Coaxial and surface modes in tokamaks in the complete cold-plasma limit

Abstract

Most treatments of ion-cyclotron radio frequency (ICRF) surface waves in Tokamaks use either the equations of magnetohydrodynamics (MHD), or the cold-plasma equations, with the approximation that the electric field parallel to the ambient magnetic field is very small. Although this assumption seems to be well justified, it eliminates the slow wave and its associated hybrid resonances, which turn out to be quite important in studying coaxial and surface modes. In particular, it was believed that coaxial modes do not exist unless there is a vacuum gap between the plasma and vessel wall. It will be shown that this is not the case when the full cold-plasma model is used, and the reason for this result will be considered. Furthermore, it will be seen that for practical purposes, the role of the plasma-vacuum boundary in confining most of the mode to the edge region seems to be taken on by the lower-hybrid resonance layer when a continuous density profile is assumed. The effect of the lower-hybrid resonance layer on the other surface waves predicted in a Tokamak seems to be small.