A novel differentially pumped UHV flange in the CESR interaction region

We describe the design and performance of a novel ultra-high vacuum flange used for the upgrade of the interaction region (IR) of the Cornell Electron Storage Ring (CESR) where the CLEO High Energy Experiment is located. The differentially pumped, Viton O-ring sealed flange requires a low sealing force, allowing remotely controlled mechanical engagement. This provides flexibility and improved safety for the existing CLEO Be-beam pipe and attached silicon vertex (SVX) detector during installation of the CESR IR UHV system. To ensure acceptable vacuum performance of the flange joint over the operational lifetime, the radiation degradation and the outgassing properties of the Viton O-rings were investigated. We concluded that there was no significant seal degradation when the Viton O-rings were exposed to 5×10⁶ Rad of γ radiation. The Viton O-ring outgassing rates were measured as a function of pumping time for various ambient air exposure times. One-dimensional finite element calculations showed that O-ring outgassing would have virtually no impact on the UHV operation. The flange also forms an axial RF seal with a canted coil spring and a radial RF seal with a electric conductive O-ring. This allows the joint to carry image currents from a storage beam up to 1 Amp. The IR UHV system including this so-called ´magic´ flange joint has been operated successfully for two years, and has been successfully engaged and disengaged twice for upgrades of the CLEO detector