Plasma stability control in a toroidal system by feedback and HF fields

The paper deals with theoretical and experimental aspects of controlling MHD plasma stability in toroidal systems by feedback systems and HF fields. Results are presented of theoretical studies on resonant excitation of helical MHD oscillations in a current-carrying plasma column (stable Shafranov modes) by the mutipole helical HF field (Ω approximately <ωsub(Hi)). The effect of such an oscillation build-up on the evolution of unstable helical modes is discussed. The conditions for exciting helical oscillations (depending on the direction of the HF field rotation, as well) and the main criteria for stabilization are given. - Results of experimental studies on kink instability stabilization by a feedback system with M=1,2,3; N=1 for various values of q(q=2π/iotasub(Σ), where iota sub(Σ) is the total angle of rotational transform) in the R-0 stellarator are presented. On stabilizing the main unstable mode for a given range of values of q, the discharge parameters are improved. A stabilizing effect is only provided by introducing a certain phase shift into the feedback circuit and by choosing the amplifier gains. Stabilization improves the plasma thermal insulation and increases its energy content. (author)