Vacuum System for the KEK Proton Synchrotron

The construction and characteristic points of the vacuum system of KEK 12 GeV proton synchrotron are reported. The whole system is divided into three, i.e., for the injector and the beam transportation line to the booster, for the booster and transportation line to the main ring and for the main ring. Of these, only the last two vacuum systems will be reported in the following. In the design and construction, many new attempts have been made and some of them will be reported. The designed value of the average pressures in the booster and in the main ring are 4 ?? 10-4 Pa and 1 ?? 10-4 Pa respectively. These values correspond to the beam lives of 300 ms (in the booster) and 10 s (in the main ring), for a beam with injection energies of 20 MeV and 500 MeV respectively. Actually, the average pressures of 6 ?? 10-5 Pa in the booster and 3 ?? 10-5 Pa in the main ring are accomplished, which are much less than the designed value.