LosAlamos Scylla ThetaPinch 1976

DC 20 Reporting Date: August 1976 Issued: August 1976

Plasma Experiments on the Scylla I-C Theta Pinch

LA-8397-MS Informal Report

Kenneth F. McKenna

UNITED STATES ENERGY RE8EAHCH AND DEVELOPMENT ADMINISTRATION CONTRACT W-740S-ENG. 3<

scientific laboratory of the University of California LOS ALAMOS, NEW MEXICO 87545

An Affirmative Aclion/Equal Opportunity fmployer

Printed in the United States of America. Available from National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 Price: Printed Copy $3.50 Microfiche $2.25

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1STO0DUCTI0S

“jOD sff«rr »T

l:iU«r report.

f i.

S c y lU 1-C is a swill scale (1-Meter) research th«rs.a pinch

developed for the experiaental investigation of basic plane*. physics processes and advanced concepts. The properties and s t a b i l i ty characteristics of the Scyll* I-C plassa, over a range or i n i t i al r i ll pressure froo JQO-500 ssTorr 0}, are discussed in this report.

ABSTRACT

Kenneth F. McKenna

PLASMA aCf’BUMEBTS 08 THE SCitXA I-C ?H£?A PI8CK

the rcisttlis of the ]«a«r-

ea” ) ttieta-

etT ). low

l.f>sti

T. 1.

after disffharge ini.iatlon.

the Jfcff.t eoii iJwtett-;-.

CSi!

in eoaparison with the sUscnarge tube dianeter, vas

used in orittr ta obtain an effective ereubsr of the

(sails flvld sJistrhar^. Unless otherwise ssseiS, tftu

;VheeaIc “f ’!« :’^\vU» I-C tlieta sn

The ScyUn I-C theta pirn* ia preiently being

tutv4 In M>nJ«ai>et.i«n uilh a high enerfif <*** l8f) J) C0g Inner to statjy the interaction of nxSuliy directed Insnr WDttr with the 4cntse (n# -v 10 islnch pinssft w l w .1’ ” pi<u!Si» iBtrracllon experiMetit will be iMva^a’«“l in s>

In the present report, the properties

jijsd Btnhility characteristics of the higit density

SryUa I-C pUtan* are 4iscusae4; exjwHstnts »er«?

eoniJ»wt«4 ewer a range *>r f i ll pivoaurcSB fro« 100-

In .» eonpanSon paper the re»u;t« of

an iftv«>sti£ntii;» «f pJaatwi <-!»4-J«8r. Cram the J’cy.ia

I«C plJinssi «?<5j!jan nrc

FJCPERTNEMTAf. ABBASSBCST ABO BIACBRSTIC5 The geottetry, energy atorage enpnbllitiee, and

e l e c t r i e n! eireuit pnr«iBeiera of the Scylla l-C them pinch «ere determined fro» opiiBlaailon stuiiiua intended to produce « *ienae C»c J 10 temperature pl.-maa {Tt> _ ‘iO eV). A acheaatic or the

thota ptneh is afeotm in Fig. I. Seylih S-C, csn-

The .’Scylla 2-C linear sheta pinch was coii3tr&-te<2

s*.tructc4 frees Ssyllae renpsnents, hits at ssxisus

i:i oriler to provide a versatile r«sewch instrument to

eitor enerisy storage of 175 W at 60-fcV primary tur.k

aupliKscnl. the larger, less flexible theia-pi rsirii pro-

voitn^c. Fifty-four 1.8-uF eapseiters feed the SQD-

grans. Swill scale research studies »«d the experS-

ca Seng, i0.5-ea <iiaseter single-turn cespreseien

sentitl investigation of atli’«r.e<?<! concepts are cwrrled

e o i l. Priaary bank operation »t &0 kV generate* *

out. on Scylla !-C in order to provide basic physics

v*<!tMM Eg of 0.36 kV/en at the inner wai; of the

inforsation and Jisgnoatic dc«e»oi«ient for the present

t.4. quart: discharge tutse and a peak crsv-

siiiil rumre asin thet«-pinch efforts.

barred coepressiti;: field of 33 kS is obtained S.O ««

j ss a 4£sttsajSB«rt.J* i#sj!»

vlife

al s» pritaa

*s aft a ne

1lirass #? ”&

The jsriatfipaS

s^hsrf:^ tafec, rtji Jiti;-.}? arjhi»is?rr-.i y p t i-

fen»t8i?4 with sft’rf-a’a1i!>j-s* f5 li

‘^i^ ”.-f-s.’.’-h irarrcss!. Is of

S- >«*

SS4«-sft

, a Si»s«?

d 5n ;.5sSi? r«jp»

(i|fts?,

tti

”« »-S-.f FSii e s s.

