Alfvén surface waves in a two-ion-species cylindrical plasma with finite edge density

Abstract

Dispersion relations are obtained for Alfvén surface waves in a two-ion-species cylindrical plasma with finite edge density. A simple step density profile is used. The results are compared with those obtained in a plasma surrounded by a vacuum layer. For modes with positive poloidal mode number m the finite-density edge region introduces an ion-cyclotron resonance and a waveguide cutoff in the surface-wave dispersion relation at the cyclotron frequency of each ion species. For negative-m modes a new dispersion-relation branch is introduced at frequencies just below the ion-cyclotron frequency of each species. The origin of these effects is analysed in terms of the wave types that can propagate in the regions of constant density, and their interaction with the Alfvén and ion-ion hybrid resonances in these regions. This interaction is shown to be determined by the polarization of the wave fields. The relevance of the results to the Alfvén-wave and ion-cyclotron-resonance heating schemes is discussed. The possibility that the excitation of high-m surface waves contributes to the anomalously large fraction of power that is found to be deposited in the plasma edge during ion-cyclotron-resonance heating is considered.