Effect of resonant magnetic perturbations on ELMs in connected double null plasmas in MAST

TL;DR

This study investigates how resonant magnetic perturbations (RMPs) influence edge localized modes (ELMs) in the MAST tokamak, revealing a threshold for mitigation, scenario dependence, and the importance of plasma response and alignment.

Contribution

It provides new insights into the threshold behavior, scenario dependence, and the role of plasma response in RMP-induced ELM mitigation in connected double null plasmas.

Findings

ELM frequency increases up to 9 times with RMPs.

A threshold current for ELM mitigation exists.

Complete ELM suppression was not achieved.

Abstract

The application of resonant magnetic perturbations (RMPs) with a toroidal mode number of n=3 to connected double null plasmas in the MAST tokamak produces up to a factor of 9 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. The effect of the RMPs is found to be scenario dependent. In one scenario the mitigation is only due to a large density pump out event and if the density is recovered by gas puffing a return to type I ELMs is observed. In another scenario sustained ELM mitigation can be achieved irrespective of the amount of fuelling. 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. The requirement for a resonant condition, that is an optimum alignment of the perturbation with the plasma, has been confirmed by performing a scan in the pitch angle of the applied field.