A-

was

the deuterliM Bftlaer series. The regaining identi- fied line* re froa excited carbon ions. Jto indica- tion of Z-pineh electrode a»terJ%l line radiation can be found; the spectrtai obtained vlth the theta-pinch preionication systea is practically indistinguishable fro that recorded with the 2-plnch syctes.

Spectra of the win plasaa diteharee *.t bO-KV

prlaary bank voltage and i00-«Torr f i ll pressure are presented in Fie. 5. these data were taken vith the iiwt preionizatlon systeas as the results shown In Pig. ii. Other than the Salsxr series, the Identifi- able lines result fro* carbon ion radiation. As in the c ue of the yreloriimion plassa spectra, iapurity r»dUt!cn fro* the 2-pinch electrode Material cannot be identified. Accordingly, the Z-pineh preionlxa- tien systest doe* not Introduce undesirable perturbing ff««t into the tMin discharce plan* resulting Tram istpurity conta»ln*tion.

««ncertt when ast«*5 * *

is th« £<j”.roituJ”.S«sn of

v i th SW-sfFcs-r pressure, is presented ‘.r, P i g. 3. SJ|tati!»2 Integration of the redue«Mi density profiien isj!iiwnte.-S that for the 100-efftarr f i ll pressure e s u e. >05 of ts<? f i ll gM» v»a Ionised by the t i ae the plneheiS p l a sm «I;UBJI ftasi ejtffutiea to the disenftree tub* v a i l. Por SOS-aTorr f i ll pressure, about SOU of the gas usit lonlsted at t h is t i a e. t ; 3 sis. The

if,

ajfm??.nsss -»r

SPECWA

int* tker

n ansi satis

«»I4-IS» »ievisj5 tthrough

vntf, ‘,r.i%Lnl*<l 10 iis « t t #r ‘.he

ife 109ffir flit pre»»are

g&teirm of the ScylU I-C prelooitation

P U & MA COWKH LUMINOSITY MEASUREMESTS

the initial imp’.osion. In this plasma regime, they

Streak camera photographs of the Scylla I-C

are identified as radial hydromagnetie oscillations

primary discharge plasma column were made with the Iseacon camera viewing the plasaa side-on at the theta- ^ir.ch coil midplane. Figure 6a presents typical streak photographs at various camera sweep rates for 100-mTorr fill pressure. The slight bow evident on the streaks is not a plasma effect but results from disruption of the camera unit due to its close prox- imity to the theta-pineh coil and thus the primary discharge magnetic fields.

The dynamics of the plasma during the implosion

and plasma column formation phase can be clearly identified from the fast sweep rate streaks where the initial radial shock wave and subsequent plasma oscil- lations are observed. The radial oscillations damp ou?, approximately 1.3 us after termination of the iaplsision. High frequency plasma oscillations have previously been observed * in high density (PQ S 100 isTorsf) theta-pinch operation at the termination of

of a cylindrical annulus of compressed plasma trapped

between external and internal fields; such an annulus

has been observed from end-on interferograms. Pheno-

menologically, at primary bank initiation the rapidly

rising external magnetic field penetrates the pre-

ionized gas before the current sheath is completely

developed and this field, which has leaked into the

gas, is subsequently trapped and compressed by the

imploding current sheath. At the termination of the initial implosion the plasma is contained within the

annulus separating the external and compressed in-

ternal fields and oscillates about an equilibrium

Assuming all the initial fill gas is swept up

during the implosion, the equation of motion of the

plasma S J M U I UE can be solved yielding an oscillation

*00 m nut

radius.

period given by

550 e?y ri-

100 m’i’orr

3S0 Xi&vv

220 m7©r

Fig. 6. Streak photographs of the Scylla I-C plasma column obtained at. different camera sweep rates and in-

itial theta-pinch fill pressures.

Vmmn O7 Fill P»t»urn

where T Is in eV. The electron teaperature, T^,

at the end of the implosion can be estimated by

adapting a snowplow model for the implosion dynamics.

