Effects of neutral beam injection on tearing mode stability: insights from hybrid simulations

Abstract In tokamak plasmas, tangential neutral beam injection (NBI) produces (a large fraction of) circulating energetic ions (CEIs) and induces plasma toroidal rotation, both of which play an important role in the stability of tearing mode (TM). In this study, the effect of NBI on TM is systematically investigated using kinetic-magnetohydrodynamic hybrid code M3D-K. Here, the effect of NBI is modeled as the combined effects of CEI and toroidal rotation. The analysis focuses on the dependency of NBI’s effect on key physical parameters, including magnetic shear, total beta and plasma shape. The modification of rotation on equilibrium is self-consistently included in simulation, which can enhance the destabilizing effect of counter-CEI on TM and has a negligible contribution to the effect of co-CEI. Furthermore, the simulation results reveal that the co-NBI always reduces TM’s growth rate due to the dominant stabilizing effect of rotation and the weak net effect of co-CEI, agreeing well with most experimental results. Whether the counter-NBI stabilizes or destabilizes TM depends on the competition between the stabilizing contribution from rotation and the destabilizing contribution from counter-CEI. Specifically, the counter-NBI tends to stabilize TM when elongation and triangularity decrease, while magnetic shear, total beta and aspect ratio increase.