MHD behaviour in high- beta p and high- beta N discharges in JT-60U

Abstract

The MHD characteristics were studied to achieve the avoidance and suppression of MHD modes associated with a beta limit over a wide range of q(4<qeff<10) in the high poloidal beta ( beta p) and high normalized beta ( beta N= beta T(%)a(m)BT(T)/Ip(MA)) regimes in JT-60U. In the high- beta p discharges (BT=4.4 T, 1.1<Ip<2.2 MA and beta p<2.3), characterized by a peaked pressure profile (p(r)) and fairly broad current profile (j(r)) (li<1.2), the beta p is limited by a beta p collapse. A mode with a fast growth time ( approximately O(10 mu s)) always occurs just before the beta p collapse, suggesting that this mode is closely correlated with the beta p collapse. From stability analyses of the ideal mode, the infernal mode (ideal low-n mode) is the most likely candidate for the MHD activity associated with the Bp collapse. In order to improve the beta to avoid the beta p collapse, high- beta N discharges were attempted by modifying the pressure and current profiles. In high- beta N discharges with low BT (1.5 T) and high li(1.3<li<1.6) with both broad p(r) and peaked j(r) characterization, magnetic fluctuations were restricted to the maxima of the stored energy in the ELMy H-mode. The pressure-driven m/n=2/1 and 4/1 modes trigger a degradation in the stored energy in the range 4<qeff<6 and qeff>7 respectively. Neither mode is considered to be ideal because of the slow growth rates. Hence, the resistive modes degrade the plasma performance in this regime.