Observation of a fast beta collapse during high poloidal-beta discharges in JT-60

Abstract

A nondisruptive \ensuremath{\beta}-limiting phenomenon in a large tokamak under a large bootstrap current fraction, up to \ensuremath{\sim}80% of the plasma current, is described; \ensuremath{\beta}=(plasma pressure)/(magnetic pressure). During long-pulse neutral-beam-heated discharges in the JT-60 tokamak, it occurs at ${\mathrm{\ensuremath{\beta}}}_{\mathit{p}}$\ensuremath{\sim}3, leading to a limit of the normalized \ensuremath{\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-${\mathrm{\ensuremath{\beta}}}_{\mathit{p}}$ 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.