# Neutral Beam Coupling with Plasma in a Compact Fusion Neutron Source
## 9 Optimum NB Injection Parameters for FNS-ST Tokamak  
The NB parameters can be optimized for the following target criteria: the NB production efficiency in the injector unit, the injected power density, the beam current drive efficiency in plasma, the overall current drive value and profile, the beam reactivity, the beam-plasma fusion rate, or the total neutron yield. Neutral beam injection can be also optimized for plasma current profile control, as it can directly generate a localized off-axis current.  
For a thin beam model, when the beam cross-section is small compared to the plasma cross-section, the best NBCD efficiency is expected when the fast ion population release and circulation are concentrated in the area where the beam axis is tangential to the magnetic field lines, i.e., the pitch cosine value is maximal. In this case the optimization is simply reduced to adjusting the values of beam energy, axis aiming, and the plasma density along the beam path. However, for a real shape (or thick) beam model, the optimization approach is quite different. This includes the parametric study of the beam driven response in plasma, which is gathered from a great number of beam rays (up to 10\({}^{9}\)); the procedure is repeated for each parameter scanned. The toroidal field topology and the beam shape effects play the major role in beam-driven response, as was described in the previous sections. Our analysis has shown that a deuterium beam with an energy of \(\sim\)150 keV and higher would be more efficient for NBCD and neutron emission in an FNS-ST tokamak, since it creates a fast ion population located closer to the plasma axis, where the ion deceleration path is maximum; however, the NB production technology has low efficiency at the energy range 150-200 keV for the preferable beamline design based on a positive ion source [21].  
The results of NB response analysis are shown in Table 2, where the beam energy and beam aiming radius are used as parameters to scan. The efficiency of axial injection (i.e., aimed tangentially to the major plasma radius, \(\rm R_{t}=R_{0}\)) of a deuterium beam injected at the energy \(\rm E_{b}=150\) keV, is the best both for NBCD and neutron emission. For lower beam energy (or higher plasma density), the beam is mainly captured at the plasma periphery. In this operation mode, the initial orbital losses of fast ions reduce the NBCD essentially (by \(\sim\)75% at \(\rm E_{b}=25\) keV), and therefore it is more efficient to steer the beam outwards from the plasma axis (to the low field side), because the beam initial losses are reduced due to increasing the initial pitch cosine of the FI population. However, for higher neutron emission, the FI deposition should be shifted inside from the plasma axis (as is clear from Figure 23a), where the internal plasma layers with more dense plasma boost the fast ion  
Figure 24: The impact of neutral beam spatial thickness and energy composition on the neutron output in FNS-ST as a function of tritium fraction in DT plasma: deuterium beam (\(\rm E_{b}=140\) keV) is shown green, tritium beam (\(\rm E_{b}=200\) keV) is shown purple. The beam energy fractions are \(\rm E_{full}/E_{1/2}/E_{1/3}=0.6/0.23/0.18\).  
reactivity. We can conclude that the maximum neutron yield from the beam-plasma fusion can be expected for NB intra-axial aiming, although the NB current drive can be reduced down by three times compared to the optimum value.