Quasineutral plasma expansion into infinite vacuum as a model for parallel ELM transport

An analytic solution for the expansion of a plasma into vacuum is assessed for its relevance to the parallel transport of edge localized mode (ELM) filaments along field lines. This solution solves the 1D1V Vlasov-Poisson equations for the adiabatic (instantaneous source), collisionless expansion of a Gaussian plasma bunch into an infinite space in the quasineutral limit. The quasineutral assumption is found to hold as long as lambda(D0)/sigma(0) less than or similar to 0.01 (where lambda(D0) is the initial Debye length at peak density and sigma(0) is the parallel length of the Gaussian filament), a condition that is physically realistic. The inclusion of a boundary at x = L and consequent formation of a target sheath is found to have a negligible effect when L/sigma(0) greater than or similar to 5, a condition that is physically plausible. Under the same condition, the target flux densities predicted by the analytic solution are well approximated by the 'free-streaming' equations used in previous experimental studies, strengthening the notion that these simple equations are physically reasonable. Importantly, the analytic solution predicts a zero heat flux density so that a fluid approach to the problem can be used equally well, at least when the source is instantaneous. It is found that, even for JET-like pedestal parameters, collisions can affect the expansion dynamics via electron temperature isotropization, although this is probably a secondary effect. Finally, the effect of a finite duration, tau(src), for the plasma source is investigated. As is found for an instantaneous source, when L/sigma(0) greater than or similar to 5 the presence of a target sheath has a negligible effect, at least up to the explored range of tau(src) = L/c(s) (where c(s) is the sound speed at the initial temperature).