Recent progress on ELM suppression in hydrogen and helium at DIII-D and ASDEX Upgrade*

ITER pre-fusion power operation (PFPO) phase operating with hydrogen (H) and helium (He) will be the first opportunity to study suppression of edge-localized modes (ELMs) by magnetic perturbations (MP). ELM suppression has been demonstrated on many tokamaks, however, only in deuterium (D). Recently, ELM suppression has been investigated in H at DIII-D and ASDEX Upgrade.

We find that on DIII-D in H plasmas a weak MP field causes a H-L transition. Diluting the H plasma moderately with He prevents the backtransition and the MP field leads to a weak density pump-out. As such, the density cannot be reduced significantly below the critical density for ELM suppression and, therefore no ELM suppression has been observed in H or H+He plasmas in DIII-D or ASDEX Upgrade.

Progress has been made extending the work reported in [1], which showed ELM suppression at a high density in a low power He plasma. Key signatures characteristic of ELM suppression in D are discovered in He such as an increase of the plasma response to the MP field, a narrowing of the pedestal and increased density fluctuations.

[1] T. Evans et al, Nucl. Fusion 57 (2017) 086016