LA-8397-MS Informal Report DC 20 Reporting Date: August 1976 Issued: August 1976 Plasma Experiments on the Scylla I-C Theta Pinch by Kenneth F. Mc Kenna scientific laboratory of the University of California LOS ALAMOS, NEW MEXICO 87545 An Affirmative Aclion/Equal Opportunity fmployer n UNITED STATES ENERGY RE8EAHCH AND DEVELOPMENT ADMINISTRATION CONTRACT W-740S-ENG. 3< 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 Thi* rvaori «*••* awparwl an *•> •rmunl irf work •a Maan* at the t'nitrd State, (iavrranwal Neither the 1'nitcd Mialra nor the Ciii Ud Slalra Kmms Krtrart-lt ai«l l>rvrlaaa»nil M- i Mrlar*. aubr Mlrartan. «r Ihrir rmatoitv*. malurs * n V warranl.i. nprm ar implied, ar ii Mion aav legal liah JKl^ «r rraaoniibility f»r |IM> arrvrarv, c«mal H*(wa«. ar wwfulnet* af anv inr«rirtai Hin. apparalin. prarfwrl, ar arm-na rfiaclaaatl. ar reprrorain thai H* UM> wauw na< inf Vinffr arivalrl> •wnctl rlah U. PLASMA a Cf'BUMEBTS 08 THE SCit XA I-C ?H£?A PI8CK Kenneth F. Mc Kenna Scyl U 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 stability characteristics of the Scyll* I-C plassa, over a range or initial rill pressure froo JQO-500 ss Torr 0 } , are discussed in this report. !. 1STO0DUCTI0S The .'Scylla 2-C linear sheta pinch was coii3tr&-te<2 i:i oriler to provide a versatile r«sewch instrument to aupli Kscnl. the larger, less flexible theia-pi rsirii pro- grans. Swill scale research studies »«d the exper S- sentitl investigation of atli'«r.e<?<! concepts are cwrrled out. on Scylla !-C in order to provide basic physics inforsation and Jisgnoatic dc«e»oi«ient for the present siiiil rumre asin thet«-pinch efforts. The Scy Un I-C theta pirn* ia preiently being tutv4 In M>n J«ai>et.i«n uilh a high enerfif <**• l8f) J) C0 g Inner to statjy the interaction of nx Suliy directed Insnr WDttr with the 4cntse (n -v 10 ea" ) ttieta- islnch pinssft wlw. 1 ''' the rcisttlis of the ]«a«r- pi<u!Si» i Btrracllon experi Metit will be i Mva^a'«"l in s> l:i U«r report. In the present report, the properties jijsd Btnhility characteristics of the higit density Sry Ua I-C p Utan* are 4iscusae4; exjw Hstnts »er«? eoni J»wt«4 ewer a range *>r fill pivoaurc SB fro« 100- "j OD sff«rr »T In .•» eonpan Son paper* the re»u;t« of an iftv«>sti£ntii;» «f p Jaatwi <-!»4-J«8r. Cram the J'cy.ia I«C pl Jinssi «?<5j!jan nrc fi. FJCPERTNEMTAf. ABBASSBCST ABO BIACBRSTIC5 The geottetry, energy atorage enpnbllitiee, and electrien! eireuit pnr«i Beiera of the Scylla l-C them pinch «ere determined fro» opii Blaailon stuiiiua intended to produce « *ienae C» c J 10 et T ). low temperature pl.-maa {T t> •_• 'i O e V). A acheaatic or the thota ptneh is afeotm in Fig. I. Seylih S-C, csn- s*.tructc4 frees Ssyllae renpsnents, hits at ssxisus eitor enerisy storage of 175 W at 60-fc V primary tur.k voitn^c. Fifty-four 1.8-u F eapseiters feed the SQD- ca Seng, i0.5-ea <iiaseter single-turn cespreseien eoil. Priaary bank operation »t &0 k V generate* * v*<!t MM Eg of 0.36 k V/en at the inner wai; of the l.f>sti t.4. quart: discharge tutse and a peak crsv- barred coepressiti;: field of 33 k S is obtained S.O «« after disffharge ini.iatlon. the Jfcff.t eoii i Jwtett-;-. in eoaparison with the s Uscnarge tube dianeter, vas used in orittr ta obtain an effective ereubsr of the (sails flvld s Jistrhar^. Unless otherwise sssei S, tftu CSi! T. 1. ;Vheea*Ic "f '!«• :'^\v U» I-C tlieta sn V* d 5n ;.5s Si? r«jp» al s» pritaa The jsriatfipa S , a Si»s«? s^hsrf:^ tafec, rtji Jiti;-.}? arjhi»is?rr-.i ypti- "« »-S-.f FSii ess. jss a 4£sttsaj SB«rt. J* i#sj!» vlife SS4«-sft *s aft ane 1lirass #? "& (i|fts?, tti was fen»t8i?4 with sft'rf-a'a 1 i!>j-s* f5 li '^i^ ".-f-s.'.'-h irarrcss!. Is of A- S- **>*•«* f^ a} vith SW-sf Fcs-r pressure, is presented '.r, Pig. 3. SJ|tati!»2 Integration of the redue«Mi density profiien isj!iiwnte.