Effect of resonant magnetic perturbations with toroidal mode numbers of 4 and 6 on ELMs in single null H-mode plasmas in MAST

TL;DR

Applying resonant magnetic perturbations with toroidal mode numbers 4 and 6 in MAST tokamak plasmas increases ELM frequency and reduces energy loss, but does not fully suppress ELMs, with observed lobe structures correlating with ELM behavior.

Contribution

This study demonstrates the effects of n=4 and n=6 RMPs on ELM mitigation in MAST, providing experimental data and modeling insights into ELM behavior and thresholds.

Findings

ELM frequency increases up to 5 times with RMPs

Complete ELM suppression was not achieved

Lobe structures correlate with ELM frequency increase

Abstract

The application of resonant magnetic perturbations (RMPs) with a toroidal mode number of n=4 or n=6 to lower single null plasmas in the MAST tokamak produces up to a factor of 5 increase in Edge Localized Mode (ELM) frequency and reduction in plasma energy loss associated with type-I ELMs. A threshold current for ELM mitigation is observed above which the ELM frequency increases approximately linearly with current in the coils. Despite a large scan of parameters, complete ELM suppression has not been achieved. The results have been compared to modelling performed using either the vacuum approximation or including the plasma response. 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 characteristics of the mitigated ELMs are similar to those of the natural ELMs suggesting that they are type I ELMs which are triggered at a lower pressure gradient. The application of the RMPs in the n=4 and n=6 configurations before the L-H transition has little effect on the power required to achieve H-mode while still allowing the first ELM to be mitigated.