Dynamic and momentum apertures are a critical parameter of storage rings, limiting the flux of x-ray sources and the luminosity of colliders. Traditional optimizers must track thousands of particles for each lattice, severely limiting the design search. MultipointBAX instead tracks single particles, each selected based on a neural-network model of the stability maps. On a fourth-generation light-source design, MultipointBAX matches a genetic-algorithm Pareto front with over two orders of magnitude fewer tracking simulations, a gain that should extend to future light sources, colliders and other large facilities, enabling more complex and higher-performance designs.

Transient beam conditions are non-optimal conditions that can result in substantial losses. Most high-intensity linear accelerators use space-charge compensation (SCC) to mitigate space-charge effects. However, implementing SCC can create temporary beam transients. Our results revealed that the most critical scenario does not occur with zero SCC, but rather sometime after it has started the SCC buildup, when beam parameters allow for transmission through the RFQ at significant current.

This paper introduces a novel concept 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.

Inverse Compton scattering produces X-rays by colliding a laser pulse with relativistic electrons. Conventional head-on geometries require low beam energies, causing wide emission cones that limit brightness. A shallow, nearly co-propagating crossing angle lets a higher-energy beam deliver the same photon energy with far tighter collimation, yet this regime had not been explored in practice. The authors derive the geometric scaling laws of shallow-angle scattering, confirm them experimentally at a 5.8° crossing angle, and use the validated framework to evaluate future high-brightness radiation sources across diverse accelerator platforms.

The first free-electron laser (FEL), demonstrated in 1977, was an oscillator design operating in the infrared (IR) spectral region. Subsequently, some of the FEL pioneers proposed a dual-oscillator FEL, powered by electron bunches selected alternately from a single rf accelerator. Here, we report on the first realization of such a dual-oscillator two-color FEL that generates pulses at two independently variable wavelengths. We demonstrate that the pulses are inherently synchronized and their wavelengths can be tuned continuously over vast ranges in the IR spectral region, thereby enabling novel pump-probe applications.

Corrugated wakefield structures have enabled advanced XFEL pulse control at low repetition rates of about 100 Hz. We report the first pulsed megahertz demonstration at the European XFEL, combining an L-shaped structure with low-beta optics and a collimator. Hard x-ray spectra at 7 keV indicate an approximate fourfold pulse shortening, and trains of 100 pulses at 1.125 MHz show uniform properties. Neutron dose measurements verify that beam-loss mitigation keeps the radiation level at the undulator entrance within safe limits.

Different attosecond XFEL schemes based on SASE can deliver distinct combinations of peak power, pulse duration, and single-spike probability. We show that these differences can be understood from the effective lasing window, which sets how far the radiation evolves into the post-saturation superradiant regime. The current-spike length sets the available window for this evolution, while peak current, energy spread, emittance, energy chirp, and transverse tilt further modify the pulse properties. These results provide a physical picture and practical guidance for electron-beam manipulation and the design of terawatt-class attosecond XFELs.

We report intrabeam scattering (IBS) measurement and modeling for gold ion beams with large transverse emittance ratios, both without and with controlled betatron coupling. To our knowledge, this is the first experimental study benchmarking IBS growth for hadron beams with large emittance ratios, as well as the first to include deliberately introduced betatron coupling. To model the IBS growth, we use the formulas developed by Lebedev and Nagaitsev.

Direct measurements of electron cloud energy spectra remain scarce, and none have been reported under real LHC beam conditions. Using Retarding Field Analysers in the LHC Vacuum Pilot Sector, we resolve the full energy distribution from secondary electrons up to keV-scale bunch-accelerated populations. A phenomenological model captures both components and their evolution through the fill cycle, benchmarked against the analytical kick approximation. These calibrated spectra and derived cumulative distribution functions provide a quantitative reference for electron cloud simulation, heat load prediction, and scrubbing studies in the LHC and future high-intensity machines.

Wakes driven by transversely asymmetric electron beams in hollow plasma channels offer a promising route to high-quality positron acceleration, yet the conditions for sustaining long-lived, positron-favorable wakes remain to be clarified. Using fully three-dimensional particle-in-cell simulations, we identify two distinct driver evolution scenarios: quadrupole polarity reversal and continuous penetration of the driver into the plasma wall. We reveal the physical mechanisms governing this evolution and establish simple criteria for stable wake formation. The results provide practical guidance for sustaining long-lived quasi-steady wakes and enabling high-quality positron acceleration.

Plasmas offer an opportunity to shrink the next generation of particle accelerators with their high accelerating gradients. As we push to higher energy, the alignment of the witness bunch in the wake becomes of critical importance. In the quasilinear regime of beam-driven plasma wakefield acceleration, the transverse motion of a misaligned witness can be quantified and predicted, and a predictive model for emittance preservation developed as a result. This model can then be applied to inform alignment tolerances for future accelerators.

Klystrons have powered particle accelerators for over 80 years, and their efficiency was long thought to have hit an industrial ceiling. This paper reviews a decade of work at CERN, together with academic and industrial partners, showing that this ceiling can be broken. New bunching methods, first explored in simulation, have now been built into working tubes, with prototypes delivering the predicted efficiency gains in real operation. Looking ahead, new tube concepts promise efficiencies above 90% for the next generation of accelerators.

