Constraining Neutral Transport Models using Measurements Based on Charge Exchange Neutral Spectroscopy

First measurements from a spectroscopy system that has been installed on DIII-D to provide information about the energy distribution of the atomic neutrals using the Doppler shift and broadening of passive Balmer-𝛼 emission will be presented. The new system complements existing low field side midplane measurements of the neutral energy distribution that have demonstrated the existence of neutrals with energies above 1keV for typical DIII-D pedestals and have shown good agreement with synthetic spectra from neutral transport modeling [1]. The new measurements will be compared with and used to constrain neutral transport models such as FIDASIM, DEGAS2, and EIRENE. The system is optimized to provide details of the neutrals as they travel upwards from the lower diverter, undergoing various atomic physics processes such as charge exchange (CX). Multistep CX between ions and recycled neutrals transfers both energy and momentum, significantly affecting the neutral mean free path, allowing them to travel deeper into the confined plasma where they ultimately provide fueling.