# Neutral Beam Coupling with Plasma in a Compact Fusion Neutron Source
## 10 Conclusions and Outlook  
To study the geometry impact on NB response and performance in tokamak plasmas, we propose a light-weighted technique of multi-ray beam tracing (LNB), which strikes a balance between the best features of the advanced NBI models, including high accuracy, flexibility, almost real-time performance, and the ability to be integrated into a calculation workflow. The LNB is implemented in BTR-BTOR software dedicated to NBI design and beam-plasma interaction in tokamaks. The approach was applied to study the beam-plasma effects in a compact neutron source based on an FNS-ST spherical tokamak with a special focus on the toroidal field and the neutral beam shaping issues. The LNB model of NB injection comprised detailed beam statistics (up to 10\({}^{9}\) particles) and evaluated the beam losses and performance in plasma by implementing straightforward and efficient methods of massive data processing. LNB analysis of FNS-ST tokamak clearly revealed that fast ion distribution in plasma and the overall NBI performance should be sensitive to beam-plasma geometrical aspects, including the plasma core toroidal aspect ratio, beam envelope geometry, beam angular divergence and axis aiming. While our approach is applicable to NBI optimization and coupling in conventional tokamak systems, shaping issues have proved to be more pronounced and essential for low aspect tokamak design, and in particular for smaller spherical systems such as FNS-ST. For a compact tokamak design, the NB optimization procedure is generally more complex due to the energy composition of the injected beam, which results in beam-plasma interaction splitting into several channels according to the beam composition. Each beam fraction produces its own fast ion population in plasma, and these populations interact in a different manner with the toroidal magnetic field and with plasma background. The optimal interval of NBI parameters is a natural trade-off between the beam costs and obtained performance. The neutral beams with lower energy are produced with much higher efficiency than those with higher energy, i.e., they are cheaper in power. However, their output in plasma in terms of current drive and neutron yield is essentially reduced, so the overall NBI efficiency is less when compared with more expensive higher energy beams. The NB performance study reveals the following issues to be addressed in a neutral beam coupling with target plasma, which applies to any beam-driven tokamak:  
* The NB production scheme depends on the injected energy range for efficient power transmission to plasma. The energy is consistent with the beam species mass, since \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Aiming Radius, m** & **Shine-Through Losses, \%** & **Fast Ions Orbital** & **Current Drive per \(\mathbf{P_{inj}}\)** & **Neutron Yield per \(\mathbf{P_{inj}}\) = 1** \\ \hline \multicolumn{5}{c}{**Beam Energy \(\mathbf{E_{b}}\) = 150 keV**} \\
0.35 & 1.6 & 42 & 134 & 1.06 \(\times\) 10\({}^{17}\) \\
0.5 & 2.8 & 29 & 170 & 1.07 \(\times\) 10\({}^{17}\) \\
0.65 & 9.4 & 12 & 151 & 8.3 \(\times\) 10\({}^{16}\) \\ \multicolumn{5}{c}{**Beam energy \(\mathbf{E_{b}}\) = 100 keV**} \\
0.35 & 0.32 & 51 & 88 & 5.75 \(\times\) 10\({}^{16}\) \\
0.5 & 0.7 & 36 & 126 & 5.71 \(\times\) 10\({}^{16}\) \\
0.65 & 3.9 & 14 & 140 & 4.66 \(\times\) 10\({}^{16}\) \\ \multicolumn{5}{c}{**Beam energy \(\mathbf{E_{b}}\) = 50 keV**} \\
0.35 & 10-4 & 73 & 28 & 7.1 \(\times\) 10\({}^{15}\) \\
0.5 & 0.01 & 56 & 50 & 6.9 \(\times\) 10\({}^{15}\) \\
0.65 & 0.3 & 20 & 96 & 6.2 \(\times\) 10\({}^{15}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Fast deuterium beam losses and performance in FNS-ST plasma \(\mathbf{B_{0}}\) = 1.5 T, \(\mathbf{T_{e}}\) = 5 keV, \(\mathbf{n_{e}}\) = 10\({}^{20}\) m\({}^{-3}\), beam cross-section 0.1 \(\times\) 0.3 m\({}^{2}\). the beam penetration and capture by plasma are mainly governed by the beam particles velocity.