Effect Of The Plasma Pressure On The Double Tearing Mode Instability In Tokamak

The tearing mode, as we all know, is dangerous macroscopic magnetohydrodynamic (MHD) instabilities such as the international Thermonuclear Experimental Reactor (ITER). Specifically, the double tearing mode (DTM) comes from the reversed magnetic shear configuration of tokamak, it is a kind of resistive instability. This kind instability reduces the plasma confinement performance, and reduces the achievable ?N limit. Here, ?N is the ratio of the averaged plasma pressure to the magnetic pressure. In order for making a tokamak device to achieve high ?N, keeping a long time or steady state discharge at the meantime, stabilizing the MHD instabilities is one of the crucial issues. However, the influence of the plasma pressure on DTM is not systematically investigated yet. Here, we investigate that effect.The effect of the plasma pressure on the double tearing mode in a toroidal geometry is numerically investigated, using the MARS-F code. It is found that there are various branch of the double tearing mode (m=2) under the different distance between q= 2 rational surfaces, and the toroidal averaged curvature contributes different stabilization effect on these branches of the DTM at finite pressure. The plasma pressure has stabilization influence on the most unstable branch, peculiarly at the low ?N region. The stabilization effect is due to resonance of the double tearing mode and the ion sound wave. Significantly, the plasma pressure will also change mode structure. Particularly, in q0=2.2(the on-axis safety factor) case, it is found that having a branch will become the kink mode from tearing mode with the increasing ?N, and in which the initially unstable DTM can generate finite rotation. When the tearing mode parameter ?ext exceeds a critical positive value, the tearing mode is unstable in the presence of the favorable magnetic curvature and the finite pressure gradient. In attention, an unstable tearing mode can generate the neoclassical toroidal viscous (NTV) torque and the resonant electromagnetic torque. As a result, an initially unstable tearing mode can intrinsically drive a steady state toroidal plasma flow, where the initial plasma is static.