FINITE MEAN-FREE-PATH EFFECTS IN TOKAMAK SCRAPE-OFF LAYERS

We address the problem of calculating the parallel electron heat conduction in a tokamak scrape off layer with non-negligible mean free path. We develop a model distribution function based on pieces of Fokker-Planck solutions, containing: a local Maxwellian at low energies, a modified Spitzer-Harm distribution at energies below where the mean free path equals the distance to the nearest wall, a bounce-averaged distribution at higher energies up to the confining potential energy, a filled-in outgoing loss-cone distribution at still higher energies, and finally an empty loss region at energies such that the mean free path exceeds the length of the field line. The model is compared with Monte Carlo simulations.