# Neutral Beam Coupling with Plasma in a Compact Fusion Neutron Source
## 2 NBI Scheme for FNS-ST and the Source Beam Structure
### NBI Options and Preferences
* The NBI negative scheme weak points included a low current density extracted from the ion source (-20 mA/cm\({}^{2}\)), a \"halo\" fraction (-15%) with higher angular spread which accompanies the beam core current and thus is fully intercepted by the beamline components, and plasma emitter non-uniformity across large surface of ion source, which can affect the overall beam transmission. A negative scheme is supposed to be more efficient for NBI energy E\({}_{\text{b}}\)\(>\) 75 keV/nucleon. * A positive beamline is preferential for NBI energy E\({}_{\text{b}}\)\(<\) 75 keV/nucleon. A higher current density (200 mA/cm\({}^{2}\)) is extracted from the beam source and thus lowers the emitter area required. A compact neutralizer can be attached directly to the ion source exit, which leads to lower gas flows and moderate cryopanels loading. Positive scheme NBI performance in power is almost a half of the negative scheme. Additionally, the positive ion source beam composition includes molecular ion fractions, which produce lower energy atoms in dissociation and neutralization (i.e., E\({}_{\text{b}}\)/2 and E\({}_{\text{b}}\)/3). These ion fractions are captured by the periphery tokamak plasma and merely drop out from current drive and neutron generation. We also note, that in a positive beamline scheme, the beam current density and volume charge are high enough, so electrostatic RID cannot be applied. Instead, a massive magnetic ion dump is used, with a large panel size capable of intercepting all the energy fractions of residual ions. For E\({}_{\text{b}}\)\(\sim\)140 keV, the residual ion power can be twice as high as the effective beam power in neutrals, so the magnetic RID appears to be the most power loaded component of the beamline. The injection port size forms a major restriction for NB power input, especially for a small tokamak operating at high neutron flux from the plasma core. The port minimum size is designed accounting for the ion source current density, the expected angular width and beam focusing methods. The design solutions also depend on the power load study for all the beamline surfaces due to beam direct interception and re-ionization, and the loads should be within the heat removal constraints along the beam ducts region. Both beamline schemes (positive and negative) were considered for the window port size justification [21] in FNS-ST tokamak. A parametric analysis of the beam power losses resulted in an injection port size (W \(\times\) H) of 30 \(\times\) 60 cm\({}^{2}\), when almost 80% of NB power is transmitted to plasma, in case of optimum (ideal) beam focusing. Since this port size is equal to the plasma minor radius (0.3 m), its reduction, at least down to 0.2 \(\times\) 0.4 m\({}^{2}\), is also discussed. As a result, for the NB injection to FNS-ST plasma, with a required neutral power range of 6-10 MW, well-developed beamline technology based on positive ions was chosen as the preferred solution [21]. The NBI design consists of four injector units. Three units are in operation, one is for maintenance and regeneration, and each injection unit delivers 2-3.5 MW to plasma in deuterium atoms with an energy of E\({}_{\text{b}}\) = 100-140 keV. The NBI unit layout is shown in Figure 1.  
Figure 1: FNS-ST neutral beam injector setup with D+ ion source (taken from [21]). The main components of each NBI unit are:  
* A positive ion source-accelerator (ISA), with a plasma emitter area of 18 \(\times\) 115 cm\({}^{2}\) and a four-grids ion-optical acceleration system. Each grid consists of five sections, with 80 beam apertures.