Fusion Neutron Energy Spectrum Measurements in Kinetic Plasmas

The first and second moments of the primary deuterium-tritium (DT) and deuterium–deuterium (DD) fusion neutron energy spectrum generated by thermonuclear plasmas with apparent ion temperatures between 2 and 18 keV have been measured on the OMEGA 60 Laser. For the low-temperature, more-hydrodynamic-like plasmas, both the DT and DD neutron energy spectrum measurements are consistent with predictions from hydrodynamic plasma models, which assume the plasma has a Maxwellian ion velocity distribution. For the high-temperature, more-kinetic-like plasmas, the DD neutron energy spectrum measurements are inconsistent with the hydrodynamic plasma model predictions. Post-shot Vlasov–Fokker–Plank (VFP) simulations predict the presence of a bimodal ion velocity distribution near peak neutron production in the high-temperature experiments, which causes the neutron energy spectrum emitted from these plasmas to deviate from the hydrodynamic, Maxwellian plasma predictions. The bimodal distribution is caused by a large counter-streaming ion population, which forms as the diffuse shock front converges in these implosions. The DD neutron energy spectrum measurements are consistent with the VFP simulation results and provide evidence of a non-Maxwellian ion velocity distribution in these thermonuclear plasmas. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.