Electron coolers with high-density electron bunches obtained via rf acceleration provide a feasible path for extending electron cooling to high energies. In such coolers, the dispersive redistribution of cooling rates between the longitudinal and transverse directions is crucial and must be rigorously calculated. This paper presents exact formulas for cooling redistribution in high-energy electron coolers. The most general formulas are derived in terms of Carlson elliptic integrals, which are further reduced to elementary functions for a key special case. These formulas enable an efficient and accurate calculation of cooling rates and were instrumental in designing the electron coolers for the Electron-Ion Collider (EIC) by enabling rapid parameter identification and optimization. Consequently, these results offer a valuable tool for the design and development of future high-energy electron coolers.

The maximum secondary electron yield, ${\delta }_{\max }$, is a key parameter governing electron cloud buildup in high-intensity proton accelerators, yet its direct in situ determination under operational conditions remains challenging. This paper presents a novel method to estimate ${\delta }_{\max }$ in the Large Hadron Collider (LHC) using electron cloud measurements performed in the Vacuum Pilot Sector. The approach exploits the characteristic evolution of electron cloud intensity during beam injection, in particular the transition from an initial build-up regime to a linear scaling of electron flux with the number of injected bunches. Electron cloud currents measured with dedicated electron pickups are compared with PyECLOUD simulations performed for varying ${\delta }_{\max }$ values. The threshold number of injected bunches required to reach the linear regime provides a robust observable from which ${\delta }_{\max }$ can be extracted with limited sensitivity to other surface-emission parameters. The method is applied to multiple LHC fills and surface materials. For conditioned copper liners, ${\delta }_{\max }$ values between approximately 1.16 and 1.30 are obtained, in agreement with independent analyses and literature values. Applying the technique over extended operation periods enables the reconstruction of surface conditioning curves as a function of cumulative electron dose. These curves show a rapid initial decrease in ${\delta }_{\max }$ before approaching an apparent plateau, which may lie below the detection limit of the method depending on the beam pipe material and location. The method is compatible with routine LHC operation and enables systematic monitoring of vacuum-surface conditioning.

Superconducting magnets enable energy-frontier accelerators by generating strong magnetic fields to steer and focus the particles. Although high-temperature superconductors such as ${\text{REBa}}_2{\text{Cu}}_3{\mathrm{O}}_x$ (rebco, RE = rare earth) hold a strong potential for generating a higher magnetic field than Nb-Ti and ${\text{Nb}}_3\text{Sn}$, the associated magnet and conductor technology for accelerator applications is still in its infancy. The U.S. Magnet Development Program is developing rebco magnet technology in collaboration with industry. Here we report an experiment of making a dipole magnet called C3 using commercial high-temperature superconducting corc wires. The magnet, following a canted $\cos \theta$ design, generated a dipole field of 5.99 T at 4.2 K in its clear aperture of 65 mm at 6.795 kA when a resistive voltage of $105\mathrm{\mu }\mathrm{V}$ appeared across one of the coils in the magnet. The stored energy was 53 kJ at the peak field. The magnet showed no degradation in the current-carrying capability at 4.2 K after the thermal cycle. We report on the detailed design, fabrication, and performance of the C3 magnet that can be of interest to potential users of this emerging technology. We also discuss issues and research needs to inform future rebco magnet development. The experiment represented another step to addressing if the high-temperature superconducting accelerator magnet technology can increase the discovery capability of future particle accelerators.

The Super Tau-Charm Facility (STCF), a proposed high-luminosity electron-positron collider in China, requires a beam current of 2 A to achieve its design luminosity. Such intense current induces severe coupled-bunch instability driven by the rf cavity accelerating modes. This paper presents a systematic analysis of these instabilities across the STCF operational energy range of 1–3.5 GeV. Without active control, the low-order coupled-bunch modes, particularly the mode $-1$, exhibit growth rates significantly exceeding the radiation-damping rate. We demonstrate that the instability can be effectively suppressed using only the proportional-integral feedback loop of the baseline low-level rf system. The key lies in setting a unit proportional gain and a loop delay of approximately half the revolution period. This configuration creates a notch-filter response that attenuates the most dangerous mode $-1$. By combining analytical calculations with particle-tracking simulations, we show that this simple and robust approach maintains beam stability for the baseline design parameters across all design energies, thereby enabling reliable high-current operation without requiring additional dedicated feedback measures, such as direct or one-turn-delay feedback. A stability assessment based on the Nyquist criterion confirms the closed-loop stability of the system and indicates sufficient gain margins. These results provide a reliable foundation for the STCF rf and feedback system design, enabling stable high-current operation under realistic constraints. The proposed approach and the associated analysis framework also offer a practical reference for other high-current storage rings facing similar instability challenges.

