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LA-UR--88-33O4

DE89 002053

TITLE EXPERIMENTAL STIJDIES OF X-RAY EMISSION PHYSICS AND HYDRODYNAMICSUSING SHORT WAVELENGTH USERS

AUTHOR(S) P. D. Goldstone, D. E. Casperson, J. A. Cobble, S. V. Coggeshall,C. C. Gomez, A. Hauer, G. A. Kyrala, P. H. Y, Lee, W. C. Mead, ●

G. T. Schappert, and A. J. Taylor

S~JtIMITTED TO 19th ECLIM

Madrid, SpainOctober 3-7, 1988

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EXPERIMENTAL STUDIES OF X-WY EMISSION PHYSICS AND HYDRODYNAMICSUSING SHORT WAVELENGTH LASERS

P. D, Golds tone, D. E. Casperson, J. A. Cobble, S. V. Coggeshall, C. C. Gomez, A. Hauer,G. A. Kyrala, P. H. Y. he, W. C. Mead, G. T. Schappert, and A. J. Taylor

Los Alamos National LaboratoryIms Alamos, New Ilexico, USA 87545

ABSTUCT

Several experimental efforts are currentlyunder way at Los Alamos to study issues ofimportance for inertial confinement fusionwith short wavelength lasers. These issuesinclude the physics of x-ray conversion andthe dynamics of short-wavelength laserinteraction with high-:! plasmas;filamentation and self-focusing processes;and the growth of instabilities in laser-driven implosions. Most of theseexperiments arz being pursued incollaboration with other laboratories ,notably the University of Rochester andbwrence Livermore National hboratory. Inaddition, we are undertaking basic studiesof the interaction of both atomi~,systeysand solids with ultra-intense (-10 W/cm )subpicosecond lasers at LQS Alamos. Theseexperiments explore the response of atomicsystems to strong fields , ❑ultiphotonexcitation, ●nd transient phenomena indense plasmas.

INTRODUCTIONWhile Los Alamos is developing KrF

lasers for(ICF) lnertiall ,2Ynfin::ent O:;::;:applicationsexperimental program has been pursued toprovide greater understanding of issuesimportant to achieving high-gain targetperformance in a tiboratory HicrofusionFacility. All of these ●xperiments haveinvolved collaboratiori with otherlabr,ratories with short-wavelength laserfac~lities. The primary e::perimentaleffortG involve 6tudie8 of x-rayconversion, hydrudynarnic instability growthand fuel-pusher mix, and filamentation andBelf-focusitlg. The first two studies willbe described in more detail brlow, Inadd;tion, preparatlorls are in progress for●xperlmenta using the Aurora KrF las~r to

atudy these and other iesues; these willnot be described here. Theoretical, laserdevelopment, and target fabrication work at

Los Alamos related to ICF is described Inseparate papers in this conference.

In addition to experiments directlyrelated to ICF, fundamental processes inlaser-plasma physics, ●tomic physics jnstrong fields, ●nd multiphoton excir.arionsare being investigated with the new LosAlamos Bright Source (LABS) laser. Currentexperiments use 700 fs, 248 ruo (KrF) pulses

?Pr ~iding2focused intensities in excess of10 W/cm . Specific experiments are beingpursued with this source to study: solidtarget interactions and x-ray production;multiphoton ionization; inner-shellexcitations via collective electronoscillations ; nuclear excitations; harmonicgeneration; above-threshold ionization; andsome x-ray laser concepts. Below, wediscuss the Study of inner-shellexcitations In ❑ ore detail.

X-RAY COtPJERSION PHYSICSX-ray conversion is an important

efficiency step for indirect-drive ICF.Together with collaborators at theUniversity of Rochester and N’RL3) we havecontinued to investigate the physics of x-ray conversion in high-Z plasmas usingshort-wavelength lasers, These experimentshave been pmrformed using spherical targetson the 351-nulRochester. ‘u; ~~;~~:~ w~!k$’l laser atindicatedthat the x-ray conversion efficiency WLIswell-reproduced by LASNEX calculations forkilojoule, 24-beam irradiations of

