Terahertz (THz) radiation may pass through dielectric materials, and this ability can be used for a variety of applications. Terahertz (THz) radiation is located between infrared and microwave radiations in the electromagnetic spectrum. FEL produces brief, high-power THz single pulses, and we provide a diagnostic approach for them. The electro optic efficacy is used as a detection method. For the THz pulse, a GaAs crystal is used as a detector. The THz pulse causes the electro-optical crystal to shift polarization, implying that the electro-optic sampling device detects 30psec pulses (or depending on the pulse length of the accelerator). The optical pulse from the electo-optic sampling is coupled to fiber, allowing the optical pulse to be stretched to the order of nanoseconds.

This project describes different techniques to manufacture THz mirrors with arbitrary surfaces. The research is part of the development of THz transmission line for the compact FEL-THz accelerator. As an initial phase flat mirrors were 3D printed with FFF (Fused Filament Fabrication) and SLA (Stereolithography Apparatus). The impact of material, layer height and layer direction to mirror’s surface quality was exanimated. In addition, various metal coating was tested, for example vacuum evaporation and metal foil. The 3D printed flat mirror’s reflection was measured in TDS (Time Domain Spectroscopy) at 1–5 THz and compared with aluminum metal plate and glass silver coated mirror. The results approve sufficient surface and coating quality. Further research is manufacture off-axis parabolic mirrors, validate with a beam profiling and manufacture arbitrary surface mirrors optimized to the current accelerator by machine learning.

The free-electron laser in Hamburg (FLASH) operates in the extreme ultraviolet (XUV) and soft X-ray region, providing photon pulses of few femtosecond (fs) duration and unprecedented intensity [1]. FLASH operates in the self-amplified spontaneous emission (SASE) regime, meaning that every pulse has a unique combination of energy, spectrum, arrival time and pulse duration. Therefore, it is critical to be able to determine these parameters for each individual pulse. The THz field-driven streaking technique has the potential to deliver single-shot pulse duration information, as well as the XUV arrival time, basically wavelength-independent and over a large dynamic range (in pulse duration and FEL energy) [2, 3]. We present the results of several campaigns measuring the single-shot pulse duration over a wide range from 10 fs to 350 fs (FWHM) [3]. Here we focus on the particular difficulties in the different pulse duration regimes. Furthermore, correlations between the pulse duration and other radiation parameters as pulse energy and spectrum are compared on a single-shot and average level as well as being compared to simulations [4]. The variable gap undulators at FLASH2 also allow to study the evolution of the XUV pulse duration for the fundamental as well as for the 3rd harmonic radiation pulse as function of contributing undulators. The best agreement between measurement and simulation was found when modeling the SASE process using an energy chirped electron pulse. Finally, a comparison of the pulse duration determined by THz streaking with an alternative pulse duration diagnostic, a transverse deflecting structure (TDS) measuring the modulation of the electron bunch (analog to [5]) is shown and the advantages, as well as limitations of both techniques, are discussed. [1] W. Ackermann et al., Nat. Photonics 1, 336 (2007) [2] Grguras et al., Nature Photonics 6, 852-857 (2012) [3] R. Ivanov et al., J. Phys. B 53, 184004 (2020) [4] I. Bermudez et al., Opt. Express 29, 10491 (2021) [5] C. Behrens et al., Nat. Commun. 5, 3762 (2014)

