The fluctuation in electron time of flight during lattice transport is crucial for various applications, such as ultrafast electron diffraction, optical stochastic cooling in storage rings, and sustaining steady-state microbunching in storage rings. A major issue impacting the time-of-flight fluctuation of the electron beam during transport is the lattice’s nonlinear effects. In this study, we introduce a design concept aimed at reducing the electron beam’s time-of-flight fluctuation in an isochronous lattice up to the subattosecond level. This approach employs a high-order achromat lattice that effectively eliminates time-of-flight contributions arising from low-order nonlinearities. We also present an optimized design example based on this concept, achieving approximately 0.3 attoseconds root-mean-square time-of-flight fluctuation (equivalent to 0.1 nm) with specific beam emittance values of ${\epsilon }_x\sim 1\mathrm{nm},{\epsilon }_y\sim 1\mathrm{nm},$ and ${\sigma }_{\delta }\sim 1\times {10}^{-3}$.

This work introduces a reinforcement learning algorithm designed for real-time luminosity optimization in collider experiments. The neural network architecture is selected from multiple candidates through systematic evaluation of their training performance. Prior to training, the input data undergo multistage preprocessing before being fed into the neural network. Our approach combines off-line pretraining on historical accelerator data with online fine-tuning during operation. By processing accelerator measurements over multisecond timescales, the reinforcement learning model dynamically adjusts the magnetic structure to maintain luminosity stability under changing beam conditions. The autonomous nature of the method eliminates the need for manual intervention, enhancing both operational efficiency and beam stability in long-term operation. Experimental validation on the VEPP-4M collider demonstrates the feasibility of the approach and provides a foundation for future development and deployment in accelerator systems.

Addressing the demands for compactness and tunability in magnet systems for fourth-generation light sources, this paper presents the design and development of a novel tunable permanent quadrupole magnet (PQM) prototype. The prototype achieves a maximum field gradient of $65\mathrm{T}/\mathrm{m}$ and demonstrates a continuous tuning range of $39\mathrm{T}/\mathrm{m}$. Within the effective aperture, normalized multipole field components are maintained below $5\times {10}^{-4}$. Experimental validation confirms that the prototype meets the engineering design requirements for the Hefei Advanced Light Facility. Compared to conventional electromagnetic quadrupoles, this PQM features a compact structure, with negligible power consumption and high magnetic field quality, representing a promising technology for advanced synchrotron radiation light sources and free-electron lasers.

The performance degradation caused by carbon contaminants has become one of the significant obstacles to the long-term, high-gradient, stable operation of the SRF cavities. This paper investigates the sources and effects of different forms of carbon contamination and recovery strategies through experiments on niobium samples and a 162.5 MHz half-wave SRF cavity. Our findings reveal that adventitious carbon and oxides are the primary chemical compounds on the niobium surface, with the work function decreasing linearly as carbon content increases. Additionally, it was observed that carbides can form through the chemical deposition of residual carbon-containing gas in a vacuum under the action of argon ions. The adsorption and deposition of carbon-containing gas molecules reduce surface work function, thereby lowering the field emission onset and quench point of the cavity. However, the combination of thermal cycling and rf conditioning facilitates the desorption of adsorbed gas molecules, leading to a full recovery of cavity performance. In contrast, carbon deposition forms strong chemical bonds with the cavity surface, necessitating more intensive removal techniques. We demonstrate that plasma cleaning effectively eliminates carbon contamination and restores cavity performance. Inspired by the formation of NbC in niobium sample experiments, we propose that, during long-term SRF cavity operation, carbides primarily originate from the chemical deposition of carbon-containing gas molecules adsorbed on the surface in the presence of energy-carrying particles. These insights deepen our understanding of carbon contaminant-induced cavity performance degradation and recovery, providing a basis for selecting suitable online performance recovery methods to maintain the cavity’s stable, high-gradient operation.

