This PDF file contains the front matter associated with SPIE Proceedings Volume 12581, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The FERMI facility has provided over the last decade pulses characterized by a high degree of coherence over the spectral range from 100 nm down to 4 nm. Thanks to the use of an external seed to initiate the FEL process, FERMI pulses are characterized by unique characteristics very much appreciated by the user community. The extraordinary degree of coherence but also the flexible temporal or spectral properties of the photon pulses combined with a high reproducibility and low temporal jitter has supported important scientific experiment in various fields. Recently experiments have been conducted extending the tuning range to wavelengths shorter than 2 nm accessing important spectral resonances. Current operations at such a high photon energy rely on the use of undulator harmonics which limit the FEL performances. A project has been initiated to upgrade the FEL facility with new accelerator modules and new FEL configurations to access this important spectral range in nominal operation at the fundamental resonance of the FEL amplifier and with full control of polarization and other photon properties currently available.

Over the last few years, tremendous progress has been gained in the generation and application of ultrashort radiation pulses. Recently, free-electron lasers generating ultrashort pulses with high peak power from the extreme ultraviolet (EUV) to the soft-x-ray region are opening a wide range of new scientific opportunities. Taking advantage of this short timescale permits probing ultrafast, out-of-equilibrium dynamics and the high intensities are key for nonlinear optics. The core structure of the extremely important light elements carbon, nitrogen, and oxygen can be accessed by soft-x-ray wavelengths by providing chemical sensitivity. Externally seeded free-electron lasers generate coherent pulses with the ability to be synchronized with femtosecond accuracy. In this contribution, we present new achievements in the generation of coherent ultrashort pulses in the range of EUV to the soft-x-ray in externally seeded FELs. In particular, we present the recently successful robust experiment at FERMI in Trieste, where few-femtosecond extreme-ultraviolet pulses were generated and characterized in terms of energy, and duration via autocorrelation.

SwissFEL at the Paul Scherrer Institute is a free-electron laser facility providing hard and soft x-rays, based on the SASE principle. In addition, the soft x-ray beamline Athos is currently extended for electron beam manipulation with external lasers, aiming to provide seeding capabilities based on the two-stage echo-enabled harmonic generation (EEHG) scheme. Completion of the installation is foreseen in spring 2023. We present the initial results on the single-stage operation for ESASE and mode-locked lasing and give an outlook on the expected performance for seeding down to 1 nm.

European XFEL is a multi-beamline x-ray free-electron laser (FEL) user facility driven by a superconducting accelerator with a nominal photon energy range from 250 eV to 25 keV. To extend the photon range towards harder x-rays, an afterburner undulator based on superconducting undulator (SCU) technology is currently being planned. This afterburner undulator would be installed at the end of the already operating SASE2 hard x-ray FEL beamline. The electron bunch is microbunched by the FEL process in the SASE2 undulators and in the SCUs drives emission either at the same wavelength or at a harmonic of the upstream SASE2 undulator. In this contribution we describe numerical simulations of the potential photon output.

Radiofrequency (RF) transverse deflection structures (TDSs) are fundamental time-resolved diagnostics in x-ray free-electron lasers. Two x-band TDSs with variable polarization of the deflecting force were recently installed after the undulators of Athos, the soft x-ray beamline of SwissFEL. This contribution summarizes the experience gained over the last few months during the commissioning of the RF system and the measurements made during operations, focusing on the setup of the entire complex RF system, the calibration and the time-resolved measurement that, combined with an energy spectrometer, provides longitudinal phase-space measurements of extreme importance for the commissioning of the complex FEL schemes implemented in the Athos beamline.

We have developed and commissioned an angle-resolved photoelectron spectrometer, based on the electron time-of-flight concept, for hard x-ray photon diagnostics at the European Free-Electron Laser. The instrument provides users and operators with pulse-resolved, non-invasive spectral distribution diagnostics, which in the hard x-ray regime is a challenge due to the poor cross-section and high kinetic energy of photoelectrons for the available target gases. We report on the performance of this instrument as obtained using hard x-rays at the PETRA III synchrotron at DESY and the SASE1 beamline at the European XFEL. We demonstrate a resolving power of 10 eV at incident photon energies up to 20 keV.

