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Abstract
The European XFEL is a state-of-the-art hard X-ray free-electron laser (FEL) facility powered by a high-energy superconducting linear accelerator. Operating at megahertz repetition rates, it can deliver thousands of X-ray pulses per second within 10 Hz bursts, enabling unique opportunities for cutting-edge experiments. At the European XFEL, FEL pulses are generated in the undulators through the Self-Amplified Spontaneous Emission (SASE) mechanism, which serves as the baseline mode of operation. The facility operates three beamlines: two dedicated to hard X-rays, SASE1 and SASE2, and one to soft X-rays, SASE3. Each hard X-ray beamline is equipped with 35 movable-gap, out-of-vacuum undulator cells. The SASE1 and SASE2 beamlines have achieved pulse energies of up to 5 mJ and 4 mJ at 9 keV, respectively, while SASE3 consistently delivers 8–10 mJ pulses at 1 keV, making the European XFEL a leader in high-intensity X-ray science. In pursuit of higher photon energy operations, significant milestones were reached in 2021. With a beam energy of 16.3 GeV, SASE1 demonstrated a pulse energy of 800 µJ at 24 keV, while SASE2 achieved a pulse energy of 340 µJ at 30 keV. To achieve narrow bandwidth and high spectral brightness, a Hard X-ray Self-Seeding (HXRSS) system was installed at the SASE2 undulator beamline in 2019 and has been in user operation since 2021. Building on the successful implementation of HXRSS in SASE2, plans are underway to equip SASE1 with an HXRSS system by 2025. The European XFEL is the first facility to demonstrate HXRSS operation at MHz repetition rates in burst mode. To accommodate the high repetition rate, a cascaded two-stage monochromator design was implemented to minimize heat load on the crystal, particularly at photon energies below 8 keV. Figure 1 illustrates the layout of the two-chicane HXRSS setup in the SASE2 beam line. This system can be used in different modes, including the SASE mode (no chicane is used), one-chicane mode (only one of the two chicanes is used) and two-chicane mode (both chicanes are used). The design of the monochromator used at the European XFEL is based on similar designs implemented at LCLS and PAL-XFEL. Currently, the first monochromator has been removed due to radiation damage affecting the encoders of its movers. To address this issue, additional molybdenum shielding was added to the monochromators during the summer of 2024. The first monochromator is now reserved as a spare for replacement if needed. However, since the majority of users require photon energies above 7 keV, this issue has not impacted user operations.