Influence of ferromagnetic walls on resistive wall mode stability in tokamaks

Abstract

The effect of a ferromagnetic wall on the stability of the resistive wall mode (RWM) in a tokamak is calculated using a simple quasi-cylindrical plasma model in which the dissipation required to stabilize the mode (in combination with toroidal plasma rotation) is provided by neoclassical poloidal flow damping. For present-day tokamaks, which possess relatively thin walls, ferromagnetism is found to have relatively little influence on the critical toroidal plasma rotation velocity above which the RWM is stabilized, which is almost the same as that calculated for a non-ferromagnetic wall. The same is true for walls of moderate thickness. In fact, ferromagnetism is only found to have a significant effect on the critical velocity in the limit of extreme wall thickness (i.e., a wall thickness comparable with the wall minor radius), in which case increasing wall permeability leads to a marked increase in the critical velocity.