AbstractThe 500 keV negative-ion based neutral beam injector for JT-60U has been operated since 1996 and has injected so far a neutral beam power of 5.2 MW at 350 keV for 0.7 s. In order to further increase the beam power and extend the beam pulse duration time, a few issues concerning the negative ion source have to be solved. The most serious issue of them is non-uniformity of source plasma in the arc chamber. Two out of some countermeasures for improving the non-uniformity have been tried. The first is changing the spatial distribution of the arc discharge through regulating arc current resistors which connect in series to each of eight filament groups. The second is masking both ends of the plasma grid with thin molybdenum plates to block beam extraction at the edges. Owing to these countermeasures, the acceleration efficiency of the deuterium negative ion beams in the accelerator has improved by more than 30% from 0.55 to 0.74.