Generalization of the Heuristic Drift Model of the SOL for Finite Collisionality

The “Heuristic Drift” (HD) model for the scrape off layer (SOL) power flux width \footnote{R.J. Goldston, Nucl. Fusion 52, 013009 (2012)} was explicitly formulated for low-gas-puff H-Mode conditions. Experimental results in these conditions have shown good agreement with the model. In 2015, however, ASDEX-Upgrade (AUG) data showed that the scrape-off width broadens as the collisionality increases \footnote{H.J. Sun et al., Plasma Phys. Control. Fusion 57, 125011 (2015)}, which is inconsistent with the HD model as formulated. We hypothesize that this broadening is due to enhanced residence time of heat in the scrape-off-layer (SOL) at higher collisionality, due to higher classical parallel thermal resistance. This allows more time for cross-field drifts to broaden the SOL. We find reasonable agreement with more extensive recent AUG data \footnote{T. Eich et al, EPS-DPP Conference, 2019} for SOL broadening at high collisionality. This broadening may play a synergistic role with turbulence in degrading global energy confinement.