In seeded free-electron lasers (FELs), harmonic up-conversion of an external seed laser can be used to generate coherent emission at short wavelengths. The harmonic generation process is sensitive to the electron beam slice energy spread, which limits the highest harmonic conversion factor. We present here an experimental study of the impact of the slice energy spread on the FEL performance in two different harmonic generation schemes: high-gain harmonic generation and echo-enabled harmonic generation (EEHG). The slice energy spread is controlled by means of a laser heater installed in the low energy region of the linear accelerator that drives the FEL. The measurements of the FEL pulse energy as a function of the electron beam slice energy spread were performed at different harmonics. Our results confirm the robustness of EEHG to the slice energy spread and open the way toward fully coherent FEL pulses in the soft x-ray domain.

We present a novel method to align a laser-ionized plasma source with ultrarelativistic electron beams in a plasma wakefield accelerator. The technique relies on analyzing plasma afterglow light at two longitudinal positions as the plasma column is scanned across the beam axis. Using this method, we aligned an 85-cm plasma source to a 10-GeV, 1.6-nC electron beam with better than $10\text{−}\mathrm{\mu }\mathrm{m}$ transverse offset and $10\text{−}\mathrm{\mu }\text{rad}$ angular error. The alignment was validated by measuring the drive beam energy loss, beam-to-wake energy transfer efficiency, and witness beam energy gain as functions of intentional misalignment. From these measurements, we determined the alignment tolerances required between the laser-ionized plasma source and the electron beams, an essential parameter for the design of plasma-based colliders and advanced light sources.

Muonium-to-antimuonium conversion experiment (MACE) aims to detect the charged lepton flavor violation (cLFV) process. A key component of MACE is the positron transport system (PTS) to collect and transport atomic positrons from antimuonium decays, which consists of an electrostatic accelerator and a solenoid beamline. Through field simulations in comsol and particle transport simulations based on geant4, the PTS can achieve a geometric acceptance of the signal of 65.81(4)% along with a position resolution of $88(1)\mathrm{\mu }\mathrm{m}\times 102(1)\mathrm{\mu }\mathrm{m}$. The system achieves a transit time of 322.4(1) ns with a spread of 6.9(1) ns, which allows for a TOF-based rejection of internal conversion backgrounds by a factor of ${10}^{-7}$. These promising results pave the way for new physics signal identification and background rejection in MACE and offer a novel paradigm for an internal transport system in high-intensity frontiers.

We propose a novel acceleration scheme for fixed-field accelerators (FFAs), in which rf buckets with harmonic numbers $h=1$ and $h=2$ are time-sequenced to form a single, continuous acceleration path. This approach completes acceleration in two rf frequency sweeps, thereby reducing the total frequency sweep range and shortening the repetition period. The feasibility of this method is demonstrated through longitudinal simulations based on parameters of the FFA at the Institute for Integrated Radiation and Nuclear Science, Kyoto University (KURNS). We also establish operational conditions under which the second-harmonic rf bucket remains stable and practically usable.

This paper presents the results of the beam dynamics design, radio frequency (rf) structure design, and rf field tuning of a 108 MHz four-vane radio frequency quadrupole (RFQ) with a piecewise linear intervane voltage. Typically, a constant or a linearly ramped intervane voltage is employed in the beam dynamics design of RFQ accelerators. However, to enhance the acceleration gradient of the RFQ, we apply a linearly increasing intervane voltage configuration in the initial part of the RFQ. Furthermore, we adopt a constant intervane voltage design in the subsequent part of the RFQ to lower the peak surface electric field. A transition section between the constant and linearly increasing voltage regions is specially designed to ensure a smooth second derivative of the voltage. The beam dynamics design indicates that the RFQ cavity measures 3.27 m in length and is capable of accelerating ${\mathrm{Bi}}^{32+}$ ions with a peak current of $50\mathrm{e\mu A}$ from 4 keV/u to 0.4 MeV/u, achieving a transmission efficiency of 95.9%. During the rf structure design, the vane base width is tailored as a function of longitudinal position to maintain the desired local frequency. It is revealed that measuring the magnetic field near the cavity wall makes it challenging to guarantee the intervane voltage distribution of the RFQ. This difficulty arises from a localized dip in the magnetic field within the transition section because of the localized dip of the vane base width, together with the additional disturbance effect of the nearby tuners on the field. As a result, establishing a voltage baseline in the transition section is not feasible. Instead, the electric field near the vane tip was measured and the field was tuned by adjusting the insertion depths of 48 tuners after the RFQ cavity was machined and assembled. After tuning, the relative error between the measured field distribution and the designed curve for the operating quadrupole mode is within $\pm 1\%$, and the dipole components are within $\pm 2\%$ of the quadrupole mode. The unloaded quality factor of the RFQ cavity was measured to be 13,179, which is 83% of the value predicted by the cst software.

