures of thg UFgand the blanket, it was thought that more accuratecalculations could be performed by using THERMOSto supply thermal neutron cross sections.

The MACH-I code was used to calculate breedingratios, critical masses, and reactor dimensions fora variety of reactor concepts. The lithium andberyllium contained in the blanket salt will actas a moderator for slowing down fission neutronsfrom the core. It was thought that additional mod-eration might be desirable and, consequently, car-bon and beryllium were added in varying amounts tothe blanket to evaluate the effects upon reactorparameters. Tables 3 and 4 show calculated breedingratios, critical masses, and reactor dimensions forvarious percentages of carbon or beryllium in theblanket. As shown, additional moderation does in-crease breeding ratios and it appears that maximumbreeding ratios occur when the blanket volume isabout 25% carbon or beryllium. Additional studiesshowed that blanket thicknesses of 100 cm or greaterbehaved as though the blanket was of infinite thick-

It is recognized that gas core reactors will un-doubtcily be built in a cylindrical geometry.Since MACH-I Is a one-dimensional code it was neces-sary to perform the survey calculations with aspherical reactor. In order to assess the effectsof analyzing two-dimensional reactors with a one-dimensional diffusion code, some of the nuclear cal-culations were repeated using the EXTERMINATOR?diffusion code which is capable of doing calcula-tions in an r-z geometry. The core capacity of theGeorgia Tech CYBER-74 computer would not allow per-forming EXTERMINATOR calculations with more than 4-energy groups. Since the MACH-I calculations wereperformed with 26-energy groups, it was deemed de-sirable to collapse the 26-energy groups used inMACH-I down to 4-energy groups and repeat the MACH-Icalculations. This enabled comparing the effectsof using 4- or 26-energy groups for calculatingbreeding ratios, reactor dimensions, and criticalmasses. Table 5 illustrates the results of thesecalculations and, as seen, there are insignificantdifferences in using 4- or 26-energy groups withthe MACH-I code. Therefore, it may be concludedthat computations using 4-energy groups with theEXTERMINATOR code should yield valid results.

Table 5 also shows the results of the 4-energygroup EXTERMINATOR calculations for a cylindricalreactor with a core height equal to the core diam-eter. As seen, the breeding ratio is slightlyhigher by going from a spherical geometry to acylindrical geometry. This is to be expected be-cause of the increased neutron leakage from acylindrical core because of the increased surface-to-volume ratio of a cylinder compared to a sphere.

249

Percent of Carbonin Blanket

Breeding Ratio

Critical Radius

Critical Mass(kg U-233)

0

1

58

379

.183

.6

Table 3 Critical parameters vs Volume percent of carbon in blanket(blanket thickness =114 cm)

25 50 75 100

1.196 1.190 1.133 0

60-* 62.6 61.4 39.2

386 463 436 114

Table 4 Critical parameters vs volume percent of beryllium in blanket(blanket thickness = 114 cm)

Percent of Be in 0 25 50 75 100blanket

Breeding Ratio 1.183 1.223 1.203 1.065 0

Critical Radius 58.6 61.8 61.1 53.4 29.8(cm)

Critical Mass 379 446 431 287 50(kg U-233)

Table 5 Comparison of critical parameters

Spherical Core Spherical Core Cylindrical Core(26 group) (4 group) (4 group)

Breeding Ratio

Critical Radius(cm)

Critical CoreVolume

Critical Mass(kg U-233)

Blanket Thickness(cm)

1

58

8.4 x

379

114

.181

.6

105 cm3

1.179

60.9

9.5 x 105 cm3

426

114

1.219

54.8

1.0 x 106 cm3

496

100

250

IV. Heat Transfer

Because of high power densities in gas core re-actors, it is necessary to analyze the core heattransfer in order to assure that unacceptably hightemperatures are not achieved in the DFg. This re-quires solving the energy equation for UFg flowingthrough a cylindrical core. Equation 1 gives theenergy equation for the UF(..

wherep = density

c_ = specific heat at constant pressureU2(r,zJ = axial velocity

T = temperatureke = k + pcpSH> total conductivityen = eddy diffusivity for heat transfer

q = volumetric heat generation rate

Equation 1 is extremely complex because UFgphysical properties are highly temperature depend-ent and the volumetric heat generation term isstrongly spatially dependent due to the variableUFg density in the core and variable neutron fluxdistributions. Equation 1 was solved for two setsof boundary conditions: (Case 1)—an insulatedliner wall in which no heat crosses the wall and(Case 2 ) — an insulated liner wall until the walltemperature reaches 920°rX for the rest of the corelength. Equation 1 was solved numerically by usingfinite difference representations for the partialderivatives and incorporating a MACH-I power dis-tribution computation for the volumetric heat gen-eration term. A marching technique was employedwhich required iteration at each axial step in orderto incorporate the temperature dependence of theUFg physical properties. Reference 8 gives a de-tailed description of the heat transfer modelingand computational techniques.

It was estimated that 9.7% of the reactor powerwould be deposited in the blanket. Consequently,for a power level of 1000 MWth, 903 MWth would begenerated in the reactor core. The UF6 inlet tem-perature was specified as 558°K and mass flow rateat 6320 kg per second: The core geometry was aright cylinder with a 100 cm radius and 200 cmlength.

Figure 2 illustrates the radial dependence ofUFg temperatures for various axial positions forthe insulated wall boundary condition (Case 1).Temperatures reach a peak at the wall because thevolumetric heat generation term is a maximum at thewall and, In particular, the fluid velocity at thewall is zero which means heat is transferred atthat location only through conduction. Figure 3illustrates core liner wall Cer.-eratures and UFgfuel temperatures at the core axis as a function ofcore length. As shown by the calculations, after50 cm down the channel length the liner wall tem-peratures exceed 920°K, which is considered unac-ceptably high.

Figure 4 illustrates the radial dependence ofUF6 temperatures for various axial locations forthe boundary conditions that liner wall temperaturesnot exceed 920°K (Case 2) . The maximum UF, temper-ature occurs at the core exit and is 1220°K, whichis far below temperatures required for substantial

ov, ionization. Figure 5 illustrates core linerwall temperatures and UF, temperatures at the core1 axi6 as a function of core length.

t

Radial Distance (cm)

Fig. 2 Temperature vs radial distance (Case 1)

Axial Distance (cm)

Fig. 3 Wall temperature vs axial distance (Case 1)

' 60 m20 en

Fig. 4

Radial Distance (cm)

Temperature vs radial distance (Case 2)

251

MlLtOWEPAT <8}°K

Axial Distance (cm)

Fig. 5 Wall temperature vs axial distance (Case 2)

The boundary condition that the liner wall tem-perature not exceed 92O°K requires wall coolingafter about 40 cm down the core length. Conse-quently, it is necessary to examine wall heat fluxesin ordec to determine the extent of the wall cool-ing. Figure 6 illustrates calculated liner wallheat fljxes as a function of channel length. Themaximum heat flux occurs at the channel exit andhas a value of 0.205 watt per square centimeterwhich 1;> a small heat flux for which it will beeasy to provide wall cooling.

nozzles for flow of UFg into and out of the core.Figure 7 illustrates the gas core reactor design.The reactor is a right cylinder with ellipsoidalheads and height equal to the diameter. It iseasily fabricated and a good geometry to work withfrom both a practical and a calculational point ofview.

UFtINLET

UF6INLET

UF6

OUTLET

UF6OUTLET

Axial Distance (cm)

Fig. 6 Hall heat flux vs axial distance(Case 2)

V, System Analysis

It was thought that.it would be desirable for theflow through the reactor core to be at a uniformvelocity so as to simplify calculations and maximizereactor performance. In order to obtain an approxi-mate uniform veiccity distribution in the core Itis necessary to employ numerous inlet and outlet

Fig. 7- Reactor configuration

The blanke.t will be pressurized to the samepressure as the core (on the order of 100 bars).The core liner is designed to withstand a pressuredifference of only 15 bars. The outside pressurevessel will need to be capable of containing the100 bar pressure plus a 20% safety ma-gin, or 120bar total. These pressures are not extreme and canbe easily accommodated. The reactor pressure ves-sel was designed according to specifications fromthe ASME Boiler and Pressure Vessel Code; SectionIll—Rules for Construction of Nuclear Power PlantComponents.' The core liner was selected at athicknee of 1.3 cm which is adequate for sustain-ing a 15 bar pressure difference at the reactoroperating temperature for a 30-year lifetime. Incase of a rapid depresscrization of the blanket,the core liner can withstand a pressure differenceof approximately 90 bars for a period of 6 minutes.

Many schemes were examined for energy conversionwith gas core breeder reactors. The UF, can beused as a working fluid for either 3i-ayton or Ran-kine cycles. However, in order to have reasonableefficiencies, a regenerator is necessary for eithercycle. High efficiencies can be achieved using UFgin Rankine cycles for the operating temperaturesselected for this study. For turbine inlet temper-atures of 850°K -nd pressures of the order of 100bars, Rankine cycie efficiencies will exceed 41%.

In order to reduce the inventory of UFg in the

252

power plant system, it is desirable to employ an*other fluid as the working fluid in the energy con-version device. Because of the adverse chemicalreaction of UFg with water, in the event of a rup-ture of a boiler tube, it is not advisable for UFgto exchange heat directly with water in a boiler.Consequently, a molten salt NaBF^-NaF was selectedas an intermediate coolant for transferring heatfrom UFg to water in a boiler. Ihe power plantschematic is shown in Fig. 8. The molten salt hasheat transfer characteristics similar to those ofwater and an additional desirable feature is thatit contains boron which is a control material usedin conventional reactors and would thus prevent thepossibility of critical!ty inside the intermediateheat exchanger.

I GATE VALVES•KWUAU.V OPEN

i8.F. HJMP

©CONDENSER

FEEOWATER HEATERS

Fig. 8 UTg breeder power plant systemschematic

The intermediate salt will be used to producesuperheated steam at a temperature of 460°C and apressure of 100 bars. The steam will be passedthrough high pressure and low pressure turbines forenergy extraction. Three feedwater heaters areemployed for extracting moisture from the turbinesand heating the feedwater before it enters theboiler. By extracting steam at optimum pressuresfor each feedwatef heater stage, the overall cycleefficiency will be 377..

VI. Conclusions

The design study has shown that it is possibleto construct a gas core breeder reactor with a highbreeding ratio, of the order of 1.2 or higher, andan overall efficiency of 37%. The plant will notrequire excessive temperatures or pressures andwill use much of the technoJogy already developedfor the molten salt breeder reactor program.

References

1. Thorn, K., Schneider, R. T., and F. C. Schwenfc,"Physics and Potentials of Fissioning Plasmasfor Space Power and Propulsion," Paper No. 74-087, International Ast^-onautical Federation 25thCongress, Amsterdam (October, 1974)

2. Williams, J. R., Clement, J. D., and J. H. Rust,"Analysis of UF& Breeder Reactor Power Plants,"Progress Report No. 1, NASA Grant NSG-7067,

Georgia Institute of Technology, Atlanta,Georgia (November, 1974)

3. Walters, R. A., and J. R. Williams, 'High Effi-ciency Laser, MHD and Turbine Power Extractionfrom a Gas Core Nuclear Reactor," Proc. South-eastern IESE Coaf.. Orlando, Florida (April,1974)

4. Schneider, R. T., "Direct Conversion of NuclearEnergy into laser Light," The 1976 IEEE Inter-national Conference on Plasma Science, Austin,Texas (May 24-26, 1976)

5. Meneley, D. A., et al., 'MACH-I, A One-Dimen-sional Diffusion Theory Package," ANL-722J (1966)

6. Toppel, B. J., and I. Baksys, "The Argonne-Revised THERMOS Code," ASL-7023 (1965)

7. Fowler, T. B., Tobias, M. L., and D. R. Nondy,"EXTERMINATOR-2; A Fortran IV Code for SolvingMultigroup Neutron Diffusion Equations in TwoDimensions," ORNL-4078 (April, 1967)

8. Clement, J. D., and J. H. Rust, "Analysis of UFgBreeder Reactor Tjwer Plants," Final Report,NASA Grant SSG-1168, Georgia Institute of Tech-nology, Atlanta, Georgia (February, 1976)

9. "ASME Boiler and Pressure Vessel Code, SectionIII—Rules for Construction of Nuclear PowerPlant. Components," American Society of Mechani-cal Engineers, New York (July, 1971)

253

station. The axial c-oniPnT-ient of the ye-loqity measured in this.v'ay at' a functionof radial position is shown in Pig. 5.For both currents, the profiles are flatwithin experi nisntal error out to 10 cm.Beyond this radius, the ion saturationcurrent drawn by the probe drops rapidlyto less than 10?S of its centerline value,making velocity measurements impossible.

The observed axial and radial profilesthus indicate that a single measurement ofcenterline velocity 25 cm downstream ofthe- anode should give a reasonable indica-tion of trends in the average exhaustvelocity, although any leakage of flowoutside of the measureable 10 cm radius atlower velocities will clearly reduce thelatter values somewhat.

Before presenting the results of thevelocity vs current survey, it should benoted that the average exhaust velocityfrom an MPD discharge can also be ex-pressed as the ratio of the total thrustto the propcllant mass flow rate. Thethrust is actually made up of two compon-ents, an electromagnetic component, asdiscussed in the introduction, and anelectrothermal component derived from thearc chamber pressure. This electrothermalcomponent was measured by Cory and foundto be proportional to tlie current to the1.5 power:t8'

Tefc = cj1.5 (4)

where c is an experimentally determinedcoefficient equal to 5.3 x 10~b N-A"1-5

for argon flow of 6 g/sec. Thusj thisaverage exhaust velocity may be expressedin two terms:

(5)

Average exhaust velocities calculated fromthis expression, and also from only theelectromagnetic term are both shown inFigs. 6 and 7 along with the experimen-tally determined exhaust velocities forthe ?.5 cm and for the 10 cm cathodes,re spectively.

The data for both cathodes lie abovethe calculated values, reflecting theexpected profile loss from outer flowleakage. As the current is increased,however, the data and the calculationsconverge, implying a significant decreasein this loss. Indeed, ion saturationcurrents measured with the time-of-flightvelocity probe for the 2.5 cm cathodeindicate that as the current increasesfrom 8 to 25 kA, the outer flow leakagedrops to a.small'fraction of the totalmass flow. For a current of 15.3 kA,Boyle(5) performed a more detailed experi-mental analysis of the total exhaust ve-locity profile and after accounting forthe profile losses, concluded that a goodaverage value of the full experimentalprofile is provided by the velocity cal-culated from Eq. 5. .To this extent, then.

the discrepancy between the data and thecalculated curve;! of Fiys. 6 ancJ 7 simplyreflects the difference between cent lineand average exhaust velocities of the flow.

Hote also that while the velocity datafor the 10. cm cathode rises CTOOt'nly withcurrtnt and gradually meets the calculatedaverage velocity line, the data for the2.5 O H cathode display a discernible changein slope at the known onset current, J* of16 kA. This fall-off beyond J* is believedrelated' to a spurious, increase in massflow rate due to ablation, which, for giventhrust lowers the average exhaust velocityfrom its pure argon value.I9' Since J*for the 10 cm cathode lies virtually atthe end of the data line (23 kA), a similarchange in slope is not discernible in thiscase.

