Prediction and analysis of neutron spectra in the burn propagation regime*

A novel set of neutron spectral features have been observed during high yield fusion experiments at the National Ignition Facility (NIF). These features span the full energy range of neutrons, from both primary fusion reactions, and down- and up-scattering events. By understanding the response of the neutron spectrum to the effects of fusion burn, we can use these features to diagnose burn propagation. In this work, we first describe modelling of the neutron spectrum emitted from burning plasmas using a combination of radiation-hydrodynamics and detailed neutron/fast-ion transport calculations. We compare the synthetic neutron spectra with measurements from the NIF neutron time of flight detector suite. In particular, we focus on the backscattered neutrons (En ~ 3.5 MeV) and the high energy neutrons (En > 15 MeV), produced by neutron and alpha up-scattering of fuel ions. The physical processes that produce these neutrons give these features unique sensitivities to plasma conditions (density, temperature, velocity). Exploiting these sensitivities will allow us to learn more about the progression of fusion burn at high yield.