Significance of Kinetic Effects for High-Z Impurity Transport in the SOL Investigated with SOLPS-DIVIMP Modelling

The fluid momentum balance for impurity ions in the Scrape-Off Layer (SOL), which has been historically approximated using only the two dominant moments (friction and thermal) for low-Z impurities, cannot be approximated in the same way for high-Z impurities, where the ‘inertial force’, i.e. the effect of impurity ion inertia, can be significant.

DIVIMP (a kinetic Monte-Carlo impurity transport code) is used to inject tungsten (W) into 12 SOLPS-ITER plasma backgrounds. These L-mode, USN, deuterium (D) + carbon (C) backgrounds incrementally increase the line-averaged electron density to span the divertor detachment threshold, with the ion BX▽B drift directed away from the closed, slot divertor. The degree to which DIVIMP satisfies the fluid momentum balance was statistically assessed via the root mean squared error percent (RMSEP) metric.

The Friction and Thermal approximation yields an RMSEP more than two times greater than that for the full fluid momentum balance for a number of cases. The most basic plasma background models were utilized in DIVIMP to determine the plasma conditions under which impurity-ion inertia shifts from being inconsequential to being important. Cross-field transport was disabled and W-ions of a single, specified charge were launched far from any surface.

Work supported by the U.S. Department of Energy under Awards DE-SC0019256, DE-SC0023378, DE-FC02-04ER54698, and DE-AC05-00OR22725