Present Status of The KEK B-Factory Vacuum System

Abstract

The KEK B-Factory (KEKB) is a two-ring electron-positron collider with asymmetric energies dedicated to proving the CP violation. The vacuum system has been working satisfactorily form the beginning of commissioning in December, 1998. The maximum stored beam currents are 1.3 A for the 8 GeV electron ring (High Energy Ring, HER), and 1.8 A for the 3.5 GeV positron ring (Low Energy Ring, LER). Most of the beam ducts are made of oxygen-free copper, owing to its ability to withstand an intense heat load and to effectively shield the radiation from high beam currents. The pumping scheme is a combination of Non-Evaporable Getter (NEG) pumps and auxiliary sputter ion pumps. An average pressure of about 1×10-7 Pa has been achieved for both rings at the maximum beam currents for both rings. The coefficient of photon-stimulated gas desorption has been decreasing steadily to less than 5×10-7 molecules photon-1 at an integrated linear photon density of about 1×1026 photons m-1. Although special care was taken to make the inside of the beam ducts as smooth as possible in order to avoid the excitation of higher order modes (HOM), the presence of high-current bunched beams has created several problems so far. The intense HOM has sometimes caused the heating of components near to some big HOM sources. Electron cloud around positron bunches led to an instability, which resulted in a vertical beam size blow-up in LER. A discussion about upgrading KEKB has just started aiming at a higher luminosity. The R & D of the vacuum components has also begun based on various experiences so far.