DIRECT ENERGY RECOVERY FROM UNNEUTRALIZED ION-BEAMS IN A NEGATIVE-ION-BEAM-BASED NEUTRAL BEAM INJECTION SYSTEM

A beam direct energy converter (BDC) is designed to recover unneutralized ion beam energies in a 500-keV negative-ion-based deuterium neutral beam 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. 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. Secondary electrons emitted from the negative ion collector for D- collection are also almost completely prevented from escaping if an auxiliary electrode controlling local electric field near the electrode surface is used.