Transport in the boundary layer of tokamaks

The plasma boundary is a complex system in which there is transport both parallel and perpendicular to the field. Limiters and divertors operate both as source and sink for the plasma species and for impurities. The main particle transport flows are discussed using simple physical models and comparisons are made with recent experimental data. Parallel transport and ion heating are consistent with classical processes, cross field transport both inside and outside the last closed flux surface is anomalous and the diffusion rates for impurities are similar to those for plasma ions. (Author)