Kinetic Effects in the Scrape Off Layer

The scrape off layer (SOL) of a confined plasma (Fig. 1a) is in many respects in a state far from, thermodynamic equilibrium: The energy diffusing outward from the plasma core across the separatrix is condensed in the thin SOL to a strong energy flux along the magnetic field. The recycling of cold neutrals from the limiter or divertor target gives rise to a strong temperature gradient from the hot midplane region to the relatively cold recycling zone. For long mean free path length in the plasma relative to the extension of the recycling zone the thermalization in the recycling zone may be incomplete. The potential gradient length in the sheath in front of the target is small as compared to the mean free path length; the velocity distributions of ions and electrons as determined by the electric field are far from being Maxwellians there. Thus the follwing topics will be discussed in this paper: Parallel transport (electron heat flow, ion viscosity) Recycling Sheath structure (grazing incidence of magnetic field, secondary electrons). Boundary conditions for fluid models Numerical illustrations for these kinetic effects are given from a 1d particle‐in‐cell code including Coulomb collisions [1]. Not treated are kinetic effects connected with instabilities, perpendicular transport, neutrals and impurities. magnified image