Ultraviolet Spectroscopy of Near-Neutral Tungsten Emission for Improved Re-deposition Measurements

To investigate tungsten (W) impurity erosion and re-deposition, a high-throughput high-resolution UV spectrometer has been installed on the Compact Toroidal Hybrid (CTH) experiment . Erosion rates from plasma facing materials can be quantified from spectroscopic measurements of emission intensity combined with S/XB coefficients provided by collisional-radiative models. Accurately estimating re-deposition requires spectroscopic observation of several near-neutral charge states, which often emit at ultraviolet wavelengths. As such, a 1.33-meter spectrometer with a high-resolution diffraction grating, UV optimized CCD camera, and improved optical throughput has been developed, providing ~4 pm resolution down to 200 nm. Tungsten emission is produced by plasma interaction with a W sample at end of a translatable probe. The probe maintains vacuum cleanliness and can extend the W sample to the CTH magnetic axis. At the W sample surface, a single tip Langmuir probe measures the plasma electron temperature and density. W III emission observed in CTH experiments will be potentially used in DIII-D experiments for more accurate measurements of W re-deposition.