Modern free electron laser (FEL) facilities often lack the ability to accurately measure the absolute flux of the x-ray pulses on a shot-to-shot basis. These measurements could be used by the machine operators for optimization, and by the users of the photon beam to better understand their data. This contribution presents methodology that combines existing, slow measurements methods currently used in gas detectors across the world and fast, uncalibrated signals from multipliers, meant for relative flux pulse-to-pulse measurements, that creates a shot-to-shot absolute flux measurement though the use of sensor-based conditional triggers and algorithms at SwissFEL

An atomically precise, super smooth, and damage-free surface is highly demanded for x-ray mirrors, multilayer optics, channel-cut crystal monochromators (CCM), and gratings. An ultra-precision optic with a figure error of several nm is crucial for single-nanometer spatial resolution, signal strength, and contrast. Moreover, sub-angstrom root-mean-square surface roughness is beneficial for high reflectivity and the lowest unwanted scattering. Additionally, a damage-free surface with no alter layer is greatly desired for CCM and grating substrates because it is essential for the high reflectivity of the CCM and the uniform etching rate of the grating’s ruling. In the manufacturing of x-ray mirrors, to obtain the desired figure error, a non-contact figuring method, such as ion beam figuring (IBF), plasma chemical vaporization machining (PCVM), or Elastic Emission Machining (EEM), is usually employed thanks to its high controllability and stability. EEM has proved the best performance in terms of achieving a low figure error and maintaining a good surface roughness. After shaping/figuring, a finishing method is usually applied to improve its surface roughness without distorting its figure error. Recently, Catalyst-Referred Etching (CARE) has realized its potential and applicability to x-ray mirror manufacturing as a finishing method. Thanks to its removal mechanism, a highly ordered surface with a root-mean-square of 0.03 nm RMS is attained. In the polishing of a CCM, because a mechanical method is usually used to polish its surface at the narrow gap, the residual mechanical damage induced a low reflectivity and low spatial resolution. PCVM with a wire electrode has recently been proposed and demonstrated its excellent performance. The damage-free surface of a CCM with a gap of less than 100µm has been successfully realized by PCVM. In this paper, recent achievements in figuring (EEM), surface finishing (CARE), and damage removal (PCVM) are presented and discussed.

Steering x-ray beams from the source towards the experiment without distorting their wavefront defines extraordinary high-quality requirements on the production of the x-ray optics. We report on how this demand settled with in situ shot-to-shot wavefront sensing optimization of KB optics at SwissFEL beamlines. This contribution presents methodology that combines moir´e interferometry and single-phase-grating Talbot interferometry. We discuss an online Kirkpatrick-Baez (KB) test plan at the Cristallina beamline based on single phase grating Talbot interferometry, demonstrating progressive optimization steps in minimizing KB wavefront distortion.

Free-electron lasers (FELs) are currently the most advanced light sources operating worldwide, thanks to their capability to lase coherent ultrashort pulses, marked by photon energies bridging the gap between the Extreme-Ultraviolet (EUV) and the Soft (SXR) and Hard (HXR) X-Rays, alongside with unique high-brightness and temporal duration lying in the femtosecond (fs) timescale. FELs can exploit, in a time-resolved approach, spectroscopies daily employed at synchrotron light sources, mostly combining EUV, SXR and HXR pulses with optical ones. Nonetheless, the next advances in ultrafast x-Ray science are strongly linked to the extension of these time-resolved schemes to perform experiments engaging exclusively EUV, SXR and HXR pulses, so triggering (and probing) matter at its (or nearby) electronic resonance(s), to reveal the microscopic mechanisms hiding behind matter phases of primary interest for broadband applications. Indeed, designing the next generation of quantum devices, as well as tailoring a new classes of biomolecules for pharmacological applications, are just two examples that can be strongly boosted by means of this optical approach. To do this, is mandatory to split and delay (in time) FELs pulses, without impacting on both the radiation coherence properties and on the photon transport. At the seeded FERMI FEL (Trieste, Italy) this goal is committed by the optical device known as AC/DC, which stands for the Auto Correlator/Delay Creator, designed to split the incoming EUV and/or SXR pulse, introducing a tunable delay between these two pulses, marked by an intrinsic resolution in the sub-fs, and aided by an opto-numerical pointing feedback system.

