A Triggering Mechanism of Fast Crash in Sawtooth Oscillation

"Full-torus, compressible, resistive MHD simulations have been performed to study the mechanism of fast crash in the sawtooth oscillation. The simulation results reveal that the q value, which at first decreases in accordance with current pea,king subject to ohmic heating, starts increasing in the q < l region due to strong excitation of nonlinear modes and becomes flattened. When the q profile is flattened in the q < 1 region, the plasma flow pushes the magnetic surface radially outwards and the poloidal magnetic field lines are driven to reconnect rapidly with each other across the q=1 surface. Consbquently, the central hot plasma is pushed out towards the wall to crash its confinement. It turns out that the m=1 plasma flow induced by the kink instability, rather than the pressure gradient, plays a decisive role in the crash process.