The influence of MHD instabilities on the COMPASS-D H-mode threshold

Significant improvements have been made to edge diagnostics on COMPASS-D enabling detailed studies of the H-mode dynamics. New experiments have been carried out with current ramps in high-power ECR discharges to test the influence of the peeling mode on the low-density H-mode transition. Observations and analysis suggest that the peeling mode is not the controlling mechanism. Density-ramp experiments indicate that the edge electron pressure gradient is correlated with transition behaviour and links are made with global observations.