Progress on edge-localized mode suppression via magnetic perturbations in non-nuclear fuels for the ITER pre-fusion power operation phase

During transitions from deuterium (D) to hydrogen (H) plasmas at ASDEX Upgrade a critical D fraction is required to maintain edge-localized mode (ELM) suppression, raising concern over the feasibility of resonant magnetic perturbations (RMPs) as an ELM suppression tool in the ITER PFPO phase. Operating ITER in H-mode requires ELM mitigation or suppression to prevent critical heat loads caused by ELMs. While access to RMP-ELM suppression is well studied in D plasmas, it has yet to be demonstrated in non-nuclear fuels (H, H+He) expected in the ITER PFPO phase. For the first time, attempts have been made to access ELM suppression with RMPs in ITER-like low collisionality H plasmas at DIII-D and ASDEX Upgrade. The DIII-D experiments operated slightly above the L-H power threshold similar to the expected conditions in the ITER PFPO phase. The RMP fields are found to trigger H-L backtransitions in H plasmas, which can be avoided by diluting the H plasma with He. The additional He combined with a weaker density decrease induced by RMPs in H as compared to D plasmas precludes access to a pedestal top density below the known RMP-ELM suppression threshold. At ASDEX Upgrade, RMP-ELM suppression has been achieved with H concentrations of up to ~38%. While the known access criteria for RMP-ELM suppression are still met above this threshold, full ELM suppression is replaced by strong mitigation. The most prominent difference between the H and D plasmas is a change of turbulence characteristics in the pedestal where Doppler reflectometry suggests a significant reduction of turbulence even with small H concentrations. In conclusion, these experiments not only identify issues and raise concern for RMP-ELM suppression in the ITER PFPO phase, but also highlight missing physics in our current understanding of RMP-ELM suppression.