Foreword to the special virtual issue dedicated to the proceedings of the PhotonMEADOW2023 Joint Workshop.

Foreword to the virtual issue papers from the PhotonMEADOW2023 workshop.

Correlation of refractive index based and THz streaking arrival time tools for a hard X-ray free-electron laser.

To fully exploit ultra-short X-ray pulse durations routinely available at X-ray free-electron lasers to follow out-of-equilibrium dynamics, inherent arrival time fluctuations of the X-ray pulse with an external perturbing laser pulse need to be measured. In this work, two methods of arrival time measurement were compared to measure the arrival time jitter of hard X-ray pulses. The methods were photoelectron streaking by a THz field and a transient refractive index change of a semiconductor.

An X-ray free-electron laser with a highly configurable undulator and integrated chicanes for tailored pulse properties.

X-ray free-electron lasers (FELs) are state-of-the-art scientific tools capable to study matter on the scale of atomic processes. Since the initial operation of X-ray FELs more than a decade ago, several facilities with upgraded performance have been put in operation. Here we present the first lasing results of Athos, the soft X-ray FEL beamline of SwissFEL at the Paul Scherrer Institute in Switzerland.

Testing high-resolution transverse profile monitors by measuring the dependence of the electron beam size on the beam energy at SwissFEL.

Transverse profile monitors are essential devices to characterize particle beams in accelerators. Here, we present an improved design of beam profile monitors at SwissFEL that combines the use of high-quality filters and dynamic focusing. We reconstruct the profile monitor resolution in a gentle way by measuring the electron beam size for different energies.

Online absolute calibration of fast FEL pulse energy measurements.

One of the challenges facing modern free-electron laser (FEL) facilities is the accurate pulse-to-pulse online measurement of the absolute flux of the X-ray pulses, for use by both machine operators for optimization and users of the photon beam to better understand their data. This manuscript presents a methodology that combines existing slow-measurement methods currently used in gas detectors across the world and fast uncalibrated signals from multipliers, meant for relative flux pulse-to-pulse measurements, which create a shot-to-shot absolute flux measurement through the use of sensor-based conditional triggers and algorithms at SwissFEL.

Deriving x-ray pulse duration from center-of-energy shifts in THz-streaked ionized electron spectra.

A fast and robust, yet simple, method has been developed for the immediate characterization of x-ray pulse durations via IR/THz streaking that uses the center of energy (COE) of the photoelectron spectrum for the evaluation. The manuscript presents theory and numerical models demonstrating that the maximum COEs shift as a function of the pulse duration and compares them to existing data for validation. It further establishes that the maximum COE can be derived from two COE measurements set at a phase of π/2 apart.

Spectral monitoring at SwissFEL using a high-resolution on-line hard X-ray single-shot spectrometer.

The performance and parameters of the online photon single-shot spectrometer (PSSS) at the Aramis beamline of the SwissFEL free-electron laser are presented. The device operates between the photon energies 4 and 13 keV and uses diamond transmission gratings and bent Si crystals for spectral measurements on the first diffraction order of the beam. The device has an energy window of 0.

Transmission measurement at the Bernina branch of the Aramis Beamline of SwissFEL.

The transmission of the optical components of the Bernina branch of the Aramis beamline at SwissFEL has been measured with an X-ray gas monitor from DESY and compared with a PSI gas detector upstream of the optical components. The transmission efficiencies of the Mo, Si and SiC mirror coatings of the Aramis beamline and the various other in-beam components were evaluated and compared with theoretical calculations, showing an agreement of 6% or better in all cases. The experiment has also shown the efficacy of the high-harmonic rejection mirrors at the Bernina branch of the Aramis beamline at SwissFEL, and characterized the transmission efficiency of the on-line spectrometer in the Aramis beamline.

Core-level nonlinear spectroscopy triggered by stochastic X-ray pulses.

Stochastic processes are highly relevant in research fields as different as neuroscience, economy, ecology, chemistry, and fundamental physics. However, due to their intrinsic unpredictability, stochastic mechanisms are very challenging for any kind of investigations and practical applications. Here we report the deliberate use of stochastic X-ray pulses in two-dimensional spectroscopy to the simultaneous mapping of unoccupied and occupied electronic states of atoms in a regime where the opacity and transparency properties of matter are subject to the incident intensity and photon energy.

The SwissFEL soft X-ray free-electron laser beamline: Athos.

