For the development of X-band deflecting structure at Shanghai Synchrotron Radiation Facility (SSRF), two units of X-band deflecting structures totally including six RF structures have been used on SXFEL successfully for ultra-fast beam diagnostics. The construction of another new FEL facility has started from 2018, which is named Shanghai high repetition rate XFEL and extreme light facility (SHINE). Four units of X-band deflectors will be installed on SHINE. The design and measurement of the first prototype has been finished, and the high power test will be carried out soon, in this paper, the design and measurement results will be presented.

The Fritz-Haber-Institut (FHI) der Max-Planck-Gesellschaft Free Electron Laser (FEL) achieved first light from its Mid-IR beamline at 18 microns on February 14, 2012. In the subsequent years, the 3 to 60 micron light has been supplied to users resulting in 96 refereed publications in Chemical Physics. In 2019, the FEL Group initiated an upgrade to add a Far-IR beamline to the system. On June 8, 2023, first light was achieved at 8 microns from this beamline which spans 4.5 to 165 microns in tunable radiation. A unique feature of this upgrade is the inclusion of a 500 MHz kicker cavity that can send the 1 GHz electron pulses alternatively into the MIR and FIR beamlines. On December 8, 2023, first light was obtained simultaneously at 18 and 55 microns respectively, thereby achieving the project goal of independently-tunable two-color lasing. We will discuss the physics and engineering design of the new FIR beamline and provide details of the radiation spectrum and parameters. We will also outline planned user experiments using this new radiation tool.

FLASH, the XUV and soft X-ray free-electron laser user facility at DESY, is in the transitional period between two substantial upgrade shutdowns within the FLASH2020+ upgrade project. FLASH consists of a common part FLASH0 (injector & superconducting linac), two FEL beamlines (FLASH1/2) and an experimental beamline FLASH3, accommodating the plasma wakefield experiment FLASHForward. The first (2021/22) shutdown was aimed at upgrading FLASH0 and install an APPLE-III undulator in the otherwise unchanged beamline FLASH2, enhancing the third harmonic at flexible output polarization. The next (2024/25) shutdown will focus on the complete exchange of the FLASH1 beamline to allow for externally seeded operation in the range from 60 nm down to 4 nm at 1 MHz bunch repetition rate (600 μs trains at 10 Hz train repetition rate). We report on the operation between the two shutdowns which was, to a large extend, dedicated to FEL operation for users and on the commissioning of the new features implemented in the last shutdown.

FLASH, the superconducting XUV and soft X-ray FEL is undergoing a substantial upgrade (FLASH2020+) with two long shutdowns 2021/22 and 2024/25. In the 1st shutdown, FLASH's 2nd bunch compression chicane (BC2) has been completely redesigned for the FLASH2020+ upgrade project. The redesign allowed the installation of two quad/skew-quad packs in each of the arms of the 4-dipole (C-type) chicane. With these corrector quadrupoles it should be possible to partially compensate linear correlations between the transverse centroids and the longitudinal position inside the bunch, so called bunch-tilts. During the limited commissioning/development shifts in a year of operation devoted to maximizing user hours we started measuring the impact of the quads on the bunch tilts and the unavoidable effects on dispersion closure and beam optics. In this contribution we report first results.

The operation of light source accelerators is a complex process that involves a combination of empirical and theoretical physics, simulations, and data-intensive methodologies. For example, the FLASH1 beamline at DESY is upgrading to an external seeding FEL light source*. We utilize special diagnostics, machine learning algorithms, and comprehensive simulations to achieve this. To optimize resources, we constantly look to improve our approach, allowing us to robustly control the accelerator and meet the desired stability of our users. Machine learning and GPU-based algorithms have become crucial, enabling us to employ advanced optimization techniques and possibly AI. However, in many cases, it is imperative to establish a robust mechanism for simulations involving large particle numbers to ensure that future upgrades and experiments can effectively and sustainably leverage these computational strategies.

