Study of D₂ Line Emission after Massive D₂ Injection into Runaway Electron Plateaus in DIII-D

Massive injection of D₂ gas into runaway electron (RE) plateaus is of interest as a possible method to reduce the severity of RE-wall strikes in ITER, motivating the study of the physics of these RE plateaus. Measurements of visible and UV D and D₂ line brightnesses in DIII-D indicate that D Ly-α is strongly (>10x) trapped. Self-consistent collisional-radiative modeling including D₂ line opacity and both thermal and non-thermal electron impact indicates that the D₂ band radiation is largely untrapped. This allows D₂ band radiation to become the dominant (>2x) power radiation channel. The modeling indicates that D₂ band radiation is caused by RE-impact excitation, with thermal electron impact nearly completely negligible. Initial fits to D₂ band structures indicate rotational temperatures of order 3000 - 4000 K, reasonably (within 2x) close to kinetic temperatures predicted by a 1D diffusion model.