Closed orbit correction is the initial step in error correction, and its results significantly influence subsequent correction of higher-order errors. This is particularly important for an steady state microbunching (SSMB) storage ring, where both the transverse dimensions approach the diffraction limit and the longitudinal length is on the order of hundreds of nanometers. The precision and speed of closed orbit correction are essential for achieving high average power and high brightness radiation in the SSMB storage ring. Here, we introduce a new model to address this issue. Our model integrates the attention mechanism with convolutional neural network (CNN). This combination enables the model to capture both global dependencies and local patterns. Furthermore, we considered the effects of various error distributions on the lattice optics function. By incorporating diverse error scenarios into the training dataset, we enhance the model’s robustness. Through simulations of the SSMB storage ring, we compared the correction results of our model with the singular value decomposition method. It achieves the desired residual closed orbit after only two iterations and shows encouraging correction outcomes across various error distributions. We tested the stability of our model by adjusting the strengths of specific correctors and incorporating beam position monitor (BPM) noise. Additionally, to address the issue of BPM failures, we integrated a Gaussian process model to ensure that our correction process remains effective even when BPMs malfunction.

The High Energy Photon Source (HEPS) is a recently commissioned 6 GeV fourth-generation synchrotron light source that utilizes a novel low-frequency superconducting radio-frequency (SRF) system. This system was designed to handle high beam power and achieve heavy damping of higher-order modes (HOMs) within strict spatial constraints. To meet these stringent requirements, we in-house developed and subsequently installed five $\beta =1$ quarter-wave SRF cavities operating at 166.6 MHz and 4.2 K with a compact envelope. The design incorporates an enlarged beampipe (505 mm in diameter) with a beamline absorber for effective HOM damping and adopts a single coaxial power coupler with on-cell coupling to handle a high continuous-wave power of 200 kW. Following rigorous cleaning and assembly protocols, the cryomodules demonstrated a peak electric field of $42\mathrm{MV}/\mathrm{m}$ with negligible radiation and maintained a low dynamic heat load of 5 to 7 W at the operating voltage of 1.2 MV, results that significantly surpassed the design goals. Installed in Summer 2025, the cavities were instrumental in the successful acceleration of a 100 mA electron beam at HEPS in September 2025. This work represents the first successful implementation of superconducting quarter-wave cavities as the main accelerating system for a high-current storage ring, thereby extending the application limits of quarter-wave cavities from their conventional use in low-$\beta$ accelerators to high-current electron storage rings and establishing a proven technology for future high-performance light sources.

Beam quality in accelerator transport lines can be significantly degraded by component imperfections such as alignment errors, magnetic field deviations, and rf phase fluctuations. In complex accelerator systems, accurately quantifying the influence of these errors and their interactions is essential for performance optimization. In this study, a variance-based global sensitivity analysis using the Sobol method is applied to the medium energy beam transport section of the CAFe facility to systematically evaluate the impact of multiple error sources on key beam quality metrics, including centroid position, envelope, Twiss parameters, and emittance. Two complementary strategies are employed: a component-level analysis that considers all error types of each element collectively, and an individual-error analysis that examines each source separately. The results show that centroid, envelope, and Twiss parameters are primarily influenced by independent error contributions, dominated by quadrupole translational and magnetic field strength errors, whereas emittance change arises from strong coupling among multiple sources. The model-independent nature of the Sobol method enables comprehensive exploration of parameter interactions, allowing clear identification of the most influential errors and providing guidance for tolerance definition, targeted correction, and system optimization. This work highlights the potential of global sensitivity analysis as a powerful framework for performance evaluation in accelerator design, commissioning, and operation.

