Time-resolved spectroscopic measurements of tungsten gross-erosion and re-deposition in the DIII-D tokamak

Measurements of tungsten (W) gross-erosion and re-deposition dynamics using a high-resolution UV optimized (200 to 400 nm) spectrometer have been performed in L- and H-mode plasmas in the lower divertor of DIII-D tokamak. W-coated graphite samples were exposed using the DiMES manipulator. Measuring several UV W I emission lines allows precise W gross-erosion measurements using atomic physics coefficients (S/XB). Higher W charged states (W II, W III) are required for W re-deposition rates. Preliminary spectroscopic analysis of L-mode plasmas (<span style="font-size:10.8333px">T_e=15-20 eV and n_e=1.5-3 10¹⁹ 10^-3) combined with post-mortem RBS measurements of net erosion indicate higher empirical S/XB values for neutral W emission (W I) lines compared to predictions from collisional radiative models. The S/XB systematic discrepancy is consistent with a low available ionization rate, in contrast to line-dependent excitation rates. Initial spectroscopic measurements from different W I lines along with predicted S/XB ratios, yields a W gross-erosion rate of ~1.5 10²⁰ atoms/m² for 20 eV, 1.5-3 10¹⁹ m^-3 plasmas. The measured W I and W II UV emission from a W sample exposed to H-mode plasmas with ELM frequencies of ~15 Hz indicate ~10 times larger W erosion rate during intra-ELM versus inter-ELM phases.

Work supported by the US Department of Energy under award numbers DE-SC0015877, DE-FG02-00ER54610, DE-FC02-04ER54698.