# Neutral Beam Coupling with Plasma in a Compact Fusion Neutron Source
## 5 Neutral Beam Shine-Through Power  
Shine-through beam losses and the associated thermal load onto the first wall serve as severe restrictions for the injected beam energy choice for efficient current drive and enhancing nuclear fusion in any tokamak. Figure 10 is an illustration that fast ion deposition within the plasma inner core, which is most favorable for current drive, is a result of a sufficient atom penetration depth into plasma. On the other hand, the beam deep dive to plasma core also means a higher fraction of non-ionized atoms, and thus raises the risk for the tokamak chamber first wall to be burnt by the residual neutral power. For the FNS-ST tokamak, a quite moderate range of beam energies of 100-150 keV is not the best for current drive and neutron generation, but this energy level ensures safer tokamak operation compared to a more efficient beam energy range above 200 keV. Maximum NBCD efficiency in tokamak is achieved normally in the diapason of energy ratio \(\rm E_{\rm b}/E_{\rm c}=2\)-5 [5], which implies the optimal beam energy for \(\rm T_{\rm e}=5\) keV should be 200-500 keV, which would be not acceptable in terms of the direct power densities faced by the first wall. The beam ionization deposition in FNS-ST plasma (thick beam 30 \(\times\) 60 cm\({}^{2}\), \(\rm E_{\rm b}=120\) keV, \(\rm n_{\rm e}=10^{20}\) m\({}^{-3}\)) is shown in Figure 13 mapped onto the horizontal (a) and vertical (b) planes containing the beam axis. The horizontal footprint proves the ionization rate to be strongly non-uniform over the beam cross section, with less total ions released along the rays coming closer to outer plasma edge. This effect raises the residual neutral power and enhances the power load asymmetry on the first wall, which is already biased due to the wall inclination with respect to the beam axis. The resulting power load profiles on the FNS-ST first wall are shown in Figure 14 for two operation cases: with ideal beam alignment and with a beam nominal misalignment of \(4/-6\) mrad (the error deviation from the ideal beam focusing in horizontal and vertical planes due to technology reasons). From Figure 14, one can expect the beam alignment errors to lead not only to the direct power losses and higher loads within the injector beamline, but also to affect the shine-through power profile, and, in some cases, they can increase the peak power density onto the first wall by \(\sim\)1.5 times (see the figure caption). Figure 12: Neutral beam ionization in plasma poloidal cross-section for deuterium beam, \(\rm E_{\rm b}=100\) keV/amu (BTOR): (**a**) thin beam \(10\times 10\) cm\({}^{2}\), (**b**) thick beam 30 \(\times\) 60 cm\({}^{2}\). Shine-through neutral power was also evaluated for the reduced injection port size 0.2 \(\times\) 0.4 m\({}^{2}\). With the expected decrease of the power injected, the shine-through power dropped by a factor of two, and the peak power density on the first wall falls by -1.5 times (see the caption to Figure 15). Figure 14: NB shine-through power density maps at FNS-ST first wall (NB port size 0.3 \(\times\) 0.6 m\({}^{2}\)) in two operation cases: (**a**) ideal beam alignment, total power—0.147 MW, peak density—1.65 MW/m\({}^{2}\); (**b**) team nominal misalignment (error) \(4/-6\) mrad in horizontal and vertical planes, total power—0.167 MW, peak density—2.43 MW/m\({}^{2}\). For BTR power density profiles, the axes X, Y and Z correspond to NBI coordinates shown in Figure 2.  
Figure 13: Neutral beam spatial deposition in FNS-ST plasma in the planes containing the beam axis (BTR plots): (**a**) horizontal plane, (**b**) vertical plane. The horizontal axis on both images indicates the length along the beam axis (meters) from the NB port. The vertical axis indicates the horizontal distance from tokamak center (**a**), and the relative height (**b**) from the level Z = \(-\)0.8 m.  
Finally, the shine-through power density profile sensibility to the beamlet horizontal focusing was evaluated. For this, all beamlets were aimed parallel in the horizontal plane (Figure 16).