Spectroscopic Neutral Density Measurements of the DIII-D Divertor and Comparison to Modeling

Initial divertor neutral density (n_D) measurements in an experiment exploring the role of neutrals on detachment in DIII-D indicate that n_D during L-mode, attached condition is ~25 - 35% of the local electron density. Neutral density measurements in tokamaks are critical to characterize particle sources and energy loss mechanisms in the plasma edge. Neutral densities are derived from calibrated line-integrated Deuterium Balmer alpha (D_α) brightness, using local n_e and T_e measurements to determine the photon emissivity coefficients and a characteristic emissivity decay length along the line of sight from UEDGE simulations. The n_D obtained is an upper estimate of the atomic density due to contributions from molecular dissociative excitation and recombination processes. To unravel the relative contributions, the derived D_α emissivity and n_D are compared to DEGAS 2, which calculates neutral (atomic and molecular) densities and emissivities from measured profiles. Atomic densities obtained with this technique and interpreted by DEGAS 2 are validated using Lyman-alpha measurements, which have negligible contributions from molecular processes. By sweeping the outer strike point past the diagnostic line-of-sights, this measurement technique can be extended into 2D.