Diagnosing reaction-in-flight DT neutrons produced in burning plasmas at the National Ignition Facility*

In the quest to exceed ignition at inertial confinement fusion (ICF) facilities, two critical quantities to diagnose are: 𝛼-heat deposition that can improve, and impurities mixed into the plasma that can limit performance. In high-density, highly-collisional ICF burning plasmas there is a significant probability that deuterium-tritium (DT) fusion products, 14 MeV neutrons and 3.5 MeV 𝛼 particles, will collide with and deposit energy onto surrounding DT fuel ions. These up-scattered ions can then undergo fusion while in- flight and produce higher energy neutrons (15-30 MeV). The resulting reaction-in-flight (RIF) neutrons can be uniquely identified in the measured neutron energy spectrum. The magnitude and shape of this spectral feature can inform on the stopping-power of the DT plasma and hence is directly proportional to 𝛼-heat deposition. In addition, the RIF spectrum can be related to mix into the burning fuel, particularly relevant for high-Z shell and other emerging target platforms at ICF facilities, such as the National Ignition Facility (NIF). The requisites for the neutron time-of-flight diagnostics at the NIF to obtain this small signal, ~10-5 times the primary DT neutron peak, will be discussed. Results from several Gain > 1 implosions will be compared to previous RIF spectra with marginal burn. Finally, comparisons of experimental data to a computational model will be made.