A single-periodic magnetically coupled standing-wave accelerating structure prototype operating at 3 GHz for low phase velocity particles has been designed as the first tank of the 30–230 MeV accelerating section for the compact proton therapy linac developed by the Institute of Modern Physics, Chinese Academy of Sciences. Fabrication and rf measurement of the prototype have been completed. High-power test of the prototype was performed. The cavity could operate stably at the effective input power of around 5.7 MW, corresponding to the accelerating gradient of $38\mathrm{MV}/\mathrm{m}$ at the pulse length of $5\mathrm{\mu }\mathrm{s}$ and the repetition rate of 70 Hz. Further increasing the duty factor to 0.1%, the accelerating gradient decreased to $24\mathrm{MV}/\mathrm{m}$ due to the uneven cooling of each single cell. The details of the high-power test setup and preparation, the process of rf conditioning, and the high-power test results are described in this paper. Limiting factors for higher accelerating gradients with high duty factors are discussed, and two novel cooling channel schemes are described.

Dynamic Coulomb expansion of dense, nonuniform particle beamlets can lead to causticlike formations in the charge density profile of the resultant beam. Such features only occur for cold beams where thermal diffusion of the particles is minimized. These phenomena have been observed experimentally for select cases with ion beams but not yet for electron beams as the electron dynamics occur over far faster timescales. We simulate the interaction of electron bunches while varying the initial transverse temperature and density profiles to determine the thresholds that characterize this pattern formation. Furthermore, we extend the simulations to geometric arrangements that are more pertinent to the nanotip cathode application and consider the effects of asymmetries and the impact of a low-density halo on the overall process. The simulations are conducted using a previously documented high-fidelity algorithm for collisional particle dynamics. These studies identify the temperature and density thresholds required for caustic formation and provide guidance for optimizing the design of nanoscale structured cathodes. We also demonstrate that the correlation between the final density patterns and the underlying beam parameters enables a virtual diagnostic tool for measuring ultralow emittance beams solely based on a transverse charge density profile.

Radio frequency (rf) control loops are vital components for the optimal operation of rf systems in synchrotron particle accelerators. Modeling these systems in macroparticle tracking simulations provides improved understanding of the complex interaction between the control loops, the beam-induced voltages in the rf cavities, and the circulating beam. This paper details an implementation of local and global rf control loops in the beam longitudinal dynamics (blond) simulation suite, which allow for the coupling of the two systems in a macroparticle tracking code for the first time. First, benchmarks of the local control loops in the CERN Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) are given. The implementation coupling the local and global loops is then tested in the SPS with momentum shifts at constant magnetic field and through the damping of phase offsets at injection into the LHC. Both test cases show the expected behavior and validate the model in blond. Finally, the SPS simulation model, which now is able to combine beam-induced voltages, global control loops and local ones, is benchmarked against measurements at the bunch-to-bucket transfer into the real accelerator. The simulations, using the coupled implementation of the loops, are able to reproduce bunch-by-bunch beam parameters at flat bottom as well as uncaptured beam generated at injection.

A development effort to improve the performance of superconducting radio-frequency half-wave resonators (SRF HWRs) is underway at the Facility for Rare Isotope Beams (FRIB), where 220 such resonators are in operation. Our goal was to achieve an intrinsic quality factor ($Q_0$) of $\ge 2\times {10}^{10}$ at an accelerating gradient ($E_{\mathrm{acc}}$) of $12\mathrm{MV}/\mathrm{m}$. FRIB production resonators were prepared with buffered chemical polishing. First trials of electropolishing (EP) and post-EP low-temperature baking of FRIB HWRs allowed us to reach higher gradient (15 MV/m, limited by quench) with a higher quality factor at high gradient, but $Q_0$ was still below our goal. Trapped magnetic flux during the Dewar test was found to be a source of $Q_0$ reduction. Three strategies were used to reduce the trapped flux: (i) adding a local magnetic shield (LMGS) to supplement the “global” magnetic shield around the Dewar for reduction of the ambient magnetic field; (ii) performing a “uniform cooldown” (UC) to reduce the thermoelectric currents; and (iii) using a compensation coil to further reduce the ambient field with active field cancellation (AFC). The LMGS improved the $Q_0$, but not enough to reach our goal. With UC and AFC, we exceeded our goal, reaching $Q_0=2.8\times {10}^{10}$ at $E_{\mathrm{acc}}=12\mathrm{MV}/\mathrm{m}$.