We present the baseline design of the electron ring collimation system for the Electron-Ion Collider (EIC) at Brookhaven National Laboratory (BNL). The system addresses beam losses in a high-current electron storage ring with superconducting (SC) final-focus magnets and sensitive detectors, where uncontrolled losses can generate heat loads, radiation, and detector backgrounds and damage. The proposed collimation insertion localizes halo particle losses through reducing interaction region beam losses from beam-gas and Touschek scattering by several orders of magnitude while keeping detector backgrounds and cryostat heat loads within acceptable limits. Multiturn particle-tracking simulations show that the collimators do not significantly impact machine acceptance or beam lifetime, and their positions and apertures can be reoptimized for future lattice configurations. Ongoing work includes incorporating crab cavities and solenoid fields into simulations, refining vacuum conditions, and optimizing collimator geometry and materials. This design establishes a robust baseline for the EIC electron ring collimation system and supports continued lattice optimization for machine operations.

The future circular electron-positron collider will provide collisions at four interaction points (IPs) along a 91-km ring, with beam energies ranging from 45.6 GeV (Z pole) to 182.5 GeV ($\mathrm{t}\overline{\mathrm{t}}$ threshold). Several processes, induced by both the collisions at the IPs and the circulating beams, contribute to the radiation environment of the experimental insertion regions, exposing the detector, machine equipment, and electronics. Quantifying these radiation fields and their effects is essential to guide design strategies. In this work, the Monte Carlo code fluka is employed to simulate the propagation of radiation showers from both IP-driven and beam-driven sources, including radiative Bhabha scattering, incoherent pair creation, beamstrahlung, synchrotron radiation, and beam-gas interactions. Power deposition, equivalent fluences, and total ionizing dose are estimated to assess the exposure of sensitive components and to characterize tunnel radiation levels at the Z-pole and $\mathrm{t}\overline{\mathrm{t}}$-threshold modes. The results show that incoherent pairs dominate the detector environment, particularly in the tracker, while radiative Bhabha losses in the superconducting final-focus quadrupoles (FFQs) require shielding to ensure safe magnet operation. Tunnel radiation levels exceed the tolerance of standard equipment at both operational modes, being dominated by radiative Bhabha and collimation losses at the Z pole, and by synchrotron radiation at the $\mathrm{t}\overline{\mathrm{t}}$ threshold. While a tungsten layer is shown to effectively protect the FFQs, the high radiation levels elsewhere indicate the need for additional shielding in dipoles and drifts, as well as dedicated absorbers downstream of the IPs and collimators.

We propose a new approach for a high-density free-neutron target, primarily aimed at nuclear astrophysics reaction studies in inverse kinematics with radioactive ions circulating in a storage ring. The target concept integrates four key subsystems: a neutron production source driven by a supercompact cyclotron utilizing ${}^9\mathrm{Be}$($p,xn$) reactions, an optimized moderator/reflector assembly using either heavy water or beryllium oxide with a graphite reflector shell to thermalize fast neutrons, a cryogenic liquid hydrogen moderator to maximize thermal neutron density in the interaction region, and beam pipe geometries that enable neutron-ion interactions while maintaining vacuum conditions for ion circulation. This integrated approach focuses on feasibility by incorporating readily available technologies. Using a commercial supercompact cyclotron delivering a proton beam of $130\mathrm{\mu }\mathrm{A}$, the design achieves thermal neutron areal densities of $\sim 3.4\times {10}^6\mathrm{n}/{\mathrm{cm}}^2$ for a proof-of-concept demonstrator at the CRYRING ion-storage ring at GSI Darmstadt. This autonomous accelerator-target assembly design enables deployment at both in-flight and ISOL facilities to exploit their complementary production mechanisms. Potential upgrades based on higher-energy and/or higher-current cyclotrons will enable an increase in areal density to $\sim {10}^9\mathrm{n}/{\mathrm{cm}}^2$. In combination with a customized low-energy storage ring and a radioactive ion-beam facility, the proposed solution could deliver luminosities above ${10}^{23}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, thereby enabling neutron capture measurements of $\sim \mathrm{mb}$ cross sections within a few days of experiment. The proposed system represents a significant milestone toward enabling large neutron-capture surveys on short-lived nuclei, thereby opening a new avenue for understanding the synthesis of heavy elements in our universe.

