A Revised Neutral Gas Shielding Model for Pellet-Plasma Interactions

A revised neutral molecule ablation model is derived to describe the evaporation of a solid hydrogen pellet in a tokamak plasma. The approach taken is based on the theory of Parks, Turnbull, and Foster who postulate that a cloud of molecular hydrogen surrounding the pellet shields the surface from incoming energetic electrons and, in so doing, regulates the evaporation rate. This treatment differs from the earlier model in that the hydrodynamic behavior of the molecular cloud is analyzed without invoking the assumption that the flow of material away from the surface is sonic everywhere. Numerical solutions of the fluid dynamic equations, which include the effects of strong electron heating locally in the gas, reveal that the flow of material away from the pellet is initially retarded relative to the solution of Parks et al., and then rapidly accelerated and rarified. This behavior is more pronounced for higher temperature plasmas and the net effect is that pellet life times might be prolonged slightly at the higher temperatures over those predicted by the approximate sonic flow model. A simple injection depth scaling law is derived and estimates of pellet fueling velocity requirements are made for several tokamaks.