The high-brightness beams being produced in current and future accelerators present new machine protection concerns with the potential for high-energy-density (HED) conditions ($>100\mathrm{J}/{\mathrm{mm}}^3$) in beam-intercepting components. Simulating HED conditions in accelerators requires utilizing a suite of physics codes for particle dynamics, particle-matter interactions, and hydrodynamics. This paper describes a method of coupling the codes elegant, fluka, and flash to simulate the effects of a rapid beam loss in the advanced photon source storage ring and the resulting interaction of the beam and collimators. This paper expands previous work [J. Dooling et al., Collimator irradiation studies at the advanced photon source, in Proceedings of the IBIC-2023 (2023), pp. 245–249] by introducing a definition of the evolving geometry of the collimator surface as well as providing methods for simulating the absorption of synchrotron radiation and tracking shower particles produced during beam strikes. We demonstrate this framework by simulating machine conditions of the APS ring before and after its recent upgrade. Simulation results are compared with observed damage to collimators and test samples taken from the APS ring.

X-ray transition radiation emitted by high-energy electrons in a thin foil placed in a strong external magnetic field is considered. The modification of the radiation properties resulting from the bending of the particle’s trajectory in the magnetic field is investigated. It is shown that, due to the interference between transition and synchrotron radiation, the total radiation spectrum can significantly differ from the simple sum of the conventional spectra of these two types of emission. Both constructive and destructive interference can occur in this case. The dependence of the magnitude of this interference on the electron energy and the acceptance angle of the radiation detector is analyzed.

One of the primary issues in designing particle accelerators is the effects of energy losses and collective instabilities caused by the conductivity of the vacuum chamber. Due to its relevance, many authors have long focused on studying the coupling impedance of lossy pipes with various geometries and metals. Most studies were developed for relativistic beams and room-temperature conductivity. In recent years, there has been increasing interest in the high-frequency impedance behavior of ultrashort bunches, as in the case of free-electron lasers, and in the anomalous conductivity at cryogenic temperatures of the vacuum chambers. This work introduces a new unifying theoretical framework for analyzing electromagnetic interactions in cylindrical pipes excited by charged particles traveling on the axis. In this context, a key achievement is the derivation of a rigorous expression for the electrical conductivity, based on the Boltzmann theory of a Fermi gas, which forms the foundation for subsequent developments. New formulas for the surface impedance of cylindrical pipes are presented, from which a more general impedance expression is derived. Our approach captures the system’s behavior across various energy and frequency regimes. Eventually, the anomalous regime is studied rigorously, providing analytical solutions for the electromagnetic field at cryogenic temperatures. Indeed, new and exact expressions for the anomalous surface impedance for carriers’ reflection coefficients $p$ = 0 and $p$ = 1 are obtained, offering deeper insight into the electromagnetic properties of such systems under extreme conditions.

On-line monitoring of beam phase and energy gain in the cavity is of great importance for the stable operation of accelerators, especially in high-power scenarios. As cavities are typically calibrated with a low-current, short-pulse beam for safety purposes, the beam behavior may deviate from the calibrated case when running high-current beams, due to the strong space charge effect and other possible factors. Motivated by the high-power operation demand at European Spallation Source and the further understanding of beam-cavity interaction, methods for beam phase and energy gain on-line measurement have been developed. The beam phase was measured based on the steady-state cavity model, as demonstrated in the half wave resonator cavity at CAFE2. In this paper, this method was not only applied to a single-cell buncher cavity but also, for the first time, extended to the more complex low-energy multicell RFQ cavity for verification. Additionally, an energy-conservation-based method for measuring beam energy gain in a cavity was proposed and validated.

The brightness of an electron emitter is fundamentally limited by the intrinsic emittance of the electrons born at a cathode. Different electron-emission mechanisms from a cathode give different values of mean transverse energy for calculating the intrinsic emittance of emitted electrons. This study reports a theory for calculating the mean transverse energy of electrons generated at a cathode via mixed thermionic, multiphoton, and field emissions. The theory successfully reveals the details of thermionically assisted multiphoton emission from a copper cathode excited by an intense infrared laser. An extension of the theory to calculate the intrinsic emittance and brightness of an electron emitter is also presented.