X-ray free-electron lasers (XFELs) of high brightness have opened new opportunities for exploring ultrafast dynamical processes in matter, enabling imaging and movies of single molecules and particles at atomic resolution. In this paper, we present a straightforward method for multiframe diffraction imaging, using the whole electron beam to generate four-color XFEL pulses with adjustable wavelength separation and time delay. The optical klystron scheme is introduced to enhance the FEL intensity and reduce the total length of the undulators. The time delay is tuned via a magnetic chicane between the undulators for different colors. Using parameters of SHINE, start-to-end simulations demonstrate the effectiveness and tunability of our method, achieving representative results such as time delays of hundreds of femtoseconds and four-color XFEL pulses spanning 1.8 to 2.7 nm with 0.3 nm intervals. The proposed scheme enables the recording of multiframe diffraction images in a single exposure, providing a new perspective for ultrafast molecular and atomic dynamics studies.

The survival of charged particles in synchrotrons requires avoiding setting the beam on machine resonances, the most dangerous of which are the integer and half-integer. Nevertheless, operationally, the transverse tunes may change dynamically, crossing these resonances and resulting in unwanted beam quality degradation and beam loss. For high-intensity beams, the process of resonance crossing is even more critical, as space charge, in addition to the incoherent effects, may also generate coherent effects. The interplay of the speed of resonance crossing, the space charge incoherent tune spread, and the half-integer resonance width is fundamental for the beam behavior. In this paper, we study the crossing of the half-integer resonance $2Q_y=9$ in the CERN Proton Synchrotron Booster (PSB) for a range of parameters using a coasting beam. We take advantage of the newly commissioned PSB to investigate the beam response experimentally. The experimental findings are also discussed with the support of simulations.

Bunch shaping in electron linear accelerators is of critical importance, as it enables the realization of prescribed projection profiles along specific coordinates of the bunch distribution for linac-based x-ray free-electron laser (XFEL) applications. It is evident that temporally flattop electron bunches yield more uniform slice properties, reduced emittance growth, and enhanced FEL gain compared to conventional Gaussian profiles. In this work, we present a data- and model-guided framework that consists of a physics-informed model and an inverse emittance model for minimizing the bunch emittance through laser pulse shaping and the control of laser and accelerator parameters in high-brightness photoinjectors. With a temporal flattop laser shape selected, a flat slice emittance distribution along the bunch has been experimentally obtained at the European XFEL. A measured central slice emittance of about $\text{0.4\hspace{0.17em}\hspace{0.17em}}\mathrm{\mu }\mathrm{m}$ at 250 pC is achieved using a 20 ps flattop laser pulse at the photocathode. Here we present the results of a commissioning experiment, marking a practical application of temporally flattop-shaped electron bunches for stable photon delivery in a user experiment week at the facility. Within limited tuning time, a pulse energy of 1.7 mJ has been achieved at photon energies of 7.1 and 7.6 keV for the two hard x-ray undulator beamlines, surpassing the user requirements. Such a data- and model-driven framework opens up new opportunities for end-to-end optimization of machine parameters, thereby enhancing the capabilities of large-scale FEL facilities.

In heavy-ion accelerators used for radioactive isotope production, the accelerated beam is typically directed onto a target and then discarded in a beam dump. To make more efficient use of the beam, recycling of the beam passed through the target is proposed in the framework of the so-called energy recovery internal target (ERIT). In the ERIT system, the target is irradiated by a circulating beam, while the energy lost in the target is recovered using rf cavities. So far, such a system has been realized only for proton beams. Here, an ERIT system for heavy-ion beams is demonstrated for the first time. A major challenge is the circulation of ions with multiple atomic charge states. After passing through the target, ions rapidly reach an equilibrated charge-state distribution, independent of the initial charge state. This stochastic charge-state conversion (SCSC) leads to rapid beam-emittance growth. To mitigate this effect, we develop a method to match the closed orbits and betatron functions of different charge states at the target location, based on a scaling fixed-field alternating-gradient (FFA) lattice. We present the design of such an FFA ring and show, through 6D beam-tracking simulations, that the transverse emittance growth induced by SCSC can be significantly reduced over a practical number of turns, whereas longitudinal emittance growth remains significant and is discussed separately.