Free electron lasers (FEL) serve a broad user community in many scientific fields ranging from atomic and molecular physics to plasma and solid state physics as well as chemistry and biology. Many experiments could benefit from a non-destructive online photon diagnostic of the provided x-ray pulses. Especially, for free-electron lasers that are operated in the self-amplified sponta- neous emission (SASE) regime, where the pulse characteristics fluctuate from pulse to pulse [1], reliable online information on the intensity, spectral distribution, and temporal structure of each individual pulse can be crucial. A fast feedback can significantly improve an on-the-fly evaluation of user experiments. In addition, subsequent sorting of measurement data by, for ex- ample, intensity or wavelength can reveal signatures of physical processes that would otherwise be hidden in the fluctuation. Finally, real-time information about the pulse can give a direct feedback for FEL beam tuning. Neural networks became popular as a powerful analysis tool in all categories of science [2]. This is due to their ability to recognize complex relationships in large datasets. There are various architectures of neural networks, each with its own focus on specific tasks. What they all have in common is that they need to be trained during a training process in order to recognize patterns and correlations. A special case of training is performed in unsupervised learning, where the network does not need any expert knowledge about the data. This can be done for example with autoencoder networks [3]. These networks consist of an encoder and a decoder. The encoder learns during the training phase to compress data to lower dimensionality, the so-called latent space, the decoder to reconstruct the input from this compressed representation. This means that, given the decoder, the latent space contains all information needed to reconstruct an in- put sample. A special form of autoencoder networks are β Variational Autoencoder (β-VAE) networks [4], that allow to balance between the goal of a perfect reconstruction of the data and a perfect disentanglement of the latent space vector components. These networks are found to be able to find the key principles in an unlabeled data set, even if these principles were not known before. We demonstrate the usage of β-VAEs to characterize SASE X-ray pulses of the free electron laser FLASH in Hamburg. We combine data from different diagnostic devices. We evaluate measured data from the online photoionization spectrometer OPIS [5], that uses 4 electron time of flight spectrometers to monitor each individual FEL pulse. In addition, we include data from an X-band transverse deflecting mode cavity diagnostic system (XTCAV). The latter is simi- lar to the XTCAV at the Linac Coherent Light Source [6]. This device measures the position and kinetic energy of the electrons after they have passed the undulator and is therefore able to monitor the differences in the temporal structure of the electron bunches due to the lasing process. We demonstrate that a β-VAE can detect key principles in the XTCAV and the OPIS data, like pulse duration and central wavelength and compare them to other diagnostic devices such as data from a gas monitor device (GMD) [7] and THz field-driven streaking [8]. Without a-priori knowledge the network is able to find directly human-interpretable representa- tions of single-shot FEL spectra, remove noise as well as reveal data artefacts and hence allows for an improved in-depth analysis of photon diagnostics data.

KAOS is the flagship optics of FERMI, the first - and presently only - fully seeded Free Electron Laser facility in the world. The name stands for Kirkpatrick-Baez Active Optical System, and it has been entirely developed in-house. After progressive revisions and upgrades, it presently empowers three out of six beamlines at FERMI, and it also serves two beamlines at FLASH, Hamburg (DiProI, LDM, MagneDyn; FL23 and FL24). Although KAOS grounds on the well-established concept of Kirkpatrick-Baez mirrors, the challenges it addressed and the needs it was built for, ultimately produced a unique system with unique features: a versatile curvature control, a broad spectral range (100 nm<λ<1 nm), and a large demagnification power (>80×). These features made KAOS an essential and mandatory tool to access the new class of scientific investigations addressed by FERMI, becoming a standard in time resolved spectroscopies, holography, and diffraction. In addition, it also enabled non-custom pump-probe spectroscopy correlation, and made the first realization of transient-grating in the XUV possible.The simple and clean mechanical design combined with the assiduous attention to online wavefront diagnostics did the rest in determining the success of KAOS over time. This contribution aims at telling how KAOS was born and grew up, showing how wavefront sensing made it work at the best, and how it will face the future challenges.

We present the study for a short period Apple-X variable polarizing undulator, with small gap of operation and high magnetic field, which will be the base module for the AQUA line of the EuPRAXIA@SPARC_LAB FEL facility, of next realization at INFN Laboratory of Frascati. The undulator allows to achieve radiation between 3 and 5 nm, the so called water-window, with a 1 GeV electron beam energy, lower than other FELs operating in the world, so giving the possibility to have a Soft X-ray source with a full polarization control in a more cost effective way and with less required space than the state of the art devices. An overview of the magnetic design is given with the main parameters and performances in terms of the field properties, tuning capabilities and the effects on the electron beam motion.