The SwissFEL soft X-ray free-electron laser (FEL) beamline Athos will be ready for user operation in 2021. Its design includes a novel layout of alternating magnetic chicanes and short undulator segments. Together with the APPLE X architecture of undulators, the Athos branch can be operated in different modes producing FEL beams with unique characteristics ranging from attosecond pulse length to high-power modes.

SwissFEL Aramis beamline photon diagnostics. Erratum.

The list of authors in the paper by Juranić et al. (2018) [J. Synchrotron Rad.

Experimental station Bernina at SwissFEL: condensed matter physics on femtosecond time scales investigated by X-ray diffraction and spectroscopic methods.

The Bernina instrument at the SwissFEL Aramis hard X-ray free-electron laser is designed for studying ultrafast phenomena in condensed matter and material science. Ultrashort pulses from an optical laser system covering a large wavelength range can be used to generate specific non-equilibrium states, whose subsequent temporal evolution can be probed by selective X-ray scattering techniques in the range 2-12 keV. For that purpose, the X-ray beamline is equipped with optical elements which tailor the X-ray beam size and energy, as well as with pulse-to-pulse diagnostics that monitor the X-ray pulse intensity, position, as well as its spectral and temporal properties.

Demonstration of femtosecond X-ray pump X-ray probe diffraction on protein crystals.

The development of X-ray free-electron lasers (XFELs) has opened the possibility to investigate the ultrafast dynamics of biomacromolecules using X-ray diffraction. Whereas an increasing number of structures solved by means of serial femtosecond crystallography at XFELs is available, the effect of radiation damage on protein crystals during ultrafast exposures has remained an open question. We used a split-and-delay line based on diffractive X-ray optics at the Linac Coherent Light Source XFEL to investigate the time dependence of X-ray radiation damage to lysozyme crystals.

SwissFEL Aramis beamline photon diagnostics.

The SwissFEL Aramis beamline, covering the photon energies between 1.77 keV and 12.7 keV, features a suite of online photon diagnostics tools to help both users and FEL operators in analysing data and optimizing experimental and beamline performance.

THz streak camera method for synchronous arrival time measurement of two-color hard X-ray FEL pulses.

The two-color operation of free electron laser (FEL) facilities allows the delivery of two FEL pulses with different energies, which opens new possibilities for user experiments. Measuring the arrival time of both FEL pulses relative to the external experimental laser and to each other improves the temporal resolution of the experiments using the two-color FEL beam and helps to monitor the performance of the machine itself. This work reports on the first simultaneous measurement of the arrival times of two hard X-ray FEL pulses with the THz streak camera.

A compact and versatile tender X-ray single-shot spectrometer for online XFEL diagnostics.

One of the remaining challenges for accurate photon diagnostics at X-ray free-electron lasers (FELs) is the shot-to-shot, non-destructive, high-resolution characterization of the FEL pulse spectrum at photon energies between 2 keV and 4 keV, the so-called tender X-ray range. Here, a spectrometer setup is reported, based on the von Hamos geometry and using elastic scattering as a fingerprint of the FEL-generated spectrum. It is capable of pulse-to-pulse measurement of the spectrum with an energy resolution (ΔE/E) of 10, within a bandwidth of 2%.

Opportunities for Chemistry at the SwissFEL X-ray Free Electron Laser.

X-ray techniques have long been applied to chemical research, ranging from powder diffraction tools to analyse material structure to X-ray fluorescence measurements for sample composition. The development of high-brightness, accelerator-based X-ray sources has allowed chemists to use similar techniques but on more demanding samples and using more photon-hungry methods. X-ray Free Electron Lasers (XFELs) are the latest in the development of these large-scale user facilities, opening up new avenues of research and the possibility of more advanced applications for a range of research.

A non-invasive online photoionization spectrometer for FLASH2.

The stochastic nature of the self-amplified spontaneous emission (SASE) process of free-electron lasers (FELs) effects pulse-to-pulse fluctuations of the radiation properties, such as the photon energy, which are determinative for processes of photon-matter interactions. Hence, SASE FEL sources pose a great challenge for scientific investigations, since experimenters need to obtain precise real-time feedback of these properties for each individual photon bunch for interpretation of the experimental data. Furthermore, any device developed to deliver the according information should not significantly interfere with or degrade the FEL beam.

Single-shot analysis of hard x-ray laser radiation using a noninvasive grating spectrometer.

We present a spectrometer setup based on grating dispersion for hard x-ray free-electron lasers. This setup consists of a focusing spectrometer grating and a charge-integrating microstrip detector. Measurement results acquired at Linac Coherent Light Source are presented, demonstrating noninvasive monitoring of single-shot spectra with a resolution of 2.