Some open Questions Concerning the Neutral-Shielding Model of a Fuelling Pellet

To obtain a better understanding of the implications and assumptions of the idealized neutral shielding model of Parks and Turnbull, the model is reformulated in a self consistent way. Due to the uncertainty of the actual ablation process occuring at the pellet surface, alternative boundary conditions are proposed. Their effect on the pellet ablation rate and the state of the ablated flow are examined by numerical analyses. The results show that the ablation rate is not sensitively affected but the ablatant state is markedly influenced by the boundary condition at the pellet surface. In particular, an increase of the energy flux received at the pellet surface by a factor of four hardly affect the ablation rate but changes the temperature and the density of the ablatant at the pellet surface by three orders of magnitude. Based on these obtained results, it is concluded that the idealized ablation model is adequate when pellet injection is used to fuel a plasma but requires modification when it is used to probe plasma properties and discharge conditions.