Powered by a storage ring with energies ranging from 240 MeV to 1.2 GeV, the Duke Free-Electron Laser (FEL) has demonstrated operation across a broad wavelength spectrum from infrared (IR) to vacuum ultraviolet (VUV): 1100 nm to 170 nm. This FEL serves as a photon source for the High Intensity Gamma-ray Source (HIGS), producing polarized, near-monochromatic, and high-flux Compton gamma-ray beams in an extensive energy range from 1 MeV to 120 MeV, with the highest flux recorded at 3.5e+10 ph/s (total) around 10 MeV. To generate high-energy gamma-ray beams above 80 MeV, the FEL must operate in the VUV region from 195 nm to 155 nm. This work describes the development and operation of VUV beam diagnostics within a nitrogen-purged enclosure, with increased difficulty as the wavelength shortens towards 155 nm. We will discuss the challenges encountered and the solutions found for VUV beam diagnostics, leading to the successful FEL lasing in the VUV region.

FEL oscillators typically employ a two-mirror cavity with spherical mirrors. For storage ring FELs, a long, nearly concentric FEL cavity is utilized to achieve a reasonably small Rayleigh range, optimizing the FEL gain. A challenge for the Duke storage ring, with a 53.73 m long cavity, is the characterization of FEL mirrors with a long radius of curvature (ROC). The Duke FEL serves as the laser drive for the High Intensity Gamma-ray Source (HIGS). As we extend the energy coverage of the gamma-ray beam from 1 to 120 MeV, the FEL operation wavelength has expanded from infrared to VUV (1 micron to 170 nm). To optimize Compton gamma-ray production, the optimal value for the mirror's ROC needs to vary from 27.5 m to about 28.5 m. Measuring long mirror ROCs (> 10 m) with tight tolerances remains a challenge. We have developed two different techniques, one based on light diffraction and the other on geometric imaging, to measure the long ROCs. In this work, we present both techniques and compare their strengths and weaknesses when applied to measure mirror substrates with low reflectivity and FEL mirrors with high reflectivity.

Reliable operation of a seeded Free Electron Laser requires the simultaneous control of several electron-beam, laser and accelerator parameters. With EEHG the complexity increases due to the second seed laser and the strong dependence of EEHG bunching to seeding parameters. With the recent upgrade of the FEL-1 line, FERMI is the first FEL facility to be operated in EEHG mode for users. This required a major work for developing software tools that could be used to easily set the FEL at the desired wavelength. We report here on the recent software developments at FERMI for the operations of the new FEL-1. An important prerequisite for EEHG is to determine both the electron beam energy spread and seed laser induced energy modulation. This is done by using HGHG time dependent bunching equations to match experimental parameters scans. With these data, optimal EEHG settings of the machine parameters are then calculated to reach the desired FEL wavelength. The requested parameters are then sent to interface tools that accurately control laser, undulator, chicane and electron beam.

Most FELs employ the mechanism of self-amplified spontaneous emission (SASE) from a relativistic electron beam to generate intense femtosecond pulses in the x-ray spectral region. Such SASE FELs are characterized by a broad bandwidth and relatively poor longitudinal coherence, and offer a rather limited control over the spectro-temporal properties. The limitations of a SASE FEL can be overcome by using an external laser to trigger the amplification process. Echo-enabled harmonic generation (EEHG), alone or in combination with the high-gain harmonic generation scheme (HGHG) is currently the most promising candidate to extend the operation of externally-seeded FELs into the soft x-ray region. Here, we discuss the plan at FERMI for the upgrade of the second FEL line in order to reach ~2 nm at the fundamental emission wavelength. In the first step, coherent radiation at ~10 nm will be generated with an EEHG layout and used as a seed in an HGHG stage on a fresh part of the electron beam. The experience with EEHG at the FEL-1 line will be an important step towards the final realization of the FERMI FEL as a reliable source of highly coherent radiation at ~2 nm and below.

