High-beta discharges in the DIII-D tokamak

Low-q (q95<3) double-null divertor discharges with values of the volume-average toroidal beta as high as 9.3% have been operated in the DIII-D tokamak [Fusion Technol. 8, 441 (1985)]. In discharges with q95≊5, values of βT/(I/aB) as high as 5 have been obtained. These discharges are shown to be at or below the stability limit to the value of beta for infinite-n, ideal ballooning modes. The discharges are significantly below the beta limit for ideal,low toroidal mode number kink modes. The kink mode beta limit is shown to be strongly dependent on the radial profiles of plasma pressure and current. The theoretical beta limit in DIII-D is shown to be in the range βT/(I/aB)=4 –5 depending on the value of I/aB, and this is consistent with the experiment. High-beta discharges have been operated with ion temperature up to 17 keV. Steady-state, high-beta, low-q operation is demonstrated by a discharge with I/aB=2.6, q95=2.7, in which βT>7% is maintained for 1.5 sec.