Effect of ion speed distribution on the spectral shape of the 2.5-MeV neutron line produced by DD fusion

Ion mean energy may be determined from Doppler broadening of the 2.5-MeV neutron line in Maxwellian deuterium plasma. Since many plasmas are non-Maxwellian a Monte-Carlo simulation has been used to predict the spectral shape produced by a variety of simple speed distributions with mean energies in the range 1–100 keV. In the case of isotropic ions considered here, the results are Gaussian to high order and their width varies as the square root of the mean ion energy. However, the linewidth varies substantially among different ion speed distributions with the same mean energy. The similarity in shape of these spectra prevent the determination of the speed distribution type from detailed measurements of the neutron spectrum, unlike the situation where co-counter beams produce anisotropic ion distributions.