We present a methodology for modeling the photoemission process in nanopatterned photocathodes using finite-difference time domain (FDTD) and 3D Monte Carlo (MC) methods adapted for tasks with 3D nanopattern geometry. Using MC, we calculated the electron transport inside the alkali semiconductor. The FDTD method was used to simulate light absorption in a photocathode. This coupled approach self-consistently models resonant effects, electron excitation, transport to the cathode surface, and emission into vacuum. Furthermore, this approach enables the study of two-dimensional (2D) nanopatterned structures. We studied semiconductor photocathodes with 1D and 2D nanopatterned metal substrates. We calculated the quantum efficiency and intrinsic emittance at photon energies from 1.8 to 2.4 eV. We have demonstrated that the quantum efficiency of a nanopatterned photocathode is significantly higher than that of a photocathode with a flat interface. We also demonstrated that the quantum efficiency of a two-dimensional nanostructured photocathode is polarization-independent. At the same time, the cathode intrinsic emittance remains unchanged upon nanopatterning.

It is significantly challenging to protect the power source against the reflected power when standing-wave rf structures operate, in particular under overcoupled conditions. Conventionally, nonreciprocal circulators, usually operating under pressurized $S{F}_6$ atmosphere, are employed to isolate the power source from the reflected power. However, breakdowns, rf losses, and thermal stress make these devices unusable in rf structures operating at high frequencies (6–12 GHz) and can limit the maximum power or the repetition rate, all of which are critical aspects for next-generation accelerators or rf photoinjectors, as an example. Furthermore, the use of $S{F}_6$ gas is becoming increasingly problematic due to its environmental impact. In this paper, two novel rf networks are proposed to compensate reflections from standing-wave structures, thus avoiding the use of circulators. They are based on specifically designed hybrids that, combined with rf structures, allow to strongly reduce the reflected power. The design of hybrids with arbitrary power ratios between different ports constitutes a multiobjective optimization problem. To address this challenge, a new optimization methodology is proposed to simplify the optimization process into a single-objective problem. These novel rf networks are then applied to a new C-band photoinjector recently developed at the Italian National Institute of Nuclear Physics INFN (Frascati, Rome, Italy) and to a new proposal of X-band rf photoinjector for National Synchrotron Radiation Laboratory (NSRL).

The optimization of the dynamic aperture (DA) of a storage ring is typically a nonconvex problem with multiple local optima. Recent studies showed that reducing the variation of resonance driving terms (RDTs) along the longitudinal position improves DA very effectively, as the reduction in the longitudinal variation of lower-order RDTs suppresses higher-order nonlinear terms. Therefore, minimizing the longitudinal variation of third-order RDTs is crucial for DA optimization. In this paper, we prove that the longitudinal variation of third-order RDTs, quantified using their rms value $f_{3,\mathrm{rms}}$ at sextupole locations, is a special convex function. In the space of sextupole strengths, the isosurfaces of $f_{3,\mathrm{rms}}$ are a series of concentric and coaxial ellipsoidal surfaces, with the central position possessing minimum $f_{3,\mathrm{rms}}$. The scanning results of a storage ring lattice show a strong consistency between the distributions of $f_{3,\mathrm{rms}}$ and DA, indicating that the optimization of DA can be regarded as a roughly approximate convex optimization problem. Based on this, a fast DA optimization method based on particle tracking is developed, where a high-quality initial population for an intelligent algorithm is generated with a Gaussian distribution based on the geometric structure of $f_{3,\mathrm{rms}}$.

Future electron-positron linear colliders require extremely small beam sizes at the interaction point (IP) to achieve high luminosity. The Accelerator Test Facility (ATF) at KEK has been developed to demonstrate the required beam focusing scheme, achieving a vertical size of 37 nm at the virtual IP using a low-emittance beam generated in a damping ring. However, as nanometer scale focusing requires specialized optics, even small kicks in the beamline can lead to significant distortions at the IP. Transverse wakefield effects have previously been observed as beam size growth with increasing bunch intensity and have been primarily interpreted as static effects arising from orbit distortion and misalignment. Dynamic wakefield effects caused by pulse-by-pulse beam orbit fluctuations have also been suggested by previous measurements and investigated through numerical simulations; however, a direct experimental and quantitative evaluation has not been reported. In this paper, we present the first experimental and quantitative study of dynamic wakefield effects on nanometer scale beams at the ATF. Controlled orbit fluctuations were introduced using steering magnets, enabling systematic measurements over a wide range of fluctuation amplitudes without changing the beam optics configuration. The resulting beam size growth was quantitatively evaluated through both experiments and simulations incorporating all wakefield sources in the beamline. The results demonstrate that dynamic wakefield effects can significantly contribute to beam size growth and must therefore be taken into account to achieve stable nanometer scale beams in future linear colliders.

