Tokamak Edge Transport Theory

Kinetic equations commonly used to study the plasma core of a tokamak do not allow a balance between parallel ion streaming and radial diffusion and are, therefore, inappropriate in the plasma edge. Standard core tokamak transport orderings allow large divergence‐free flows in flux surfaces, but only weak radial flows. Alternate orderings are required in the edge region where radial diffusion must balance large parallel ion flows to divertor target plates or limiters. Similarly, core transport formulae cannot be extended to the edge region without qualitative alteration. Here we address the necessary changes by considering a large parallel flow ordering appropriate for the scrape‐off layer of a tokamak. By deriving and solving a novel kinetic equation, we construct distinctive transport laws for an impure, collisional edge plasma. We find in particular a surprising form for parallel transport in the scrape‐off layer, in which the parallel flow of particles (and heat) are driven by a combination of the conventional gradients, as well as viscosity terms and new terms involving products of radial derivatives of the parallel mean velocity with density and temperature gradients. The new terms are not relatively small, and could affect understanding of limiter and divertor operation.