Laboratory Plasma Devices.pdf

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LABORATORY PLASMA DEVICES

R. JONES

Plasma Physics Research Institute, Physics Department, University ofNatal, Durban,Rep. ofSouthAfrica

N OR T H 4 I OL L A N D PUBL I SH I NG C O M P A N Y -A M S T E R D A M


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PHYSICS REPORTS (Review Section of Physics Letters) 61 , No. 5(1980)295-325. NORTh-HOLLAND PUBLISHING COMPANY

LABORATORY PLASMA DEVICES

R. JONES*

Plasma Physics Research Institute, Physics Dept., University ofNatal, Durban,

Rep. ofSouth Africa

Received October 1979

Contents:

I. Introduction 297 4. Closing remarks and acknowledgements 32 4

2. General selection criteria 29 7 References 32 4

3. Characteristic operating parameters of common laboratoryplasma devices 30 9

Abstract:

We review the characteristics of the common typesoflaboratory plasma and the devices used to generate them. After discussing the relative

importance of the various plasma parameters we give brief descriptions of the salient features of each device. This review is intended to aid the

reader in the selection of a plasma source suitable for his own (unspecified) program ofresearch.

* Present address: Plasma Physics Research Laboratory, Box 44 , Colonia, New Jersey 07067 , U.S.A.

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R.Jones, Laboratory plasma devices 29 7

1. Introduction

Amongst the many hundreds of laboratory plasma generators which have been constructed

throughout the world [1] one can discern various distinct family trees. In order to design a bench topplasma system for laboratory research purposes [2] one must first compare the needs imposed by his

program objectives with the performance and critical parameters obtainable from each family ofdevices. The purpose of the present review is simply stated; it is to provide astarting point from which

to make an intelligent selection in the design of a laboratory plasma device specially suited to the needs

of a given researcher.

In section 2 we will begin the review by listing and discussing some of the general selection criteria

which should be considered. Some of these items directly involve the plasma properties which can be

obtained while others are related to economic constraints or matters of reliability, versatility, etc.Keeping his own priorities firmly in mind the reader can then apply these general criteria to 17 common

plasma sources which we catalog and describe in section 3 .

2 . General selection criteria

It is useful to begin with adiscussion of the various features which may be considered desirable in alaboratory plasma generator. Any list of such criteria [2] would certainly include:

1 . Range of plasma density obtainable.

2 . Degree (and range) of ionization.3 . Range of electron temperature and degree to which the distribution is Maxweilian.4 . Range of ion temperature and degree to which that distribution is Maxwellian.5. Range of magnetic field (ifdesired).

6 . Degree of plasma quiescence.7 . Homogeneity obtainable.

8. Range of gases employable.9 . Reproducibility.10 . Cost to build and maintain the source.

11 . Ease of engineering.

12 . Ease of operation, etc.

These criteria are, of course, strongly interrelated and their optimization is , often times, mutually

exclusive. It is important at this point for the reader to stop, consider his research objectives, abilities,and resources, and then assign some sort of relative priority (weighting) to each of these ideal criteria.

Later on, when faced with a choice between two desirable characteristics one should be in aposition tosay what he values most highly for his own purposes, and what other features he is willing to

compromise. The required physics parameters and the available manpower and budget are usually theeasiest to quantify and a good place from which to start.We will now proceed to discuss each of the basic criteria in its turn.

1 . Range ofplasma densityThe absolute plasma density, ~e, will be important in any plasma physics experiment, at the very


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2 9 8 R. Jones, Laboratory plasma devices

least, insofar as it affects the Debye length:

/ T ~h/2~ I____ADI 2

\4lTnee

where Te is the electron plasma temperature and e is the electronic charge. If one is to have a plasma at

all n~must be large enough that, for given Te, AD is much smaller than the dimensions of theexperimental chamber. Other constraints may be imposed by the diagnostics one hopes to employ.

Standard microwave interferometry, for instance, places a lower bound on the product of plasmadensity and microwave path distance traversed, which is , commonly, iO~cm2. Unless one is willing

to generate a very long (or large) plasma this translates into a restriction on minimum density. Similarly,

if one wishes to use standard Langmuir probes [3] the electron saturation current:

Ie.~t=neeAp (21e)h/2 (2 )

where A~is the probe collection area and me is the electron mass, may set an upper bound on tie (for

given Te) in order to avoid overheating and damaging the diagnostic.The electron and ion plasma frequencies:

n e2 1/2=(~~) (3)

and

(njZ2e2\1~12

irm~ 1 (4)

where Z is the ionic charge state, may also be important to a researcher and place limits on tie n1 . If

fle is too large the wavelengths of any oscillations under investigation may then become too small(compared to the probe) to measure.

