In-depth computational impurity transport analysis in the SAS-VW divertor using the Monte Carlo GITR code

This study analyzes the inverse relationship between tungsten (W) gross erosion intensity and leakage probability using the Global Impurity Transport (GITR) Monte Carlo code to investigate the mechanisms driving W transport from the divertor target through the scrape-off layer (SOL) and private flux region (PFR). Physics mechanisms driving leakage of eroded W into the core need to be understood to minimize power depletion caused by line emission and Bremsstrahlung radiation. Slotted divertors have been designed to address both of these problems. The small-angle slot V-shaped W-coated (SAS-VW) divertor in the DIII-D tokamak provided an opportunity to investigate W erosion and migration in a reactor-relevant plasma environment.

Four cases were investigated computationally and experimentally with the SAS-VW to examine the impacts of the Bx▽B drift direction and the OSP location on the net divertor erosion trends and W leakage into the core. All four cases were simulated using SOLPS and the 3D Monte Carlo transport code GITR, and results were compared against gross erosion estimates calculated using emission spectroscopy via the S/XB method.

Under detached conditions, less W is eroded (1x10¹² – 1x10¹⁵ m^-2s^-1) from the divertor target as expected, but long ionization mean free paths resulted in poor entrained deposition and significant leakage (~12%) of W into the core through the near-SOL close to the X-point. Under attached divertor conditions, significant W gross erosion (1x10¹⁸ – 1x10¹⁹ m^-2s^-1) is counteracted by high deposition in the divertor region (~85%), and the ExB drift and friction force driving entrained deposition onto the carbon tiles in the PFR without a significant ITG force to act in the opposing direction.