Characteristics of voltage holding and outgassing on the accelerator of JT-60 N-NBI ion source

Kikuchi, Katsumi; Akino, Noboru; Ikeda, Yoshitaka; Usui, Katsutomi; Umeda, Naotaka; Oga, Tokumichi; Kawai, Mikito; Mogaki, Kazuhiko

The 500 keV negative-ion based neutral beam injector (NBI) has been operated to heat plasma and drive plasma current on JT-60U since 1996. The ion source was designed to accelerate the negative ions up to 500 keV. During the last 10 years, the accelerated voltage of the negative ion beam has been limited to 400 keV by breakdowns in the accelerator. To understand the breakdown phenomena, the characteristics of the voltage holding of the ion source were studied without beam extraction. Outgassing with the main species of m/e=28 was observed when high voltage was applied even without breakdowns. It was noticed that the fraction of the main species at breakdown was almost the same as without breakdowns. Conditioning reduced the outgassing and resulted in improvement of the voltage holding capability. Inside the ion source, a brightening was observed even without breakdown. The brightening intensity was suppressed by increasing the D pressure in the accelerator in the range of 10 Pa to 0.5 Pa. Since the voltage holding was also improved with D pressure, breakdowns seemed to correlate with the brightening phenomena in the accelerator. This report gives the preliminary results of outgassing and brightening measurements when the high voltage was applied on the accelerator without beam extraction.