Neutrals’ effects on stability

Abstract

The physics of the neutral atoms is incorporated into a generalized description of edge instabilities in tokamaks and plasmas with a small neutral fraction. The description includes ideal and resistive ballooning modes; modes driven by a radial electron temperature gradient when the plasma is in contact with conducting material surfaces such as limitors or divertor target plates, the destabilizing effect of the parallel variation in the E×B drift frequency, and effects due to the flow of the neutral gas. The analysis considers the neutral dynamics in both the short and long neutral mean-free path limits (relative to the wavelength of the instability), since the perturbed ion–neutral coupling depends on collisionality. Moreover, parallel and cross-field variations in the equilibrium temperatures, densities, and potential are retained as well as the corresponding diamagnetic effects. In the short neutral mean-free path limit, the ion and neutral viscosities and heat fluxes must be retained, while in the long neutral mean-free path limit the neutrals are not perturbed, but the ion viscosity and heat flux must still be considered. The possible destabilizing impact of the new heat flux and viscous terms on stability is demonstrated.