The charge-state distribution (CSD) of the projectile ions through solid targets in the intermediate energy range ($1\mathrm{MeV}/\mathrm{u}<E<4\mathrm{MeV}/\mathrm{u}$) has a major impact on the collision of the ion atom and accelerator physics. We explore the mean charge states taken from the empirical formula [G. Schiwietz et al., Femtosecond dynamics–snapshots of the early ion-track evolution, Nucl. Instrum. Methods Phys. Res., Sect. B 225, 4 (2004)] are only good for projectile ions with $Z_1\le 16$. To solve this issue, we develop a model in which instead of a single formula, if we employ four formulas, the comparative picture between experimental and empirical data becomes impressive. Furthermore, the CSDs obatained using the mean charge state and the Gaussian distribution function having distribution width as given by Novikov and Teplove [Methods of estimation of equilibrium charge distribution of ions in solid and gaseous media, Phys. Lett. A 378, 1286 (2014)] show good agreement with the experimentally measured CSDs for the entire range of projectile ions. We believe that accurate CSDs will be highly useful for both ion-atom collision and accelerator physics.

The spectral emission characteristics of a proof-of-concept hybrid metallized diamond(001) photocathode are presented. The quantum efficiency (QE) is shown to be determined by the photoinjection efficiency across the ohmic contact at the back metallized face, whereas the mean transverse energy (MTE) of the photoemitted electrons is consistent with an (optical)phonon-assisted and momentum-resonant Franck-Condon mechanism [Franck-Condon electron emission from polar semiconductor photocathodes, Phys. Rev. Appl. 23, 054065 (2025)] following electron drift transport to the untreated diamond(001) front emission face. Emission is observed from both the lower and upper conduction bands of diamond with positive and negative electron affinity, respectively. A potential route to the realization of a red-visible photocathode based on this concept with a sub-50 meV MTE and a QE greater than 0.1% is discussed.

Anomalies in radio-frequency (rf) stations can result in unplanned downtime and performance degradation in linear accelerators such as SLAC’s Linac Coherent Light Source (LCLS). Detecting these anomalies is challenging due to the complexity of accelerator systems, high data volume, and scarcity of labeled fault data. Prior work identified faults using beam-based detection, combining rf amplitude and beam position monitor data. Due to the simplicity of the rf amplitude data, classical methods are sufficient to identify faults, but the recall is constrained by the low-frequency and asynchronous characteristics of the data. In this work, we leverage high-frequency, time-synchronous rf phase data to enhance anomaly detection in the LCLS accelerator. Due to the complexity of phase data, classical methods fail, and we instead train deep neural networks within the Coincident Anomaly Detection (CoAD) framework. We find that applying CoAD to phase data detects nearly 3 times as many anomalies as when applied to amplitude data, while achieving broader coverage across rf stations. Furthermore, the rich structure of phase data enables us to cluster anomalies into distinct physical categories. Through the integration of auxiliary system status bits, we link clusters to specific fault signatures, providing additional granularity for uncovering the root cause of faults. We also investigate interpretability via Shapley values, confirming that the learned models focus on the most informative regions of the data and providing insight for cases where the model makes mistakes. This work demonstrates that phase-based anomaly detection for rf stations improves both diagnostic coverage and root cause analysis in accelerator systems and that deep neural networks are essential for effective analysis.

Generating narrow energy spread beams from laser wakefield accelerators with minimal dark current is important for many applications. We present an automated experiment using Bayesian optimization to control the laser spectral phase and focal position, achieving significant enhancement of the beam quality. Starting from a broadband spectrum, the optimized configuration achieved low-energy spread with negligible dark current above 20 MeV. Particle-in-cell simulations reveal that fine control over the laser spectral phase, particularly third-order dispersion, directly impacts the timing and localization of ionization injection due to its effect on the initial laser intensity envelope and its subsequent evolution. A positively skewed temporal profile enhances the plasma potential depth at the right location, enabling localized injection and reducing the energy spread and dark current. These findings highlight the impact of algorithmic optimization and the powerful, underutilized role of temporal shaping in beam quality control.

