A model for the ablation rate of a solid hydrogen pellet in a plasma

Abstract

It is shown that the ablation of a solid hydrogen pellet subject to a plasma is likely to produce a quasi-steady dense neutral gas cloud. The total integrated density of the cloud is such that the plasma electrons lose essentially all their energy in the cloud. The electron energy flux is degraded by inelastic collisions and elastic backscattering with the neutral molecules, providing local heating and acceleration of the neutral gas. Only a small fraction of the energy flux reaches the surface of the pellet, raising the pellet's surface temperature to a point where the energy flux at the pellet's surface is in balance with the energy lost through vaporization. The vaporization rate, in turn, determines the total integrated neutral gas cloud density. The scaling laws derived from the model indicate that the pellet lifetime varies as: where τp is the lifetime of the pellet and Te, ne, and rp0 are the electron temperature, density of the plasma, and initial pellet radius, respectively. A good agreement is found between this model and the ORMAK pellet injection experiment.