Parallel transport in a tokamak scrape-off layer
Résumé
Sudden transient events - such as edge-localized modes (ELMs) - routinely occur in the tokamak edge during H-mode confinement. ELMs are plasma relaxations that cause a sudden drop in density and temperature of the pedestal plasma, leading to a significant loss of the stored plasma energy. Once the ELM-driven plasma pulse has crossed the magnetic separatrix, it travels mainly parallel to the magnetic field lines and ends up hitting the divertor plate, after a delay typically of the order of a few hundred microseconds. In the present paper, the fully nonlinear parallel dynamics in the scrape-off layer is solved by means of a self-consistent kinetic model (Vlasov-Poisson equations). The Vlasov equation is integrated numerically using an Eulerian method, which guarantees a high resolution in velocity space, even in regions of low plasma density such as the sheaths. The model is applied to the study of parallel transport during edge-localized modes (ELMs), with particular emphasis on the energy fluxes on the divertor plates. The numerical results are compared to analytical estimates based on a free-streaming model, and to numerical simulations performed using two different approaches: a particle-in-cell code and a one-dimensional fluid code. An interesting feature is the observation of an early electron energy flux, due to suprathermal electrons escaping the ions' attraction.
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