In electron coolers devices, at low energies the electron beam is immersed in a continuous solenoid, which provides a cold and tightly focused beam in the cooling and transport sections. At low energies, the electron motion satisfies the adiabatic criterion when the typical length of the magnetic field variation is much larger than the Larmor spiral length. At relativistic energies, this condition generally cannot be fulfilled, so a special method of transporting the electron beam is required. This article describes one such method, which involves the use of special “resonant” elements to guide the electron beam along the desired trajectory.

We demonstrate the first realization of electron beam and x-ray pulse shaping with a corrugated wakefield structure at a pulsed-mode megahertz-repetition-rate hard x-ray free-electron laser (XFEL). This is achieved by an L-shaped corrugated structure with low-beta optics and a collimator. Variable streaking direction and signature of quadrupole wakefield compensation are observed by controlling the position of the electron beam relative to the corrugated plates. Spectral analysis at 7 keV indicates a reduction of the reconstructed x-ray pulse duration by an approximate factor of 4 compared to the standard self-amplified spontaneous emission mode of the European x-ray free-electron laser. X-ray pulses with uniform properties along a train of 100 pulses at a 1.125 MHz repetition rate are generated under 10 Hz pulse mode. These results are a step toward tailoring the temporal and spectral domain of x-ray pulses at megahertz repetition rates with high flexibility and compatibility.

Plasma-based accelerators offer high accelerating gradients and scalability through staging or long plasma sources, which makes them good candidates for future accelerator and collider concepts. Proton-driven accelerators in particular have the potential to bring particles to high energy in a single stage. In the quasilinear regime, where the plasma wake is only partially evacuated, a witness bunch of electrons drives a cavitated wake that acts to preserve the emittance of the portion of the witness inside this self-blowout. In the case of a misalignment between the driver and witness, this behavior can persist, but its effectiveness is reduced. In this paper, we study transverse witness dynamics in this regime and develop an analytical model to describe the witness motion. We show that the degree of emittance preservation can be predicted by estimating the beam density after phase-mixing. Particle-in-cell simulations using the AWAKE Run 2c baseline parameters show excellent agreement with the predictive model developed. This work allows alignment constraints to be set both for the AWAKE experiment and other wakefield acceleration schemes operating in the quasilinear regime.

The stability of vertical emittance is a critical performance metric for modern high-brightness synchrotron light sources. This paper presents a real-time, reinforcement learning (RL)-based correction framework based on deep reinforcement learning by employing dueling deep Q-network with a discrete action space to dynamically adjust skew quadrupole magnets. To ensure operational safety and accelerate convergence, we introduce a curriculum learning strategy and leverage a high-fidelity simulation of the Shanghai Synchrotron Radiation Facility for systematic hyperparameter optimization. Experimental results demonstrate that the agent, trained in situ on the real machine, successfully achieves stabilization of the beam-size readback at the submicrometer fluctuation level and exhibits rapid adaptability to target changes. This work validates the feasibility of using RL for robust, online accelerator optimization during user beam delivery.

Arclike bunch compressors are important components in recirculating accelerator linacs, energy recovery linacs, and high-brightness light sources. However, their design is complicated by longitudinal nonlinear optics and coherent synchrotron radiation (CSR), including both steady-state CSR (ss CSR) in bending magnets and CSR in the subsequent drifts (dr CSR). These effects can degrade the longitudinal compression quality, generate current spikes, and induce transverse emittance growth. In this work, we develop an analytical model for evaluating CSR effects in complex arclike compressors without relying on the small-angle approximation. The proposed method includes both ss and dr CSR contributions and is applied to the mitigation of CSR-induced transverse displacements as well as to longitudinal matching with nonlinear compression. Based on these methods, a two-stage arclike compressor using an asymmetric quadruple-bend-achromat structure is designed. Numerical simulations show that the proposed compressor can compress a $100\mathrm{pC}$ bunch to nearly $50\mathrm{fs}$ while maintaining negligible emittance growth. The analytical model is further used to study CSR-induced modifications to the compression process, including rms bunch length and peak-current behavior. Benchmarking against bmad simulations demonstrates good agreement, confirming the model as an efficient tool for compressor design and optimization.

