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 β 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 β (β is the ratio of thermal to magnetic pressures). The joint effects of the safety factor and β, 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 β. Multiple n kink branches can coexist in the plasma. At a higher β value (β_{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 β limit imposed by the kink modes in the HL-2A limiter discharge is close to that of the ballooning mode, and the maximum normalized β, β_{N}, is both β_{N}^{c}≈2.0, whereas the kink β limit in the divertor discharge (β_{N}^{c}=2.5) is much higher than the ballooning β limit (β_{N}^{c}≈1.9). This indicates that the introduction of a divertor configuration in HL-2A is effective in increasing the kink β limit rather than the ballooning β limit. The maximum β that can be achieved by the HL-2A divertor discharge is affected by the internal ballooning modes and is somewhat lower than the kink β limit, except for the high-performance discharge.