Observation of D₂ molecule line emission after massive D₂ injection into runaway electron plateaus in DIII-D

Molecular deuterium line emission is observed in both the visible and ultraviolet (UV) wavelength ranges after massive (> 100 Torr-L) injection of D-2 gas into post-disruption runaway electron (RE) dominated plasmas in the DIII-D tokamak. D-2 UV line emission is found to be the dominant source of radiated power, surpassing D Ly alpha. Interpretive modeling with a collisional-radiative model (CRM) indicates that D-2 radiation surpasses D radiation because Ly alpha is strongly trapped, while D-2 UV lines are mostly untrapped. The CRM also indicates that the D-2 line emission is completely dominated by RE impact (rather than thermal electron impact), so the D-2 line emission can serve as a good diagnostic for the spatial localization of REs. Analysis of D-2 visible lines indicates that the D-2 molecules in the plasma are thermally equilibrated with the background plasma, with vibrational, rotational, and kinetic temperatures all near 0.3 eV. D-2 spectroscopy therefore serves as a convenient diagnostic of background plasma temperature. Measurement of D-2 radiated power also appears to serve as a useful diagnostic for constraining neutral transport modeling.