Description
Resonant Magnetic Perturbations (RMPs) has been demonstrated as a robust tool for suppression of Edge Localized Modes (ELMs) in present tokamaks, such as EAST [1], while its effects on fast-ion losses challenge the achievements of ITER’s main goal for fusion gain Q=10 operation. Key issues to achieve optimized control with balanced suppression effect and fast-ion losses are the deep understanding of the physics in both ELM suppression and fast-ion loss induced by RMPs and looking for possible differences between them. Progresses in experiments and modelling in the last decade reveal that the optimized RMP ELM control effects occur in the RMP configuration that can generate strongest resonant harmonics near the plasma edge with the dominant contribution from tearing-like or peeling-like plasma response. However, the significant contribution from side-band and even far non-resonant components of RMPs on fast-ion loss has been observed in the simulation [2], which suggests that fast-ion loss may be dominantly contributed from the non-resonant harmonics that are enhanced by resonant field amplification (RFA) effect due to kink-like plasma response. Key physics behind is that the drift surfaces of energetic ions and the magnetic flux surfaces are different, which generates different contributions of resonant and non-resonant harmonics on the topology of particle orbits and magnetic field. Based on this physics hypothesis, we propose a new strategy for optimizing RMP coil configuration on ELM suppression. It is found from the simulation that there is a separation, ~70o, between optimized phasing (δϕ_UL, coil current phase difference between the upper and lower coil arrays) for ELM suppression and the phasing with maximal fast-ion loss for an n=2 RMPs in EAST. To achieve the same ELM control effects, the difference in fast-ion loss rate can be a factor of 2-4. For n=4 RMPs in the example equilibrium, the strongest resonant phasing coincides almost with the minimal fast-ion loss, which is naturally optimized against fast-ion loss and emphasizes again the advantage of this high-n RMPs for ELM suppression. This finding provides a strategy for integrated RMP ELM control in the future ITER.
[1] Y. Sun et al, “First demonstration of full ELM suppression in low input torque plasmas to support ITER research plan using n = 4 RMP in EAST”, Nucl. Fusion 61, 106037 (2021)
[2] Y. N. Zhang et al, ‘Influence of the far non-resonant components of high-n resonant magnetic perturbations on energetic passing ions loss’, Nucl. Fusion 64, 046012(2024)