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In the field of beam physics, two frontier topics have taken center stage due to their potential to enable new approaches to discovery in a wide swath of science. These areas are: advanced, high gradient acceleration techniques, and x-ray free electron lasers (XFELs). Further, there is intense interest in the marriage of these two fields, with the goal of producing a very compact XFEL. In this context, recent advances in high gradient radio-frequency cryogenic copper structure research have opened the door to the use of surface electric fields between 250 and 500 MV m−1. Such an approach is foreseen to enable a new generation of photoinjectors with six-dimensional beam brightness beyond the current state-of-the-art by well over an order of magnitude. This advance is an essential ingredient enabling an ultra-compact XFEL (UC-XFEL). In addition, one may accelerate these bright beams to GeV scale in less than 10 m. Such an injector, when combined with inverse free electron laser-based bunching techniques can produce multi-kA beams with unprecedented beam quality, quantified by 50 nm-rad normalized emittances. The emittance, we note, is the effective area in transverse phase space (x, p x /m e c) or (y, p y /m e c) occupied by the beam distribution, and it is relevant to achievable beam sizes as well as setting a limit on FEL wavelength. These beams, when injected into innovative, short-period (1-10 mm) undulators uniquely enable UC-XFELs having footprints consistent with university-scale laboratories. We describe the architecture and predicted performance of this novel light source, which promises photon production per pulse of a few percent of existing XFEL sources. We review implementation issues including collective beam effects, compact x-ray optics systems, and other relevant technical challenges. To illustrate the potential of such a light source to fundamentally change the current paradigm of XFELs with their limited access, we examine possible applications in biology, chemistry, materials, atomic physics, industry, and medicine-including the imaging of virus particles-which may profit from this new model of performing XFEL science.

A Nanobeam X‐ray Experiments (NXE) instrument was developed and installed at the hard X‐ray beamline of the Pohang Accelerator Laboratory X‐ray Free Electron Laser. This instrument consists of a diagnostic system, focusing optics, an X‐ray diffraction endstation and a femtosecond laser delivery system. The NXE instrument enables sophisticated X‐ray experiments using nanofocused X‐rays. At a 9.5 keV X‐ray energy, the beam was successfully focused to 390 nm × 230 nm at the focal plane using Kirkpatrick–Baez mirrors. Following the successful commissioning experiments in December 2021 and April 2022, the instrument became available for regular user experiments in January 2023. The first user experiment was conducted in January 2024. This article provides detailed information on the beamline optics, the NXE instrument, and its performance and capabilities. The Nanobeam X‐ray Experiments (NXE) instrument at the Pohang Accelerator Laboratory X‐ray Free Electron Laser (PAL‐XFEL) is introduced. The NXE instrument enables users to conduct X‐ray experiments with nanofocused X‐rays.

An experimental platform for dynamic diamond anvil cell (dDAC) research has been developed at the High Energy Density (HED) Instrument at the European X‐ray Free Electron Laser (European XFEL). Advantage was taken of the high repetition rate of the European XFEL (up to 4.5 MHz) to collect pulse‐resolved MHz X‐ray diffraction data from samples as they are dynamically compressed at intermediate strain rates (≤103 s−1), where up to 352 diffraction images can be collected from a single pulse train. The set‐up employs piezo‐driven dDACs capable of compressing samples in ≥340 µs, compatible with the maximum length of the pulse train (550 µs). Results from rapid compression experiments on a wide range of sample systems with different X‐ray scattering powers are presented. A maximum compression rate of 87 TPa s−1 was observed during the fast compression of Au, while a strain rate of ∼1100 s−1 was achieved during the rapid compression of N2 at 23 TPa s−1. A MHz X‐ray diffraction set‐up for the investigation of material behaviour under dynamic compression in a diamond anvil cell at intermediate strain rates has been developed at the High Energy Density (HED) instrument at the European XFEL.

X‐ray free electron lasers (XFELs) serve as advanced light sources and have become essential for investigating ultrafast dynamic phenomena in physics and materials with extraordinary resolution. Owing to the XFEL's ultrafast characteristics and short wavelengths, an arrival‐timing tool is crucial for pump–probe experiments. To address this, we have developed a timing diagnostic tool employing both spectral‐encoding and spatial‐imaging methods at the SBP beamline for the newly constructed Shanghai Soft X‐ray Free Electron Laser Facility (SXFEL). This timing tool was experimentally validated, proving that the spectral‐encoding technique could achieve single‐pulse measurement with an accuracy of under 40 fs [root mean square (RMS)], and exhibited a timing‐jitter measurement of 90.3 fs (RMS) at the CSI endstation of SXFEL. Furthermore, the spatial‐imaging approach used both polished‐ and rough‐surface GaAs crystals, which simplifies implementation in X‐ray pump–probe experiments, and allows for the characterization of X‐ray pulse arrival times at the endstation without rotating the sample stage. These findings confirm that the timing diagnostic tool provides dependable high‐precision temporal characterization of X‐ray pulses at SXFEL, facilitating high‐accuracy X‐ray pump–probe experiments. The development of a timing diagnostic tool at the CSI endstation of the Shanghai Soft X‐ray Free Electron Laser Facility achieves precise temporal characterization for advanced pump–probe experiments. The tool's unique design enables X‐ray pulse monitoring without sample‐stage rotation with both spectral‐encoding and spatial‐coupling methods, offering a reliable solution for X‐ray pump–probe experiments at the CSI endstation.