Table-top beamlines based on high-order laser harmonics (HHs) are nowadays lab-based facilities commonly used both for ultrafast experiments on its own sake and in preparatory experiments conceived, ab initio, for large-scale facilities such as FELs. Differently from FELs, HHs are emitted in a broad spectral range, requiring for most experiments the selection of a single harmonic. The monochromatization should preserve the temporal structure of the femtosecond pulse in a so-called time-delay-compensated monochromator (TDCM), where a couple of gratings is used in a configuration to compensate for the pulse-front tilt. At present, TDCMs in the extreme ultraviolet (15–100 eV) are realized using six optics at grazing incidence: two plane gratings and four toroidal mirrors. The gratings are illuminated by a collimated beam and the mirrors are used to collimate and focus the beam in the two sections of the monochromator: intermediate slit and target area. Here we present the design of a TDCM with four optical elements: two gratings, a cylindrical(spherical) mirror and a toroidal mirror. The gratings are used in the off-plane geometry and are illuminated by a divergent beam. The optical design is discussed in detail giving all the parameters for the definition of the configuration. We present the design of a TDCM for the 15-60 eV region, being a physical realization in progress. The main topics discussed are the beam size at the target area, the residual temporal broadening, the error budget for the alignment and the expected throughput. The design has advantages in terms of costs, compactness, alignment stability and throughput.

Time-resolved spectroscopies have provided insights in the quest for understanding the fundamental properties of quantum materials and towards controlling their functional properties through light-matter interaction. In this regard, Free Electrons Lasers (FELs) have developed as a powerful tool to perform ultrafast x-ray spectroscopy to obtain energy and momentum-resolved information. In this contribution, we introduce the SwissFEL soft-x-ray condensed matter experimental endstation, named Furka, which is dedicated to Time-Resolved X-ray Absorption (TR-XAS), resonant X-Ray Diffraction (TR-RXRD) and Resonant Inelastic x-ray Scattering (TR-RIXS) experiments to study quantum materials. The current status of the endstation and the first results from the commissioning phase will also be discussed.

Microfluidic liquid sheet jets have rapidly grown in popularity for extreme ultraviolet and soft x-ray spectroscopies as they are vacuum stable, constantly refreshing, and are easily able to reach sub-micron optical path lengths required for transmission measurements. We have recently demonstrated the generation of a new class of sheet jet comprised of two liquids (a “liquid heterostructure”) by colliding two jets of one liquid onto opposite sides of third jet of another liquid. The resulting structure is a layered sheet jet where a thin sheet of one liquid is completely enveloped by a larger sheet of a separate liquid. If the component liquids are miscible, the thin component layers result in fast diffusive mixing on submillisecond time scales based on measurements using FTIR microscopy. If the component liquids are immiscible, the resulting structure contains well-defined, large-area liquid-liquid interfaces with a minimized bulk liquid background as determined from ellipsometry and FTIR microscopy measurements. The inner liquid layer in these structures was found to be as thin as tens of nanometers, comparable to the thinnest sheet jets that can be produced. These new heterostructures provide the same benefits as conventional sheet jets for XUV and SXR spectroscopy and could enable new mix-and-probe spectroscopic techniques or support developing methods such as XUV/SXR second harmonic generation for examining buried liquid interfaces.

High data rates produced by custom-built burst mode detectors available at the European XFEL present challenges for data processing. One crucial processing step required for subsequent analysis is detector calibration and data correction. We present an overview of the calibration and correction software infrastructure developed at the European XFEL to accommodate the specific needs at the facility. We discuss the distinction between offline and online processing, outlining the roles and interesting technical properties of calibration software in both contexts. Recent improvements to the online processing software allow for correcting full detector data stream in near real time, enabling more comprehensive online analysis methods for improved steering of experiments.

Bragg crystals are widely used in free electron lasers to diffract and isolate an extremely narrow spectral range from the initial self-amplified spontaneous emission (SASE) signal. By rotating the crystal along one of its axes, multiple reflections can be generated as per Bragg’s law. A measurement model based on Bragg’s law was established at the SASE2 beamline of the European XFEL after extensive hours of hard x-ray self-seeding (HXRSS) operation. This model characterizes the crystal’s range of motion and reflections. In this work, two computer vision techniques, the Hough transform and template matching, are implemented to identify the absolute photon energy from photon diagnostic images. By comparing the spectrometer correlation scans with the model, the actual operational energy can be determined from the measured data using these techniques, enabling accurate calibration as well as the ability to freely scan the photon energy with a self-seeded beam over a wide energy range.

