Fueling Efficiency in Pellet Fueled Plasmas on DIII-D, AUG, and LHD and Implications for Larger Burning Plasmas

Pellet fueling experiments on medium sized fusion experiments, both tokamaks and helical devices, have been examined from the perspective of fueling efficiency, defined as the fraction of the pellet mass that is measured as a plasma particle increase from a single pellet. This fueling efficiency has been determined as a function of pellet penetration depth, injection location and magnetic configuration in these different yet similar plasma volume devices and we find that it has a strong dependence on penetration depth. This adds significant uncertainly in projecting the fueling efficiency of shallow pellet penetration expected in burning plasmas with H-mode pedestals, the triggering of ELMs, and different magnetic topology. We describe these dependencies and point out where more research is needed and what the implications are for divertor operation, pumping, and the fuel cycle operation of future reactors.