2. Degree ofionizationIfone is interested, primarily, in the behaviour of a pure or fully ionized plasma one would like to

minimize the effects of plasma-neutral particle interactions. Such criteria will place lower bounds on thedegree of ionization which cart be tolerated. Since the various collision cross-sections depend upon thefilling gas employed we must restrict our general comments to qualitative observations. The plasma

frequency (and perhaps the cyclotron frequency as well) should, for instance, far exceed the neutral

collisionfrequency. This criterion is obviously easierto observe forelectrons and electron phenomena thanfor ions.

Since neutral collision frequencies are proportional to n~,the neutral density, (and not n ~j2,analogous to the plasma frequency) it is, however, best to work at lo w plasma density if the degree ofionization, tie/Pin, is fixed for some reason. In many common gas discharge devices the degree ofionization may, in fact, be limited by the phenomena of arc starvation. Arc starvation [41occurs when


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R.Jones, Laboratory plasma devices 2 9 9

the mean free path of a (refueling) neutral entering the plasma volume:

A= (5)ne(oVe)

becomes less than the (desired) plasma dimension, L:

AL (7)

fle(OVe)Te

where ( )r, is an average taken over the Maxwellian distributed electrons of temperature Te. V~,maybe taken to be the average neutral gas speed (nearly the room temperature value in many discharge

devices). In the next section we will derive discharge equilibrium equations which allow us to recast (7) in

the form:

VnTion loss

n~ L

where r10 ~~ is the average ion containment (or loss) time. For low gas pressure (low n~)unconfined

discharges the Langmuir free fall model [5] allows us to write:

loss=3L/C5 (9)

where C5 is the ion acoustic speed, Cs= (Te/mi)2. For a reasonable choice of parameters it becomes

difficult to obtain ionization fractions much above ne/nn 3 VJC. -~0 .1 . (Note the temperature depen-dence.)

This limitation on the degree of ionization obtainable in the common gas discharge plasma can be

relaxed in several ways, however. In higher temperature plasmas (T, or Te) ions tend to be neutralized

on the wall and return as relatively energetic neutrals [6]. These neutrals have large V~and hence A.Furthermore, if T

1 , the ion temperature, is raised then V~,can be increased by ionneutral collisions.Mechanical means [7] are also available with which to increase fle/fln. The plasma could be created

in a high neutral pressure environment, under conditions of low fle/Pin, but be allowed to stream (alonga magnetic field for instance) into a differentially vacuum pumped experimental volume (fig. 1).

Differential pumping can be costly and it is advisable to use tubular apertures between the experimentalvolume and the source and plasma dump regions (fig. 1 ). (Note that the plasma current into the dump

just balances the neutral current out; niviAa~pure=nnvnApumpingduct.) The ionizing plasma present in

the tube significantly enhances the vacuum pumping impedance of the aperture and so reduces theoverall pumping requirements [71.(Such action is also useful in Tokamak divertors.)


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3 0 0 R.Jo nes , Laboratory plasma devices

F

A E D

iip~_I _ ____ 1 ~ >< I ~F>( I ~5 E1 such as the spark source [40]shown in fig. 1 0 . Such photons can crossan y magnetic field present but must not be overly attenuated before reaching the target gas. For this

reason the spark source is placed inside the vacuum container. Furthermore, only a small fraction ofthepower input to the source appears as light output so pulsed operation is employed.

Intense vacuum sparks suffer considerable wear an d reproducibility is degraded. The spark, itself, is adischarge and can mix with the photoplasma unless prevented from doing so by an intervening region oftransverse magnetic screening fields. (Time-of-flight separation could also be used.) Photointensity alsofalls offas one moves away from the (poorly collimated) source, an d plasma inhomogeneity will result


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31 6 R.Jones, Laboratory plasma devices

High Voltage Peedthrough

Alumina Spacer

M olybdenum Ca thode

0

Molybdenum Anode

Fig. 10 . Spark-type UV source as developed at U.C.L.A. Designed for mounting in a 1 cubic meter vacuum chamber.

