A NONLINEAR MODEL FOR THE SINGULAR SURFACE RESPONSE

The issues concerning the response of a plasma, at or near a singular surface, to a magnetic perturbation with a phase velocity different fr om the plasma flow velocity, are important for a number of phenomena. Among these are ideal and nonideal magnetohydrodynamic stability of pl asmas with shear flow or a flow relative to a resistive wall, sensitiv ity of rotating plasma to field errors, and the ''locked mode'' phenom enon. Models for the singular surface response have been tested agains t results from ''magnetic braking'' experiments in DIII-D [R. J. La Ha ye ct al., Nucl. Fusion 32, 2119 (1992)]. Previous models are found un able to account for all of the experimental observations. A new heuris tic nonlinear model presented in the paper may account for the observa tions. A key element in the model is turbulence developed at the singu lar surface; the turbulence is assumed driven by the singular layer di ssipation and is assumed to impede the singular current through an ano malous resistivity. When the perturbation amplitude is sufficiently la rge, a positive feedback mechanism exists, since in the regime of inte rest, dissipation increases with decreasing singular current. For smal l perturbation amplitudes this mechanism is not operative so that prev ious models for the response may be valid. (C) 1996 American Institute of Physics.