EuPRAXIA@SPARC_LAB is a new Free Electron Laser (FEL) facility that is currently under construction at the Laboratori Nazionali di Frascati of the INFN. Fermilab is contributing to the project with the design, manufacturing and qualification of a prototype conduction cooled superconducting undulator (SCU) that, if successful, could be integrated in the final machine. The design of the SCU capitalizes on the extensive experience present at Fermilab on cryomodules. Specifically, the system is based on the warm strongback concept developed for the PIP-II project which enables a modular design with multiple undulator coils integrated in a single vacuum vessel. This publication focuses on the overall design concept of the magnet system, its modularity, cost reduction potential and industrialization strategy.

Optimal FEL gain in a seeded FEL requires the careful alignment of different components. As for SASE FELs, the gain is optimized when the electron bunch travels in a straight line along the axis of each undulator in the radiator section. We have recently developed an alignment strategy for the optimization of the FERMI FELs which combines the beam-based alignment of the magnetic elements (undulators and quadrupoles) with the collinear alignment of spontaneous emission from each undulator. The method is divided into 3 steps. In the first step, we measure the undulator spontaneous emission with a spectrometer to fine-tune each undulator gap and set the best electron beam trajectory for collinear emission of each module. In the second step, the alignment of the undulator axis on the electron trajectory previously defined is achieved by looking at the undulator focusing effect. Finally, the seed laser is superposed on the electrons and aligned to maximize the bunching along the defined direction. This procedure can lead to an improvement in the control over the electron beam trajectory and results in a more efficient FEL process characterized by more stable and larger energy per pulse and a cleaner optical mode. A description of the method with the obtained results are reported in this work

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.

Development and characterization of an angle-resolved photo-electron spectrometer, based on the electron Time-of-Flight concept, designed for hard X-ray photon diagnostics at the European free-electron laser is described. The objective with the instrument is to provide beamline 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 often very high kinetic energy of photo electrons for the available target gases. In this contribution we describe development of the device, including electron trajectory simulations, and first tests with hard X-rays at the PETRA III synchrotron where we have characterized the performance and optimized the voltage settings for resolution and electron detection efficiency. We demonstrate a resolving power of better than 5 eV up to at least 20 keV photon energy.

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.

The superconducting undulator (SCU) based on the second-generation high-temperature superconducting (HTS) tapes is a promising application for building tabletop free-electron lasers (FELs). The short period < 10 mm undulators with a narrow magnetic gap < 4 mm are especially relevant. The advantage of the HTS tape is that it shows both high critical current density and high critical magnetic field. Each tape has 50 µm thickness and 12 mm width and is further scribed by a laser to achieve a meander structure, hence, providing the desired magnetic field pattern. Thus, a new approach to a superconducting undulator has been presented in the past and is further developed at KIT: each coil is wound with a single 15 m structured HTS tape. As a result, 30 layers of scribed sections lay above each other, and therefore, provide the required magnetic field. The results of the magnetic field measurements together with the results of the numerical investigation will be presented and discussed.

We were able to realize a compact microtron accelerator with 5 MeV electron beam acceleration energy and a hybrid electro-magnetic undulator that can vary the magnetic field of 1.07 T at 0.74 T. The electron beam is accelerated by an RF electric field of a 1-cell acceleration cavity and recirculated by a uniform magnetic field of the microtron main chamber. Through the re circulation process, the electron beam energy 5 MeV, energy spread 0.5%, pulse width 5 μs, and electron beam acceleration current are 48 mA. Hybrid electro-magnetic undulator set Iron buses in hybrid planar undulator structures and generate electro-magnet effects.The undulator has an iron pole in the form of a comb and can easily install one turn of electromagnet coil using a copper tube. This undulator can adjust the applied current to adjust the magnetic field strength to 0.74–1.07T, when the standard deviation of the maximum magnetic field strength distribution is very accurate with less than 0.5%. The trajectory of the electron beam of the undulator inside has the stable less than the 3 mm.

