Seeded free-electron lasers (FELs) have become indispensable tools across numerous scientific fields, owing to their high coherence and stability. Unlike self-amplified spontaneous emission, seeded FELs require a prebunched electron beam to be formed upstream of the radiator, enabling the subsequent generation of fully coherent radiation. This bunching process can be numerically calculated based on the specific components and their parameters used. To facilitate the discovery and optimization of such FELs, in this paper we present an automated optimization method for the microbunching process in seeded FEL schemes. It is demonstrated that the method can automatically reproduce established seeded FELs, such as echo-enabled harmonic generation, eliminating the need for prior physical analysis. Moreover, the method yields new findings in seeded FELs that go beyond mere replication. By incorporating more kinds of components and exploring a broader set of optimization objectives, the optimization can be extended to explore more possibilities.

In the formulation of synchrotron radiation (SR), the Wigner function represents the photon flux density in the four-dimensional phase space, and thus defines the brilliance and coherence. Although we have to rely on a numerical method to evaluate the Wigner function for a realistic electron beam, a convolution approach is available under the assumption that each electron travels along a straight line apart from a sinusoidal motion driven by an undulator magnetic field. As a result, the computation of Wigner functions for typical SR sources is usually completed within an acceptable time. It should be noted, however, that the above assumption is violated in a multiple-beam-waist (MBW) configuration, in which the lattice function is designed to have more than one beam waist position where the vertical betatron function is optimized to accommodate a narrow-gap undulator. In such a case, we have to carry out a Monte Carlo simulation, which requires a huge amount of time to get a reliable result. To overcome this difficulty, we introduce a theoretical-numerical method aimed at reducing the cost of computing Wigner functions in the MBW configuration. We show that the computation time can be saved by 2–4 orders of magnitude by means of the proposed method.

The 3-GeV rapid-cycling synchrotron (RCS) at Japan Proton Accelerator Research Complex (J-PARC) supplies a high-intensity beam to the Main Ring (MR) synchrotron. Under the present operating conditions, the RCS extraction bunch is intentionally injected into the MR rf bucket in a mismatched condition. This mismatch causes the bunch to dilute within the MR bucket, which rapidly increases the bunching factor $B_f$, defined as the ratio of average to peak current, and raises it to the desired value immediately after injection. Increasing the bunching factor $B_f$ is essential for reducing beam losses caused by space-charge effects. Although this scheme has been effective for improving $B_f$, the dilution process also produces oscillations in the bunch profile and leads to local reductions of $B_f$, which contribute to beam losses immediately after MR injection. To achieve higher beam power in future MR operation, it is therefore desirable to increase $B_f$ before injection into MR, so that beam losses at the beginning of acceleration can be suppressed. A method called “bunch lengthening” has been proposed to stretch the RCS extraction beam bunch. This method stretches the bunch and increases the $B_f$. It is based on driving a second-harmonic rf voltage in the latter part of acceleration and placing the beam at the unstable fixed point. Experiments on bunch lengthening with low-intensity beams, conducted in RCS and MR, demonstrated the stretching of the RCS extraction beam. The $B_f$ increased from approximately 0.2 to 0.4 in both simulations and beam experiments. These results indicate that the longitudinal beam matching between RCS and MR has improved. To further optimize bunch lengthening, the deformation process of the stretched bunch was investigated. The second-harmonic rf voltage was applied twice with optimized timing, resulting in improved longitudinal matching. A detailed presentation of the bunch lengthening and the results of demonstration tests conducted on the J-PARC synchrotrons is provided. The examination results on the optimization of bunch lengthening are also reported.

The High Energy Photon Source (HEPS), a newly commissioned 6 GeV fourth-generation synchrotron light source, employs five 166.6 MHz quarter-wave $\beta =1$ superconducting cavities in its storage ring. Each cavity requires a fundamental power coupler capable of delivering 200 kW continuous-wave power—a specification largely exceeding demonstrated limits for existing on-cell couplers in transverse electromagnetic (TEM)-type superconducting cavities. This paper presents the complete development, validation, and operation of these couplers from prototype to series production. The design resolves four competing constraints: high power handling, low cryogenic heat load, mitigation of field-emission damage, and cleanroom assembly compatibility. Overheating at the cavity-coupler interface was identified as the primary degradation mechanism during prototype testing. Consequently, Series-A and Series-B design iterations were developed to eliminate interface overheating and window frosting, respectively. These modifications reduced the maximum interface temperature from 15.4 to 10.8 K and halved the average 4.2 K heat load to 1.5 W under nominal operation. All 10 series couplers were validated via high-power testing, with five currently operating stably in the HEPS storage ring at a 150 mA beam current. This represents the highest beam-operation power level reported to date for on-cell couplers in TEM-type superconducting cavities, expanding the application of nonelliptical superconducting cavities in high-current accelerators.

The Super Tau-Charm Facility (STCF), a next-generation electron-positron collider, adopts an on-axis swap-out injection scheme that demands high-charge electron bunches with exceptionally low emittance and minimal energy spread. This work presents a comprehensive and systematic beam dynamics design for the STCF electron injector. The bunching system, based on subharmonic bunchers and a traveling-wave buncher, is optimized using the nondominated sorting genetic algorithm III to efficiently explore the high-dimensional parameter space. Start-to-end simulations incorporating space-charge forces, wakefield effects, and alignment errors are performed, with beam-based alignment (BBA) applied to correct orbit deviations. The optimized injector delivers 8.5 nC bunches at 3.5 GeV with a normalized transverse emittance below $70\text{mm mrad}$ and an rms energy spread below 0.5%, satisfying the stringent STCF requirements. The high transport efficiency achieved in simulations alleviates the design pressure on the electron source, while the excellent beam quality inherently suppresses beam halo effects. By integrating systematic multiobjective optimization with high-fidelity simulations, the proposed design overcomes the intrinsic limitations of thermionic cathodes while capitalizing on their proven stability, robustness, and cost-effectiveness, offering an efficient engineering solution for high-charge beam delivery in next-generation colliders.

Scaling fixed-field alternating-gradient accelerators (scaling FFAs) are promising candidates for high-intensity proton drivers, for which space charge effects are a key limitation of beam performance. For long bunches in circular accelerators, 2.5D space charge methods provide an efficient alternative to full 3D PIC methods by exploiting the large longitudinal-to-transverse aspect ratio of the bunch. In scaling FFAs, however, the reference closed orbit varies with particle energy, and the transverse and longitudinal directions are not directly available in cylindrical coordinates. Therefore, the longitudinal slicing and transverse planes required by a 2.5D model must be reconstructed dynamically, rather than defined from a single fixed reference orbit, which poses a central challenge for efficient 2.5D modeling in high-intensity scaling FFAs. This paper presents a new 2.5D space charge modeling method for high-intensity scaling FFAs and its implementation in the PyFFA code. The central idea is to replace the fixed reference orbit used in conventional 2.5D models with a dynamically reconstructed local reference frame based on a precomputed family of energy-dependent closed orbits. During tracking, this frame is updated according to the instantaneous particle state, enabling transverse Poisson equations to be solved consistently in this dynamic local coordinate system. The resulting space charge field is then transformed back to global cylindrical coordinates and applied in the tracking equations. Benchmarks of space charge fields, tune shifts, rms envelope evolution, and longitudinal phase-space evolution show good agreement with analytical results, validating the field calculation and its coupling to beam dynamics for long, arc-shaped bunches in scaling FFAs. The implemented model enables efficient studies of space charge effects on beam dynamics in high-intensity scaling FFAs.

Understanding and mitigating high-order nonlinearities is essential for ensuring long-term beam storage in high-energy colliders. The stability of particles at the Large Hadron Collider (LHC) is indeed strongly influenced by nonlinear effects, particularly at injection energy where the beam occupies a large fraction of the vacuum chamber. Among these effects, third-order chromaticity, primarily driven by decapolar fields, limits dynamic aperture and beam lifetime. Despite corrections based on magnetic measurements and model predictions, a discrepancy remains between the measured and expected third-order chromaticity, indicating the presence of residual decapolar components. In this study, the decapolar fields in the LHC at injection energy are investigated and their impact on beam dynamics assessed. Chromatic amplitude detuning and decapolar resonance driving terms are directly measured for the first time, using forced oscillations driven by an ac dipole. These measurements provide new insights into the source and impact of decapolar fields in the LHC and indicate that refined correction strategies could enhance beam lifetime, particularly for high-brightness operation in future runs.