(unless one installs numerous sparksources, which would prove very difficult to pulse simultaneouslyand reliably).

9. MultidipoleplasmasFrom f i g . 2 it is clear that low pressure gas discharges are only possible if sustained by primary

ionization [9].In order to stabilize any primary electron beam-plasma instabilities, however, one mustinsure an isotropic primary distribution in the experimental volume. One way to do this was to arraynumerous source filaments about the plasma. Alternatively, surface magnetic (multidipole) cusps canbe added to the chamber wall so as to scatterthe primaries. This has the added advantage ofconf ining theprimaries and increasing the ionization that they are capable of [41].Such a device is illustrated in f i g .1 1 .

Because of the enhanced primary residence time [42] the energy efficiency is improved over the

multifilament device and can rival that observed for Maxwellian electron sustained discharges (i.e.

50% of the input power appears as plasma thermal energy). Plasma electrons and ions are alsoconfined but evidently this is of lesser importance [43].

At high neutral gas pressure cross field diffusion widens the cusp leak width and the multidipoles arerelatively less efficient. At ~102torr the addition of surface cusps, at an expense of hundreds ofAmerican dollars, may only double the ambient plasma density. At lower pressures (~10~torr) density

improvements of one or two orders of magnitude can be obtained.Since the cusp fields only penetrate a short distance into the plasma the core is left nearly field free

[41]. The small volume occupied by magnetic field also makes permanent magnets more economical

than solenoids (under most conditions).

10 . The Penning dischargePenning suggested that ionizing electrons be confined to oscillate along magnetic field lines an d

between negatively biased electrodes. Plasma ions would be lost freely (but relatively slowly) to these(hot or cold) cathodes and electrons could only diffuse radially, across the magnetic field, to an exterioranode. Such a Penning, Phillips, Reflex or P.I.G. discharge [22,44, 45] is shown in f i g . 1 2 and, l ikethe multidipole device, can be said to have a magnetically insulated anode [22].

Once again, the magnetic insulation is particularly useful in confining an y primary electrons present.


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N SN

Filamer*

9Chamber/Anode

Fig. 11. Themultidipole source asdevelopedbyLimpaecher. (Only afew representative surfacemagnetsare shown.)Typical plasma volume: 1 cubicmeter.

Unfortunately, the ion loss to the end electrodes, which is limited to the ion saturation value:

I, = 2irr~n0ZeC~ (36)

may not automatically balance the electron diffusive loss across the magnetic field:

= 2irleD1n0 (37)

and instabilities may have to develop in order for the plasma to maintain itself neutral (22). Also, the

system will not work at all in the absence of an ambient magnetic field.

T A

_ _ H-

Fig. 12. A typical reflex plasma device with magnetically insulated central anode disc A, filament F, cathode C, magnets M, and vacuum chamber V

Typical length: 1 2 meter, typical plasma diameter: 2 5 cm.


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318 R. Jones, Laboratory plasma devices

A more quiescent discharge can be regained by decreasing the (relative) electron confinement insome way (and with it the discharge density and/or efficiency). This can be done by reducing the

electrical bias on one or both ends, increasing neutral ~,ressure,decreasing magnetic field, lengthening

the device, etc.The presence of an oscillating but nonisotropic primary population also gives rise to beam-plasma

instabilities and the reflex discharge (with or without strong electrostatic axial confinement) can also be

called abeam-plasma discharge [46, 47] . Beam-plasma instability may not interfere with certain typesof experiments or one may manufacture the plasma in one chamber of the device and then allow it to

flow into a second, experimental chamber (fig. 1) [13].In the later case one may hope to confine RFnoise to the source chamber via ambient plasma - RF damping or by use of a conducting grid placed atthe aperture between chambers (to reflect RF waves).

In such beam-plasma devices [13]RF fields transfer energy from beam particles to the plasma proper[12]. Obviously a magnetic confining field may be useful but is no longer essential in this limit. The

energy utilization efficiency of these devices ca n be quite good; as much as 5080% of the input powermay go into plasma thermal energy [12, 26 ] . As with any cathode one can apply various chemicalcoatings in an attempt to lower work functions and increase current emission. A simple way of doing

this is to use an aerosol spray gun to paint on a mixture of lanthanum hexaboride in methanol. Themethanol evaporates leaving behind the chemical film covered cathode, ready for insertion into the

plasma device. A slow run-up to cathode operating temperature and subsequent discharge operationactivates the cathode.

