Modification of neutron emission spectra and determination of fuel ion ratio in beam-injected deuterium-tritium plasma

To stably control and operate a deuterium-tritium (DT) fusion reactor, it is important to accurately obtain the fuel ion ratio n_t/n_d (n is the number density). Previously, fuel ion ratio diagnostic methods using deuteriumdeuterium (DD) and DT neutron emission rates have been studied. The reaction rate coefficients and neutron emission spectra, i.e., the shape and peak value, are strongly influenced by external plasma heating such as neutron beam injection (NBI) heating. In this paper, we consider the fuel ion ratio diagnostics by directly measuring the neutron emission spectra, including both beam-thermal and beam-beam fusion reactions, in deuterium beaminjected DT plasma. We also evaluate the slowing-down neutron component, i.e., noise, to examine whether the above method can be used for plasma diagnostics. On the basis of Boltzmann-Fokker-Planck and Monte Carlo neutron transport analyses, the applicability of the method to the beam-injected DT plasma is discussed. It is shown that the feasible plasma parameter regions of measuring for fuel ion ratio diagnostics is increased by beam injection and by adopting neutron detector channels shifted to the high-energy side.