Resonant effects on the loss of energetic trapped ions induced by low-n resonant magnetic perturbations

Abstract

The fast ion dynamics and the associated loss channels under resonant magnetic perturbation (RMP) fields are investigated numerically with newly upgraded Monte-Carlo code ORBIT-RF. Special attention is paid to the effect of low-n () perturbations on the confinement of energetic trapped ions in an equilibrium from EAST experiments. Here, n donates the toroidal mode number. It is found that the dominant loss mechanism under n = 2 RMP is essentially different from that under n = 1 RMP. For n = 2 RMP, the enhanced losses are mainly due to the single resonance with . Here, and are, respectively, the precession and bounce frequencies. For n = 1 RMP, the losses are mainly due to orbital stochasticity near the trapped-passing boundary, which is mainly promoted by the nonlinear resonances. It is found that the loss fraction has a sine-like dependence on the phase difference between the upper and lower coil currents under both n = 1 and n = 2 RMPs, which suggests the possibility of active control of fast ion profile by designing the spectrum. The loss rate under n = 2 RMP shows a linear dependence on the perturbation amplitude, while the loss rate under n = 1 RMP shows a quadratic dependence. The results here highlight the importance of nonlinear resonances in fast ion confinement under RMP, in addition to that of linear resonances frequently emphasized before.