11 . Shock tubesPulsed plasmas can also be generated in chemical or magnetic shock tubes [48]. In the chemical

shock tube a low molecular weight gas at high pressure is separated from a higher molecular weight gasat low pressure by a diaphragm. The gas pressure difference is programmed so as to burst the

diaphragm and the higher pressure component acts as a piston to drive a shock in the low pressure

target chamber. Since the escape speed ofthe driver piston can exceed the sound velocity in the targetgas a strong shockforms which ionizes the shocked target material.

Higher temperature plasmas can be produced by using a magnetic shock tube. Typically a capacitorbank is discharged between two electrodes placed a t one end of the partially evacuated tube. This sparkmay be simply struck between two point electrodes or, for example, the plasma focus geometry [49]

can be used. In the later arrangement the electrodes are two concentric metal cylinders, coaxial with the

surrounding glass vacuum tube and with each other. Gas breakdown occurs across the insulator thatsupports the electrodes an d the resulting radial current sheet is driven down the annular ga p between theelectrodes by an azimuthal self magnetic field.

Very intense plasmas can be formed in this way using quite inexpensive apparatus. A single onekilojoule (fast) capacitor charged to 1 0 kilovolt is quite adequate and vacuum requirements are

minimal ( 1 ton). The plasma is very short lived, however, and difficult to diagnose. Various

instabilities may also occur.

Plasmas having thermonuclear characteristics can be (briefly) formed in an intense focus near theend ofthe coaxial electrodes. For the purposes of the present review (and table 1 ), however, we concernourselves with the research plasma parameters usually generated in the larger volume target region.

12 . RFdischargeTime varying electric (and magnetic) fields can accelerate charged particles and input energy to


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R.Jones, Laboratory plasma devices 319

sustain a discharge [50].These fields may be established by antennas emersed in the plasma itself or onemay try to transmit RF power across an intervening spatial gap and into the plasma. A gas filledmicrowave cavity operated at some appropriate RF frequency is one common experimental arrangement.

The discharge regime varies according to the RF frequency and amplitude, the presence of any

plasma resonance processes, the antenna coupling method, and the plasma (and neutral gas)parameters. At very low frequency an d high f i l l pressures one may simply excite transient glowdischarges of alternating polarity. Where wall impact takes place ionization may occur or secondaryelectrons may be produced. As one proceeds to higher frequency electric fields one eventually reaches apoint where the reflexing electrons can no longer oscillate between the chamber walls.

The energy utilization efficiency of an RF discharge depends, of course, on the ability of the antennastructure to electronically couple to the plasma. That is , the antenna impedance must be matched to aneffective plasma impedance [28].Furthermore, if the antenna is not physically embedded in the plasmaitself then one must first couple to waves which can propagate across the vacuum gap, reach the

plasma, and ultimately couple in turn to actual plasma oscillations [51].If the RF field amplitude (and by implication, the RF input power) is very high then one can of

course, drive forced oscillations (which are not normal modes of the plasma dielectric medium).Energy economy, however, suggests lower input power and coupl ing to some one of various possibleplasma waves. An antenna then, will be chosen so as to operate at the frequency of, and to have thespatial periodicity (wavelength) compatible with , some known normal mode of the (desired) plasma.Furthermore, unless one is interested in research on strong turbulence or the l ike, we restrict ourattention to those plasma waves which will be damped in a distance which is shorter than thedimensions of the discharge, thereby giving up their field energy to plasma thermal energy. This

becomes more difficult in a collisionless plasma. Conversely, if the input RF waves are damped to oquickly all the energy may heat the plasma edge, creating an inhomogeneous discharge.

