It is about “Development for Various Application at Compact ERL as a high-current CW SRF linac in KEK”. As an introduction, the author will talk about the merit of the superconducting RF (SRF) cavity and also talk about our applied research based on Compact ERL (cERL) in KEK, which uses the Nb superconducting cavity and can make energy recovery operation. The cERL's characteristic using the high-current beam has the variety of applications; industrial applications using high-intensity terahertz light and mid-infrared FEL (free-electron laser). In addition, the high current CW beam irradiation was conducted for basic research on domestic production of nuclear medicine, strengthening of asphalt, and the highly efficient production of nanocellulose from wood in cERL. After talking these applications of cERL, next we will talk about “Future plan for applied research using superconducting accelerators”. One is the EUV-FEL light source development for EUV-lithography and the other is the development of compact superconducting RF accelerator based on Nb3Sn for high-power beam irradiation.

The scheme with quadrupole lenses is presented for realization relativistic strophotron type Free electron laser. Equations of motion are solved and trajectories are found. It is shown, that movement of electrons in presented scheme is stable in both transverse directions.

THz sources are typically very limited in power, making high-power sources scarce. One of the most promising THz sources are the Free Electron Lasers (FELs), which can generate high-power THz radiation using an undulator structure. Undulator radiation is an incoherent synchrotron spontaneous emission whose energy is proportional to the number of particles in the beam (𝑁). By longitudinally bunching the charged particle beam, a coherent spontaneous emission is generated and referred to as a super-radiant emission. Unlike spontaneous emission, super-radiant energy yield is proportional to N^2. However, like typical FELs, the energy conversion efficiency is rather low. Here, we demonstrate a novel THz source structure based on a radiative interaction scheme of super-radiance – Tapered Enhanced Super-radiance (TES), which employs a tapered (amplitude) undulator in the zero-slippage condition. This method yields a significantly more powerful and efficient THz radiation source. An optimization algorithm was developed to obtain a tapering rate that yields the most efficient energy conversion from the electron beam to the radiation field.

FLASH, the XUV and soft X-ray free-electron laser at DESY, is currently undergoing the 2nd of two long upgrade shutdowns within the FLASH2020+ project. The 1st half of 2024 was dedicated to user operation. The upgrade shutdown started in June 2024, and we plan to come back to beam operation in August 2025. Here we will discuss the operational highlights of the first half of 2024, briefly describe the new features being implemented, and report on the shutdown status.

Laser-plasma accelerators (LPAs) produce high-quality electron beams with the GeV-level of the energy, the high peak currents and low emittance, making them ideal for compact novel free-electron lasers (FELs). However, the large angular divergence and energy spread of these beams pose challenges for efficient beam transport and overall FEL performance. This study explores the use of an active plasma lens (APL) as a capture block to improve the transport of LPA-generated beams into an undulator. Initial beam parameters were based on published results from LWFA studies. In this report, we present the results of the modeling conducted to design an efficient LPA-based electron beamline and optimize the FEL regime for the extreme ultraviolet (EUV) range. Our goal is to achieve saturation of the photon beam power within a single unit. The results show that the APL enables efficient beam transport and facilitates the generation of high-brightness coherent X-rays. This work underscores the potential of APLs in developing compact FELs and advancing LPA beams. This technology is essential for creating a new generation of FELs at ELI-ERIC in the Czech Republic and within the EuPRAXIA project.

FEL simulation codes are essential tools for simulating various FEL scenarios. Among the algorithms, the orbit-averaged algorithm is the most widely used due to its speed and low computational cost. The averaged algorithm simplifies physics model, so present codes such as GENESIS and SIMPLEX have limitations to model accurate features like FEL polarization and variations in electron beam current. Such details are possible when using an unaveraged algorithm (like PUFFIN). In this presentation we introduce an approach based on the averaged algorithm to simulate arbitrary FEL polarization and non-fixed electron beam current. Our method involves initializing all particles at the beginning of the simulation and performing particle calculations at each simulation step to account for the non-fixed electron beam. Additionally, arbitrary FEL polarization is calculated by modifying the FEL equations using the elliptical undulator formula and the GENESIS algorithm. Finally some demonstration will be shown comparing the performance of PAL-XFEL and results from other simulation codes to highlight the capabilities of this approach.

The free-electron laser (FEL), powered by an accelerator and equipped with an undulator, produces intense coherent radiation at ever-shorter wavelengths. Whilst the hard x-ray regime represents the current state of the art, the gamma-ray regime remains the next objective. Gamma-ray lasers, deemed one of the most profound and intriguing challenges in physics by the 2003 Nobel Laureate, hold the key to unlocking the largely unexplored nuclear domain. This article introduces a novel scheme that harnesses FEL harmonics, offering a pathway for existing x-ray FELs to operate as gamma-ray lasers.

Orbit alignment plays an important role in free-electron laser (FEL) facilities, particularly in the collimation section, where multipoles are strategically positioned near the collimators as part of the specialized optics design. At the European XFEL, a strong dependence of lasing performance on the orbit in the collimation section has been observed. This study focuses on calibrating the central positions of the collimators using an orbit bump scanning technique combined with beam loss detection. Additionally, the influence of orbit alignment in the collimation section on lasing performance was systematically investigated, offering valuable insights into optimizing FEL operation.

UK XFEL is a multi-stage project to pursue ‘next-generation’ XFEL capabilities, either through developing a new facility in the UK or by investing at existing machines. The project’s Science Case envisages a step-change increase in the number of simultaneous experiments, with transform-limited (‘laser-like’) x-rays across a wide range of pulse durations and photon energies (up to ~20 keV) being delivered together with an array of synchronised sources, at high repetition rate to approximately ten FELs (evenly spaced pulses at approximately 100 kHz per experiment, with flexibility). A subset of applications require increased pulse energy and higher photon energies at low repetition rate or in short bursts. The project is now in the final year of its three-year conceptual design and options analysis phase, in which it has produced a conceptual design to efficiently meet these requirements, as well as conducting an analysis of the costs, socio-economic factors, and sustainability of the different investment options. The conclusions of this study are expected to be of general interest to the community.

The polarization of the gamma-ray beam plays a critical role in experimental photonuclear research by probing angular momentum. For example, the multipolarities of the 80Se(g,n)79Se reaction can be assigned by measuring cross-sections relative to the plane of polarization*. Dynamic control over gamma beam polarization will open new opportunities in nuclear research, particularly by allowing relative asymmetries to be calculated without the uncertainty introduced by relative detector efficiency. A gamma-ray beam with rotational linear poarization and high polarization purity (Plin ~ .99) has been demonstrated at the High Intensity Gamma-ray Source (HIGS)**. Without active tuning by an accelerator physicist, polarization quality is degraded due to decoupling of the free-electron laser (FEL) axis and the electron beam orbit. The FOAMS is an active feedback system that is sensitive to the small centroid motions of the FEL optical axis. Measurement uncertainty characterization has been conducted. Ongoing work will utilize this feedback system to automatically sustain controllable gamma-ray polarization for nuclear physics experiments.

FLASH is an XUV and soft X-ray free-electron laser (FEL) facility that comprises a superconducting linear accelerator with a beam energy of up to 1.35 GeV which drives two FEL beamlines in parallel and the plasma wakefield accelerator experiment FLASHForward. Within the upgrade program FLASH2020+, a laser heater was installed upstream of the first bunch compression chicane to mitigate microbunching instability in the linear accelerator by a controlled increase of the uncorrelated energy spread. The effect of the laser heater on microbunching instability and final energy spread has been verified with a transverse deflecting structure. In this paper, we describe the layout of the laser heater and report on improved operational aspects. It has been shown that the laser heater eliminates coherent contributions to visible transition radiation in transverse beam size measurements and, thus, contributes to better electron beam matching. In addition, an increase in the FEL output power is demonstrated, especially for first operation of the 3rd harmonic afterburner with variable polarisation.

Generating few- or sub-femtosecond radiation pulses in a free-electron laser (FEL) requires precise control of the longitudinal phase space density of the driving electron bunch, as the FEL process depends strongly on the bunch current and energy spread profile. In an experiment conducted at FLASH in Hamburg, Germany, an energy modulation with linearly changing amplitude is imprinted onto part of the bunch by a laser pulse in an undulator upstream of the first bunch compression chicane. In subsequent longitudinally dispersive sections, a short current spike is created, as the linearly modulated region is compressed more strongly than the rest of the bunch. Measurements with a transverse-deflecting X-band cavity verify the creation of a short current spike, whose duration falls below the temporal resolution of the measurement setup of approximately 7 fs.

We have been studying about a pre-bunched FEL in the THz region. In the pre-bunched FEL, the electron bunches being compressed shorter than the oscillation laser wavelength, it is expected that we can generate short-pulse THz laser pulses with high peak intensity. A broadband spectrum and high-intensity characteristics, which cannot be realized by conventional FEL, are expected. The pre-bunched FEL experiments were conducted using the THz-CUR at Kyoto University Free Electron Laser (KU-FEL) consists of an existing electron rf gun (ECC-RF-Gun), which can produce short electron bunches adequate for pre-bunched FEL, and a 10-period undulator. We installed an optical cavity and performed beam tests at the lasing frequency of 0.2 to 0.4 THz. As the results of beam test, we observed the coherent stacking of coherent THz pulses inside the cavity, however, FEL oscillation has not been achieved yet. We will report on our pre-bunched FEL project, experimental setup, beam test results and future prospects.

The PITZ facility at DESY in Zeuthen has demonstrated the first operational high peak and average power THz self-amplified spontaneous emission (SASE) free electron laser (FEL). The current setup displays the onset of saturation at a central frequency of 3THz using a 3.5m long LCLS-I undulator. However, the THz user community has expressed the need for carrier-envelope phase (CEP) stability and the availability of few-cycle THz pulses to complement the currently demonstrated long pulses. In this work, simulations are conducted to evaluate and optimize FEL performance by incorporating a Cherenkov waveguide to seed the process. The waveguide parameter space is scanned to vary energy modulation depth and frequency, after which the performance is estimated using the space charge tracking algorithm, ASTRA, and the FEL simulation code, Genesis1.3. The optimized parameters allow saturation to be reached much earlier, while also significantly increasing the shot-to-shot stability. Down the line, the implementation of such a scheme would facilitate generation of few-cycle, CEP-stable THz pulses to be used in user experiments.

The accelerator and free-electron laser (FEL) laboratory at the University of Hawai’i at Manoa (UHM), established by John Madey, has been in standby since his passing in 2016, with operations further paused during the pandemic. Recent efforts aim to recommission the facility, which includes a thermionic gun, an S-band linear accelerator reaching 45 MeV, and a Mark III undulator FEL oscillator producing tunable infrared light. Previously, 3 μm infrared light from this undulator demonstrated the capability to generate 10 keV X-ray photons via inverse Compton scattering. Current upgrades include enhancements to vacuum systems and linac controls. Future plans focus on enhancing cathode performance, developing 3D FEL simulations for superradiance studies, achieving phase coherence with interferometer optics, and using waveguides for THz generation. Recent GINGER simulations explored FEL oscillator output under varying Desynchronization conditions, demonstrating pulse train formation. The revived UHM accelerator will advance FEL science and train the next generation of researchers.

The accelerator and Compton gamma-ray source research program at Duke Free-Electron Laser Laboratory (DFELL), TUNL, is focused on the development of the storage ring-based free-electron laser (FEL) and a state-of-the-art Compton gamma-ray source, the High Intensity Gamma-ray Source (HIGS) driven by the storage ring FEL. With a maximum total flux of about 3.5E10 gamma/s and a spectral flux of more than 1,000 gamma/s/eV around 10 MeV, the HIGS is the world's highest-flux Compton gamma-ray source. Operated in the energy range from 1 to 120 MeV, the HIGS is a premier Compton gamma-ray facility in the world for a variety of nuclear physics research programs, both fundamental and applied. In this work, we will describe our recent FEL development to enable the production of gamma rays in the higher energy range from 100 and 120 MeV. We will also provide a summary of our recent activities in accelerator and FEL physics research and Compton gamma-ray source development.

The electron beam linear accelerator (linac) at the free-electron-laser (FEL) laboratory of the University of Hawai‘i at Mānoa, originally developed by Prof. John Madey, has undergone recommissioning. The S-band linac delivers 45 MeV electron beams with 170 mA pulse current and 4–8 $\mu$s pulse duration to drive an infrared FEL oscillator. Recent efforts include restoration of the microwave thermionic gun with a new LaB$_6$ cathode, upgraded vacuum and RF systems, and development of a Python-based beam dynamics model to recover operational magnet settings and optimize beam transport. These upgrades address key technical challenges to restart the accelerator and pave the way for future FEL experiments, including coherent pulse shaping and inverse Compton scattering x-ray generation.

National Institute of Advanced Industrial Science and Technology (AIST) has collaborated with Nihon University to study generation of high-intensity terahertz waves using coherent radiations at the Laboratory for Electron Beam Research and Application (LEBRA) at Nihon University. In a straight section for parametric X-ray (PXR) generation, developments of various types of coherent radiation sources and a study of superimposed coherent radiation using a ring-type resonator have been conducted. Coherent edge radiation (CER) generated in the downstream bending magnet is transported to an experimental room using the PXR beamline and is used for spectroscopic measurements and imaging experiments in an acrylic box filled with dry air. In a straight section for an infrared free-electron laser (FEL), CER generated by a downstream bending magnet during FEL oscillations is extracted from an FEL resonator by a toroidal mirror with a hole.* The extracted CER is reflected by a sapphire substrate coated with Indium-tin oxide and transported to the room using an FEL beamline. In this presentation, the status of the two THz beamlines at Nihon University's laboratory LEBRA will be described.

Feasibility design of THz beamlines for the use of the superradiant THz free electron laser driven by the NSRRC high brightness photo-injector has been studied. The Accelerator Test Area (ATA) building, where the photo-injector installed, will be transformed into a THz user facility that meets radiation safety regulations. Narrow-band intense superradiant THz radiations with pulse energy as high as 20 μJ and tunable central frequency from 0.6 to 1.4 THz, generated by injecting an ultrashort electron beam into a U100 planar undulator, can be a useful tool for nonlinear and time-resolved pump-probe experiments. There will be two stages for user experiments. Phase I will be opened for users with the experimental station installed right after the THz sources in the accelerator tunnel. Another THz beamline, which is currently being designed to maintain the quality of THz radiations after propagation over longer distances, will be built for user experiments in Phase II. This report briefly describes the beamline design and the operation of user experiments in Phase I.

Multi-alkali antimonide photocathodes, particularly potassium–cesium-antimonide, have gained prominence as photoemissive materials for electron sources in high-repetition-rate FEL applications due to their properties, such as low thermal emittance and high sensitivity in the green wavelength. To explore the potential of these materials in high-gradient RF guns, a collaborative effort was undertaken between DESY PITZ and INFN-LASA to develop and study multi-alkali photocathode materials. A batch of three KCsSb photocathodes and one NaKSb(Cs) photocathode was grown on molybdenum substrates using a sequential deposition method in the new preparation system at INFN LASA. These cathodes were successfully transferred and tested in the high-gradient RF gun at PITZ. Following the tests, a post-operational optical study was conducted on all the cathodes. Based on these findings, efforts are underway to optimize the fabrication recipes for KCsSb and NaKSb(Cs) photocathodes to achieve lower field emission and longer lifetimes. This contribution summarizes the experimental results of the production, operational performance, and post-usage analysis of the current batch of cathodes.

Undulators assembled from quasi-helices consisting of readily available magnetized ring rare-earth sectors are proposed. "Radially" magnetized sectors create a stronger field on the axis than longitudinally magnetized ones. The field value weakly depends on the number of sectors per undulator period. An experimentally studied prototype Halbach-type helical undulator of "radially" and longitudinally magnetized quasi-helices consisting of ring NdFeB sectors with a period of 2 cm and a comparatively large inner diameter of 8 mm provides a field of about 0.6 T on the axis. By reducing the inner diameter to 5 mm, it is possible to obtain a field twice as large. When assembling such an undulator, it is convenient, while maintaining the positions of all ring sectors, to use a division of the undulator not into quasi-helices, but into cylindrical sectors shifted along the axis and rotated relative to each other. Permanent undulators from ring sectors can provide a higher velocity of transverse electron oscillations than planar ones, and therefore seem promising for increasing the efficiency of FELs in various frequency ranges.

High-field micro-undulators are one of the key elements in most compact Terahertz and X-ray FEL projects. In our works, helical undulators of several helices, each made of a single piece of rare-earth magnet, are proposed for this purpose. We demonstrated previously the possibility of high-precision manufacturing helices with centimeter periods using the Wire Electric Discharge Machining. In this paper, we will discuss an experimental prototype micro-undulator of two oppositely longitudinally magnetized NdFeB helices with a period of 6 mm and an inner hole diameter of 1 mm, creating a transverse field close to 1 T. The magnitude of the field and/or the inner diameter of the helices can be significantly increased by using hybrid systems with two longitudinally pre-magnetized rare-earth and two pre-unmagnetized steel helices. We are currently developing methods for manufacturing, assembling and measuring the parameters of such systems with periods of 6 and 3 mm and a field of 1 T and will demonstrate the corresponding results in the presentation.

