Paper VT-WeM3
Construction Status of the SuperKEKB Vacuum System

Wednesday, October 21, 2015, 8:40 am, Room 230B

The SuperKEKB, the upgrade project of KEKB, is an electron-positron collider with asymmetric energies, that is, 7 GeV electrons and 4 GeV positrons. The goal luminosity is 8x10³⁵ cm^-2s^-1, approximately 40 times higher than that achieved in the KEKB. The construction of the new vacuum system for the SuperKEKB has been in progress since 2010, as a key item of the upgrade. Most of the vacuum components, especially in the positron ring, are newly fabricated to manage the electron cloud effect (ECE), and to reduce beam impedance, which are essential to keep the low-emittance beams stable even for high beam currents. The design and the manufacturing of the major vacuum components, including beam pipes, bellows chambers, gate valves, vacuum pumps and so on, have been completed. Approximately 1240 new beam pipes were made of aluminum-alloy or pure copper depending on the intensity of the irradiated synchrotron radiation. They basically have two antechambers at both sides of a central beam channel. The main pump at arc sections of the ring is a strip-type NEG installed in one of the antechambers. Approximately 1130 NEG modules were prepared with various lengths of from 0.7 to 3 m consisting of three NEG strips. New bellows chambers and gate valves have basically a comb-type RF-shield and have the same cross sections to the connecting beam pipes. Countermeasures against the ECE, such as the coating of TiN film, the grooved surface, the clearing electrode and so on, were prepared for the beam pipes of the positron ring. All of aluminum-alloy beam pipe have TiN coating inside to reduce the secondary electron yield. Approximately 150 beam pipes for dipole-type magnets have grooved structure at top and bottom of the inner surface. Beam pipes for wiggler magnets have 116 clearing electrodes inside in total. The installation of these components into the KEKB tunnel has almost finished. The MO-type flanges, which have structurally little step inside, were adopted to the connection flanges between the beam pipes and the bellows chambers. The air-leak rate at the first fastening was less than 5 % out of more than 5000 times fastening in total during the pre-baking, the TiN coating and the connection in the tunnel. The final activation of NEG pumps in the tunnel has finished approximately 70 % of the ring until October. After the NEG activation, the average pressures decreased to on the order of 10^-8 Pa, where the beam pipes were not baked in situ. The vacuum system will be ready in next January, 2016. The construction status SuperKEKB vacuum system, including various experiences during the construction stage, will be reported.