A novel cooling mechanism for heavy-ion beams, termed charge-exchange cooling (CEC), is proposed. Phase-space-dependent zoned charge exchange combined with charge-state-dependent closed-orbit distortions (CODs) enables rapid, autonomous, turn-by-turn contraction of the phase-space distribution for continuous-wave heavy-ion beams. The resulting dynamics forms a stochastic affine-map system with state-dependent switching and a net negative drift. CEC is implemented in the charge stripper ring 1 (CSR1), under development at the RIKEN RI Beam Factory, where zoned charge exchange and charge-state-specific CODs are readily incorporated. Proof-of-principle studies based on two- and multicharge-state models, together with realistic CSR1 simulations, demonstrate fast turn-by-turn cooling under practical machine conditions and establish CEC as a new beam-cooling principle with potential implications for future accelerator design and applications.

We consider the radiation emitted by a purely longitudinally accelerated relativistic electron bunch and find, contrary to the usual expectation of negligible emission, that for ultrashort compact bunches within reach of current experiments, a significant and measurable radiation pulse is produced. The underlying mechanism for the enhanced emission is coherence in the single-electron emission over $\mathcal{O}({10}^8)$ electrons. We derive analytic descriptions of the emission spectrum and energy for acceleration fields and bunch conditions encountered in rf-driven particle accelerators, and in the extreme high-gradient conditions of plasma-driven acceleration. Both the bunch duration and the duration of the acceleration have a role in describing a phase-matching limitation to the coherence, with the latter dependent on the acceleration field strength and the initial energy of the electrons. In extremely compact bunches, just beyond those currently realized in the laboratory, the radiation losses approach the Larmor radiation for a massively charged particle ($Q\sim {10}^{9}e$). In such conditions, the implied energy of the longitudinal acceleration radiation may approach the energy available from the electric field driving the acceleration, raising questions about a connection to mechanisms of field depletion.

SuperKEKB collimator system protects the superconducting final focusing magnets (QC1 and QC2) from quenching caused by beam halo particles. For effective shielding, the minimum aperture of vertical collimators is set smaller than the physical aperture of QC1. However, such collimators become dominant sources of beam coupling impedance, which can limit the maximum beam current due to impedance-driven instabilities. To address this issue, a nonlinear collimator (NLC) was introduced in February 2024. NLC is designed to reduce both beam backgrounds and collimator impedance, thereby suppressing beam instabilities caused by short-range wakefields. In this paper, we describe the results on impedance effects, beam background reduction, and beam lifetime when the nonlinear collimation system is turned on and off. These results validate the NLC concept and offer a promising approach for future high-luminosity colliders.

Since the emergence of laser-plasma accelerators (LPAs), substantial work has been dedicated to using LPAs to drive free-electron lasers (FELs) for a broad range of applications. Despite recent breakthroughs, which have proven the fundamental feasibility of operating FELs with an LPA source, stable FEL operation over multiple hours without operator input had yet to be achieved. In this work, we report significant improvements to the stability of a hundred terawatt laser system, resulting in successful demonstration of reliable, long-term operation of an LPA-driven FEL in the self-amplified spontaneous emission (SASE) regime at 420 nm. The LPA source delivered 100 MeV electron beams at 1 Hz with high stability over more than 10 h, enabling over 8 h of continuous FEL operation without operator input. The acquired data were subsequently used to investigate correlations between the measured undulator radiation and parameters of the drive laser, plasma source, and electron beam. The revealed connections between LPA and FEL performance gave important additional insights into ways to further improve and stabilize the system, thus demonstrating the capability of our setup to serve as a powerful platform for future studies of LPA-driven FEL operation. The one-of-a-kind integration of multiple stabilization concepts onto the LPA facility discussed in this manuscript yielded significant accelerator and light source improvements (with residual correlations suggesting even more is possible), which will positively impact LPA applications at large.

