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The average photon flux is one of the main parameters of any photon source. At the European XFEL (EuXFEL), this is continuously monitored in the tunnels by X-ray Gas Monitors (XGMs) [1-3]. However, to measure the absolute value of the X-ray flux at the sample location in the scientific end-stations, in general a smaller device is required since the XGMs and their associated differential pumping stages are very space demanding. Therefore, a miniature solid-state room-temperature calorimeter based on a design by AIST, Japan [4-8] was commissioned with beam at the Spectroscopy and Coherent Scattering (SCS) instrument where it could be located directly downstream of an XGM. This allowed for validation of the gas-based measurements with a solid-state physics method with an independent absolute calibration. The calorimeter was then applied at the High Energy Density (HED) instrument to characterize the beamline transmission and to calibrate a commercial laser power meter (LPM) [9] as a secondary relative monitor for X-ray measurements. This contribution describes the technical parameters and capabilities of the calorimeter and LPM and presents the measurements taken at SCS and HED.

European XFEL has initiated a project to facilitate two-color pump-probe spectroscopy with X-rays at the SASE3 soft X-ray undulator. To this end, the controllable temporal delay between the two X-ray pulses is obtained by a magnetic chicane and an optical delay line (ODL) that delay the electron and X-ray beams, respectively. This article focuses on describing the alignment process of the ODL and its related specification. The distance between the experimental station and the ODL is nearly 500 m, thus if one of the mirrors in the ODL is misaligned, the focus spot experiences a noticeable shift. Moreover, a pulse train can include up to 2700 X-ray pulses with a minimum spacing of 222 ns. An increase in temperature per pulse would just add up if there was no heat removal during that time and the material could be damaged during a pulse train. The question arises whether the X-ray optics can survive the high X-ray peak power during a single pulse. Finally, the present study also investigates the damage issue.

X-ray Free-Electron-Lasers (XFEL) as the Linac Coherent Light Source (LCLS) in the USA, SACLA in Japan, and the European XFEL under construction in Germany are 4th generation light sources which allow research of at the same time extremely small structures (Ångström resolution) and extremely fast phenomena (femtosecond resolution). Unlike the pulses from a conventional optical laser, the radiation in these sources is created by the Self-Amplified Spontaneous Emission (SASE) process when electron bunches pass through very long segmented undulators. The shot noise at the origin of this process leads to significant pulse-to-pulse variations of pulse intensity, spectrum, wavefront, temporal properties etc. so that for user experiments an online monitoring of these properties is mandatory. Also, the adjustment of the long segmented undulators requires dedicated diagnostics such as an undulator commissioning spectrometer and spontaneous radiation analysis. The extremely high brilliance and resulting single-shot damage issue are difficult to handle for any XFEL diagnostics. Apart from the large energy range of operation of the facility from 280 eV to 25 keV in FEL fundamental, the particular challenge for the European XFEL diagnostics is the high intra bunch train photon pulse repetition rate of 4.5 MHz, potentially causing additional damage by high heat loads and making shot-to-shot diagnostics very demanding. This contribution reports on the facility concepts, recent progress in instrumentation development, and the optimization of diagnostics performance with respect to resolution/accuracy, shot-to-shot capabilities and energy range.

We suggest optical schemes for the European X-ray Free Electron Laser facility (XFEL.EU) in Hamburg: a single element X-ray spectrometer on the basis of a reflection zone plate (RZP) for single-shot diagnostics; and a two-element soft X-ray spectrometer on the basis of two RZPs to carry out Resonant Inelastic X-ray Scattering (RIXS) experiments. With this setup, a full map of the sample spectrum is obtainable in a single measurement. The main advantage of using zone plates is the possibility to enable dispersion and focusing in one step. Moreover, highest possible X-ray transmission is achieved by using the minimum number of optical elements. Taking into account the European XFEL beam parameters, our simulations, concerning the RIXS experiment, produced very promising results, reaching an energy resolution (E/ΔE) of up to 30,000 at photon energy of 1 keV. When applied as a single shot spectrometer the energy resolution for RZP is of the same order of magnitude.