Beam acceleration test in negative‐ion based NBI system for JT‐60U

Beam extraction and acceleration test in the Negative Ion Based Neutral Beam Injector for JT‐60U has been started using one ion source that is designed to produce a 500 keV, 22 A D− ion beam. Deuterium negative ions are produced in a cesium‐seeded semi‐cylindrical plasma generator and accelerated by a multi‐aperture three‐stage electrostatic accelerator. In the preliminary experiment of beam acceleration, the D− ion beam of 13.5 A was successfully accelerated to 400 keV for a pulse duration of 0.12 s. The negative ion beam power was 5.4 MW. The operating gas pressure in the plasma generator was as low as 0.22 Pa. The highest energy beam of 460 keV, 2.4 A, 0.44 s was also obtained. The ratio of extracted electron current to extracted negative ion current is estimated Ie/ID−<1. It was confirmed that the electron leak from the extractor to the accelerator is suppressed efficiently by the effects of biassing, electron trapping gap and magnetic field.