The High Energy Photon Source (HEPS) is a recently commissioned 6 GeV fourth-generation synchrotron light source that utilizes a novel low-frequency superconducting radio-frequency (SRF) system. This system was designed to handle high beam power and achieve heavy damping of higher-order modes (HOMs) within strict spatial constraints. To meet these stringent requirements, we in-house developed and subsequently installed five $\beta =1$ quarter-wave SRF cavities operating at 166.6 MHz and 4.2 K with a compact envelope. The design incorporates an enlarged beampipe (505 mm in diameter) with a beamline absorber for effective HOM damping and adopts a single coaxial power coupler with on-cell coupling to handle a high continuous-wave power of 200 kW. Following rigorous cleaning and assembly protocols, the cryomodules demonstrated a peak electric field of $42\mathrm{MV}/\mathrm{m}$ with negligible radiation and maintained a low dynamic heat load of 5 to 7 W at the operating voltage of 1.2 MV, results that significantly surpassed the design goals. Installed in Summer 2025, the cavities were instrumental in the successful acceleration of a 100 mA electron beam at HEPS in September 2025. This work represents the first successful implementation of superconducting quarter-wave cavities as the main accelerating system for a high-current storage ring, thereby extending the application limits of quarter-wave cavities from their conventional use in low-$\beta$ accelerators to high-current electron storage rings and establishing a proven technology for future high-performance light sources.

A single-periodic magnetically coupled standing-wave accelerating structure prototype operating at 3 GHz for low phase velocity particles has been designed as the first tank of the 30–230 MeV accelerating section for the compact proton therapy linac developed by the Institute of Modern Physics, Chinese Academy of Sciences. Fabrication and rf measurement of the prototype have been completed. High-power test of the prototype was performed. The cavity could operate stably at the effective input power of around 5.7 MW, corresponding to the accelerating gradient of $38\mathrm{MV}/\mathrm{m}$ at the pulse length of $5\mathrm{\mu }\mathrm{s}$ and the repetition rate of 70 Hz. Further increasing the duty factor to 0.1%, the accelerating gradient decreased to $24\mathrm{MV}/\mathrm{m}$ due to the uneven cooling of each single cell. The details of the high-power test setup and preparation, the process of rf conditioning, and the high-power test results are described in this paper. Limiting factors for higher accelerating gradients with high duty factors are discussed, and two novel cooling channel schemes are described.

Electron storage rings based on multibend-achromat (MBA) lattices can achieve exceptionally low natural emittances. At the Paul Scherrer Institute in Switzerland, the Swiss Light Source facility underwent an upgrade project, replacing its triple-bend achromat ring with a seven-bend achromat ring. Due to the compact nature of MBA lattices, magnetic fields can be influenced by neighboring magnets. These magnetic crosstalk effects turned out to be significant during the design phase: the integral fields of the magnets could be altered by several percent. We present how we incorporated compensations for these effects into the design and how we tuned the magnets prior to installation. The crosstalk compensations were validated through the accelerator commissioning.

We show that the emittance of fourth-generation 6 GeV machines such as PETRA IV is close to what is theoretically achievable due to beam intensity limitations from space charge and intrabeam scattering. By investigating these limitations, in particular their scaling with the bare lattice emittance and the beam energy, we argue that achieving further significant emittance reduction is only possible by increasing the beam energy. We show further that electron energy increase is also the most efficient path toward higher radiation brightness.

The $\overline{\mathrm{P}}\mathrm{ANDA}$ experiment at the High Energy Storage Ring (HESR) of the Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, will be a unique setup to study the strong interaction in antiproton-proton collisions. The combination of a windowless cluster-jet target and a high-intensity antiproton beam, cooled by stochastic cooling down to momentum spreads of $4\times {10}^{-5}$ in the presence of the target, is ideal to study, e.g., the line shape of exotic candidates in the charmonium spectrum, such as the ${\chi }_{c1}(3872)$. For first studies on the performance of the cluster-jet target and the stochastic cooling, both were installed at the COoler SYnchrotron (COSY) at the Forschungszentrum Jülich, Germany. Detailed studies on the beam quality for different target settings with different densities were performed and will be presented in this paper. The results show that the cluster-jet target is able to provide the required hydrogen target thickness of more than $2\times {10}^{15}{\mathrm{atoms}/\mathrm{cm}}^2$ for the $\overline{\mathrm{P}}\mathrm{ANDA}$ experiment and that the stochastic cooling is able to efficiently cool the beam in presence of such a target thickness, although the system is not yet complete compared to the setup foreseen for the HESR. The presented results strengthen the theoretical understanding of the beam-target interaction and stochastic cooling performance and will provide important input for operation later at the HESR.