Electron cloud effects constitute a major performance limitation in high-intensity particle accelerators such as the Large Hadron Collider (LHC). A key ingredient for understanding and modeling these effects is the energy distribution of electrons impinging on the vacuum chamber walls. In this paper, we present direct measurements of electron cloud energy distributions obtained in the LHC Vacuum Pilot Sector (VPS) using retarding field analyzers with a beam pipe diameter of 8 cm, i.e., twice the LHC arc one. The VPS provides a dedicated, fully instrumented test environment for electron cloud studies under real LHC beam conditions. The measured energy spectra systematically exhibit a low-energy component associated with secondary electron emission and a higher-energy component originating from bunch-driven acceleration. To characterize these features, a phenomenological model is developed to reproduce the observed distributions up to 3 keV. Model parameters are extracted from a large dataset of LHC fills, with a representative 2025 physics fill used for detailed illustration. The evolution of the accelerated-electron peak is analyzed throughout the fill cycle and compared with predictions from the analytical kick approximation model. Good agreement is observed—particularly during the stable beam phase in one beamline—confirming that bunch-induced kicks dominate the electron energy gain in the field-free regions, as for the VPS. Discontinuities in the peak energy at operational transitions, as well as the recurrent observation of narrow accelerated peaks in the other beamline, are reported and suggest a dependence of the beam optics on the resulting energy gain. Finally, cumulative energy distribution functions are derived to quantify the fraction of electrons exceeding given energy thresholds. These results establish a quantitative experimental reference for energy-resolved electron cloud modeling in the LHC, providing direct inputs for electron cloud simulation benchmarking, heat load estimates, ESD modeling, and residual electron background assessment in ion diagnostics.

The Linac Coherent Light Source II (LCLS-II) is designed to generate 4 GeV electron bunches at an up-to-1-MHz-high repetition rate to drive x-ray free electron lasers. It has been in user operation since 2023 and has progressively ramped up the beam repetition rate to 93 kHz to date. The LCLS-II 1-MeV electron source was successfully commissioned between 2018 and 2020. The full-scale LCLS-II injector, operating above 75 MeV, began commissioning in 2022 and transitioned to operations a few months later. The LCLS-II injector has demonstrated the ability to deliver a high-brightness, low-emittance electron beam at high repetition rates, meeting key performance targets. However, several operational challenges have emerged during commissioning and early user operations. These include substantial gun dark current, the dependence of bunch charge generation and free-electron laser pulse intensity on the beam repetition rate, the formation of cathode quantum efficiency craters and bumps, significant emittance growth through the laser heater chicane, beam breakup, and the presence of beam halos. This paper presents recent progress in addressing these challenges, along with injector emittance measurements and optimizations, and the first demonstration of operating a laser heater undulator at the third-harmonic resonance.

Space charge is one of the main challenges for high-intensity linear accelerators (linacs). To overcome this, most linacs have adopted the space charge compensation (SCC) scheme at the low-energy beam transport (LEBT). However, SCC is a buildup process that produces beam transients, i.e., nonoptimal beam conditions, which can lead to significant beam degradation and losses. Furthermore, most LEBTs are equipped with a chopper to provide a beam structure intended for the downstream sections and for the gradual ramp-up of power. However, operation of the chopper will destroy the SCC, leading to additional beam transients. SCC studies on beam transients focus primarily on the LEBT section, resulting in insufficient analysis of the downstream section, where beam transients can cause significant losses. This work addresses this gap by analyzing beam transients generated in the LEBT and their propagation throughout the linac to assess beam losses. This analysis challenges conventional notions by revealing that the worst-case scenario does not occur when the SCC is zero, but rather within twice the compensation time (${\tau }^{*}$). This is because, although mismatch and emittance growth are at their maximum at the beginning, 80% and 450%, respectively, the current transmitted to the superconducting linear accelerator is minimal (30%); consequently, beam instabilities caused by the mismatch are less severe. Most nonoptimal beams were intercepted by the cone collimator and the radio-frequency quadrupole at energies below the activation threshold (2.16 MeV), with a peak power loss density that remained within cooling capacity limits. Furthermore, any residual risk was safely eliminated by the collimators in the medium-energy beam transport. As a result, transient losses in the superconducting linac were estimated to be below $1\mathrm{W}/\mathrm{m}$ and were confined to the low-energy section.

During the past decade, a comprehensive research and development (R&D) program on high-efficiency klystrons has been carried out in collaboration with industry. Initial prototypes have undergone testing, with experimental results showing significant promise. This paper is an extended and updated version of the work presented in Catalan-Lasheras et al. [JACoW IPAC2024, FRYD1 (2024)]. After a brief overview of the R&D program, it presents the latest experimental findings and discusses future developments, including novel vacuum tube topologies aimed at next-generation collider applications.

