Integrated study of magnetohydrodynamic stability in the HL-2A tokamak

The magnetohydrodynamic (MHD) instability in the HL-2A tokamak has been comprehensively studied, including ideal MHD ballooning and kink modes, as well as the operational beta limit imposed jointly by them. It shows an internal ballooning mode appearing in the plasma with parabolic pressure and current density profiles exhibited in the case with a higher beta (beta is the ratio of thermal to magnetic pressures). The joint effects of the safety factor and beta, as well as the structural features of various n (toroidal mode number) kink modes were investigated. The combination of the axial and edge safety factors, (q(0),q(a)), leads to a corresponding equilibrium, and its stability is determined by both the safety factors and beta. Multiple n kink branches can coexist in the plasma. At a higher beta value (beta(p) = 2) for the case q(0) = 0.95, n = 1,2,3 and 4 kinks are generally internal modes, dominated by the (m[=n],n) branch. Here m is the poloidal mode number. When q(0) =1.05 and q(0) = 1.3, the n > 1 kink mode is generally an external mode. The larger q(0) is compared to one, the more unstable the system becomes, and the more complex the mode structure changes with n. The beta limit imposed by the kink modes in the HL-2A limiter discharge is close to that of the ballooning mode, and the maximum normalized beta, beta(N), is both beta(c)(N) approximate to 2.0, whereas the kink beta limit in the divertor discharge (beta(c)(N) = 2.5) is much higher than the ballooning beta limit (beta(c)(N) approximate to 1.9). This indicates that the introduction of a divertor configuration in HL-2A is effective in increasing the kink beta limit rather than the ballooning beta limit. The maximum beta that can be achieved by the HL-2A divertor discharge is affected by the internal ballooning modes and is somewhat lower than the kink beta limit, except for the high-performance discharge.