Vacuum properties and operation stability of the radio-frequency quadrupole accelerator in Japan Proton Accelerator Research Complex linac

The Japan Proton Accelerator Research Complex accelerator comprises an injector linac, a 3-GeV rapid-cycling synchrotron (RCS), and a 30-GeV main ring. The linac provides a 400-MeV negative hydrogen ion beam to the RCS. For beam power upgrade, a new radio-frequency quadrupole (RFQ) was designed to increase the peak beam current from 30 to 50 mA. This RFQ was fabricated and installed in the front end of the linac beam line in the summer of 2014. Since then, the RFQ has been operated without serious problems for more than two years. However, operational stability of the RFQ can be improved further. The RFQ operation trips occasionally owing to sparking between vane tips. An increase in residual gas components is observed during beam operation. A part of the beam lost in the RFQ hits the inner surface of the cavity, which induces outgassing from the surface. Moreover, the trip rate depends on the beam operating condition. Under a higher trip rate, a larger increase in the carbon-related residual gas components was observed. Therefore, the authors concluded that the sparking occurred at the high-voltage vane tip, which had been contaminated by the carbon-related contaminants.