High-Resolution Ultraviolet Spectroscopy for Erosion Measurements of Tungsten and Tantalum

The erosion of Plasma Facing Components (PFC) in magnetically confined experiments can be quantified via a spectroscopic technique relating observed spectral line intensities to material influx via atomic physics coefficients (S/XBs). Failing to sufficiently resolve measured spectral lines due to impurity line blending, pressure broadening, and Zeeman splitting can lead to incorrect inferred erosion rates. A 1.33-meter focal length spectrometer has been outfitted with a high-resolution diffraction grating and a new UV optimized camera providing ~4 pm wavelength resolution down to 200 nm. Spectral data is collected by observing the plasma material interaction of a W or Ta Langmuir probe system installed on the Compact Toroidal Hybrid (CTH) experiment. Comparisons of new highly resolved W spectra to previous spectra reveals >20 previously unresolved or blended W spectral lines from 245 nm to 265 nm. High-resolution measurements of Ta emission in CTH are presented along with identifications in the visible and UV regions of Ta spectral lines for the potential benefit to erosion measurements. Initial calculations of Ta ionization cross-sections are utilized to estimate the ionization mean free path of Ta providing a preliminary comparison of expected Ta and W re-deposition rates.