Cockcroft-06-831-Vacuum Systems for the ILC helical undulator

The International Linear Collider (ILC) positron source uses a helical undulator to generate polarised photons of ~10 MeV at the first harmonic. Unlike many undulators used in synchrotron radiation sources, the ILC helical undulator vacuum chamber will be bombarded by photons, generated by undulator , with energies mostly below the 1 harmonic. Achieving the vacuum specification, of ~100 nTorr, in a narrow chamber, of 4 to 6 mm, with a large length, of 100 – 200 m, makes the design of the vacuum system challenging. This paper describes the vacuum specifications , calculations of the flux and energy of photons irradiating the undulator vacuum chamber and considers possible vacuum system design solutions for two cases: cryogenic and room temperature. 1 ASTeC, Daresbury Laboratory, Warrington, WA4 4AD,England 2 Cockcroft Institute, Warrington, WA4 4AD,England 3 Department of Physics, University of Liverpool, Oxford St., Liverpool, L69 7ZE, UK 4 DESY-Hamburg, Notkestraße 85, 22607 Hamburg, Germany 5 CCLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK 6 Institute of Particle Physics Phenomenology, University of Durham, Durham DH1 3LE, UK, and CERN, CH1211 Genève 23, Switzerland EUROTeV-Report-2006-086 Cockcroft-06-83