Impact of mid- Z gas fill on dynamics and performance of shock-driven implosions at the OMEGA laser

Shock-driven implosions with 100% deuterium (D₂) gas fill compared to implosions with 50:50 nitrogen-deuterium (N₂D₂) gas fill have been performed at the OMEGA laser facility to test the impact of the added mid-Z fill gas on implosion performance. Ion temperature (Tᵢₒₙ) as inferred from the width of measured DD-neutron spectra is seen to be 34%±6% higher for the N₂D₂ implosions than for the D₂-only case, while the DD-neutron yield from the D₂-only implosion is 7.2±0.5 times higher than from the N₂D₂ gas fill. The Tᵢₒₙ enhancement for N₂D₂ is observed in spite of the higher Z, which might be expected to lead to higher radiative loss, and higher shock strength for the D₂-only versus N₂D₂ implosions due to lower mass, and is understood in terms of increased shock heating of N compared to D, heat transfer from N to D prior to burn, and limited amount of ion-electron-equilibration-mediated additional radiative loss due to the added higher-Z material. This picture is supported by interspecies equilibration timescales for these implosions, constrained by experimental observables. The one-dimensional (1D) kinetic Vlasov-Fokker-Planck code ifp and the radiation hydrodynamic simulation codes hyades (1D) and xrage [1D, two-dimensional (2D)] are brought to bear to understand the observed yield ratio. Comparing measurements and simulations, the yield loss in the N₂D₂ implosions relative to the pure D₂-fill implosion is determined to result from the reduced amount of D₂ in the fill (fourfold effect on yield) combined with a lower fraction of the D₂ fuel being hot enough to burn in the N₂D₂ case. The experimental yield and Tᵢₒₙ ratio observations are relatively well matched by the kinetic simulations, which suggest interspecies diffusion is responsible for the lower fraction of hot D₂ in the N₂D₂ relative to the D₂-only case. The simulated absolute yields are higher than measured; a comparison of 1D versus 2D xrage simulations suggest that this can be explained by dimensional effects. The hydrodynamic simulations suggest that radiative losses primarily impact the implosion edges, with ion-electron equilibration times being too long in the implosion cores. The observations of increased Tᵢₒₙ and limited additional yield loss (on top of the fourfold expected from the difference in D content) for the N₂D₂ versus D₂-only fill suggest it is feasible to develop the platform for studying CNO-cycle-relevant nuclear reactions in a plasma environment.