Ideal and resistive stability near the beta limit in the DIII-D

The maximum achievable beta in the DIII-D tokamak scales linearly with normalized current, ..beta../sub T/(%) less than or equal to 3.5 I/aB (MA/m/T). This scaling has been confirmed in neutral beam heated single null divertor H-mode discharges over the range of normalized current, I/sub N/ = I/a/B (MA/m/T), from 0.6 q/sub 95/ > 3. q/sub 95/ is the inverse of the rotational transform at 95% of the enclosed poloidal flux. Although similar discharges have been obtained at low q, q/sub 95/ approx. 2, and I/sub N/ approx. 3.1, the beta limit has not been reached in these discharges owing to insufficient heating power and energy confinement time. Discharges that approach the beta limit in DIII-D exhibit one of three distinct types of behavior: (1) quiescent operation, (2) sudden disruption, and (3) beta saturation and collapse. The three discharges were part of a series, having identical shape, plasma current, and q/sub 95/, and with the wall conditions as identical as possible. One discharge was stable near the beta limit with ..beta.. /sub T/ > 3 I/aB for 1 sec, another ended in a sudden disruption at 3.37 sec, and a third discharge,more » similar to the previous two with the exception of a small increase in the electron density prior to neutral power injection, shows a decrease in beta beginning at 2.75 sec, and saturation at a lower value of beta. This paper discusses these conditions further. 9 refs., 2 figs.« less