The nonlinear evolution of neo-classical tearing modes (NTMs) following sawtooth crashes is numerically investigated using the three-dimensional, toroidal, resistive MHD code. The simulation results show that seed magnetic islands do not appear immediately after a sawtooth crash. Instead, there is a transition phase where ideal kink-like magnetic perturbations gradually evolve into tearing-mode-like structures at the resonant surface, during which the island grows rapidly. After this phase, the island enters the modified-Rutherford regime, characterized by slow nonlinear growth. The simulation results are qualitatively consistent with experimental observations from ASDEX Upgrade (Igochine et al 2014 Phys. Plasmas21 110702) Additionally, we find that large islands can significantly modify sawtooth behavior: the amplitude of sawtooth oscillations decreases as the island grows. This is due to mode locking between the NTM and the internal kink mode through toroidal coupling, which alters the core dynamics and suppresses sawtooth activity.
This paper investigates how neoclassical tearing modes (NTMs) interact with sawteeth in fusion plasmas. The simulations show that NTMs do not appear immediately after a sawtooth crash, but gradually evolve from ideal kink-like perturbations. Large NTM islands can also modify sawtooth behavior by mode locking with the internal kink mode.