A nonideal error-field response model for strongly shaped tokamak plasmas

Abstract

A model is developed that describes the error-field response of a toroidally rotating tokamak plasma possessing a strongly shaped poloidal cross-section. The response is made up of nondissipative ideal and dissipative nonideal components. The calculation of the ideal response is greatly simplified by employing a large aspect-ratio, constant pressure plasma equilibrium in which the current is entirely concentrated at the boundary. Moreover, the calculation of the resonant component of the nonideal response is simplified by modeling each resonant surface within the plasma as a toroidally rotating, thin resistive shell that only responds to the appropriate resonant component of the perturbed magnetic field. This approach mimics dissipation due to continuum damping at Alfvén and/or sound wave resonances inside the plasma. The nonresonant component of the nonideal response is neglected. The error-fields that maximize the net toroidal locking torque exerted on the plasma are determined via singular value decomposition of the total response matrix. For a strongly dissipative plasma, the locking torque associated with a general error-field is found to peak at a beta value that lies above the no-wall beta-limit, in accordance with experimental observations.