Neutron-source fidelity for laser-driven D--D lithium-blanket tritium-breeding tests

Compact deuterium–deuterium (D–D) neutron sources can provide controllable irradiation fields for lithium-blanket studies, although their broad joint energy and angle distributions differ from the conventional $2.45$~MeV isotropic representation. We couple particle-in-cell (PIC) simulations of target-normal-sheath-accelerated deuterons with a thick-target $D(d,n{)}^3$He source model and Monte Carlo neutron transport. For natural lithium, the seven two-dimensional sources change tritium production per source neutron by $-2.5\mathrm{\%}$ to $+54.1\mathrm{\%}$ relative to the ideal source. The matched three-dimensional calculation gives an increase of $43.5\mathrm{\%}$ and lowers the corresponding ratio from $1.5406$ to $1.4350$. Source substitutions show that the difference is predominantly spectral, since the real spectrum alone gives a factor of $1.4199$, while using the real neutron emission directions in place of isotropic emission adds only a further factor of $1.0106$ in the three-dimensional case. The real spectrum lowers the ${}^6$Li contribution by $6.9\mathrm{\%}$, but the accessible ${}^7$Li$(n,Xt)$ response exceeds this loss. Enrichment to $90\mathrm{\%}$ ${}^6$Li keeps the total change within $\pm 1.5\mathrm{\%}$. In the matched three-dimensional converter and blanket calculation, direct $D(d,p)$T production is $0.8458$ tritons per source neutron and accounts for $98.1\mathrm{\%}$ and $86.9\mathrm{\%}$ of the combined production for natural and enriched lithium, respectively. High-density polyethylene moderation raises tritium production by about one order of magnitude but first weakens and then reverses the increase in blanket tritium production. The analysis quantifies source-model effects in compact breeding tests.