Measurements of radiated power by molecular deuterium in the DIII-D divertor

Radiated power from molecular deuterium emissions have been shown to be quantitatively minor compared to calibrated line radiation from deuterium and impurities in both attached and detached divertor conditions in the DIII-D tokamak. Molecular deuterium is a constituent of recycling flux from plasma-material surfaces, in particular in the divertor target region where the flux is the highest. The Lyman-Werner molecular band produced by excitation of D₂ is a broad spectral feature in the VUV which complements the Fulcher band in visible wavelengths. Previous results measured in DIII-D with an absolutely calibrated broadband EUV/VUV spectrometer and modeling suggested that the Lyman-Werner band may account for up to 20% of the radiated power from the plasma, however without the ability to resolve the band. This motivated a new, high resolution VUV instrument which was installed on DIII-D and cross calibrated with the original instrument. In experiments, however, the integrated L-W band constituted <2% of total radiated power. Relationship between the VUV and visible Fulcher band emissions with local conditions (T_e, n_e) are presented, and suggest that the original interpretation of the VUV data underestimated a wide, secondary line profile due to instrumental effects in its detector.