Simulation investigates how neoclassical tearing modes impact sawtooth oscillations, suggesting significant interactions. Findings imply altered core dynamics due to mode locking.
The nonlinear evolution of neo-classical tearing modes (NTMs) following sawtooth crashes is numerically investigated using the three-dimensional, toroidal, resistive MHD code (CLT). 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 q=2 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 [53]. Additionally, we find that large m/n=2/1 islands can significantly modify sawtooth behavior: the amplitude of sawtooth oscillations decreases as the 2/1 island grows. This is due to mode locking between the 2/1 NTM and the 1/1 internal kink mode through toroidal coupling, which alters the core dynamics and suppresses sawtooth activity.
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Huang et al. (2025) studied this question.
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