1. Supported by Office of Science Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAERI Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin 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. 47 th APS – DPP Meeting Oct 24-28, 2005 Denver, Co.

2. 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

3. 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 direction V inj

4. Mueller, D. APS’05 4 Target shapes SN plasma shape ExB is up Strike points bridge insulator Different insulator configurations V inj –HIT-II IGBT supplies –NSTX Capacitor plus crowbar circuit HIT-II NSTX

5. Mueller, D. APS’05 5 HIT-II results indicate persistent Ip after CHI Successful in demonstrating persistent plasma current –When inj > tok, where inj = I inj /  inj, tok = I p /  tok tok inj I p (kA) V inj (kV) I inj (kA) 0 0.8 0 2 1 2 0 4 80 120 inj > tok inj < tok TIME(ms) 1416 18 16 1814 0 0.8 0 2 1 2 0 4 80 120

6. Mueller, D. APS’05 6 HIT-II results indicate broader current profile after CHI EFIT analysis indicates that l i is lower after I inj = 0 l i is leading term in current distribution and indicates a broader current profile

7. Mueller, D. APS’05 7 NSTX progress towards edge current drive Modifying shape avoids arcs and increases toroidal current

8. Mueller, D. APS’05 8 Early NSTX experiments clarify path forward inj > tok, feedback on I p lowers V loop (and I p ) - persistent I p not readily discernable n=1 tearing mode starts before CHI is applied

9. Mueller, D. APS’05 9 Identified needed improvements in the approach used on NSTX Need to apply constant V loop, using feedback on I p –confuses determination of CHI driven current Faster turn-off of I inj 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 n e,T e,  s at 2 poloidal and radial positions B , B r, B   at one location -This can provide crucial information regarding models of the CHI mechanism by directly measuring the candidate fluctuations ~ ~ ~ ~ ~ ~