:p;’;:!5°var~,cm$~e intensir.y range 4 x 10~~’~Conversion efficiencies in

;:;::S:’’7:’;;IZ ~,cmY●chie ed for lnt.ensfties

However, thp

conversf on ●fflclency for more nollunlforrn6-beam lllumlnatfon, using lower erlergy and

smaller targets to preserve incidentintensity, was significantly reduced.Subkilovolt x-ray spectra were alsomeasured using a transmission gratingspectrograph and were significantlydifferent from I.ASNEX calculations. Ourrecent experiments were aimed atdetermining the relative effects ofillumination uniformity and energy/targetsize in the 6-beam/24-beam discrepancy. Inaddition we have studied whether x-rayconversion could be ●nhanced by varying t}.etarget Z or by significantly reducinginitial target density, as suggested bysome simulations.

Figure 1 shows the previous (1985)data of reference 4, together with apreliminary analysis of newconversion data

x-rayfor gold, bismuth, and

uranium targets. No significant change inx-ray conversion efficiency is observed.

Figure ? shows that the reduced x-rayconversion for the 6-beam data was●pparently due to the nonuniformity ofillumination. Repeating the previous 24-team ●xperiments and then reducing thelaser energy and target size to the valuesused in the previous six-beam experimentsproduced high x-ray conversion ●fficiencyin both cases, consistent (to within theuncertainties of this first analysis) withthe prev~ous 24-beam results. The 24-beamillumination produces rms nonuniformitiesof -20t; the six-beam illumination producedan rms nonuniformity of -SOB and had“macroscopic” intensity variations of -2:1with a lateral ~cale size comparable to thetarget radius,

Here uni form illumination Llianprovided hy t-he ?h nominal Omega beams can

;;;:;0 .ved try the use of distributed p}lase

Aowever, a compar[sor, of gold x-ray con~jersionconditions at -4 ‘H’oJ”&Ns’%E: ‘:;change in the cor,vers{.on●fficiency. Theseaxperimenta ❑ay provide an indication ofthe ❑agnitude or scale mlze of theillumirrar.lon nonun~forrnf[y nc,censary toproduce d~trlmental effert~ 011 x-mycorrverslon.

Compnra(ive ❑ea~uremcnt H ofconvers 1on efffclcllcy for fdolid go

+9anti

1.4-7.4s de~~#i[y gold targets, mt )0 ‘ al~rl

1015 U/cm*, showed no enhancement of x-rayconversion for the low-density targets.

140st spectroscopic results from thisexperiment are still being analyzed.However, It is apparent that thesubkilovolt spectra are in generalagreement with the previous data, and that~ubtle differences in the width of the N-and O-band features

‘kj11 appear as a

function of target 6ize.

./7

HYDRODYNAMIC INSTABILITY GROWTHMost laser-driven measurements of

hydrodynamic instability growth have usedplanar geometry because of itsdiagnosibility. However, there have beenrecent significant advances in x-raybacklighting technology, x-ray gated-irnaging technology, target fabrication, andnon-LTE theoretical ❑odeling. As a resultit is possible to obtain and interpretspectroscopically resolved, <1oo ps“snapshots” of medium-Z tracer ❑aterialsembedded as a shell or band into a low-Zpusher. Ultimately, high spatial- andspectral- resolution imaging ❑ay allowdetailed studies of mix and the state ofthe fuel-pusher interface. Ims Alamos’effort to develop and use these techniquesinvolves collaboration with severallaboratories .

An experiment has9een designed in

collaboration with ~,47 to use x-ray

imaging techniques for studies ofinstability growth in spherical geometry.Diagnostic techniques correlative to theseexperiments are being tested on the Omegalaser at Rochester and at KMSFusion, Inc.,in collaboration with those institutions.

Conceptually, in these experiments athin x-ray absorbing tracer layer isembedded in a low-Z shell (e.g. plastic).A flash x-ray backlighting source Is thenused to produce an (absorption) image ofthis layer at characteristic times duringthe implo~ion. Initial ❑easurements willstress proving the technique by imaging thegrowth of simple perturbations (e.g. bumpson th? surface of a low-Z shell) that havelarge amplitudes (in the nonlinear regime)as the shell is Imploded to ~pproximately1/3 icis initial radius. Estimates,assuming half-classical growth, suggestthat an initial perturbation of 7 ❑icronsamplitude would grow to approximately 40microns , which is expected to be readilyobservable . These techniques vill beextended in the future to combinenpectroacopy with imaging.

