Two kilometers at 10/sup -10/ Torr The CERN intersecting storage rings for protons

The Intersecting Storage Rings (ISR) in Geneva are a new research facility for experiments with colliding proton beams of energies of up to 28 GeV and intensities of up to 20 A. The ISR vacuum system consists of two interlacing doughnuts formed by elliptical pipes 160*50 mm and circular pipes of 160 mm diameter, of a total length of 2 km. The chambers are made out of stainless steel and can be baked in situ at 200-300 degrees C. An average pressure of 10/sup -10/ Torr has been attained. This is necessary in order to achieve the required beam life of several days. At the intersecting regions where the proton- proton collisions are being observed and where proton-gas collisions produce a disturbing background, equivalent nitrogen pressures of 10 /sup -11/ Torr are required and have been actually achieved. Experiences are reported on the construction, commissioning and operation of this vacuum system.