Kyma S.p.A. was awarded the design and production of the APPLE-X undulator for SABINA project at INFN - Laboratori Nazionali di Frascati. SABINA (Source of Advanced Beam Imaging for Novel Applications) is a project aimed at the enhancement of the SPARC_LAB research facility. The two user lines that are going to be implemented are; a power laser target area and a THz radiation line. Here we present the magnetic design and a novel mechanical implementation of this APPLE-X undulator for the THz/MIR radiation line. Undulator is made from three 1.35 m long sections. Each section consist of an APPLE-X magnetic array with 55 mm undulator period, a minimum gap of 10 mm and a mechanical frame. The undulator design is both compact and lightweight. This is achieved by novel mechanical design and implementation of the multiple dynamic corrections through the motion control system.

PolFEL will be the first free-electron laser facility in Poland. It will be driven with RF continuous-wave superconducting linac including an SRF injector furnished with a lead film superconducting photocathode. PolFEL will provide a wide wavelength range of electromagnetic radiation from 0.6 mm down to 60 nm. The linac will be split into three branches. Two of them will feed undulators chains dedicated for VUV, and IR radiation emission, respectively, and a single THz undulator will be settled in the third branch. The design of the THz undulator has been recently accomplished. It consists of a 1560 mm long permanent magnet’s structure ordered as a Halbach array of 8 periods. Large blocks dimensions, gap flux zeroing at full opening and 0.5 THz – 5 THz wavelengths range imposed on the undulator significantly influenced the final shape of the device, including blocks holders, girders and frame robustness unto magnetic forces, and hindered manufacturing and assembling processes. The following publication presents the challenges and solutions that were accompanying the conceptual phase.

A helical undulator provides a stronger FEL coupling than common planar geometries as the beam’s transverse velocity never vanishes. However, a significant challenge lies in tuning and measuring the fields with limited access to the beam axis along the undulator. Confirming the good field region off axis is difficult without space for 3D hall probe scans, and is important for low energy beams used to create THz radiation. We present our tuning procedures developed for the meter-long THESEUS undulators, consisting of two orthogonal permanent magnet Halbach arrays shifted by a quarter period relative to one another. The hall probe and pulsed wire measurements are guided by the general field expansion of helical undulators to correctly tune fields on and near the axis.

Diffraction gratings are an essential instrument used at free-electron laser facilities in soft and tender x-ray ranges. Their application ranges from monochromators and analyzers to self-seeding and pulse compression. These gratings are typically around 50-200 mm, up to 500mm in length with pitches from a few micrometers down to a few 100 nm, made on flat or curved substrates. Blazed gratings exhibiting higher efficiency are made by the ruling technique, however, the production of high-quality blazed gratings has become a significant bottleneck due to challenges in their fabrication and few suppliers. In this presentation, we report on a novel method for production of next-generation X-ray diffraction gratings based on gray-tone electron-beam lithography (EBL) and thermal oxidation of silicon. We can take advantage of the greatly enhanced flexibility regarding the grating design, allowing for enhanced optical performance as well as novel optical functionalities. This new technology will enable researchers all around the world to exploit fully the unique opportunities provided by the dramatically enhanced brilliance and coherence of a new generation of light sources.

