Direct energy recovery from unneutralized ion beams in a negative-ion-beam-based neutral beam injection system

This paper reports on a beam direct energy convertor (BDC) designed to recover unneutralized ion beam energies in a 500-keV negative-ion-based deuterium neutral injection system for the Fusion Experimental Reactor of Japan Atomic Energy Research Institute. A newly developed three-dimensional beam transport code KUNABE-3 is used. Due to approximately equal fractions of unneutralized D+ and D- beams flowing from the gas neutralizer, electrostatic electron suppression is efficient. Also, magnetic separation and deflection of both species by a 1-kG magnetic field are efficiently applicable. Under suitable energy recovery conditions, perfect collection of both positive and negative ion beams is theoretically achievable, even for a collector voltage of ±480 kV, resulting in 96% energy recovery efficiency. Within ±10% deviation from the reference parameters, the designed BDC shows excellent performance for such parameters as magnetic fields, incident beam energies, and gas line densities