Impact of ocean-generated microseism on the European X-ray Free Electron Laser

In this thesis, it is shown that ocean-generated microseism significantly impacts the synchronisation performance of the European X-ray free-electron laser (XFEL), which is based on a high-energy linear accelerator. Using superconducting technology, it generates hard X-ray pulses at megahertz frequencies. Arrival time stability refers to the precision and consistency of the timing of the X-ray pulses, ensuring that they arrive at the target with minimal variation in their temporal alignment. Ocean-generated microseism is low-frequency seismic noise caused by the interaction of ocean waves with the seabed. Distributed acoustic sensing (DAS) is a technology that turns optical fibres into a sensor array for monitoring acoustic signals along their length. By measuring tiny changes in light reflection caused by acoustic disturbances, DAS can provide real-time information about vibrations over long distances.Notably, the bunch arrival time monitors (BAM) show noise between 0.05 Hz and 0.5 Hz, which has a significant impact on the arrival time stability at European XFEL. The impact can be more than 25 fs peak-to-peak. By correlating European XFEL bunch arrival time data with seismic DAS measurements, it is shown that the noise is of seismic origin. Next, both primary and secondary ocean-generated microseism were identified using seismometers and a numerical ocean wave model. Whereas secondary microseism has a strong impact on the bunch arrival time, primary microseism has no notable effect. This is attributed not only to the smaller amplitudes of secondary microseism but also to the fact that primary microseism has a higher Love wave to Rayleigh wave ratio. The effect on the bunch arrival time is caused by Rayleigh waves, while Love waves have a negligible influence on the bunch arrival time. In the presented cases, the noise originates from the North Atlantic and/or the North Sea. The amplitude of the noise depends on the weather conditions in the North Sea and the North Atlantic and is generally much stronger in winter. In conclusion, this work shows that ocean-generated microseism is a significant bottleneck that must be addressed to achieve femtosecond bunch arrival time stability.