Direct Measurements of Thermal Neutrals and their Impact on Particle Source Rates Using Spectrally Resolved Balmer-Alpha Measurements on DIII-D

Charge exchange between thermal ions and edge neutrals transfers energy and momentum between the populations giving rise to thermal neutrals with energies approximating the ions in the pedestal region. The increased mobility of these neutrals allows them to penetrate deeper into the confined plasma providing fueling inside the pedestal top. Spectrally resolved D-α measurements from a 16 ch edge main-ion CER system confirms the presence of these thermal neutrals. Spectral discrimination allows the measurement of thermal neutral densities inside the confined plasma while largely avoiding tomographic inversion challenges due to bright cold emission from the scrape off layer, which is not Doppler broadened. The FIDASIM code provides a forward model of the measured spectrum and allows the underlying neutral densities and localized ionization/source rates to be calculated. The spectra are accurately reproduced and the simulations show 40% of the radially integrated ion source located inside the pedestal top at the measurement location, largely due to the thermal neutrals. These results will be supplemented with more detailed neutral modeling using DEGAS2 and EIRENE and comparisons to 2D measurements based on a Ly-α emission using the recently installed LLAMA diagnostic.