OBSERVATION OF A FAST BETA-COLLAPSE DURING HIGH POLOIDAL-BETA DISCHARGES IN JT-60

A nondisruptive beta-limiting phenomenon in a large tokamak under a large bootstrap current fraction, up to approximately 80% of the plasma current, is described; beta = (plasma pressure)/(magnetic pressure). During long-pulse neutral-beam-heated discharges in the JT-60 tokamak, it occurs at beta(p) approximately 3, leading to a limit of the normalized-beta lower than the Troyon limit. The MHD feature is characterized by a large-amplitude partial relaxation with a fast growth time. A hollow current profile evolution in the high-beta(p) regime plays an essential role in the MHD stability, analysis of which shows that the ideal n = 1 kink-ballooning modes can be unstable just before the collapse.