Dual-frequency radio-frequency systems are widely deployed in high-brightness synchrotron radiation light sources to achieve electron bunch lengthening. However, the transient beam-loading (TBL) effect induced by bunch gaps causes significant synchronous phase shifts and severely compromises bunch-lengthening performance. To address this, we propose an active compensation scheme that suppresses voltage fluctuations in both the main and harmonic cavities by modulating only the driving voltage of the main cavity via a feedforward approach. To elucidate the underlying physical mechanisms, we establish a gap-induced linear decomposition analytical model, which decomposes the TBL-induced beam voltage into steady-state and transient components. Based on this model, we derive an optimization method to minimize power requirements, enabling the main-cavity TBL compensation to achieve performance levels comparable to those of broadband-cavity TBL compensation. The efficacy of this framework is demonstrated through two case studies: for an steady-state microbunching storage ring with a 7.5-MHz hardware half-bandwidth, the required single-cavity peak power is maintained at 158 kW, with normalized average radiation power improved from 78% to 97%; for the KEK light source storage ring with a 6-MHz bandwidth, the single-cavity peak power is optimized to 148 kW, successfully restoring the average bunch length from 31.1 to 40.9 ps. The proposed scheme establishes a highly efficient and cost-effective paradigm for TBL suppression, providing a promising technical pathway for next-generation light sources.

Conventional radio-frequency (rf) transverse deflecting structures provide high-resolution longitudinal beam diagnostics, but require externally generated high-power rf, waveguide distribution, synchronization, and input coupling at the operating frequency. We propose design criteria for a beam-driven resonant passive transverse deflector that does not require an external rf source. A leading drive bunch excites long-range wakefields in an off-axis periodic copper structure and a delayed witness bunch experiences the transverse wake near a zero crossing. The concept is based on the large temporal slope available from high-frequency wake components. A structure designed for installation after the second bunch compressor in the three-bunch-compressor layout of the European XFEL is optimized to place the zero crossing of the drive-bunch-induced transverse wake potential approximately one rf-bucket spacing of the $1.3\mathrm{GHz}$ linac behind the drive bunch. The selected geometry produces a multimode transverse kick dominated by TM-like modes. We use time-domain wake simulations, frequency-domain decomposition, cell-number scaling, mechanical-tolerance scans, orbit-offset studies, and uniform thermal scaling to determine the operating point and its sensitivity. For this geometry, the zero crossing occurs at $s_0\simeq 230.6\mathrm{mm}$, with a per-cell temporal slope of $S_{\text{cell}}=1.186\mathrm{mV}/(\mathrm{pC}\mathrm{fs}\mathrm{cell})$. For a compact $1\mathrm{m}$ structure operated with a $250\mathrm{pC}$ drive bunch and a $700\mathrm{MeV}$ witness beam, the estimated temporal resolution is about $33\mathrm{fs}$.

The long-range transverse resistive-wall (RW) wakefield is a key driver of transverse coupled-bunch instabilities in high-current storage rings. Simulating these instabilities accurately is typically time-consuming, because conventional time-domain tracking codes such as Pelegant store the turn-by-turn bunch-centroid history and perform discrete convolutions with the wakefield, which poses a bottleneck for long-term tracking. To overcome this limitation, this work reformulates the long-range RW wakefield calculation, converting the explicit history-dependent convolution into a recursive update by representing the wakefield as a sum-of-exponential functions. This approach avoids storing the bunch-centroid history and is well suited to parallel graphics-processing-unit architectures; it has been implemented in the tracking code STABLE. Benchmark tests, using parameters of the Hefei Advanced Light Facility (HALF), show that STABLE with the proposed method maintains high accuracy in the growth rate while achieving a speedup of at least 2 orders of magnitude over Pelegant. Leveraging this computational efficiency, we conduct a systematic study of RW instabilities for HALF, with particular focus on the interplay among filling pattern, chromaticity, and harmonic-cavity bunch lengthening.

Bulk niobium has long been the material of choice for superconducting radio-frequency applications. An alternative approach is the superconductor-insulator-superconductor (SIS) multilayer structure, which enables the use of brittle high-$T_c$ materials, such as NbTiN. At present, SIS coatings are limited to flat samples, with the single-cell TESLA cavity representing a key milestone. Extending coating processes to nonflat geometries is expected to introduce macroscopic inhomogeneities in coating thickness. We model these variations using Gaussian random fields parameterized by a spatial correlation length and generated by solving a stochastic partial differential equation. The resulting field is incorporated into the boundary condition of the cavity eigenvalue problem, from which quantities of interest—such as the quality factor—are computed. This procedure is repeated for eight correlation lengths, with 2048 samples per correlation length, where the resulting quality factors are recorded. Our results show that the quality factors follow a normal distribution. The standard deviation increases with the correlation length and can be statistically distinguished. In contrast, the mean values remain largely unchanged, with only a few significant differences. In extreme cases, depending on the correlation length, the quality factor may differ from the uniform case by 2%–6%.

