Breakdown Phenomena in the Negative Ion Source of 500 keV Negative-Ion Based NBI System on JT-60U

The breakdown phenomena in the 500 keV electrostatic accelerator with large FRP insulators for the fusion application was examined without the beam acceleration. When the high voltage was applied, the light emission with a broad peak at a wavelength of 420 nm was mostly emitted inside the accelerator even though breakdown did not occur. It was found that the voltage holding capability has a strong correlation with the light intensity, and that stable voltage holding was realized under the conditions in which light emission was well suppressed