Neutral Beam Current Ratio in the Neutron Source FNS-St

The FNS-ST spherical tokamak [1] is of particular interest for development as a neutron source, since it can provide higher steady-state neutron flux densities than classical tokamaks [2].When choosing the device operation parameters, it is assumed that the non-inductive current drive in FNS-ST may be achieved with an energetic neutral beam injection (NBI).The choice of a neutral beam (NB) production scheme is mainly determined by beam energy; neutral beams with energy E b > 140 keV can be more effective in plasma heating and current drive, while they can be efficiently produced only from negative ions.For small tokamak reactors, positive-ion-based injectors with smaller dimensions seem more attractive, thus the lower energy interval of 100-140 keV is considered too, and the final choice of operating energy will be based on the total NBI efficiency optimization.In the framework of this study, injection efficiency was estimated in terms of current ratio, i.e. the ratio between the beam current output in plasma and the beam source current input.The analysis is done by a simple approach (LNB, light neutral beam), used for the direct estimation of fast ion (FI) current.The LNB technique has much in common with the model presented in [3], but allows for NB energy optimization without recourse to any scaling laws.It leverages the analytical expressions from [4] to calculate FI thermalization and phase space distributions, and obtain FI current profiles.It has proved to be applicable to selecting the injected beam energywith account of the NB production scheme, and in agreement with the shine-through losses reduction.