For the development of new synchrotron light sources, new operation modes are considered. For a successful application of these schemes, however, an understanding of the effects involved is essential. The KIT storage ring, Karlsruhe Research Accelerator (KARA), allows the implementation of new operating modes, including tests using a variety of high-performance beam diagnostics devices. Negative momentum compaction optics has been established at KARA at various beam energies. Systematic studies of the resulting beam dynamics showed significant differences to the dynamics at equivalent positive momentum compaction factors. Moreover, a longitudinal instability occurs at low absolute values of the momentum compaction factor, caused by coherent synchrotron radiation, which is the dominant longitudinal impedance at KARA. This work outlines the status of the operation with negative momentum compaction factors at KARA and presents the current-dependent bunch length at negative momentum compaction. Furthermore, the observation of the instability and its dynamics, including significant deviations from threshold predictions, is shown and compared to the equivalent instability at positive momentum compaction.

Magnetically insulated transmission lines (MITLs) are critical components for efficiently transporting power from pulsed power sources to a diverse range of loads, including magneto-inertial confinement fusion and radiation production. This study investigates the behavior of planar MITLs, taking into account the source voltage and source and load impedances, using two-dimensional theory and simulations. Self-limited behavior, where the MITL operating impedance (voltage divided by current) is insensitive to the value of load impedance, is extended to small-to-large gap distance MITL transitions. Load-limited behavior, in which the load impedance is below the self-limited impedance, causes the MITL operating impedance to adjust to match the load impedance and can be extended to describe large-to-small transitions. In both cases, the operating impedance of the larger MITL section changes to match the self-limited operating impedance of the smaller section. This behavior extends to multiple transitions and to both low and high impedance loads. Assuming equilibration occurs (pulse duration larger than MITL transit time), the entire MITL operating impedance becomes the self-limited impedance of the smallest-gap MITL section or the load impedance, whichever is lower.

A novel cooling mechanism for heavy-ion beams, termed charge-exchange cooling (CEC), is proposed. Phase-space-dependent zoned charge exchange combined with charge-state-dependent closed-orbit distortions (CODs) enables rapid, autonomous, turn-by-turn contraction of the phase-space distribution for continuous-wave heavy-ion beams. The resulting dynamics forms a stochastic affine-map system with state-dependent switching and a net negative drift. CEC is implemented in the charge stripper ring 1 (CSR1), under development at the RIKEN RI Beam Factory, where zoned charge exchange and charge-state-specific CODs are readily incorporated. Proof-of-principle studies based on two- and multicharge-state models, together with realistic CSR1 simulations, demonstrate fast turn-by-turn cooling under practical machine conditions and establish CEC as a new beam-cooling principle with potential implications for future accelerator design and applications.

We consider the radiation emitted by a purely longitudinally accelerated relativistic electron bunch and find, contrary to the usual expectation of negligible emission, that for ultrashort compact bunches within reach of current experiments, a significant and measurable radiation pulse is produced. The underlying mechanism for the enhanced emission is coherence in the single-electron emission over $\mathcal{O}({10}^8)$ electrons. We derive analytic descriptions of the emission spectrum and energy for acceleration fields and bunch conditions encountered in rf-driven particle accelerators, and in the extreme high-gradient conditions of plasma-driven acceleration. Both the bunch duration and the duration of the acceleration have a role in describing a phase-matching limitation to the coherence, with the latter dependent on the acceleration field strength and the initial energy of the electrons. In extremely compact bunches, just beyond those currently realized in the laboratory, the radiation losses approach the Larmor radiation for a massively charged particle ($Q\sim {10}^{9}e$). In such conditions, the implied energy of the longitudinal acceleration radiation may approach the energy available from the electric field driving the acceleration, raising questions about a connection to mechanisms of field depletion.

High-quality and prebunched electron beams can produce coherent x-rays with high intensity and narrow bandwidth, which are essential for modern light sources. An all-optical scheme based on plasma-based acceleration for producing bright electron beams that are prebunched on the nanometer scale is proposed. By using a density modulation created by two low intensity counterpropagating lasers, the phase velocity of the plasma wake excited by an intense driver laser in a uniform plasma can be modulated at a frequency twice that of the colliding lasers, thus turning the injection on and off. The injected electrons are microbunched at the Doppler-shifted modulated wavelength, corresponding to the phase velocity of the gradually expanding wakefield. It is demonstrated that by controlling the properties of the drive and colliding lasers, beams with exotic prebunched structures can be produced, which may have critical applications in ultrafast high power x-rays. This extremely compact, all-optical scheme for producing ultrabright prebunched electron beams may therefore enable novel applications for ultrafast x-ray users and arouse general interest in various fields.