Additional evidence for increased abla-tion at the onset current can be seen inthe voltage-current characteristics forthe two cathodes, as shown in Figs. 8 and9. Here, ths extrapolated value of theterminal voltage for zero current is takento represent the electrode fall voltageand has been subtracted from the terminalvoltage data shown in the figures. Thisquantity, the terminal voltage less theelectrode fall, represents the potentialdrop over the discharge plasma body andis made up of contributions from ohmiclosses, Lorentz terms and others. For anMPD thruster operating in an ideal electro-magnetic mode, the Lorentz terms shoulddominate the plasma potential drop andscale with the exhaust velocity times themagnetic field. The self-magretic field,in turn, scales with the current, and theexhaust velocity with the current squared,so that this component of plasma potentialwill be cubieally dependent on the current.For a thruster operating in a pure electro-thermal mode, in contrast, ohmic lossesshould dominate the plasma potential drop,which would then scale linearly withcurrent. Any real thruster has both elec-tromagnetic and electrothermal componentsto its thrust, so that the plasma potentialdrop should scale somewhere between alinear and a cubic dependence on current,according to the dominant process in anyregion. The data in Figs. 8 and 9 showthis is indeed the case, with electromag-netic processes clearly favored over mostof the regime studied.

The additional evidence for ablationbeyond J* appears in the discerniblechange of slope for the 2.5 cm cathodejust above, its onset current of 16 kA.Since the exhaust velocity is inverselyproportional to the total mass flow ratefor a given thrust, as ablation increases,the velocity should decrease. Thisdecrease will be reflected in a drop inthe Lorentz voltage term and hence, in theterminal voltage. The decay in the(V-Vo) ys J dependence beyond J* in Fig. 8thus correlates well with the similar'decay in the u vs J slope in Fig. 6.

254

GEORGIA INSTITUTE OF TECHNOLOGY RESEARCH ON THE GAS CORE ACTINlDETRANSMUTATION REAGTOR (GCATR)*

J. D, Clement, j. H. Rust, and A. SchneiderGeorgia Institute of Techuology

Atlanta, Georgia 30332

and

• F. HohlNational Aeronautics and Space Administration

Langley, Virginie 23665

The Gas Core Actinide Transmutation Reactor(GCATR> offers several advantages including (1) thegaseous state of the fuel nay reduce problems ofprocessing and recycling fuel and wastes, (2) highneutron fluxes are achievable, (3) the possibilityof using a molten salt in the blanket may also sim-plify the reprocessing problem and permit breeding,(4) the spectrum can be varied from fast to thermalby increasing the moderation in the blanket so thatthe trade-off of critical mass versus actinide andfission product burnup can be studied for optimiza-tion, and (5) the U233-Th eycle, which can be used,appears superior to the U '-Fu cycle in regard toactinide burnup.

The program at Georgia Tech is a study of diefeasibility, design, and optimization of the GCATR.The program is designed to take advantage of ini-tial results and to continue work carried out byGeorgia Tech on the Gas Core Breeder Reactor underNASA Grant-1168. In addition, Che program willcomplement NASA's program of developing UFg-fueledcavity reactors for power, nuclear pumped lasers,and other advanced technology applications.

The program comprises:

(lj General Studies — Parametric survey calcu-lations will be performed to examine the effect ofreactor sp 'Ctrum and flux level on the actinidetransmutation for GCATR conditions. The sensitiv-ity of the results to neutron cross sections willbe assessed. Specifically, the parametric calcula-tions of the actinide transmutation will includethu mass, isotope composition, fission and capturerates, reactivity effects, and neutron activity ofthe recycled actinides.

(2) GCATR Design Studies — This task is a majorthrust of the proposed research program. Severalsubtasks are considered: optimization criteriastudies of the blanket and fuel reprocessing, theactinide Insertion and recirculation system, andthe system integration.

The total cost of the GCATR in a nuclear wastemanagement system will be evaluated and comparedto the cost of alternate strategies presently beingconsidered.

This paper presents a brief review of the back-ground of the-GCATR and ongoing research which hasjust been initiated at the Georgia Institute ofTechnology.

Research sponsored by N.A.S.A.

I. Introduction

The high level radioactive wastes generated inthe operation of nuclear power plants contain bothfission products and actinide elements produced bythe non-fission capture of fissile and fertile iso-topes . The fission products, atoms of mediumatonic weight formed by the fission of uranium orplutonium, consist mainly of short term (3C yearsor less half-life) isotopt;, including Sr 9 0 andCs 1 3 7. T c " and I 1 2 9 are long-lived fission prod-ucts. The actinide components of radioactivewastes, including isotopes of Np, Am, Cm, and Fuand others are all very toxic and most have ex-tremely long half-lives. The amount oi long-livedradioactive material expected to be produced issubstantial. Smith(1) has estimated that In 1977.150 kg of Am243, 150 kg of Am241, and 15 kg of Cm244

will be produced. The actinides cause waste man-agement difficulties at two stages in the fuelcycle. Some are carried over with the fissionproducts during fuel reprocessing, but also somedilute plutonium wastes will ap-ear from fuel manu-facturing plants. Thus at the entrance to thewaste facility are found a mixture of transuranicactinides combined with shorter-lived and tempor-arily more hazardous fission products.

The safe disposition of the radioactive wastesis one of the most pressing problems of the nuclearindustry. Any viable plan must meet the three re-quirements of

(1) technical soundness(2) reasonable economics(3) public acceptance.

II. Background

The strategies which have been suggested forhigh-level nuclear waste management encompass

(1) terrestrial disposal (geologic, seabed,ice sheet)

(2) extraterrestrial disposal, and(3) nuclear transmutation,

or some combination of these methods, such as ter-restrial burial of the short-lived fission productsand extraterrestrial disposal or nuclear inducedtransmutation of the long-lived actinides. Fapersdiscussing all of these methods were presented atthe Haste Management Symposium in December 1974."'The technical soundness of terrestrial disposal isa controversial topic, and also public acceptanceis questionable. Extraterrestrial disposal iscostly. Hence, there is increasing interest in

255

nuclear transmutation as a potential solution tothe nuclear waste disposal problems, i'igure i sum-marizes the nuclear waste mansgeraen'.. schemes whichare under consideration.

Fi£. 1 Schematic representation of schemesfor nuclear waste management

The first published suggestion for the use ofneutron-induced transmutation of fission productswas made in 1964 by Steinberg et al.,O who con-cerned themsulves only with the transmutation ofKr85, Sr90, and Cs137. Their calculations assumedthat the krypton and cesium fission product wasteshad been enriched to 90% in Kr85 and Cs137. Thiswas necessary due to the relatively small thermalneutron cross sections of these two nuclides andtheir small concentration with respect to theirstable isotopes found in spent fuel. The Sr ": anal-ysis was based on the presence of Sr9" and Sr°9

which has a half-life of 53 days. If the strontiumwastes are allowed to decay for one year beforebeing returned to the reactor, then all the Sr89

portion will decay to Y 8 9 (stable) which can bechemically separated £rom the Sr90. However, evenwith these modifications to the waste isotopJ.c com-position, Steinberg et al, indicate that a thermalneutron flux of 1 0 " n/cm -sec is required beforedie halving time of Sr90 can be reduced from thenormal half-life of 28.1 years to 1 year. A fluxof 10^7 n/cm^-see was indicated to be necessary be-fore the halving time could be reduced from thenatural half-life of 33 years to 1 year. The halv-ing time describe* the "total" time spent in areactor for the inventory of a particular isotopeto be reduced to half its value.

ft)In another work, Steinberg and Gregory con-

sidered the possibility of fission product burnup(specifically Cs1 7 and Sr90) in a spallation reac-tor facility. In this scheme a nuclear power reac-tor is used to "drive" a high-energy proton accel-erator with the resultant (p,xn) spallation reactionsof the proton beam with the target producing theextreme fluxes of lo''7 n/cm -sec necesnary for fis-fion product burnup. However, in addition toeconomic disadvantages this concept faces seriousmechanical and material design problems.

Claiborne ' * " was the lirst investigator toreport detailed calculations of actinide recyclingin light water reactors. Claiborne studied actiniderecycling in light water reactors (LWR) operatingon 3.3% uZ35-u238 fuel cycle. He concluded that itwas not practical to burn the fission products, be-cause' the neutron fluxes were too low and "develop-

ing jpecial burnt - reactors with required neutronfluxes of 10" n/em^-sec or in thermonuclear nu-clear reactor blanket 5 is beyond the limits ofcurrent technology."<••>/

For purposes of comparison, Claiborne ex-pressed radioactive waste hazards in terms of thetotal water required to dilute a nuclide or mixture

: of nuclides to its SCG (Radiation ConcentrationGuide Value, also known as the Maximum PermissibleConcentration, MPC). Using this criterion, thewaste from a PWR spent-fuel reprocessing plant isdominated by fission products for about the first400 years. After the first 400 years the actinidesand their daughters are the dominant factor. Theamericium and curium components of the actinidewaste are the main hazards for the first 10,000years, after which the lci.g-lived Np and itsdaughters become the controlling factor. Thesedata assume that 99.5% of the U and Pu has been re-moved from the waste.

Claiborne indicates that, if 99.5% of the C andPu is extracted, then a significant reduction it:the waste hazard can be achieved by also removing99.5% of the other actinides, mainly americium,curium, and neptunium.

For the purpose of calculations, Claiborne as-sumes that recycling takes place in a typical PWRfueled with 3.3% enriched U and operated with aburnup of 33,000 MWd/metric tonne of uranium. Thebumup was assumed continuous at a specific powerof 30 MW/metric tonne over a three year period.The calculations also ignored intermittent opera-tion and control rods and assumed that the neutronflux was uniform tb-oughcut a region. The recycledactinides were addeu uniformly to the 3.3% enrichedfuel. The actual calculations were performed by amodified version of the nuclide generation and de-pletion code ORIGEN.' ' The calculations are baso^on three energy groups (thermal, 1/E energy distri-bution in the resonance region, and a fast group)with three principal regions in the reactor. Eachregion was in the reactor for three years whilebeing cycled from the outside to the center so thatthe innermost region is removed each year.

The "standard" Chat was used for comparing theeffect of the actinide recycle on the actinidewaste hazard was the waste obtained after removingeither 99.5% or 99.9% of the U and Pu at 150 daysafter discharge and sending the remaining quanti-ties to waste along with all the other actinides,and all actinide daughters generated since dischargefrom the reactor. The results of Claiborne's cal-culations are presented in the form of a hazardreduction factor which he defines as "the ratio ofthe water required for dilution of the waste to theRCG for the standard case to that required to dilutethe waste after each successive irradiation cycle."

The contributions of the actinides, fission prod-ucts, and structural materials to the total wastehazard are shown in Table 1. Table 2 shows the ef-fect of recycling the actinides in terms of thehazard reduction factor for two cases of actinideextraction efficiency. Note that the values decreaseasymptotically with increasing recycles. This isdue to the buildup of actinides in the system untildecay and burnup equal production, after about 20cycles. Figures 2 and 3 compare the effect of re-cycling in a LWR versus no-recycle for the shortand long time hazards.

256

Table 1 Relative contribution of actinides and their daughters to the hazard measure of the waste andof each actinide and i t s daughters to actinide was-e with 99.5% of U + Pu extracted'5'

Nuclides toWaste

ActinidesFission ProductsStructural

AtericitniCuriumNeptuniumJ.5% U + 0.5% PuOther

io2

0.399+0.04

51410.2

5 x 10"3

Water requiredwater required

5 x 102

All Components

9451

for dilution tofor the mixture)

104

of Waste :b

9460.2

Actinide Waste:1"

56370.3

1 x W3

245912

5 x 10 - 2

the RCGa (% of totalfor decav times (vr)

105

9820.03

89

30

of:

106

99

4 x 10"4

81891

nil

aUsing CFR RCGs and recommended default values for the unlisted nuclides.Round-off may cau^d column not to total 100.

Table 2 Effect of recycle of actiaides other than U and Pu ou the hazardmeasure of waste from PWR spent fuel processing'-1'

RecycleHo.

012345102030

012345102030

Water requir&d.' for dilution to RCGa, ratio of standardto recycle" case (hazard reduction factor) for

io2

9.33.27.67.26.85.8'5.15.0

58443836333227—--

103

Actinide

1512108.47.46.64.73.83.6

Actinide

735948403531221817

iecav times (vr) of:

104

Extraction Efficiency.

1813119.38.37.55.84.94.6

Extraction Efficiency,

896452443936272221

105

, 99.5%:

282018171717171717

, 99.9%:

1379687848333838282

1C6

524644434242424242

256224213210209208207206206

(8)Using CFR RCGs and recommended default values for the unlisted nuclides.

Chemical processing assumed at 150 days after reactor discharge; one cyclerepresents 3 years of rea.-.tor operation.

257

10' p

a

S10"

20 30 *O SO CO

Fig. 2 ' Short-term cumulative hazard of actinidewaste from 60-year-operation of a typicalPWR(5>

10' 10" 10' 10°

TIME AFTER DISCHARGE, yr

Fig. 3 Long-term cumulative hazard of actinidewaste from 60-year operation of a typicalPWR» ^

For the PWR examined, the decrease in the averageneutron multiplication was only 0.8%. By increasingthe fissile enrichment by only 2% (from 3.3 to 3.4%enrichment) the loss in reactivity can be compen-sated for.

The recycling of reactor actinide waste will in-crease radiation problems associated with chemicalprocessing and fuel fabrication because of the in-creased radioactivity of the reactor feed and dis-charge streams. However, the increased actinideinventory in a reactor will probably have littleeffect on the potential danger in design basis ac-cidents because the actinides are not volatile and,therefore, will not be significantly dispersed intothe environment by any credible reactor accident.

Claiborne also states that the recycle of acti-nides in MFBR's should produce even higher hazard.reduction factors because of the better fission-to-capture ratio of the aetinides in the presence of afast flux. He also states that the recycling ofactinides is well suited for fluid fuel reactors,such as the MSBR, because of the on-stream continu-ous reprocessing.

A technical group at Battelle Northwest Labora-tories I9' extended Claiborne*s work to provide adetailed review of the alternative method for longterm radioactive waste management. Section 9 oftheir report was devoted to Transmutation Process-ing and covered four categories: (1) accelerators,(2) thermonuclear explosives, (3) fission reactors,and (4) fusion reactors. The study identified re-cycling in thermal power reactors as a promisingmethod and went on to state, "consideration shouldalso be given to evaluating the merit of havi sspecial purpose reactors optimized for destroyingactinides."<9)

(10)

(IDAs reported in a review paper by Raman,(

evaluations made by Raman, Nestor, and Dobbsshow that actinide inventories can be reduced fur-ther by recycling in a U233-Th23z fueled reactor.This is made possible because neutron captures bythe fertile Th23z produce the fissile U 3. Neutroncaoture by U 2 3 3 results in higher U isotopes untilU " 7 is reached. Plutonium and transplutonium iso-topes are generated to a far lesser extent in aU 2 3 3-Th" z reactor than in a uz35-U238 reactor.Raman also stressed the need for more accuratecross section measurements.

