The role of stochasticity in sawtooth oscillations

Stochastization of field lines, resulting from the interaction of the fundamental m/n=1/1 helical mode with other periodicities, plays an important role in sawtooth oscillations. The time-scale for stochastic temperature diffusion is shown to be sufficiently fast to account for the fast sawtooth crash. The enhanced electron and ion viscosities arising from the stochastic field lines are calculated. The enhanced electron viscosity always leads to an initial increase in the growth rate of the mode-the 'magnetic trigger'. The enhanced ion viscosity can ultimately lead to mode stabilization before a complete temperature redistribution or flux reconnection has occurred. A dynamical model is introduced to calculate the path of the sawtooth oscillation through the parameter space of shear and amplitude of the helical perturbation, including a stochastic trigger to an enhanced growth rate, stabilization by ion viscosity and a prescription for flux reconnection at the end of the growth phase