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CHI during ohmic discharges in spherical tori

D. Mueller, M.G. Bell, Princeton Univ.W.T. Hamp, A.J. Redd, B.A. Nelson,

T.R. Jarboe, R. Raman, Univ. of Wash.

S.A. Sabbagh, Columbia Univ. 47th APS – DPP Meeting

Oct 24-28, 2005Denver, Co.

Mueller, D. APS’05 2

Coaxial helicity injection (CHI)

• Used to provide solenoid-free plasma startup– transient CHI (Raman et al.)

• Steady-state CHI– Relaxation current drive

• Edge current drive– Control edge SOL flows– Improve stability limits– Induce edge rotation– Provide means to study current penetration mechanisms

• Persistent current• Well-characterized plasma

Mueller, D. APS’05 3

Experimental setup

Direction of toroidal and poloidal fields and the applied electric field are chosen so that

1) ExB direction is into the vessel

2) Current following the field lines connecting the the electrodes will have Ip in the desired directionVinj

Mueller, D. APS’05 4

Target shapes

• SN plasma shape• ExB is up• Strike points bridge

insulator • Different insulator

configurations

• Vinj

– HIT-II IGBT supplies

– NSTX Capacitor plus crowbar circuit

HIT-II NSTX

Mueller, D. APS’05 5

HIT-II results indicate persistent Ip after CHI

• Successful in demonstrating persistent plasma current– When inj > tok , where inj = Iinj / inj , tok = Ip/ tok

tok

inj

Ip(kA)

Vinj(kV)

Iinj(kA)

0

0.8

0

2

1

2

0

4

80

120

inj > tokinj < tok

TIME(ms)14 16 18 16 1814

0

0.8

0

2

1

2

0

4

80

120

Mueller, D. APS’05 6

HIT-II results indicate broader current profileafter CHI

• EFIT analysis indicates that li is lower after Iinj = 0• li is leading term in current distribution and indicates a

broader current profile

Mueller, D. APS’05 7

NSTX progress towards edge current drive

• Modifying shape avoids arcs and increases toroidal current

Mueller, D. APS’05 8

Early NSTX experiments clarify path forward

• inj > tok, feedback on Ip lowers Vloop (and Ip)

- persistent Ip not readily discernable

• n=1 tearing mode starts before CHI is applied

Mueller, D. APS’05 9

Identified needed improvements in the approach used on NSTX

• Need to apply constant Vloop, using feedback on Ip

– confuses determination of CHI driven current

• Faster turn-off of Iinj would be helpful

• Need to avoid tearing mode in target plasma– observation of coherent n=1 activity

• Dynamo probe tip on fast reciprocating probe will measure

ne,Te, s at 2 poloidal and radial positions

B, Br, Bat one location

- This can provide crucial information regarding models of the CHI mechanism by directly measuring the candidate fluctuations

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