In this model the ions are assumed to gain eijual

uh«r« M it the line density of ions of mass *i In the

kinetic and thermal energies during the implosion

annului, and B li the external Magnetic field. Con-

while the electrons retain their initial preloniza-

sidering the 100-mTorr fill pressure case, the period

tion temperature; ohmic heating of the electrons is

of the first oscillation, after the implosion, is

assumed small. At the termination of the implosion

about 220 ns and the average external magnetic field

the ions rapidly transfer energy to the electrons,

strength over this time interval i 13 KG. With this value of nagnetic rield and N < no>ib2 « W» 1 016

via collisions, until temperature equilibratior. is

obtained. The ion energy at the end of the implosicn

cm” (b is the discharge tube radius), Eq. (l) pre-

is then 3/2 kT * ?(]/? o.Vj), where v. is the inward

dicts an oscillation period of 235 ns, in very good

radial velocity of the ions. Estimating the pinch

agreement with the experinental result. The experiment-

time from the streak photographs to be t r 330 r.s

ally observed periods of the two successive oscilla-

(again for PQ * J00 mTorr) and tailing Vj ; b/t ,

tions are 160 and 130 ns respectively und the cor-

responding analytical values are 150 and 100 .is.

Accordingly, it does appear that the radial oscilla-

tions of the dense plasma column observed in Scylla

I-C are consistent with the theory of field trapping

during the implosion phase.

Damping of the radial oscillations can be at-

tributed to resistive field diffusion through the

plasma. Neglecting convective effects, the diffusion

equation can be written in one dimension as

.iiff

(1)

tions, l.i us.

(2)

…

,j %

where o is the plasma conductivity and x Is the co-

ordinate perpendicular to B. The one-dimensional

approximation overestimates the diffusion time since

diffusion occurs mare rapidly in cylindrical geometry.

The characteristic tine over which the magnetic field

*iiffuses is, from Ei|. (2),

where the diffusion length scale, !., can be taken ns

half the plasma radius L * r /?. From Spitser the

conductivity is Riven by

0 * 1.8 * 10 TnH

the kinetic temperature of the ions before energy

equlpartition with the electrons is approximately ho

eV. Thus, after temperature equilibration the elec-

tron teaperature is about 20 eV. Over the damping

time of the radial oscillation.1 the plasma is eon-

pressed by the rising main magnetic field so that

the electron temperature will increase over this lice

interval. Using the 20-eV electron temperature es-

timated above will thus *et a lower limit on the cal-

culated magnetic field diffusion time. Talcing

In JJ ; 8, Te r 20 eV and r ; 0.6 cm (obtained from

end-on interferograms to be discussed), the estimated

diffusion time is about ’,’.*> us, in reasonable spree-

Rent with the observed damping time of the oscilla-

Fror. the oireajt photographs of Fig. 6, the coa-

prossed plusma column is observed to be highly stabie

and reproducible for a period of approximately 8.5 ust

after this time ttn ra>? instability is observed.

The time required for an Alfvfn wave to propagate

from the theta-pinch ends to the midplane is abou*.

equal to the onset time of the m*2 instability, indi-

cating that the instability may be a result of pias-

aa rotation iiklucvd by end-shorting of the platen 7 8

radial electric fields. ” The plasma column stabil-

ity characteristics remain the same over a wide

range of initial fill pressures (100-900 mTorr) as

demonstrated in Fig. 6b. VI. I’LASMA DEHSTTY MEASUREMENTS

The time and spatial evolution of the main dis-

charge plusnn density distribution was determined

with the ruby laser interferoneter in an end-on single-

during the early stages of the colunn formation

phase. At later tiner., the plasma loses Its annular

structure through field diffusion, the flutes disap-

pear, and a well confined high density plasm column

is observed for several microseconds. The beginning

pass configuration. Figure 7 presents a time sequence

arate plasma discharge and the entire 3.8-cu disaeter

of plasma column lnterferograms obtained for 100-mTorr

of the discharge tube was illuiainatcd by the inter-

fill pressure. The straight background fringe pat-

tern vas generated on each interferograa by a small

The plassa column formation phase (t*0.5 us,

angular deviation of the scene beam Introduced be-

Fi.f. 7) is dominated by high e-nuober («>6) flutes,

tween exposures taken vtth and without plasma.

with coae of the flutes extending froa the plasm

Plasma electron refraetivity results in displacement

column to the di scharge tube wul1. The annular dis-

of the background fringes, with one fringe displaee-

tributlon of plnsma density, previously indicated to

oent corresponding to a change in J a dt of

be a result of ‘.nipped magnetic fields, is observed

Each interferogriuB of Fig. 7 ua3 obtained from a sep-

1 017 electrons em”2

(5)

ferometer laser light.

t.Sflt

Fig. 7* Tiae sequence of plasma colunn interferograas obtained at 100-nTorr fill pressure.

of the plasma coluan breakup and filamentatlon (m»2) Is evidenced at t > 8.5 us.

