A short mean-free path, coupled neutral-ion transport description of a tokamak edge plasma

The ion and neutral densities and temperatures in the edge region of a diverted plasma can vary over two or more orders of magnitude causing atomic physics processes to modify the usual short mean-free path transport descriptions of magnetized plasmas. The short-mean-free path ordering is particularly relevant to the rather cold, moderately dense plasmas of interest in gas target divertor operation. Under such conditions, charge-exchange, ionization, and recombination couple the ions to the neutrals and alter the transport coefficients of both species. The coupling modifications to the ions are evaluated in the short mean-free path limit by a minor adjustment in standard procedures when neutral–neutral collisions are negligible. The short-mean-free path coupling modifications to the neutrals are evaluated by assuming the charge-exchange rate coefficient is velocity independent. A closed neutral-ion fluid description is obtained by assuming temperatures sufficiently high that ion energy and momentum exchange with the electrons is unaffected by atomic processes and densities sufficiently low that electron–neutral collisions may be neglected.