The Role of Main Ion Isotope Mass on Neutral Fueling and the Density Pedestal Structure*

Edge ionization source measurements and plasma profiles on the DIII-D tokamak for dimensionally matched hydrogen (H) and deuterium (D) H-mode pedestals show a clear isotope-mass effect and give experimental evidence for an influence on the density pedestal structure. For all electron pedestal densities studied, hydrogen penetrates 40% deeper than deuterium. Neutral penetration decreases with increasing electron density pedestal. For low density, we find that the hydrogen isotope-mass penetration increase widens the density pedestal in comparison to deuterium. As the electron pedestal density increases, the ratio of H/D penetration remains constant, but the overall magnitude of the isotope-mass penetration difference diminishes and projects to be negligible for a reactor density pedestal. Simultaneous High-Field Side (HFS) and Low-Field Side (LFS) ionization source measurements confirm an asymmetry in which the majority of the fueling occurs on the HFS, at the diagnostic location that demonstrates the isotope-mass fueling increase in H. However, on the LFS, differences in the neutral profiles suggest an additional isotope-mass driven transport effect in the pedestal is present.

Predictive capabilities for tokamak edge density profiles remain limited and fail to adequately model the complex plasma response to a change in isotope mass. By leveraging the new Lyman-alpha diagnostic on DIII-D, this work allows experimental comparison to models which use the neutral penetration to set the density pedestal width. Quantitative comparisons of edge main-ion neutral penetration in H and D plasmas allow us to disentangle the isotope-mass effect on edge fueling and on the density pedestal structure, to constrain modern 2D edge codes in their modeling of current devices, and to improve their predictive capabilities for future startup and operation of burning plasmas devices.