(EX/4-2) Prevention of the 2/1 Neoclassical Tearing Mode in DIII-D

R. Prater1), R.J. La Haye1), T.C. Luce1), C.C. Petty1), E.J. Strait1), J.R. Ferron1), D.A. Humphreys1), A. Isayama2), J. Lohr1), K. Nagasaki3), P.A. Politzer1), M.R. Wade1), A.S. Welander1)
1) General Atomics, San Diego, California, United States of America
2) Japan Atomic Energy Agency, Naka, Japan
3) Institute of Advanced Energy, Kyoto University, Kyoto, Japan

Abstract.  Onset of the m/n = 2/1 neoclassical tearing mode (NTM) has been prevented in high-performance DIII-D discharges using localized electron cyclotron current drive (ECCD). Active tracking of the q = 2 surface location, using real-time equilibrium reconstructions with motional Stark effect data, allows the current drive to be maintained at the rational surface even in the absence of a detectable mode. With the application of this technique in DIII-D hybrid discharges, the 2/1 mode is avoided and good energy confinement is maintained for more than 1 second with beta at the estimated no-wall stability limit for ideal kink modes (βT approximately equals 4.2% and normalized beta βN approximately equals 3.2). A real-time correction of the ECCD location, taking account of refraction effects as the density varies, has recently been implemented and will be incorporated into future experiments.
* Work supported by the US DOE under DE-FC02-04ER54698.

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