3-D MHD studies of sawtooth oscillations and pressure driven resistive modes in tokamaks

Three-dimensional MHD codes are used to study sawtooth oscillations and pressure driven resistive modes in tokamaks. The sawtooth simulations at high temperature agree with the Kadomtsev model, but compare well with experiment only if neoclassical resistivity is used. An analytic formula for q0(t) and a semi-empirical sawtooth period scaling are presented. At high β, the helically twisted hot spot has a toroidal bulge at the large major radius side, characteristic of a high εβp instability. This bulge drives various magnetic islands, resulting in a stochastic annular region. This effect, also seen in experiment, can explain the sawtooth heat pulse 'anomaly' and the 'high β disruption'. Linear studies of pressure driven resistive modes give results consistent with various MHD effects measured in 'supershots'. The stabilizing effect of the pressure in the layer would be mitigated by finite initial islands in the experiment. This is modeled either by locally flattening the pressure profile, or by setting the ratio of specific heats, Γ = 0. Nonlinear studies show that the pressure tends to be destabilizing with multiple low mode number singular surfaces. With one low mode number singular surface present, it is often stabilizing. In supershot type discharges, finite pressure induces many islands and stochastic regions, resulting in many spikes on the pressure profile. This would cause anomalous electron heat conductivity, and may also explain the 'beta collapse' phenomenon. (author). 13 refs, 6 figs