Simulations of beta-induced Alfvén eigenmode mitigation by off-axis energetic particle distribution

Abstract

The effect of different off-axis energetic particle (EP) slowing down distribution on beta-induced Alfven eigenmode (BAE), driven by the on-axis EP distribution, is systematically studied using kinetic-magnetohydrodynamic code M3D-K. The aim is to analyze the optimal parameter region for controlling AEs via varying EP distribution parameters. The simulation results reveal that by modifying the gradients of the EP distribution, the off-axis EP can further destabilize or mitigate the on-axis EP driven BAE, depending on the off-axis EP distribution's parameters: deposition profile, EP beta, pitch angle, injection velocity and direction. When the off-axis EP is deposited outside the mode center, and its injection velocity is sufficiently large to satisfy the resonance with BAE, the stabilization of BAE is achieved. This stabilizing effect is directly proportional to the off-axis EP beta, while excessive off-axis EP beta can trigger a new EP-driven instability located outside the BAE. Furthermore, to achieve a stronger stabilizing effect, the pitch angle distribution and velocity direction of the off-axis EP should be close to those of the on-axis EP. For instance, compared to the counter-passing off-axis EP, the co-passing off-axis EP can lead to a more effective mitigation of the BAE driven by the co-passing on-axis EP.