MHD stability studies in reversed shear plasmas in TFTR

MHD phenomena in reversed shear plasmas in TFTR are described during each of the three phases of the evolution of these discharges: the current ramp, high power neutral beam heating and after the beam power has been reduced. Theoretical analysis of discharges which disrupted in the high-β phase indicates that the β - limit is set by the ideal n = 1 infernal/kink mode. The mode structure of the disruption precursor reconstructed from the electron temperature data compares favorably with the predicted displacement vector from the ideal MHD model. In contrast, disruptions during the early and late phases are due to resistive instabilities, double tearing modes coupled to high-m edge modes. The resistive interchange mode, predicted to be unstable in reversed shear plasmas, is not seen in the experiment. Neo-classical tearing mode theory is shown to describe the non-disruptive MHD phenomena. A nonlinear resistive MHD simulation reproduces off-axis sawtooth-like crashes during the post-beam phase. The dependence of the β-limit on the pressure peakedness and qmin is discussed, showing a path to stable higher-β regimes