X-ray absorption spectroscopy (XAS) enables the study of the electronic and geometric structural properties of matter. Such investigations can now be realized with femtosecond temporal resolution owing to the availability of X-ray free-electron lasers (XFELs) [1]. However, most XFELs currently utilize self-amplified spontaneous emission (SASE), which causes strong shot-to-shot fluctuations of their intensity and spectral distribution. Consequently, SASE fluctuations represent a challenge for the precise normalization of the measured absorption signal to the incident photon flux. Here, we have developed normalization schemes utilizing diffractive optics that overcome the SASE fluctuations. The diffractive optics are used to split the incoming XFEL SASE beam into two or three identical copies (±1 and 0th orders). By placing the (solid or liquid jet) sample in one of the diffracted beams, the absorption and reference signals are recorded simultaneously, thus enabling efficient data normalization on a shot-to-shot basis. In this contribution, we will present diffractive optics for two normalization schemes at SASE XFELs. First, a three beam geometry based on beam-splitting silicon off-axis zone plate [2] for soft XAS implemented at the Spectroscopy and Coherent Scattering beamline at the European XFEL to study L2,3 edges of transition metals will be presented. Secondly, a two-beam configuration for hard X-ray transient absorption spectroscopy of liquid jets (K-edge, an aqueous solution of [Fe(C2O4)3]3-) will be reported. Here, a beam-splitting transmission diamond grating for focused hard X-rays in combination with bent silicon <220> crystal was experimentally tested at the ALVRA station at the SwissFEL (ΔE/E ≈ 3% bandwidth). The results demonstrate high-quality K-edge transient XAS of [Fe(C2O4)3]3- solution without the need to scan the monochromator [3]. These normalization schemes pave the way for ultrafast L- and K-edge XAS measurements of transition metals at XFELs.

FELs deliver rapid pulses on the femtosecond scale, and high peak intensities that fluctuate strongly on a pulse-to-pulse basis. The fast drift velocity and high radiation tolerance properties of chemical vapor deposition (CVD) diamonds make these crystals a good candidate material for developing a high frame rate pass-through diagnostic for the next generation of XFELs. We report on two diamond based diagnostic systems being developed by a collaboration of a UC campuses and National Laboratories supported by the University of California and the SLAC National Laboratory. For the first of these diagnostic systems, we have developed a new approach to the readout of diamond diagnostic sensors designed to facilitate operation as a passthrough detection system for high frame-rate XFEL diagnostics. Making use of the X-ray Pump Probe (XPP) beam at the Linac Coherent Light Source (LCLS), the performance of this new diamond sensor system has been characterized and compared to that of a commercially available system. Limits in the magnitude and speed of signal charge collection are explored as a function of the generated electron-hole plasma density and compared to results from a TCAD simulation. A leading proposal for improving the efficiency of producing longitudinally coherent FEL pulses is the cavity-based X-ray free electron laser (CBFEL). In this configuration, the FEL pulses are recirculated within an X-ray cavity in such a way that the fresh electron bunches interact with the FEL pulses stored in the cavity over multiple passes. This creates a need for diagnostics that can measure the intensity and centroid of the X-ray beam on every pass around the recirculatory path. For the second of these diagnostic systems, we have created a four-channel, position-sensitive pass-through diagnostic system that can measure the intensity and centroid of the circulating beam with a repetition rate in excess of 20 MHz. The diagnostic makes use of a planar diamond sensor thinned to 43 µm to allow for minimal absorption and wave-front distortion of the circulating beam. We present results on the response and position sensitivity of the diagnostic, again measured using the LCLS XPP beam.

The European X-Ray Free Electron Laser (XFEL) operates three Undulator Systems to generate high-brilliance and high repetition X-ray pulses. Each System consists of multiple 5-m long undulator segments separated by 1.1-m long intersections. Such intersections contain vacuum systems, diagnostic and correction equipment for the electron's trajectory, and phase shifters (PS) [1] to match the phase of the electron beam and SASE photons. An array of Radfets monitors the absorbed doses at the entrance of each undulator segment since the start of operation in 2017 [2] but no dosimetry is available in the intersections. Recently, some SASE3 phase shifters stopped working and their motors had to be exchanged. This may have been caused by radiation damage. In this work we used Gafchromic films to measure radiation doses and its spatial profile in the intersections and PS vicinity. The measurements showed that significantly higher radiation doses are absorbed in the intersections as compared to the entrance of the next downstream undulator segment and significant radiation is also found near mechanical motors and electronic circuitry. We performed Monte Carlo simulations using the Geant4 code to investigate the composition of the radiation field in the intersections of the Undulator Systems and correlate it with the Gafchromic measurements and possible radiation damage to PS encoders and motors.

