Resistive wall mode stabilization by differential rotation in an analytic tokamak

Abstract

It is well known that stabilization of the resistive wall mode (RWM) may allow fusion power to be significantly increased for a given magnetic field in advanced tokamak operation. The principle of stabilization of the RWM by rotation has been established both experimentally and theoretically. Recent experimental results have indicated stabilization of the RWM has been achieved with very small levels of rotation using balanced neutral beam injection. The framework of Connor et al (Connor et al 1988 Phys. Fluids 31 577) is used to develop two ideal plasma analytic toroidal models where stepped pressure profiles and careful ordering of terms are used to simplify the analysis. The first model has one resonant layer in the plasma and two resistive walls and the second has two resonant layers and one resistive wall. The RWM can be stabilized with slow rotation (∼0.5%ΩA, where ΩA is the Alfvénic rotation frequency) of a secondary resistive wall. A secordary rotating resistive wall can only stabilize the plasma if a perfectly conducting wall at that location would stabilize the plasma. Differential rotation in the plasma is investigated by rotating two resonant layers in the plasma at different rates. It is found in this model that differential rotation of the outer resonant surface can stabilize the RWM, with no rotation required at the inner surface. However, stabilization is not possible with the outer surface static and the inner surface rotating. Experimental rotation profiles for balanced neutral beam shots have higher levels of rotation near the plasma edge showing differential rotation between the outer surface and the wall may be part of the RWM stabilization mechanism.