The Importance of Charge Exchange Neutrals for Fueling DIII-D Plasmas

Spectrally resolved passive Balmer-α (Dα, Hα) measurements from the DIII-D sixteen channel edge main-ion CER system confirm the presence of higher energy charge exchange (CX) neutrals (“thermal” neutrals) in addition to the recycling cold neutrals in the edge region of DIII-D plasmas. CX between ions and edge neutrals transfers energy and momentum between the populations giving rise to neutrals with energies approximating the ions in the pedestal region. Multiple CX events in succession provide a pathway of increasing energy and velocity, permitting a neutral to get deeper into the plasma, allowing fueling in what may otherwise be considered opaque scrape off layers. The spectrally resolved measurement provides key information about the density and velocity distribution of these thermal neutrals while avoiding several challenges that are associated with filter based measurements. The neutral dynamics, and dependence on parameters such as ion temperature, density, Zeff, and isotope mass are modeled with the FIDASIM collisional radiative code, which produces synthetic spectra with a shape that is in close agreement with the measurements. The local neutral densities and ionization source rates are determined by matching the intensity of the measured and modeled thermal emission.