Magnetic shielding effects in pellet plasma interactions

The principal goal of the analysis of the interaction between solid D-T pellets and a thermonuclear reactor grade plasma is the determination of the ablation rate, and the pellet lifetime as functions of the reactor plasma parameters and the pellet size. This analysis can be separated into four parts: (1) the shielding of the pellet surface by the ablated material; (2) calculation of the ablation dynamics for the reduced energy flux; (3) analysis of the time evolution of the shielding cloud and ablation rate; (4) analysis of the motion of an ablating pellet across the reactor plasma. The latter two questions have not as yet been considered in any detail. In computations of the fuel deposition profile, the pellet velocity has always been assumed to be constant and equal to the injection velocity. Transients in the time evolution of the cloud of ablated material have also been neglected, generally on the assumption that these transients are short compared to the pellet lifetime, and so a quasi-steady-state situation exists at all times. In this paper we consider the first of these questions, the shielding of the pellet surface by the ablation cloud