The recycling of actinides in fast reactors hasalso jeen studied.(12,13,14,15) Greater actinideburnup is achievable in a fast reactor than in athermal reactor because the fission-to-c "ture ratiois generally higher as shown in Table 3, ; i a 1973review paper in Science, Kubo and Rose^6) suggestedthat recycling of actinides in an LMFBR has severaladvantages over recycling in a thermal reacfor in-cluding the possibility that extreme chemirs'j sep-arations may not be required because fewer acti-nides are produced in a fast spectrum.

Paternoster, Ohanian, Schneider, Thorn, andSchwenk(l'»l°>19) have studied tee use of the gascore reactor for transmutation of fission pro-'u. tsand actinide wastes. The fuel was UFg enriched to6% in U"^. The four meter diameter core was sur-rounded by a reflector-moderator of D2O with athickness ot 0.5 meter. The initial mass was 140kg of XS235F6. Adjustable control rods were locatedin the outer graphite reflector and the radioactivewastes were loaded in target ports. Figure 4 shows'results of calculations, comparing both actinideand fission product waste in current tWR's with thegaseous fuel power reactor. Notice that after 800-1000 days, the actinide wastes in the gaseous corereactor are an order of magnitude less than thosein a LHR.

dy sponsored by NASA, Clement, Rust, and>Z1J analyzed a gas core breeder reactor-ThZJZ fuel cycle. One- and two-

In a stud'Williams < "using a Udimensional calculations were carried out for a UFBfueled core surrounded by a molten salt blanket.Figure 5 shows a diagram of the reactor. The me-dium fission eueigy in the core was found to be300 keV, and there was some spectrum softening inthe blanket.

258

Table 3 Fission-to-capture ratios of actinides infast and thermal reactors v-0)

Isotope Half-LifeYears

FastSpectrum

ThermalSpectrum

Np

Am'

Am

Am'

237

241

242m

243

244

2.14 x 10

433

152

7370

17.9

0.213

0.115

4.85

0.309

1.25

1.26 x 10

4.48 x 10

1.12

4.87 x 10

0.068

I

I

Is

Fig.

IM 4H <•• IM 1IH U» IM 1IM tlH

DAYS OF REACTOR OPERATION4 Radioactive waste production of 3425 Mw(t)

fission power reactors^''

mnul

Fig. 5 UFs gas-core reactor

Table 4 is a brief summary of some importantdates in the history of the GCATR. As previouslystated, the buraup of fission products requiresthermal neutron fluxes of the order of 101' nlaar-sec. In the United States, the two reactors withthe highest neutron fluxes are the ORNL High FluxIsotope Reactor fHFIR)( > and the SRI. High FluxReactor (HFR)^"' uhich have maximum neutron fluxesof 3 x 10 1 5 and 5 x 10 1 5 u/cm2-sec, respectively.Both of these reactors employ solid fuels and haveessentially reached the upper limit in neutronfluxes because of heat transfer limitations. Inaddition, when operating at these neutron fluxesthe refueling intervals are of the order of twoweeks. The gas core reactor does not have the in-core heat transfer limitations posed by solid corereactors employing a coolant aud, consequently,higher neutron fluxes should be achievable. Inaddition, reactor shutdown for refueling is notnecessary because new fuel can be continuously addedto the UFg during reactor operation. Therefore, agas core reactor may be the only practical reactorfor consideration of fission product bumup if sucha scheme of waste disposal is considered desirable.

Table 4 Some dates in the history of GCATR

1960-73 - NASA sponsored research on gas-corereactor for rocket propulsion

1964 - Steinberg first suggests neutron-inducedtransmutation

1972 - Claiborne's studies of actlnide recycling

in LWR's

1973 - documentation of ORIGEN program

1974 - recycling studies in LWR's, UiFBR's,HTGR's by Croff, Raman, et al.

1974 - suggestion of GCATR by Paternoster,Ohanian, Schneider (University ofFlorida) and Thorn (NASA)

1974-75 - UF, breeder reactor study at GeorgiaTech sponsored by NASA

1976 - GCATR study at Georgia Tech sponsored byNASA

Table 5 summarizes some of the advantages of theGCATR which appear to make it an attractive candi-date for actinide transmutation.

Table 5 Some advantages of the gas-core reactor

(1) The gaseous state of the fuel significantlyreduces problems of processing and recyclingfuel and wastes.

(2) High neutron fluxes are achievable.

(3) The possibility of using a molten salt in theblanket may also simplify the reprocessingproblem and permit breeding.

(4) The spectrum can be varied from fast to thermalby increasing the moderation in the blanket sothat the trade-off of critical mass versus ac-tlnide and, fission product bumup can be studiedfor optimization.

233(5) The U -Th cycle, which can be used, is su-

perior to the U"-"-Pu cycle in regard to acti-nide burnup.

259

III. GCATR Research Program

Ihe overall objective of the NASA sponsored pro-gram is to study the feasibility, design, and opti-mization of a GCATR. The program involves threeinterrelated and concurrent tasks, as listed inTable 6.

Table 6 NASA sponsored GCATR research atGeorgia Tech

General Studies.Update cross-sectionsSensitivity analysisParametric survey

Reactor and System DesignDesign criteriaReactor subsystem

(a) 2 3 3UF 6(b) plasma core

Fuel and actinide insertion andrecycling

Economic AnalysisComparison of GCATR with other

strategies

TASK 1 General Studies

Ramanv has pointed out the need for more accu-rate cross section data and the necessity of as-sessing the sensitivity of the calculational resultsto the uncertainties in cross sections. This taskwill include the following subtasks:

A. Literature Survey and Cross Section Tabula-tion—A literature survey will be carriedout and the best available cross sectionsof the fission products and actinides willbe tabulated. Improved values will be usedas they become available.

B. Implementation of a Versatile DepletionProgram—ORIGEN^ or a similar computercode will be implemented or developed. Adepletion code which solves the equations of

. radioactive growth and decay and neutrontransmutation for large numbers of isotopesis required. ORIGEN has been used previ-ously for LWR's, LMFBR's, MSBR's, and HTGR's,and may also be suitable for the GCATR.

C. Parametric Survey Calculations—Parametricsurvey calculations will be performed toexamine the effort of reactor spectrum, andflux level on the actinide transmutation forGCATR conditions. The sensitivity of theresults to neutron cross sections will beassessed. These studies will be related to

. the nuclear analysis work of TASK 2. Spe-cifically, the parametric calculations ofthe actinide transmutation will include themass, isotope composition, fission and cap-ture rates, reactivity effects, and neutronactivity of the recycled actinides. Table 7summarize:: the most important parameters tote investigated.

Table 7 Most important parameters to beInvestigated

(1) The mass and composition of the actinidesbeing recycled

(2) The rate at which the recycled actinides arefissioned and transmuted in the reactor

(3) The effect of the recycled actinides on fissionreactor criticality and reactivity

(4) The effect of the recycled actinides on theout-of-reactor nuclear fuel cycle (i.e., fab-rication, shipping, reprocessing, actinideinventory, etc.)

TASK 2 GCATR Design Studies

This task is a major thrust of the proposed re-search program. Four subtasks are considered:

A. Optimization Criteria StudiesB. Design Studies of the Reactor SubsystemC. Design Studies of the Blanket and Fuel Re-

processing and Actinide Insertion and Recir-culation System

D. System Integration

In subtask A, Optimization Criteria Studies,consideration will be given to understanding thetrade-offs that are made to achieve a given result.For example, is the GCATR to be used only for acti-nide burnup? Should we also include breeding(U 3-Th) or fission product transmutation? If wereduce the mean neutron energy to achieve fasterfission product burnup, how much do we sacrifice inactinide burnup? Should the reactor also be usedto produce power? If so, how much power? What arethe optimization criteria?

In subtask B, Design Studies of the Reactor Sub-system, a multidisciplinary approach similar tothat in Refs. 20, 21 will be carried out Involving:

(1) Materials(2) Nuclear Analysis(3) Thermodynamics and Heat Transfer(4) Mechanical Design.

Results of this task will be used iteratively withthe parametric study described in TASK 1. In pre-vious work'20,21) one-dimensionaT and two-dimensionalsurvey calculations were carrie< out for a UFg-fueled core surrounded by a molten salt blanket,and a preliminary mechanical design was developed.This work will be extended to include the insertionof fission products and accinides in various loca-tions in the reactor. The effect of other reactorcomponent changes such as using different reflectormaterials (carbon, beryllium, deuterium oxide) ormodifying the molten salt reflector by the additionof moderator will also be evaluated. Best availablecross section data from TASK 1 will be utilized.A preliminary reactor design will be developedtaking into account thermal and mechanical designconsideration*.

In subtask C, a preliminary design of the UFgand blanket reprocessing system (if molten salt)will be prepared and performance of the systemsanalyzed. Equilibrium fuel and blanket compositionsincluding fission products and actinides will becomputed. These results will provide necessary

260

Information on equilibrium core and blanket compo-sitions for use in the nuclear analyses.

Subtask D, System Integration, Involves puttingall the sub-components together in a workable sys-tem taking account of critical!ty, shielding, andeconomic, considerations.

TASK 3 Comparison of the GCATR with Othor NuclearHaste Management Strategies

The cost of the GCATR shall be evaluated interms of mills/kwhre. The cost can be broken downinto the components:

(1) solid and liquid storage(2) shipping(3) interim retrievable storage separations(4) separation(5) disposal or elimination in GCATR

The total cost of the management system will becomputed and compared to the cost of alternatestrategies presently being considered.

As of June 1976, the research program has beenunderway for only two months. Table 8 summarizesthe status of the program at this time.

Table 8 Summary of Georgia Tech GCATRresearch program to date

General Studies1. Actinide cross sections have been updated2. ORi'CEN has been implemented and modified3. Some! sensitivity results and parametric

studies have been obtained

Reactor Studies1. Series of rruclear design codes have been

implemented 2332. Several configurations of

are being analyzedOTfi reactor

IV. References

1. Smith, J. A., "Proceedings of the Califomium-252 Symposium," USAEC Report CONF-681032,p. 179 (1969)

2. 'Waste Management Symposium," Huclear Technol-ogy. 24 (December 1974)

3. Steinberg, M., Molzak, G., and Menovita, B.,'Neutron Burning of Long-Lived Fission Prod-ucts for Haste Disposal," BNL-8558, BrookhavenNational Laboratory (September 1964)

4. Gregory, M. V. and Steinberg, M., "A NuclearTransmutation System for Disposal of Long-Lived Fission Product Waste in an ExpandingNuclear Power Economy," BNL-1195 (November1967)

5. Claiborne, H. C , 'Neutron Induced Transmuta-tion of High-Level Radioactive Wastes," ORNL-TM-3964 (December 1972)

6. Claiborne, H. C , "Effect of Actinide Removalon the Long-Term Hazard of High-Level Waste, 'ORNL Report TM-4724 (1975)

7. Bond, W. D., Claiborne, H. C , and Levze,R. E., 'Methods for Removal of Actinides fromHigh Level Wastes," Nuclear Technology. 24,362-370 (December 1974)

8. Bell, M. J., 'ORIGEN--The ORNL Isotope Genera-tion and Depletion Code," ORNL-4628, Oak RidgeNational Laboratory (May 1973)

9. Schneider, K. J. and Plat, A. M., "Bigh-LevelRadioactive Waste Management Alternatives,"BNWL Report 1900 (1974)

10. Raman, S., "Some Activities in the UnitedStates Concerning the Physics Aspects of Acti-nide Waste Recycling," Review Paper from ORNL(1975)

11. Raman, 5., Nestor, C. W,, Jr., and Dabbs,J. W. T., "The U233.UJ232 rieactor o8 a Burnerfor Actinide Wastes," Proc. of the Conf. onNuclear Cross Sections and Technology. Washing-ton, D. C. (March 1975)

12. Breen, R., "Elimination cf Actinides withLHFBR Recycle," Westinghouse Advanced ReactorsDivision, private communication reported inRef. 10

13. Beaman, S. L., "Actinide Recycle Evaluations,"General Electric Energy Systems and TechnologyDivision, private communication reported inRef. 10

14. Croff, A. G., 'Parameter Studies ConcerningActinide Transmutation in Power Reactors,"Transactions of the American Nuclear Society.22, 345 (November 1975)

15. Davidson, J. W. and Draper, E. L., Jr., "Costsfor Partitioning Strategies in High-LevelWaste Management," Transactions of the AmericanNuclear Society. 22, 348 (November 1975)

16. Kubo, A. S. and Rose, D. J., 'On Disposal ofNuclear Waste," Science. 182. No. 4118, 1205-1211 (December 1973)

17. Paternoster, R., Ohanian, M. J., Schneider,R. T. and Thorn, K., "Nuclear Waste DisposalUtilizing a Gaseous Core Reactor," Transactionsof the American Nuclear Society. 19. 203 (Oc-tober 1974)

18. Paternoster, R. R., "Nuclear Waste DisposalUtilizing a Gaseous Core Reactor," Master'sThesis, University of Florida (1974)

19. Schwenk, F. C. and Thorn, K. T., "Gaseous FuelNuclear Reactor Research," Paper presented atthe Oklahoma State University Conference onFrontiers of Power Technology (October 1974)

20. Clement, J. D., Rust, J. H., a<sd Williams,J. R., "Analysis of UFg Breeder Reactor PowerPlants," Semi-annual Report NASA Grant NSG-1168 (October 1975)

21. Rust, J. H. and Clement, J. D., "UFg BreederReactor Power Plants for Electric Power Gener-ation, " groc• Third Symposium on UraniumPlasmas. Princeton University Conference,June 10-12, 1976

261

22 . Binford, F . T . , e t a l . , llHie High-Flux IsocopeReec tor ," ORNL-3572 (Rev. 2 ) (May 1968)

2 3 . Crandall , J . Xi.,' 'The Savannah River High FluxDemonstracion," USt.EC Report CP-999 (1965)

262

APPLICATION OF GASEOUS CORE REACTORSFOR TRANSMUTATION OF NUCLEAR WASTE

B. G.. Schnitzler, R. R. Paternoster? and R. T. SchneiderUniversity of FloridaGainesville, Florida

Abstract

An acceptable management scheme forhigh-level radioactive waste is vital tothe nuclear industry. The hazard potentialof the trans-uranic actinides and of keyfission products is high Jue to theirnuclear activity and/or chemical toxicity.Of particular concern are the very long-lived nuclides whose hazard potentialremains high for hundreds of thousands ofyears. Neutron induced transmutationoffers a promising technique for the treat-ment of problem wastes. Transmutation isunique as a waste management scheme in thatit offers the potential for "destruction"of the hazardous nuclides by conversion tonon-hazardous or more manageable nuclides.The transmutation potential of a thermalspectrum uranium hexafluoride fueled cavityreactor was examined. Initial studiesfocused on a heavy water moderated cavityreactor fueled with 5% enriched U235 -Fg andoperating with an average thermal flux of6 x 1014 neutrons/cm2-sec. The isotopesconsidered for transmutation were 1-129,Am-241, Am-242m, Am-243, Cm-243, Cm-244,i-ai-245, and Cm-246.

