Study of the resistive wall mode in DIII-D

Recent MHD calculations predict that, for a plasma with sufficient rotation, a resistive wall can provide stability up to the βN limit predicted assuming the wall were ideal. In the region of βN between the wall-at-infinity limit and the ideal-wall limit, an MHD instability branches into two modes: a plasma mode, that is nearly stationary with respect to the plasma at the resonant surface, and a resistive wall mode (RWM), that is nearly stationary with respect to the wall. Experiments conducted in the DIII-D, PBX-M and HBT-EP tokamaks have demonstrated that plasmas with a nearby conducting wall can remain stable above the beta limit predicted with wall-at-infinity, and have reported observations of instabilities with the characteristics of a resistive wall mode. In the experiments described in this paper, improved diagnostic measurements and plasma operational techniques, giving broader current density profiles and high toroidal rotation, have provided direct identification of the resistive wall mode. These experiments were designed to ease the requirements on total beta in favor of maximizing the wall stability enhancement factor, EW, increasing the duration of the wall-stabilized phase, and ensuring shot-to-shot reproducibility with the available heating power