A knowledge of solid hydrogen pellet lifetimes in a plasma is critical to the design of devices to refuel tokamak fusion reactors. When the pellet is injected into the plasma, the ablated material from the pellet undergoes a transonic flow since it is heated while it expands. Calculations are done on the behavior of the transonic flow for various plasma conditions and pellet sizes. From these calculations, the ablation rate and lifetimes of the pellet are determined. A scaling law is given which allows pellet lifetimes to be easily calculated for any plasma conditions. The results of these calculations give good agreement when compared with experiments.

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.

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NOTICE This document contains information of a preliminary nature. I t is subject to revision or correction and therefore does not represent a final report.

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