On the origin and nature of the alpha knock-on tail in neutron spectra from ignited implosions at the NIF

In Inertial confinement fusion (ICF) experiments at the National Ignition Facility (NIF), the energy deposited by α-particles, generated by deuterium–tritium (DT) fusion reactions, play an essential role in heating the hot spot to achieve ignition, burn propagation, and energy gain. When these α-particles interact with the thermal D and T fuel ions, both via Coulomb and nuclear-elastic scattering, they transfer energy and generate supra-thermal populations of D and T ions, which in turn can react with thermal ions and generate an alpha knock-on (AKN) component in the neutron spectrum. This component is observed in neutron spectra, measured with the MRS, from ignited implosions at the NIF. The magnitude of the AKN component relative to the primary neutron yield, called AKN ratio, provides a direct assessment of α-heating of the fuel ions. Data indicate that the AKN ratio scales approximately with T_e³, which is in agreement with analytical scaling. The MRS data also indicate that the slope (-1/λ) of the AKN high-energy tail scales with areal density (ρR) as 1.7×ρR, which illustrates that λ increases with increasing ρR (and density) because of an enhanced slowing down of alphas and supra-thermal D and T ions, while the observed T_e variations have an insignificant impact on λ. This observation, which is in qualitative agreement with theory, is the first observation of how ρR (and density) impacts λ. In addition, the hot-spot-size and burn-duration data, when applied to existing burn-wave models, indicate that the burn-wave propagating through the surrounding main fuel is supersonic in most cases. On the basis of the observed fusion yields, the fuel burn-up fractions differ from the simple 1D description of the fuel burn-up fraction based on measured ρR, which is currently under investigation.



This work was supported by LLNL under Contract No. B656484.