Ultracold plasmas (UCPs) form a new exotic category of plasmas that can be produced by photo-ionizing laser-cooled atoms in a magneto-optical trap (MOT) near-threshold. With densities up to $10^{18}$ m$^{-3}$, temperatures as low as $\sim$100 $\mu$K for the ions, and $\sim$1 K for the electrons, they are the ideal model plasmas to study fundamental processes in plasma physics, such as (the competition between) three-body recombination, disorder-induced heating, and collisional and collisionless microwave heating. To study these plasmas, conventional diagnostics such as Langmuir probes are not suitable, and tools from the field of atomic and particle physics are employed instead: charged particle diagnostics for electrons and ions, and laser-induced fluorescence and absorption imaging for ions. However, these diagnostics are limited by the charged particle’s time-of-flight to the detector or require optical transitions available to lasers and cameras, such as present in alkaline earth metals, to work. At TU/e, we recently developed a novel diagnostic that combines some of the advantages provided by the previous diagnostics. The diagnostic is based on a 5 GHz resonant microwave cavity and uses the shift in the resonance frequency of the cavity, induced by the UCP, to determine the electron dynamics of the plasma. This diagnostic allows us to study the dynamics simultaneously non-destructively, very fast (ns temporal resolution), with high sensitivity, and is a potentially interesting device for other types of plasmas as well, such as plasmas induced by extreme ultraviolet irradiation.

Electromagnetic wave undulators have the advantage of a shorter period compared with the permanent magnet undulators when operating at high frequency, therefore producing FEL radiation at the same wavelength with less electron energy. This paper investigates the properties of a Ka-band microwave undulator, and the factors that affect the choice of the high-power drive sources, through the design and beam dynamic study of a 36GHz cavity-type microwave undulator proposed for the CompactLight X-ray FEL. The future research is to prototype a millimeter-wave undulator operating at ~100GHz, which will have an undulator period of about 1/10 of the state-of-the-art permanent magnet undulators. The millimeter-wave undulator will allow the generation of soft X-ray radiation at much lower beam energy, such as hundreds of MeV, enabling a reduction in the cost of a compact XFEL facility.

DEIMOS (Dichroism Experimental Installation for MagnetOptical Spectroscopies) is the beamline built at French Synchrotron SOLEIL facility intended for soft X-rays magnetic and natural dichroism spectroscopies. It has been designed to enable most challenging measurements in terms of X-rays sample sensitivity and signal detection level. The energies accessible on DEIMOS beamline rank from 350 eV up to 2500 eV, with all polarizations (circular left and right, linear), covering the absorption edges of the elements most relevant to the magnetic nanostructure scientific community, i.e. the first (3d) and second (4d) rows transition metals L-edges, the rare earth elements M-edges and nitrogen, oxygen and sulfur K-edges. While its main end station, a 7 Tesla cryomagnet, allows for measurements down to sub-Kelvin temperature up to room temperature, its second end station, a 2 Tesla electromagnet, is currently under renovation thanks to the partnership between Italian power supply constructor OCEM Power Electronics and French electromagnet manufacturer SEF Technologies. The coil is made of hollow copper to allow direct cooling in order to achieve the required 2 Tesla field strength. The magnetic model of this coil has been studied and validated before manufacturing. The new power supply will have a four quadrant fast switching topology, with a high stability output. To increase the reliability, the architecture is a proven modular technology coming from previous realizations running in other facilities. Once renovated, the so-called MK2T end station will allows for fast switching (1 Hz) between +/-2 Tesla. It will be aimed to host most peculiar inserts such as variable temperature liquid cell, high temperature (1000 K) and multiferroic inserts. Commissioning is expected as early as autumn 2022 and the facility could be available to users through standard review of the SOLEIL program committee, at the upcoming call for proposal.

