Design study of a neutral beam injection system for JAERI experimental fusion reactor

Design study has been made of a 200keV, 32MW deuterium neutral beam injection system for JAERI Experimental Fusion Reactor (JXFR). The NBI system consists of four main components; high current ion sources, neutralizing cells, direct energy converters, and evacuating systems. Neutral beams are injected for 110sec along four beam lines. Each beam line has five ion sources and one spare source. The ion source is hollow cathode type of long operation period at low pressure (about 2.5 x 10-3Torr) and the high gas efficiency (about 63%). The extraction system consists of four acceleration grids with an extraction area of (9.4 x 77.7)cm2. In-line direct converters are used in order to recover the unneutralized ion beam energy. There are two negative electrodes for electron suppression and one positive electrode for ion collection. Computer analysis of beam trajectories was made to find proper shape and geometry of the electrodes. Heat load of the collector was found to be below 100 W/cm2 by calculation. Each beam line is evacuated by two pumping systems, main cryopumps of 1.1 x 106l/s at 10-4Torr and subsidiary turbo-pumps of 105l/s at 8 x 10-4Torr. The main cryo-pumps consisting of four independent cryo-rooms are placed under the beam line. Over all power efficiency of about 25% is realized. It was shown that the secondary particles created by beam-gas interaction would affect seriously power efficiency of the NBI system. (author)