Overview of photo-neutralization techniques for negative ion beam injectors in future fusion reactors

In fusion reactor technology, the Neutral Beam Injection (NBI) system is one of the principal systems for plasma heating. The main issue in current NBI devices is the adoption of a gas neutralizer, whose efficiency for negative ion beams is limited to about 30%. Conversely, the efficiency of a photo-detachment-based NBI system can be as high as 90%. In a gas or plasma neutralizer, the extra electron in the negative ion is detached by stripping, while in photo-neutralizers it detaches by absorbing a visible or near-infrared photon. The main technological challenge is the level of required optical power (several MW), which forbids direct illumination of the ion beam and implies the exploitation of light trapping techniques (e.g., resonant or non-resonant cavities). Therefore, research activities focus on the development of suitable coherent sources and enhancement cavities. The present work discusses the different photo-neutralization concepts so far developed, describing their working principles, advantages and critical points as well as new possible methodologies to realize photo-neutralization for nuclear fusion reactor applications.