The implementation of a helical afterburner undulator at DESY's VUV-FEL source is part of the current FLASH2020+ upgrade program. The device shall be installed downstream of the present FLASH2 SASE undulators and will provide radiation with variable polarization from 1.33 nm to 1.77 nm (890-700eV) and thus also cover the L-edges of the 3d transition metals Fe, Co, and Ni. Despite a moderate energy upgrade of the machine to 1.35 GeV, the required wavelengths and tunability range can only be reached by a high magnetic performance of the undulator. We report on design and development of an APPLE-III undulator with 17.5 mm period length operating at a minimum magnetic gap of 8 mm which will make use of a magnetic force compensation scheme. A short prototype has been built to verify and iterate both the mechanical and magnetic concept. Details on keeper design, prototype results and the tuning concept will also be discussed. The full length device is presently under construction and shall also verify this concept for the future seeding undulators at FLASH1.

The XFELs with an anomalously high peak brilliance are opening the way to a number of novel X-ray photon research paths. At SPring-8 Angstrom Compact Free-Electron Laser (SACLA) [1], the XFEL pulses with high stability and short pulse duration (6-7 fs) have been regularly provided thanks to the unique electron gun, accelerator, and undulator systems [2]. By focusing these XFELs to 1um-100nm, the peak intensity has been dramatically increased and new phenomena in hard X-ray nonlinear optics have been explored, such as observation of saturable absorption [3], two-photon absorption [4], and the atomic inner-shell laser emission [5]. To further promote the study in the ultra-intense X-ray laser field, we have developed a focusing system that achieves sub-10nm spot size and 10²² W/cm² intensity. For the sub-10 nm focusing optics, an advanced Kirkpatrick-Baez (AKB) mirror system based on Wolter-type III geometry [6] has been adopted. The AKB consisting of one-dimensional Wolter mirrors can satisfy Abbe’s sine condition, which leads to a reduced coma aberration and a high tolerance to the incident angle error. We have designed and developed the AKB mirror system for SACLA BL3-EH4c at a photon energy of 9.1 keV. One of the remarkable challenges for the development was the fabrication of the mirrors with 1-nm accuracy. We applied an X-ray wavefront correction scheme [7] for the precise fabrication, and achieved wavefront accuracy of λ/15 rms which satisfies Maréchal’s criterion. Ptychographic probe measurements revealed the focusing spot size of 6.6 nm (horizontal) × 7.1 nm (vertical), indicating eventually attained focused intensity of 1.21 × 10²² W/cm². References: [1] T. Ishikawa et al., Nat. Photon. 6 (2012). [2] For example, I. Inoue et al., Phys. Rev. Lett. 127 (2021). & T. Osaka et al., Phys. Rev. Research 4 (2022). [3] H. Yoneda et al., Nat. Commun. 5 (2014). [4] K. Tamasaku et al. Nat. Photon. 8 (2014). & K. Tamasaku et al., Phys. Rev. Lett 121 (2018). [5] H. Yoneda et al., Nature 524 (2015). [6] J. Yamada et al., Opt. Express 3 (2019). [7] S. Matsuyama et al., Sci. Rep. 8 (2018).

