Numerical study on toroidal mode coupling and triggering of neoclassical tearing modes by sawteeth

Numerical simulations have been carried out to understand the seeding of neoclassical tearing modes (NTMs), using a single fluid model and the four-field equations. The paper starts by considering a simplified physics situation: the destabilization of a linearly stable m/n = 3/2 mode by an unstable m/n = 2/2 mode (m/n being the poloidal/toroidal mode number). Within the single fluid model the mode coupling is found to be weakened by relative mode rotation and enhanced by finite plasma beta value. For the four-field equations in case of a sufficiently low electron diamagnetic drift frequency, the results are similar to those from the single fluid equations. For a sufficiently large diamagnetic drift frequency, three regimes are found: (a) suppression regime for moderate plasma beta values, in which the originally unstable 2/2 mode is stabilized by its coupling to the originally stable 3/2 mode; (b) oscillation regime for a sufficiently large beta value, in which the 3/2 mode has the feature of an ideal mode, and the diamagnetic drift frequency reaches the shear Alfven frequency outside the tearing layer; (c) NTM regime for sufficiently large beta value and bootstrap current density. Consistent with these findings, in our simulations 3/2 modes are triggered by sufficiently strong sawtooth crashes at high beta value and/or a low electron diamagnetic drift frequency. A sufficiently large bootstrap current density is then required for the 3/2 island to grow into the NTM regime.