Following the successful commissioning of the soft X-ray planar undulator system (SASE3), the European XFEL user community expressed a strong demand to extend the radiation properties and provide the possibility to obtain variable polarization modes. It was therefore decided to build a helical afterburner behind the SASE3 system in collaboration with Paul Scherrer Institute (PSI). The final installation of the undulators in the tunnel took place at the beginning of 2024. Since then, a series of measures have been taken to commission four APPLE-X undulators, which form the base for the helical afterburner. The main objective is to maximally suppress the linear polarization of planar undulators in order to obtain the purest radiation from helical undulators. The methods and results of the optimization to achieve a maximum contrast between the pulse intensity generated by planar and helical undulators as well as the operating experience are presented.

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.

Highly polarized MeV gamma rays, produced by Laser Compton Scattered (LCS) of a polarized laser with an electron beam, offer a unique probe for basic and applied physics research. As the polarization characteristics of these gamma rays vary with the position of the beam cross section, it is essential to understand the polarization properties when using polarized gamma rays * . However, detailed measurements of the two-dimensional spatial polarization distribution have not yet been conducted. In the UVSOR synchrotron facility, a polarimeter was developed to measure the spatial polarization distribution of linearly polarized gamma rays. The polarimeter is based on asymmetry measurements of the Compton scattering cross section. In this conference, we will report on measurement results of the spatial polarization distribution of linearly and circularly polarized LCS gamma-rays. The polarization axis of the polarized gamma rays was clearly measured to vary with scattering and azimuth angle. In the near future, we plan to use the developed polarimeter to also measure the spatial polarization distribution of gamma rays generated by an axially symmetric polarized laser ** .

The insertion devices (IDs) can severely affect the beam dynamics of a storage ring. Recently, a new elliptically polarizing undulator(EPU) is installed in the Hefei Light Source II (HLS-II) storage ring. The effects of this EPU can be modeled using the kick map method. In this paper, we present the kick map of the EPU with vertical mode and how it affects the beam dynamics. Since the HLS-II storage ring is compact, only four quadrupoles in the same straight section can be used to compensated the ID effect. The compensation result is also reported in this paper.

Precise determination of the center-of-mass energy at the Future Circular Collider (FCC-ee) operating at the Z and W resonance energies relies on resonant spin depolarization techniques, which require a sufficient level of transverse beam polarization in the presence of machine imperfections. In this study, the FCC-ee lattice is modeled and simulated under a range of realistic imperfections, complemented by refined orbit correction and tune-matching procedures. The equilibrium polarization is computed within these realistic machine models to investigate the causes of polarization loss and explore potential improvements through lattice optimization. Additionally, spin tune shifts, which contribute to systematic errors, are estimated to support the precision requirements of the energy calibration experiment.

For the FCC-ee collider it is planned to, in regular intervals (minutes), measure the average beam energy of the circulating electron and positron beams with a relative precision of $10^{-6}$ or better, using the method of resonant depolarisation with pre-polarized pilot bunches. In this article, we study basic systematic effects and ultimate uncertainties that may arise in this kind of measurement. To do so, we carry out simulations for a simple model representing an ideal situation, where an ensemble of particles with energy spread is subjected to synchrotron oscillations and to perfect spin motion. We assume an initial spin orientation in the vertical direction for all particles. The behavior of the spin is explored as an exciter frequency is swept slowly or rapidly, and in either direction,through the spin resonance.

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-Ion Collider (EIC), which is being designed by BNL, JLab and other partners, will be a particle accelerator that collides electrons with protons and nuclei to produce snapshots of those particles' internal structure. It will collide polarized high-energy electron beams with hadron beams in the center-of-mass energy range of 20-140 GeV. The electron beam, employed as a probe, will reveal the arrangement of the quarks and gluons that make up the protons and neutrons of nuclei. The EIC will allow us to study the "strong nuclear force", the role of gluons in the matter within and all around us, and the nature of particle spin. This talk will describe the Electron-Ion Collider design and construction at Brookhaven National Lab.

