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 ${\mathrm{\beta }}_{\mathit{p}}$∼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-${\mathrm{\beta }}_{\mathit{p}}$ 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.