Application of the Gaede effect to the neutralizer of DTT neutral beam injector

Neutral Beam Injectors (NBIs) are key components in the additional heating of the tokamak plasmas. In this scheme, fast charged ions are accelerated electrostatically by suitable grids and then neutralized through a gas target in a dedicated neutralizer, in order to penetrate the strong confining magnetic field. NBIs require a powerful Gas Vacuum System (GVS) in order to operate, and any simplification to this element has a positive impact on the wall-plug efficiency of the NBI, as well as on the overall procurement. In this paper, new design concepts that rely on the Gaede effect are proposed for the neutralizer: their objective is the reduction of the gas conductance through the component by means of appropriate shaping of the internal walls, while maintaining enough clearance for the fast beam to pass through. In this way, the same gas target can be achieved with a reduced neutral gas input, decreasing the throughput that the GVS must be able to evacuate. This work gives a description of the application of this concept to the Divertor Tokamak Test (DTT)’s own NBI, which has been carried out by comparing different design options by means of several simulations with various vacuum codes. Due to the significant improvement that the Gaede effect is expected to bring to the NBI gas economy, this design application has been patented and will be proposed to be implemented in the NBI of the DTT facility, under construction in Frascati (Italy).