Measurement of Radial Loss and Lifetime. of Microwave Plasma in the Octupo1e. J. C. Sprott PLP 165. Plasma Studies. University of Wisconsin DEC 1967

Transcription

1 Measurement f Radial Lss and Lifetime f Micrwave Plasma in the Octup1e J. C. Sprtt PLP 165 Plasma Studies University f Wiscnsin DEC 1967

2 1 The number f particles in the tridal ctuple was measured as a fl.d1ctin f time by three independent measurements. Thes measurements were used t calculate the fractin f plasma lst t the walls the lifetime f the plasma and the apprximate electrn temperature. A cld in plasma was prduced in the usual manner by a 15 ]lsec pulse f 3 GHz micrwave pwer. The micrwave pulse began 11 ]lsec after the start f the 5 ]lsec magnetic field pulse. The start f the micrwave pulse is called t = O. The magnetic field rse t a peak value f 37 gauss at the utside wall midplane at t = 1 ]lsec and fell t zer at 4 ]lsec. A plasma with density strngly peaked near the separatrix was thus prduced. The flux f particles t the hps and hangers was measured by using tw pairs f hps as a flating duble prbe biased t 9 vlts. At higher bias vltages a discharge ccurred as evidencedby large bursts f particles t the hps. The flux f particles t the wall was measured by tw methds giving identical results. One methd used a circular cllecting baffle which extended frm the bttm wall t crit (abut 2.5 em) and encircled the entire majr axis f the trid. The cllectr was biased negative with respect t the wall and used as a single prbe. The ther methd frm which the data presented here was btained cnsisted f biasing the hps +9 vlts with

3 2 respect t the wall and measuring in saturatin current t the wall. The result f the tw measurements is shwn in Figure 1. rt is apparent that the flux f particles t the wall is 23% f the ttal particle lss during the time that the field is rising and that the fractin is sharply decreasing. When the magnetic field starts t decrease (tv 1 ].lsec) the field lines mve ut tward the wall carrying a large amunt f plasma t the wall. When the magnetic field is almst zer a burst f particles strikes the hps r hangers. The small radial lss fr the micrwave plasma is in striking cntrast t the radial lss f gun plasma which may amunt t as much as 5% f the ttal lss as indicated by identical measurements. The explanatin f this difference may cntain infrmatin crucial t the understanding f plasma cnfinement in tridal multiples. The number f particles in the machine at time t can be determined by integrating the flux f particles t e hps an h ger d wall frm t t : N (t) = 1:. r<x> [2 rh. (t) + rw. (t)] dt. e t 1 1 (1) N(t) btained in this way is pltted in Figure 2. Over a cnsiderable range (5 25 ].lsec) N(t) decreases expnentially with a lifetime f 3 msec.

4 3 The number f particles in the machine can als be determined by using a micrwave cavity perturbatin technique GHz radiatin was used and the frequency shift f sme high rder mdes was measured a functin f time as the plasma decayed. The number f particles N(t) present at time t is then given by f(t) f net) _ Tn; 2 e = '12: 27ff mv N(t) (2) where V is the vlume f the cavity (3. x 1 5 cm 3 ). N(t) btained by this methd is als shwn in Figure 2. The values are abut 5% higher than the particle lss measurements. 'This difference may indicate that sme f the lst particles are nt being cllected as fr example if sme f the ins strike the psitively biased hangers when the hps are used as a duble prbe. Over the range 5 25 sec the lifetime is identical t that btained frm the particle lss measurements. The third methd f measuring N(t) cnsisted f using a single 1/8 x 1/8 cylindrical Langmuir prbe biased t 45 vlts t measure in saturatin currentas a functin f space and time. N(t) can then be fund frm 5 N(t) = n($t) V'( )d 5 (3)

5 4 where V (1/!) = dv/d1/! and n(1/! t) is btained frm the in saturatin current by. = nea '2 MC 1 'T (4) where A is the cllecting area T e the in mass. is the electrn temperature and M is Since there is cnsiderable dubt abut the reliability f the prbe measurement f T e equatin (3) was used t estimate T e frm the values f N(t) btained by the ther'methds. A plt f in saturatin current vs 1/! at varius times is shwn in Figure 3. Nte that the plasma is initially peaked n the separatrix (5 sec) mves in tward the hps as the field increases (1 sec) and then mves ut t the wall as the field decreases (3 sec) The value f T e (t) btained in this way is shwn in Figure 4. This figure is nt meant t be taken t seriusly because it is nly a crude estimate f T e Fr example as the plasma decays the sheath thickness changes and the effective cllecting area f the prbe changes. Furthermre azimuthal density variatins which are knwn t be appreciable during the first 1 sec have been ignred. The pint f this calcualtin is t

6 5 shw first that the electrn temperature is prbably cnsiderably less than the 1 ev read by prbes and secnd that the rate f temperature decay is prbably quite fast in agreement with theretical predictins based n inizatin and excitatin lsses. This preliminary wrk has raised a number f imprtant questins which remain t be answered by future experiments: 1. Why is the radial lss f micrwave plasma s much less than the gun plasma? By what mechanism are particles reaching t the wall in the tw cases? 2. What fractin f the particle flux t the hps and hangers is t the hps? By hw much wuld levitated hps r successfully guarded hangers increase the lifetime? Why is the lifetime nt significantly increased by present guarding techniques? 3. Why d prbes nt read the crrect electrn temperature? Des this errr relate t the yet unreslved flating ptential paradx? What is the true electrn temperature? 4. T what extent can the differences in behavir f the micrwave and gun plasma be attributed t high backgrund gas pressure and lw electrn temperature as ppsed t merely a difference in in temperature?

7 8' & r rt)

8 i t prbe at p = $ arbitrary scale) x PARTCLE LOSS MCROWAVE PERTURBATON d' '.. x '. _ T' = 3 msec t Cj'sec.) x 35 a 4

9 (J (1) n ::l. (1) (f) =t O C E t CJ d lo C d + (1) n ::l. C' i i i 3 t') 8. C C r() v l{)

10 V {) t<) CD (f) :l + t<) x CJ {) {) d