Introduction

The proper management of high-levelradioactive waste is an important issuewith which the nuclear community must deal.In the face of rising serial pressure, theidentification of a satisfactory long-termmanagement scheme is vital to the health ofthe industry.

A number of high-level, long-termwaste management schemes have been pro-posed; none are entirely satisfactory.The ultimate standard against which anyproposed management scheme must be judgedis whether or not ic will prevent anynuclear waste material from ever posing athreat to man's well-being. Managementschemes are invariably aimed at accom-plishing this goal by isolation of thewaste material from man and from theenvironment for whatever period of time itremains hazardous. Transmutation is theonly exceptipn to the isolation rationale.Transmutation is unique in that it offersthe potential of converting hazardousnuclides into non-hazardous forms by thesame nuclear processes responsible fortheir creation.

Waste Characterization

Until recently, spent fuel reproces-sing had been directed toward recovery of

most of the uranium and plutonium present.Recovery fractions were governed by trade-offs between the value of the recoveredmetal and sharply increasing costs withhigher recovery fractions. The remaininghigh-level waste was treated collectivelyby storage until future implementation ofa long-term management scheme. With thistechnique, the entire high-level wastemass must be treated as though it had thehalf-life of the longest lived nuclide,the chemical toxicity of the most toxicnuclide, etc.

A recenu study1 on waste managementalternatives examined the requirementsand potential benefits of separating thehigh-level vraste stream into two fractionsbased on half-life. The short-livedfraction would contain only those nuclideswhich would decay to radiologically non-toxic levels in 1000 years. At the end ofthe 1000-year decay period, this fractionof the high-level waste would require nomore control than that necessary from achemical standpoint. The long-lived frac-tion would contain the actinides and a fewkey fission products not meeting the"short-lived" criteria. Different manage-ment techniques might then be employed forthe two fractions. Some options, such astransmutation, might require additionalseparation by element or chemical groupsof elements.

Fission Products

The fission products are character-ized by high specific activity sadrelatively low neutron absorption crosssections. They are generally short-livedcompared to the actinide elements. Theyare neutron rich and decay toward a stableneutron-proton ratio by successive betadecays. Typical decay chains are illus-trated in Figure 1. For thermal fissionof U-235, there is slightly less than 1%fission yield into the 129 mass chain.There is negligible direct yield ofxenon-129. The shorter-lived chain mem-bers rapidly decay into iodide-129 whichhas a half-life of 15.9 million years.Within the fission product group, a fewlong-lived nuclides, such as 1-129, controlthe long-term waste hazard.

Actinides

The trans-uranic actinides are formedby those neutron absorption reactions inthe fuel which do not result in fission.The most common such reaction competingwith fission is radiative capture. The

* Now at Los Alamos Scientific Laboratory

263

tniLE

Xl.&ltfms

*>*

,1a

53.lt»

JOHJM

\rs

MU

I13

7t129

«.3lH

8.9 KM

\

XIS m7.5 WN

6.6 HI.

Sum

\

\

0.8 sic

1 7 , .

\

*0.SW

FltlUE 1, TYPIOl FISSIOM PKKUCI KCC OUHS

newly formed nucltde may then undergospontaneous or neutron induced fission,decay by particle emission, or anotherneutron absorption reaction such as radi-ative capture may take place. In thismanner, higher and higher mass numbertrans-uranics are built up in the fuel.

The heavy metals present the most se-vere long-term management problems. Theyare characterized by high chemical toxicityand very long half-lives. The waste hazardassociated with the heavy metals remainshigh for hundreds of thousands of years.In general, the actinides possess sub-stantial neutron reaction cross sections.

Transmutation

Transmutation is any process by whicha nuclide is changed into another nuclide.

- The decay of a radioactive nuclide is anatural transmutation process, but it maynot occur on a sufficiently short timescale to be useful for waste management.Transmutation may also be artificiallyinduced by neutrons, photons, or chargedparticles. Of the known methods of arti-ficially inducing transmutation, onlyneutron induced transmutation appears t:ooffer near term technical and economicfeasibility.

Transmutation of the long-lived wastefraction will involve treatment of theactinides and some key fission products.For the fission products, the motivationfor treatment is illustrated by consider-ing iodine-129. The 1-129 decay bottleneckillustrated in Figure 1 may be relieved byirradiation. Radiative capture reactionsin the 1-12? will drive it to 1-130. Theshort-lived (12.4 hours) 1-130 will thenbeta decay to xenon-130 which is stable andnon-hazardous. The very slowly decayingnuclides in a particular mass chain areshifted to a more rapidly decaying chain.

A number of competing reactions mayalso occur. The major reaction paths areindicated in Figure 2. Table 1 contains asymbol key for the figure. For 1-130,

radiative captures producing 1-131 competewith the natural beta decay producing Xe-130. Similar competitive reaction pathsmay be readily observed for other nuclides.In general, the rate at which the trans-mutation proceeds will be governed by themagnitude of the neutron flux. The re-action rate may also be altered, sometimessubstantially, by tailoring the neutronspectrum to exploit neutron absorptionresonances. Particular reaction paths canbe enhanced by altering the spectrum. Theeventual distribution of nuclides is stronglydependent on the neutron spectrum.

The final nuclide distribution willbe very significant from a waste managementstandpoint. The variety of possible re-action paths generally precludes predictionof the final distribution by observation.The effect of a spectral change must beevaluated by following the isotopic balanceas a function of time during the irradia-tion and subsequent decay period. Nuclideconcentrations, transmutation rates, andthe waste hazard may then be calculated.

For the actinides, the transmutationrationale is different. Contrary to thefission products, there is little to begained by radiative capture reactions inthe actinides. To build even higher acti-nides from them is of questionable value.The ultimate objective is to inducefission. Actinide fission cross sectionsgenerally decrease with increasing energy,but at a fafter rate for most actinides.Therefore, fission can be promoted overradiative capture by the employment of aharder neutron spectrum. As with thefission products, the optimum spectrumcannot be predicted by simple observationand detailed calculations must be per-formed.

A number of transmutation schemeshave been proposed for waste management.Detailed transmutation calculations6 in-dicate that actinide recycle in currentlyoperating light water reactors (LWR)would be effective in reducing the long-term hazard associated with the actinidewastes. The liquid metal fast breederreactor (LMFBR) may prove even moreeffective for actinide recycle.' Transmu-tation of most fission product wastes isnot feasible for either the LWR or theLMFBR. Both the attainable fluxes and thefission product cross sections are too lowfor practical application. Neutrons pro-duced in a CTR device would be most effec-tive for transmutation of both actinidesand fission products, but near termimplementation is not possible.7

The requirements for very high neutronfluxes and the desirability of spectrumtailoring make the cavity reactor .logicalcandidate for transmutation of both fis-sion product and actinide wastes. Auranium hexafluaride fueled heavy watermoderated cavity reactor was considered ininitial studies. The reactor was assumed

264

to operate at an average thermal flux ofapproximately 6.4 x 10T* neutrous/cm2-seefor a five-year period. The assumed fuelenrichment was 5%. The transmutationrates of several waste isotopes presentin the discharged fuel from currentlyoperating light water reactors werestudied using the isotopic generation anddepletion code ORIGEN.2

The isotopes selected for evaluationof the gas core transmutation potentialwere 1-129, Am-241, Am-242m, Am-243, Ctn-243,Cm-244, Cm-245, and Cm-246. Americium andcurium were chosen since they.control theactinide waste hazard for very long decaytimes. The fission product iodine-129 was

chosen since it is a major contributor tothe long-term waste hazard. The amountsof the initial waste loading were detPi-mined by the composition of 20 typicallight water reactor fuel loads stored fora decay interval of ten years. The wastequantities employed in the study are there-fore approximately equivalent to that

55Cs133

12854

(Data compiled fromReferences 2 - S)

FIGURE 2. IODINE-129 TRANSMUTATION REACTION PATHS

TABLE 1: TRANSMUATION FIGURE SYMBOL KEY

El

A

Z

(T¥0

sigth

siSres

element symbol

mass number

atomic number

electron emission

positron emission

excited state half-life

ground state half-life

thermal cross sectionfor (n,y) absorption

resonance integral for(n,Y) absorption

(F*) fraction of (n,Y) transitionsto an excited state of theproduct, nuclide

(F) fraction of (n,Y) transitionsto a ground state of productnuclide

[B+] fractional decay by positronemission

[B~] fractional decay by electroneraiss ion

[B"*] fractional decay by electronemission to an excited state ofthe product nuclide

NOTE: designates an isomeric transition; the fraction of excited state decays by isomerictransition is 1 - [B+] - [B~] - [B~*]

265

generated in 60 reactor years of operation. C 2 3 8 present in the reactor fuel.

As previously indicated, the transmuta-tion rates and the final isotopic balancesare strong functions of the neutron spec-trum. For th .•> study, three referencespectra were employed. A relatively hardspectrum typical of that expected in thecore and a thermal spectrum typical of thatpresent in the heavy water moderator wereused. In addition, an intermediate spec-trum typical of that found in light waterreactors was used for comparison purposesIt is emphasized that only the spectra istypical of the LWR; the magnitude of theflux in the gas -core is substantiallyhigher than that attainable in the lightwater reactor.

Gas Core Transmutation Results

Iodine-129

The initial loading of the 1-129 was400 kilograms. The mass of 1-129 remain-ing as a function of irradiation time forthe three reference spectra is shown inFigure 3. The curve labeled $t correspondsto irradiation in the core spectra and $1to irradiation in the moderator spectra.

Americium-

Curves for the transmutation of theamericium isotopes in the spectrum presentin the reflector-moderator are shown inFigure 4. The initial loadings of thethree americium isotopes considered ereindicated. The Am-243 concentrationapproaches a steady state value after ap-proximately twe years. This behavior isdue to Am-243 production by successiveradiative captures and beta decays in the

INITIAL LOADING:

I129 - 100 KG

The effect of the neutron spectrum onthe transmutation of Am-243 is illustratedin Figure 5. The reflector-moderator isindicated by the subscript 1 and the coreby the subscript 3.

Curium

The curium transmutation calculationsare illustrated in Figure 6. The signifi-cance of the neutron spectrum employed ismost vividly illustrated in the case of

INITIAL LOADINGS:

AM 2' 1 - S3.31 KG

An2'2" - 1.58 «G

- 172.51 <G

10°

10*

I

10°

0 360 720 JOSO M 0 1800

DAYS OF IRRADIATION

FJEUJE 1 . AHERICIUM TMUSniTATIOH VERSUS IRRADIATION TIBE

INITIAL LOADING:

-1 ' - 172.54 KG

0 360 • 720 1080 1410 1800

DAYS OF IRRADIATION

FIGURE 5. 10DME-129 TRANSMUTATION VERSUS IRRADIATION T1BE

360 720 1030 1110 1800

DAYS OF IRRADIATION

FIGURE 5. ABERICIU11-213 TRATISHUTATIOH VERSUS IRRADIATIONTIBE

266

INITIAL LOADINGS;

CM2"3 - 0.117 M

Oi2'" - 38.M «

Ci.2*5 - 3 . M «

<C»»5)

720 1080 11W 1800

DAYS OF IRRADIATION

FI61i«E 6. CU»!U: IKAKSBI'TATION VERSUS IFlRABIATIDK T i l t

curium-244. After an irradiation periodof approximately six months, the Cm-244waste masses differ by a factor of 30 de-pending on whether the core or moderatorspectrum is employed.

Higher Actinide Build-Up

Successive radiative captures and betadecays in both the reactor fuel and theisotopes to be transmuted can produce sig-nificant quantities of higher mass numberactinides. The production curves forsignificant transmutation by-products areshown in Figure 7.

rcofte

0 ?6C 720 1030 1W0 1300

DAYS OF IRRADIATION

FIGURE 7. HIGHER ACT/HIDE BUILD-UP HBSUS IMADIA;JUI HUE

Waste Hazard Reduction

The success of a particular transmu-tation scheme can be judged in terms ofthe overall reduction in the waste hazard.The relative inhalation hazard (RIHH) isdefined as the volume of air (cubic meters)required to dilute the radioactivity inthe waste to the Radiation ConcentrationGuide levels listed in Title 10 of theCode of Federal Regulations.* Similarly,the relative ingestion hazard (RIGH) isdefined in terms of the volume of waterrequired for dilution. The primary hazardassociated with long-term waste disposalis the danger of dissolution in groundwater or dispersal in the atmosphere andsubsequent uptake by man. The quantity ofair or water required to dilute the wasteto a concentration low enough for unre-stricted use can be used as a crude measureof the waste hazard.

The relative ingestion hazard associ-ated with the americium and curium wastesis illustrated in Figure 8. The top curverepresents the no irradiation case and thelower three curves, irradiation by the threereference spectra. The lower curves repre-sent the hazard measure of the actinides tobe transmuted, the by-products of thetransmutation, and the new wastes generatedby operation of the reactor for five years.

Conclusions

Transmutation of 1-129 by a five-yearexposure to a thermal neutron flux of6.4 x 1014n/cm2-sec results in nearlyorder-of-magnitude reductions in the wasteinventory of this nuclide. A five-yeartransmutation of americium and curiumwastes produces order-of-magnitude decreasesin the overall hazard potential of actinidewastes generated in 60 reactor years ofLUR operation.

For this study, no attempt was made tooptimize transmutation rates. Currentstudies are examining the transmutationpotential of a fast spectrum cavity reactor.Other fuel cycles will be considered and anattempt made to optimize the irradiation time.

10'

CHARGE DISCHARGE « I I 3 l

FIVE YEJ8SIRRADlt'lIOH

FIGlFx a . RELATIVE WGESTION HAZARD (Rlffl) OF W O I O I H AMD C i lH IM 'HASTES

1 0 ' 10-' 1 0 5

YEARS AFIEH DISCHARGE10"

267

References

1: K.J. Schneider and A.M. P la t t , "High-Level Radioactive Waste ManagementAlternatives", BNWL-1900 (1974).

2. M.J. Bel l , "ORIGSS - The ORNL IsotopeGeneration and Depletion Code", ORNL-4628 (May 1973).

3. ORYX-E, ORIGEN Yields and Cross Sec-tions — Nuclear Transmutation andDecay Data from ENDF/B", Oak RidgeRadiation Shielding Information Center,DLC-38 (Sept. 1975).

4. N.E. Holden and F.W. Walker, "Chart ofthe Nuclides", 11th Edition, GeneralElectr ic Company (April 1972).

5. Radiological Health Handbook, U.S.Department of Health, Education andWelfare (Jan. 1970)

6. H.C. Clairborne, "Neutron InducedTransmutation of High Level RadioactiveWaste", ORNL-TM-3964 (1972).

7. W.C. Wolkenhauer, (ed.) "The ControlledThermonuclear Reactor as a FissionProduct Burner", BNWL-1685 (1972).

8. Code of Federal Regulations, Ti t le 10,Part 20.

DISCUSSION

T. S. LATHAM: Have you looked at what the secularequilibrium buildup of actlnide becomes as afunction of the reprocessing time In the UFg i t -self? This suggests a class of reactor that m?ymake most of the actinlde problem go away.