Relying on atomic-scattering factors from evaluated databases, a new model for the reflectivity of x-rays on solid surfaces has been developed for FLUKA v4-6.0. This model accounts for the variation of reflectivity as a function of the photon energy, its incidence angle, and linear polarization; surface-roughness effects are also taken into account. FLUKA reflectivities agree well with those obtained from state-of-the-art codes used for the characterization of optical devices, both for homogeneous solids and for multilayer mirrors. This new capability renders FLUKA a nearly one-stop shop for synchrotron radiation simulations: emission from bending magnets and wigglers, photon transport and interaction, electromagnetic (and hadronic when applicable) shower development in complex geometries, as well as x-ray reflection at designated solid surfaces, can now all be accounted for in a single FLUKA run. This streamlined FLUKA simulation workflow greatly simplifies the plethora of simulation tools that Monte Carlo practitioners previously needed to rely on. Two application scenarios of this new reflectivity model are showcased: first, the use of a multilayer mirror to deflect x-rays from an optical hutch onto an experimental hall at the MINERVA beamline of the ALBA synchrotron and second, the assessment of the photon flux near the interaction point at CERN’s Future Circular Collider (in its electron-positron stage) as a result of upstream x-ray reflections.

Pulsed high-power radio frequency (rf) systems frequently employ pulse compressors to augment peak output power, thereby reducing the required number of rf sources and enhancing cost efficiency. A prominent class of these devices utilizes single resonant cavities supporting two degenerate modes to achieve high performance within a compact footprint. However, the inherent coupling of these modes complicates experimental characterization, a challenge exacerbated by the fact that these units are typically nondismountable. This paper proposes a novel analytical method to disentangle the complex electromagnetic data encoded within these systems. By separately extracting the parameters of the individual degenerate modes and the characteristics of the associated waveguide network, the method provides a granular view of device performance. This approach enables a more comprehensive interpretation of measurement data, offering a pathway for the precise tuning and optimization of monolithic rf pulse compression systems.

A new beam delivery system has been developed to explore the feasibility of extending the energy reach of Compact Linear Collider (CLIC) to 7 TeV, either through an extension of existing CLIC linacs or by integrating novel acceleration technologies. This study provides key insights for the design of future multi-TeV collider concepts, including a potential 10 TeV plasma-wakefield acceleration-based collider. Particular emphasis is placed on developing a more compact final focus system capable of achieving a luminosity higher than that of previous designs. An in-depth analysis of higher-order optical aberrations and their impact on beam size and luminosity performance is presented.

Historically, the design and operation of secondary beamlines at CERN’s North and East Areas relied on simplified particle distributions and a beamline acceptance determined solely by collimators. The formalism presented here makes as few simplifying assumptions as possible and can be used to match and design secondary beamlines without time-consuming HPC tracking studies. In this study, the formalism is applied to the development of a new neutrino beamline within the Physics Beyond Colliders framework at CERN. The code is expressed generically, providing tools for designing secondary beamlines with small relative losses.

In order to reduce the increase in bunch energy dispersion caused by the beam loading effect, beam loading compensation in the acceleration structure is required. In this paper, a novel scheme is proposed to use microwave pulse compression to compensate for the beam loading effect in the traveling wave structure. The compression system consists of an energy storage cavity and a correction chain of six cavities working in S band. Through parameter optimization, the optimal coupling factors of the energy storage cavity and the correction cavity are obtained to generate microwave pulses with increasing amplitude. Through this scheme, the S-band traveling wave acceleration structure achieves an acceleration voltage of 50.14 MV with a power input of 16.6 MW and a beam current of 2.5 A. The relative energy dispersion is 0.77%, and the effective system power gain reaches 10.56, significantly surpassing that of conventional microwave pulse compression systems.

