The vacuum system of the BEPCII storage ring

The design, fabrication and performance of a vacuum system for the BEPCII storage ring are described in this paper. The arc beam pipes and their antechambers are made by machining upper and lower chamber halves, then welding them at the periphery to form a complete chamber. The photon absorbers and the pumping stations are located in the antechambers. The special vacuum components, such as photon absorbers, RF shielding bellows, and the in-vacuum wiggler, have been manufactured according to the characteristics of the BEPCII storage ring and shown to perform well. The vacuum systems of the BEPCII storage ring have been assembled and run over a two years period, and they had accumulated 1000 A h of beam dose in the colliding mode as of the end of July 2009. On average, the dynamic pressure rise of the BEPCII storage ring is less than 3×10−7 Pa/A. The effect of a cloud of photoelectrons around the positron beam was not found in the high beam current, because the beam pipe with an antechamber in the positron ring was coated by a TiN layer of about 100 nm in thickness. Ion currents of a few specially positioned ion pumps were slight high in the high beam current, which can be explained as an effect of electron multiplying.