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

• Published in: Plasma physics and controlled fusion 2013-08, Vol.55 (8), p.85003-21
• Main Authors: Moulton, D, Ghendrih, Ph, Fundamenski, W, Manfredi, G, Tskhakaya, D
• Format: Article
• Language: English
• Subjects: Controlled fusion; Elm; Exact solutions; Gaussian; Mathematical analysis; Mathematical models; Plasma (physics); Transport
• ISSN: 0741-3335; 1361-6587
• DOI: 10.1088/0741-3335/55/8/085003

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 λD0/σ0 0.01 (where λD0 is the initial Debye length at peak density and σ0 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/σ0 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, τsrc, for the plasma source is investigated. As is found for an instantaneous source, when L/σ0 5 the presence of a target sheath has a negligible effect, at least up to the explored range of τsrc = L/cs (where cs is the sound speed at the initial temperature).