Radiotherapy utilizing very-high-energy electron (VHEE) beams in the range of 50–300 MeV has gained significant interest over the past two decades due to their advantageous dose characteristics, deep tissue penetration capabilities, and potential for ultrahigh dose-rate treatments. Laser wakefield accelerators (LWFAs) are particularly well suited for generating VHEE beams in a compact setup, thanks to their substantially higher accelerating gradients compared to conventional radio-frequency accelerators. To meet the demands of clinical applications, a comprehensive dosimetry study of LWFA-generated VHEE beams in a preclinical treatment configuration is essential. In this study, a VHEE beam with a maximum energy over 160 MeV was produced using a compact and stable LWFA prototype operating at 1 Hz. The beam was subsequently transported through a quadrupole triplet to eliminate low-energy components and reduce pointing jitter, followed by a scatterer and collimator to create a uniform circular radiation field with an 8-mm diameter. Using this beam, we measured the three-dimensional dose distribution within a solid-water phantom and evaluated the feasibility of multifield intensity-modulated irradiation by delivering 400 VHEE beams from 20 different angles into the phantom. This resulted in a 16-mm-diameter dose plateau peak, with the relative uniformity of approximately 2.8%, and the entrance dose was only 20% of the peak value. These findings demonstrate the feasibility and robustness of LWFA-based VHEE beams for treating deep-seated tumors, highlighting the need for dedicated engineering and preclinical studies in this promising direction.

Optimization of beamlines and lattices is a common problem in accelerator physics, which is usually solved with semianalytical methods and numerical optimization routines. However, these are usually of the gradient-free or finite-difference type, whose computational cost grows quickly with the number of optimization parameters. On the other hand, the cost of gradient-based optimization can scale well with the number of parameters, but only if the computation of the gradient is itself efficient (e.g., not via finite differences, which are inefficient). Recently, there has been an emergence of so-called “differentiable” codes that efficiently provide gradients. Nevertheless, analysis and benchmarking comparisons of these techniques have largely been absent from the literature. In this work, we develop our own differentiable code via autodifferentiation. We analyze and benchmark differentiability against finite differences in two test cases, a space-charge FODO cell from the literature and a realistic future beamline at CERN. The analyses and benchmarking are done both theoretically and empirically. Finally, we embed the gradient provided by the differentiable code in gradient-based optimization routines and compare with gradient-free methods. This work offers the first such analysis and benchmarking and thus contributes toward the development of more efficient and performant particle accelerators.

The issue of concern in ultralow-emittance synchrotron light sources is the growth of emittance and the shortening of the Touschek lifetime as a result of intrabeam scattering (IBS). One of the most reliable methods for mitigating these IBS effects is to lengthen the bunches using fundamental and harmonic cavities. In such double rf systems, the effect of bunch lengthening is limited by the fluctuation of the fundamental and harmonic rf voltages. In particular, the voltage fluctuations caused by gaps in the bunch filling pattern critically influence the bunch lengthening. To mitigate such a transient beam-loading (TBL) effect, a compensation method using a broadband cavity has been proposed. In this paper, we present a conceptual design of a broadband “kicker” cavity that can be used to compensate for the TBL effect. We then report the manufacture of a low-power model cavity and the results of our evaluation of its performance to confirm the design of the kicker cavity. The measured external-$Q$ value of the cavity was sufficiently low to achieve the requisite broad bandwidth and was in agreement with simulation results. We also confirmed that the rf absorbers sufficiently damp the harmful higher-order modes.

