The magnetized plasma–wall transition (PWT) and its relation to fluid boundary conditions

The magnetized plasma–wall transition (PWT) region typically exhibits three characteristic subregions: the “Debye sheath”, the “magnetic presheath”, and the “collisional presheath”. The fluid boundary conditions for transport codes (simulating, e.g., the scrape-off layer (SOL) of a tokamak) are usually applied at the “magnetic presheath entrance”, where in the simplest model the ion velocity parallel to the magnetic field equals the local sound velocity. After reviewing the basic time-independent and collisionless models of the magnetized PWT, various extensions will be discussed which are due to E×BE×B, ∇B and diamagnetic drifts, nonuniformity of the electric field parallel to the wall, and turbulence effects. In practically all cases considered, quantitative results can be obtained only by massive application of numerical methods of solution.