Transverse beam profile monitoring in high-radiation areas is challenging due to camera degradation. A proposed solution employs a multimode fiber (MMF) to relay optical signals from the radiation zone to a shielded area, where a standard CMOS camera can operate safely. However, MMF transmission introduces significant distortions, producing complex speckle patterns at the output. We present a machine-learning–based method to reconstruct the transverse beam distributions from these patterns. The model was trained solely on synthetic data generated via stochastic Gaussian mixture simulations, in which the samples were displayed on a laser-illuminated digital micromirror device (DMD) and relayed through a 5-m MMF. The same setup was used for testing, with beam images on a scintillating screen from CERN’s CLEAR facility replayed on the DMD. The model achieved a 1.49% mean relative root mean square error across four key beam parameters—77% lower than the 6.59% baseline, demonstrating its potential for diagnostics in radiation-constrained environments.

The design of radio frequency (rf) cavities in particle accelerators constitutes a multiobjective optimization problem (MOOP) characterized by strict equality constraints and computationally expensive evaluations. Here, a multiobjective Bayesian optimization (MOBO) framework is applied to rf cavity design for the first time to efficiently identify high-quality Pareto-optimal solutions under limited simulation budgets. To maintain high sample efficiency of the MOBO framework in problems with extremely strict equality constraints, two constraint-handling strategies are investigated: a penalty-based scalar acquisition function (sAF) strategy and a novel two-stage acquisition strategy (tAS). The tAS divides the sampling process into two stages. In the first stage, a vectorized acquisition function (vAF) is combined with a multiobjective evolutionary algorithm (MOEA) to efficiently search for candidate points in the full high-dimensional objective space. In the second stage, a sAF is employed to select the most promising point from candidate points for actual evaluation. Results on a test problem and the design of a practical 499.65 MHz rf cavity show that tAS outperforms penalty-based sAF in handling equality constraints within the MOBO framework. Furthermore, by using the MOBO framework with tAS, high-quality Pareto-optimal solutions are obtained within 1000 simulations, demonstrating a significant advantage of efficiency over conventional rf cavity optimization algorithms. Additionally, tAS allows for the flexible adjustment of MOEA, vAF, and sAF configurations, thereby providing flexible adaptation to other MOOPs in particle accelerators.

We revisit the nonlinear lattice design approach for the National Synchrotron Light Source II (NSLS-II) storage ring. By suppressing chaos, we identify alternative sextupole configurations to the original design, which relied on the conventional strategy of simultaneously minimizing resonance driving terms (RDTs) and amplitude-dependent detuning (ADD). These alternatives achieve comparable performance while requiring fewer sextupoles. A detailed comparison of two representative solutions is presented and supported by experimental validation. Our results show that the dynamic aperture correlates more strongly with global chaos than with individual RDTs, and that the importance of minimizing ADD may have been overstated in earlier design strategies.

The rapid advancement of laser compression techniques has brought increasing attention to few-cycle laser-plasma interactions. Through comprehensive particle-in-cell simulations, we demonstrate that compressing laser pulses to sub-10-femtosecond durations significantly enhances transient electrostatic shocks. Crucially, the plasma dynamics are strongly modulated by the laser carrier-envelope phase (${\mathrm{\Phi }}_{\text{CEP}}$). Our results reveal a pronounced cosinelike dependence of the proton cutoff energy on ${\mathrm{\Phi }}_{\text{CEP}}$, with certain values of ${\mathrm{\Phi }}_{\text{CEP}}$ yielding superior performance due to enhanced electron displacement and stronger charge-separation fields. Phase-space analysis uncovers expanding “source”-like electron orbits, whose gyration radii correlate with ${\mathrm{\Phi }}_{\text{CEP}}$-dependent energy transfer. We further develop an analytical model connecting the electrostatic shock strength, inferred from the proton energy, to the laser-plasma interaction parameters. This model is validated through extensive parameter scans over target density, thickness, laser intensity, and ${\mathrm{\Phi }}_{\text{CEP}}$. Our findings establish a predictive framework for ${\mathrm{\Phi }}_{\text{CEP}}$-controlled plasma dynamics in short-pulse laser interactions, providing insight into mechanisms relevant to laboratory high-energy proton sources and origin of energetic cosmic rays in astrophysical environments.

