Phase-space spectrum analysis of fast-ion resonances with externally applied 3D magnetic perturbations in the EAST tokamak

The spectral configurations of resonant magnetic perturbations (RMPs) play a key role in the transport and confinement of fast ions. In this paper, we aim to provide a quantitative investigation on how the RMP spectrum affects fast-ion resonances in the EAST tokamak. To achieve this, we implement the action-angle transform in the ORBIT-RF code and establish a new spectral analysis in phase space. The advantages are twofold. First, accurate estimate of the drift orbit island width for both passing and trapped ions is performed without resorting to Poincaré plots. Second, this approach enables explicit evaluation of the coupling effects between particle motions and RMPs. The fast-ion losses are analyzed and compared between the vacuum and response RMP cases. For passing ions, the peripheral resonance is contributed by a wide range of poloidal harmonics of RMPs, and in particular there is a second peak in Hamiltonian contributions in the response RMP case. This phenomenon is not only related to the radial excursion of the particle orbit but also links to the angular drift motion. While the finite orbit width (FOW) effect is familiar in neoclassical transport theories, the effect of the curved orbit in drift surface is seldom emphasized. For trapped ions, the difference in loss rates can be explained by the relative phases of Hamiltonian contributions. The contributions are mostly in phase in the vacuum case but out of phase in the response case. The analysis procedure thus offer a new perspective on optimization of RMP spectrum.