A novel X‐ray free electron laser (XFEL) diffraction setup for use with diamond anvil cells (DACs) at the Linac Coherent Light Source (LCLS) is described. The new diamond window at the Matter at Extreme Conditions (MEC) instrument allows hard X‐ray experiments on DACs to be performed in air. The platform is described along with alignment and calibration procedures, and details of the X‐ray beam and diagnostics. Example data are presented, including a reversible XFEL‐induced phase transition in CsPbI3. The DAC setup was commissioned at MEC, but is applicable to most LCLS instruments where the unique pulse structures available at LCLS offer access to new ultrafast experimental techniques at high pressure. A setup for fielding high‐pressure samples contained in diamond anvil cells (DACs) at the LCLS X‐ray free electron laser is described and example powder X‐ray diffraction data are presented. Future prospects for experimental capabilities in DACs enabled by the LCLS, such as short pulse laser interactions and multiple pulse modes, are discussed.

The European XFEL is a hard X-ray free-electron laser (FEL) based on a high-electron-energy superconducting linear accelerator. The superconducting technology allows for the acceleration of many electron bunches within one radio-frequency pulse of the accelerating voltage and, in turn, for the generation of a large number of hard X-ray pulses. We report on the performance of the European XFEL accelerator with up to 5,000 electron bunches per second and demonstrating a full energy of 17.5 GeV. Feedback mechanisms enable stabilization of the electron beam delivery at the FEL undulator in space and time. The measured FEL gain curve at 9.3 keV is in good agreement with predictions for saturated FEL radiation. Hard X-ray lasing was achieved between 7 keV and 14 keV with pulse energies of up to 2.0 mJ. Using the high repetition rate, an FEL beam with 6 W average power was created.The first operation of the European X-ray free-electron laser facility accelerator based on superconducting technology is reported. The maximum electron energy is 17.5 GeV. A laser average power of 6 W is achieved at a photon energy of 9.3 keV.

The characterization of X‐ray focal spots is of great significance for the diagnosis and performance optimization of focusing systems. X‐ray free‐electron lasers (XFELs) are the latest generation of X‐ray sources with ultrahigh brilliance, ultrashort pulse duration and nearly full transverse coherence. Because each XFEL pulse is unique and has an ultrahigh peak intensity, it is difficult to characterize its focal spot size individually with full power. Herein, a method for characterizing the spot size at the focus position is proposed based on coherent diffraction imaging. A numerical simulation was conducted to verify the feasibility of the proposed method. The focal spot size of the Coherent Scattering and Imaging endstation at the Shanghai Soft X‐ray Free Electron Laser Facility was characterized using the method. The full width at half‐maxima of the focal spot intensity and spot size in the horizontal and vertical directions were calculated to be 2.10 ± 0.24 µm and 2.00 ± 0.20 µm, respectively. An ablation imprint on the silicon frame was used to validate the results of the proposed method. A new spot characterization method is proposed, based on coherent diffraction imaging that can accurately determine the focal spot size of a single X‐ray free‐electron laser pulse. This method was successfully applied to characterize the focal spot size at the Coherent Scattering and Imaging endstation of the Shanghai Soft X‐ray Free Electron Laser Facility.

Many X-Ray Free-Electron Lasers (X-FELs) have been designed, built and commissioned since the first lasing of the Linac Coherent Light Source in the hard and soft X-ray regions, and great progress has been made in improving their performance and extending their capabilities.

Pump–probe experiments with subfemtosecond resolution are the key to understanding electronic dynamics in quantum systems. Here we demonstrate the generation and control of subfemtosecond pulse pairs from a two-colour X-ray free-electron laser. By measuring the delay between the two pulses with an angular streaking diagnostic, we characterize the group velocity of the X-ray free-electron laser and show control of the pulse delay down to 270 as. We confirm the application of this technique to a pump–probe measurement in core-ionized para -aminophenol. These results reveal the ability to perform pump–probe experiments with subfemtosecond resolution and atomic site specificity. Researchers have demonstrated the generation and control of subfemtosecond pulse pairs from a two-colour X-ray free-electron laser and conducted pump–probe experiments in core-ionized molecules.

With the introduction of x-ray free electron laser sources around the world, new scientific approaches for visualizing matter at fundamental length and time-scales have become possible. As it relates to magnetism and 'magnetic-type' systems, advanced scattering methods are being developed for studying ultrafast magnetic responses on the time-scales at which they occur. We describe three capabilities which have the potential to seed new directions in this area and present original results from each: pump-probe x-ray scattering with low energy excitation, x-ray photon fluctuation spectroscopy, and ultrafast diffuse x-ray scattering. By combining these experimental techniques with advanced modeling together with machine learning, we describe how the combination of these domains allows for a new understanding in the field of magnetism. Finally, we give an outlook for future areas of investigation and the newly developed instruments which will take us there.