MHD phenomena in reversed shear discharges on ASDEX Upgrade

Ideal and resistive MHD instabilities degrade the confinement or even lead to disruptions in ASDEX Upgrade reversed shear discharges. Double tearing m odes temporarily arise when the minimum q value passes the q = 2 surface. T hey usually become stabilized with increasing distance between the rational surfaces or by the large pressure gradient at the inner rational surface p rovided by central electron heating. Large pressure gradients in the region of weak shear between the two q = 2 surfaces drive infernal modes unstable . These modes can couple to external kink modes if the q value at the plasm a edge is close to low order rational values. The resulting global mode usu ally causes disruptions. An optimized current profile has to avoid, therefo re, close double rational q surfaces as well as large pressure gradients in the weak shear region.