On the one hand, MHD instabilities in advanced tokamak scenarios are favourable as they can contribute to the stationarity of the current profilesand act as a trigger for the formation of internal transport barriers (ITBs). In particular, fishbone oscillations driven by fast particles arising from NBIare shown to trigger ITBs in low and reversed magnetic shear discharges. During the whistling down period of the fishbone oscillation the transport is reduced around the corresponding rational surface, leading to an increased pressure gradient. This behaviour could be explained by the redistribution of the resonant fast particles resulting in a sheared plasma rotation due to the return current in the bulk plasma, which is equivalent to a radial electric field. On the other hand, MHD instabilities limit the accessible operating regime. Ideal and resistive MHD modes such as double tearing modes, infernal modesand external kinks degrade the confinementor even lead to disruptions in ASDEX Upgrade reversed shear discharges.Localized electron cyclotron heating and current drive are shown tosignificantly affect the MHD stability of this type of discharge.
MHD phenomena in reversed shear discharges on ASDEX Upgrade