Impact of Resonant Magnetic Perturbations on the L-H Transition on MAST

TL;DR

This study investigates how resonant magnetic perturbations (RMPs) influence the power needed to achieve H-mode in MAST, revealing that RMPs delay or prevent the transition and significantly increase the power threshold, especially at higher RMP fields.

Contribution

The paper provides the first detailed experimental analysis of RMP effects on the L-H transition power threshold on MAST, quantifying the increase in required power and its dependence on RMP configuration and magnitude.

Findings

RMPs delay or prevent L-H transition at high fields.

Power threshold increases by 20-60% with RMPs.

Higher RMP fields lead to larger increases in the L-H power threshold.

Abstract

The impact of resonant magnetic perturbations (RMPs) on the power required to access H-mode is examined experimentally on MAST. Applying RMP in n=2,3,4 and 6 configurations causes significant delays to the timing of the L-H transition at low applied fields and prevents the transition at high fields. The experiment was primarily performed at RMP fields sufficient to cause moderate increases in ELM frequency, f mitigated/f natural~3. To obtain H-mode with RMPs at this field, an increase of injected beam power is required of at least 50% for n=3 and n=4 RMP and 100% for n=6 RMP. In terms of power threshold, this corresponds to increases of at least 20% for n=3 and n=4 RMPs and 60% for n=6 RMPs. This 'RMP affected' power threshold is found to increase with RMP magnitude above a certain minimum perturbed field, below which there is no impact on the power threshold. Extrapolations from these results indicate large increases in the L-H power threshold will be required for discharges requiring large mitigated ELM frequency.