Beam-driven plasma wakefield acceleration represents a promising strategy for producing high-energy electron and positron beams in future accelerator facilities. A key challenge in realizing high-quality acceleration is the production of high-charge, high-stability double-bunch beams, as existing methods are often limited to low charges or suffer from significant charge loss. In this paper, we propose a coherent synchrotron radiation-tolerant beam-merging scheme using twin isochronous beamlines that share a common dipole magnet to generate a high-charge double-bunch beam with excellent stability. Numerical simulations indicate that the proposed scheme generates a 5 nC/1 nC electron double-bunch beam that exhibits no charge loss, with emittance growths of both bunches constrained to approximately $0.1\mathrm{mm}\mathrm{mrad}$, and an interbunch timing jitter maintained below $\pm 2\mathrm{fs}$. The robustness of the scheme is evaluated, and its potential use in large-scale accelerator facilities is emphasized for supplying high-quality drive beams for plasma wakefield accelerators.

Real-time monitoring and predictive modeling of complex accelerator subsystems demand data-efficient approaches that can capture essential dynamics from minimal measurements. This study introduces a framework based on kernel-based dynamic mode decomposition that constructs surrogate dynamical models from a single reference waveform, enabling accurate time-series reconstruction with substantially reduced data requirements compared to conventional methods. The approach formulates the system dynamics as a continuous ordinary differential equation (ODE) in a delay-embedded phase space, where a sparse dictionary and polynomial kernel provide both computational efficiency and physics-aligned interpretability through the underlying ODE structure. The method is validated on three beam kicker systems and two pulse-forming network (PFN) modulator systems at the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC, demonstrating accurate waveform reconstruction using only one training waveform per system. A comparative study against exponential smoothing and linear trend models confirms the method’s superior predictive capability, particularly in extrapolative tail regions. These results establish a lightweight, scalable foundation for real-time waveform monitoring, anomaly detection, and potential integration into broader facility-level digital twin frameworks in accelerator science and beyond.

Beam stacking allows a fixed field alternating gradient accelerator (FFA) to increase the extracted beam current while also enabling a flexible time structure, making FFAs a promising candidate for future spallation neutron sources and other high intensity beam applications. For successful beam stacking, beam loss caused by rf knockout must be prevented. rf knockout can occur during beam stacking because of the finite dispersion function at the rf cavity location, which is unavoidable in a scaling FFA. In this work, the rf knockout resonance is characterized and through a series of experiments at the ISIS Neutron and Muon Source, we show that it is possible to suppress losses due to rf knockout.

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.

Multipactor (MP) discharge is a critical limitation in superconducting radio-frequency (SRF) cavities, impacting the quality factor and cryogenic stability. In this article, we present a novel mitigation strategy to mitigate MP discharge by utilizing specific higher order quadrupole modes in single spoke resonator (SSR) cavities, designed for the MEHIPA-II linac. We demonstrate that the presence of a specific quadrupole mode leads to the effective suppression of MP in that SSR cavity and show that the quadrupole modes have minimal influence on the proton beam across the 40–200 MeV energy range. Analytical proofs based on field symmetry, supported by numerical simulations, confirm that the excitation of the quadrupole field results in zero net transverse kick and negligible emittance growth of the proton beam. This approach provides a flexible and alternative solution against persistent electron activity, overcoming a long-standing limitation to stable, high-gradient SRF operation.

The work studies the electromagnetic field of a thin extended (filament) bunch of charged particles formed over some finite time. The main attention is paid to the analysis of the radiation field. We show that the radiation field is the main part of the total field in the relatively thin spherical layer with the thickness determined by the time interval of the bunch formation. The energy characteristics of the radiation, including the angular distribution of the power, the power and the total energy of the radiation, as well as the spectral energy characteristics, are calculated and analyzed. The radiation under consideration is compared with the radiation of a variable point charge moving with a constant velocity, as well as with the bremsstrahlung of a point charge having constant magnitude.

We present a methodological study for measuring electron-bunch temporal profiles using coherent transition radiation Michelson interferometry. A numerical frequency-filter model is established through imaging calculations of the experimental setup, accounting for the electron-bunch transverse profile, finite angular acceptance, and detector size. An adaptive genetic-algorithm-based forward-fitting approach is implemented to retrieve arbitrary bunch temporal profiles, demonstrating advantages over the classical Kramers-Kronig minimum-phase method. The frequency-filter model, together with the adaptive fitting framework, provides a robust tool for electron-bunch temporal profile characterization in beam physics studies.