We report on the design and performance of a two-color dual-oscillator infrared free-electron laser (FEL). The midinfrared (MIR) FEL at the Fritz Haber Institute has been upgraded to include a second oscillator FEL beamline that permits lasing in the far-infrared (FIR) regime from 4.5 to $175\mathrm{\mu }\mathrm{m}$. In addition, a 500 MHz kicker cavity has been installed downstream of the electron accelerator. It allows deflection of electron bunches of up to 50 MeV energy alternately left and right by an angle of $\pm 2°$. It can, thus, split the high-repetition rate (1 GHz) electron bunch train from the accelerator into two bunch trains of 500 MHz repetition rate each; one is steered to the MIR FEL and the other one to the new FIR FEL. In this two-color mode of simultaneous, synchronized operation, the wavelengths in both FELs can be tuned independently over wide ranges of up to a factor of 4 each by undulator gap variation. In addition, two-color operation is also available at reduced repetition rates (e.g., 55.6 MHz of both MIR and FIR pulses), as needed for some applications. This unique two-color mode opens up a wealth of novel user applications, such as MIR-FIR pump-probe experiments.

In a rapid cycling synchrotron (RCS), closed-orbit correction is an important task for maintaining high beam quality and machine stability. Since the orbits in RCS change during energy ramping, response matrix-based orbit correction is performed in segments, which can result in the corrector current ramp rates exceeding safety limits and accuracy loss in orbits. To address these issues, this paper employs machine learning for orbit correction in RCS. An adjacent-difference-constrained neural network (ADCNN) was built and tested in both simulations and machine experiments at the China Spallation Neutron Source (CSNS). The correction results demonstrate the feasibility of using neural networks for RCS orbit correction. The ADCNN exhibits a correction effect equivalent to that of response matrix-based correction while effectively limiting the current ramp rates of the correctors.

The electron-ion collider (EIC), to be constructed at Brookhaven National Laboratory, will collide polarized, high-energy electron beams with hadron beams, achieving peak luminosities of up to $1.0\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. To reach such high luminosity, the EIC will employ flat-beam collisions at the interaction point. The design transverse emittance ratio will be about $10:1$ in the hadron storage ring. Thanks to stochastic cooling and precise decoupling, we successfully generated and accelerated gold ion beams with a large emittance ratio of $11:1$ in the relativistic heavy ion collider. In this article, we present results of intrabeam scattering (IBS) measurements and modeling for large-emittance-ratio gold ion beams, both without and with controlled betatron coupling. To model the IBS growth, we use the formulas developed by Lebedev and Nagaitsev.

The square matrix-based convergence map (CM) method has proven effective in characterizing nonlinear dynamics in several 4D dynamical systems. However, when time-dependent perturbations, such as crabbing kicks in colliders, are present, a comprehensive 6D analysis becomes essential to accurately capture the coupling between transverse and longitudinal motions. In this work, we extend the CM method to the full 6D phase space by employing an eigen-decomposition-based formulation of the square matrix combined with iterative procedures. The proposed 6D CM approach is first validated using a simplified crabbing map and subsequently applied to the dynamic aperture study of the future Electron-Ion Collider. The CM results show qualitative consistency with frequency analysis in identifying major resonance structures and stability boundaries by using only one-turn map, suggesting that the 6D CM can serve as a fast indicator for nonlinear beam dynamics analysis.

Plasma wakefield acceleration in hollow plasma channels has emerged as a promising approach for positron acceleration, since an electron beam can drive wakes with a transversely uniform accelerating field and no intrinsic defocusing force for positrons. Recently, it was proposed that a transversely asymmetric electron beam can excite quadrupole-dominated wakefield in a hollow channel, enabling the formation of accelerating and focusing fields suitable for positrons [S. Zhou et al., Phys. Rev. Lett. 127, 174801 (2021)]. However, the self-consistent evolution and stability of such asymmetric drivers, which are crucial for sustaining a usable wake over long distances, remain insufficiently understood. In this work, we investigate the evolution of wakefield driven by asymmetric electron beams in hollow plasma channels using fully three-dimensional particle-in-cell simulations. We identify two distinct unstable scenarios: a reversal of quadrupole field polarity and continuous penetration of the driver into the plasma wall. By analyzing the transverse dynamics of the driver and the restoring forces provided by the channel ions, we establish simple physical criteria that ensure stable propagation. These results clarify the fundamental constraints governing asymmetric-driver evolution, provide practical guidance for realizing long-lived, quasisteady wakes in hollow plasma channels, and indicate that the scheme can preserve mm mrad-level normalized positron emittance.

We present an on-line tuning strategy for the isotope separator and accelerator postaccelerator that presets machine optics with a digital twin and then performs Bayesian optimization for steering during operation with beam. The model computes end-to-end configurations in seconds and interfaces with the control system under device bounds, slew-rate limits, and loss interlocks. We report three experimental case studies demonstrating that decoupling optics from steering yields faster and more reliable convergence than a fully Bayesian optics-plus-steering baseline under identical conditions. Across these cases, the number of iterations required to reach high-transmission solutions was substantially reduced, with final average transmissions in the mid- to high-90% range. By removing optics from the steering optimization, the dimensionality of the parameter space is reduced, convergence becomes more predictable, and operational safeguards are easier to enforce.

