External kink instabilities are suppressed in a tokamak experiment by either ͑1͒ energizing a distributed array of independently controlled active feedback coils mounted outside a segmented resistive wall or ͑2͒ inserting a second segmented wall having much higher electrical conductivity. When the active feedback coils are off and the highly conducting wall is withdrawn, kink instabilities excited by plasma current gradients grow at a rate comparable to the magnetic diffusion rate of the resistive wall.

A new sawtooth control mechanism relying on toroidally propagating ion cyclotron resonance frequency waves: Theory and Joint European Torus tokamak experimental evidencea) Phys. Plasmas 17, 056118 (2010); 10.1063/1.3363201 Sawtooth control using beam ions accelerated by fast waves in the DIII-D tokamakClosed and open loop control techniques were applied to growing m/nϭ2/1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak-Extended Pulse ͑HBT-EP͒ ͓J. Fusion Energy 12, 303 ͑1993͔͒. HBT-EP combines an adjustable, segmented conducting wall ͑which slows the growth or stabilizes ideal external kinks͒ with a number of small ͑6°wide toroidally͒ driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control from 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplifiers. The phase instability has been observed and is well modeled by the single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equations. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the theory. Suppression of the 2/1 island growth has been demonstrated using an asynchronous frequency modulation drive which maintains the inertial flow damping of the island by application of rotating control fields with frequencies alternating above and below the natural mode frequency. This frequency modulation control technique was also able to prevent disruptions normally observed to follow giant sawtooth crashes in the plasma core.

The High Beta Tokamak-Extended Pulse ͑HBT-EP͒ experiment ͓J. Fusion Energy 12, 303 ͑1993͔͒ combines an internal, movable conducting wall with a high-power, modular saddle coil system to provide passive and active control of long wavelength magnetohydrodynamic ͑MHD͒ instabilities. Systematic adjustment of the radial position, b, of the conducting wall elements in relation to the surface of the plasma ͑minor radius a͒ resulted in the suppression of ␤-limiting disruptions for discharges in which b/aϽ1.2 and a positive plasma current ramp was maintained. Conducting wall stabilization of kink instabilities was observed in discharges with strong current ramps and in plasmas with ␤ values near the Troyon stability boundary. The frequency of slowly growing modes that persisted in wall-stabilized discharges was controlled by applying oscillating mϭ2, nϭ1 resonant magnetic perturbations. A compact, single-phase saddle coil system permitted modulation of the rotation velocity of internal m/nϭ2/1 instabilities by a factor of 2.

The characteristics of external kink instabilities observed during wall stabilization studies in the HBT-EP tokamak have been compared with the predictions of ideal MHD theory, in order to examine the stabilizing role of a resistive wall that is segmented both toroidally and poloidally. The reconstructed equilibria, for discharges with different plasma-wall configurations, are consistent with external and internal magnetic measurements, measured soft X ray profiles and measured equilibrium wall eddy currents. The stability analysis of these equilibria predicts patterns of instability induced eddy currents for a model wall that is continuous and perfectly conducting, and these patterns are in good agreement with the ones observed on the HBT-EP segmented wall. These eddy currents account for the observed stabilization of fast ideal modes when the wall is fully inserted, consistent with the prediction of marginal stability.