-S that for the 100-efftarr fill pressure esue. •>05 of t*s<? fill g M» v»a Ionised by the tiae the plnehei S plasm «I;UBJI ftasi ejtffutiea to the disenftree tub* vail. Por SOS-a Torr fill pressure, about SOU of the gas usit lonlsted at this tiae. t ; 3 sis. The vntf, ',r.i%Lnl*<l 10 iis «tt#r '.he SPECWA Of ««ncertt when ast«*5 * * is th« £<j".roitu J".S«sn of , int* tker ajfm??.nsss -»r n ansi satis «»I4-IS» »ievisj5 tthrough the deuterli M Bftlaer series. The regaining identi- fied line* *re froa excited carbon ions. Jto indica- tion of Z-pineh electrode a»ter J%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 b O-KV prlaary bank voltage and i00-«Torr fill 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 cue of the yreloriimion plassa spectra, iapurity r»d Ut!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 t Min discharce plan* resulting Tram istpurity conta»ln*tion. if, g&teirm of the Scyl U I-C prelooitation *l* ife 109ffir flit pre»»are V. PU&MA COWKH LUMINOSITY MEASUREMESTS Streak camera photographs of the Scylla I-C 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-m Torr 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 (P Q S 100 is Torsf) theta-pinch operation at the termination of the initial imp'.osion. In this plasma regime, they are identified as radial hydromagnetie oscillations 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 radius. Assuming all the initial fill gas is swept up during the implosion, the equation of motion of the plasma SJMUIUE can be solved yielding an oscillation period given by 100 m'i'orr 220 m7©r 3S0 Xi&vv 550 e?y ri- •00 m nut Vmmn O 7 Fill P»t»urn Fig. 6. Streak photographs of the Scylla I-C plasma column obtained at. different camera sweep rates and in- itial theta-pinch fill pressures. (1) uh«r« M it the line density of ions of mass * i In the annului, and B li the external Magnetic field. Con- sidering the 100-m Torr fill pressure case, the period of the first oscillation, after the implosion, is about 220 ns and the average external magnetic field strength over this time interval i 13 KG. With this value of nagnetic rield and N < n o >ib 2 «W» 10 16 cm" (b is the discharge tube radius), Eq. (l) pre- dicts an oscillation period of 235 ns, in very good agreement with the experinental result. The experiment- ally observed periods of the two successive oscilla- 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 at (2) 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), .iiff ... 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 Tn H ,j % where T Is in e V. 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 kinetic and thermal energies during the implosion while the electrons retain their initial preloniza- tion temperature; ohmic heating of the electrons is assumed small. At the termination of the implosion the ions rapidly transfer energy to the electrons, via collisions, until temperature equilibratior. is obtained. The ion energy at the end of the implosicn is then 3/2 k T * ?(]/? o. Vj), where v. is the inward radial velocity of the ions. Estimating the pinch time from the streak photographs to be t r 330 r.s (again for P Q • J00 m Torr) and tailing Vj ; b/t , the kinetic temperature of the ions before energy equlpartition with the electrons is approximately ho e V. Thus, after temperature equilibration the elec- tron teaperature is about 20 e V. 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-e V electron temperature es- timated above will thus *et a lower limit on the cal- culated magnetic field diffusion time. Talcing In JJ ; 8, T e r 20 e V 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- tions, l.i us. 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 m Torr) 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- pass configuration. Figure 7 presents a time sequence of plasma column lnterferograms obtained for 100-m Torr fill pressure. The straight background fringe pat- tern vas generated on each interferograa by a small angular deviation of the scene beam Introduced be- tween exposures taken vtth and without plasma. Plasma electron refraetivity results in displacement of the background fringes, with one fringe displaee- oent corresponding to a change in J a dt of * 10 17 electrons em" 2 (5) Each interferogriu B of Fig. 7 ua3 obtained from a sep- arate plasma discharge and the entire 3.8-cu disaeter of the discharge tube was illuiainatcd by the inter- ferometer laser light. The plassa column formation phase (t*0.5 us, Fi.f. 7) is dominated by high e-nuober («>6) flutes, with coae of the flutes extending froa the plasm column to the di scharge tube wul1. The annular dis- tributlon of plnsma density, previously indicated to be a result of '.nipped magnetic fields, is observed 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 t. Sflt Fig. 7* Tiae sequence of plasma colunn interferograas obtained at 100-n Torr 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 m Torr 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 a Torr is difficult due to the large number of closely spaced fringes. At P Q « 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 Q) b) OA OS 12 TUBE RADIUS (em) Fig. 8. Plasaa column interferogram and reduced density profile for Po • 300 m Torr, t » 2.7 us. interferograas at 500 a Torr 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 10 17 cm" 3 at P o • 100 m Torr, * 1.5 * 10 17 ca" 3 at P Q • 250 a Torr, and •»- 2.3 * 10 i7 cm" 3 at 500-a Torr fill. Spatial in- tegration over the reduced density profiles obtained at 100-a Torr and 250-m Torr 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 P o « 500 m Torr. In an attempt to generate a density minimum on the plasma column axis, and thus provide a "light pipe" for the C0 g 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 k C 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-a Torr 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 a Torr. At 100-a Torr 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 e V 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 e V at t « 0.3 Us, obtained without forward bias for P » 100 m Torr, indicates o ' 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 in electron temperature, the peak plasma density de- creases when the bias field is applied; at I • 1 ps, n = 1 x io 1T cm" 3 without bias and n : 0.6 x 10 17 -3 e cm with bias, assuming a plasma column length of 100 cm. Since the main magnetic field, B, is inde- pendent of applied bias and 2nk T « 6B 2 /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 radi U3 r eff 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 a) Without Bk* Field b) With Bias Field Fig. 9- Plasma column iaterferograms taken with and without an applied bias field. estimate of the plasma kinetic temperature can be obtained. The diamagnetic loop-probe system consists of two closely spaced, Magnetic pick-ups. A small magnetic probe (HA i 0.3 CM) 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 report 3 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 CE, between % 1.5 and t> 2.5 us is nearly insen- sitive to fill pressure, changing only 15£ between 100-n Torr fill (0.23 cm) and 5no-m Torr fill (0.19 CM). VIII. DETERMINATION OF PLASMA B From the definitions of excluded flux and the Q pressure balance equation it can be shown that the area of excluded flux, *r^ ff , i B related to the plasma S through the expression (6) ical techniques were: 6 A • O.ltti with P Q - 100 m Torr Assuming that the plan** temperature la Independent of radius, pressure balance yields »(r) (7) 'A For where i> A and 8 A «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 as » funciion or r eff and the 1/e plasma radius a non-Gaussian profile such as shown in Fig. 8b, n!r)/n A must be detemined graphically and the result substituted into Eq. 16) which can then be numerical- ly integrated to deteniinc (L. Tor the Scylla I-C density profiles at maximum co Hpresulon, •. • 2.0 us, the B A determined by numer- and * 0.33 with P Q « 250 m Torr. The value of obtained assuming a Gaussian density profile and calculating the 1/e radius using the peak density value were: $ A * 0.63 for 100-a Torr fill and B. • 0.37 for 250-ji Torr 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 B A 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 P Q « ^00 «Torr is Gaussian, the calculated B A is then : 0.17. IX. OEr ERMIXATIOM OF FUSKA ELECTROX TEMPERXTUHE The compressed plasma tenperature is deterained fro* the eensured values of plasma density on axis n., the aeasured external aagnetic field B, and the calculated values of S... From pressure balance, 14 •o oa 0.4 0.2 40 M Fig. 10. Excluded flux radius obtained at 100- m Torr fill pressure. (8) In the high density plasma column the thermal equili- bration tin!