## 3 Possibility of Injecting Tritium with Fast Atomic Beams  
For the FNS-ST project, the heating beams with different isotope compositions can be developed based on either positive or negative ions, which are accelerated to the required energy and subsequently neutralized in the gas target. This is possible due to the transitional range of atomic energies required for the most efficient current drive and heating of the FNS-ST plasma [13]. In [14], both options for the beam formation were analyzed, and it was recommended to form the neutral beam from the positive ions. In the \"classical\" neutral beam injector based on the positive ions, the beam source is connected directly to the gas neutralizer. The same hydrogen isotope is used for beam neutralization and formation, and the initial gas is also composed of it. For the T or D + T injector, this operating principle can also be used. However, the option of the use in the neutralizer of gas with the isotopic ratio different from that in the source should beconsidered to reduce the amount of T\({}_{2}\) in the gas supply system of the injector and FC of the facility (see, for example, [18]). The gas flow injected into the ion source must ensure the production of a sufficient amount of deuterium and tritium ions to feed the bulk plasma. In this case, it is not necessary to use T\({}_{2}\) as the gas forming the neutralizing target. Both pure D\({}_{2}\) and a D + T mixture could be used there to simplify the system for gas treatment in the closed or open general fuel cycle (FC) of the injection system. At the same time, in the systems for gas supplying, pumping, and processing (the isotope separation system), the amount of tritium should also be minimized to meet the basic safety requirements. The design of the injector operating with a D + T mixture should be optimized to deliver the required amount of D\({}^{0}\) + T\({}^{0}\) to the bulk plasma with the minimum amount of T\({}_{2}\) stored in the injector and gas supply system. To maximize the injection of T\({}^{0}\), the ion source current should be maximal, and it is necessary to provide the required isotope composition of the accelerated beam. In addition, the efficient neutralization of ions and efficient atom transportation along the beam path must be ensured. A distinctive feature of the beam injector based on the positive ions is the presence of the D\({}_{2}\)\({}^{+}\) (15 \(\div\) 20%) and D\({}_{3}\)\({}^{+}\) (\(\div\) 10%) molecular ions in the ion beam produced in the source. After dissociation and neutralization in the neutralizer, these ions produce atoms with energies E/2 and E/3 (in addition to the atoms with energy E). In the case of the D + T injectors, the T\({}^{+}\), T\({}_{2}\)\({}^{+}\), T\({}_{3}\)\({}^{+}\), D\({}^{+}\), D\({}_{2}\)\({}^{+}\), and D\({}_{3}\)\({}^{+}\) ions leave the source and accelerate. Consequently, the heating beam will contain the deuterium and tritium atoms with energies E, E/2, and E/3. If we assume that the DT molecules are present in the working gas, then the D (2E/5) and T (3E/5) components will occur. In the FNS-ST project, it is supposed that an additional power of 10 MW can be input into the plasma, while in the basic operating regimes, the input power will be lower (6 MW). Such powers can be delivered using three 3.5 MW injectors. The operating scenario for the injectors is as follows: two injectors operate simultaneously, while the third one is in the regeneration mode. Increasing the additional heating power will require the use of four such injectors. The duration of the continuous injector operation is limited by the explosion safety condition. In the course of pumping out the gas entering the injector chamber, the gas becomes frozen on the cryo-panels, and its amount can reach a critical value for which, in the event of the emergency breakthrough of atmospheric air into the vacuum chamber and rapid warming of the panels in the injector volume, an explosive mixture can form.