Investigations of time-resolved tungsten erosion and re-deposition using UV spectroscopy

A new high-resolution, high-throughput spectroscopic system optimized for UV wavelengths has been installed on the DIII-D tokamak for tungsten (W) erosion and re-deposition studies. It is composed of a CCD camera with high sensitivity in the UV range and a spectrometer with a spectral resolution of 0.16 Å around 250 nm. The spectrometer allows simultaneous observation of multiple emission lines from different lower W charge states within the UV wavelength range, required for W re-deposition measurements. Preliminary observations have been made in DIII-D plasmas. The total number of W atoms eroded from a surface (gross-erosion) can be determined from spectroscopy of neutral W emission using the S/XB method. The fraction of re-deposited W requires observing emission from higher charge states. Thus, at least W I and W II emission are needed to estimate W net erosion rates. Measurements of multiple spectral lines can reduce uncertainties and quantify the effect of metastable levels on the radiated spectrum. Spectroscopic measurements of time resolved W erosion and re-deposition rates are benchmarked by cumulative gross and net erosion from a DiMES probe. Further, experiments to quantify tungsten erosion and re-deposition rates during intra- and inter-ELM periods will be presented.