## 3 Calculation Results
### Particle Flows  
The particle balance in the plasma is shown in Figure 2. To provide the given density, plasma fueling is necessary at the level 1.5 \(\times\) 10\({}^{21}\)-1.3 \(\times\) 10\({}^{22}\) particles/s depending on confinement and target density. In all regimes, the 30 MW-power deuterium neutral beam injection (NBI) provides flows at the level of (36-37) \(\times\) 10\({}^{19}\) particles/s (absorption in the plasma taking into account the \"shine-through\" particles). The particle flow from the divertor is (2.2-2.3) \(\times\) 10\({}^{21}\) particles/s. The beam and neutrals from the divertor (that are provided by gas puffing) can provide the required density only in the case of high particle  
Figure 1: Dependence of the ratio \(D/\chi_{e}\) on the value \(\tau_{p}/\tau_{E}\). The dashed lines show the conventional limitation of parameter range: over the vertical axis taken from [18] and over the horizontal axis taken from [20]. Colors indicate calculated values for different values of electron density <\(n_{e}\)>: \(\bullet\)—6.0, \(\bullet\)–6.5, \(\bullet\)–7.0, \(\bullet\)–7.5, \(\bullet\)–8.0 \(\times\) 10\({}^{19}\) m\({}^{-3}\). confinement. In the range \(\tau_{p}/\tau_{E}\) = 1-2, plasma fueling with pellets is needed. These values are lower than the ones calculated earlier via the parameterization of calculations by the ASTRA code [8] in the range \(\tau_{p}/\tau_{E}\) = 1.5-3. Injection parameters and source profiles are discussed in detail in Section 3.3. For the fueling of DEMO-FNS, deuterium and tritium pellets are considered, which can be introduced separately or by using the same injection system [21,22]. In the absence of forced isotope content control in the divertor plasma by controlled gas puffing (\(f_{div}{}^{T}\) = \(f_{core}{}^{T}\)), the fraction of tritium in the plasma is determined mainly by the ratio of the flows \(S_{pel}{}^{T}\) and \(S_{pel}{}^{D}\). In the described calculations, the parameters of injection of tritium pellets were chosen to provide the given ratio \(n_{D+}\):\(n_{T+}\) = 1:1, i.e., the fraction of tritium in the core plasma is \(f_{core}{}^{T}\) = 0.5,  
\[f_{pel}^{T}=\frac{S_{pel}^{T}}{S_{pel}^{D}+S_{pel}^{T}} \tag{14}\]  
The obtained data on pellet flows is shown in Figure 3. It is seen that, in the case of low density and high particle confinement, tritium-only pellets are sufficient to maintain  
Figure 2: (**a**) Variation of total ion flows \(S_{tot}\) and flows from separate particle sources: (**b**) from the beam \(S_{beam}\), (**c**) from the pellets and (**d**) flow of neutrals through the separatrix \(S_{sep}\) in the working window of densities \(<\)\(n_{e}\)\(>\) and confinement parameters \(\tau_{p}/\tau_{E}\). the plasma parameters (together with the injection of deuterium form NBI). In all other cases, injection of both deuterium and tritium pellets is necessary, and their ratio should be \(f_{pel}{}^{T}\) = 0.5-0.8. The performed calculations show that, in regimes with high confinement and low density, it is impossible to provide the required isotope composition of the core plasma because, in this case, all the density is provided by the beam and flow of neutrals through the separatrix (gas puffing), and the tritium flow through the separatrix is zero. This, once again, indicates that the parameter range of \(\tau_{p}/\tau_{E}\) was chosen correctly. Additional calculations can be carried out when ions are separated in SOLPS-ITER calculations (in this approach, the separation of the total flow through the separatrix \(S_{sep}\) was carried out proportionally to \(f_{div}{}^{T}\), similar to [7,8]), yet currently this appears unnecessary. In all other cases, the flows \(S_{sep}{}^{D}\) and \(S_{sep}{}^{5}\) turn out to be approximately equal, as is shown in Figure 4.  
Thus, by increasing the calculation precision due to the use of ion equations, it was shown that regimes with high confinement and low density can be non-optimum from the viewpoint of plasma fueling and control of the tritium fraction.