An X-ray free-electron laser oscillator (XFELO) is a next generation X-ray source promising radiation with full three-dimensional coherence, nearly constant pulse to pulse stability and more than an order of magnitude higher spectral flux compared to SASE FELs. In this contribution, the concept of an R&D project for installation of an XFELO demonstrator experiment at the European XFEL facility is conceptually presented. It is composed of an X-ray cavity design in backscattering geometry of 133 m round trip length with four undulator sections of 20 m total length producing the FEL radiation. It uses cryocooled diamond crystals and employs the concept of retroreflection to reduce the sensitivity to vibrations. Start to end simulations were carried out which account for realistic electron bunch distributions, inter RF-pulse bunch fluctuations, various possible errors of the X-ray optics as well as the impact of heat load on the diamond crystals. The estimated performance and stability derived from these simulations shall be reported and foreseen issues shall be discussed.

The SASE3 soft X-ray beamline at the European XFEL is equipped with the grating monochromator allowing to reduce SASE FEL bandwidth and to improve longitudinal coherence at the experiments in the photon energy range 250 eV - 3000 eV. The design of the monochromator is challenged by a demand to control both photon energy resolution and temporal resolution; the aim to transport close to transform-limited pulses poses very high demands on the optics quality, in particular on the grating. Presently, the monochromator operates with two gratings: the low-resolution grating is optimized for time-resolved experiments and allows for moderate resolving power of about 2000 - 5000 along with pulse stretching of few to few tens of femtoseconds RMS, and the high-resolution grating reaches resolving power of 10000 at a cost of larger pulse stretching. The examples of time-resolved experiments and experiments performed in high photon energy resolution mode are presented. In addition, being operational in spectrometer mode, the monochromator is regularly used for the spectral characterization of the FEL beam including photon pulse length retrieval.

The European X-ray Free-Electron Laser (EuXFEL) is a unique FEL facility that provides X-ray pulses of high spectral brilliance and high photon flux at MHz repetition rate. However, the high peak power, produced in trains of up 2700 femtosecond pulses at a rate of 10 Hz, induces a periodic temperature increase of the hard X-ray monochromators, thereby reducing their transmitted intensity. To address this limitation, a diamond channel cut monochromator (DCCM) was proposed as an alternative to the currently used silicon monochromators. The heat load effect of typical EuXFEL pulses at 300 K and 100 K was simulated by finite element analysis (FEA) and indicates that the significant reduction of the transmitted intensity occurs after a higher number of pulses when compared to silicon. The DCCM first prototype was manufactured from an HPHT IIa type diamond single block and characterised by rocking curve imaging (RCI). The RCI results demonstrated the high crystalline quality of the DCCM with rocking curve widths of the same order as the width predicted by the dynamical theory and a uniformly reflected intensity over the surface. The performance as a monochromator was demonstrated by measuring the double bounce reflection. The resulting images after two successive reflections showed a diffracted beam of the same size and parallel to the incident beam and confirmed its applicability.