For the European free electron laser XFEL project extremely short bunches (25µm) with very small emittance in transversal and longitudinal direction are essential in order to facilitate the SASE (self-amplified spontaneous emission) process. In order to maintain the conditions for SASE it is important to reduce emittance broadening effects like wakefields to a minimum. Wake fields can be produced by geometric changes of the vacuum chamber profile, as well as by the surface impedance, surface roughness and dielectric (oxide) layers of the chamber material.
Special attention is needed for the undulator regions of the XFEL, where more than 700m of vacuum chambers with very small apertures are foreseen. Here it is planned to use an extruded aluminium profile. A special property of aluminium is, that it naturally generates a protective oxide layer that has to be as thin as possible. For a detailed understanding of the oxide formation and thickness appropriate analysis techniques have to be developed and used to characterize the surface.
A range of surface and bulk sensitive analysis techniques have been used on a prototype extruded aluminium chamber, with and without surface coating of copper and gold. Small sample pieces have been cut from the chamber and analyzed by Rutherford Backscattering Spectroscopy (RBS), Secondary Ion Mass Spectrometry (SIMS) and Elastic Recoil Detection Analysis (ERDA). X-ray spectroscopy (XAS), Nuclear Reaction Analysis (NRA) and Atomic Force Microscopy (AFM) will be used.
The experimental methods give complementary information about the undulator chamber material. Ion-beam techniques as RBS and ERDA are suitable for depth profiling and analysis of elements in the bulk and surface, such as contaminants. NRA is suitable for lighter elements, like profiling of oxide layers. AFM provides information of the surface roughness. In this report we present experimental results and discuss the suitability of the planned extruded aluminium profile, with or without coating, for use as undulator chamber in the XFEL project.
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