HIGH POLOIDAL BETA-LONG-PULSE EXPERIMENTS IN THE TOKAMAK FUSION TEST REACTOR

Experiments have been performed in the Tokamak Fusion Test Reactor [D. M. Meade et al. in Plasma Physics Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 9] with neutral beam injection of up to 4 sec. duration, which is comparable to the time scale for resistive redistribution of the plasma current profile. These plasmas were created using a rapid decrease of the plasma current which initially created a plasma with enhanced stability and confinement. As the current profile evolved, a significantly reduced beta limit was observed. The high epsilonbeta(p) plasmas had up to 90% of the current driven noninductively which significantly broadened the current profile during the long pulse lengths. These experiments demonstrated that high beta(N) plasmas could not be sustained for times longer than the resistive relaxation of the outer current region which at early times after the current ramp-down carried negative current. At later times in lower beta(N) discharges, beta collapses were sometimes observed as the current profile broadened at beta(N) approximately 1.5. The appearance of disruptions was consistent with the predictions of ideal magnetohydrodynamics (MHD) stability analyses.