Ideal beta limits of negative shear plasma in JT-60U

Ideal magnetohydrodynamic (MHD) stabilities are studied for the negative shear plasmas with steep pressure gradient observed in JT-60U. In order to improve the ideal -limit, effects of the location of a minimum value of the safety factor, , and of the location of the maximum pressure gradient on ideal MHD stability are investigated. When the maximum pressure gradient is inside the surface, the ideal -limit is determined by the n = 1 kink-ballooning mode. When the maximum pressure gradient is on or outside the surface, the infernal and high-n ballooning modes become more unstable than the n = 1 mode. It is found that the ideal -limit is improved when the maximum pressure gradient is inside the surface and the edge pressure gradient is high. The experimentally observed -limit in negative shear plasmas in JT-60U is consistent with the numerically obtained -limit determined by the n = 1 mode.