Analysis of W erosion and migration in the DIII-D SAS-VW divertor using GITR and comparison with optical emission spectroscopy*

A case study was performed on W erosion and migration in the SAS-VW divertor on DIII-D. The study is specific to an upper single-null magnetic equilibrium with the OSP on the progressive angle of the W side of the divertor, a Bx▽B drift towards the divertor, and the reported plasma parameters. Gross W erosion along the W surface was estimated using emission spectroscopy data and results from a 3D Monte Carlo transport code, GITR. Both experimental shots yielded a somewhat constant gross W erosion flux estimate of ≈1e18 m−2s−1. Meanwhile, GITR simulation results yielded fluxes of 4.67e17, 1.42e19, and 1.53e19 m−2s−1 for spectroscopic Views 1, 2, and 3 respectively. The GITR simulation may be under-predicting W erosion in View 1 due to a simple sheath model and is otherwise over-predicting W gross erosion by a factor of 10. Sheath physics dominates W transport and redeposition near the surface; ionization in the Chodura and Debye sheaths is a good predictor for prompt redeposition. A series of simulations with varying permutations of forces turned on were performed to investigate W migration away from the surface. The fraction of simulated W impurities that escape the sheath are driven into the private plasma by the ExB drift, and driven further into the slot toward the vertex by Fokker-Planck collisional friction forces. This case study is the first of four that will demonstrate the impact of Bx▽B drift direction and strikepoint selection on W gross erosion, local redeposition trends, and overall leakage out of the divertor.