We present studies on enhanced SASE at the soft x-ray beamline Athos at SwissFEL. The generation of a series of current spikes with 790 nm spacing is injected into the Athos undulator beamline. Due to strong space-charge forces, the optimum performance occurs with a strong positive taper of the undulator field. There is a clear modal structure in the observed spectra, indicating the presence of a train of attosecond FEL pulses with no intrinsic phase-locking. The space-charge dynamics causes an elongation of the current spike and a redshift in the bunching spectra, which determines the optimum taper gradient.

High-gradient normal-conducting radio-frequency (rf) cavities are crucial components of the rectilinear cooling channel in a muon collider, providing the accelerating field necessary to sustain the rapid six-dimensional ionization cooling essential to achieve high luminosity. To reach this goal, a muon collider requires approximately ${10}^{12}$ muons per bunch. At such high beam intensities, beam loading induced by bunch-driven wakefields can significantly reduce the cavity accelerating gradient and compromise beam stability. This paper focuses on the conceptual rf design and beam-induced-effect analysis of copper cavities with so-called beam windows developed to meet the beam dynamics requirements of the rectilinear muon cooling channel. We first present the design and optimization of the cavities, evaluating their rf performance and power requirements. Thereafter, we analyze cavity wakefields and beam loading effects, proposing a new numerical method for their compensation, which takes the transverse distribution of the beam into account.

NbTiN thin films are promising candidates for superconducting multilayer structures aimed at surpassing the intrinsic field limits of bulk Nb in superconducting radio-frequency (SRF) cavities. In this work, ${\mathrm{Nb}}_{0.75}{\mathrm{Ti}}_{0.25}\mathrm{N}$ films grown by plasma-enhanced atomic layer deposition were systematically investigated with respect to their crystalline structure, surface morphology, and field emission behavior in their as-deposited state, as well as after annealing in vacuum at $900°\mathrm{C}$ and $1000°\mathrm{C}$, respectively. X-ray diffraction reveals that annealing induces significant grain growth, a pronounced (111)-texture, and a strong reduction of residual stress, with the $1000°\mathrm{C}$ samples exhibiting the most uniform and relaxed microstructure. Optical profilometry shows a clear decrease in macroscopic roughness by approximately 20%–25% after annealing, while atomic force microscopy measurements confirm that the nanoscale roughness remains low and largely unaffected by the heat treatments. Field emission scanning microscopy demonstrates that annealing substantially increases the onset field for electron field emission from values below $60\mathrm{MV}/\mathrm{m}$ in the as-deposited state to $180–220\mathrm{MV}/\mathrm{m}$ for annealed films, correlating with improved crystallinity, reduced stress, and enhanced film-substrate adhesion. Complementary ultraviolet photoelectronspectroscopy measurements yield a work function of ${\mathrm{\Phi }}_{\mathrm{NbTiN}}=(4.32\pm 0.07)\mathrm{eV}$, supporting the interpretation of the field emission scanning microscope data. The combined results show that high-temperature annealing, particularly at $1000°\mathrm{C}$, significantly enhances the structural and functional performance of NbTiN films, underscoring their potential for application in superconducting multilayer coatings for next-generation SRF cavities.

Superconducting magnets enable energy-frontier accelerators by generating strong magnetic fields to steer and focus the particles. Although high-temperature superconductors such as ${\text{REBa}}_2{\text{Cu}}_3{\mathrm{O}}_x$ (rebco, RE = rare earth) hold a strong potential for generating a higher magnetic field than Nb-Ti and ${\text{Nb}}_3\text{Sn}$, the associated magnet and conductor technology for accelerator applications is still in its infancy. The U.S. Magnet Development Program is developing rebco magnet technology in collaboration with industry. Here we report an experiment of making a dipole magnet called C3 using commercial high-temperature superconducting corc wires. The magnet, following a canted $\cos \theta$ design, generated a dipole field of 5.99 T at 4.2 K in its clear aperture of 65 mm at 6.795 kA when a resistive voltage of $105\mathrm{\mu }\mathrm{V}$ appeared across one of the coils in the magnet. The stored energy was 53 kJ at the peak field. The magnet showed no degradation in the current-carrying capability at 4.2 K after the thermal cycle. We report on the detailed design, fabrication, and performance of the C3 magnet that can be of interest to potential users of this emerging technology. We also discuss issues and research needs to inform future rebco magnet development. The experiment represented another step to addressing if the high-temperature superconducting accelerator magnet technology can increase the discovery capability of future particle accelerators.

