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The ablatant state of a hydrogen pellet during its passage in a tokamak

By considering the ablatant to be a 4-species (H2, H, H+ and e−) reacting gas and assuming the validity of local thermodynamic equilibrium, the ablatant state of a hydrogen pellet during its passage in a tokamak is studied. Results of analysis showed that the pellet ablation rate is sensitive neither to the specific assumption regarding the variation of the ratio of the specific heats, γ in the ablated cloud, nor to the precise ablation process at the pellet surface. Except a proportional constant depending on the effect of dissociation and ionization, the scaling of the pellet ablation rate with respect to the plasma state and pellet radius is similar to that of the 1-species neutral shielding model; the overall ablation rate is reduced by approximately 15%. The ratio ri/rp of the ionization radius, ri to the pellet radius, rp is shown to be related to the pressure, pe and the equilibrium constant of ionization, Ki(Te) by pe/Ki(Te) of the plasma electrons, and approaches a constant value, (≈ 11), when pe/Ki(te) > 2 × 10−12. For combinations of pellet radius, plasma state and magnetic field strength of interest, irrespective of the ionization process, at an ablated cloud radius r ≈ 2rp, nearly 80% of the ambient electron energy flux is absorbed. Under some situations when there is a lack of balance between the energy input of the plasma electrons and the heating and expansion of the ablatant, a weak shock (flow Mach number, M = 1) can develop near the pellet during the expansion of the ablatant.