Steady state operation of an ampere-class hydrogen negative ion source

A cesium-seeded volume negative ion source producing H(-) ion beams of 800 mA has been operated continuously at a high current density of 20 mA/cm(2). The ion source consists of a magnetically filtered multicusp plasma generator and a multiaperture extractor. The ion source has a frame-cooling-type plasma grid, which is continuously able to keep the temperature at optimum using radiation from filaments and arc discharge. The ion source produces about 150 mA of H(-) in operation without cesium (pure volume operation). The negative ion yield is enhanced by more than a factor of four by injecting 600 mg of cesium. It is important to keep the plasma grid surface temperature at about 300 degrees C, where the negative ion yield has the maximum. The plasma generator has six tungsten filament cathodes of 1.2 mm in diameter. To estimate a lifetime of the filaments, weight and diameter of the filaments were measured after continuous operation. It was found that evaporation is the dominant wearing-out process, and no significant sputtering effect such as the self-sputtering, cesium sputtering, and chemical sputtering was observed. (C) 2000 American Institute of Physics. [S0034-6748(00)62002-6].