Figure 8a presents a plasm coluan lnterferograa taken at t * 2.7 us for m fill pressure of 300 mTorr and ia typical of the high fill pressure data. The plasma electron density profile, derived frosi the in- terferograa of Pig. 8a Is shown in Fig. 8b. The density profile deviates from a Gaussian, exhibiting a long tail which, at high fill pressures, extends to the discharge tube wall. Extracting exact plasaa profiles fro* the interferograu taken at fill pres- sures greater than 300 aTorr is difficult due to the large number of closely spaced fringes. At PQ « 500 «Torr the total nuaber of fringes was -* 77 at maximum compression, t « 2.0 us. The peak density on axis could be estimated fro* staple fringe count- ing and the peak density determined frca the end-on

interferograas at 500 aTorr agreed within 20* with the values derived froa a Halted number of side-on interf erograms.

From the analysis of many interf erograas, the

average plasaa density at maxiaum compression, t - 2.0 us, was estimated to be -v 1.1 x 1 017 cm”3 at Po * 100 mTorr, * 1.5 * 1 017 ca”3 at PQ * 250 aTorr, and *»- 2.3 * 1 0i7 cm”3 at 500-aTorr fill. Spatial in- tegration over the reduced density profiles obtained at 100-aTorr and 250-mTorr fill, indicated that all of the fill gas was ionized and contained within the observable plasaa density distribution at the tiae of maximum compression; only 70S ionlzation and contain- ment was indicated froa both end-on and side-on data at Po « 500 mTorr.

In an attempt to generate a density minimum on

the plasma column axis, and thus provide a “light pipe” for the C0g laser beam in the laser-plasma in- teraction experiment, a forward bias field was applied. The bias field had a quarter cycle time of approximately £3 us and a magnitude of ^ 1.8 kC at the tine of primary bank initiation.

End-on interferograms taken at t * 1.0 us with

and without the applied bias field, for 100-aTorr fill pressure are presented in Fig. 9. A well de- fined density minimum on axis results when the bias field is epplied. Density ainlauas could not be identified for fill pressures greater than 300 aTorr. At 100-aTorr fill, the minimum persists for a period of approxiaately 3 Us, its duration being Halted by magnetic field diffusion. An estiaate of the plasaa column temperature can be made from the known dif- fusion tiae and the plasam radial dimensions deter- mined from the interfergraa of Fig. 9b- Solving for T by combining Eqs. (3) and (U), and taking 3 us, r 12 « 0.1>5 cm, and In a s 8, an electron tem- perature of about 13 eV is determined for the ob- served plasma at t « 1 Us. Comparing this value vith the estimated electron temperature at the end of the implosion T = 20 eV at t « 0.3 Us, obtained without forward bias for P » 100 mTorr, indicates

that the bias field significantly reduces the elec- tron temperature. Such a. result is expected since the presence of the bias field decreases the inward radial velocity of the ions during the implosion thus decreasing the total ion energy at the termina- tion of the implosion phase. In addition to a drop

’

TUBE RADIUS (em)

Fig. 8. Plasaa column interferogram and reduced density profile for Po * 300 mTorr, t » 2.7 us.

in electron temperature, the peak plasma density de-

estimate of the plasma kinetic temperature can be

creases when the bias field is applied; at I * 1 ps,

n = 1 x io1T cm”3 without bias and n : 0.6 x 1017

The diamagnetic loop-probe system consists of

cm with bias, assuming a plasma column length of

100 cm. Since the main magnetic field, B, is inde-

pendent of applied bias and 2nkT « 6B2/8IT, in the

Scylla I-C plasma, the plasma 6 also decreases sig-

nificantly when the bias field is applied. VII. PLASMA EXCLUDED FLUX MEASUREMENTS

The effective radiU3 re ff at which magnetic flux

is excluded from the compressed plasma was determined

from diamagne-tic loop-probe measurements. The ex-

cluded flux data, when appropriately combined with

the known plasma density distribution, are used to

determine the plasma 6 (ratio of plasma pressure to

magnetic field pressure). From tbe 0 measurement an

-3

two closely spaced, Magnetic pick-ups. A small

magnetic probe (HA i 0.3 C M) aeasures the changing

magnetic flux Just outside tbe discharge tu%e and,

since the probe area is fixed, the integrated signal

gives the magnetic field external to the piassn. A

single-turn loop encircling the discharge tube re-

sponds to the changing Magnetic flux inside the dis-

charge tube and therefore Measures the Magnetic flux

excluded by the plasma. The signals froa the loop

and probe are nulled in the absence of plasma by means

of a passive differencing circuit. The integrated

difference signal is divided by the integrated probe

signal to give the net flux excluded by the plasma.

