Scrape-off layer flows with pressure gradient scale length ∼ρp

Abstract

A heuristic model for the plasma scrape-off width balances magnetic drifts against parallel loss at cs/2, resulting in a SOL width ∼ρp. Tsep is calculated from Spitzer–Härm parallel thermal conduction. This results in a prediction for the power scrape-off width in quantitative agreement both in magnitude and scaling with recent experimental data. To achieve the ∼cs/2 flow assumed in this model and measured experimentally sets requirements on the ratio of upstream to total SOL particle sources, relative to the square-root of the ratio of target to upstream temperature. The Pfisch–Schlüter model for equilibrium flows has been modified to allow near-sonic flows, appropriate for gradient scale lengths of order ρp, resulting in a new quadrupole radial flow pattern. The strong parallel flows and plasma charging implied by this model suggest a mechanism for H-mode transition, consistent with many observations.