Neutrals' effects on stability

The physics of the neutral atoms is incorporated into a generalized descrip tion of edge instabilities in tokamaks and plasmas with a small neutral fra ction. The description includes ideal and resistive ballooning modes; modes driven by a radial electron temperature gradient when the plasma is in con tact with conducting material surfaces such as limitors or divertor target plates, the destabilizing effect of the parallel variation in the ExB drift frequency, and effects due to the flow of the neutral gas. The analysis co nsiders the neutral dynamics in both the short and long neutral mean-free p ath limits (relative to the wavelength of the instability), since the pertu rbed ion-neutral coupling depends on collisionality. Moreover, parallel and cross-field variations in the equilibrium temperatures, densities, and pot ential are retained as well as the corresponding diamagnetic effects. In th e short neutral mean-free path limit, the ion and neutral viscosities and h eat 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 s till be considered. The possible destabilizing impact of the new heat flux and viscous terms on stability is demonstrated. (C) 1999 American Institute of Physics. [S1070-664X(99)04008-2].