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

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