The sequence formed by the fractional tune resonances up to order $n$ exactly coincides with the Farey sequence, $F_n$. In circular accelerators, it is important to place the tunes in regions free of resonances. From numerical evaluations, it is well known that the largest resonance-free gaps happen next to low order resonances. In this report, we provide a mathematical demonstration of this statement for any order $n\in \mathbb{N}$ showing that the resonance gaps are strictly ordered according to the order of the adjacent lowest order resonance up to some given value. We present a connection between the resonance sequence and one of the most important unsolved problems in mathematics, the Riemann hypothesis (RH). If RH is true, it implies that the resonance sequence is significantly more regularly spaced than if it were built at random; however, this has not yet been demonstrated. A new analytical estimate of the number of resonance lines up to order $n$ in the two-dimensional tune diagram is also derived and compared to previous estimates.

The Vlasov solver provides an efficient method to analyze transverse mode coupling instability (TMCI). However, many traditional Vlasov solvers are based on the small amplitude approximation, and are applicable only to single rf (radio frequency) systems, whereas harmonic cavities are widely utilized in colliders and synchrotron light sources to improve beam stability. In this paper, we have developed a new Vlasov solver suitable for any double rf system to study TMCI. The results of the Vlasov solver are in good agreement with the simulation results obtained using elegant.

Laser-driven ion beams offer significant advantages over radio-frequency accelerated beams, making them particularly promising for biomedical applications. Key features include their ultrashort pulse duration, on the order of picoseconds, and high peak fluxes ranging from ${10}^{11}$ to ${10}^{13}$ particles per shot. These characteristics enable the delivery of ultrahigh dose rates, potentially reaching the FLASH therapy regime. The future implementation of laser-driven ion beams in clinical settings relies on the development of advanced focusing and beam transport systems capable of precisely controlling parameters such as energy range, beam focus, and dose distribution to meet stringent therapeutic requirements. However, the integration of ultrashort laser-driven ion beams into medical treatments presents challenges, particularly due to their broad energy spectrum and high angular divergence. We present 3D simulation-based studies investigating the focusing effects of different high-current solenoid configurations on laser-plasma-accelerated proton beams. To identify the optimal focusing solution, we designed, analyzed, and compared high-current solenoids of varying dimensions and shapes, both as stand-alone magnetic elements and as part of two-solenoid focusing systems. Our analysis focuses on magnetic fields between 6 and 9 T, generated by the high-current solenoids. The study considers a proton beam with an energy range from a few MeV to 20 MeV and an initial divergence of 21°. Results are evaluated in terms of collection efficiency, beam focusing position, and beam profile. The optimized beamline configuration was then used to assess the dose distribution in a cylindrical water target with a volume of $78{\text{mm}}^3$ by calculating the absorbed dose and dose delivery rates, demonstrating the potential of laser-plasma-accelerated proton beams for radiotherapy applications.

High-gradient acceleration is a key research area that could enable compact linear accelerators for future colliders, light sources, and other applications. In the pursuit of high-gradient operation, rf breakdown limits the attainable accelerating gradient in normal-conducting rf structures. Recent experiments at the Argonne Wakefield Accelerator suggest a promising approach: using short rf pulses with durations of a few nanoseconds. Experimental studies show that these $\mathcal{O}(1\mathrm{ns})$ rf pulses can mitigate breakdown limitations, resulting in higher gradients. For example, an electric field of nearly $400\mathrm{MV}/\mathrm{m}$ was achieved in an $X$-band photoemission gun driven by 6-ns-long rf pulses, with rapid rf conditioning and low dark current observed. Despite these promising results, the short-pulse regime remains an underexplored parameter space, and rf breakdown physics under nanosecond-long pulses requires further investigation. In this paper, we present analytical and numerical simulations of dark current dynamics in accelerating cavities operating in the short-pulse regime. We study breakdown-associated processes spanning different time scales, including field emission, multipacting, and plasma formation, using simulations of the $X$-band photogun cavities. The results reveal the advantages of using short rf pulses to reduce dark current and mitigate rf breakdown, offering a path toward a new class of compact accelerators with enhanced performance and reduced susceptibility to breakdown.