Non-uniform rotation and the resistive wall mode

Advanced Tokamak Concepts [C. Kessel, J. Manickam, G. Rewoldt, and W. M. Tang, Phys. Rev. Lett. 72, 1212 (1994)] have been designed assuming that the ''Resistive Wall Mode'' (RWM) is stable. It has recently been shown that the RWM can be stabilized by a combination of strong uniform plasma rotation and visco-resistive dissipation. In this paper we examine the consequences of a sheared flow on the RWM, and contrast the results to the case of uniform flow. It is shown that, as for uniform flow, the rotation initially further destabilizes the resistive wall mode, but for higher rotation velocities the growth rate is reduced, and the presence of plasma dissipation may completely stabilize the mode. However sheared rotation allows the possibility of the RWM coupling to and converting into a Kelvin-Helmholtz mode. It is shown that the position of the wall with respect to the critical position for stabilization of the external kink mode is of crucial importance.