THRESHOLD FOR NEOCLASSICAL MAGNETIC ISLANDS IN A LOW COLLISION FREQUENCY TOKAMAK

A kinetic theory for magnetic islands in a low collision frequency tok amak plasma is presented. Self-consistent equations for the islands' w idth, w, and propagation frequency, w, are derived. These include cont ributions from the perturbed bootstrap current and the toroidally enha nced ion polarization drift. The bootstrap current is independent of t he island propagation frequency and provides a drive for the island in tokamak plasmas when the pressure decreases with an increasing safety factor. The polarization drift is frequency dependent, and therefore its effect on the island stability cannot be deduced unless omega is k nown. This frequency is determined by the dominant dissipation mechani sm, which for low effective collision frequency, nu(eff)=nu/epsilon<om ega, is governed by the electrons close to the trapped/passing boundar y. The islands are found to propagate in the electron diamagnetic dire ction in which case the polarization drift is stabilizing and results in a threshold width for island growth, which is of the order of the i on banana width. At larger island widths the polarization current term becomes small and the island evolution is determined by the bootstrap current drive and Delta' alone, where Delta' is a measure of the magn etic free energy.