SASE3-Soft X-Ray Beamline at European XFEL

The European XFEL in Hamburg will be comprised of a linear accelerator and three Free-Electron-Laser beamlines (SASE1, SASE2 and SASE3) covering the energy range from 250 eV to 24 keV. It will provide up to 2700 pulses in trains of 600 microsecond duration at a repetition rate of 10 Hz. SASE3 beamline is the soft X-ray beamline (0.25 - 3 keV) and delivers photon pulses to SQS (Small Quantum System) and SCS (Spectroscopy & Coherent Scattering) experiments. The beamline is able to operate in both monochromatic and non-monochromatic mode. The latter provides the inherent FEL bandwidth at higher intensities. The beamline from photon source to experimental station is about 450 m long. The length of the beamline is related to the optics single-shot-damage issue. The almost diffraction-limited beam is propagated along the beamline with very long (up to 800 mm clear aperture), cooled (with eutectic bath) and super-polished (50 nrad RMS slope error and less than 3 nm PV residual height error) mirrors. The VLS-PG (variable line spacing - plane grating) monochromator covers the entire beamline energy range and its optical design is guided by the optimization of the energy resolving power, the minimization of the pulse broadening and the maximization of optics damage tolerance. Grating substrates are 530 mm long, eutectic cooled and present outstanding surface quality. The VLS parameters of the blazed profile are also a real challenge under manufacturing and measuring point of view. Adaptive optics in the horizontal (the second offset mirror) and vertical (monochromator premirror) plane are foreseen in the optical layout to increase the beamline tunability and to preserve the highly coherent beam properties. Beamline optical design, expected performance and also mechanical aspects of main beamline components are reported.