EuPRAXIA@SPARC_LAB is a new multi-disciplinary user-facility that is currently under construction at the Laboratori Nazionali di Frascati of the INFN in the framework of the EuPRAXIA collaboration. The electron beam will be accelerated by an X-band normal conducting linac followed by a Plasma WakeField Acceleration (PWFA) stage. It will be characterized by a small footprint and it will drive two FEL beamlines for experiments, one in the VUV (50 to 180 nm) and the other in the XUV-soft x-rays (4 to 10 nm) spectral region. As an ancillary beamline, we are also including a betatron source in the x-ray from laser-plasma interaction. We present the status update of our facility.

Two-color is an emerging mode of operation in x-ray Free Electron Laser (FEL) having the potential to expand the pump and probe experiments toward fs temporal resolution and to site-selective spectroscopy. A novel and quick to setup method for two-color x-ray generation is presented here. The approach is experimentally demonstrated at the hard x-ray branch of SwissFEL (Aramis) using a laser emittance spoiler(LES). A short laser pulse is overlapped at the peak of the primary photocathode laser causing a local increase of the electron bunch emittance. In this configuration, the x-ray FEL emission occurs only for the two unspoiled parts of the bunch. Due to the fact that the electron bunch acquires an energy chirp along the acceleration, the spoiled beam produces two x-ray pulses of individual duration of few tens of fs having 1% photon energy separation and a time delay controllable up to 100 fs. Different from other techniques, the present method relies on the standard FEL configuration and it does not require a dedicated accelerator and undulator setting. With the LES, we achieved high efficiency, high energy and spectral stability paired to an independent control of the duration and of the relative intensity of the two colors. The LES enables shot-to-shot switching between one and two-color FEL and further, it is compatible with high repetition-rate FELs, as it is not associated to electron beam losses such as in other two-color methods.

The harmonic lasing technique was proposed to extend the harmonic up-conversion number, and a reverse taper undulator was then adopted to enhance the performance of the harmonic lasing process. But in a typical experiment, energy spread always plays an essential role in the exponential gain of a high-gain free electron laser. In this paper, phase shifters are employed at the Shanghai Soft X-ray Free Electron Laser Facility to reduce the damage effect of the energy spread and avoid the degradation of the final FEL performances. Three-dimensional simulations are conducted to verify the feasibility of the phase shifters’ ability to prevent high energy spread and obtain FEL with superior coherence.

Broadband light monochromatization in the extreme ultraviolet requires typically the use of diffraction gratings at grazing incidence, where the grating is rotated and eventually translated to perform the wavelength scanning. Several configurations have been proposed, using plane or concave gratings with uniform or variable line spacing. We propose here a low-cost experimental setup in which a thin plane diffraction grating is bended to an almost cylindrical shape to perform at the same time the spectral selection and the focalization of the selected spectral component. In such a way, the number of optical elements needed to realize the monochromator is reduced to two: the grating and the focusing mirror. A bendable grating has been tested in the 13-50 eV region, very good focal properties have been measured with very low residual aberrations. Furthermore, the shape of the bended surface has been measured using a wavefront sensor in order to confirm the effectiveness of the bending technique. The proposed solution can be used to achieve monochromatization both in large-scale facilities such as FELs or synchrotrons, but also in table-top setups, such as those exploiting high-order harmonic generation, that are often used for the preparatory phase of experiments to be later performed at FEL facilities.

The European X-Ray Free Electron Laser (EuXFEL) is a unique facility that provides femtosecond x-ray pulses of high pulse energy at MHz repetition rate. However, the high peak power results in a high dynamical heat load in the optical components, like monochromators, which reduces the intensity of the transmitted pulses significantly as compared to the full capacity of the EuXFEL source. To address these challenges at the high photon energy instruments of EuXFEL, we propose a diamond channel cut monochromator as an alternative to the standard Si monochromators. Diamond has a lower absorption cross-section at high photon energies and a higher thermal conductivity compared to Si, making diamond a promising candidate for x-ray optics applications under high heat load conditions. Here, we present a finite element model (FEM) of the temperature increase in diamond and the resulting thermal expansion to estimate the changes in the diffraction profile and the expected monochromator transmission depending on the number of pulses.

