An analysis of the ablation rate for solid pellets injected into neutral beam heated toroidal plasmas

A new model for calculating the ablation rate of a solid pellet injected into a neutral-beam-heated toroidal plasma is discussed. It is an extension of the neutral cloud shielding model introduced by Parks and Turnbull. Fluid equations describing a transonic stationary flow of neutral particles ablated from the pellet surface in the presence of both electron heat flux and of flux due to fast ions are derived and solved as an eigenvalue problem. The effect of fast ions on the ablation rate is significant in low electron temperature and low density plasmas such as edge plasmas when the fast ion density is rather high. When the ablation due to fast ions is dominant and electron heat flux is negligible, the ablation rate is given by a scaling relation similar to that obtained in the case of electron heat flux only. An example of an ablation rate profile is shown for a PDX tokamak-like plasma in a self-limiting approximation for electron temperature and density. The results are compared to those of the Milora model which includes fast ions. (author)