Plasma Boundary Layer in Limiter and Divertor Tokamaks

Experimental results and theoretical models for the boundary layer of toroidal confinement devices are described. Emphasis is put on the scrape-off layer of poloidal divertor tokamaks which have demonstrated a significant reduction in impurity content as compared to limiter operation. The competition of transport processes parallel and perpendicular to the field lines, determining the profiles of plasma density, temperatures and impurity concentrations, is described. The role of hydrogen recycling in the divertor chamber is underlined, which decides the flow Mach number in the scrape off, the plasma parameters in the target plate vicinity and the regime of energy transport along the field lines. Limiter and divertor operation in ASDEX are compared and the favourable results of the latter explained by the reduced impurity production of the target plates and their distance from closed field lines, which impurity ions would have to overcome by diffusion against the plasma flow into the divertor chamber.