Characterization of deuterium neutral density profiles in the National Spherical Torus Experiment Upgrade*

Neutral density profiles are measured on the outboard midplane of the National Spherical Torus Experiment Upgrade (NSTX-U) using a two dimensional camera looking at the D-α emission. In magnetically-confined fusion devices, assessing the distribution of neutrals is critical to estimate particle sources and energy loss mechanisms in the edge plasmas, which play an important role in defining the structure of the H-mode density and temperature pedestals. Neutral densities are calculated by inverting the line-integrated D-α brightness and using local measurements of electron density and temperature to infer atomic rate coefficients. Neutral distributions on the low-field-side midplane are compared for L-mode and H-mode discharges in NSTX-U, including the evolution during sawtooth and ELM cycles. Neutral density profiles with boronized wall conditions in NSTX-U are compared with those measured in lithium-conditioned discharges in NSTX. The particle source profiles will be inferred from simulations with the Monte Carlo neutral transport code DEGAS2 and the multi-fluid edge transport code UEDGE, constrained by the experimental D-α emissivities.