Ideal MHD stability of high βp plasmas and βp collapse in JT-60U

Abstract

Analyses of ideal MHD stabilities of high βp plasmas show that the occurrence of βp collapse in JT-60U is consistent with the violation of the low n kink mode stability boundary. It is demonstrated theoretically that the pressure profile, internal inductance, li, qs/q0 and ϵβp influence the ideal stability limit, g, imposed by the low n kink mode and the infinite n ballooning mode; here, g = βt/(I/aBt). Peaking of the pressure profile decreases the stability limit g to low values (2-1.5) in both high li (≈ 1.2) and low li (≈ 0.8) plasmas, though the maximum g value does not strongly depend on ϵβp and qs/q0. In a high li plasma with peaked pressure, the stability limit is determined by internal low n kink modes, especially by the infernal mode in the low q0 region, while, in the low li plasma, it is determined by the infinite n ballooning mode. Broadening of the pressure profile combined with the high li (≈ 1.2) significantly increases the stability limit g (≈ 5) and makes the displacement more global, though the g value decreases with ϵβp for fixed values of qs/q0. Experimentally, by the broadening of the pressure profile with high li (>1.2), high g (≈ 4) plasmas are obtained and no βp collapse is observed, which is consistent with non-violation of the ideal MHD stability boundary