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 modes temporarily arise when the minimum q value passes the q = 2 surface. They usually become stabilized with increasing distance between the rational surfaces or by the large pressure gradient at the inner rational surface provided 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 plasma edge is close to low order rational values. The resulting global mode usually causes disruptions. An optimized current profile has to avoid, therefore, close double rational q surfaces as well as large pressure gradients in the weak shear region.