The Role of Thermal Charge Exchange Neutrals in Fueling on DIII-D and Future Reactors

Spectrally resolved measurements of the Deuterium Balmer-$alpha$ (Ba-$alpha$) line have been used on DIII-D to uncover details of the neutral energy distribution, demonstrating the existence of neutrals with energies in excess of 1keV for typical DIII-D pedestals and showing good agreement with synthetic spectra from neutral transport modeling. Multistep charge exchange (CX) between ions and recycled neutrals transfers energy and momentum, creating thermal neutrals with energies approximating the ions at the pedestal top. This increases their mean free path by orders of magnitude, allowing fueling in what may otherwise be considered opaque scrape off layers. These atoms play a central role in core edge integration by affecting energy, momentum, impurity and particle transport as well as permitting increased spectral radiation from low-Z impurities and increasing the energy of neutrals impinging on the wall. The Doppler shifts of the Ba-$alpha$ emission from these neutrals allow them to be directly detected despite cold recycling neutral emission being orders of magnitude larger. Simulations constrained by these measurements demonstrate their importance for particle transport studies on DIII-D, accounting for 40% of the integrated ion source inside the pedestal top and being significantly larger than the source from typical NBI fueling. Simulation scans show that the particle source broadens with increased Ti, but narrows with increased ne, Zeff, and isotope mass. For ITER, main chamber thermal neutrals are attenuated more rapidly but will still play an important role. Additionally, the same CX process will apply to auxiliary fueling methods. Contrary to expectations, the benefits of smaller scales for fueling compact high field devices are largely offset by the higher densities assumed in modeling due to the larger nGW density limits.