Effect on OSP location and Bx▽B drift direction on W erosion and migration in the DIII-D SAS-VW divertor using GITR and optical spectroscopy*

W erosion and migration from the V-shaped W-coated Small Angle Slot divertor (SAS-VW) were analyzed using the 3D Monte Carlo transport code GITR. The W-coated outer side of the slot consists of 2 nearly vertical surfaces connected by a progressive angled surface.

Simulations were performed for a DIII-D discharge with a Bx▽B drift towards the divertor and the outer strikepoint (OSP) on the progressive angle. Results indicate that the ExB drift drives W ions from the near-SOL into the private flux region, where friction dominates and pushes them deeper into the slot, resulting in ~99.9% of W depositing in the divertor region though W gross erosion is ~1e19 m-2s-1.

 

In subsequent simulations with the OSP on the slot vertex and the same Bx▽B drift, the near-SOL is wider. With the private plasma farther from the W surface, the ExB drift is not strong enough for most sputtered W ions to cross the outer leg. The magnetic and ▽Ti forces dominate W transport into the far-SOL. Local re-deposition was poor, though W gross erosion is ~1e18 m-2s-1, a factor of 10 lower than in the progressive angle case.

Finally, with a Bx▽B drift away from the divertor and the OSP on the progressive angle, detached conditions were achieved, leading to W gross erosion fluxes of ~1e12 m-2s-1, lengthening the W ionization mean free path, and resulting in a high W neutral population and poor redeposition of W ions. The balance of W gross erosion and redeposition in the divertor contribute to W leakage out of the divertor, which will be reported for all cases.