# Neutral Beam Coupling with Plasma in a Compact Fusion Neutron Source
## 10 Conclusions and Outlook
* The neutral beam power which can be injected to the tokamak is generally reduced not only by the injection port dimensions but also by the beam source current density and the accelerated beam angular dispersion. When the NB production scheme is chosen, the total beamline geometry needs to be optimized to deliver maximum power to plasma. * The neutral beam direct losses in plasma include the shine-through power, which should be evaluated within the entire range of beam-plasma operation scenarios, including the cases with reduced plasma density and current drops. * The initial beam losses in plasma also include the fast ion Larmor excursion beyond plasma LCMS bound, as well as their probability to be captured to the trapped \"mirror\" orbits. * The real beam shape, which includes the beam envelope dimensions and the internal angular distribution, essentially affects the beam footprint and resulting plasma response when compared to simplified beam geometries like \"thin\" and/or \"parallel\" beam models. The reduced beam models can be reasonably applied only to beam-driven tokamaks with an expected small contribution from NB to current drive and neutron generation. * The beam contribution to neutron generation is highly sensitive to the plasma temperature due to the \"hot tail\" growth with temperature in fast ion energy distributions. * While the beam driven current is known to be highly dependent on the plasma temperature and density, the fast ion current in low aspect tokamaks can be essentially reduced by the toroidal effects, which are expressed in higher initial fast ion losses and in the toroidal velocity variation (bouncing). * For a multi-component neutral beam, a specific contribution of each beam fraction needs to be evaluated. Together with the beam actual shape, the beam species composition may reduce the expected NB efficiency down by approximately two times (as shown in FNS-ST case);
* The optimum beam parameters, when evaluated for different criteria, typically do not coincide. For example, in our FNS-ST study, the NB axis optimum tangential aiming for the current drive and for the neutron generation appeared to be located at the opposite sides from the plasma magnetic axis. Conceptualization, E.D. ; Data curation, A.P. ; Methodology, E.D. ; Project administration, B.K. ; Resources, A.S.; Software, E.D. ; Supervision, B.K. and A.S.; Validation, A.P. ; Visualization, E.D. ; Writing--original draft, E.D. ; Writing--review & editing, E.D. and A.P. All authors have read and agreed to the published version of the manuscript. This work was partially supported by NRC Kurchatov Institute. Not applicable. The work was partially supported by the National Research Center \"Kurchatov Institute\". Software testing was performed using the computing resources of the federal collective usage center Complex for Simulation and Data Processing for Mega-science Facilities at NRC \"Kurchatov Institute\", [[http://ckp.nrcki.ru/](http://ckp.nrcki.ru/)]([http://ckp.nrcki.ru/](http://ckp.nrcki.ru/)) (accessed on 23 February 2022). The authors declare no conflict of interest.