* between electrons end ions is small (« 1 MS) so that k(T e *Tj) : 2k T e . The plasma elec- tron teoperature can then be determined directly from Eq. (8). 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 m Torr to 500 m Torr. 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. -1 and B, the pla:;m.i temperature T g * T. a P Q , the pltuu dtnilty u P* /2 and the plassa o P ~ 3/5 A *t the tiae of magnetic field maximum, t * 2.0 us. XI. PLASMA PARAMETERS WITH A 5-CM DIAMETER THETA- PINCH COIL 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 It O-k V primary bank voltage which produced a peak field of U6 k G, with a quarter cycle time of 1.1» us, and an initial E o of 0.U6 k V/cm at o 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-m Torr fill pressure data, from fringe counting. At fill pres- sures above 100 m Torr the plasma density was too high to be resolved by the end-on interferograms. For 100-m Torr fill, the peak plasma density n ft 2 1.5 x 5x10% 50 r UK) 17 10.5 O.I 0 100 200 300 400 500 600 P 0 <m T0HR) 10 17 cm" 3 at t <v< 1.1 us, slightly prior to peak field. The plasma 6 ft 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 e V 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 17 3.2 x 10 with a corresponding electron temperature of 17 e V and $ A of 0.2k is indicated for the 500-m Torr fill pressure case. In the Scylla I-C laser-plasma experiment the inverse bremsstrahlung absorption length, &_}., is a fundamental parameter. For C0 2 laser radiation I . can be expressed as 1.03 x 10 35 (T e ) 3/2 at n e 2 to(l.39 T e ) 3/2 cm ,(9) where n is in cm and T in e V. Considering the 100-m Torr fill pressure data, the absorption length for the plasma parameters obtained with the 10.5-cm diameter coil, i. , - 612 cm, is slightly smaller than that indicated for the 5-cm diameter coil, l^ = 657 cm; at 500-m Torr fill pressure H &b ~ l U 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 3/2 2 T /n the absorption length is not significantly altered by decreasing the coil diameter. ACKNOWLEDGMENT 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 maintaining the experimental apparatus and S. E. Linzey for his technical assistance. Fig. 11. Scylla I-C plasma parameters at the time of maximum compression, t = 2.0 Ms. 10 REFERENCES 1. K. F. Hc Kenna, 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. 2. T. M. York and K. F. Hc Kenna, "Laser-Plasma Interactions in the Scylla I-C Experiment, Pre- liminary Analysis," Los Alamos Scientific Lab- oratory report LA-5957-MS (May 1975). 3. K. F. Mc Kenna and T. H. York, "Plasma End Loss Studies in Scylla I-C," Los Alamos Scientific Laboratory report LA-61*12-MS (August 1976). 4. G.B.F. Niblett and J. S. Green, "Radial Hydro- magnetic Oscillations," Proc. Phys. Soc. LXXIV, 6, 737 (1962). 5. H.A.B. Bodin, T. S. Green, G.B.F. Hiblett, N. J. Peacock, J.M.P. Quinn, J. A. Reynolds, "The Influence of Trapped Field on the Characteris- tics of a Magnetically Compressed Plasma," Duel. Fusion, £, 521 (l?6l). 6. L. Spitzer, ?hy&ic A oi Tatty Ionized Gaie A, 2nd Ed., Interscience Pub., New York, (1962). 7. R. F. Cribble, W. E. Quinn, and R. E. Siemon, "Plasma Experiments with a Three-Meter 6 Pinch," Phys. Fluids, 1U., 20U2 (1971). 8. K. S. Thomas, H. W. Harris, F. C. Jahoda, G. A. Sawyer, and R. E. Siemon, "Plasma Experiments on the Linear Scyllac Theta Pinch," Phys. Fluids, 17, 131b (1975). 9. W. R. Ellis, J. P. Freidberg, R. E. Siemon, E. L. Zimmermann, in "LASL Controlled Ther- monuclear Hesearch Program, Los Alamos Sci- entific Laboratory report LA-525O-PR, 38 (June 1973). 11