Development of a 400 keV multi-stage electrostatic accelerator for neutral beam injectors

A three-stage electrostatic accelerator has been tested up to 400 keV. The structure of the accelerator is the same as that of the 500 keV accelerator for the JT-60U negative-ion-based neutral beam injection (N-NBI) system. It was confirmed that the heat loads were mainly due to secondary particles generated by the stripping of H- ions in the accelerator, and suppressed effectively by reducing the operational gas pressure. The heat loads at the source pressure of 0.3 Pa, which is the design pressure of the JT-60U ion source, were evaluated to be 3.4 % (the first acceleration grid), 4.3 % (the second acceleration grid) and 2.4 % (the grounded grid) of the input electric power. A H- beam of 0.18 A has been accelerated successfully up to 400 keV for 1 s from 9 apertures. The accelerated H- current density was 13 mA/cm2, the same current density of the JT-60U source. Higher H- beam current of 0.5 A was also obtained at 350 keV from 49 apertures. (orig.)