Field line stochasticity during the m=1 reconnection

A full three-dimensional nonlinear magnetohydrodynamic (MHD) simulation using a high number of modes supports a model of the m=1 asymmetric resistive reconnection in tokamaks. The current sheet associated with the internal kink mode exhibits a toroidal modulation mainly due to a finite toroidal pressure effect that leads to an asymmetric m=1 magnetic island. The main effect of the modulation is to drive the growth of (m±1,m) magnetic islands in the plasma on a time scale imposed by the m=1 reconnection process. As the reconnection proceeds, different magnetic island chains with m of order 10 overlap, leading to an annular stochastic region around the m=1 separatrix. The diffusion in this stochastic layer depends exponentially on the current state of the reconnection, and can reach a value high enough to expel the electronic temperature in less than about 100 μsec if the modulation is greater than about 1.0, that is, for low-shear and/or high-pressure gradient equilibrium values on the q=1 surface