Demonstration of 500 keV beam acceleration on JT-60 negative-ion-based neutral beam injector

Hydrogen negative ion beams of 486 keV, 2.8A and 507 keV, 1A have been successfully produced in the JT-60U negative ion source with multi aperture, multi grid accelerator of three stages. This is the first accomplishment of the H beam acceleration up to 500 keV at high-current of > 1 A. These successful productions of the high-energy beams at high current have been achieved by overcoming a poor voltage holding of the large negative ion sources with the grids of ~2 m 2 . To improve voltage holding capability, the breakdown voltage for the large-area grids has been carefully examined by changing the gap lengths. It was found that a vacuum insulation distance for the large-area grids was quantified to be 6-7 times longer than that for the smallarea electrodes (0.02 m 2 ) utilized for the vacuum insulation design of the JT-60U negative ion source. From this result, the gap lengths have been tuned to sustain 600 kV with the margin of 20% to the rated value of 490 kV under the condition that the good beam optics is sustained in calculation. This improvement realizes a stable voltage holding and a short conditioning time. In the beam acceleration tests, the stable voltage holding is also confirmed. Moreover, no degradation of the beam optics and stripping loss has been actually confirmed. These results indicate that the high energy multi-stage accelerator with multiple grids is feasible for JT-60SA and ITER by tuning the gap length.