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

Abstract

The plasma response to external resonant magnetic perturbations is measured as a function of stability of the resistive wall mode ͑RWM͒. 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/nϭ3/1 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* Ͻ3, 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 180°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.

Measurement of the Resistive-Wall-Mode Stability in a Rotating Plasma Using Active MHD Spectroscopy

“…In the absence of plasma, γ τ RFA, for C coil pulses yields a value of γ 0 which is found to be in good agreement with the observed decay time [8]. The dynamic response to externally applied pulsed perturbations is also used in HBT-EP [9], where it has been successfully compared to the Fitzpatrick-Aydemir RWM dispersion relation [14]. The experiment is now generalized by applying an external field with various angular frequencies ω ext .…”

“…This is consistent with dissipation mechanisms based on coupling to sound waves and subsequent ion Landau damping [4] or kinetic damping [7], but it generally excludes dissipation models based on resistive MHD. The RWM stability has also been actively probed by measuring the plasma response to externally applied resonant magnetic field pulses [8,9]. Here, we generalize the measurement of the plasma response to externally applied fields by probing the entire relevant range of frequencies.…”

In situ “artificial plasma” calibration of tokamak magnetic sensors

“…The HBT-EP experiment studies the physics and control of beta-limiting MHD instabilities such as the resistive wall mode (RWM) [12][13][14] . The RWM is an ideal external kink mode whose growth rate has been slowed to a magnetic diffusion time set by the presence of a nearby conducting wall.…”

Effects of kinetic resonances on the stability of resistive wall modes in reversed field pinch

“…Converted from the ideal MHD mode in the presence of the wall with finite conductivity, and growing on the time scale of the magnetic field penetration time through the wall, RWM sets pressure limit in the advanced scenario of the tokamak device [1,2,3]. It also limits the discharge duration of the Reversed Field Pinch (RFP) device [4,5,6].…”