Performance of the Negative ion source for JT-60U

The 500keV Negative-ion based neutral beam injector for JT-60U started the operation in 1996 and has injected so far a neutral beam power of 5.2MW at 350keV for 0.7sec. However there are some issues to be conquered for increasing furthermore the beam power and beam pulse duration time. One of those is source plasma non-uniformity in the arc chamber. Improving the issue, three countermeasures have been conducted. The first is to change an arc discharge distribution through changing arc current resistors which connects in series to each of eight filament cathode groups. The second is masking both ends of the plasma grid with thin molybdenum plates to block beam extraction at the edges. The third is changing arc discharge mode from space charge limited discharge to emission limited which is relatively independent on the plasma condition in the arc chamber. The first two countermeasures have been determined to be effective against the source plasma non-uniformity, and the third is being examined.