Conditioning characteristics of DC500kV large electrostatic accelerator in negative-ion based NBI on JT-60U

Voltage holding capability of a 500 kV, 22 A three-stage electrostatic accelerator, where large-area grids of 0.28 m² and large FRP insulators of 1.8 m in diameter are used, was examined. High voltage was independently applied to each acceleration stage. The voltage holding capabilities of 130 kV were obtained for each acceleration stage. Namely, there is no particular weak stage for voltage holding capability. To identify the breakdown location in each stage, the grids were removed. Breakdown voltages without grids, i.e., for the FRP insulator itself reached 170 kV, that is the design value for each stage. These results show that the breakdown voltage of the accelerator was mainly determined by the vacuum gap between the large-area grids. In this paper, the influence of non-uniform electric field and multi-stage grids on the voltage holding capabilities was also discussed.