Design study of a negative-ion-based neutral injector for the large helical device

A design for a high power negative-ion-based neutral beam system for the large helical device (LHD) is described. A total power of 20 MW with a pulse length of 10 s is required for neutral beam injection in the LHD. The characteristic of this system is its rather low energy (125 keV for hydrogen) for the use of negative ions, so a large amount of negative ions is needed. The basic design of the beamline is based on the grounded gas neutralizer. A large expansion chamber between the ion source and the neutralizer, and additional ion dumps are needed for the use of negative ions. The beam acclerating power supplies are similar to those of the positive ion system because of the similar beam energy. Some additional power supply is needed for the production of negative ions. The construction of the neutral beam injector will start in 1995.