Diagnosing hot-spot ρR and asymmetries in ICF implosions at the National Ignition Facility using directional secondary DT-neutron spectra

Obtaining a fundamental understanding of the impact of fuel-ablator mix, drive asymmetries and capsule-engineering features on the performance of an Inertial Confinement Fusion (ICF) implosion is essential to the Program at the National Ignition Facility (NIF). Traditionally, x-ray techniques have been used to probe these phenomena through measurements of the hot-spot shape. A new window on hot-spot ρR and asymmetries has been opened through measurements of directional secondary DT-neutron spectra. In contrast to the x-ray image that reflects the plasma electron profile, the yield of the secondary DT neutrons reflects triton's average range in the deuterium plasma. As these techniques have different sensitivities to fuel-ablator mix, drive asymmetries and capsule-engineering features, they are very complementary. In this groundbreaking work, it is demonstrated that absolute secondary-DT neutron spectra provide accurate information on the convergence ratio (CR) that correlates with the true convergence of an implosion. It was also shown that the secondary-DT-neutron-derived CR properly captures the deleterious effects of drive asymmetries and engineering features. The CR inferred from x-ray imaging techniques was shown to respond differently to these phenomena, albeit highly correlated with the CR inferred from secondary DT-neutron technique. This is most likely an indication the ablator gets mixed into the fuel when the implosion is asymmetric causing increased brightness in the central region. In addition, it was shown that the relative widths of directional secondary-DT neutron spectra are strongly correlated with a mode-2 asymmetry of the hot spot, which is supported by multi-dimensional simulations. This work was supported in part by the US DOE, LLE, LLNL and DOE NNSA COE.