Effect of thermal ³He minorities on knock-on tail formation and the resulting neutron emission spectrum modification in deuterium-tritium plasmas

The knock-on tail formations in fuel-ion velocity distribution functions by energetic alpha particles (by the T(d,n)⁴He reaction) and protons (by the D(d,p)T and ³He(d,p)⁴He reactions) are investigated by simultaneously solving the Boltzmann-Fokker-Planck (BFP) equations for deuteron, triton, ³He, alpha particle and proton in an ITER-like (DT) plasma admixed with a small amount of ³He. As a result of the ³He inclusion, a fraction of the transferred energy from energetic ions to thermal deuterons and tritons via nuclear plus interference (NI) scattering is reduced. Owing to the NI scattering of the energetic protons by fuel ions, the latter are knocked up to higher energies. The knocking-up effect of fuel ions is enhanced with increasing ³He concentration. It is shown that if ³He with relative concentration of 4.2%, i.e.,n3_He/n_e, is included in T_e = 20 keV, n_e = 9.5 × 10¹⁹ m^-3 plasma, the magnitude of the knock-on tail in deuteron distribution function in 300 keV-3 MeV energy range is reduced by about 15% from the value when ³He is not externally supplied. Such knock-on tail reduction also results in alternation of the non-Gaussian neutron emission spectrum with energies less than ∼13 MeV and above ∼15 MeV.