Understanding edge-localized mode mitigation by resonant magnetic perturbations on MAST

Sustained edge-localized mode (ELM) mitigation has been achieved using resonant magnetic perturbations (RMPs) with a toroidal mode number of n = 4 and n = 6 in lower single null and with n = 3 in connected double null plasmas on MAST. The ELM frequency increases by up to a factor of eight with a similar reduction in ELM energy loss. A threshold current for ELM mitigation is observed above which the ELM frequency increases approximately linearly with current in the coils. A comparison of the filament structures observed during the ELMs in the natural and mitigated stages shows that the mitigated ELMs have the characteristics of type I ELMs even though their frequency is higher, their energy loss is reduced and the pedestal pressure gradient is decreased. During the ELM mitigated stage clear lobe structures are observed in visible-light imaging of the X-point region. The size of these lobes is correlated with the increase in ELM frequency observed. The RMPs produce a clear 3D distortion to the plasma and it is likely that these distortions explain why ELMs are destabilized and hence why ELM mitigation occurs.