B. G. SCHNITZLER: Me did look at the actinidebuildup in a typical l ight water reactor asopposed to the actinide buildup in the gas .core.There is very l i t t l e difference In the fissionproduct buildup, but as for the act inides, theywil l level out, and we can only assume that theent i re UFg mass was reclrculated.

2 6 8

GAS CORE REACTORS FOR COAL GASIFICATION*

By

Herbert WeinsteinIllinois Institute of Technology

Chicago, IL 60616

Abstract

The use of nuclear process heat for coal gasifi-cation is a subject of" considerable interest today.The concepts presently under study employ a processheat stream limited in temperature by the maximumallowable temperature of the reactor fuel elements.Because of this temperature limitation, the ratesof the gasification reactions must be enhanced bythe use of catalysts. The chemical plant associ-ated with the production of either coal gas orhydrogen then becomes complex and expensive.

In this paper, the concept of using a gas corereactor to produce hydrogen directly from coal andwater Is presented. It is shown that the chemicalequilibrium of the process is strongly in favor ofthe production of H2 and CO in the reactor cavity,indicating a 98% conversion of water and coal atonly lSOCK. At lower temperatures in themoderator-reflector cooling channels the equilib-rium strongly favors the conversion of CO andadditional B2O to CO2 and H2> Furthermore, it isshown the H2 obtained per pound of carbon has 23%greater heating value than the carbon so that somenuclear energy is also fixed. Finally, a gas corereactor plant floating in the ocean is concepru-alized which produces H2, fresh water and sea saltsfrom coal.

Introduction

It lias become apparent in recent years that theUnited States must utilize its coal resources moreeffectively to offset the problems caused by dwind-ling petroleum and natural gas resources. Some ofthe more attractive processes proposed for coalutilization are gasification—the production ofsynthetic natural gas—and hydrogen production.The problems associated with these processes arebeing worked on today. These problems include lowefficiency and the complexities brought on by theuse of very large scale catalytic reactors. It hasbeen projected for gasification processes in whichthe coal is also used to generate process heat thatthe maximum possible efficiency is 67%, which meansthat one-third of the heating value of the coal islost in processing and must be disposed of aswaste heat.

The concept of utilizing nuclear energy for coalgasification can be traced back to the early 196O's.1.2.3,4 Currently, this concept has seen renewedinterest in Europe. Most of the related researchis directed towards the utilization of process heatfrom high temperature gas cooled reactors (HTGCR).The maximum temperatures reached by the coolantgas in these reactors is less than 1500°K. Becauseof this temperature limitation, the chemical ratesof the gasification reactions must still beenhanced by the use of catalysts.

* Work supported under NASA Grant NSG7039

It would appear that both coal gasification andhydrogen production could be done more directly andmore efficiently in a gas core cavity reactor thanin any indirect process heat application. Thereare several factors that maka ;his approach seempromising. In the gas core reactor, the pulverizedcoal and steam would flow along together directlythrough the thermal radiation field from the fis-sioning plasma. In fact, the coal would providethe necessary shielding from thermal radiation re-quired to protect the cavity walls. The resultanttemperature of the steam-coal mixture can be highenough for the direct pyrolysis of water. The coalwill combine with the oxygen as the mixture iscooled after leaving die reactor. However, thecoal-steam mixture need be heated only to 1500°Kto 2000°K at a 500 psi cavity pressure to effectthe production of hydrogen and CO without the useof catalysts. The temperature of the mixture inthe cavity should in fact be such as to obtain themost desirable mixture of 113, CO, CO2, H2O andother organic compounds at the reactor exit.

The most straightforward application of thisconcept would be in the production of Hydrogen.The idea of using hydrogen as a new type of cleanfuel has been gaining acceptance. The potential ofthis process as discussed here is for producingfuel hydrogen in massive quantities. It should benoted that there also is potential for producingother valuable organic compounds directly in a gascore reactor.

The object of this work is to present the con-cept of using a gas core reactor to produce hydro-gen from water and coal. The chemical equilibriais shown to be favorable for this reaction atmodest gas core reactor temperatures. The chemicalkinetics are not discussed because non-catalyticrate data is not readily available in the litera-ture. An entire process is conceptualized in orderto point out that the elements of the processappear feasible. The areas where intensive inves-tigation is required are pointed out. The partic-ular scheme chosen is one where the plant is float-ing in the ocean, using distilled sea water for theprocess and the ocean as a waste heat sink. Thehydrogen and fresh water obtained from the wasteheat are piped directly to the mainland. The seasalts obtained in the process can be returned tothe mainland on the barges that bring coal to theplant.

Chemistry

The Heating Values of Products.The resulting products of the water-coal reac-

tions in the gas core reactor can be strongly in-fluenced by the choice of temperature path for thereactant stream. The choice of products is basedor. the desirability of the fuel and the amount ofnuclear energy that is chemically fixed in the fuel.It is desirable to fix as much nuclear energy aspossible not only to increase our energy resources

269

(1) W20

(2) c +

(3) 2C

(4) C +

(5) C +

1/2

COfH2

TABLE I

Candidate Chemical Reactions

Reaction Fuel

Lower heating value5per mole of fuel

K calFixed Nuclearenergy per mole C

Z Increasein RV ofCoal

CH,

CH.

CO+H,

"7.8

191.8

95.9

125.4

115.6

(a)

(b)

118.8

12.4

41.9

42.6

104%

27.

33%

23%

(a) 191.8/2 (b) 2 x 57.8

but also to minimize the waste heat disposal prob-lem. Candidate reactions ai-e listed in Table 1along with the fixed nuclear energy data. Thefixed nuclear energy is expressed in two ways.First, as.the number of kllocalories fixed per mole.of carbon and second, as the resultant percentageincrease in the heating value of carbon. The firsttwo reactions are presented Just to demonstratethe maximum possible fixed energy yields. Ttiefirst of jhese is the pyrolysis of water withoutthe use of coal to scavenge the oxygen. All of thecuel- heating value is fixed nuclear energy. Thesecond reaction is for the production of methaneand oxygen from coal and water. Again coal is notused to scavenge O2 and the increase in heatingvalue of the coal is about 100%. However, thismethane formation reaction xs more truly repre-sented by reaction 3. In this case the amount offixed nuclear energy is negligible. Reaction 4shows the production of a CO+H2 fuel gas. The per-cent of nuclear energy fixed per pound of carbonheating value is 33%. While this is a significantincrease, the problems associated with CO as a fuelmake it an unlikely prospect as a final product.The last reaction is the production of H2 and COjand is the one ussd in this report because itrepresents a reasonable compromise of fuel utilityand fixed nuclear energy. It will, however, occuras a two step process with reaction 4 being thefirst step. This is discussed in the next section.

Chemical Equilibrium Calculations

At elevated temperatures (>1500°K), the princi-pal products in a C-H-0 system are H2, CO, 003.Methane formation is suppressed above 1000°K.Since the effects of nuclear radiation and theplasma field are neglected, only these products areconsidered. The overall reaction is:

I.) AHnO + bC •* cH20 + dC + eH2 + f CO + gCO2 and thebasic reaction steps are:

2) CO + H 2O^±CO 2 + H2

3) C + H2O^±.CO + E 2

The equilibrium equations are:

NCOK.3

and

K,"2 ~* P P^ *C0 H20 CO 20

Her<=, a is chemical activity, N is number of moles,P is pressure and Subscript T stands for total.The activity of carbon is taken as one.

An H_ balance and an 0 2 balance complete the equa-tion set

( N ) initial = (N 0 + eq.

(H n) initial (2 N,C02 eq.

Solving these equations with the K values ofreference 6, and a total pressure of 500 psi yieldsthe results shown in Figure 1. This value ofpressure should be high enough for criticality andpressure level only affects in a small way thetemperature at which conversion is completed.Equilibrium calculations below 900°K were notcarried out because oi the complexity of the calcu-lations. Without these results it is Impossible todetermine the gas quenching paths required to yieldthe maximum value of the product stream. However,the reaction rates for the reverse reactions whichwould return the system to the initial reactantconcentrations are certainly slow enough so thatquenching is possible.

The results are presented for two cases. Figurela is for equimolar feed and Figure lb is for afeed ratio of 2 moles H20/mole C. It is seen thatfor the equimolar case the equilibrium is suchthat conversion to CO and H 2 can be essentiallycomplete above 1500°K. Chemical reaction ratesat these temperatures are expected to be extremelylarge so that this equilibrium concentration shouldexist in the reactor cavity. For the case of molarfeed ratio of 2, the coal ia completely converted

270

to CO and C02 with about 57% of the water convertedto hydrogen. It can be seen from the equilibriumconstant data that as the percentage excess ofsteam is Increased, the fraction of coal convertedto CO2 increases. The trade off becomes then,either separating large amounts of steam from theproduct stream or separating the CO and recyclingit with a second steam feedstrearn. The chemicalequilibrium of the latter case can be investigated:

co ±

ized coal reactanc will bematerial. Because coal ccvolatile hydrocarbons, abevolved at very iov tempssteam itself will begin tc

very suitable seeding•' , a large amount of. £, gasep will, bees. In fact therip some of Mirm if

and the equilibrium equation is:

The results of these calculations are shown inFigure 2 for molar feed ratios of steam to CO of 1and 2. The equilibrium for this reaction isfavored by lower temperatures,.however, methaneformation becomes significant below 900°K.It is apparent from the graph that between 900°Kand 1000°K, conversions of 70S are readily attained.Again, Increasing the steam-CO ratio favors theconversion but the same separations problem is in-volved. This temperature range is compatible withwhat would be expected in the moderator-reflectorcooling channels close to the cavity. The secondstage of the conversion process could be effectedin passages next to the cooling channels. Thisreaction is exothermic however, and would add tothe heat load on the cooling channels in thisregion of the reflector, but the problem should notbe significant.

The Process Concept

The conceptual application of the process isdescribed in this section. The fissioning plasmareactor station is fixed in the ocean, off shoreof the continent. The off-shore location ischosen mainly for safety and environmental pro-tection reasons. Coal is barged to the facilityand sea water enters directly. The hydrogen andfresh water are piped directly to shore. Wasteheat is disposed of to the ocean and sea saltresulting from the desalination is returned toshore in the barges. See Figure 3.

Figure A is a schematic of the various processelements which are described below. The choice ofmethod for accomplishing the transformation in eachelement is not an optimal ore but rather the onemost obvious to the author.

Gas Core ReactorThe temperature field required for the chemical

reactions is modest in the context of proposed gascore reactors. It should be readily achieved ineither an open or closed cycle reactor. Therefore,both the coaxial flow GCR and the "light bulb" GCRare compatible concepts. Further, it does notappear that breeding of fuel or magnetohydrodynamicpov?er generation are precluded from also beingcarried out along with the hydrogen production inthe reactor facility.

The seeding of the working fluii1 in the cavityto make it opaque to thermal radiation is not anadded complexity in this application. The pulver-

the coal is soft enough. Furthermore, the cnsl inthe large amounts present may have some neutronmoderating effect, decreasing the critical massrequirements.

Water Supply and Cooling Requirements

no attempt has been made to estimate the energygeneration rate in the GCR. There will, however,be a large excess of energy generated over thatactually fixed chemically in the fuel. This excesswill come from cooling of the moderator-reflectoras well as the reaction products to suitable tem-peratures. It is expected that some of this wasteheat will be used to obtain the process steam bydistillation. Distillation is chosen rather thansome less energy-expensive desalination process inorder to keep the reactor cavity as free of saltas possible. The rest of the waste heat is usedfor desalination of sea water by some less expen-sive process. The final sink for waste heat afteras much useful work as possible has been obtainedis the ocean.

Process Elements

There are many process elements such as heatexchangers, and physical separations processesthat make up the facility along with the reactor.They are described briefly according to alphabetickey on Figure 3. (a) Distillation of seawater forreactant steam. This should be a multiple dis-tillation process in order to obtain a reactantstream which is as salt-free as possible, (b)Just downstream of the reactor is a heat exchangerin which fuel, and possibly some coal ash,is con-densed, (c) A cyclone follows the condenser inwhich the spent fuel and coal ash are removed.

(d) The product gases, still relatively hot, arecooled in a gas turbine. Electrical power forfacility use is generated from the power derived.(e) The cold gases are filtered to remove any re-maining solids. (f) The gases are cooled to roomtemperature in another heat exchanger and excesssteam is condensed and passed to the :.O stream.(g) The cool gases are passed through a refriger-ated charcoal bed tc remove radioactive gases suchas iodine. Two beds, operating in flip-flopfashion, can be used. One operates in an adsorb-ing mode while the other is being regenerated.(h) The hydrogen is removed in a pair of molecularsieves operating in flip-flop fashion and piped tothe mainland, (i) The CO stream from separatorII meets the condensed water stream from (f) and amake up steam stream from the distillation ur.itand is piped to the reactor reflector. The productstream from the reflector goes to a heat exchanger•..'here the gases are cooled and excess water con-densed, (j) The hydrogen in the cooled gases isremoved in separator III. This is another pair ofmolecular sieve units operating In flip-flopfashion. The CO and CO remaining is then ventedto the atmosphere.

Every element is not described in the abovesection. The heat removed in the heat exchangersalong with heat removed in the reflector is usedin the distillation column and an ancillary de-salination plant.

271

Closure

The concept described in this paper Is crudelyformulated and presented. However, the basic IdeaIs very sound?>°»9'10 The high temperatures of thegas core reactor can be used to effect valuable,endothermic chemical reactions in a non-catalyticmanner. The example used here was chosen becauseof its great current Interest. However, many otherreactions for the production of a whole range ofimportant organic compounds can be carried outinstead of or along w.L'-A the production ofh; irogen.

References

1. Harder, H., Fischer, K., Nuclear Process HeatProgrammes in Germany, Proceedings of the 1st

- National Topical Meeting on Nuclear Process HeatApplications^ Nov. 1974 (Also Nuclear Hews; ;No. 4, p. 56-62, May 1975).

2. Cohen, S., Nuclear Process Heat and Direct CoalGasification,Proceedings of the 1st NationalTopical Meeting on Nuclear Process Heat Appli-cations, Nov. 1974.

3. Schulten, R., Synthetic Natural Gas as a NuclearEnergy Carrier Over Long Distances, Umsch. Wise.Tech., 2i, No. 2, 53-54 (1973).

4. Schulten, E.; Kugeler, K., Coal Gasification &Other Nuclear Process Heat Applications,Proceedings of the 1st National Topical Meetingon Nuclear Process Heat Applications, Nov.,1974.

5. Hougen, 0. A., Watson, K. M., and Kagatz, R. A.Chemical.Process Principles. 2nd Ed., J. Wiley& Sons, Inc., New York, 1962.