The conceptual design of the laser-plasma-based soft X-ray Free Electron Laser at ELI-Beamlines involves the integration of a novel high-power, high-repetition-rate laser, plasma source, compact LPA-based electron beam accelerator, dedicated electron beam line with relevant diagnostics, undulator beam line, photon beam line with required diagnostics, as well as a photon beam distribution system. The proposed concept of the whole setup is optimized to produce high-quality, coherent X-ray pulses with femtosecond duration in the ‘water-window’ wavelength of the photon radiation, which will be used by the photon user community for expiring research in the field of biology to study biological structures and processes at the cellular and molecular level at high resolution. In addition, the laser-plasma-based soft X-ray FEL will extend the abilities of users in material science to study nanostructures and thin films. In the frame of this report, we present the conceptual design for the full setup, which will be incorporated into the existing infrastructure of ELI-Beamlines. Furthermore, we discuss the key obstacles and the role of this project in the EuPRAXIA joint activity.

The plasma accelerator-based Free Electron Laser research program at ELI-ERIC (ELI-Beamlines, Czech Republic) intends to utilize the unique qualities of plasma accelerators to build FELs with remarkable brightness, coherence, and pulse length. The program is based on the novel high-power, high-repetition-rate laser system, which is under preparation at ELI-Beamlines. The program entails expanding the LUIS experimental setup to test and validate the performance of the laser-plasma accelerator-based extreme ultra-violet (EUV) FEL, integrating a high-power laser, plasma source, and electron beam transport line with relevant diagnostics to create a comprehensive test bed for the development of the EuPRAXIA LPA-based FEL. The plasma accelerator-based FEL development program at ELI-Beamlines represents an innovative effort to expand the capabilities of FEL technology and open new possibilities for scientific research and industrial applications. In the frame of this report, we provide an overview of the relevant developments at ELI-ERIC (ELI-Beamlines) as well as the main challenges of this program.

A second hard X-ray beamline (HX2) at the PAL-XFEL (Pohang Accelerator Laboratory, X-ray Free Electron Laser) has been proposed to meet the increased demands of XFEL science. A photon energy ranging between 1.5 to 10 keV was determined to cover low photon energy with enhanced FEL pulse energy of about 3.0 mJ, and to cover mostly used range between 8 to 10 keV simultaneously. Accordingly, baseline design of the electron beamline was completed using MAD-X code. Here, to avoid physical overlap of the beamline elements, a dog-leg transport line is installed. In addition to first-order optics design, complete start-to-end simulation is performed to understand the evolution of the 6D electron beam phase space and to optimize the beam parameters such as energy chirp, energy spread, and emittance at the entrance of the undulator. In this study, we will show the start-to-end simulation by using Impact-T for injector section and ELEGANT for the remaining sections from linac modules to the end of the HX2 undulator line. Particularly, we will discuss whether coherent synchrotron radiation effects along the dog-leg section is suppressed so that the beam phase space distortion is minimized. Plus, we will introduce planned R&D activities such as AI/ML-based injector operation (virtual machine) and various studies on the XFEL modes such as multi-bunch operation, enhanced SASE (ESASE), and THz FEL.

Scientific opportunities with very hard XFEL radiation demands dedicated facility development towards FEL operation in the sub-ångström regime. Very hard X-rays provide capabilities of high Q-range coverage and high penetration, and also allow access to the K-edge spectroscopy of high-z materials. Production of such X-rays using FELs takes advantage of general FEL characteristics such as large coherence, short pulse option, variable pump-probe delay control and higher brightness compared to conventional storage ring sources. R$\&$D activities in the characterization and production of high energy photon beams beyond 25 keV has been launched since 2021 at the EuXFEL. Photon beams of 30 keV have been produced, characterized and delivered to experimental hutches. In this paper, we give an overview of the overall development. Obtained results will be discussed.

The UK XFEL project is now mid-way through its three-year Conceptual Design and Options Analysis (CDOA) phase. The purpose of this phase is to develop concepts to meet the required ‘next-generation’ XFEL capabilities identified in the project’s peer-reviewed Science Case, developed by UK scientists. The envisaged next-generation features are a step-change in both the number of simultaneous experiments and in their capability – through multiple, combinable FEL sources delivering transform limited pulses across a wide range of photon energies and pulse durations, together with a comprehensive array of synchronised sources including high power lasers and particle beams. The project is assessing options to achieve this either via a new UK-based facility or by investment at existing XFELs, based on criteria that include performance, cost, and environmental sustainability. The project is holding a series of Town Hall meetings and workshops around the UK (see https://xfel.ac.uk) and is expanding collaborations nationally and internationally.

