General remarks concerning the ablation of a fuelling pellet

According to the general accepted view, the main shielding effect of a hydrogen (or its isotopes) pellet subjected to the impact of plasma electrons is due to the presence of a highly dense cloud around the pellet. Assuming the stopping power process is valid for particles of any species and using dimensional reasoning, the particle ablation rate of a pellet subjected to the impact of particle beam of that species at the energy E and number density n can be shown, is given by N=(n/m)1/3(1/m)1/6E1/2f(r2/Ai) where m is the molecular weight of the pellet material, Ai is the energy flux attenuation cross section of the incident particle of species with respect to the target particle of the ''ith constituent'' of the ablatant. When most energy of the incident particle is effectively absorbed in a spherical shell around the pellet, the ablation rate N can be simplified to N=(n/m)1/3r4/3fi(E). Generalization to a multi-species ablatant does not change the scaling law of pellet ablation rate. Finally, the ablation rate of a pellet subjected to the impact of particles of known energy distribution can be obtained by taking the proper moment of Eq. (2). The possible existence of an effective spherical energy absorbing region of the gas-shielding model for the pellet ablation is examined by considering the ablatant as composed of our species: H2, H, H+ and e-. The ablated cloud expands spherically if the local collisional mean free path of the cold ablated electrons is much less than the local electron Larmor radius. Results of the analysis showed that for magnetic field strength prevailing in most of the present tokamaks such a region exists at a distance of about 2r where approximately 80% of the incoming electron energy flux is absorbed. 28 refs, 2 figs, 1 tab