A study of tungsten migration in the Alcator C-Mod divertor

A toroidally-continuous row of bulk tungsten (W) tiles was installed near the typical location of the outer strike-point in the Alcator C-Mod divertor and was removed for analysis after two campaigns (3151 plasma discharges). External beam (in-air) particle induced X-ray emission (PIXE) analysis has been used to observe campaign-integrated W migration patterns in the otherwise Mo divertor using the W as a tracer for divertor erosion and transport. With a set of high spatial resolution PIXE measurements (∼3 mm resolution), poloidal profiles of W migration have been constructed. Redeposition of up to 1.5 × 1021 atoms/m2 (∼20 nm equivalent thickness) is observed on the outer and inner divertor, and in the private-flux region. The campaign-integrated deposition on the analyzed tiles corresponds to net W erosion of ∼60 nm (>0.014 nm/s). The migration profiles indicate sputtering and prompt redeposition in the outer divertor, neutral and ion transport through the private-flux region, and transport in the scrape off layer.