Pursuing on-chip particle accelerators has long been a focal point for researchers, and the inverse Smith-Purcell dielectric laser accelerator holds great promise as a viable option. However, achieving the cascade acceleration and focusing of subrelativistic electrons on a chip remains one of the most formidable challenges. In this paper, we propose a dynamic phase method to address this issue. This method takes advantage of the phase slippage between the electron and the acceleration wave, enabling transverse alternate focusing, defocusing, and continuous acceleration. Simulation results show that a 29.50 keV electron beam with a 20 eV spread can achieve an acceleration gradient of $39\mathrm{MV}/\mathrm{m}$ with a captured efficiency of 95% within a 120-μm distance in a 200-nm-wide channel. Compared with the alternating phase method, this approach streamlines the fabrication process by removing the need for incremental adjustments of the phase velocity at each accelerating stage to match the electron energy. We anticipate that applying the dynamic phase-driven approach will significantly propel the development of on-chip accelerators.

In this study, we employ hydroforming technology to develop a 1.3 GHz one-cell $\mathrm{Nb}/\mathrm{Cu}$ full-seamless cavity to reduce cost and improve performance. Superconducting radiofrequency (SRF) cavities made of pure niobium are expensive because of the material’s high cost and the complex manufacturing process, which involves electron beam welding. To overcome these challenges, extensive research has been conducted on using copper substrates coated with niobium, leveraging hydroforming to eliminate welding and seams, thereby reducing cost and simplifying production. The hydroforming process involves expanding oxygen-free copper tubes in two steps, followed by vacuum annealing to recover elongation. This method exhibited high productivity, with forming times of 51 and 78 s for the two steps. However, the process introduced surface roughness, compromising optimal rf performance. Following hydroforming, the cavities were coated with niobium via high-power impulse magnetron sputtering. This coating method ensured excellent film adhesion and can withstand high-pressure water rinsing. We conducted rf measurements of the cavities, achieving an accelerating gradient of $12\mathrm{MV}/\mathrm{m}$ at 4.2 K. The quality factor was slightly below requirements, attributed to surface roughness from the hydroforming process. The maximum accelerating gradient reached $15.7\mathrm{MV}/\mathrm{m}$ at 1.85 K with no field emission. We demonstrate the advantages of full-seamless cavities, including reduced manufacturing costs, high reproducibility, and the elimination of welding seams that can introduce defects. However, challenges such as surface roughness and its impact on the rf performance must still be addressed. This study makes a significant contribution to the development of cost-effective, high-performance SRF cavities for coating studies.

We report the first direct experimental observations of resonance island formation in a hybrid multibend achromat (HMBA) storage ring lattice. Using a low-emittance electron beam, we achieved high-precision measurements that resolve the onset, evolution, and transient dynamics of particle capture near resonance islands. These measurements are compared with simulation predictions, enabling a direct validation of HMBA lattice models. The experimental results also provide new insight into the capture and trapping mechanisms governing the island population, as well as beam lifetime in resonance islands. Together, these findings establish a detailed experimental basis for understanding nonlinear dynamics in next-generation HMBA-based synchrotron light sources.

Electron storage rings based on multibend-achromat (MBA) lattices can achieve exceptionally low natural emittances. At the Paul Scherrer Institute in Switzerland, the Swiss Light Source facility underwent an upgrade project, replacing its triple-bend achromat ring with a seven-bend achromat ring. Due to the compact nature of MBA lattices, magnetic fields can be influenced by neighboring magnets. These magnetic crosstalk effects turned out to be significant during the design phase: the integral fields of the magnets could be altered by several percent. We present how we incorporated compensations for these effects into the design and how we tuned the magnets prior to installation. The crosstalk compensations were validated through the accelerator commissioning.