Measurement of the response of external kink modes to resonant magnetic perturbation in HBT-EP Tokamak plasmas

This thesis reports an experimental investigation of external kink modes in tokamak plasmas surrounded by a segmented conducting wall and measurements of the plasma response to applied external resonant magnetic perturbations near resistive wall (RWM) stability limit in HBT-EP Tokamak.;RWM instabilities having an m/n = 3/1 helical mode structure are observed in plasmas with high edge current. As the discharge evolves, the edge safety factor, q*, approaches 3, the RWM transitions through marginal stability, and grows to large amplitude. RWM rotation stabilization and the mode deceleration due to the breaking torque from the wall are observed and characterized.;The plasma response to external resonant magnetic perturbations is measured as a function of stability of the external kink mode. The magnetic perturbations are produced with a flexible, high-speed waveform generator that is preprogrammed to drive an in-vessel array of 30 independent control coils and to produce an m/n = 3/1 helical field. Both quasi-static and "phase-flip" magnetic perturbations are applied to time-evolving discharges in order to observe the dynamical response of the plasma as a function of external kink stability. The plasma resonant response depends upon the evolution of the edge safety factor, q*, and the plasma rotation. For discharges adjusted to maintain relatively constant edge safety factor, q* < 3, the amplitude of the plasma response to a quasi-static field perturbation does not vary strongly near marginal stability and is consistent with the Fitzpatrick-Aydemir equations with high viscous dissipation. Applying "phase-flip" magnetic perturbations that rapidly change toroidal phase by 180 degrees allows observation of the timescale for the plasma response to realign with the applied perturbation. This phase realignment time characterizes the natural damping time of the stable external kink mode, and this time is observed to increase at marginal stability, as predicted by theory. This excited plasma resonant response is easily measured and provides direct detection of the damping rate and toroidal mode frequency of the marginally stable RWM.