Test fabrication of a copper beam duct for the KEK B-factory

A trial model of a copper beam duct for the KEK B-Factory was fabricated and gas desorption rates were measured. The duct consists of a cooling channel, a beam channel and a pump channel, which were extruded separately and welded each other by the electron beam welding. The duct received only ultrasonic cleaning in a freon bath. The ultimate total pressure was about 4×10^-10 Torr after baking at 150°C for 48 hours and the thermal gas desorption rate was calculated to be about 1.5×10^-12Torr. l/(s·cm ²) in N₂ equivalent. The measurement of the photon stimulated gas desorption rate was performed using synchrotron radiation with a critical energy of 26.3 keV from the TRISTAN Accumulation Ring. The photo-desorption coefficients of the order of 10^-2 molecules/photon were observed for main gases desorbed at the initial stage. These high gas desorption rates indicate that more careful surface preparation must be necessary