The interaction of error fields and resistive wall modes

An investigation is presented of the effect of an error field on the growth of a resistive wall mode (RWM). This issue is of importance to many toroidal confinement devices, but particularly to the proposed International Thermonuclear Experimental Reactor (ITER) [ITER Physics Basis, Nucl. Fusion 39, 2175 (1999)], where if advanced tokamak performance is required, then the RWM instability is especially vulnerable. The paper includes a discussion of error field amplification when RWM marginal stability is approached, and it is pointed out that error field current is the fixed quantity in this calculation, not the error field flux, which is essentially an eigenvalue of the calculation. The key issue concerns the effect of plasma rotation on the RWM. The distinction is made between the resistive wall tearing mode (RWTM), which is stabilized by modest plasma rotation, and the ideal RWM, which is not. The importance of the joint torques that are present when an error field and an RWM of the same helicity coexist is investigated. Using torque balance a simulation is presented of the effect of reducing error field amplitude on the duration of a stable discharge. This shows features in accord with experimental observations from the DIII-D device [J. L. Luxon , Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p.159]. (C) 2004 American Institute of Physics.