## 3 Calculation Results
### Gas Puffing into the Vacuum Vessel
\[S_{out}^{T}+S_{out}^{D}+S_{puff}^{T}+S_{puff}^{D}=c_{p}\times p_{n}+S_{sep}^{D} +S_{sep}^{T}. \tag{17}\]At a fixed value of gas injection \(S_{GIS}\) = 1600 \(\times\) 10\({}^{19}\) s\({}^{-1}\), the flow into the pumping system of the vacuum vessel will increase to 50 m\({}^{3}\) Pa/s for the scenario with natural ELMs and to 65-80 m\({}^{3}\) Pa/s for the scenario with particle losses in convective ELMs. In this case, the geometry of the pumping nozzles and the chosen pumping equipment need to provide the required conductivity and pumping rate. In the DEMO-FNS project, pumping will be carried out from the upper and lower divertors through 36 pumping nozzles, of which  
* the upper pipeline contains 18 branches, each of which consists of a 264-mm-diameter opening, a 264-mm-diameter 6900-mm-long pipeline, a 90\({}^{\circ}\) bend, and a further length of 304-mm-diameter 2200-mm-long pipeline;
* the lower pipeline also contains 18 branches, each of which contains a 264-mm-diameter opening, a 264-mm-diameter 5330-mm-long pipeline, a 90\({}^{\circ}\) bend, and a further length of 304-mm-diameter 2284-mm-long pipeline. The total hydrogen conductivity of these pipelines is 34 m\({}^{3}\)/s (at \(T\) = 20 \({}^{\circ}\)C) and 44 m\({}^{3}\)/s (at \(T\) = 200 \({}^{\circ}\)C). When pumps with a total nominal hydrogen conductivity of 111.6 m\({}^{3}\)/s are used (18 + 18 turbomolecular pumps with a capacity of 3.1 m\({}^{3}\)/s), the total pumping efficiency is \(\sim\)30 m\({}^{3}\)/s. When the total nominal pumping efficiency is increased two times (by using cryopumps), the total effective pumping rate increases only by \(\sim\)15%. In order to be able to pump out substantially larger flows, it is necessary to increase the conductivity of the pumping channels, i.e., increase their cross section and decrease their length. The analysis of the gas flow rate \(S_{GIS}\) acceptable for the DEMO-FNS project showed that the current limitation of \(c_{p}\)-30 m\({}^{3}\)/s can provide gas injection up to \(S_{GIS}\) = 20 Pa m\({}^{3}\)/s, or 1000 \(\times\) 10\({}^{19}\) s\({}^{-1}\) for the considered working window (Figure 12). Consequently, at the calculated flows from the plasma onto the DEMO-FNS wall (that are lower than flows calculated for ITER), the requirements for gas injection exceeds the engineering limitations of the pumping system for some of the considered scenarios. Specification of gas conditions at the first wall is extremely important to optimize the gas injection and engineering solutions in the pumping system. In calculations, one should also account for the fact that the violation of the top-bottom symmetry [38] in the double-zero configuration caused by the radiation-condensation instability (RCI) can lead to up to 5 times less efficient pumping. This means that nearly all pumping will be carried out from only one (colder) divertor, while the pumps connected to the other (warmer) divertor will be blocked. In this case, the total conductivity of the pumping system will decrease to 35 Pa m\({}^{3}\)/s, which will allow us to conduct gas injection within 10 Pa m\({}^{3}\)/s in the regimes with convective ELMs and within 20-30 Pa m\({}^{3}\)/s in the regimes with natural ELMs. To provide pumping in the range of 50-85 Pa m\({}^{3}\)/s, which corresponds to fixed gas injection of 33 Pa m\({}^{3}\)/s, in case of disruption of divertor symmetry, the required pumping rate is \(c_{p}\) = 40-70 m\({}^{3}\)/s (depending on the operation scenario). Taking into account the fixed value of \(S_{GIS}\) and the connected increase in flows in the gas injection system (relative to the estimates that were made to date) will not affect the flows in the fuel injection systems and the hydrogen separation system that contain the maximum gas reserves. However, this will lead to an increase in the amount of tritium accumulated in the pumping, processing, and gas injection systems (in detail, this phenomenon is discussed in Section 3.6).