## 6 Conclusions  
A consistent approach was taken in simulations of the operating regimes of the FNS-ST facility. The simulations were performed using the SOLPS4.3 and ASTRA codes for different bulk plasma densities and diffusion particle diffusivities. For the cases of using the heating beams consisting of D + T or pure D atoms at the compact neutron source FNS-ST, the dependence of the FNS neutron yield on the fraction of tritium in the bulk plasma was analyzed for the first time. The consistent modeling using the FC-FNS code made it possible to calculate the required fluxes of the fuel components into the plasma, supplied by different injection systems (pellet injectors and fast atomic beams). For both types of the heating beams, in the window of the working parameters \(n_{e}\) and \(D/\chi_{e}\), the maximum possible fraction of tritium in the bulk plasma was calculated, at which all deuterium particles were injected into the plasma with the beam. For the scenarios of using the D + T or D beams, the neutron yields were calculated as functions of the \(n_{e}\) and \(D/\chi_{e}\) parameters. For the regimes with the maximum neutron yields, the accumulation of tritium at the site (in the fuel cycle) was calculated for different types of heating beams. It was shown that for the D + T beams, the maximum neutron yield of 4.5-5.5 \(\times\) 10\({}^{17}\) s\({}^{-1}\) could be obtained at the bulk plasma density of \(n_{e}\) = 7.0-8.5 \(\times\) 10\({}^{19}\) m\({}^{-3}\). The maximum neutron yields were reached in the range of \(D/\chi_{e}\) = 0.2-0.4, corresponding to the good confinement of particles in the plasma. In this case, the amount of tritium accumulated in the FC of the facility is expected to range from 260 to 370 g, including up to 150 g accumulated in the neutral beam injectors. For the D beam, the maximum neutron yield of 4.5-5.5 \(\times\) 10\({}^{17}\) s\({}^{-1}\) corresponded to the density range of \(n_{e}\) = 7.0-10.0 \(\times\) 10\({}^{19}\) m\({}^{-3}\) in the regimes with not very good particle confinement, \(D/\chi_{e}\) = 0.4-0.6. The expected amount of tritium in the FC ranged from 90 to 200 g. At the same time, in the range of the tritium fraction \(f_{core}{}^{T}(max)\) < 0.5, the use of the D beam may result in low neutron yields (2-4 times lower than the maximum one), although it provides the minimum tritium content at the facility site. Based on the above estimates, we conclude that in the density range of \(n_{e}\) = 7.0-10.0 \(\times\) 10\({}^{19}\) m\({}^{-3}\), the use of the D beam is more reasonable. First of all, this is due to the lower tritium inventory in the FC, technical advantages associated with the absence of tritium in the injectors, and the corresponding less strict safety requirements for the operation of the injection system for plasma heating. In this case, the neutron intensities comparable in magnitude with the maximum ones (which is the priority issue for the fusion neutron source) for the D + T beam can be obtained in a wider range of plasma densities. We note that the range of parameters favorable for using the D beam corresponds to somewhat worse confinement of particles in the bulk plasma, which reduces the D ion density in the core and, correspondingly, increases \(f_{core}{}^{T}(max)\). **Author Contributions:** FC-FNS calculations, S.A.; ASTRA calculations, A.D.; SOLPS4.3 calculations, A.K. ; writing--original draft preparation, S.A.; writing--review and editing, S.A., A.D. and A.K. ; project administration, S.A. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was supported in part by the Russian Science Foundation (project no. 18-72-10162). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable.