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.

We present a noninterferometric method for measuring the transverse electron beam size at the European XFEL based on synchrotron radiation intensity autocorrelation. The technique enables cell-by-cell diagnostics along the SASE1 undulator using existing hardware: a commissioning monochromator and a spatial x-ray detector. We extend the results of [A. Trebushinin et al., companion paper, First observation of synchrotron radiation spikes for transverse electron beam size measurements at a free-electron laser, Phys. Rev. Lett. 135, 215001 (2025).] focusing on data analysis procedures and technical implementation. Our method allows for extraction of the beam size from noisy measured intensity distributions with a signal-to-noise ratio as low as 0.9 and is capable of distinguishing between the projected and slice sizes. To validate our approach, we introduced a mismatch in the magnetic lattice, inducing a controlled sinusoidal variation in the beam size along the undulator. The corresponding modulation was clearly reproduced in the electron beam size values extracted from the autocorrelation measurements. This method offers a practical solution for transverse electron beam size diagnostics in high-energy linacs, with direct application to the operation and tuning of free-electron laser facilities.

Tapering enhanced superradiance (TES) is a novel radiation scheme that significantly improves the energy conversion efficiency in free-electron lasers. The undulator tapering design and optimization are crucial to the success of this efficiency boost and are fairly complicated tasks. Here, we demonstrate how to optimize the linear tapering rate using analytical magnetic field maps derived from pure permanent magnet (PPM) equations. A TES undulator (waveguided tapered undulator) is designed to operate at 0.5 THz at the zero-slippage regime, ensuring phase synchronicity and group velocity match between the electron beam and the radiation field. The superradiance condition is fulfilled by compressing the electron bunch to emit coherently. The results reveal a significant enhancement in energy conversion efficiency for both ideal and realistic beams. The real beam simulations use the ORGAD FEL parameters from the Schlesinger Accelerator Center at Ariel University. This work shows the potential of the TES method to achieve higher radiation energies through careful undulator design and optimized tapering and provides a complete model for this purpose.

The low relative charge-to-mass ratio offset of 0.065% between fully ionized helium-4 and carbon-12 ions enables simultaneous acceleration in hadron therapy synchrotrons. At the same energy per mass, helium ions exhibit a stopping range approximately 3 times greater than carbon ions. They can therefore be exploited for online range verification downstream of the patient during carbon ion beam irradiation. One possibility for creating this mixed beam is accelerating the two ion species sequentially through the LINAC and subsequently “mixing” them at injection energy in the synchrotron with a double multiturn injection scheme. This work reports the first successful generation, acceleration, and extraction of a mixed helium and carbon ion beam using this double multiturn injection scheme, which was achieved at the MedAustron therapy accelerator in Austria. A description of the double multiturn injection scheme, particle tracking simulations, and details on the implementation at the MedAustron accelerator facility are presented and discussed. Finally, measurements of the mixed beam at delivery in the irradiation room using a radiochromic film and a low-gain avalanche diode detector are presented.

The nonlinear space-charge effect plays a significant role in high-intensity accelerators and has been extensively studied using multiparticle tracking methods. In this paper, we present a novel 2.5-dimensional symplectic space-charge solver specifically designed for long beam bunches. We begin by detailing its application to a transverse Gaussian density distribution under open boundary conditions in a straight system, where a semianalytical expression is derived. We then demonstrate the solver’s adaptation to arbitrary distributions in open space, as well as within rectangular and round conducting pipes. Finally, we discuss the extension of this solver to circular accelerator systems. This study shows that the fast 2.5-dimensional solver can be a good approximation to the fully three-dimensional solver for long bunches in large circular accelerators.

The SPring-8 Angstrom Compact free-electron LAser (SACLA) linear accelerator utilizes a dc gun with a thermionic cathode, known for delivering high beam quality, excellent operational stability, and low maintenance requirements. In the second part of our design study, we present a comprehensive beam dynamics analysis of the SACLA dc electron gun and outline a roadmap toward achieving submicrometer ($0.1\mathrm{\mu }\mathrm{m}$) beam emittance. We provide both analytical and numerical estimates of four key sources contributing to emittance growth: (i) image = charge effects on the cathode, (ii) nonlinear space-charge forces arising from imperfect beam edge geometry, (iii) aberrations in the accelerating field, and (iv) solenoid field aberrations due to longitudinal–transverse momentum exchange. The analysis yields a closed-form expression for the superposition of correlated emittance growth mechanisms, highlighting an “interference” term that amplifies the net emittance when space-charge and solenoid effects coexist. To address the challenge of emittance growth, we propose a compact hybrid magnet that ensures strong longitudinal confinement of the magnetic field. This confinement allows placement of the hybrid magnet in close proximity to the gun exit, thus mitigating emittance growth effectively, while having a negligible magnetic field on the cathode. The hybrid magnet design is achieved by symmetrically positioning permanent ring magnets on either side of a solenoid, enabling fine control over the field distribution and “squeezing in” it longitudinally. Our findings offer new insights applicable to low-voltage continuous-wave very high-frequency guns and superconducting radio-frequency electron guns.

Superconducting radio-frequency (SRF) cavities with high-loaded-Q are susceptible to ponderomotive instabilities arising from nonlinear electromechanical coupling. Under low-level rf feedback control, stability analysis is challenging and has, until now, lacked comprehensive experimental verification. In this work, we linearize the coupled cavity electromagnetic and mechanical equations about the operating point, deriving explicit small-signal stability criteria for both open- and closed-loop operation. A characteristic equation approach is used to map the oscillatory instability threshold in the detuning–gradient plane. Time-domain simulations confirm the predicted onset of self-excited oscillations at these thresholds. Furthermore, experiments on a 162.5 MHz SRF linac (CAFE2) show that the observed oscillation onset agrees with our analytical predictions within a small margin. By combining analytical modeling, numerical simulation, and direct experimentation, this study providescomprehensive validation of oscillatory ponderomotive instability theory under feedback control, offering practical guidance for the stable operation of high-gradient SRF accelerators.

In low and medium energy electron storage rings, intrabeam scattering (IBS) is a significant collective effect that can lead to an increase in equilibrium emittance, thereby diminishing the light source’s flux. Typically, the bunch length of the electron beam in storage rings is considered constant around the ring circumference. However, in the concept of steady-state microbunching storage rings, the bunch length can vary significantly around the ring in an effort to achieve high coherence in the longitudinal dimension. In this paper, we have extended the Bjorken and Mtingwa (B-M) model to accommodate this requirement, making it versatile enough to be used for arbitrary planar storage rings. Additionally, this paper addresses the simplification of equilibrium emittance calculations involving IBS through certain approximations, making it possible to directly calculate equilibrium emittance by a formula rather than conventional iteration method. This method reveals the physical relationship between equilibrium emittance and the IBS growth rates, and also allows for the IBS effect optimization of the ring lattice by directly using equilibrium emittance as the optimization target rather than the IBS growth rate.

Calculating the effects of coherent synchrotron radiation (CSR) is one of the most computationally expensive tasks in accelerator physics. Here, we use convolutional neural networks (CNNs), along with a latent conditional diffusion (LCD) model, trained on physics-based simulations to speed up calculations. Specifically, we produce the 3D CSR wakefields generated by electron bunches in circular orbit in the steady-state condition. Two datasets are used for training and testing the models: wakefields generated by three-dimensional Gaussian electron distributions and wakefields from a sum of up to 25 three-dimensional Gaussian distributions. The CNNs are able to accurately produce the 3D wakefields $\sim 250–1000$ times faster than the numerical calculations, while the LCD achieves a gain of a factor of $\sim 34$. We also test the extrapolation and out-of-distribution generalization ability of the models. They generalize well on distributions with larger spreads than what they were trained on but struggle with smaller spreads.

High-intensity positron sources are critical for next-generation electron-positron colliders, where positron beam quality and characteristics directly impact the luminosity. Accurate modeling and validated simulation tools for positron sources are essential to optimize their performance. However, modeling a positron source tends to be complex, as it involves multiple interdependent stages, from positron production and capture dynamics to beam transport through the injector linac to the collider ring. In this work, we present a start-to-end simulation tool developed for positron sources modeling. The model was benchmarked against existing simulation tools and validated through experimental measurements conducted at the SuperKEKB positron source. Key operational parameters were systematically scanned to evaluate the simulation model performance, including the primary electron impact position on the target, solenoid field strength around the capture linac, and rf phase settings. The primary figure-of-merit for all validation tests was the positron yield at the end of the SuperKEKB positron capture section. Our model reproduces the measured positron yield ($0.603\pm 0.003$) under nominal operating conditions, with the simulation result of 0.603 lying well within the 0.5% statistical uncertainty. Furthermore, the simulation results demonstrate a very good agreement with experimental data and other simulation tools, confirming the model’s reliability and establishing a framework for future positron source studies.