The FLARE project aims to investigate special operation modes of the laser heater at the free-electron laser FLASH in Hamburg that enable the generation of few- or possibly sub-femtosecond soft X-ray pulses. To this end, laser pulses of the laser heater are split and then recombined after one pulse has been delayed. By controlling the interference of both pulses via their temporal overlap, a longitudinally non-uniform heating of the electron bunches can be achieved. Utilizing this, two short-pulse generation schemes are to be implemented as part of the FLARE project. In the first scheme, the energy spread of the bunch is increased to a degree that inhibits lasing, leaving only a small unheated region which emits a short FEL pulse. The second scheme works by imprinting an energy modulation with a linearly increasing amplitude onto the longitudinal phase-space distribution of the bunch. In subsequent magnetic chicanes, this phase-space structure results in a localized compression of the bunch, creating an extremely short current spike, which might be used to produce an X-ray pulse on the same time scale. The FLARE setup as well as first experimental results are presented.

A single-pass THz free-electron laser (FEL) at the Photo Injector Test facility at DESY in Zeuthen (PITZ) was designed and implemented for a proof-of-principle experiment on a tunable high-power THz source for pump-probe experiments at the European XFEL. THz pulses are generated at a radiation wavelength of 100 μm within a 3.5 m long, strongly focusing planar LCLS-I undulator. High gain is achieved by driving the FEL with high brightness beams from the PITZ photoinjector at 17 MeV and a bunch charge of up to several nC. In addition to the mechanisms of self-amplified spontaneous emission (SASE), seeding of the THz-FEL by electron bunch modulation at the photocathode is also being investigated. The experimental results, including the gain curves and spectral properties of the THz-FEL radiation, are presented in comparison with theoretical predictions and numerical simulations.

The paper introduces design and optimization of a high-repetition-rate infrared terahertz free-electron laser (IR-THz FEL) facility, which leverages optical resonator-based FEL technology to achieve a higher mean power output by increasing pulse frequency. Electron beam of the facility will be generated from a photocathode RF gun injector and further accelerated with a superconducting linear accelerator. Taking into account the collective effects, such as space charge, coherent synchrotron radiation (CSR), and longitudinal cavity wake field impacts, beam dynamics simulation for the injector, the accelerator, as well as the bunch compressor, has been done with codes of ASTRA and CSRTrack. With optimized microwave parameters of the linac, current profile with good symmetry has been obtained and the peak current can reach 100 A.

A new infrared Free-Electron Laser (FEL) facility FELiChEM has been established as an experimental facility at the University of Science and Technology of China. It consists of two free electron laser oscillators which produce mid-infrared and far-infrared lasers covering the spectral range of 2-200 μm at the present stage. The output power is a crucial parameter for users, and it is usually achieved by an out-coupling hole located in the center of a cavity mirror. Nevertheless, the spectral gap phenomenon has been observed in FEL oscillators with partial waveguides as the output power is highly dependent on the mode configuration before the out-coupling mirror. Such power gaps have an adverse effect on experimental results since numerous experiments require continuous spectral scanning. In this paper, we propose the utilization of multiple out-coupling holes on the cavity mirror, instead of relying solely on a central out-coupling hole, to reduce the adverse impact of the spectral gap phenomenon.

To explore and detect novel effects and mechanisms inherent in materials, a tenable wavelength terahertz free electron laser (THz-FEL) is integrated into the terahertz near-field high-throughput material property testing system (NFTHZ). Employing a compact linear accelerator capable of adjusting electron energy within the range of 10 to 18 MeV as the injector, pre-bunching of electron bunches is implemented to create longitudinal spacing structures by manipulating the driving laser. By appropriately correlating the forming factor of electron bunches, electron beam energy at the undulator entrance, and the undulator K value, a terahertz free electron laser with peak power in the megawatt range and a central wavelength spanning from 0.5 to 5 THz can be achieved. This article provides an overview of the development progress of THz-FEL within the NFTHZ framework.

