Diagnosing Ion Distributions using Primary Neutron Spectra

Recent measurements of primary neutron spectra on the OMEGA and NIF facilities have demonstrated unexpected features - the ratio of first to second spectral moments was higher than expected for hydrodynamic plasma in which the reacting ions have a Maxwellian distribution. These observations occurred in different plasma regimes. For NIF, the observation occurred in layered cryogenic DT experiments in which a burning plasma was achieved. For OMEGA, the observation occurred in low pressure, gas-filled, glass capsules.

This work investigates the feasibility of using primary neutron spectra measurements to identify the form of ion distribution functions in ICF plasmas. The iFP code is used to simulate OMEGA experiments. This shows ions accelerated by the shock create non-Maxwellian ion distributions that produce the observed spectra. These distributions are anisotropic in velocity space. However, the observed ratio of moments could also be obtained from isotropic, non-Maxwellian distribution functions. In contrast, the observed ratio of spectral moments from the NIF experiments can only be obtained from anisotropic, non-Maxwellian ion distribution functions. This result provides a stricter test for hypotheses seeking to explain the physical cause of the observed spectra.