From this measurement the effective cross-sectional

area of plasma from which magnetic flux is completely

excluded can be determined. The dlamagnetic loop-

probe system was calibrated with * copper rod of

known diameter which was inserted into the discharge

tube and simulates the plasma column.

Four loop-probe systecs were arranged along the

axis of the discharge tube. Only the results obtain-

ed from the system located at the coll aidplane will

be discussed here; the axial variation of excluded

flux is detailed in the companion report3 which dis-

cusses the Scylla 1—7 end-loss experiments. The ex-

cluded flux radius derived from the loop-probe data

taken at the coil midplane with 100-MTorr fill

pressure is presented in Fig. 10. The implosion

phase, damped oscillation phase and quiescent plasma

column phase observed in the streak photographs can

also be identified from the tine history of the ex-

cluded flux radius. The excluded flux data of KJg.

10 are typical of those obtained at higher fill pres-

sues. The relatively constant minimum value of r _ .,

0.22 C E, between % 1.5 and t> 2.5 us is nearly insen-

sitive to fill pressure, changing only 15£ between

100-nTorr fill (0.23 cm) and 5no-mTorr fill (0.19 C M ).

obtained.

VIII. DETERMINATION OF PLASMA B

pressure balance equation it can be shown that the area of excluded flux, *r^f f, iB related to the plasma S through the expression

From the definitions of excluded flux and the

b) With Bias Field

Fig. 9- Plasma column iaterferograms taken with

and without an applied bias field.

a) Without Bk* Field

substituted into Eq. 16) which can then be numerical-

IX. OErERMIXATIOM OF FUSKA ELECTROX TEMPERXTUHE

ly integrated to deteniinc (L.

The compressed plasma tenperature is deterained

Tor the Scylla I-C density profiles at maximum

fro* the eensured values of plasma density on axis

coHpresulon, *. * 2.0 us, the BA determined by numer-

n., the aeasured external aagnetic field B, and the

calculated values of S… From pressure balance,

ical techniques were: 6A * O.ltti with PQ - 100 mTorr and obtained assuming a Gaussian density profile and

0.33 with PQ « 250 mTorr. The value of

calculating the 1/e radius using the peak density

value were: $A * 0.63 for 100-aTorr fill and B. *

0.37 for 250-jiTorr fill. The difference between the

B. deteralned nuaerlcally and that calculated assi»-

lng a Gaussian profile decreases as the fill pressure

increases, being ^ ?9X at 100 BTorr and 11? at 250

MTbrr. The convergence of the BA values generated by

the two techniques indicates tha*. the density pro-

files aore closely approxioate a Gaussian shape as

the fill pressure is increased. Assuming that the

profile at PQ « ^00 «Torr is Gaussian, the calculated

Assuming that the plan** temperature la Independent

of radius, pressure balance yields

where i>A and 8A «r* the plasma density and 0 on axis.

For B Gaussian density distribution, substitution of

Eq. (7) Into Kq. (6) yields an exact colutlon for 8 ‘A For

as » funciion or re ff and the 1/e plasma radius

a non-Gaussian profile such as shown in Fig. 8b,

n!r)/nA must be detemined graphically and the result

»(r)

0.2

0.4

(6)

(7)

BA is then : 0.17.

Eq. (8).

(8)

-1

In the high density plasma column the thermal equili-

bration tin!* between electrons end ions is small

(« 1 M S) so that k(Te*Tj) : 2kTe. The plasma elec-

tron teoperature can then be determined directly from

X. SCYLLA I-C PLASMA PARAMETERS AND SCALIMG

Figure 11 suonarizes the Scylla I-C plasma para-

meters n , T , and P, obtained at maximum compression

over the investigated range cf fill pressure, P ,

rron 100 mTorr to 500 mTorr. The error bars displayed

on the plasma density data reflect the assumed un-

certainty in the data reduction and indicate the er-

ror associated with the calculated value of T and B«.