We study circular-mode optics as a framework for transporting and accelerating intrinsically flat beams while maintaining a round transverse projection in physical space. Motivated by the strong degradation of flat beams under low-energy space-charge forces, we develop a coupled-optics description of circular modes, analyze their formation and preservation, and derive lattice design conditions for conserving canonical angular momentum and coupling phase. These conditions motivate rotationally invariant transport and ring cells based on solenoids, symmetric alternating-gradient structures, and indexed dipoles. We further derive space-charge tune-shift and tune-spread expressions for circular-mode beams in uncoupled and coupled lattices, showing that they preserve the leading-order space-charge behavior of round beams while retaining intrinsic eigenemittance flatness. Particle-in-cell simulations with TRACK and impactx confirm the generation, transport, and acceleration of circular-mode beams in representative lattices, including regimes with strong space charge. Circular-mode optics therefore provides a promising route toward high-intensity hadron beams and high-luminosity collider applications.

High-field conditioning is the process by which radio-frequency structures in particle accelerators and other high-gradient devices reach their operating fields, yet the underlying physical mechanism remains an open question. Models and indirect measurements point to subsurface dislocation dynamics, but large-area structural measurements have been missing. We present electron backscatter diffraction measurements spanning millimeter-scale regions on a copper cathode conditioned at pulsed direct-current fields up to $\sim 80\mathrm{MV}/\mathrm{m}$ in a sloped-anode geometry, which imposes a known gradient of field exposure across a single electrode. Across nine regions of interest spanning this exposure range, the mean intragrain misorientation of field-exposed regions exceeds that of unexposed references by $\sim 75\%$; the difference is reproduced by three independent misorientation metrics and confirmed by Kolmogorov-Smirnov tests. To our knowledge, this is the first large-area observation of structural differences between conditioned and unconditioned regions of a high-field electrode. The misorientation separates into three tiers (high-field center and edge, low-field periphery, and unexposed reference) that match the spatial profile of the conditioning-state variable $E_S$ predicted by Monte Carlo simulations. These observations point to the evolving subsurface dislocation population as a candidate physical basis of conditioning.

The magnetic-field node of a ${\mathrm{TM}}_{020}$-mode cavity permits compact higher-order-mode (HOM) damping, but displacement of the coupling slots can expose the absorber to appreciable accelerating-mode leakage. A ferrite-SiC absorber is investigated numerically to combine broadband damping with improved thermomechanical response in a 1.5 GHz cavity. A transmission-line formulation relates material ordering to input impedance, and coaxial full-wave calculations show that placing SiC ahead of ferrite suppresses the pronounced low-frequency absorption dip of the reverse arrangement. The concept is integrated into a radially folded, six-sector absorber, with parameter sweeps resolving the effects of material dimensions, slot height, and sector angle. For the six higher-order modes (HOMs) tabulated, external quality factors are below 150, while the aligned accelerating mode has an external quality factor of $1.20\times {10}^7$. Frequency-resolved loss maps show increasing SiC participation across the transition near 2 GHz. At the same imposed absorber load of 2.65 kW, the hybrid reduces peak temperature rise from 97.0 to 79.9 K and maximum first principal stress in ferrite from 146.0 to 78.6 MPa, reductions of 17.6% and 46.2%, respectively. These results connect electromagnetic loss redistribution to improved thermal and mechanical response and support the hybrid arrangement as a compact damping option for ${\mathrm{TM}}_{020}$-mode cavities.

Microcoulomb-class electron beams from laser-plasma accelerators (LPAs) driven by kilojoule-class laser systems can enable high-flux electron radiography and copious secondary particle production. However, electron beam spatial properties such as transverse beam nonuniformity and large spatial scale at application sites can limit their utility. In this work, we demonstrate spatial conditioning of microcoulomb-class electron beams using electromagnetic solenoids as focusing optics. By applying $\sim 10\mathrm{T}$ solenoidal fields, we focused the electron beam onto a detection plane and measured a fluence enhancement factor of 6 over unconditioned electron beams. The chromatic focusing of the solenoids improved the resolution of electron beam radiographs produced using the focused electron beam. We also demonstrate a method using angled electromagnetic solenoids to simultaneously condition LPA electron beam spatial and spectral properties. These features are attractive for applications because they enable the spatial separation of a narrow-energy-spread population of the electron beam from the bulk of the electron beam.

