Effects of filament positions on the arc discharge characteristics of a negative hydrogen ion source for neutral beam injector

The influence of filament positions on the arc discharge characteristics of a negative hydrogen ion source for a neutral beam injector were studied by simulations and experiments. We have made a simulation code named "PRIMELOC2,'' which calculates the orbits of primary electrons emitted from cathode filaments, including collisions with H-2 gas molecules filled in the source, to obtain the spatial distribution of ionization points. We applied this code to a large negative hydrogen ion source having an external magnetic filter for LHD-NBI No. 1 (neutral beam injector 1 for a large helical device). As a result of the calculations, we have found that the moving area of emitted primary electrons differs markedly according to the filament positions. Calculation results agreed well with the experimental results of high-power arc discharge and the spatial distribution of plasma parameters obtained by Langmuir probe measurements. By applying PRIMELOC2 to filament-arc ion sources, we can choose appropriate areas in arc chambers for filament positioning. (C) 1999 American Institute of Physics. [S0034-6748(99)00405-0].