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 Wmigration 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× 10atoms/m (∼20 nm equivalent thickness) is observed on the outer and inner divertor, and in the private–flux region. The campaignintegrated 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.