Simulation studies of interactions between neoclassical tearing modes and sawteeth

Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>q</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> 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. Plasmas 21 110702) Additionally, we find that large <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>m</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>n</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> islands can significantly modify sawtooth behavior: the amplitude of sawtooth oscillations decreases as the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> island grows. This is due to mode locking between the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> NTM and the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> internal kink mode through toroidal coupling, which alters the core dynamics and suppresses sawtooth activity.