Magnetic Surfaces In A Steady State Tokamak

We study a toroidal plasma in a tokamak with externally given toroidal and poloidal magnetic fields as well as self-consistently interacting internal currents. The unperturbed magnetic surfaces are described by the well-known nonlinear standard map.'' When the magnetic field is allowed to carry an internal current, the self-interaction of these currents disturbs the integrity of the magnetic surfaces. We carry out a computational study of the effects of the interacting internal current filaments, measuring the diffusion of field lines from the unperturbed surfaces, and find the self-interaction to be a significant effect which always serves to increase diffusion. Perfect surfaces are difficult to maintain even when magnetic islands do not overlap. Diffusion from the current interaction dominates when current fluctuations reach (approximately) 10% of the applied field.