Tungsten erosion studies utilizing ultraviolet spectroscopy in Wendelstein 7-X

Ultraviolet spectroscopy of tungsten (W) emission for investigation of erosion and re-deposition dynamics have been obtained in the Wendelstein 7-X (W7-X) stellarator. Erosion fluxes can be quantified in-situ by time-resolved line emission spectroscopy combined with S/XB coefficients from collisional-radiative atomic physics models. W7-X has mostly carbon plasma facing components (PFCs), with only a select number of W tiles recently installed to gain experience operating with partial W PFCs. W I and W II spectral emission is observed at UV wavelengths in W7-X when the heat flux on an array of W baffle tiles is purposely increased by modifying the island divertor topology using control coils in the high-mirror magnetic configuration. Nevertheless, W emission intensities are frequently just above the noise floor, making inferences of erosion challenging. No increase in W radiation was observed when trace levels of impurities (N₂, Ne, & Ar) were injected. A correlation between W I and W II intensities with a quasi-continuum W band spanning 4 to 7 nm suggests that eroded W from the baffle tiles may be transporting into the core without affecting plasma performance. Future upgrades are planned to improve the throughput of W emission measurements for the next W7-X experimental campaign.