SECOND RESULTS FROM THE WISCONSIN NON-CIRCULAR RFP. J.C. Sprott

Transcription

1 SECOND RESULTS FROM THE WSCONSN NON-CRCULAR RFP J.C. Sprott PLP 978 May 986 Plasma Studes Unversty of Wsconsn These PLP Reports are nformal and prelmnary and as such may contan errors not yet elmnated. They are for prvate crculaton only and are not to be further transmtted wthout consent of the authors and major professor.

2 SECOND RESULTS FROM THE WSCONSN NON-CRCULAR RFP by J.C. Sprott ncluded here are copes of the posters presented at the 986 EEE nternatonal Conference on Plasma Scence n Saskatoon, Canada, May 9-2, 986. (Ref: EEE Catalog No. 86CH237-6, page 79, 986). The data represent more recent and non-overlappng results than those presented n PLP 969. The major new results are the extenson of the 2 ka/8 msec dscharges to 3 ka/ msec and estmates of the plasma densty, temperature, and confnement tme. The changes were brought about by operaton at hgher polodal bank voltages (44 volts vs 35 volts), core basng, correcton of some feld errors at the polodal gap, mproved vacuum condtons, and better control of the gas puffng. Some mprovements n plasma parameters have been obtaned, but there s stll no evdence of a quet perod when the feld reverses, and the resstvty s stll 5- tmes the ZT-4 value at the same plasma current. Reducton of some known large feld errors at the polodal gap dd not lower the resstvty. The most lkely cause s the nflux of mpurtes as evdenced by a rsng resstvty and densty, and a rsng level of oxygen and alumnum radaton durng the pulse. The dscharges contnue to show slow mprovement wth surface cleanlness and are easly spoled by vacuum accdents. nterferometer measurements ndcate a typcal lne- averaged densty of 5 x 2 cm -3, whch s consstent wth the fll pressure of -.2 mtorr (gauge). Measurements of the oxygen lne radaton, Doppler broadenng of

3 -2- carbon, and the neutral charge exchange spectrum suggest a peak temperature of T e - T - ev, whch mples Z eff - 5. For a plasma current of 3 ka, a loop voltage of 6 volts and a volume of 8.6 m 3, these numbers correspond to an energy confnement tme of Ths estmate may be low because t comes from data at the tme of peak current whch s near the end of the dscharge after a sgnfcant mpur ty nflux has occurred. Near-term plans call for nstallaton of stanless, torodal lmters n May '86, nstallaton of dvertor rngs n July '86, and nstallaton of the Thomson scatterng system n September '86. The nstallaton of the new MST vacuum vessel s scheduled for Aprl '87, w th frst RFP plasmas expected n October of '87.

4 ABSTRACT By removng the nternal rngs from the Levtated Octupole vacuum vessel, a large, non-crcular RFP was produced. The major radus s.39 m, and the cross secton s about m 2. The devce s unconventonal n that the vacuum vessel, whch conssts of 5-cm thck alumnum wth a sngle polodal and torodal gap, serves as the vacuum lner, conductng shell, and polodal and torodal feld cols. A torodal feld of up to about kg can be produced, and the polodal feld s drven by a 6 kj capactor bank through a 2-volt-second ron core. Dscharges are ntated wth 2 volts per turn usng self-reversal of the torodal feld n order to prevent arcng of the polodal gap whch s exposed to the plasma. The gap s protected wth a 2-cm wde strp of ceramc. The best RFP dscharges have a peak current of -2 ka and a duraton of - msec. The torodal feld reverses when the current reaches - ka, makng ths one of the lowest current densty RFP's n exstence. The current ramps up to the fnal value over resstve dffuson tmes and termnates only because the volt-second lmt of the ron core s reached. The F-8 trajectory les sl ghtly to the rght of the A=constant theory as do all other RFP devces. Dscharges have been produced wth 8 up to 2.5 and F as low as -.8. A feature of the devce s that t s capable of producng dscharges wth plasma current of ka and msec duraton over a wde range of safety factor from the q> tokamak lmt to the deeply-reversed, RFP lmt. The hghest current dscharges (-3 ka ) are obtaned at q-.5. 3

