Kinetic and transport theory near the tokamak edge

Conventional transport orderings employed in the core of a tokamak plasma allow large divergence-free flows in flux surfaces, but only weak radial flows. However, alternate orderings are required in the edge region where radial diffusion must balance the rapid loss due to free streaming to divertor plates or limiters. Kinetic equations commonly used to study the plasma core do not allow such a balance and are, therefore, inapplicable in the plasma edge. Similarly, core transport formulas cannot be extended to the edge region without major, qualitative alteration. Here the necessary changes are addressed. By deriving and solving a novel kinetic equation, distinctive collisional transport laws for the plasma edge are constructed. It is found that the new edge ordering retains the radial diffusion and parallel flow of particles, momentum, and heat to lowest order in the conservation equations. To higher order a surprising form for parallel transport in the scrape-off layer is found, in which the parallel flow of particles and heat are driven by a combination of the conventional gradients, viscosity, and new terms involving radial derivatives. The new terms are not relatively small, and could affect understanding of limiter and divertor operation.