Some features of the disruption instability in reversed shear TFTR plasmas

The behaviour of MHD perturbations before and during disruptions in TFTR reversed shear plasmas with qmin » 2 was analysed. In the qmin region, tearing modes, wavelike modes, and mixed tearing plus wavelike modes are followed by disruption. Sometimes a helical snake (helix) appears at the X point of the qmin island. The local outward electron energy transport near the X point can be explained by the development of `positiveʹ magnetic islands (islands with positive current perturbations). It is proposed that the disruption is initiated when the X point of the magnetic islands coincides in one toroidal position near the torus equator.