Drifts, currents, and power scrape-off width in SOLPS-ITER modeling of DIII-D

The effects of drifts and associated flows and currents on the width of the parallel heat flux channel (λ_q) in the tokamak scrape-off layer (SOL) are analyzed using the SOLPS-ITER 2D fluid transport code. Motivation is supplied by Goldston's heuristic drift (HD) model for λ_q, which yields the same approximately inverse poloidal magnetic field dependence seen in multi-machine regression. The analysis, focusing on a DIII-D H-mode discharge, reveals HD-like features, including comparable density and temperature fall-off lengths in the SOL, and up-down ion pressure asymmetry that allows net cross-separatrix ion magnetic drift flux to exceed net anomalous ion flux. In experimentally relevant high-recycling cases, scans of both toroidal and poloidal magnetic field (B_tor and B_pol) are conducted, showing minimal λ_q dependence on either component of the field. Insensitivity to B_tor is expected, and suggests that SOLPS-ITER is effectively capturing some aspects of HD physics. Absence of λ_q dependence on B_pol, however, is inconsistent with both the HD model and experimental results. The inconsistency is attributed to strong variation in the parallel Mach number, which violates one of the premises of the HD model.