The negative-ion based neutral beam injection system developed by JAERI for JT-60U has been intended to generate a 500 keV and 10 MW beam for 10 seconds with two ion sources. Technical efforts to decrease heat loads on the accelerator grids, to improve non-uniformity of the source plasma, to increase voltage holding of the accelerator, and to optimize the operational parameters have been conducted intensively. A step-down control of the filament voltage was very effective to keep the acceleration current constant during the pulse duration. To suppress degradation of voltage holding of the accelerator, protection spark gaps of ~178 kV for each stage, its gap length is 73 mm, were confirmed to be effective.

As results of these improvements, as of June 2002, the maximum injection power has reached 6.2 MW, and a pulse length has attained 10 seconds of the design value with a 2.6 MW beam.