SHINE (Shanghai Hard X-ray FEL Facility) is a high repetition rate X-FEL facility under construction in Shanghai, China. The facility is based on an 8 GeV CW superconducting linac and plans to have 3 undulator lines and 10 experimental stations in phase-I, covering the photon energy range of 0.4 – 25 keV. Mass production of the components and installation of the machine are in course. User experiments are expected to start in 2025. This presentation summarizes the proposed configuration and the status of R&D and production for the critical components and systems, discussing the key technologies. The current status of the project and  the plans leading to the completion will be presented, outlining the major scientific goals of the facility.

An Argonne-SLAC collaboration is working on the design of a superconducting undulator (SCU) demonstrator for a free-electron laser (FEL)*. A SCU magnetic structure consisting of a 1.5-m-long planar SCU magnet, and a superconducting phase shifter have been designed. A novel three-groove correction scheme has been implemented for the SCU magnet. A compact four-pole phase shifter with magnetic shields was also designed. This paper presents the calculations of the magnetic performance of the phase shifter and a planar SCU magnet, which include magnetic field and field integrals with end corrections.

In the framework of the SABINA project (Source of Advanced Beam Imaging for Novel Applications), a new Free Electron Laser line will be realized at the Laboratori Nazionali di Frascati (LNF). It will be based in the SPARC_LAB laboratory with the purpose to supply radiation in the Thz/MIR range to external user. The line layout foresees two correctors between the three APPLE-X undulators devoted to providing angular and position offset correction to the beam aiming to maximize the efficiency of the FEL process. They will steer the electron beam both in the X and Y axis at the mrad level, and they will be integrated with Beam Position Monitors to perform the trajectory correction and the position monitoring at the same point. This paper presents the magnetic design of the two correctors performed by OPERA 3D software, the mechanical design, the manufacturing together with the magnetic measurement performed at the magnetic laboratory facility in LNF using a Hall probe system.

Orbital angular momentum (OAM) photon beams are excellent tools for non-contact optical manipulation of matter in a broad photon energy range. A free-electron laser (FEL) oscillator is well-suited for studying OAM beams with various features including a wide spectral coverage, wavelength tunability, two-color lasing, etc. Here, we report the first experimental demonstration of superposed OAM beams from an oscillator FEL. Lasing at around 458 nm, we have generated superposed OAM beams up to the fourth order as a superposition of two pure OAM modes with opposite helicities. These generated beams have a high beam quality, a high degree of circular polarization, and high power. Using external rf modulation with frequencies from 1 to 30 Hz, we also developed a pulsed mode operation of the OAM beams with a highly reproducible temporal structure. FEL operation showcased in this work can be extended to higher photon energies, e.g. using a future x-ray FEL oscillator. The operation of such an OAM FEL also paves the way for the generation of OAM gamma-ray beams via Compton scattering.

The FERMI free-electron laser (FEL) facility has recently achieved a significant milestone with the successful implementation of the echo-enabled harmonic generation (EEHG) scheme in the FEL-1 amplifier line. This advancement is part of a broader upgrade strategy aiming at expanding the covered spectral range of the facility to the entire water window and beyond. Through this upgrade, the maximum photon energy of FEL-1 has been doubled and spectral quality has been enhanced. The updated FERMI FEL-1 is the first user facility operating in the spectral range 20-10 nm utilizing the EEHG scheme. It will serve also as the ideal test bench for conducting new machine studies in the perspective of future developments. In this contribution, we present the results obtained during the commissioning phase and the first user experiments.

The compact ERL (cERL) is operated at mid-energy region around 17 MeV for beam studies on industrial applications since 2017. Toward the future high power FEL source for EUV lithography, high current beam operation was demonstrated at low bunch charge after install of undulators as a first step. It is critical to reduce beam loss not to exceed 20 uSv/h outside the shield wall of the cERL acceleration room, however, it can increase especially at the arc sections, the undulators, and superconducting cavities for decelerating. Therefore, 16 high-speed loss monitors are located along the whole beam line as the machine protection system. Recently, machine learning is applied for beam tuning to reduce all loss monitor signal. In addition, we tried the energy recovery operation while undulator light is amplified at a high bunch charge around 60 pC.

