Physics design of the Injector Source for ITER NBI

Two Neutral Beam Injectors (NBI) are foreseen in ITER to provide a substantial fraction of the heating power necessary to ignite Thermonuclear Fusion reactions inthe magnetically confined plasma. The development of the NBI system at unprecedented parameters (40 A of negative ion current accelerated upto 1MV) is one of the most challenging tasks of the whole project so that before the installation of the NBI in ITER a full scale prototype will be realized, tested and optimized at the Test Facility in construction in Padova (Italy). The beam source is the key component of the system and the design of the multi-grid accelerator is the goal of a multi-national R&D effort. In particular the beam steering is a challenging aspect being a tradeoff between request of the optics and real grids with finite thickness and thermo-mechanical constraints due to the cooling needs and the presence of permanent magnets. At the Test Facility in Padova an intensive R&D activity, well coordinated with ITER and the other partners, has been carried out in order to provide ITER with the elements to come to a final decision about the design. A novel approach based on innovative scheme for magnetic deflection has been developed. This scheme added to the electrostatic deflection commonly applied and successfully tested in several accelerators, offers significant simplification in the grid design and in flexibility as it works in a wide range of acceleration voltages. In the paper a review of the accelerator physics and an overview of the whole R&D physics program aimed to the development of the injector source is presented.