Experimental measurements on DIII-D of hydrogen neutral penetration lengths () on the high field side (HFS) are longer by a factor of than for deuterium consistent with the thermal velocity ratio for neutrals at the same temperature . This ratio is constant for both low and high pedestal electron density. At low pedestal density m, the neutral penetration length is greater than the density pedestal width for both isotopes, and the additional 40% increase of neutral penetration in hydrogen widens the pedestal by the same amount. As the density pedestal height increases m, the neutral penetration lengths drop below the density pedestal widths for both isotopes, and the increased penetration of hydrogen has no increased effect on the pedestal width compared to deuterium. Extrapolating to future reactor-relevant high electron density pedestals, the isotope-mass change in neutral fueling on the HFS from the deepest neutral penetration of hydrogen, to the shortest neutral penetration of tritium will be negligible (0.2–0.4 cm) in comparison to estimates of the density pedestal width (6–8.5 cm).
This paper investigates how the mass of hydrogen isotopes affects the penetration of neutral particles into the plasma and the structure of the density pedestal in the DIII-D tokamak. The results show that hydrogen neutrals can penetrate deeper than deuterium, but this effect becomes negligible at high plasma densities relevant for future fusion reactors.