Impact of the isotope effect on divertor detachment and pedestal structure in DIII-D

Dedicated H-mode hydrogen experiments has been conducted at DIII-D and are compared to a similar deuterium dataset under a range of divertor conditions, revealing significant effects of the isotope mass on divertor conditions and pedestal behavior. H-mode confinement is found to be systematically lower in hydrogen and experiences a more pronounced degradation with increased fueling. Hydrogen is found to have a higher ne,sep at similar fueling rates. When comparing hydrogen and deuterium discharges under similar divertor regimes, the hydrogen cases are shown to have a higher electron density and lower electron temperature in the outer divertor. Hydrogen and deuterium density ramps demonstrate a 20% higher ion current and of a 17% higher upstream density at detachment onset in hydrogen. The trends of divertor conditions with isotope mass through detachment are studied through a 2 point model analysis and detailed 2D interpretive SOLPS-ITER modeling. Both approaches are able to reproduce key experimental trends. Highlighted in the modeling are a significant role of the ion sound speed, the momentum balance, and carbon sputtering and radiation. An analysis of the particle source in the 2D modeling confirms a higher core particle source for hydrogen, consistent with the experimental hydrogen pedestal structure. However, deuterium is shown to have a lower ionization mean free path at the target than in hydrogen at the same upstream density, contrary to expected scaling with isotope mass. An experimental analysis of the pedestal stability and neutral fueling demonstrates that neither of these effects are sufficient to explain the pedestal differences between isotopes nor the hydrogen pedestal structure respectively. These results indicate that increasing ion mass is likely beneficial for divertor performance in future devices, reducing heat and particle fluxes as well as the ne,sep required for detachment.