Influence of the far non-resonant components of high-n resonant magnetic perturbations on energetic passing ions loss

The influence on the loss of energetic passing ions by the n = 4 resonant magnetic perturbation (RMP) is investigated through numerical simulations (here, n is the toroidal mode number of the RMP field). Dedicated efforts have been made to uncover how the plasma response modifies the loss fractions and underlying mechanisms. The stochastization of the drift surfaces and the particle loss fractions increase significantly under the response field, in which the resonant component is significantly shielded. In order to better understand how the response field contributes a considerable drift island width, the respective effect of each poloidal component $m_\mathrm{b}$ on the outermost drift island $m_\mathrm{p}/n = 9/4$ are compared (here, $m_\mathrm{b}$ and $m_\mathrm{p}$ are poloidal mode numbers of the RMP components and drift island, respectively). Contrary to the intuition that the components with $m_\mathrm{b} = m_\mathrm{p} \pm 1$ should dominate the sideband resonance, some of the components which fulfill $|m_\mathrm{p}-m_\mathrm{b}|\gg 1$ have the dominant contributions under the response field. There are mainly two reasons accounting for this phenomenon, i.e. the drift motion of particles and the enhancement of amplitudes of non-resonant high- m components by the plasma response near the edge due to the resonant field amplification (RFA) effect. The former made it possible for particles to resonate with the far non-resonant components, and the latter significantly enhanced the perturbation field experienced by particles. The above results imply that the RFA effect is more critical than the stochastization of magnetic topology in the fast ion losses under RMP.