Abstract and subjects
Shock-driven implosions with 100% deuterium (D2) gas fill compared to implosions with 50:50 nitrogendeuterium (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.