Resistive wall modes and nonuniform wall rotation

The resistive wall mode (RWM) poses a threat to many plasma confinement devices. The continuous rotation of the wall relative to the plasma makes it appear perfectly conducting, because of the skin effect, but this is ineffective if the perturbation locks to the wall. This raises the question of whether a nonuniformly rotating wall is more effective. In this paper we discuss the effect of such nonuniform wall rotation, in both the toroidal and poloidal directions, on resonant and nonresonant RWMs. In the case of toroidal rotation it is shown that at large wall velocity both the resonant and nonresonant RWMs are stabilized, even though the nonresonant mode rotates with the maximum wall velocity. In the case of poloidal rotation RWMs do not lock to the wall and have a complicated behavior at intermediate velocities. However they are again stabilized by large wall velocity.