First observation of RMP ELM mitigation on MAST Upgrade
In this work, the first experimental attempts at controlling Edge Localised Modes (ELMs) via the application of resonant magnetic perturbations (RMPs) on theMAST Upgrade tokamak are reported. Moderate ELM mitigation was achieved in low betaN by the application of n=1 3D fields, at the cost of inducing a ’benign’ locked mode. At higher betaN however, this locked mode caused plasma termination which precluded mitigation access. Initial experiments applying an n=2 field had no effect even at maximum field amplitude. However applying a rigid toroidal shift to this field caused a locked mode disruption. This suggests that the n=2 applied field is superposed onto a substantial n=2 error field, which must be rigorously accounted for in optimal configuration predictions. In the absence of a rigorous model of the n=2 error field, an empirical scan of the phase shift between the upper and lower rows deltaphi was undertaken to quantify the deltaphi dependence of the locked mode coil current threshold ILM, and compare this with predictions produced using the linear MHD model MARS-F. The RMP coil current was ramped for 6 values of deltaphi, and although none of these ramps resulted in a mitigated phase, the deltaphi dependence of ILM was loosely bounded, and the optimum phase for core interaction was found to be within 60 degrees of the linear MHD prediction. The influence of the n=2 error field is the most likely cause of the deviation. However mitigation with n=2 fields was not established.