Innovative Concepts in the Design of DTT Neutral Beam Injector.

The main purpose of the Divertor Tokamak Test facility (DTT) is to study alternative solutions to mitigate the issue of the power exhaust, under integrated physics and technical conditions relevant for ITER and DEMO. One of the most complex and innovative subsystems of the entire project is certainly the negative-ion-based Neutral Beam Injector (NBI), meant to couple to the plasma deuterium neutrals (D0) with an energy of 510 keV and an injected power of 10 MW. This contribution describes the conceptual design of the beamline for the DTT NBI system, with a particular focus on the new technical solutions adopted to fulfill the requirements and maximize the performances. The DTT NBI is required to operate with high efficiency in several operating scenarios, i.e. in a large range of beam energies, between 10% and 100% of the nominal value (510 keV). To reach this challenging goal, an innovative accelerator design, featuring sphere- shaped grids and hyperlens grids, has been developed. The implementation of this design concept of the accelerator has recently become possible thanks to recent improvements of the additive manufacturing technology. A second relevant innovation of the DTT NBI compared to the existing devices regards the vacuum pumping system, which will be based on Non-Evaporable Getter (NEG) pumps. This will represent the first application of the NEG technology to an NBI for the heating and current drive system of a fusion experiment, with a possible simplification of the overall construction compared to typical solutions with cryogenic pumps. A detailed quantitative analysis of the main design choices will be provided, together with the lines of the project future developments.