Preliminary tests of some of theseideas have been initiated on the NOVA laserat LLNL. An unfilled shell with a 5000-Angstrom KC1 tracer layer was backlitduring its implosion. The backlit imagewas projected onto the ant of an x-ray

streak camera by a Uolter x-ray microscope.A shadoti of the limb of the shell isapparent on one side, but not on the other,possibly suggesting breakup of the shell.The backlight was well-resolved in timefrom the spike of radiation at finalcollapse of the shell.

THE lJ3SAMMOS BRIGHT SOURCEAtomic excitations in strong fields,

and other fundamental issues in laser-matter interactions, are being investigated

with the Los Alamos Bright Source (1.A.BS),subpicosecond, ultra-high-flux laser. 87

The system in its current incarnation (LABSI) uses KrF amplifiers to produce 20-Id,700-fb pulses at neveral Hz. Using f/1optics spot areas of -5 micron 2 have be~yachi veal,

5resulting in irradiances of >10

W/cm . A ❑edification and upgrade of LABS(IABS II) Is under way, using XeCl as anamplifying medium, to as much as 1 J in 150fs at 1 Hz. Several experiments are inprogress on LABS I. Hultlphoton ionizationof Xe gas has been investigated and theresults compared with theoretical models.The laser interaction with solid targets(or ❑ ore accurately, in~eracti~n with the

small plasma produced at the solid surfaceby the low-level ASE preceding thesubpicosecond pulse), and the production ofpicosecond bursts of kilovo x-rays, hasbeen discussed elsewhere. 9,ih) Here wediscuss the results of a search for inner-shell excitations produced via collectiveoscillations of the valence electrons inthe strong laser field,

SEARCH FOR HULTIPHOTON-INDUCED INNER-SHE1,LEXCITATIONS

Boyer and Rhodesll) recentlyconjectured that ●tomic inner-shellexcitations could be induced by laser-driven coherent oscillations of the outer-shell electrons, if the laser fie dstrength exceeds an atomic 1

un~\2)eQa;Lambropoulous, however, suggestedsequential ionization of the outer shelloccurs as the laser intensity rises andthese channels become energeticallyallowable, thus competing with possiblecollective oscillations. However, inner-shell ●xcitatio~~s will radiatively decay in<<1 ns, making a measurement of prompt,high-energy photons a reasonable signatureof the production ofexcitations .

inneri~~~~j

Several of the authorshave untiertaken a photon-countingexperiment to search for inner-shellexcitations , ●nd these results are brieflysummarized here.

The LABS I 1 er was focused to -3 xY1017 W/cm* (E-3e/ao ) i~~a 0.12 torr Xe gas

target. The maximum ionization stateachievableexperiment15YastomeT~re:ll:n a ‘eparateRadiative

transitions characteristic of inner-shellexcitations would therefore be photons inthe range 90-140 eV (0->N transitions) and5OO-1OOO eV (N->M transitions). Filtered❑icrochannel plate (MCP) andMCP/phosphor/photomultiplier detectors wereused to search for photons in these energyranges and determine, with l-ns resolution,the time distribution of these photons withrespect to the laser pulse. A schematic ofthe setup is shown in figure 3. Three runswith different filters or geometries were

accomplished, with >104 las r shots in eachA run with -lo -5run. torr of Xe

established a background count rate af -0.3events per 103 1~.sershots.

in each of the three data runs,several delayed photons were ot,se~ed. butno prompt photons. The earliest events inthe three runs Gccurred 2 ns, 3 ns, and 6ns after the laser pulse. From this nullresult an estimate can be ❑ade of theprobability of prcducing an inner-shellexcitation which radiatively decays byprompt x-ray enission. Using conservativeestimates of filter transmissions anddetector efficiency, knowing the detectorsolid angle, and knowing the number of Xeatoms in the focal volume (spot area xRayleigh range ). this yrobabilitv isestimated to he <5 x 10- per Ite‘atom.~m;a:$ laser intensities corresponding to

iriner shell excitation bycollective ●lectron oscillation of theouter shell(s) is a negligible process.

