The Generalized Heuristic Drift Model of the Tokamak Scrape-Off Layer Width

Abstract

A closed-form algorithm is developed for generalizing the "Heuristic Drift" (HD) scrape-off layer (SOL) width calculation, which matches low-collisionality SOL results, to high collisionality. Then, a transition density at which collisional effects will broaden the SOL by ~25% above the original HD prediction is calculated, and it is shown analytically that the minimum broadening from collisional and finite target temperature effects is a factor of ~1.22. Finally, the effect of a flux tube of finite width is accounted for and the generalized HD SOL width prediction is compared to recent experiments at ASDEX Upgrade. Estimates of edge shearing rates are presented and the edge shear is shown to be comparable to the interchange growth rate at the L-H mode transition.