As of July 2025, the SuperKEKB accelerator, which collides 7 GeV electrons with 4 GeV positrons to abundantly produce particles such as B mesons and $\tau$ leptons, holds the world record for the highest instantaneous luminosity. Continuous operation and upgrades are underway to achieve even higher luminosities. Maintaining a high instantaneous luminosity requires sustaining high beam currents in the storage rings, which in turn demands efficient beam injection from the injector. In particular, a high injection efficiency, defined as the ratio of the beam current successfully accumulated in the ring to the current delivered from the beam transport line, must be ensured. In the present study, we developed a tool to automate the injection tuning process using Bayesian optimization, a machine-learning-based technique, in order to improve the injection efficiency. During test operations conducted in November-December 2024, this tool successfully enhanced the injection efficiency by up to 32%.

Inverse Compton scattering (ICS) at shallow angles can be used to tune the spectrum and increase the brightness of the emitted radiation. Here, we investigate the geometric scaling laws governing this interaction and experimentally validate them by demonstrating shallow-angle ICS of a 780 nm laser crossing a 4.7 MeV electron beam at 5.8°. Angular and spectral measurements of the emitted radiation show excellent agreement with analytical and numerical predictions. To further optimize this interaction, we consider the use of velocity-matched pulse-front tilt to decouple the total yield from the interaction bandwidth. The resulting framework is used to evaluate the performance scaling of three distinct driving accelerator architectures: a radio-frequency linac, a laser wakefield accelerator, and a storage ring synchrotron. These results establish a scalable, practical pathway for deploying next-generation, high-brightness compact light sources across diverse accelerator platforms.

A novel concept is introduced to reduce the longitudinal emittance in a low momentum compaction factor (low-alpha) storage ring, enabling the stable storage of electron bunches shorter than 100 nm. This design strategy can be applied to any quasi-isochronous storage ring to achieve very high radiation power through the longitudinal coherence of the emitted radiation. This paper addresses the nonlinear dynamics and aperture optimization for this type of storage ring. An optimal design example based on our analysis is presented. Single-particle tracking results demonstrate that an electron beam with an equilibrium root-mean-square bunch length of approximately 80 nm can be maintained in this ring, featuring a dynamic aperture larger than 1 mm. Additionally, preliminary considerations of intrabeam scattering effects in this ring are discussed.

Permanent magnet electron optics offer many advantages over electromagnets and are being increasingly used in high-energy (GeV) electron accelerator designs. Here, we identify the advantages of permanent magnet electron optics for low-energy (keV) electron accelerators. We explore the applications of a class of designs based on axially magnetized permanent magnets, which offer a variety of advantages such as short focal lengths (few mm), while also preventing apparent emittance growth resulting from starting particles in a magnetic field. The proposed design philosophy is applied to an accelerator based on the ultracold electron source. The design is shown to be “emittance preserving” even for very short focal lengths (approximately 5 mm) at an emittance level better than 1 nm rad, while the short beamline ($\approx 12\mathrm{cm}$) limits space-charge effects. Two remedies for the mitigation of typical manufacturing and alignment challenges are considered. The performance of the design (related to parasitic aberrations) is enhanced by the proposed techniques. Applications of this design philosophy can improve the performance of ultrafast electron diffraction setups with minimal manufacturing effort.

We demonstrate that graph neural network embeddings of injector configurations provide a practical operational reference space for the Continuous Electron Beam Accelerator Facility injector at Jefferson Lab. Using 137,389 snapshots spanning January 2022 through March 2023, we show that injector operation occupies a small number of persistent, well-separated neighborhoods in a 16-dimensional learned state space rather than a featureless continuum. Density-based clustering identifies ten persistent operating regimes with strong operational run alignment, and regime persistence statistics confirm that these regimes are stable over timescales of hours to weeks. Large relocations between neighborhoods are rare and episodic; 99.6% of 1-h operating windows fall within an empirically derived jitter baseline. Geometric outlier screening narrows a year-long dataset to a small set of intervals warranting operational review, and nearest-neighbor retrieval enables case-based reasoning over the historical archive. A controlled beam study validates that deliberate injector reconfiguration traces coherent, interpretable trajectories in embedding space. Together these capabilities demonstrate that machine-state embeddings support holistic operational monitoring in ways that single-channel inspection cannot.