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

Photo-neutralization of negative ions ( H^-/D^- ) is now regarded as a promising technique to increase the efficiency of neutral beam heating systems in future fusions reactors. 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 optical power required (several MW), which forbids the use of direct illumination and requires some form of light trapping: the research focuses on the development of suitable coherent sources and enhancement cavities. This overview discusses the different 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. Graphical abstract