LCLS-II first stage commissioning is completed in the summer of 2023, with demonstration of 93 kHz electron beam and 1 kHz FELs using the superconducting CW linac. Operation-based electron beam and FEL commissioning has been continued with the goal of ramping up beam rate, improving the FEL performance, and developing advanced FEL operation modes. We started 33 kHz x-ray FELs to user experiments from 2025. The latest machine performance, commissioning challenges, and next-step plan will be discussed in this paper

PAL-XFEL is planning to install second hard X-ray undulator line (HX2) to meet the high beamtime demand from the users. The photon energy range for the second hard X-ray beam line is from 2~ to 11 keV which is lower than the first hard X-ray photon energy range (2 ~ 20 keV). The required undulator parameters are 35 mm period, max Keff=3.48 at 9.00 mm gap, ~ 3.0 m magnetic length with phase error less than 5 degrees. In addition to the existing conventional undulator design, horizontal gap vertical polarized undulator (HGVPU) concept is also being considered. HGVPU is well developed by LCLS-II team and applied in LCLS-II. In this report, we summarize the VPU design for PAL-XFEL HX2, and reports progress in the prototyping.

Superconducting undulators (SCUs) may be capable of generating stronger magnetic fields at shorter periods than can be achieved using permanent magnet undulators. Therefore, the range of x-ray wavelengths that an XFEL facility can generate for users could be expanded by exploiting SCU technology. Prototyping work is ongoing at STFC to build a helical superconducting undulator (HSCU) with 13 mm period and 5 mm magnetic gap designed for future XFEL facilities. As part of this work, a test cryostat has been built to cool 325 mm long prototype magnets to 4 K and to measure the field profile of the HSCU using a cryogenic Hall sensor. The magnetic field measurements are necessary to confirm the peak-to-peak field quality and trajectory wander of an electron beam through the device. These quantities must be measured to understand the impact of the HSCU on the FEL radiation output. The trajectory wander can be minimised through the use of field integral corrector coils at either end of the HSCU coil. We present here a description of the test cryostat and the results of the magnetic field measurement regime performed on the prototype HSCU coil.

In low-energy FEL beamlines, like SXFEL-SBP at the Shanghai Synchrotron Radiation Facility and FLASH1 and FLASH2 at DESY, SASE undulators with perfectly reasonable strength may dynamically affect the optics of the Focusing-Undulator-Defocusing-Undulator (FUDU) cells, pre-matched for a given fixed set of undulator parameters, so violently that a dynamical re-adjustment of the FUDU quadrupoles becomes mandatory. Here we refine and generalize a result reported at the FEL conference 2024. Our almost-analytical result allows implementation in the control system, and is valid for fairly general symplectic coupling-free perturbing matrices. In an approximative sense it includes undulators changing along the beamline and even missing undulators in given cells.

Intra-beam scattering (IBS) has recently gained significant interest in the community of free electron lasers (FELs), as it is believed to produce an increment in the sliced energy spread (SES), which is detrimental to FEL performance. To control and contain this phenomenon, it is important to include IBS in the design phase of an FEL through appropriate numerical simulation. Most existing codes that simulate IBS were developed for long-term tracking in circular lattices, assuming Gaussian bunches. Unfortunately, this assumption doesn’t capture the rapid bunch evolution of electron bunches in photoinjectors. To address this limitation, the tracking code RF-Track has recently been updated to include IBS, using a novel hybrid-kinetic Monte Carlo method. This paper presents benchmarks performed to verify the implementation. The predicted SES increment in the beam due to IBS using RF-Track has been compared against a kinetic approach used in a different tracking code and, secondly, against a semi-analytical model. The results showed a good agreement, setting RF-Track as a tool to understand and control the SES growth in photoinjectors and, in particular, in FEL.

We present an overview of recent and upcoming enhancements to the optical electron beam diagnostics stations at the Novosibirsk Free Electron Laser (FEL) facility. These diagnostic stations are designed to measure key beam parameters, including beam energy spread, length and emittance, at the third FEL of Novosibirsk FEL. Currently, the stations for measuring electron beam energy spread and undulator radiation spectrum are in the commissioning phase, with initial results already obtained. The new optical diagnostics are essential for the precise tuning of the magnet system used in electron outcoupling experiments. This paper provides a comprehensive overview of the new diagnostic systems, discusses the preliminary measurement results of beam parameters, and outlines the experiments planned for the near future.

The paper presents a design of an autocorrelator manufactured to measure the duration of infrared picosecond pulses of radiation from the 3rd laser of the Novosibirsk Free Electron Laser facility, as well as the results of testing the autocorrelator when measuring the duration of picosecond pulses in the visible range. The results and future plans for future experiments using developed autocorrelator