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

Abstract

Molecular deuterium line emission is observed in both the visible and ultraviolet (UV) wavelength ranges after massive (> 100 Torr-L) injection of D2 gas into post-disruption runaway electron (RE) dominated plasmas in the DIII-D tokamak. D2 UV line emission is found to be the dominant source of radiated power, surpassing D Lyα. Interpretive modeling with a collisional-radiative model (CRM) indicates that D2 radiation surpasses D radiation because Lyα is strongly trapped, while D2 UV lines are mostly untrapped. The CRM also indicates that the D2 line emission is completely dominated by RE impact (rather than thermal electron impact), so the D2 line emission can serve as a good diagnostic for the spatial localization of REs. Analysis of D2 visible lines indicates that the D2 molecules in the plasma are thermally equilibrated with the background plasma, with vibrational, rotational, and kinetic temperatures all near 0.3 eV. D2 spectroscopy therefore serves as a convenient diagnostic of background plasma temperature. Measurement of D2 radiated power also appears to serve as a useful diagnostic for constraining neutral transport modeling.