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

A heuristic model for the plasma scrape-off width balances magnetic drifts against parallel loss at c_s/2, resulting in a SOL width ∼ρ_p. T_sep 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 ∼c_s/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 p_p, 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.