The Relativistic Heavy Ion Collider (RHIC) Run 24 was 27 cryo weeks, operating with collisions at the STAR and sPHENIX detectors. The primary mode was polarized protons at 100 GeV, where there was 22 weeks of physics production. sPHENIX continued commissioning, becoming fully operational after 13 weeks and the addition of isobutane to their TPC gas mixture. STAR had a low luminosity run followed by twenty weeks of high luminosity and radially polarized beams. To reduce the beam-beam parameter and maximize the number of collisions within a small vertex region at sPHENIX, sPHENIX planned to operate with a crossing angle. For 8 weeks, collisions were only at sPHENIX until the beam-beam parameter was sufficiently low to support the additional collisions at STAR. A significant number of power dips earlier in the run greatly affected machine performance and reliability. At the maximum achieved performance, the luminosity was limited by four factors simultaneously: accelerating RF cavity intensity limit, intensity from the injectors, losses at rebucketing, and dynamic aperture. Despite these difficulties, sPHENIX and STAR were able to collect sufficient data commensurate with their goals.

Axions, originally introduced to solve the strong CP problem, are leading dark matter candidates appearing in various Standard Model extensions. At low masses, axion-like particle (ALP) dark matter behaves as a classical field, potentially detectable when its frequency resonates with a beam's spin-precession frequency. The JEDI collaboration's proof-of-principle experiment at COSY set upper limits on oscillating EDMs caused by ALPs, though no signals were observed. This presentation discusses COSY results and recent efforts to explore the feasibility of conducting axion search experiments using existing accelerators at GSI/FAIR with polarized hadron beams.

Negative Electron Affinity (NEA) activated GaAs photocathodes are the only one capable of generating spin-polarized electron beam larger than 90%. However, the NEA layer currently made from mainstream cesium (Cs) and oxygen (O) is chemically unstable, the NEA-GaAs photocathode has a rapid quantum efficiency degradation over time or electron beam. As a result, it requires an operating vacuum pressure of below 10-9 Pa and has a short lifetime. Recently, a new NEA layer using heterojunctions with semiconductor thin films of alkali metals and antimony (Sb) or tellurium has been proposed. Recent works have shown that the deposition of the NEA layer was realized using cesium, antimony and oxygen. In this work, we attempted to introduce Sb at two different timing. One is introduction from the beginning, and the other one is introduction after Cs and oxygen deposition. We systematically investigate the deposition temperature and antimony thickness to find the optimal conditions for improving quantum efficiency and lifetime. We will report the latest results.

Vertically polarized superconducting wigglers enable unique hard X-ray experiments based on horizontal optical setups. However, their implementation in modern low-emittance storage rings has been limited due to significant emittance growth. We present a vertically polarized superconducting multipole wiggler designed to reduce the impact on beam emittance. By limiting the magnetic field to 2-3 T and shortening the period length using Nb3Sn wires with higher critical current density compared to conventional NbTi, the beam orbit amplitude and the resultant emittance growth are reduced. As a case study for the future light source project at KEK, PF-HLS (Photon Factory Hybrid Light Source), we discuss a design based on vertical circular winding coils with a sub-100 millimeter period length and a sub-100 micrometer orbit amplitude, as well as its influence on beam emittance.

At the MAX IV 1.5 GeV ring, two APPLE II undulators with period lengths of 84 mm (Bloch) and 95.2 mm (FinEstBeams) cover minimum photon energies of 7 eV and 4 eV, respectively. Operating below 80 eV, the polarization state is distorted significantly by the beamlines' optical elements. A combination of helical and linear inclined modes during undulator operation - the so-called universal mode - can compensate for the distortions. In this paper, we describe how we compensate for the effect of the undulators on the beam orbits and ring optics when operating in universal mode. Additionally, some of the achieved commissioning results at both beamlines will be shown.

The next generation of synchrotrons will have undulators with shorter periods, stronger magnetic fields, and thus higher radiation power. Consequently, concepts for reducing on-axis heat load will become more relevant. One possible idea is to introduce so-called APPLE “knot” undulators that shift the main energy peak off-axis. Thanks to almost on-axis injection, APPLE X undulators with a round vacuum chamber can be used for the upgraded SLS 2.0 at the Paul Scherrer Institute (PSI). This contribution presents an adaptation of the APPLE “knot” concept tailored to the needs of SLS 2.0 in the form of two-meter-long APPLE X undulators with a 36 mm period length and a gap of 11.5 mm. Our design faces the challenge of dealing with up to 16 different magnetization angles introduced by combining and merging NdFeB magnets into four arrays with peak fields around 1 T. Consequently, the magnetic design and the first measurement results are discussed with an outlook on magnet optimization.

