Progress in the Understanding of Primary Neutron Spectra Moments

Recent experiments on NIF and OMEGA have identified the presence of non-Maxwellian ion distributions in the stagnation phase of ICF experiments. The physical mechanisms generating these distributions remain poorly understood. Identification was achieved by measuring the ratio of first and second moments of the time-integrated primary neutron spectra. Theoretical models show that the measured ratio values cannot be produced by plasmas composed of Maxwellian distributions.



This work reports on recent progress in the modelling of spectra moments and the underlying physics. There are two main results. First, further characterization of the non-Maxwellian ion distributions is achieved by calculating moments of orders beyond the first and second. Secondly, the moment method is also incorporated into a time-of-flight analysis to include the effects of temporal variations during the stagnation phase. It is shown that time-of-flight detectors at multiple distances from the target could be used to calculate the correlation of ion temperature with time. This correlation can be used as a metric of implosion performance.