First observation of RMP ELM mitigation on MAST Upgrade

Abstract

The first experimental attempts at controlling Edge Localised Modes (ELMs) via the application of resonant magnetic perturbations (RMPs) on the MAST Upgrade tokamak are reported. Using the linear MHD model MARS-F, the phase shift between the upper and lower coil rows ΔΦ was optimised for toroidal mode number n=1 and n=2 fields, to provide forward guidance to experiments. In low βN discharges, the application of n=1 3D fields caused the ELM frequency fELM to increase by over a factor 20 relative to the reference, and also induced a locked mode, which did not cause a plasma termination nor an H-L back transition. However when βN was raised, this induced locked mode caused plasma termination which precluded mitigation access. Initially, applying a numerically optimised n=2 field had no effect. However applying a rigid toroidal shift to this field caused a locked mode disruption, demonstrating the presence of a substantial n=2 error field. Coil current ramps were conducted with ΔΦ set at 6 different values, resulting in either locked mode disruptions or no effect, but mitigation with n=2 fields was not established.