A high-power input coupler was developed at KEK to deliver 100 kW-class radiofrequency (rf) power in the continuous-wave (cw) mode at 1.3 GHz to a superconducting cavity. To mitigate thermal loads during high-power rf transmission, the coupler was engineered with active water cooling integrated into critical components, including a warm inner conductor, outer conductor, antenna, and rf window. The newly developed coupler underwent rf conditioning of up to 145 kW in the pulsed mode and 100 kW in the cw mode using a dedicated test stand at room temperature. The coupler successfully transmitted world-record cw 100 kW rf power at 1.3 GHz continuously for 10 h without any interlock event. During operation, the temperature rise of the warm inner conductor, the most thermally stressed component, was limited to $15°\mathrm{C}$ whereas the maximum observed temperature increase was $19°\mathrm{C}$ at the doorknob. Further power scaling was not constrained by the coupler itself. These results demonstrate that the newly developed coupler can reliably sustain cw rf transmissions exceeding 100 kW with excellent thermal performance.

Storage ring-based steady-state microbunching (SSMB) is a promising approach for generating high-average-power coherent radiation, while the instabilities driven by coherent undulator radiation in the laser modulator (LM) are important for the ring performance. In this paper, we investigate the longitudinal single-bunch multiturn LM instability using cavity-mode decomposition techniques. The evolution of the wakefield in the longitudinal beam dynamics equations is derived, and the instability growth rates are analyzed. Numerical simulations show excellent agreement with the theoretical model, validating the mode-decomposition approach. These findings provide critical insights into the design and operation of SSMB storage rings, suggesting effective mitigation strategies to suppress the instability and enhance the overall performance.

In modern accelerator facilities, coherent synchrotron radiation (CSR) presents a fundamental limitation to maintaining high-brightness electron-beam quality during deflection, with accurate modeling of its effects remaining an outstanding challenge. This study introduces a novel optimization method using an integral approach to suppress CSR-induced transverse emittance degradation under two representative bunch scenarios: ultrashort bunches and high-charge long bunches. We present analytical solutions for suppressing CSR effects in the dipoles across several quadruple-bend-achromat (QBA) layouts and establish general conditions for mitigating CSR-induced emittance growth in the subsequent drift sections of isochronous beamlines. Our analysis demonstrates that the QBA is the minimal configuration capable of simultaneously suppressing transverse emittance degradation and preserving the longitudinal beam distribution. Beam tracking results show negligible emittance growth for multikiloampere bunches at a 16° deflection. Furthermore, multicell simulation confirms the exact scalability of this approach. While previous designs suffer severe emittance degradation upon cascading, the QBA maintains exceptionally preserved beam quality.

Attosecond pulses from x-ray free-electron lasers (XFELs) have opened new opportunities for probing ultrafast electronic dynamics on the Ångstrom-attosecond spatiotemporal scale. Most attosecond XFEL concepts rely on generating an ultrashort high-current spike through either external laser modulation or accelerator-based beam manipulation. Despite their different implementations, these approaches share the same essential physics, namely, that the XFEL amplification is confined to a short effective lasing window within the electron beam. However, existing studies are often scheme-specific and do not yet provide a unified quantitative picture of how fundamental electron-beam properties constrain high-power attosecond performance. In this work, we investigate the general physics and scheme-independent requirements for generating high-power attosecond x-ray pulses from a short current spike. From the perspective of postsaturation superradiant evolution, we show that the electron-beam length governs both the attainable peak power and the pulse duration. We further examine the distinct roles of slice energy spread, slice emittance, energy chirp, and transverse beam tilt. Our results reveal the trade-off between peak power, pulse shortening, and single-spike probability, and provide facility-independent guidelines for optimizing electron-beam phase-space manipulation toward terawatt-class attosecond XFEL operation.

In the SuperKEKB accelerator, higher-order-mode (HOM) power absorbed by the HOM dampers of the superconducting (SC) cavities limits the attainment of the design electron beam current of the High Energy Ring. As regards cylindrical ferrite dampers installed in the SC cavities, we observed that their effective loss factors considerably depended on the bunch fill pattern. To investigate this behavior in detail, wakefield simulations were conducted. HOM power is generally evaluated from the numerically calculated coupling impedance, which provides only the total beam power loss within the structure. However, when multiple absorbers are present, the absorbed power of each absorber remains unknown. To address this limitation, we introduce calculation methods that utilize the local electromagnetic fields to evaluate the absorbed power. In this study, the theoretical basis of the methods is presented and applied to the SuperKEKB SC cavity.