CONCLUSIONSExperiments, ❑any in collaboration

with other laboratories, are elucidatingimportant issues in ICF and in fundamentallaser-matter jnteract.ions from thenanosecond to the sub-picosecond regime.In both regimes, the experimental studiesof x-ray emission suggest areas cf atomicand plasma physics needing furthertheoretical explanation. Experimentalstudies of hydrodynamics and fundamentalInstability growth processes are reachingnew levels of sophistication and similarlyhold promise for testing theoretical❑odels .

ACKNOWLEDGEMENTSIn addition to our colleagues and

collaborators ntited in the references, wewish to thank D. C. Cartwright, D.Giovanielli, R. J. Trainor, and F. A. Horsefor their support of various aspects ofthis work. We also wish to thank R. L.HcCrory and E. II. Campbell for theirsupport of collaborative experiments withtheir laboratories.

REFERENCES1) Rosocha, L, A., et al., “Aurora:Multikilojoule KrF laser system prototype

for Inertial Confinement Fusion,” FusionTechnology ~, 497 (1987)2) McDonald, T., ●t al., “High power KrFldse~ development at Los Alamos,” elsewherein this volume3) Bradley, D. K., Jaanimagi, P. A.,Knaue~, J., Marshall, F. J., Pien, G.,Richardson, M. C., Seely, J. F., andFeldman, U.4) Goldstone, P. D., et al., “Dynamics ofhigh-Z plasmas prodcced by a short-

r r

wavelength laser, ” Phys. Rev. Lett. ~, 56

(1987)

5) See Laboratory for Laser LnergeticsQua terly Report, DOE/DP 40200-65, 1987,pp. 1-1o6) Goldstone, P. D., ●t al,, “X-rayemission from high-Z spherical laserplasmas: implications for plasma dynamica,”in SPIE 01. 031..-

Pla~inas, 1:87, pp. 54-61Rays from LaseI

7) Bach, D., Glendenning, G,, Wallace, R.,and Kilkenny, J.8) Roberts, J. P., Taylor, A. J., Lee, P.H. Y., and Gibson, R. B,, “High-irradiance248-rim laser system,” Opt, Lett. 1~, ?34(1988)9) Cobble, J. A., et al., “Kilovolt x-rayspectroscopy of a sub-picos+:cond-laser-excited mource,a ●ubmitted to Phys. Rev.Lett,10) Gomez, C. C., ● t al., “Study of laser-plasml interactlor, w~,th picosecond lasers,”precenr.ed at the 18th ●nnual AnomalousAbsorption Conference, L’Esterel, Quebec,Ctiqqada,June 27-July 1, 198711) Boyer, K., ●nd Rhodes, C. K,, Phys.Rev. Utt. ~, 1490 (1985)12) La.mbropoulous, P., Phya. Rev, Lett. ~~,2141, (1985)13) Lee, P. H. Y., Casperson, D. E.,

Schappert, G. T., ‘Are there ❑ultiphotor.induced inner-shell ●xcitations?,’”submitted to Phys. Rev. Lett.16) Lee, P. H, Y., Casperson, D. E , ●mdSchappert, G. T., “Collisionless x-raygeneration from picosecond laser-gasinteraction,” presented ● t the OSA TopicalMeeting on Short Wavelength Radiation, N.Falmouth, MA, September 198Ll15) Kyrala, G. A,, Caspersorl, D. E,, he,P. H. Y., Jones, L. A., Taylor, A, J,, andSchappert, C. “r., “Multiphoton 1onproduction in intense fields, ” presented atthe 4th Internatlcnal Conk2rence 011Multiphoton Procesees, Boulder, CO, July1907

_,... -.

Fig. 1. Preliminary ●nalysis of x-ray

.~~~i~~~~~ efficiency data compared withresults

Fig. 2. law-energy experiments (150-250 J)can produce high x-ray Conversion ifillumination uniformity is go~d (e.g. 24-bean Omega)

Fig. 3. Schematic of apparatus used tosearch for mltiphoton- induced inner-shellexcitations at Los Alamos Bright Source

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