In this paper we introduce and discuss the EXtra-Xwiz pipeline for the semi-automated analysis of serial femtosecond crystallography data collected at the European XFEL. EXtra-Xwiz wraps the CrystFEL software suite, exposes data in a CrystFEL-compliant format, handles the interaction with the local high-performance computing cluster and simplifies certain experiment schemes such as the pump-probe one. Alongside with the integration of EXtra-Xwiz into the European XFEL ecosystem, future plans and developments are also briefly discussed.

Plasma accelerators can generate electron beams with a smaller physical footprint and properties un-achievable with conventional modulation methods. The Laser Plasma Accelerator (LPA) platform at the SASE (self-amplified spontaneous emission) line of Shanghai Soft X-ray Free-Electron Laser Facility (SXFEL) has been being constructed since 2020. The synchronization of the LPA platform and the electron beam from SXFEL has been achieved in 2022. The acceleration, and the energy modulation of the electron beam of the LPA platform is observed by an energy spectrometer. The designing and the commissioning of the LPA platform is ongoing. Simulation of the LPA system can be performed by the code Quick PIC, and the accelerated or modulated electron beam can be used to generate XFEL with different properties, such as ultrafast XFEL, large bandwidth XFEL and so on. In this paper, we will present the possibility of generating ultrafast XFEL by the electron beam modulated by the LPA platform. Preliminary start-to-end simulation result shows that we can achieve ultrashort XFEL pulses with the pulse duration of 4.8 fs by simple method, and the pulse duration can be further reduced by changing the setup of the LPA.

The majority of user experiments at the high repetition-rate free electron laser (FEL) facility FLASH are of pump-probe type, combining the extreme ultraviolet (XUV) or soft x-ray radiation from the FEL with ultrashort pulses generated by optical lasers. In this contribution, we demonstrate the advantages of using high-power Yb:YAG lasers with subsequent nonlinear pulse compression stages based on multi-pass cells (MPC). The approach enables the combination of hundreds of kHz to MHz repetition-rates, hundreds of watts of average powers and excellent intensity stabilities. We present the characteristics of the MPC-based pump-probe laser at the FLASH plane-grating beamlines. Furthermore, we report pulse compression to 8.2 fs pulse duration and the seeding of an optical parametric amplifier generating mid-IR radiation tunable from 1.4 µm to 16 µm.

We present the temporal characteristics of the split-and-delay unit at FLASH2 via visibility measurements which characterize the temporal resolution of the combined system of the FEL and the split-and-delay unit. The use of the split-and-delay unit at FLASH2 allows the users at the beamlines FL23 and FL24 at DESY to perform such pump-probe experiments. By using wavefront beam splitting, grazing incidence mirrors, and two different coatings the whole spectral region of FLASH2 is covered, and even harmonics up to 1800 eV are transmitted with a transmission of T>0.06. It is concluded that user experiments with a pump-probe scheme from the picosecond regime down into the sub-femtosecond region can be carried out.

The European X-ray Free Electron Laser (EuXFEL) began its user operation five years ago, opening and offering new research possibilities. The facility delivers high brilliance, ultra-short, spatially coherent x-ray pulses with a high repetition rate to six instruments (FXE, SPB/SFX, MID, HED, SCS and SQS) by means of three different beamlines (SASE 1, SASE 2 and SASE 3). One of the first detectors used for early-stage experiments was the Adaptive Gain Integrating Pixel Detector (AGIPD), custom designed to meet the challenging needs of scientific instruments. The AGIPD is a megahertz-rate integrating hybrid megapixel camera with a per-pixel adaptive gain amplification, allowing the integration of up to 104 of 12 keV photons per pixel in its low gain stage. Currently, three scientific instruments, namely SPB/SFX, MID and HED employ the AGIPD systems, the latter mentioned using a prototype, half-megapixel camera with an upgraded version of readout ASICs. The AGIPDs at EuXFEL are successfully used for experimental techniques like serial femtosecond crystallography, MHz single particle imaging, MHz x-ray photon correlation spectroscopy or MHz diffraction of materials under high pressures in a diamond anvil cell. Since September 2017, the AGIPD is continuously used and has become an established detector technology, with further advancements and developments planned. Delivering quality experimental data requires reliable and reproducible detector characterisation and calibration that have to be performed regularly with a continuous improvement of correction methods in close collaboration with scientific instruments. This work summarises five years of experience operating the AGIPD detectors at the EuXFEL scientific instruments. It gives an overview of scientific capabilities and examples of successful studies performed with AGIPD detectors. Moreover, challenges concerning detector calibration and characterisation are presented.