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 particle 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 a heuristic closed-form
prediction for the power scrape-off width that is in reasonable quantitative agreement
both in absolute magnitude and in scaling with recent experimental data from deuterium plasmas.
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 (April 2011
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Download PPPL-4619 (pdf 184KB 9 pp)
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