Present status of vacuum system for Tristan

The Tristan's electron-positron collision ring at High-Energy Physics Research Institute has been operating for two and a half years since October 1986 to March this year or 1989. Various efforts have been carried out for enhancing its performance, including the improvement in the distributed ion pumps (March-April 1987), extension of the radio-frequency wave cavity (August-September 1988) and installation of a superconduction cavity. Accordingly, the beam energy was increased from the initial 23GeV to 30.4GeV in the fall of 1988, and the beam current reached a maximum of 14mA in 1988. The luminosity also increased to 1.3 X 1031 in 1988, and the time-integrated beam current reached 27A·h at the end of March 1989. For the vacuum system for Tristan, troubles associated with the initial faults have been eliminated. Regular inspection and maintenance has resulted in a decreased number of troubles that could cause shutdown, though checking should be performed continuously for radiation damage. The average beam life has exceeded five years, and the pressure increase is currently of the order of 10-8 Pa/mA. Further improvements should be made in the future to increase the beam life. (N.K.)