Partial and full reconnection during sawtooth activity and disruptions

Reconnection phenomena in the form of sawteeth and major and minor disruptions have been studied on the TFTR and Tokapole II tokamaks. On TFTR most sawteeth are caused by a reconnection process, although the timescale is often much faster than the Sweet-Parker reconnection time, (τRτA)1/2. However, on Tokapole II the central safety factor, q(0), has previously been observed to remain fixed at 0.7 during the sawtooth oscillations. This result implies the absence of total reconnection to the plasma center during sawteeth on the Tokapole II tokamak. It is found that the sawtooth crash enhances the magnetic stochasticity outside the reconnection volume, complicating the analysis of heat pulse propagation. However, when the effects of the enhancement in transport are properly accounted for, it is found that the heat pulses propagate consistently with the power balance thermal diffusivity. Major disruptions at high β are found to have some similarity to sawteeth in that they typically have (m,n) = (1,1) precursors and stochasticity may also be involved in the final disruption phase. In the Tokapole II tokamak, direct measurements of the q profile have been made through repetitive minor disruptions. It is found that q(0) increases from 0.75 to 1.3 during the minor disruption. Precursor oscillations are consistent with the simultaneous presence of the (m,n) = (2,1) and (3,2) modes. (author). 19 refs, 7 figs