Design study of a neutral injection system for the JAERI Experimental Fusion Reactor (JXFR)

Design study of a 33 MW, 200 keV deuterium neutral beam injection system for the JAERI Experimental Fusion Reactor (JXFR) is presented. The NBI system consists of high current ion sources, neutralizing cells, direct energy converters, an evacuating system with cryo-pumps and drift tubes. Neutral beams are injected for 310 seconds along four beamlines. Some optimizations and parameter studies of neutralizing cell length, gas flow rate, operating pressure in ion sources, total pumping speed and pressure in direct converters are carried out. A hollow cathode discharge is responsible for producing a source plasma at low pressure (4.5 x 10-3 Torr) and with high gas efficiency (40%). The injector is evacuated by a cryo-condensation pump consisting of 6 independent cryo-rooms, whose pumping speed is about 2.0 x 105 l/s at the entrance of the pumping duct. In-line direct energy converters are equipped to recover the energy of residual ion beam, so that an overall power efficiency of about 40% is realized, since the ion beam is neutralized only by 17% at 200 keV. Items clarified during development in this design work are summarized