X-ray Optical Delay Line at EuXFEL and Investigation of $B_{4}C$ Damage Threshold

A two-color pump-probe experiment is essential for investigating complex chemical reactions and electronic dynamics in quantum systems. To facilitate this at the European X-ray Free Electron Laser (EuXFEL), a Magnetic Chicane (MC) was installed between the undulators to deflect the electron beam; however, using only the MC does generally not allow for zero and negative temporal delay between the two pulses of different wavelengths. The X-ray Optical Delay Line (ODL) was developed to provide tunable temporal delays (positive, zero, and negative) and adjustable wavelengths in pump-probe experiments. The ODL consists of four flat mirrors made of B4C (50 nm) on a Si substrate, designed to deflect the photon beam and will be installed parallel to the Magnetic Chicane. The specifications of the ODL, both from mechanical and optical perspectives, are detailed, supported by wavefront propagation, ray tracing, and finite element analysis (FEA) simulations to predict its performance. The first mirror of the ODL features a fine motorized pitch rotation mechanism, in addition to the fourth mirror, which includes motorized pitch, roll, and vertical movements. COMSOL ray tracing simulations confirm that this motorized movement is adequate for aligning the mirrors. The first mirror is water-cooled to mitigate excessive heat and reduce spontaneous background radiation. The heat load and mechanical structure causes a bump of 9.5 nm on the first mirror (M1), based on simulations. The quality of the mirror surfaces was measured using a Fizeau interferometer. The roughness was assessed before and after coating using a White Light Interferometer. SRW (Synchrotron Radiation Workshop) wavefront propagation simulations further demonstrated that the mirrors maintain excellent quality, resulting in no degradation of the beam. The European XFEL operates at a repetition rate of up to 4.5 MHz with femtosecond pulse duration, presenting challenges for mirror durability under both single-shot and multi-pulse conditions. The absorbed dose was calculated for single-shot damage and determined that for higher pulse energies, users should open some undulators upstream of the ODL to prevent damage. For multi-pulse conditions, COMSOL heat transfer simulations assessed the accumulated temperature over time. To address damage concerns, experiments were conducted at the SQS instrument to investigate the damage thresholds. Results indicated that the fluence threshold for single-shot damage of B4C (50 nm)/Si-substrate at 1 keV and a grazing angle of 9 mrad is 0.34 μJ/μm2, compared to 0.16 μJ/μm2 for uncoated silicon, highlighting the advantages of B4C as a coating. The experiments covered grazing angles of 9, 15, and 19 mrad, resulting in measuring an electron collision length of 8.6 nm, indicating enhanced heat transport through the material and increased damage thresholds. These findings offer critical insights for designing more resilient optical components in XFEL facilities. Additionally, Monte Carlo and COMSOL simulations were performed to estimate the energy deposition depth and temperature profiles over time and depth within the material. A safety diagram outlining safe operating parameters for future ODL users was created, indicating operating limits based on photon energy and pulse energy to guide decisions on opening undulators and prevent mirror damage.