An Heuristic Drift-Based Model of the Power Scrape-Off Width in H-Mode Tokamaks
Authors: Robert J. Goldston
Abstract: An heuristic model for the plasma scrape-off width in H-mode plasmas is introduced. Grad B and
curv B drifts into the SOL are balanced against sonic parallel flows out of the SOL, to the divertor plates. The overall mass flow pattern
posited is a modification for open field lines of Pfirsch-Shlüter flows to include sinks to the divertors. These assumptions result in an estimated
SOL width of 2aρp/R. They also result in a first-principles calculation of the particle confinement time of H-mode plasmas, qualitatively
consistent with experimental observations. It is next assumed that anomalous perpendicular electron thermal diffusivity is the dominant source of heat
flux across the separatrix, investing the SOL width, defined above, with heat from the main plasma. The separatrix temperature is calculated
based on a two-point model balancing power input to the SOL with Spitzer-Härm parallel thermal conduction losses to the divertor. This
results in an heuristic closed-form prediction for the power scrape-off width that is in remarkable quantitative agreement both in absolute
magnitude and in scaling with recent experimental data. Further work should include full numerical calculations,
including all magnetic and electric drifts, as well as more thorough comparison with experimental data.
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Submitted to: Nuclear Fusion (February 2011)
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Download PPPL-4604 (pdf 573 KB 9 pp)
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