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 c s /2, resulting in a SOL width ∼ ρ p . T sep is calculated from Spitzer–Harm 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–Schluter model for equilibrium flows has been modified to allow near-sonic flows, appropriate for gradient scale lengths of order ρ 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.