Bunch length is an important parameter for free-electron laser (FEL). The deflecting RF cavity was used in the beam length diagnostic instrument. In this paper, we present the design of a 3-cell rectangular deflecting RF cavity for a compact terahertz (THz) free-electron laser (FEL) facility. The 3-cell deflecting cavity has a residual orbit offset of zero as compared to single-cell deflecting cavity. Rectangular deflecting cavity does not need to lock the dipole polarisation direction as compared to cylindrical cavity. The time resolution of the measurement system can reach 500 fs. In this paper, the cavity design is carried out using CST and the results of cavity analysis are presented. Particle tracking is performed with the Astra code and the space charge effect is taken into account.

The mechanisms that drive short-range modulations in the longitudinal phase space of accelerated electron bunches, otherwise known as the microbunching instability, have undergone intensive study. The various collective interactions between charged particles within the bunch, and their environment, can degrade the quality of these bunches, eventually making them unsuitable to drive light sources such as free-electron lasers (FELs). Although the most common method for removing this instability at X-ray FELs – namely, the laser heater – has proven to be very useful in improving the performance of these facilities, alternative methods to achieve this goal are active areas of research. In this contribution, we present experimental evidence of the influence of transverse Landau damping on mitigating the microbunching instability.

Version 4 of the widely used time-dependent FEL code Genesis 1.3 has been released. The C++ code keeps the entire bunch in memory and thus allows for self-consistent effects such as wakefields or long-range space charge fields. With sufficiently allocated distributed memory, Genesis 1.3 can represent each individual electron. This solves the problem of the shot noise statistics at any arbitrary frequency in the simplest way and allows for sorting and redistribution of particles among the computer cores for advanced FEL applications such as the Echo-Enabled Harmonic Generation schemes. This presentation reports on the new physics added to the code as well as features which simplify the setup of the simulations as well and the ability to link user-made libraries to adapt to the specific needs of each user.

Hefei Infrared Free-Electron Laser device (IR-FEL) is a user experimental device dedicated to energy chemistry research that can generate high brightness mid/far infrared lasers. It is driven by an S-band linear accelerator with a maximum electron energy of 60 MeV. The stability of the final output laser is determined by the energy stability and spread of the electron beam, and the Low-Level RF control system (LLRF) is opitimized to improve the energy stability of the electron beam. There are two klystrons in the linear accelerator of IR-FEL, and the periodic oscillation of out power output of the klytrons is existed (approximately ± 0.2%~2% for amplitude). The oscillation period of two klystrons are exchanged in the case of exchanging the filament power supplies of two klystrons. The pulse-to-pulse feedforward and in-pulse feedback algorithm are developed to compensate the periodic fluctuations of the output power of the klystrons, and the IQ demodulation is changed to the Non-IQ demodulation (13/3) to separate and suppress the odd harmonic. After the optimization, the stability of klystron output signal has been improved from 0.12%/0.07° (rms) to 0.04%/0.09° (rms).

The SABINA project will add a user facility to SPARC_LAB at INFN in Frascati (Rome). For the THz line, an electron beam is transported to the APPLE-X undulators to produce photon pulses in the ps range, with energy of tens of µJ, with linear or elliptical polarization. Each undulator has four magnetic arrays that can be moved radially simultaneously to set the operating gap. Two arrays can also move longitudinally for phase displacement. A structural analysis of this unique mechanical structure has been performed by the production company (KYMA S.p.a) to ensure good field quality and beam trajectory. To support those, a set of tests has been performed with FBG acting as strain sensors in Frascati. An FBG is a phase grating inscribed in the core of a single-mode fiber, whose Bragg-diffracted light propagates back along the fiber. Any deformation of the grating affects its pitch, which changes the diffracted Bragg wavelength thus giving information about the occurred deformation. Application of the technique at the state-of-the-art level allows to perform strain measurements with 1 µStrain resolution. Such analysis and results will be presented in this contribution.