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.

Femtosecond science has opened up new opportunities for probing atomic and molecular dynamics across multiple disciplines, while attosecond science further extends such capabilities to investigate the dynamical processes of electron motion in matter with unprecedented temporal resolution. In such situations, ultrashort electron beams serve as effective tools, providing complementary probing capabilities to ultrashort laser beams. In this paper, we propose a scheme that employs THz modulation to generate a mega-electron-volt (MeV) relativistic electron microbunch train. The scheme integrates the well-established undulator technology commonly used in the accelerator community with a tailored sawtooth THz field to manipulate the electron-beam phase space. To assess the feasibility of this scheme, we have developed an efficient and effective particle tracking model that incorporates the relevant physical effects, including the transport dynamics of relativistic MeV electrons interacting with the modulation field inside the undulator, as well as the beam longitudinal space charge effects. The simulation results show that, based on the proposed scheme, a microbunch train can be generated with a beam kinetic energy of approximately 3 MeV, a microbunch length of about 1.9 fs full width at half maximum, a microbunch charge of up to 2 fC, and a tunable microbunch spacing of around 0.2 mm. The output microbunch length can be controlled by tuning the downstream longitudinal dispersion, while adjustment of the upstream dispersion manipulates the microbunch structure into a periodic comblike train, each accompanied by one or more pairs of temporal satellite pulses. We believe that the proposed scheme and the analysis shall provide useful insights for generating few-femtosecond or even attosecond electron bunches.

In a storage ring, focusing perturbations mainly from quadrupole strength errors and sextupole alignment errors induce $\beta$-beat, breaking the periodic symmetry of lattice and deteriorating the performance of nonlinear dynamics. There is a general negative correlation between $\beta$-beat and nonlinear performance. In this paper, the impact of $\beta$-beat-induced asymmetry on nonlinear dynamics of storage ring is studied with multi-bend achromat (MBA) lattices. For on-momentum dynamic aperture (DA), the asymmetry parameters are introduced to quantitatively characterize the breaking of lattice symmetry, which are calculated from the Fourier harmonics of the $\beta$-functions and resonance driving terms (RDTs). Compared to the commonly used $\beta$-beat, the asymmetry parameters exhibit stronger correlations with reduced DA, which is further enhanced when considering the frequency diffusion rate. In addition, the commonly used $\beta$-beat serving as a reasonable indicator of DA degradation is physically explained based on RDT variations. For off-momentum nonlinear dynamics, the half-integer resonance crossing of chromatic tunes is found to have a small effect on six-dimensional (6D) local momentum aperture for the hybrid MBA lattice with $\beta$-beat, when the tune shifts with momentum are reasonably controlled. Moreover, the reduced off-momentum DAs with $\beta$-beat have stronger correlations with the off-momentum asymmetry parameters than the on-momentum ones.

Spin-polarized beams are important for some nuclear and high-energy physics experiments, such as those planned for the future Electron-Ion Collider. However, maintaining polarization during the acceleration of a charged-particle beam is difficult because the periodic nature of circular accelerators leads to spin-orbit resonances where the spin-precession frequency is a sum of integer multiples of the orbital frequencies. Usually, the dominant depolarization mechanisms are first-order spin-orbit resonances, and the depolarization associated with crossing such a resonance can be computed using the Froissart-Stora formula. However, accelerating polarized hadron beams to high energy requires special magnet structures called Siberian snakes. When these are implemented to maintain a spin-precession frequency of one-half the revolution frequency, there will be no first-order spin-orbit resonance crossings. The dominant depolarization mechanisms are then higher-order spin-orbit resonances. The Froissart-Stora formula can be applied to higher-order resonances when the slope of the amplitude-dependent spin tune (ADST) is constant. However, the slope of the ADST often changes at the moment of resonance crossing. This work introduces a generalization of the Froissart-Stora formula which is applicable when the slope changes in this manner. The applicability of this formula is demonstrated through tracking simulations of a higher-order resonance crossing in both a toy model and the Relativistic Heavy Ion Collider. It is additionally shown that the Froissart-Stora formula is mathematically equivalent to the Landau-Zener formula for the diabatic transition probability in two-level systems with a linearly increasing energy gap and constant coupling. This work, therefore, also extends the Landau-Zener formula to the case of changing slope.