Kinetic theory of plasma scrape-off in a divertor tokamak

The structure of the plasma boundary, in a tokamak with a poloidal divertor, is considered. The assumption, of hot, collisionless ions, and cold, collisional electrons is shown to be self-consistent. The density gradient length is shown to be of the order of ρpi, the ion poloidal gyroradius, at typical poloidal angles. Plasma diffusion across the separatrix, and particle motion along the field lines to the collector plates, are described by a kinetic equation. From the solution of the boundary-layer problem, the ion flux and ion energy flux, to the plates, are calculated. The ratio of these fluxes, and the corresponding ratio for the electrons, complete the specification of divertor boundary conditions, to be used in tokamak transport codes.