Q-profile flattening due to nonlinear development of resistive kink mode and ensuing fast crash in sawtooth oscillations

Presented is a new full-torus, compressible, resistive MHD simulation model that can adequately explain the mechanism of the fast crash in the sawtooth oscillation. The simulation results reveal that the q value, which at first decreases in accordance with current peaking subject to ohmic heating, starts increasing in the q < 1 region due to strong excitation of nonlinear resistive kink modes and tends to be bottom-flattened. When the q profile is largely flattened in the q < 1 region, the dynamic pressure of the m=1 kink mode pushes the hot core plasma in the main flow direction and the hot core deviates from the magnetic axis. Then, a strong plasma compression occurs at the stagnation point of the directed plasma flow which is coincident with the m=1 rational surface. Consequently, the poloidal magnetic field lines are driven to reconnect rapidly with each other across the q=1 surface, and the deviated hot core is pushed out towards the wall to destroy its confinement. The driven magnetic reconnection leading to the crash is found to occur as a result of the q-profile flattening, in contrast to Kadomtsev's model where the q-profile becomes flattened as a result of reconnection exchanging the magnetic flux. The crash is governed by the MHD time scale, about 100 poloidal Alfven transit times, which is much faster than the Kadomtsev reconnection time. The resistive kink mode and its nonlinear behavior control the whole process of the sawtooth oscillation. Especially, the m=1 plasma flow induced by the resistive kink instability, neither the magnetic force nor the pressure force, plays a decisive role in triggering the crash. (author)