B y way of example, the total power coupled from the antenna to the plasma ca n be found byintegrating the Poynting vector over the antenna area:

P=JS.dA. (38)

For a magnetized plasma and electrostatic waves in the regime w~.~1 w~one can use Maxwellsequations an d the standard dielectric tensor to give the real time averaged components [28]:

5. = ~- 0K11E~ (39)

S~=~rEOK~lE~ (4 1 3 )

with

k~/k~=K~1/K~ (41)

where the notation is standard and z is the magnetic field direction. The antenna is taken to be a


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320 R. Jones, Laboratory plasma devices

distance d away from the plasma and an additional evanescence layer of thickness a is assumed. For an

antenna of characteristic wavelength 2~r/k~we obtain:E~k~Ve~e~. (42)

Writing a in terms of the plasma gradient scale length dN/dx:

= w2

0m0 43

a_e2dNIdx )

and using the definition P ~V2/R we obtain the effective antenna impedance:

R A~ exp[+4ird/A~] exp[+(4irw20m0)/(e

2 dN/dxA5)] 44

4i~0A [w(w2/w2_ 1)~][(1 w~~/w2)Lf2]

(A factor of2 appears to account for waves propagating in either direction.) This equation assumes cold

plasma theory. A better agreement with experiment has required a warm plasma correction where theterm

1)1~2

in (44) is replaced by:

Re(zz)2 (45)

with

~ (46)

where Z(u) is the FriedConte plasma dispersion function [52],with u = (w/k5XT0/m0)

2.For a laboratory plasma (Te = 1 to 1 0 eV) of density 107_ lO b cm3 and B = fe w hundred gauss

(where there is adequate plasma-wave damping) experimental power balance measurements haveestablished that at least 75% ofthe power input to the antenna is absorbed by the plasma provided [28that the antenna length is at least:

I=

where:

ZY=(a+j$)2

Z~Rohmic+~WL (47)

1 .1Y~+j~


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R.Jones, Laboratory plasma devices 32 1

Fig. 13 . Typical RF couplers. Left: Dipole and currentloop, and Right: Coaxial cavity, with coaxial feedsC, plasma P, electrical insulator I, and vacuum

chamber V. Typical plasma diameter: 2Scm.

are calculated for the appropriate (FEM) antenna mode. R o h m j c accounts for the resistive loss in theantenna elements. Because ofthe dependence on R we find that 1 is strongly dependent on (increaseswith) d+a .

13 . Electron cyclotron resonance dischargeIn the special case of magnetized plasmas one can take advantage of the excellent coupling of

external antenna structures to plasma waves occurring near the electron (or ion) gyrofrequency. Onevery popular antenna geometry is the helical Lisitano coil [53] but most any reasonable configurationseems to work adequately. Fig. 1 3 shows the half wavelength long resonant cavity employed by the

F

M /

H

IFig. 14 . An alkali metal 0 machine having solenoid magnet M, plasma column P, vacuum chamber V , hot plate H, oven outlet 0, and filamentheaters F. Typical length: 1 meter, typical plasma diameter: 25cm.


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3 2 2 R.Jones, Laboratory plasma devices

author to generate an E.C.R.H. plasma at afrequency of 3 GHz [54]. A coaxial geometry was employedfed by semi-ridged coaxial cable. The frequency chosen tookadvantage of inexpensive surplus military

magnetron C.W. power tubes but implies a fairly strong magnetic field at resonance (1070 gauss).

Similar tubes are available at low cost in common commercial microwave ovens.

Operation is also possible at lower magnetic fields if resonance is achieved with some multiple of the

cyclotron frequency. Unfortunately, the coupling efficiency drops at these higher resonances and one istrading off magnetic field cost for a larger microwave power tube. The tapered center conductor (fig. 1 3)is intended to improve plasma uniformity since E.C.R.H. plasmas can tend to be hollow. (The hollow

center conductor design was chosen solely to permit energy analyzer access to the plasma [54].)Coupling efficiencies for E.C.R.H. generated plasmas generally range between 10 and 80%; a

substantial loss occuring in the coaxial cables (type N in our device) connecting the magnetron to theplasma device, and then leading the power on into the vacuum chamber (0.28 inch O.D. semi-ridged

copper jacketed cable in our experiment). All cable connections should be carefully adjusted if input

power is expected to exceed 50 watt. Cable reflections can be serious and lead to electricalbreakdowns. Fortunately, one can maintain a reasonable research plasma (Te 5 eV, tie ~1O~cm

3)with only 10 or 20 watt of microwave power. Since microwave source tubes operate over a rather

limited range of frequencies only a narrow band of magnetic fields can be employed in these devices.

