Effects of atomic processes on fuel pellet ablation in a thermonuclear plasma

Abstract

The neutral-ablation model describing ablation of solid hydrogen fuel pellets in thermo-nuclear plasmas has been extended to determine the species composition of the ablatant consistent with the fluid profiles, and has been used to assess pellet shielding mechanisms. Aside from neutral-ablation shielding, none of the shielding mechanisms studied is very effective. Dissociation of hydrogen molecules extends pellet lifetimes by 10-20%. Ionization reduces the temperatures in the ablation by about a factor of three, but extends pellet lifetimes by only a few per cent. Excitation of atoms is not important because the fraction of excited atoms does not exceed a few per cent anywhere in the ablation. Bremsstrahlung and recombination radiation are negligible energy losses. Where emitted abundantly, line radiation is trapped within the optically thick ablatant. Incident electron heat is reduced by less than 10% in overcoming the electric potential of the pellet. Magnetic shielding is treated elsewhere.

Two-dimensional simulation of pellet ablation with atomic processes

“…In keeping with Ref. 19, the CAP code finds that the energy sinks made available by the latent heats of dissociation and ionization lead to only fractionally lower ͑ϳ10%͒ ablation rates.…”

“…19, and generalizes the validity of the quasisteady assumption under a wider set of plasma-pellet parameters. We carried out simulations over a nominal parameter range in order to compare the ablation rates between the cases without (G iso w/o ) and with (G iso with ) atomic processes.…”

“…Instead, the ablation flow evolves towards a quasistationary ''double-transonic'' flow solution with a shock discontinuity, and thus the flow singularity encountered in Ref. 19 is resolved. Then, the CAP code reinforces the validity of the quasisteady approach pertaining to the inertially dominated part of the flow domain.…”

“…The code has been first benchmarked with the results of the previous 2D code 11 and good agreement was obtained for the ablation rate G. Dissociation and ionization in the ablation flow can also be treated by using the assumption of local thermodynamic equilibrium ͑LTE͒ as in Felber et al, 19 who included atomic processes in the original TF model. In keeping with Ref.…”

“…19 Reference 19 also found that when the pellet is subjected to high heat fluxes, the Mach number can dip below unity, i.e., the flow becomes singular preventing a continuation of the exterior flow solution beyond that point. We have found that the Mach number never dips below unity.…”