Cavity-Based X-ray Free-Electron Lasers (CBXFELs) employ an X-ray cavity formed by crystal mirrors such that X-ray pulses receive periodic FEL-amplification and Bragg-monochromatization. CBXFELs enable improved longitudinal coherence and spectral brightness over single-pass self-amplification of spontaneous radiation (SASE) FELs [1,2] for high-repetition rate FELs. Construction and alignment of a stable low-loss cavity of Bragg-reflecting mirrors has been considered a daunting challenge and has not seen previous experimental implementation of large X-ray cavities in the hard X-ray regime. In this work, we demonstrate stable operation of a low loss 14-m-roundtrip rectangular cavity of four Bragg-reflecting diamond (400) mirrors. 9.831 keV X-rays from the Linac Coherent Light Source (LCLS) were in-coupled into the cavity via a thin diamond transmission grating. X-ray ring-down was characterized using fast photodiodes and a nanosecond-gated camera. Intra-cavity focusing was introduced to further stabilize the cavity, enabling observation of X-ray storage at >50 round trips. This experiment demonstrates feasibility of a stable low-loss hard X-ray cavity that will support future CBXFEL tests and operation [3].

Free-electron lasers (FELs) are currently the most advanced class of light sources, by virtue of their unique capability to lase high-brightness and ultrashort pulses characterized by wavelengths spanning the Extreme-Ultraviolet (EUV), the Soft (SXR) and Hard (HXR) X-Ray spectral domains, alongside with temporal duration lying in the femtosecond (fs) timescale [1]. Specifically, the advent of FELs light sources has recently allowed to perform, in a time-resolved fashion approach, both established spectroscopies, daily employed at synchrotron light sources, and novel non-linear optical methods, mostly combining FELs and laser pulses. Nonetheless, the next step to push the ultrafast X-Ray science standards is widely recognized to be linked to go beyond the current time-resolved schemes, so performing experiments engaging exclusively EUV, SXR and HXR pulses. Indeed, exciting (and probing) matter at its (or nearby) electronic resonance is largely speculated to be one of the key for discriminating and revealing the microscopic mechanisms hiding behind some of the most exotic phases of physical, chemical, and biological systems. Such a challenge calls the design of optical devices capable to both split and delay (in time) FELs pulses, without impacting on their coherence properties, and fully user-friendly in terms of preserving the perfect overlap of the resulting focal spots, even in the few microns spatial domain, a well-known trademark for focusing EUV, SXR and HXR pulses at FELs light sources [2]. At the seeded FERMI FEL (Trieste, Italy) this goal is committed by the novel optical device known as AC/DC, which stands for the Auto Correlator/Delay Creator. AC/DC is purposely designed to double the incoming FEL photon beam into two exact pulse replicas, splitting it by inserting a grazing incidence flat mirror, and further delaying in time, in a controlled way, one of the two pulses, with an intrinsic temporal resolution of approximately 360 attoseconds. A detailed description of AC/DC is highlighted here. Specifically, strong emphasis is dedicated to the opto-mechanical design and the laser-based feedback system, purposely designed and implemented to compensate in real-time any potential drift and pointing mismatch affecting the FEL optical trajectory, ascribable to both mechanical imperfections and residual paraxial errors appearing during a temporal delay scan [3]. [1] Bostedt C., Boutet S., Fritz D.M., Huang Z., Lee H.J., Lemke H.T., Robert A., Schlotter W.F., Turner J.J., Williams G.J., Linac Coherent Light Source: The first five years. Rev. Mod. Phys.88, 015007 (2016) [2] Manfredda M., Fava C., Gobessi R., Mahne N., Raimondi L., Simoncig A., Zangrando M., The evolution of KAOS, a multipurpose active optics system for EUV/Soft X-rays, Synchrotron Radiation News, 0, 0, (2022) DOI: 10.1080/08940886.2022.2066432 . [3] Simoncig A., Manfredda M., Gaio G., Mahne N., Raimondi M., Fava C., Gerusina S., Gobessi R., Abrami A., Capotondi F., De Angelis D., Menk R., Pancaldi M., Pedersoli E., Zangrando M., AC/DC: The FERMI FEL Split and Delay Optical Device for Ultrafast X-rays Science, Photonics, 9(5), 314, 2022