Characteristics of X-point Lobe Structures in Single-Null Discharges on MAST

TL;DR

This study investigates the characteristics of X-point lobe structures caused by resonant magnetic perturbations in MAST, demonstrating how lobes extend with coil current and plasma rotation, and comparing observations with vacuum and plasma screening models.

Contribution

It provides detailed imaging and analysis of lobe structures in MAST, and compares experimental data with models to improve understanding of RMP effects and plasma screening.

Findings

Lobe structures extend linearly with coil current above a threshold.

Lobes extend radially as plasma rotation decreases, indicating reduced screening.

Vacuum models overestimate lobe extent; plasma screening improves predictions.

Abstract

Lobe structures due to the application of resonant magnetic perturbations (RMPs) have been observed using wide-angle imaging of light from He1+ ions in the vicinity of the lower X-point in MAST. The data presented are from lower single-null discharges where RMPs of toroidal mode number, n, of 4 and 6 were applied. It has been found that, above a threshold value, the lobe structures extend radially, linearly with the coil current, both in L-mode and H-mode. It is observed that after the application of the RMP, as the toroidal rotation in the confined plasma decreases, the lobes extend radially, suggesting the plasma is less effectively screening the RMP field. Comparing the imaging data with results from vacuum modelling shows that this technique can accurately predict the number and poloidal location of the lobes, but over-estimates their radial extent. More accurate estimates of the extent of the lobes can be made by accounting for plasma screening of the RMP field. Qualitative agreement between simulation and experiment is found if it is assumed that the RMP penetrates 2% in normalised radius from the last closed flux surface.