The isotope effect on pedestal structure: fueling is less impactful than transport

We present results consistent with the picture that increased pedestal pressure with isotope mass is a result of transport rather than fueling using a database of stationary ELM-y hydrogen and deuterium discharges on DIII-D. The isotope effect is an empirically observed decrease in heat and particle transport with increasing hydrogenic isotope mass. The change in transport manifests partially in the electron channel where hydrogen plasmas have decreased η_e=Lne/L_Te primarily through increased mean L_Te at similar mean L_ne. The effect of increased particle transport must be disentangled from the effect of increased fueling and changing divertor conditions since hydrogen neutrals penetrate further and physically sputter less carbon. Fueling increases the ratio n_e^SEP/n_e^PED of reducing the pedestal stability particularly on the ballooning side, yet ELM frequency increases with η_e faster for hydrogen than deuterium. Additionally, while n_e^SEP/n_e^PED is similar for both isotopes, η_e increases independent of species implying the fueling is not the primary driver of the isotope effect and we investigate this using new Lyman-α emission main ion source measurements from the LLAMA diagnostic. This work will inform models of pedestal structure for reactor-relevant hydrogen startup in machines such as ITER and SPARC.