6. Rossini, F. D., PiKer, K. S., Arnett, R. L.,Braun, R. M., and Pimental, G. C , SelectedValues of Physical and Thermodynamie Propertiesof Hydrocarbons and Related Compounds. CarnegiePress, Pittsburgh, 1953.

7. Von Heek, K, H.; Juentgen, H., Reactions of Coalin High-Temperature Plasmas, GlueckaufForschungsh, 33, No. 2, 45-47, (1972).

8. Sheer, Charles; Konnan, Samuel, Arc Synthesisof Hydrocarbons, Advances in Chemistry Merles.No. 131, Coal Gasification, 42-53, (1974).

9. Damo, Akibumi, Producing Hydrogen with NuclearFuel, Chem. Econ. and Eng. Rev., June, 1974.

10. Bal, S., Swierczek, R., and Musialski, A.,Power-Coal Gasification in Plasmochemical FlowReactors, Koks. Smola, Gag. 1971 16(4) 96-100.

500 1000 1500Equilibrium Temperature "Ka) Initial Feed:

2000

1 mo H.O/mo.C.

500 1000 1500 2000"K

Equilibrium Temperature °K,b) Initial Feed: 2 mo H20/mo C*

Figure 1. Equilibrium Conversionvs. Temperature for a C-H-0System at 500 psi.

272

110

.9

§18

I- 'Formation of conald- JJerable amounts of \^methane distort curves^i this region. \

H20/C0

600 700 800 900 1000

Equilibrium temperature, °K

Figure-Z.-Equiyexlam Conversion vs.Temperature' fop the. reactionCO + H.O - CO'V H,.

Coal Barge• /

Sea Salts to Barge

Fissioning PlasmaReactor Station

Hot SeawaterDischarge

Pipelines to Shore

Figure 3. Gas Core Reactor CoalGasification Facility

electricalPower

Nuclear Fu<

teflector CoolingCO, H2, H

Spent Fuel toRecycle, Coal

to Removal

RadioactiveGases

CO + CO2

to ExhaustSea Water

Figure 4. Schematic of Hydrogen Production Facility.

273

DISCUSSION

R. T. SCHNEIDER: In Europe last year, I saw anopen peat lignite mine in Germany which had justbeen started.. Forty five percent of Germanelectricity is generated with lignite. The pitsthey have started at this rate will last them500 years. The mining company was looking intogas cooled reactors to do what you have just said.Someone asked the question, "Why do you want tosave 20% of the coal if you have that much?" Thatis the question they will want to ask you perhaps.

H. TOINSTEIN: That is not that damning aquestion. The environmental impact of burningpeat and low grade coal Is sizeable, and it wouldbe easy to show that the hazards involved inlarge-scale power production around the worldwith coal is greater than with nuclear energy.The whole idea of producing important andexpensive organics can' be treated with a gas corevery nicely. Hydrogen is the easiest thing tomake.

274

CONSIDERATIONS TO ACHIEVE DIRECTIONALITY FOR GAMMA RAY LASERS

S. Jha« V. of Cincinnati, Cincinnati, OHJ. Blue, NASA-Lewis Research Center, Cleveland, OH

Abstract

This study concerns a method of align-ment of nuclei for a gamma ray laser and ameans of achieving preferential emission ofradiation along the crystal axis. Theseconsiderations are Important because itprobably Is not possible to achieve re-flection of gamma radiation in order tohave photons make multiple passes throughan active region. Atomic alignment hasbeen achieved by materials researchers whohave made composite structures composed ofneedle-like single crystals all with thesame orientation and all pointing in thesame direction contained in a matrix ofcobalt or nickel. The proposed method ofpreferential emission of radiation alongthe aligned needles is to have a symmetricfield gradient at the nucleus and a sequenceof excited levels of spin and parity 2* and0+. The proposed scheme will reduce thedensity of excited states required forlasing and reduce the linewi&th due tolnhomogenous broadening. Mossbauerabsorption experiments intended to testthese ideas are outlined.

I. Introduction

Historical Review

Recent review articles have indicatedthe substantial difficulties that exist inachieving a gamma ray laser.1'2 The 3tateof knowledge of the interactions of thenucleus and its surroundings was insuf-ficient to allow those who conceived theconcept of the gamma ray laser to fjreseemany of the problems.'•"

Perhaps the most difficult of theoriginally perceived problems, how to obtaina density of Inverted states sufficient tohave lasing, is still with us. There is,however, no known fundamental limitation toachieving high densities and man's con-tinuing effort to make his power sources(fission reactors, fusion reactors andparticle accelerators) more powerful maysomeday solve the intensity problem.5

An originally unappreciated problem,that fluctuating nuclear interactions withthe surroundings that broaden nuclearlevels to the extent that resonant absorp-tion and stimulated emission are unlikely,is the subject of this paper. The staticinteraction of the nuclear quadrupole

moment with an Internal electric fieldgradient is considered as a means toachieve:

1) energy level split Sing2) desirable directivity of the emitted

gamma radiation3) a practical physical material with

geometry suitable for a laser.

The conclusion of this paper unfortunatelywill not give a solution to all of theseproblems but is rather a suggestion as towhat nuclear energy level configuration isdesirable for a gamma ray laser. Thesecont'iderations will serve to guide ourfuture experimental research studies.

General Considerations for PopulationInversion

To achieve population inversion of nueJearlevels is not difficult if one takes ad-vantage of beta decay and nuclear iso-merism. This is illustrated with the decayof n 3 S n .

115d

A freshly separated source of i:L3sn,willdecay into the isomerie level of 11JIn andpopulation inversion will exist for severalhours; eventually the ground state popu-lation Increases and exceeds that of the•isomeric level.

Population inversion can be achieved bynuclear reactions as shown by the example:

(njf)

_» 50d

79 keV

mm, 72S

""•In

•supported by NASA Grant: NSG 3091

275

Energy -Requirements W(e) = 3 (cos2e + 4 cos^e) for Am = 0

The energy of the lsomeric level is aconsideration since the parasitic absorp-tion by atomic electrons is decreased formore energetic photons whereas the irosssection for stimulated emission is pro-portional toX and therefore favors low.energy transitions. Another considerationfavoring low-energies is that photonenergies must not be decreased by the re-coil of the emitting nucleus. The so-called recoilless fraction decreasesexponentially with increasing energy,becoming negligible above 150 keV. Inthis note we are proposing the use of onlycertain kinds of nuclear levels becauseone can thereby achieve: 1) unidirectionaltransmission of gamma rays. 2)by removinglevel degeneracy, photons with less spread. in energy than otherwise and 3) an energymatching of the phot®n with the levelsthat are to undergo stimulated emission.

II. Proposal

Electric Quadrupole Transitions

Gamma radiation emitted between nuclearlevels of spin and parity 2 + and 0 + or 0 +

and 2 + actually involves transitions to orfrom sublevels of the 2 + state since thatlevel splits into m = + 2, + 1 and 0. Ifthere is an electric field gradient qwhich is axially symmetric with respect tothe crystal axis then the sublevels splitas shown in figure 1, and the threetransitions im = + 2, Am = +, 1, andAm = 0 are shifted in energy as shown.

These radiation patterns are shown infigure 2.

Am= 11 Am t £•

0*o

4e2qQ

Fig. 1 Energy level sp l i t t ing of 2+ level in af ie ld gradient and also showing the threetransitions to a 0+ level.

The angular d i s t r i b u t i o n of the gammarad ia t ion with respect t o the symmetryaxis i s given by

W(e) = i (1 - cos % ) for Am = + 2

W(e) = i (1 - 3 cos2e + 4 cos ^d) for

Am = + 1

Fig.2 Electric quadrupole angular distributionsfor the three components of a 2*1 eve I.

Combining the information of the twofigures shows that the im = +_ 1 componentis emitted alona the crystal axis with anenergy E g - i/gerqQ . If the energyof the gamma ray is not too high and theDebye temperature of the solid is appro-priate then there may exist a largefraction of recoilless photons. When oneof these photons interacts with anothernucleus in the isomeric state then stimu-lated emission will occur. However, onlythe &m = + 1 transition will have anenergy match with the incident? photon.The gamma rays arising from the other twotransitions would emerge perpendicular tothe symmetry axis.

These ideas have been tested by carryingout Mossbauer absorption experiments forthe 2+-*-0+ transition. The absorption ofthe recoilless photon in a WS2 singlecrystal (hexagonal close packed) which hasan electric field gradient along the c axisat each tungsten nucleus gave the resultsshown in figure 3a. ° Only a single com-ponent, Am — +_ 1, is resonantly absorbedwhen the incident photon is oriented along

276

c ax-li. A similar result is shown infigure 3b for the photon of the 2+-»-0+

transition in 1'°Hf, which was absorbedin single crystal of1 hafnium metal.

VELOCITY (mm/ste)

Fig.3a Mossbauer transmission experiment of the2+-i-0+ transition in WS, for the casewhen the incident photon Is aligned withc axis of the crystal.

Velocity (mm/sec)

Fig.3b Same as 3a except the 2+-t-0+ transitionis transmitted In Hf metal single crystal.

In both cases the results confirmed thatthe Am = + 1 transition is absorbedparallel to the symmetry axis.

Filamentary alignment of the NuclearIsomers

It was recognized by the inventors ofthe gamma ray laser concept that thealignment of the active atoms on a singleaxis could solve the problem of "the non-existance of mirrors with which to form acavity. Whisker crystals were suggested

as a possible materials configuration.^

The idea of the whisker configurationcan be combined with the anisotroplcdistribution of the Am = + 1 quadrupoletransition. What is required to do this 3san electric field gradient along thewhisker filament and the crystal structureof the filament to be symmetric about thewhisker axis. The concept is shown infigure k.

Schematic illustration of nuclei In Isomericstates * * , aligned in filaments and emittingquadrupote radiation when an electric fieldgradient is present and is In the filamentarydirection.

Most of the photons from Am=±2 and Am=0transitions will be emitted out of thewhisker; those few emitted along the whiskeraxis will be shifted in energy from theAm=±l photons. The axially directed, Am=ilphotons will have less energy spread thanwould photons from a degenerate 2+ level.

Directlonally Solidified Eutectics (DSE)

Materials scientists are working on thedevelopment of reinforced materials wherethe strengthening members are filaments orlamellae of a precipitated eutectic. Theprecipitates are single crystal andaligned and are often spaced in a ratherregular array In the matrix alloy. Sucha material Is shown in figure 5, a SEMview of a directionally solidified rod ofnominal composition Co-15 Cr-20 Ni-10.5Hf ~.7C; prepared by V.G. Kim of NASA'sLewis Research Center." In this case theprecipitate Is HfC and the matrix materialhas been etched away to show the protrudingHfC filaments.

277

Additional considerations favor the0+-*2 sequence, i.e., the isomeric stateshould be the 0+ and the lower state 2 +

and both states should be above the groundstate. With this arrangement, there willbe no nuclear absorption of the recoillessgamma rays and there will be no broadeningof the 0 level due to the stochasticfield gradient or magnetic fields at thenucleus since a 0 state has no moment.The only factor then broadening the 0+

level will be the inhomogenous isomershift. We prefer the 2 + level to be short-lived, then its natural width will befar larger than the isomershift, thus anenergy match between the recoilless gammaray and the energy difference between0++2+ is assured and stimulated emissioncan occur.

The question as to whether the preferrednuclear level arrangement exists can onlybe answered by research studies inisomerism. A number of known 0+->-2+ icomersare given below.

Fig.5 Structure of directionally solidifiedTaC-15Cr-20Ni-Co alloy.

1.52J* HeV

The DSE alloys offer an obvioussolution to, the gamma ray laser require-ment for aligned nuclei. The requirementof symmetry and an electric field gradientalong the filament can be achieved ineither two ways:

1) the precipitate can have a crystalstructure which has isotropy about acrystal axis and the crystal axis isparallel to the filament axis; an exampleof such a structure is tungsten disulfide,a hexagonal close packed crystal, or

2) the precipitate can have an isotropicstructure, e.g., cubic, and be strainedin the filamentary direction by differ-ential expansion coefficients between theprecipitate and the matrix. The strainof the crystal would be expected to destroythe symmetry in the strain direction andgive rise to an electric field gradient.

Mossbauer absorber measurements willbe made using the HfC reinforced compositeas the absorber of the 93keV gamma fromthe 2+-*0'; transition in " o H f . If theHfC has become noncubic, this should bedetectable in the shape of the absorptionline.

Preferred Level Arrangement

As we have indicated, a long-livedIsofer having a spin sequence of levels2+->-0+ or 0 + 2 + in a host having anaxially symmetric field gradient willprovide directed gamma radiation.

..937 HeV 22.8ns

.862 HeV

70 G e2+

,1.2158 MeV 3.6ns

1.0396 HeV

188,'Kg.82*1 HeV

.A13 HeV

Isomeric states with longer lifetimesshould be sought for the gamms ray laser.In addition to the traditional islands ofi'omerism, many quasiparticle states,bandheads of decoupled bands and nuclearlevels with large changes in nuclear shapemay provide the longer-lived isomericstate of the 0+->-2+ type.

278

Conclusion

The rationale for searching for anuclear isomer with a 0+ spin and parityand decaying by means of a 0 -»-2+ t rans i -tion has been given. At this point nosuitable isomer is known, "he direction-al i ty of emission which a l£.ser requirescan be achieved in filamentary structureswith symmetry about the filaiTientary axis.Materials research in directionallysolidified, precipitate reinforced, alloysshould be watched for developments whichmay have the symmetry and electric fieldgradient required to achieve directionalgamma ray emission.

References

1. V. A. Busheuv and R. N. KuzminSov. Phys. Vsp., Vol. 17, 912, M. -Ju (1975)

2. G. C. Baldwin and R. V. Khokhlov,Phys. Today 28, 32 (1975)

3. B. V. Chirikov, Sov. Phys. - JETP 17.1355 (1963)

4. W. Vali, V. Vali, Proc. IEEE 51 182(1963)

5. M. N." Yakimenko, Sov. Phys. Usp. 17.651 Mar-Apr (1975)

6. Y. W. Chow et a l . Phys. Ltrs. 30B,171 (1969)

7. G. C. Baldwin et a l . , Proc, IEPF jjl.,1247 (1963)

8. Y. G. Kim, NASA TM X-71751

DISCUSSION

R. T. SCHNEIDER: I t there such a thing asstimulated emission in a crystal?

S. JHA: In the nuclear case, stimulated emissionhas not been experimentally demonstrated. Butthere i& no reason why there should not bestimulated emission. If the nucleus is sostrongly bound that i t excites without recoiling,the gamma ray leaves with full energy; if i t nowmeets an atom which is so strongly bound that i tcan accept th is gamma ray without recoiling, thenstimulated emission should take place.

J. BLUE: Nuclear resonant absorption has beenobserved, and i sn ' t i t probable that spontaneousemission has happened but not been observed? Ifa nucleus is going to resonantly absorb a photonto an upper level , then we know the fraction ofnuclei excited to that level , that should returnemitting photons.

