Dynamical plasma response of resistive wall modes to changing external magnetic perturbations

The plasma response to external resonant magnetic perturbations is measured as a function of stability of the resistive wall mode (RWM). 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 RWM stability. The evolving stability of the RWM is estimated using equilibrium reconstructions and ideal stability computations, facilitating comparison with theory. 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 quasistatic 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 180degrees allows observation of the time scale for the plasma response to realign with the applied perturbation. This phase realignment time increases at marginal stability, as predicted by theory. This effect is easily measured and suggests that the response to time-varying external field perturbations may be used to detect the approach to RWM instability. (C) 2004 American Institute of Physics.