5 The plasma noncrcularty ( ndented at the mdplane ) provdes an opportunty to gan expermental nformaton on whether fluctuatons n an RFP are current-drven or pressure-drven nstabltes generated by unfavorable polodal curvature. To ths end, we are measurng the edge magnetc fluctuatons n the separate good and bad curvature regons on a gven magnetc surface. Results wll be presented for both reversed and non-reversed dscharges ( at varous q values ). The resstvty of the RFP dscharges s lower than non- reversed dscharges but a factor of hgher than other RFP's wth the same current. The resstvty correlates strongly wth vacuum condtons, ndcatng a need for more agressve cleanng and mpurt y control. The tme-dependence of the plasma electrcal parameters agrees wth a smple electrcal crcut model n whch the plasma resstvty s gven for all tmes by R p = const/l p where the constant s typcally 5- volts and depends on the surface cleanlness. Plans call for mprovng the cleanlness of the machne, mprovng the electrcal crcuts and measurng the densty and temperature. Over the longer term, new nternal rngs wll be nstalled to attempt RFP operaton wth a magnetc lmter ( or polodal dvertor). Thereafter, the vacuum vessel wll be replaced wth a new crcular vessel wth R =.5 m and a =.52 m. The new devce, called MST, s scheduled for completon n late

6 -2-5) The plasma resstance drops, and the conductvty temperature rses when the plasma enters the reversed-feld state. However, there s no notceable quet perod, and the resstvty s stll an order of magntude hgher than n ZT-4. The conductvty temperature strongly correlates w th vacuum condtons, ndcatng a need for more aggressve dscharge cleanng. Removal pumpng rate per unt wall surface area s qute low (-/sec/4m 2 ). 6) Numercal crcut modelng usng the expermentally observed F-6 curve and a plasma resstance of R p = 65/ at all tmes produces a remarkably accurate predcton of the plasma electrcal waveforms. f the plasma resstvty can be lowered to the ZT-4 value, small mprovements n the electrcal crcuts should allow 5 ka/2 msec RFP dscharges. 7) Temperature and densty (and hence confnement tme) have not yet been measured. The conductvty temperature never exceeds 2 ev, but shows mprovement wth surface cleanlness. Densty s apparently low as evdenced by the optmum H 2 fll pressure of -. mlltorr. The ablty to start up at low pressure s greatly enhanced by the use of -5 watts of 245 MHz ECRH preonzaton.

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15 u p g u o t s 2 99 " 9!.UlaUU,.'UfuuutlUUH.r,r:rH:J,uJauHU"rlUUnrrllllll.rrn.UUlUUfll,J."UHfHHUlrlHU,L " Polodal Gap Uoltage (Uolts).. ""... 'T''''.. "... """"''''''T ''''''''''""".. "'.. '"l'''''''"'... ",.. ".. "!''''''''... "'''".. "''!'"...,,.. ""''''''''''''j'''''''''''''''' "... '!"... "... ""'... ""...,,".. ".. ''... ".. ""'.. ",,.. \! ""!,,... ON LY RFP SHOT"S ( t.. '".. ) "" V PG = "l' (R../ a..2) p 3/9 't.... FroW\ 2;- - '- M "" 9 PlasMa Cuent (ka) rr.u,uu'uuu:tu,uuu'uhuuhl f,fllllnulllu,un'""h,nu:,u"uu,uu,u,,,,,,uuuurrul,lu'.. '"... " " " " luf! " " 4

16 ,+8 F &.f.lf. k V 6: POLOOAL FEL.D CCU/T COUPLe.D Vp.. F (9) t.5 F 28 V TORODAL FELD CRCUT 2. : 5

17 .6

18 NEAR-TERM PLANS: 7/86 9/86 9/86 4/87 4/87 :./87 :.:./87 7