Eddy-current characterization and plasma rotation control in wall-stabilized tokamak discharges

Measurements of eddy-currents flowing on the surfaces of a segmented, adjustable, and close-fitting conducting wall are used to identify MHD instabilities which precede plasma disruptions and to characterize the eddy-currents present during wall stabilization. The eddy-currents on the plasma side of the wall are measured to be similar to those predicted for an ideal axisymmetric wall and account for the effectiveness of segmented walls for wall-stabilization. Dynamic control of slowly growing internal instabilities which persist during wall stabilization is investigated by energizing a small number of highly localized saddle-coils. The toroidal rotation frequency of the resonant mode is strongly modulated by the application of both oscillating single-phase and rotating two-phase saddle-coil currents. When the toroidal phase of a rotating magnetic perturbation is advanced by 180 degrees, the growth of the phase-instability associated with tearing mode stabilization is detected in preparation for active feedback experiments.