Effect of an error field on the stability of the resistive wall mode

A simple model of the resistive wall mode (RWM) in a rotating tokamak plasma subject to a static error field is constructed, and then used to investigate RWM stability in a DIII-D-like [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] plasma. An error field as small as 10 G (i.e., about 5x10-4 of the toroidal field) is found to significantly increase the critical plasma rotation frequency needed to stabilize the RWM. Such an error field also profoundly changes the nature of the RWM onset. At small error-field amplitudes, the RWM switches on gradually as the plasma rotation is gradually reduced. On the other hand, at large error-field amplitudes, there is a sudden collapse of the plasma rotation as the rotation frequency falls below some critical value. This collapse is associated with a very rapid switch-on of the RWM