With the large variation in plasma 8 observed

over the investigated range of fill pressures, it is

difficult, to analytically derive expressions which

predict the scaling of the plasoa parameters with fill

pressure. However, empirical relations can be ob-

tained from the data presented in Fig. 11. Accord-

ingly, in the Seylla I-C theta pinch with constant

discharge lube :imi roil diameters and constant E.

Fig. 10. Excluded flux radius obtained at 100-

mTorr fill pressure.

and B, the pla:;m.i temperature Tg * T. a PQ , the

u P*/2 and the plassa

1 017 cm”3 at t <v< 1.1 us, slightly prior to peak field.

*t the tiae of magnetic field maximum, t * 2.0 us.

The plasma 6ft was determined as discussed above and

found to obtain a peak value of t> O.63. The plasma

temperature, estimated from pressure balance, ob-

tained a maximum value of *>< 100 eV at t i» 1.1 us.

Assuming that the plasma parameters scale in the same

manner in both theta-pinch coils, a peak density of

3.2 x 10 with a corresponding electron temperature

of 17 eV and $A of 0.2k is indicated for the 500-mTorr

In the Scylla I-C laser-plasma experiment the

inverse bremsstrahlung absorption length, &_}., is

a fundamental parameter. For C 02 laser radiation

time of 1.1» us, and an initial Eo of 0.U6 kV/cm at

1.03 x 1 03 5( Te)3 /2

XI. PLASMA PARAMETERS WITH A 5-CM DIAMETER THETA-

The previously discussed plasma experiments

wer« conducted with a 10.5-em diameter theta-pinch

coll. In order to investigate the plasma paraaeters

at a higher initial Eg and peak magnetic field

strength, the coil diameter was reduced to 5 cm. A

less effective crowbar of the main field was obtain-

ed with the smaller diameter coil and only the

plaina properties at peak field, prior to crowbar

initiation, will be discussed here. The experiments

were carried out at ItO-kV primary bank voltage which

produced a peak field of U6 kG, with a quarter cycle

the discharge tube inner wall.

Plasma interferograms, obtained with the 5-cm

diameter soil, showed physical features identical to

those taken with larger bore coil; the plasma for-

mation phase exhibited flute-like instabilities which

disappeared in about 1 ps. The plasma column was

well defined and of high density. The peak plasma

density on axis was estimated, for 100-mTorr fill

to be resolved by the end-on interferograms. For 100-mTorr fill, the peak plasma density nft 2 1.5 x

PINCH COIL

pltuu dtnilty

5x10% 50 r

UK)17

o P~3 /5

2to(l.39 Te)3 /2

fill pressure case.

I . can be expressed as

ACKNOWLEDGMENT

3/2 2

1 0 .5

O.I

P0<mT0HR)

Fig. 11. Scylla I-C plasma parameters at the

time of maximum compression, t = 2.0 Ms.

(9)

cm ,

where n is in cm and T in eV. Considering the

100-mTorr fill pressure data, the absorption length

that indicated for the 5-cm diameter coil, l^ = 657

cm; at 500-mTorr fill pressure H&b ~ lU cm for both

coil configurations. Accordingly, in the Scylla I-C

theta pinch significant changes in the inverse

bremsstrahlung absorption length could not be pro-

duced by reducing the theta-pinch coil diameter; al-

though higher densities are generated in the smaller

diameter coil, the increased initial Eg results in

higher plasma column temperatures and since £ . o

T /n the absorption length is not significantly

altered by decreasing the coil diameter.

The author wishes to thank Dr. K. B. Freese for

identifing the line spectra, W. T. Armstrong for his

aid in the data reduction, E. L. Zimmermann for

pressure data, from fringe counting. At fill pres-

for the plasma parameters obtained with the 10.5-cm

sures above 100 mTorr the plasma density was too high

diameter coil, i. , - 612 cm, is slightly smaller than

100 200 300 400 500 600

maintaining the experimental apparatus and S. E.

Linzey for his technical assistance.

REFERENCES

K. F. HcKenna, E. L. Zionermanc, and K. B.

Freese, “Laser-Plasma Interaction in the Scylla I-C Theta Pinch,” Third Topical Conference on Pulsed High-Beta Plasmas, Culham, England, Sept. 9-12, 1975.

T. M. York and K. F. HcKenna, “Laser-Plasma

Interactions in the Scylla I-C Experiment, Pre- liminary Analysis,” Los Alamos Scientific Lab- oratory report LA-5957-MS (May 1975).

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