Plasma edge fluid models for recycling at near tangential surfaces

Abstract 2D-plasma edge fluid modeling has become a common tool to investigate aspects of plasma surface interaction (PSI) in fusion devices. Recycling at exposed surfaces is included, typically, via Bohm-type boundary conditions for the plasma flow field along the B-field. Only very crude prescriptions are in use for near tangential surfaces. Such structures are, however, crucial for limiter configurations, for the baffles in divertors and for main chamber recycling. Experimentally the recycling at such surfaces is often found to be much stronger than predicted if sheath theory is extrapolated to very small field line inclination angles. In this paper we present extensions of the B2-EIRENE code system to study such effects in detail, and we apply this to evaluate, quantitatively, some of the assumptions behind one particular (`funneling-') model proposed earlier for plasma fluxes onto tangential surfaces.