In the SuperKEKB superconducting (SC) cavities, cylindrical ferrite dampers have been installed for higher-order-mode (HOM) damping. While the HOM power increases as ramping up beam current, we observed that its increasing rate depends on the bunch fill pattern. To investigate this, wakefield simulations were conducted. HOM power is generally evaluated from the coupling impedance, which provides only the total beam power loss. When multiple absorbers are present, the absorbed power of each absorber remains unknown. Then, we introduce power calculation methods that utilize the local electromagnetic fields. In this paper, the theory and application to the SC cavity are presented.

We propose creative applications of axially magnetized permanent magnet rings for low-energy (keV) electron optics with an emphasis on preventing apparent emittance growth in a magnetic field, ease of integration, and minimization of parasitic aberrations. Extensive analysis of tolerances, and preliminary magnetic field measurements indicate that ‘extraordinary performance’ can be achieved using off-the-shelf ‘fridge’ magnets. Our design philosophy offers a different perspective on the otherwise arduous challenges of mechanical alignment of accelerator components.

Closed-orbit correction in rapid cycling synchrotrons is complicated by orbit evolution during the energy ramp, where conventional time-sliced response-matrix correction can drive corrector current ramp rates beyond operational limits. We introduce an adjacent-difference-constrained neural network that embeds the ramp-rate constraint in model structure. Simulations and machine experiments at the China Spallation Neutron Source (CSNS) achieve correction performance comparable to the response-matrix method while reducing the maximum corrector current ramp rate by 54%.

Maintaining vertical emittance stability is essential for modern high-brightness synchrotron light sources. Here, we present a real-time deep reinforcement learning correction framework using a Dueling Deep Q-Network and curriculum learning to dynamically adjust skew quadrupoles. $In$ $situ$ experimental validation at the Shanghai Synchrotron Radiation Facility demonstrates beam-size stabilization at the sub-micrometer fluctuation level, proving the practical feasibility of online reinforcement learning during active user beam delivery.

The LCLS-II injector has delivered a high-brightness, low-emittance electron beam at high repetition rates, meeting key performance targets. However, several operational challenges have emerged during early user operations, including large gun dark current, the formation of quantum-efficiency craters and bumps, dependence of free-electron laser pulse intensity on the laser repetition rate, emittance growth, and beam breakup and halos. This paper presents recent progress in addressing these operational challenges, along with injector emittance measurements and optimization efforts, and the first demonstration of operating the laser heater undulator at the third-harmonic resonance.

In electron cooling 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 relativistic energies, the adiabatic criterion generally cannot be fulfilled, so alternative electron beam transport is needed. Several methods exist for reducing transverse momentum: reduction in the amplitude of the impact at each perturbation; compensating for the influence of edge fields at the entrance and exit of the magnetic element; and adjusting the Larmour phase in the transport channel to minimize the sum of all local perturbations. A realizable strategy may involve using all of these methods.

The convergence map (CM) method is extended from 4 dimensions (4-D) to the full 6 dimensional (6-D) phase space to analyze nonlinear beam dynamics in the presence of time-dependent perturbations such as crabbing kicks in colliders. The approach enables efficient and accurate identification of high-order resonances and synchro-betatron coupling effects that can be missed by conventional frequency map analysis and is applied to a dynamic aperture (DA) study of the future Electron Ion Collider (EIC). The method successfully identifies resonance structures and evaluates DA for the EIC using only a one-turn transfer map.

Arclike bunch compressors can produce ultrashort electron bunches, but coherent synchrotron radiation (CSR) can degrade beam quality during compression. We develop an analytical model incorporating steady-state and drift CSR to guide compressor design and longitudinal matching. For Gaussian bunches, theoretical calculations provide a preliminary assessment of the longitudinal impact of CSR during compression, including changes in bunch length and peak current. Simulations demonstrate that a two-stage arclike compressor designed using the theory developed in this work can compress a 100 pC bunch to an rms duration of about 50 fs with negligible emittance growth.

Modern accelerators generate machine states that are high-dimensional and tightly coupled, making single channel monitoring an incomplete picture of true operating conditions. Building on graph neural network embeddings of the CEBAF injector, this work shows that 14 months of operational history collapses into a small number of persistent, physically interpretable regimes within a 16-dimensional learned space. The embedding supports stability baselining, outlier screening, and historical case-based retrieval, and a controlled beam study confirms it tracks deliberate reconfiguration coherently — demonstrating a practical operational reference space for holistic machine state monitoring.

Better optimizers are not what automated accelerator tuning needs most; smaller problems are. Experiments at TRIUMF’s ISAC postaccelerator demonstrate that a validated beam-physics model can directly determine the machine optics, removing those parameters from black-box optimization altogether. The lower-dimensional problem that remains converges faster and more reliably, illustrating a broader principle: model what can be predicted from physics and optimize only what cannot.

At SuperKEKB, the nanobeam collision scheme and high beam currents result in short beam lifetimes, making efficient top-up injection essential for sustaining luminosity. We apply Bayesian optimization to improve the injection efficiency during physics data taking while imposing operational constraints to avoid beam aborts. Beam tests demonstrated improvements in injection efficiency of up to 32%, showing that automated optimization can be safely integrated into collider operation.