Validating ultraviolet spectroscopic measurements of tungsten erosion rates during negative triangularity plasmas in DIII-D

Ultraviolet (UV) and visible (VIS) spectroscopic measurements of tungsten (W) net erosion were obtained in DIII-D using negative triangularity plasmas with divertor electron densities of n_e,div~5×10¹⁹ m^-3 and temperatures T_e,div ~25 eV. Net erosion from divertors can be measured by atomic line emission spectroscopy of neutral (W I) and near-neutral (W II, W II) UV emission lines combined with S/XB ratios from Collisional-Radiative (CR) models. S/XB results from W I emission lines for n_e,div~1-2×10¹⁹ m^-3, T_e,div ~ 15 – 35 eV are consistently larger than CR predictions. Negative triangularity plasmas, with increased power loads to the divertor, allow W erosion experiments at high n_e,div, relevant for ITER. W samples deposited on graphite and optimized for net-erosion measurements were exposed using the Divertor Materials Evaluation System (DiMES). Gross erosion rates of ~0.1-1 nm/s and re-deposition fractions of ~70% were measured by post-mortem Rutherford backscattering spectrometry for a 15 mm diameter W coatings. Emission from the W I spectral line at 400.88 nm was measured by two different instruments in order to benchmark previously measured S/XBs for UV emission lines. S/XBs for UV W I lines demonstrate a clear n_e,div dependence, in agreement with CR predictions.



*Supported by the US Department of Energy under Award Numbers DE-SC0015877, DE-FG02-00ER54610 and DE-FC02-04R54698.