14 . Ion cyclotron resonance dischargeI.C.R.H. sustained plasmas [55] operate in a manner similar to E.C.R.H. devices but at lower RF

frequencies, f~/f~= m~/m1.Antenna structures differ for this reason and, most important perhaps, ions

are directly heated.

Megahertz power oscillators are (again) readily available and coaxial cables are a suitable method oftransferring the power. Stix coils (i.e. spatially periodic induction coils [51]),simple wire loops, or

metal plates situated in close proximity to and parallel with the plasma have been used as antennae (fig.

1 3) and prove relatively efficient for generating (or heating) laboratory research plasmas. The efficiency

of I.C.R.H. may be a bit below that of E.C.R.H. and again the frequency and magnetic field are not as

widely variable as one might like.

15. Rotatingplasma devicesCrossed electric and magnetic fields can accelerate charged particles by driving an ExB drift

motion. This occurs to a greater or lesser degree in most magnetized discharges since the existence of a

plasma densitygradient transverse toB implies, by the Boltzmann relation (eq. (15)), a radial electric field.In Penning discharge geometry (fig. 1 2) it is possible to impose a relatively large radial electric field by

simply making the anode inside diameter larger than the cathode (outside) diameter and so sustain a

substantial ExB azimuthal plasma rotation (modified Penning discharge).This heating method has the advantage of driving ion as well as electron drift and, hence, heating

ions directly [56]. In fact, T, can be made to exceed T~by a substantial amount (e.g. an order of

magnitude or more) in such devices.

Unfortunately, one may not fully thermalize this drift energy and the rotation is also a source of freeenergy that can drive plasma instabilities [57].Rotating plasmas may also be inhomogeneous (appearingfor example, as a rotating radial spoke or density maximum [56]) due either to the formationmechanisms (such as cathode/anode spots) or to macroscopic instabilities (such as exchange and

KelvinHelmholtz instabilities).


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16 . LaserproducedplasmasLaser-pellet irradiation makes possible the rapid creation of dense, energetic, and highly localized

plasmoids. Such plasmoids can, for instance, be created~in situ deep within asystem of confinement

fields or they can be allowed to expand and be studied in the afterglow [58].Because of the very small initial size, the extreme density required for laser light absorption, and the

inherently transient character (required by the overall low energy efficiency) laser initiated plasmas are

rather difficult to diagnose. The plasma formation process and subsequent expansion mechanisms are,themselves, poorly known and worthy of extensive study in their own right [58].The ambipolar electric

fields established during the plasma expansion phase may also be responsible for setting up convective

cells in the confinement volume.

Extremely pure and intense plasmas can be created by laser pellet heating but the composition islimited by the availability of solid state target materials. The experimental device itself must be quite

complex, perhaps comparable to the alkali metal Q devices that will be described in the nextsubsection. Laser produced plasmas seem especially suited, then, to thermonuclear plasma confinement

studies and other closely related experiments.

17. Alkali plasmas

If a neutral atom impacts a hot metal surface it may be ionized upon contact and return as an ion,

accompanied by a thermionically emitted electron [59, 6 0 ] . This process is energetically allowedprovided that e4w >E~,where 4~is the work function of the metal surface. Such surface ionizedplasmas were among the first reasonably quiet plasmas (i.e. dne(t)/ne low) produced in the laboratory

and they were given the name Q machines, the Q standing for quiescent.

These devices (fig. 14) produce plasmas which are in fairly good thermal contact with the source hotplate (usually tungsten) and hence have T T

1 Te 0.2 eV. This can be varied only by application of

some auxiliary heating method. The fuel neutrals are often input through a nozzle as an atomic beam(originating from a specially designed, and complicated, source oven) and percent ionizations approach-

ing 100% are obtainable at reasonable power levels and densities (up to ~b013cm3). The electron

thermionic emission rate must be adjusted so as to nearly match the atomic beam flow rate and the

desired outgoing plasma flux. Relative influx rates can be used to adjust the plasma potential or percentionization somewhat.Recombination is an important loss mechanism in Q devices and one defines the recombination

coefficient by the relation:

(48)

For this cylindrical magnetically confined system [60] plasma flows from the source hot plate according

to the usual fluid equation:

m~n~[3V1/3t+ViVi/dZ]~Tit9fle/3Z. (49)

The equation of continuity for ions is:

3n1/t+ 3(n~V1)/3Z+an~=0 (50)


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32 4 R. Jones, Laboratory plasma devices

where V1 is the ion drift velocity, which, for aMaxwellian distribution:

n(V)=no(V=0)exp(mV2/T~) (51)

gives:

(1 mV2/2T) V/8Z= an0 exp(mV

2/4T) (52)

and so (with m1V~4T):

V ano(L Z) (53)

where V= 0 at the end plate and L is the plasma column length. Equations (51) and ( 53 ) serve to modelthe degree of (axial) plasma homogeneity attainable in 0 devices.

