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Operating high-gain FELs with ultra-low bunch charges around 1~pC or even less enables the generation of ultra-short, coherent pulses with significantly higher brightness, opening the path to explore processes at attosecond timescales [1]. Similar requirements exist for next-generation ultrafast electron diffraction (UED), which demands sub-pC bunches and arrival-time monitoring with single-digit femtosecond precision.
Bunch arrival-time monitors (BAMs) based on electro-optical (EO) methods are already established at facilities such as FLASH, EuXFEL, ELBE, SwissFEL, and others [2,3]. They provide quasi non-invasive measurements relative to a sub-fs synchronized laser pulse, reaching 3.5~fs resolution for 250~pC bunches at EuXFEL \cite{ref14}. However, adapting these systems for low-charge operation is necessary, since sensitivity strongly depends on bunch charge \cite{ref15}.
A previous project (05K19RO1) developed new broadband pickup structures exceeding 100~GHz bandwidth, showing promising results [4]. These concepts were further validated as a demonstrator at ELBE [5]\cite{ref18,ref19}. The proposed work aims to realize Mach–Zehnder-type EOMs and pickup structures with integrated networks to achieve sub-10~fs resolution for sub 1~pC bunches [6]. Prototypes will be tested at facilities such as REGAE (DESY), FLUTE (KIT), ELBE (HZDR) to enable applications at EuXFEL, FLASH, and future FEL facilities.
[1]: S. Reiche et al., "Development of ultra-short pulse, single coherent spike for SASE X-ray FELs," Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, pp. 45 - 48, 10 May 2008.
[2]: F. Löhl et al., "Electron bunch timing with femtosecond precision in a superconducting free-electron laser," Physical Review Letters, vol. 104, no. 14, p. 144801, April 2010.
[3]: S. Schulz et al., "Few-Femtosecond Facility-Wide Synchronization of the European XFEL," in 39th Free Electron Laser Conference (FEL2019), Hamburg, Germany, 2019.
[4]: A. Penirschke et al., "Concept of a novel high-bandwidth arrival time monitor for very low charges as a part of the all-optical synchronization system at ELBE," in Proceedings of 8th Int. Beam Instrum. Conf (IBIC2019), Malmö, Sweden, 2019.
[5]: B. E. J. Scheible et al., „Real-time measurements of the RF-path of an electro-optical bunch arrival-time monitor with integrated planar pickup structure with low-charge electron beams at ELBE,“ Proceedings 15th International Particle Accelerator Conference, pp. 2407-2410, 05 2024.
[6]: M. K. Czwalinna et al., "Beam Arrival Stability at the European XFEL," Proc. of the 12th Int. Particle Acc. Conf., p. 3714 – 3719, 2021.
