PPPL-4855

Scrape-off Layer Flows With Pressure Gradient Scale Length ~ Ρp

Authors: Robert J. Goldston

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.

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Accepted to: Journal of Nuclear Materials, (January 2013)

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