The vacuum systems for the European X-ray free electron laser project XFEL

The European X-Ray Free Electron Laser XFEL, a new international research facility, will be build in close connection with DESY/Hamburg. The XFEL will generate extremely brilliant and ultra short pulses of X-rays with tunable wavelengths down to 0.1 nm, and exploit them for revolutionary scientific experiments at various disciplines. Therefore electron bunches are brought to high energy of about 20 GeV through a superconducting linear accelerator, and conveyed to up to 250 m long undulators where the X-ray pulses are generated by SASE (Self-Amplified Spontaneous Emission). The beam vacuum system of the XFEL will operate at temperatures of a few K in the areas of the superconducting accelerating structures, thus requiring an insulating vacuum system. In addition to standard UHV requirements the vacuum system for this facility needs to preserve the cleanliness of the superconducting cavity surfaces, which are prepared with procedures similar to semiconductor industry. Thus the preparation of all vacuum components for the 1.6 km long main linac includes cleaning of the components in a clean room to remove particles, installation into the machine under local clean rooms and special procedures for pump down and venting. In the room temperature vacuum system the requirements to the aluminium vacuum chambers of the 700 m long undulators are challenging with respect to very low surface roughness and thickness of oxide layers