S. JHA: I t is possible to create conditions,given appropriate energy and appropriatematerials that wi l l have 100% recoil less emission,such that a l l the gamma rays wil l be r eco i l l e s i .

279

PRINCETON UNIVERSITY CONFERENCE ON PARTIALLY IONIZED PLASMASIncluding the THIRD SYMPOSIUM ON URANIUM PLASMAS

June 10, 11, and 12, 1976WORKSHOP WORKING GROUP SUMMARY REPORT

GROUP I

PLASMA GENERATION AND CONFINEMENT

Chairman: J. S. KendallGroup Members: J. Blue, P. W. Levy, M. Stoenescu, S. Sutherland, and A. Kazi

Status of the Field:

In the past several years, gas-core uraniumplasma generation and confinement studies havecentered on two major reactor schemes — thevortex-stabilized uranium hexafluoride (UFg)

plasma contained in an ergon buffer gas, whichis a closed-cycle system, and the coaxi.,1 open-cycle gas core reactor. Significant progress hasbeen made on vortex-stibilized confinement ofUF& injected into an argon plasma, Cong-term

(~ 40 min) operation at high uranium particledensities has produced encouraging results withrespect to the degree of deposition on the fused-silica walls of the container. However, consi-derable work remains in this area before a high-power closed-cycle plasma core reactor experimentis performed.

The advancement of both open- and closed-cycle gas core reactor research might be aidedby more complete understanding of the complexfluid dynamics and its coupling with radiativetransport phenomena in the reactor cavity.

Proposed Goals and Research Directions:

Major directions recommended for the proposedresearch on g&; core plasma confinement are: '

(1) The fissioning uranium plasma

<•' (2) Transparent materials research

The Fissioning Plasma

Analytical and experimental research shouldbe continued to determine the dominant physicalprocesses relevant to the fluid dynamics andradiative heat transfer both within and at theedge of the fissioning plasma region. Developmentof a high-pressure vortex-stabilized optically-thick uranium plasma is a central feature to suchan endeavor.

Priority in future research should be givento the vortex-stabilized concept over the coaxialconcept for the following reasons:

(a) The virtual nonsxistence of potentiallyattractive terrestrial applications forthe coaxial flow concept; the coaxialconcept being better suited to thedesign of a nuclear rocket for spacemissions requiring high thrust densitiesand high specific impulses.

(b) The higher levels of uranium confinementachieved using the vortex scheme.

(c) The apparent absence of a "restoringforce" in the fuel region of the corefor the coaxial scheme.

Reactor experiments should be complementedby concurrent experiments aimed at establishingempirical, values of nonequilibrium transportcoefficients for fissioning plasmas potentiallysuited to nuclear punped laser applications.

Transparent Materials Research

Closed-cycle gas core reactors require useof transparent materials that remain optically-transparent over a wide spectral region andretain-meehanical integrity at high temperatureswhile subjected to intense nuclear radiation.Currently most research has focused on very purefused silica which appears to be a potentiallyattractive confinement material. Two areas offocus are suggested in future research:

(a) Radiation damage and color center forma-tion studies should be extended to muchhigher temperatures, radiation levels,and neutron and electron fluxes appro-priate to the highest power fissioningplasmas. Such studies should be aimedat predicting the absorption and mecha-nical properties of the confining mate-rial, as well as gauging its lifetime andreliability.

(b) The possibility of using materials otherthan fu.sed silica such as single crystalberyllium oxide should be investigated.

New Concepts and Directions:

The National Aeronautics and Space Admini-stration conducts and sponsors considerableresearch in fissioning plasma reactors. To cora-pleaent this research, it is suggested thataccelerator experiments be considered to evaluateUFg radiation decomposition by fission fragments,

to obtain spectra of emitted radiation and to studymaterials problems. Accelerator experiments withheavy particles bombarding transparent windowsmay be useful' for investigation of transparencyunder conditions analogous to reactor conditionswhere there are significant numbers of displacedatoms in the window material.

280

GROUP II

PLASMA CHARACTERISTICS

Chairman: J. H. LeeGroup Members: C. K. Choi, N. L. Krascella, D. Williams


At the present time, the available informa-tion on UF& and Uranium plasma characteristics

under non-equilibrium conditions is limited(Ref. H. Helmick, LASL). The analytical investi-gation of the transport of radiation and thedistribution of energy within reactor systems ishampered by a lack of accurate knowledge on suchessentials as cross sections and radiative ]"fa-times for transitions involving electronic,vibrational and rotational excitation and alsoon the dissociation of CUPA) molecules by fission

fragment impact. While some elastic crossiactions for electron-molecule (UF^) collision

pi jcesses are known [see the paper by S. Trajmaret. al. in this Proceedings), little work isdone for inelastic cross sections, involvingionization exitation, etc.

Similarly, although a significant amount ofdata exists on spectral emission characteristicsof fissioning uranium plasmas under equilibriumcondition., non-equilibrium radiation effects dueto deposition of large quantities of fissionfragment energy into the fissioning (UFg) plasma

have not been extensively examined on either atheoretical or an experimental basis. In addi-tion, it is of paramount", importance that theFp.itial and spectral content of radiation emittedby the fissioning plasma be known to aid thedesign of in-reactri- experiments as well as thedesign and feasibility assessment of plasma corereactor applications. The above comments applyparticularly sttongly to. nuclear pumped laserresearch, which cannot go forward as rapidly inthe absence of detailed knowledge of non-equilibrium plasma characteristics.

In particular, more extensive work isrequired in the future regarding in-core measure-ments of density, temperature and neutron fluxeswith both space and time resolution. At thepresent tine, there are well established tech-niques for measuring all of these quantities butonly in a spatially and temporally averaged form.Density measurements reported so far have tendedto be confined to regions near the axis of theflowing (UF6) system. Profiles of density do

exist for unenriched (UFg) but it is of some use

to measure such profiles in a reacting systemwhich includes the effect of a neutron flux onthe density.

In summary, substantial data exists forlow pressure, low temperature (UFg) plasmas.

However, such data lacks certain essential

features of non-equilibrium plasmas and is alsonot compiled in a comprehensive manner. It wouldbe of .great benefit to the plasma core reactorresearch community to obtain a comprehensivecompilation of all of the relevant data on (UF6)plasma characteristics.

For the progress toward the ultimate goalof U-235 plasma-core reactor, • basic studies ona metalic U-235 plasma are also necessary. Theemission characteristics of metallic uraniumplasua in UV and soft x-ray ranges have beenreported (see thejpaper by M. D. Williams, et al.in this Proceedings). However, the effect offissioning has no; been observed due to the lowreaction rate in the plasma. A study of metallicuranium plasma in a high-flux neutron field isdesired.


The proposed research on fissioning UFfi and

uranium plasma characteristics may be divided intofour areas:

a) Measurement of inelastic cross-sections,vibrational and rotational energy levels,and dissociation by fission fragmentsfor both (UF6) and its decomposites.

b) Measurement of the Spectral Emission fromfissioning (UFg) plasmas. Initial Los

Alamos Scientific Laboratory (LASL)reactor experiments will probably rotoperate at temperatures sufficient toproduce appreciable emission in thefissioning (UF&). Therefore, simulation

experiments utilizing the GODrVA-BallisticPiston combination should be acceleratedto provide initial (UFg) spectral emission

data in a fissioning gas oysr s. range oftemperatures and pressures as well asneutron fluxes. These initial experimentswould provide valuable baseline data forthe design of similar subsequent experi-ments in various cavity reactor programs.Finally, the ultimate goal of this researchshould be to incorporate extensive radia-tion emission studies in the actualfissioning gaseous (UF,) experimentalseries.

c) To complement the X-ray diagnostictechniques at United Technologies ResearchCenter, in-core measurements should bemade of density, temperature and neutronfluxes with space and time resolution.These measurements are essential to the

281

understanding of reactor start-updynamics and to. evaluate the effects onplasma parameters of neotron coupling.A good diagnostic would be a pulsedlaser which can be focused down from arather large .aperture to spots through-out the volume of the (UF6) tank. The

back sc ttering of this coherent radia-tion yields temperatures and densitiesof various species throughout the plasma.

* The pulse shape will provide temporalresolution. Other diagnostics may alsobe readily applied to complement theabove.

d) Study on a metallic uranium plasma, as along-range program, should be continued.Especially the non-thermal effect offissioning in the uranium plasma shouldbe determined to obtain a referencedata for reactor development and nuclearpumping of lasers in the plasma reactor.

e] Assignment of a special task group forthe purpose of compilation and review

of (UF6) and U plssma data. The

group will also conduct revisionsusing new data aa It becomes avail-able. The group may identify andr jcommend necessary measurements.


• The use of tunable dye lasers is suggestedfor determination of CU^g) plasma characteristics,

especially under reactor conditions. Elect"beam probes may also o-nplement the laserdiagnostics.

282

GROUP III

NUCLEAR PUMPED LASERS

Chairman: F. HohlGroup Members: G. Cooper, R. De Young, J. Early, D. Lorents, D. McArthur, S. Suckewer,

P. Thiess, S. Trajmar, H. A. Hassan


The potential for achieving high powerdensity, low wavelength lasers by direct nuclearpumping has been keeping the area of nuclearpumping active since about 1961. Several reviewsof the field have appeared recently (Thorn, K; andSchneider, R. T.: AIAA J. 10, 400 (1972):Schneider, R. T.; and Thorn, K.: Nuclear Tech. 27 ,34 (1975)). However, it has only been in thepast 2 years that considerable progress has beenmade toward achieving high-power direct nuclear-pumped lasers. Present research uses neutronsfrom high flux reactors to induce nuclear reaction

in either U or B laser tube wall coatings,

or in He mixed with the lasing gas (see thepaper by R. De Young, et al. in this Proceedings).

The first two nuclear pumped lasers using

U wall coatings were demonstrated in 1974by researchers at Sandia Laboratories (McArthur,D. A.; and Tollefsrud, P. B,: Appl. Phys. Lett.26, 187 (1975) and at the Los Alamos ScientificLaboratory (Helmick, H. H.; Fuller, J.; andSchneider, R. T.: Appl. Phys. Lett. 26, 327(1975)). The Sandia group observed lasing ofCO at the 5.24 and 5.6 tin vibrational bonds. Atpresent they have achieved outputs of 100 wattsat relatively high efficiencies. The Los Alamosgroup achieved lasing of a He-Xe laser at 3.5Urn. The lowest wavelength and lowest neutronthreshold for a nuclear pumped laser has beenachieved by the University of Illinois group(De Young, R. J.; Wells, W. E.; Miley, G. H.:and Verdeyen, J. T.: Appl. Phys. Lett. 28,194 (1976)). They achieved lasing of a Ne-N7

o ft c

system at 8629 A and 9393 A. The first volumejumped laser was achieved only recently by theNASA-Langley Group (Jalufka, N. W., DeYoung,R. J.; Hohl, F.; and Williams, M. D.: Appl. Phys.Lett. 28, (197S)) who used the He3(n,P)3H vol-umetric reaction to pump a He -Ar system whichlased at 1.79 um. Table I summarizes the nuclear-pumped laser results as of June 1976.

In addition to experimental work, consider-able progress is being made in modelling nuclearpumping of lasers. Energy deposition, electrondistribution, and excitation of various lasertransitions are being predicted. Details ofthat work are given in the Proceedings.

It should be noted that all results achievedso far demonstrate only "proof of principle."What must be demonstrated next is that high-power nuclecr-pumped laser can be competitivewith more conventional lasers.


Broadly speaking, the goals for NuclearPumped Laser Research fall into two categories:

1) To demonstrate the advantages of usingnuclear pumping over other possiblepumping mechanisms;

2) to determine what limitations, if any,are imposed on the lasing system by thecharacteristics of the pumping siurce,i.e., the fissioning plasma.

More specifically, the research goals may besummarized as follows:

a) To achieve high efficiency combined withhigh power CK laser output;

b) to extend the lasing wavelength range toshorter wavelengths such as the visibleor ever, ultraviolet region of the spectrum;

c) to investigate the possibility ofvolumetric pumping using Uranium Hexa-fluoride (UFfi) - laser gas mixtures;

d) to investigate the compatibility of laser' threshold flux requirements with the

actual reactor flux levels.

Special recommendations for research aimedat achieving the abeve goals are as follows:

1) Identify laser media that are most amen-able to nuclear pumping.

2) Conduct experiments to establish howwell electron-beam pumping of ;• lasermedium simulates nuclecr pumping.

3) Obtain valuable kinetic data such as onradiative processes, electron collisionprocesses and excitation and chargetransfer processes. Use such data tomodel candidate laser systems. Analy-tical modelling should of course runconcurrently with ongoing experimentalresearch.

4) Investigate the relative merits of self-critical and externally driven neutronpumping schemes.

5) Identify the kinetics of the CarbonMonoxide nuclear pumped laser with aview to optimising and scaling thislaser to higher powers.

283

6) Investigate the advantages of nuclearpumping of noble gas-excimer lasermedia.

7) In a carefully diagnosed nuclear pumpedlaser system, compare volumetric versussurface-coated pumping sources fordifferent laser media.

8) Conduct further detailed investigationsof the effects of electric fields onthe performance characteristics ofnuclear pumped lasers,


Nuclear pumped laser research has madesignificant progress in the past two years. Thefeasibility of producing a laser pumped byfission fragments has been clearly demonstrated.Spurred by these successes, nuclear pumpedlaser research may now continue in many poten-tially fruitful directions. A variety of conceptsand innovative research directions may beactively pursued.

Whereas it is highly desirable to have acritical laser-reactor using UF- as a fuel, such

a system is complicated by the love vapor pressureof UFg at temperatures'appropriate for lasing.It is therefore of interest to study otherpossible fuels such as the transuranic elementswhose nuclei offer much higher fission crosssections, or liquids which contain the transuranicelements. Since fusion reactors are potentialsources of neutron and charged particle fluxes,their application to nuclear pimped lasersshould be investigated.

Nuclear radiation enhancement wf ,i elec-trically pumped laser has been experimentallydemonstrated. Preliminary studies at Los AlamosScientific Laboratories on the effects of anelectric field on a nuclear pumped laser indicatethe potential for. substantial enhancement of thelaser output at a minimal cost of electricalenergy. It is therefore of great interestto investigate further the physics and economicsof electric field enhancement of nuclear pumpedlasers.

Finally, nuclear pumped flowing gas andgas dynamic lasers are also worthy of furtherresearch. Kfiile such laser systems do sufferthe inefficiencies of foil excitation, theydo offer the possibility of making very largelasers for which there are many potentialapplications.

