# 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 choice of NB production technology and the beamline design depends on the energy range of the particles that can be transmitted to plasma, ensuring the efficient CD and neutron yield. The injected beam energy should be also consistent with the beam species mass (e.g., hydrogen, deuterium, or tritium), and with the plasma content. The cross-section of beam particle ionization in plasma depends on its relative velocity over the target plasma species. It naturally implies similar beam ionization profiles for different hydrogen isotopes when their energy ratio corresponds to their mass ratio. The atomic beam is produced by extraction of ions, their acceleration to high energy and neutralization on a gas target, while other neutralization techniques are still not elaborated enough to be widely applied. Depending on the required beam energy, there are two possible design solutions for NBI [21]. For energies below 75 keV/nucleon (150 keV in deuterium), a positive ion-based technique is more efficient, while for higher beam energies, negative ion-based solutions are preferable. The neutralization yield per ion current normalized to a source current of 1 A in the \"positive\" scheme decreases rapidly with energy, so NB production with Eb > 75 keV/nucleon from positive ions is associated with high power losses (~70% of ions to waste), although this energy level is better for tangential beam penetration into FNS-ST dense plasma (densities n\({}_{\rm e}\)\(\sim\)1\(\cdot\)10\({}^{20}\) m\({}^{-3}\)). An alternative injector scheme based on negative ions is more attractive for the energy range 100-500 keV/nucleon, with high neutral yield (~60%), which is almost independent of the ion energy. \"Negative\" beamlines are more efficient in beam transmission to plasma, since a negative ion beam has lower angular dispersion compared to a positive ion beam. However, the positive ion current density is significantly higher than the current density achieved for negative ion sources, so the positive beamline design is typically much more compact, the injector technology is better understood and widely implemented, and the positive ion scheme is preferential even for the beam energy falling within the \"transition\" range E\({}_{\rm b}\) = 75-100 keV/nucleon (where the neutralization efficiency drops to 20-30%). A detailed NBI design schemes comparison for FNS-ST [21] resulted in the following conclusions:  
* Regardless of the beamline design, the injector chamber should be mounted without inclination, i.e., horizontally, since the mass of the chamber with the beamline components is dozens of tons, and their installation on an inclined platform with proper adjustment with further remote maintenance is almost impossible. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value/Range** \\ \hline Aspect ratio R/a, m/m & 0.5/0.3 \\ \hline Elliptic elongation k & \(\sim\)3 (2.75) \\ \hline Toroidal magnetic field, T & 1.5 \\ \hline Electron temperature T\({}_{\rm e}\), keV & 1–5 \\ \hline Average plasma density n\({}_{\rm e}\), 10\({}^{20}\) m\({}^{-3}\) & 0.5–1 \\ \hline Plasma current I\({}_{\rm p}\), MA & 0.8–1.5 \\ \hline NB tangential injectors number & 3–4 \\ \hline NB total power P\({}_{\rm b}\), MW & 6–10 \\ \hline NB port size W \(\times\) H, m\({}^{2}\) & 0.3 \(\times\) 0.6 (0.1 \(\times\) 0.3) \\ \hline D\({}^{0}\)/T\({}^{0}\) atoms energy, keV & 100–200 \\ \hline Ion source current, A & 80 \\ \hline NB injection current I\({}_{\rm[lin]}\), A & 30–40 \\ \hline Axis inclination angle, degree & 0–30 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: FNS-ST tokamak design parameters. * The \"negative\" scheme has a high efficiency in power, which is practically independent of the beam energy. The efficiency of beam transmission (for nominal NBI port dimensions) is high enough; for residual ions deflection and dumping (RID), and the negative scheme applies an electrostatic device (ERID) of fairly compact size.