Observation and control of resistive wall modes

Abstract

Two approaches to achieving long timescale stabilization of the ideal kink mode with a real, finite conductivity wall are considered: plasma rotation and active feedback control. DIII-D experiments have demonstrated stabilization of the resistive wall mode (RWM) by sustaining β greater than the no-wall limit for up to 200 ms, much longer than the wall penetration time of a few milliseconds. These plasmas are typically terminated by an m = 3, n = 1 mode as the plasma rotation slows below a few kilohertz. Recent temperature profile data show an ideal MHD mode structure, as expected for the RWM at β above the no-wall limit. The critical rotation rate for stabilization is in qualitative agreement with recent theories for dissipative stabilization in the absence of magnetic islands. However, drag by small amplitude RWMs or damping of stable RWMs may contribute to an observed slowing of rotation at high β, rendering rotational stabilization more difficult. An initial open loop active control experiment, using non-axisymmetric external coils and a new array of saddle loop detectors, has yielded encouraging results indicating a delayed onset of the RWM.