At the 1.5-GeV synchrotron light source DELTA operated by TU Dortmund University, intense and ultrashort radiation pulses in the vacuum ultraviolet and terahertz regimes are generated by the interaction of femtosecond laser pulses with electron bunches in an undulator. The resulting periodic modulation of the electron energy is converted into a density modulation in a dispersive section, giving rise to coherent radiation emission at harmonics of the seed wavelength in a second undulator. This process is known as coherent harmonic generation (CHG). This paper describes the experimental setup and concentrates on the spectral aspects of CHG radiation under variation of properties of the seed laser pulse and the dispersive section. CHG spectra at the second harmonic of the 800 nm seed wavelength were recorded using a grating spectrometer equipped with an image-intensified CCD camera. Numerical simulations to model the CHG spectra for different group-delay dispersion and third-order dispersion were carried out, and a convolutional neural network was implemented to predict these parameters from measured CHG spectra. The results show that the tunable laser parameters directly translate into the spectrotemporal properties of the coherently emitted pulses.

The High Energy Photon Source is the first fourth-generation light source in China, featuring a hybrid multibend achromat lattice. This innovative design incorporates high gradient quadrupoles with an aperture of 26 mm, a gradient of about $80\mathrm{T}/\mathrm{m}$, and high-order field harmonics of less than $4\times {10}^{-4}$. This paper details the optimized magnetic design, precise fabrication techniques, batch field measurements, and a method for field harmonic compensation utilizing a magic finger. A comparative analysis was conducted on nine types of the 480 pure quadrupole magnets that share the same cross section but differ in iron lengths. These variations include laminated or solid cores and closed or open yokes. The cross section, characterized by a narrow concave and convex pole contour, a tapered pole root, and saddle coils, is designed to mitigate magnetic saturation while enhancing the iron filling ratio. The impact of iron saturation on the transfer function and the evolution of current-dependent multipole fields has been systematically investigated, along with the effects of mechanical errors and inhomogeneities in the magnetic properties of the iron core on harmonics. We demonstrate a comprehensive engineering solution for high gradient small-aperture quadrupole magnets, which has been validated through mass production.

We report the experimental implementation of a data-driven chaos indicator (DDCI) [Y. Li et al., Data-driven chaos indicator for nonlinear dynamics and applications on storage ring lattice design, Nucl. Instrum. Methods Phys. Res. A 1024, 166060 (2022)] for online optimization of the National Synchrotron Light Source II storage ring. The DDCI quantifies the predictability of electron beam dynamics using turn-by-turn beam position monitor data. A surrogate model of the one-turn map is first trained, and its out-of-sample predictive uncertainty is then employed as a measurable indicator of chaos. By tuning sextupole magnets to mitigate nonlinear effects, a clear enlargement of the dynamic aperture is achieved, accompanied by a corresponding improvement in injection efficiency.

Undulator tapering is an important technique in free-electron lasers and synchrotron radiation sources, used to enhance radiation efficiency, broaden spectral bandwidth, and perform other related functions. However, conventional tapering methods, such as gap variation or electromagnetic control of individual magnets, involve complex mechanical structures or power supply systems, posing challenges in tuning the taper profile while maintaining good field integral performance in practice. This paper presents a beat frequency undulator that enables dynamic control of undulator tapering over a wide range. The concept employs two permanent magnet arrays with slightly different periods, whose magnetic field superposition generates a longitudinal beat frequency envelope, creating a tunable taper profile without gap adjustment or electromagnetic tuning. By longitudinally shifting one array, the taper strength and direction can be reconfigured in real time. This approach offers inherent advantages in mechanical simplicity and maintains a field integral comparable to that of a standard planar undulator, providing a robust and flexible solution for undulator tapering.

The inverse-Cherenkov dielectric laser accelerator (ICR-DLA) holds great promise as a compact, on-chip accelerator for a wide range of future applications. However, two significant challenges—bunch dispersion/deflection and phase slippage—have hindered its development in the subrelativistic regime. In this paper, we propose an approach that addresses both issues simultaneously by utilizing a single laser pulse to illuminate a staged dielectric prism. Our method leverages the phase slippage experienced by subrelativistic electrons during high-gradient acceleration, allowing these electrons to encounter alternating focusing and defocusing forces throughout the acceleration process. This technique enables stable, long-range bunch transport within a miniaturized acceleration channel. We show that cascading focusing and acceleration of subrelativistic electrons can be achieved in a multistage acceleration structure, laying a foundation to bridge the gap between subrelativistic and relativistic regimes, which is crucial for the realization of a practical on-chip particle accelerator.

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.