Effect of thermal³He minorities on knock-on tail formation and the resulting neutron emission spectrum modification in deuterium–tritium plasmas

Abstract

The knock-on tail formations in fuel-ion velocity distribution functions by energetic alpha-particles (by the T(d,n) 4 He reaction) and protons (by the D(d,p)T and 3 He(d,p) 4 He reactions) are investigated by simultaneously solving the Boltzmann-Fokker-Planck (BFP) equations for deuteron, triton, 3 He, alpha-particle and proton in an ITER-like deuterium-tritium (DT) plasma admixed with a small amount of 3 He. As a result of the 3 He inclusion, 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 latters are knocked up to higher energies. The knocking-up effect of fuel ions is enhanced with increasing 3 He concentration. It is shown that if 3 He with relative concentration of 4.2 %, i.e. n 3He /n e = 0.042, is included in T e =20 keV, n e =9.5×10 19 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 3 He 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.

Knock-on Tail Formation Due to Nuclear Elastic Scattering and Its Observation Method Using <i>γ</i>-Ray-Generating ⁶Li+d Reaction in Tokamak Deuterium Plasmas

“…In such a case, NES effects on slowing down of energetic ions would not be negligible compared with those due to Coulomb collisions. We have evaluated the NES effects on burning plasma properties for several situations that energetic ions exist [5][6][7], on the basis of the Boltzmann-Fokker-Planck (BFP) analysis model [5,[8][9][10]. Many calculations have predicted that NES accelerates slowing-down process of fast ions and enhances the fractional energy deposition from fast to bulk ions [11][12][13].…”

“…In this paper, we focus our attention on a verification scenario of knock-on tail formation using 0.48-MeV γ-rays by 6 Li(d,p) 7 Li*, 7 Li*→ 7 Li+γ reaction [19][20][21][22] in a deuterium plasma. In Fig.…”

“…In Fig. 1 the cross section of the 6 Li(d,pγ) 7 Li reaction is shown as a function of the relative energy of reactants. The cross section rapidly increases with increasing relative energy.…”

“…The cross section rapidly increases with increasing relative energy. Owing to the existence of small fraction of knock-on tail component, "velocity-averaged" 6 Li(d,pγ) 7 Li reaction rate coefficient would be significantly influenced. It is expected that by using the γ-raygenerating reaction for various plasma conditions, we can quantitatively estimate the magnitude of the knock-on tail.…”

“…We consider the knock-on tail in deuteron velocity distribution function by energetic protons in proton-beaminjected deuterium plasmas. Previously we roughly evaluated the 6 Li(d,pγ) 7 Li reaction rate coefficient assuming a uniform plasma [16]. In this paper we develop more 6 Li(d,pγ) 7 Li cross section as a function of relative energy [19][20][21][22].…”