GaAs cathodes with thin-film Negative Electron Affinity (NEA) surfaces affixed have been used to generate spin-polarized electron beams for decades, but still suffer from short lifetimes. Heterojunction NEA surfaces have shown promise in improving cathode lifetimes, but further optimization of cathode activation and surface deposition is possible. Here we report the results of cathode activation with evaporative deposition of a CsTe surface performed at Hiroshima University.

To study the electric dipole moment of light nuclei, it is necessary to maintain the direction of the spin along the particle's motion along the ring. The first obvious solution to this problem is to use elements with an electric field that rotates the spin in the direction opposite to the spin rotation in a magnetic field. The most successful solution in this case is the Wien filter, which ensures spin rotation while maintaining the co-direction of the spin and momentum. In this case, the ring structure consists of arcs with bending magnets and straight sections on which Wien filters with crossed electric and magnetic fields are installed. The paper considers various versions of a magneto-optical structure that implements the "Quasi-frozen" spin method for studying the electric dipole moment of deuterons and protons. This approach can be used in developing an upgraded Nuclotron structure.

One of the possible proofs of CP violation beyond the Standard Model may be the discovery of permanent Electric Dipole Moments (EDM) of elementary particles. To search for the EDM of charged particles, the Frozen Spin (FS) concept was first proposed at BNL. The implementation of the latter involves the creation of a special storage ring in which the spin vector is preserved along the momentum and precesses due to the EDM only. In a magnetic storage ring initially not dedicated to measure the EDM, it is also possible to study the EDM by inserting electrostatic or E+B elements that compensate for the spin rotation in the bending magnets in a so-called Quasi-Frozen Spin (QFS) mode. Magneto-optical structures fulfilling the QFS condition can be used in application to study the proton and deuteron EDM and for axion search at the NICA accelerator complex. The main features of the implementation of the QFS concept are discussed, the method of measuring the EDM in the frequency domain, as well as the main spin dynamics properties of the lattice are covered.

The variation of polarized light is a critical characteristic of synchrotron radiation sources. To accommodate diverse user needs and enable helicity switching, a soft X-ray beamline has been designed to alternate the helicity of polarized undulator radiation. This is achieved by switching between two undulators, configured to provide right and left circularly polarized radiation, respectively. To separate and select these two circularly polarized photon beams, six kicker magnets are installed in the straight section. This paper details the design considerations, fabrication processes, and field measurement results of these kicker magnets, emphasizing their role in achieving seamless helicity switching and supporting the beamline’s functionality for cohabitation of multiple users.

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.

To mitigate the risk of radiation damage induced failure while operating the LHC beyond its initial integrated luminosity target, changes to the triplet polarity and crossing angles have been applied in the two main experimental interaction regions of the LHC. This allows for a more distributed radiation deposition in the insertion region magnets, which should allow their survival until they are replaced as part of the High Luminosity LHC upgrade from 2026-2030. These changes in the optics during 2024 and 2025 came with important challenges regarding machine commissioning and optics correction. In this paper, we discuss our experience of linear optics correction for the various triplet polarity configurations and review the implications for nonlinear optics corrections.

The transition from 3rd to 4th generation synchrotron light sources can primarily be characterized by a significant reduction in horizontal emittance. This enables a nearly uniform transverse X-ray beam profile and a brilliance that approaches the diffraction limit. A consequence of the upgrade to Diffraction Limited Storage Rings (DLSRs) is that the traditional emittance measurement techniques lack the resolution required to accurately measure emittances in the picometer-radian range. At the Advanced Photon Source Upgrade (APS-U), we explore the use of high harmonics of undulator radiation for precise emittance characterization. Previously at the Advanced Photon Source (APS), vertical emittance measurements, validated through SPECTRA simulations, were performed. This drove the desire to measure the horizontal emittance at the APS-U. Simulations performed in SPECTRA and Synchrotron Radiation Workshop (SRW) guide our experimental strategy for characterization. We present measurements of both the horizontal and vertical emittance at the APS-U, including variations across different bunch timing modes. We conclude by discussing the advantages of this approach over traditional methods.