Molecular contribution to the Dα emission in the divertor of the ASDEX Upgrade experiment

Spectral line emission from the deuterium Balmer α line and from molecular deuterium in the Fulcher band, d³Π_u→a³∑_g⁺, was studied in the outer divertor region of the ASDEX Upgrade plasma with a pair of relatively calibrated spectrometers. A dependence of the signals on the proximity of the plasma strike point and on divertor gas puffing was noted. The rotational spectrum of the lowest diagonal vibrational molecular bands indicated rotational temperatures of ca. 1000 K and hence, through calculation of relative upper state populations, a ground state vibrational temperature of ca. 4000 K. Calculating the particle fluxes, the molecular deuterium was found to contribute up to 1% of the Dα intensity, the relative importance of the molecules at some divertor heights was affected by the position of the strike point.