XP #802 - Biased Electrodes

S.J. Zweben, R.J. Maqueda, L. Roquemore, R.J. Marsala, Y. Raitses, R. Kaita, C. Bush

R.H. Cohen, D.D. Ryutov, M. Umansky (LLNL)

Team XP review 1/11/08

• Upgrades for 2008

• Physics goals for 2008

• Proposal for 2008

• Possible concerns

Hardware Upgrades for 2008

• Two positive supplies increased from ~10 A to ~30 A

• Added radial array of probes to measure local SOL

• Will now have fast camera views of BEaP electrodes

radial probe array

cable shield

GPIpuffer

BEaP

B

BEaP Goals for 2008

• Measure effect of increased positive bias (x3 in I, x2 in V)

• Measure effect on local SOL with new radial probe array

• View effects of biasing on D light near BEaP electrodes

• Evaluate effect of double-probe biasing (floating electrodes)

• Evaluate effects during NBI and RF (via piggy-backing ?)

BEaP XP for 2008 (~1/2 day)

Repeat #124688, I=0.8 MA, B=4.5 T, outer gap ~5 cm -> 1 cm 1st hour: single-electrode voltage scan up to +100 V (≤ 30 A)

monitor local array, local D light at BEaP, GPI imageslook for effects on plasma (impurities, radiation, divertor)after this, check probe-GPI correlations for alignment

2nd hour: optimize alignment based on shots #1-5 (i.e. adjust Ip)try four electrode biasing +/-/+/- (VExB outward at array)

3rd hour: reconfigure for double-probe biasing (pending review)

4th hour: repeat best case, or try with NBI (using ~5 cm gap)

Approximate Shot Sequence

shot # plasma type bias voltage (1,2,3,4) comments

1 OH, 0.8 MA, 4.5 kG 0, 0, 0, 0 recover #124688

2 same 0, 0, +25, 0

3 same 0, 0, +50, 0

4 same 0, 0, +75, 0 stop if near 30 amps

5 same 0, 0, +100, 0 stop if near 30 amps

6 same 0, 0, 0, 0 repeat shot #1

7 same +25, -25, +25, -25

8 same +50, -50, +50, -50

9 same +50, -50, +50, -50

10 same +75, -75, +75, -75

11 same +100, -100, +100, -100

12 same +25, -25, +25, -25 try double probe bias

13 same +50, -50, +50, -50 try double probe bias

14 same +75, -75, +75, -75 try double probe bias

15 same +100, -100, +100, -100 try double probe bias

16 same but with 1 NB

+25, -25, +25, -25 best case of above

17 same but with 1 NB +50, -50, +50, -50 best case of above

18 same but with 1 NB +50, -50, +50, -50 best case of above

19 same but with 1 NB +75, -75, +75, -75 best case of above

20 same but with 1 NB +100, -100, +100, -100 best case of above

Possible Concerns

• Electrode heating: P(2008) ≤ 3 kW @ 50% for 0.2 sec=> maximum T (surface) ~ 75 ºC during shot=> maximum T (bulk) ≤ 40 ºC after shot

=> should be no problem with melting

• Arcing: will look at electrode currents and fast camera

• Impurity influx: will monitor iron with SPRED and Ptot with bolometerno increase in Ptot in 2007 run

• Floating double-probe operation with voltage transients ?

(review before implementation)