Inter-ELM pedestal evolution on MAST and impact of Resonant Magnetic Perturbations

TL;DR

This study investigates the evolution of the pedestal in MAST, examining how resonant magnetic perturbations affect pressure gradients, pedestal width, and plasma stability, revealing asymmetries and transport changes during ELM mitigation.

Contribution

It provides new insights into how RMPs influence pedestal structure, symmetry, and transport properties during ELM mitigation on MAST, with detailed measurements of displacement and width changes.

Findings

RMPs cause outward displacement and increased pedestal width.

Application of RMPs reduces pressure gradient dPe/d{}.

Pedestal symmetry is broken by RMPs, affecting edge stability.

Abstract

The peak pressure gradient in the pedestal (dPe/d{\psi}) on MAST varies little between edge localized modes (ELMs), although it varies between discharges due factors such as gas fuelling and plasma current. The pressure pedestal width in flux space on the high-field side (HFS), both during the inter-ELM period and amongst different plasma discharges is consistent with a scaling of . In flux space very similar dPe/d{\psi} and {\Delta}pe are observed on the HFS and low-field side (LFS) in single null configuration. This symmetry is broken by the application of resonant magnetic perturbations (RMPs). During ELM mitigation by application of RMPs changes in the edge transport barrier position and width are observed. These changes are dependent on the intensity of the RMP and on the toroidal location with respect to the RMP phase. An outward displacement of up to 30 mm and increase in the edge pedestal width of up to 50% with respect to the coils case off are observed. Increased particle transport causes a decrease in ne,ped, and hence Pe,ped, as is observed on other devices. The combination of an increase in {\Delta}pe on the LFS and decrease in Pe,ped results in significantly reduced LFS dPe/d{\psi} when these perturbations are applied to the plasma edge. A decrease in dPe/d{\psi} on the HFS is also observed due to RMP; however, this decrease is caused solely by the Pe,ped decrease whilst no expansion of {\Delta}pe on the HFS is observed.