# Neutral Beam Coupling with Plasma in a Compact Fusion Neutron Source
## 7 Fast Ion Distributions in Energy
Figure 22c illustrates the NBCD scans against tritium fraction in DT plasma target. In the FNS-ST neutron source design, up to 99% of the primary neutrons are expected to come from fusion between the fast beam ions and thermal background plasma, while thermal (cold) plasma is evaluated to produce ~1% of total fusion rate; the thermal share in neutron yield can be boosted by rising the plasma temperature. The fast ion distribution in energy space (Figure 21) allows one to obtain the relative fast ion velocity distribution in plasma, and to calculate the beam-plasma fusion rates. The neutron source operation scenario presumes that the fast ion slowing down time exceeds the plasma energy confinement time; thus, the probability of a fusion act involving a fast ion (which can be called  
Figure 22: Radial plasma profiles (temperature T\({}_{\text{e}}\), density n\({}_{\text{e}}\)), fast ion deceleration time (\(\tau_{\text{s}}\)), ion source intensity, and fast ion current in FNS-ST driven by a thin deuterium beam with different energy E\({}_{\text{b}}\): (**a**) 200 keV; (**b**) 50 keV; (**c**) deuterium (E\({}_{\text{b}}\) = 140 keV) and tritium (E\({}_{\text{b}}\) = 200 keV) beam driven current against tritium fraction in DT plasma, for the NBI power P\({}_{\text{b}}\) = 6 MW. F1 \"reactivity\") exceeds the probability of fusion between thermal plasma ions by factor of 10-100. The neutron emission rate in beam-plasma fusion depends on the hot tail contribution to the EDF, the ion deceleration time, and the target ion density (tritium target for deuterium beam, and deuterium target for tritium beam). Typically, the operation windows for optimum NBCD, and for the beam-plasma fusion, correspond to different parameter ranges, and multi-parametric optimization is needed to find the parameters intervals possible overlaps. The neutron source distribution from a deuterium beam envelope 0.1 \(\times\) 0.3 m\({}^{2}\) (E\({}_{\text{b}}\) = 200 keV) is shown in Figure 23, for different cases of NB steering into FNS-ST. When the beam axis is targeted to the tangential point with a radius less than the plasma major radius (a), the neutron source distribution is most uniform over the plasma core, while outward beam targeting (c) results in neutron emission concentrated in a radial layer. For a beam directed on-axis (b) the neutron source has a peak in the vicinity of magnetic axis. For a neutral beam produced from positive ions and composed of three energy components (E\({}_{\text{full}}\), E\({}_{1/2}\), and E\({}_{1/3}\)), the specific contribution of each beam fraction to neutron yield was evaluated. The beam spatial thickness and the energy composition impact on the neutron output in FNS-ST plasma is shown in Figure 24. The atomic beam with cross-section 0.3 \(\times\) 0.6 m\({}^{2}\) and composed of three fractions produced half the neutron yield compared to the thin mono-energetic beam option (at E\({}_{\text{b}}\) = E\({}_{\text{full}}\)). Figure 23: Neutron source distribution from a deuterium beam 0.1 \(\times\) 0.3 m\({}^{2}\) (E\({}_{\text{b}}\) = 200 keV) for different beam tangential aiming: (a) \(\text{R}_{\text{t}}\) = \(\text{R}_{0}-0.5\text{a}\); (b) \(\text{R}_{\text{t}}\) = \(\text{R}_{0}\); (c) \(\text{R}_{\text{t}}\) = \(\text{R}_{0}+0.5\text{a}\).