A negative ion-based beam injector comprising a negative ion source and an accelerator. The ions produced by the ion source are pre-accelerated before injection into a high energy accelerator by an electrostatic multi-aperture grid pre-accelerator, which is used to extract ion beams from the plasma and accelerate to some fraction of the required beam energy. The beam from the ion source passes through a pair of deflecting magnets, which enable the beam to shift off axis before entering the high energy accelerator. The negative ion-based beam injector can be combined with a neutralizer to produce about a 5 MW neutral beam with energy of about 0.50 to 1.0 MeV. After acceleration to full energy, the beam enters the neutralizer where it is partially converted into a neutral beam. The remaining ion species are separated by a magnet and directed into electrostatic energy converters. The neutral beam passes through a gate valve and enters a plasma chamber.