Exploration of Non-Maxwellian Effects of Alpha Heating on Neutron Spectra Using the Vlasov-Fokker Planck code iFP

In nuclear fusion reactions, a set amount of energy is liberated and distributed amongst the products in a manner dictated by kinematics, momenta and energies of the fusion reactants. Within a plasma, the reactants take on a distribution of velocities, affecting the width and peak location of the neutron energy spectra produced in both DT and DD fusion reactions. For a Maxwellian plasma, the relationship between neutron-energy peak location and peak width forms a concave-down curve. Even for nonuniform plasmas, any observed neutron spectra from a plasma well-described by hydrodynamics must be an average of points along the curve and thus lie below the curve. However, recent work in analyzing the neutron spectra of high yield NIF shots has revealed that the DT neutron data lie significantly above the curve when alpha heating is considerable. In this work, the Los Alamos developed kinetic code iFP is utilized to explore the effect of alpha heating on neutron spectra. With the help of iFP’s fully kinetic burn model and neutron spectrum calculator, the effect of small angle coulombic alpha collisions on neutron spectra is investigated as a potential source of the anomaly.