To provide excellent conditions for the scientific user experiments at European XFEL, a good beam stability is essential. But to accomplish this task, it is needed to overcome some difficulties and challenges. Especially, the very long beamlines – up to about 1 km of photon distribution tunnels – of the hard x-ray beamlines are amplifying the effect of vibrations. Each small displacement of an optical component might result in a beam motion in the experimental end station of a non-neglectable order of magnitude and therefore disturbing or even making impossible the execution of the experiments. With increasing number and continuously improving beam quality since the start of user operation at EuXFEL, experiments are becoming more demanding and therefore vibration issues are more and more relevant. Different vibrations were reported from the scientific instruments, like occasionally occurring large horizontal beam motions of several hundred microns or shift of the beam position from week to week. These disturbances may require extra tuning time and lead to a loss of usable beamtime. Because they are not always present, it is very challenging to analyse the root causes. Different studies have been undertaken to determine the vibration at different beamline components and to identify possible sources. At the hard x-ray beamlines, SASE 1 and SASE 2, Laser Doppler Interferometers have been installed to observe the vibrations present at the x-ray mirrors and other optical components. With external excitation, sensitive resonance frequencies of the mirror chambers were studied in detail. Continuous monitoring with seismometers and microphones gives information about the basic background sound spectra and local noise sources.
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