Table 1. Summary of Nuclear-Pumped Laser Results

NUCLEAR PUMPEDLASERS

(Hay 1976)

Y-PumpedHF laserLos Alamos

r- PumpedXe Ampl i f ied

spontaneousEmission

Livermore

Fission FragmentCO Laser

Sandia Labs

Fission FragmentHe-Xe Laser

Los Alamos

B1 0(n,d,) L I 7

Ne-Ng LaserUniv. of I l l inois

He3(n,p)H3

He-Ar LaserNASA-Langley

H'YRENGm

-1700 8

5.1-5.6 um

3.0-4.2 um

8629 %and

9393 8

5.79 u

PRESSURE(TORR)

1.3

5,2007,800

10,50013,000•5,700

100

200

75-400

200-700

ThERMW.FLUX

THRESHOLD

(n/cmz-sec

- 5 x l O W

3xlO 1 5

i x l O 1 5

1.4xlO 1 6

ENERGYDEPOSITEE

(J/a>TIME

PERIOD

• 92312 nsec

Sxl 03. to1SxlOJ

17 sec

200150 usec

50150 psec

313 to1000

10 msec

24 to 270150 us

DURATIONOF

LASEROUTPUT

12 nsec

31-10 nsecA.S.E.

50 usec

235 usec

6 srnec

550 usec

PEAK POWERDEPOSITED

(W/*)

8 x 1010

2.7 X 1011

to „9 X lO 1 '

1.3 X 106

3.3 X 105

3 x 10*to 5

1 x 10*

.6 x 105

t 0 «.8 x 106

PEAK LASEROUTPUT

TO]5 x 108

92.3

2 - 60.2

> .012.8 x 10'5

1.5 x 10"3

2.4 x 10"5

0.05

284

GROUP IV

REACTOR CONCEPTS, SYSTEMS, AND APPLICATIONS

Chairman: H. WeinsteinGroup Members: T. S. Latham, M. Suo, E. Maceda, S. Chow, H. Helmick


Plasma-Core Nuclear Reactor concepts may beconveniently divided into c sed cycle and opencycle systems. Open cycle systems are predom-inantly of the coaxial flow type in which walljet injection of propellant keeps the fissioningplasma in the central region of the cavity sur-rounded by a moderator-reflector shell and highpressure buffer gas. Closed cycle systems, suchas the Nuclear Light Bulb, confine the fissioningfuel by means of a tangentially injected swirlflow of a buffer gas. In both cases the transferof power from the fissioning fuel to the propellantor buffer gas is by radiative heat transfer.Obviously, in the coaxial system, such transfercan go unimpeded, whereas, in the closed cyclesystem, limited transparency of the confiningcell limits radiative energy fluxes. On theother hand, the closed cycle concept offerscomplete fuel confinement, while in the coaxialopen flow system, flow mixing can lead to fuellosses and furthermore, the efflux from thereactor of radicactive material relegates thisconcept to solely space power application withno realistic terrestrial power applications.

The fuel used in these reactors may be apure fissionable fuel, a compound such asUranium Hexafluoride (UF6) or perhaps a hybrid

composed of a solid fuel-UFg combination.

At the present time there are few enoughconcepts and no well established basis forordering them in terms of probability of success.Consequently, all concepts should be vigorouslyinvestigated. The real basis of priority for agiven concept derives from its applicabilityto a particular mission, be it ground-based orspace-based.

The applications for plasma-core nuclearreactor systems are numerous. They fall intothree major categories:

, a) Space propulsion and power

b) Terrestrial power concepts

c) Indirect applications

Space Propulsion and Power

The gas-core nuclear reactor is ratherversatile in that it can provide (for examplein a coaxial flow configuration) direct propul-sion for a spacecraft by acceleration of pronell-ant to high velocities resulting in thrust levelsand specific impulses for exceeding thoseattainable by conventional chemical rockets.Simultaneously the nuclear power plant can also

supply electrical power for the maintenance cfspacecraft functions. Alternatively, a closedcycle system may be used to generate electricalpower for use in ion thrusters or magnetopl-isma-dynamic gas accelerators or even to generate ahigh power laser beam which may then be usedeither to transmit power to other regions inspace or else to provide thrust via a laser-heated thrust chamber.

Terrestrial Power Concepts

The closed cycle reactor system is bestsuited to ground-based power needs. Gas-corenuclear reactors offer a new technology for theuse of high temperature (~4000 K) working fluids.There is also the attractive possibility ofdirect -oupling of the radiant energy in thereacto. to output laser radiation and other directconversion schemes. The compactness of feasiblegas-core fissioning reactor systems combined withtheir efficient operation at temperatures oforder 4000 K make them viable alternatives tofusion reactor systems which are necessarilyburdened by mammoth dimensions and plasma

temperatures in excess of 10 K.

Indirect Applications

The gas-cere nuclear reactor system is anexcellent source for indirect power conversionsuch as radiochemical, thermochemical, andphotochemical schemes, since it produces radiantpower over a very wide and easily tunable band-width of the electromagnetic spectrum. Notnecessarily"being restricted to power conversionschemes, the gas-core reactor is 2lso an excellentsource of high temperature process heat formetallurigal uses and also for photochemicalproduction processes.

An Ideal Fis»ion Power Reactor must satisfy

the following requirements: *• '

a) Low Critical Mass ~ 10kg

b) Small Units Possible (MW range)

c) Fuel Circulation and On-Site Processing

d) Burnup of Transuranium Actinides andCertain Fission Fragments

e) Increased Fuel Utilization

"•MCarlheinz Thorn, "Gaseous Fuel Nuclear ReactorResearch" invited paper at 1976 IEEEInternational Conference on Plasma Science,May 24-26, 1976, held at Austin, Texas.

285

GROUP IV, page 2

f) High Efficiency Power Generation

g] Breeding of U-233 from Thorium

h) Low Fission Fragment Inventory

i) High Temperatures for Process Heat (coaigasification, steel and hydrogen produc-tion, etc.)

j) Non-equilibrium Radiation for Photo-chemistry and Hydrogen Production

k) Nuclear Pumped Lasers

1) Advanced Propulsion in Space

m) Nuclear Safeguards - Non proliferationof Atomic Weapons

The Gaseous Fuel Reactor is ideally suitedto all of these requirements and is worthy ofconsiderable attention in the future. Figures 1and 2 summarize the essential features ofConventional Nuclear Power Plants and GaseousFuel Reactors and highlight the advantages ofdeveloping Gaseous Fuel Reactors in preferenceto present day nuclear power plants.


In planning ? well balanced program ofresearch into cavi'y reactor systems, the basisof priority should be on the most suitablesystem concept for a particular application.There are few enough concepts at the present timeand no • —tablished basis for ordering themin te. jjrobability of success, so that allconcej-^j snould be vigorously investigated. Thepriorities for program planning may be classifiedas follows:

a) Maintenance of a program of research inbaseline technology such as cavity reactorexperiments, f'uid mechanics, fusl hand-ling, laser pumping aspects, and plasmaexperiments of fundamental nature on, forexample, plasma characteristics etc.

b) Choice of a set of reference systems thatare competitive with other systems forparticular applications.

c) Maintenance of an effort into the develop-ment of a broad spectrum of innovativereactor concepts to supplement currentideas.

d) Maintenance of a program to developsupporting technology such as inmaterials, handling, fuel recycling, etc.

e) Total benefit evaluation of different•concepts. Such an evaluation must takeinto account the following considerations:

(i) minimal environmental impact

(ii) nuclear safeguards

(iii) efficient energy resource utilization

(iv) exploitation of low grade nuclear fuelsand breeding possibilities.

Based on these priorities and program goals,research should revolve around five key areasas outlined b^low:

a) Materials

b) Plasma Confinement and Radiative HeatTransfer

c) Fuel Recycling and Processing

d) System Confinement and Radioactive MaterialSeparation

e) Integrity of the boundary of the reactorflow system -- Mechanical Design Features.

Kcrsstve REACTIVITY

Figure 1 Conventional Nuclear Power Plant

TOWEHFUNT

QUICK SHUTDOWNAND SAKTV t WFOR CQttKACl WIATIQH

Figure 2 Gaseous IJuel Reactor

286

PRINCETON UNIVERSITY CONFERENCE ON PARTIALLY IONIZED PLASMASIncluding the THIRD SYMPOSIUM ON URANIUM PLASMAS

June 10, n and 12, 1976

List of Participants

M. A. AkermanNuclear Engineering ProgramUniversity of IllinoisUrbana, IL 61801

J. H. AndersonNuclear Engineering ProgramUniversity of IllinoisUrbana, IL 61801

Michael J. AntalDepartment of Aerospace &

Mechanical SciencesPrinceton UniversityPrinceton, NJ 08540

C. BathkePlasma Physics LaboratoryPrinceton UniversityPrinceton, NJ 08540

G. K. BienkowskiDepartment of Aerospace &


J. BlueNASA Lewis Research Center21000 Brookpark RoadCleveland, OH 44135

S. M. BogdonoffDepartment *>f Aerospace &


C. ByvikMail Stop 231ANASA Langley Research CenterHampton, VA 23669

S. I. ChengDepartment of Aerospace &

Mechanical SciencesI'rinceton UniversityPrinceton, NJ 08540

C. ChoiUniversity of Illinois214 Nuclear Engineering LaboratoryUrbana, IL 61801

S. Chow75 Montgomery Street, Apt. J.8ANew York, NX 10002

K. E. ClarkDepartment of Aerospace &

Mechanical SciencesEQ D3O9Princecon UniversityPrinceton, NJ 08540

J. D. ClementSchool of Nuclear EnglneeiingGeorgia Institute ot TechnologyGeorgia, GA 30332

G. W. CooperG.iseous Electronics LaboratoryUniversity of Illinois607 East Healey StreetChampaign, IL 61820

J. F. DavisDepartment of Nuclear Engineering

&. ScienceUniversity of FloridaGainesville, FL 32611

R. De YoungNASA Langley Research CenterHampton, VA 23365

D. H. Douglas-HamiltonAVCO Everett Research Laboratory201 Lowell StreetWilmington, MA 01887

J. T. EarlyLawrence Livermore LaboratoryUniversity of CaliforniaLivermore, CA

J. GreyAmerican Institute of Aeronautics

& Astronautics1290 Avenue of the AmericasNew York, NY 10019

H. A. HassanDepartment of Aerospace &

Mechanical SciencesUniversity of North CarolinaP. 0. Box 5246Raleigh, NC 27607

H. H. HelmickP. 0. Box 1663, Group A5Los Alamos Scientific LaboratoryLos AJamos, NM 87544

287

R. V. HessLaser Molecular Physics BranchHASA Langley Research CenterMail Stop 160Hampton, VA 23360

F. HohlMail Stop. ISOHASA Langley Research CenterHampton, VA 23660

R. G. JahnSchool, of Engineering & Applied

SciencePrinceton UniversityPrinceton, NJ OJ354O

J. F. JaainetUnited Technologies Research CenterSilver LaneEast Hartford, CT 06108

S. JhaDepartment of PhysicsUniversity of CincinnatiCincinnati, OH 45Z21

A. H.Army Post Radiation FacilityBldg. 860Aberdeen Proving Ground, MD 21005

J. S. KendallUnited Technologies Research CenterSilver LaneEast Hartford, CT 06108

N. L. KrascellaUnited Technologies Research Center400 Main StreetHartford, CT 06108

M. KriahnauDepartment of Aerospace &


S. H. LamDepartment of Aerospace it

Mechanical ScliutssPrinceton UniversityPrinceton, NJ 08540

T. S. LathamUnited Technologies Research CenterSilver LaneEf.st Hartford, CT 06108

J. LaudenslagerJet Propulsion Laboratory4800 Oak Grove DrivePasadena, CA 91103

J. P. LaytonThe Aerospace Systems LaboratoryPrinceton UniversityPrinceton, NJ 08540

J. H. LeeBox 1807Department of Physics & AstronomyVanderbilt UniversityNashville, TN 37235

P. W. LevyBrookhaven National LaboratoryUpton, Long Is land, OT 11973

G. LewickiJet Propulsion Laboratory4800 Oak Grove DrivePasadena, CA 91103

D. C. LorentsMolecular Physics CenterStanford Research InstituteMenlo Park, CA 94025

E. MacedaNuclear Engineering Laboratory"diversity of IllinoisUrbana, IL 61801

T. C. MaguireDepartment of Nuclear EngineeringUniversity of FloridaGainesville, FL 32611

J. L. MasonThe Gart'ett Corporation9851 Sepulveda Blvd.Los Angeles, CA 90009

D. A- MeArthurDivision 5423Sandia LaboratoriesAlbuquerque, NM 87115

R. B. MilesDepartment of Aerospace &


G. H. Miley214 Nuclear Engineering LaboratoryUniversity of IllinoisUrbana, IL 61801

T. G. MillerHigh Energy Laser LaboratoryRedstone Arsenal. AL 35809

D. H. MonsonMail Code N23O-3NASA Ames Research CenterMoffett Field, CA 94035

E. V. PawllkJet Propulsion Laboratory4£00 Oak Grove DrivePasadena, CA 91103

R. J. Rodgerstnited Technologies Research Center400 Main StreetEast Hartford, CT 06108

288

W. C. RomanUnited Technologies Research CenterSilver LaneEast Hartford, CT 06108

G. R. RussellMail Code 122-123Jet Propulsion Laboratory4800 Oak Grove DrivePasadena, CA 91103

J. H. RustSchool of Nuclear EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332

R. T. SchneiderDepartment of Nuclear EngineeringUniversity of FloridaGainesville, FL 32611

B. G. Schnitzler202 Nuclear Sciences BuildingUniversity of FloridaGainesville, FL 32611

F. C. SchwenkCode RRNASA HeadquartersWashington, DC 20546

W. A. SirignanoDepartment of Aerospace &


D. E. SmithSayre Hall, Forrestal CampusPrinceton UniveristyPrinceton, NJ 08540

T. M. SmithSayre Hall, Forrestal CampusPrinceton UniversityPrinceton, NJ 08540

D. E. SterrittTennessee Valley Authority216 Haney BuildingChattanooga, TN 37401

R. C. StoefflerUnited Technologies Research CenterSilver LaneEast Hartford, CT 06108

M. L. StoenescuSayre Hall, Forrestal CampusPrinceton UniversityPrinceton, NJ 08540

S. SuckewerForrestal CampusPrinceton UniversityPrinceton, NJ 08540

M. SuoPower System TechnologyUnited Technologies Research CenterSilver LaneEast Hartford, CT 06103

S. SutherlandUniversity of Illinois214 Nuclear Engineering LaboratoryUrbana, IL 61801

T. B. TaylorInternational Research & Technology Corp.1501 Wilson BoulevardArlington, VA 22209

P. E. ThiessUniversity of IllinoisChampaign, IL 61001

K. rbomCode RRNASA HeadquartersWashington, DC 20546

L. ToburenBattelie Northwest LaboratoryBox 999Richland, WA 99352

S. TrajmarMail Code 183-401Jet Propulsion Laboratory4800 Oak Grove DrivePasadena, CA 91103

W. F. von JaskowskyDepartment of Aerospace &


H. We insteinIllinois Institute of TechnologyChicago, IL 60616

P.. J. WilburMechanical Engineering DepartmentEngineering CenterColorado State UniversityFort Collins, CO 80521

M. D. WilliamsNASA Langley Research CenterHampton, VA 23365

J. R. WilliamsCollege of EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332

H. P. YockeyAberdeen Proving Ground, MD 2100.5

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