Field line diffusion and loss in a tokamak with an ergodic magnetic limiter

A numerical study of chaotic field line diffusion in a tokamak with an ergo dic magnetic limiter is described. The equilibrium model field is analytica lly obtained by solving a Grad-Schluter-Shafranov equation in toroidal pola r coordinates, and the limiter field is determined by supposing its action as a sequence of delta-function pulses. A symplectic twist mapping is intro duced to analyze the mean square radial deviation of a bunch of field lines in a predominantly chaotic region. The formation of a stochastic layer and field diffusivity at the plasma edge are investigated. Field line transpor t is initially subdiffusive and becomes superdiffusive after a few iteratio ns. The field lines are lost when they collide with the tokamak inner wall; their decay rate is exponential with Poisson statistics. (C) 2001 American Institute of Physics.