0plasmas [61] are limited to materials for which ediw >E~and are generally magnetized. Since theappropriate working gases ate -high atomic number alkali metals the corresponding ion gyroradii are

rather large. In order to obtain a plasma many gyroradii in diameter one must either operate at veryhigh magnetic field or else employ large diameter devices. Either of these limits implies substantial

solenoids and accompanying power supplies.Due to the low electron temperature the 0 plasma will tend to be fairly collisional even when

nearly fully ionized. Finally, these devices are by far the most complex laboratory plasma sources we

have described. Vacuum requirements can be especially exacting; if one desires 90% ionization at a

plasma density of, say, 1010 cm~then the base pressure must be ~5 X iO~torr.

4 . Closing remarks an d acknowledgements

The material summarized in this review was assembled by the author during most of calender year

1 978 . The author compiled this collection of data to assist him in the design of a new, general purpose,plasma research device which he hopes to build shortly. We have found it a useful asset during thepresent machine design phase of our work and hope that it may prove similarly valuable to the reader.

The author acknowledges financial support from the South African Atomic Energy Board, Pelin-

daba, South Africa. We also acknowledge useful experiences working on the various plasma experi-

ments at the Stevens Institute of Technology, Hoboken, New Jersey, U.S.A., the General Atomic

Company, San Diego, California, U.S.A., and the University of Natal, Durban, Natal, R.S.A.

References

[1] See, for example, World Survey of Major Facilities in Controlled Fusion Research ( I .A.E.A., Vienna, 1976) .

[2] F.F. Chen, Lecture Notes from the Plasma Physics Summer Institute (Princeton University, 1964) p. 35 .1 3 1 P.M. Chung, L. Talbot and K.J. Touryan, Electric Probes In Stationary And Flowing Plasmas (Springer-Verlag,Berlin, 1975) .[4]G. Fuchs, I.E.E.E. Trans. Nuc. Sci. 1 9 (1971) 160.[5] L. Tonks and I. Langmuir, Phys . Rev. 34 (1929) 876 .

[6]R. Jones, Phys. Lett. 67A (1978) 194.[71R. Jones, Vacuum 2 9 ( 1 9 7 9 ) 27 .1 8 ] L. Spitzer, Physics of FullyIonized Gases (Wiley, N ew York, 1962) .[9] R. Jones, J. md. Inst. Sd. 6 0 (1978) 71 .


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R.Jo nes , Laboratory plasma devices 3 2 5

[10] R. Jones, Pramana 12 (1979)1.

[11] R. Jones, Plasma Physics 21(1979) 399 .[12] R. Jones, Nuovo Cimento 40B (1977) 261 .

[13] M. Seidl, W. Carr, D. Boyd and R. Jones, Phys. Flu. 19 (1976) 78 .

[141T. Honzawa, J. Phys. Soc. Japan 4 2 (1977) 1077 .

[151R. Jones, Plasma Physics 21(1979) 505 .

[16] I. Langmuir, The Collected Works, ed. C . G. Suits (Pergamon, New York, 1961)Vol. 5, p. 151.

[17] D.A. Dunn and S.A. Self, J. AppI. Phys. 35 (1964 ) 113.[18] R. Jones, Plasma Physics 20 (1978) 717 .

[19] R. Jones, Plasma Physics 19 (1977) 259.

[20] T. Honzawa, I. Phys. Soc. Japan 3 6 (1974)1674.

[21] R. Jones, Pramana 13 (1979) 329 .[22] R. Jones, J. Plasma Physics 20 (1978) 221 .

[23] R.J. Roth, Phys. Flu. 12 (1969) 260 .[24] A. Caruso and A. Cavaliere, Nuovo Cimento 2 6 (1962) 1389.

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