SuperKEKB collimator system protects the superconducting final focusing magnets (QC1 and QC2) from quenching caused by beam halo particles. For effective shielding, the minimum aperture of vertical collimators is set smaller than the physical aperture of QC1. However, such collimators become dominant sources of beam coupling impedance, which can limit the maximum beam current due to impedance-driven instabilities. To address this issue, a nonlinear collimator (NLC) was introduced in February 2024. NLC is designed to reduce both beam backgrounds and collimator impedance, thereby suppressing beam instabilities caused by short-range wakefields. In this paper, we describe the results on impedance effects, beam background reduction, and beam lifetime when the nonlinear collimation system is turned on and off. These results validate the NLC concept and offer a promising approach for future high-luminosity colliders.

Technical details of a recently proposed system for direct current (dc) induction acceleration are described. This dc induction acceleration system comprises two induction cells, each with two insulating gaps; one acts as an acceleration gap, and the other is placed on the outer cage of the induction cell to act as a resetting gap. Both gaps are bridged by diodes. The system is energized by a bipolar switching power supply. Continuous dc acceleration is limited by the saturation of magnetic materials distributed along the beam line, such as the beam-guiding magnets that act as effective absorbers of the magnetic flux that is released into the surrounding area upon resetting of the induction acceleration cell core. A secondary loop current (the primary loop current being the current that excites the magnetic core of the induction acceleration cell) necessarily flows on the metal beam-chamber surface distributed along the accelerator ring. This secondary loop current tends to degrade the resetting of the induction acceleration core and must be canceled by an artificially added tertiary loop current, whose loop passes symmetrically through the guiding magnets but bypasses the dc induction acceleration cells. In this way, resetting of the induction cores is secured at the expense of magnet saturation. dc induction acceleration can be performed via intermittent operation to release the absorbed extra flux. The effects of this externally absorbed flux on the beam orbit and optics are discussed. Applications of the dc induction acceleration are of practical interest. In addition, proposals are made for a dc induction microtron and a fixed field alternating gradient accelerator for heavy ions, which could replace existing electrostatic accelerators such as Van de Graaff or tandem accelerators, and an isochronous storage ring with dc induction acceleration for electrons/positrons, which could lead to the ultimate form in efficient continuous wave free-electron laser.

Since the emergence of laser-plasma accelerators (LPAs), substantial work has been dedicated to using LPAs to drive free-electron lasers (FELs) for a broad range of applications. Despite recent breakthroughs, which have proven the fundamental feasibility of operating FELs with an LPA source, stable FEL operation over multiple hours without operator input had yet to be achieved. In this work, we report significant improvements to the stability of a hundred terawatt laser system, resulting in successful demonstration of reliable, long-term operation of an LPA-driven FEL in the self-amplified spontaneous emission (SASE) regime at 420 nm. The LPA source delivered 100 MeV electron beams at 1 Hz with high stability over more than 10 h, enabling over 8 h of continuous FEL operation without operator input. The acquired data were subsequently used to investigate correlations between the measured undulator radiation and parameters of the drive laser, plasma source, and electron beam. The revealed connections between LPA and FEL performance gave important additional insights into ways to further improve and stabilize the system, thus demonstrating the capability of our setup to serve as a powerful platform for future studies of LPA-driven FEL operation. The one-of-a-kind integration of multiple stabilization concepts onto the LPA facility discussed in this manuscript yielded significant accelerator and light source improvements (with residual correlations suggesting even more is possible), which will positively impact LPA applications at large.

The compression of relativistic electron beams by laser fields requires an in-depth understanding of ponderomotive processes. However, existing theoretical frameworks fail to capture the full complexity of these modulation processes. This paper develops a covariant oscillation-center formulation of the ponderomotive four-force and derives a laboratory-frame closed-form expression for its longitudinal component evaluated on the axis of a tightly focused radially polarized beam. This force, commonly described through cycle averaging that neglects instantaneous field phases, has validity conditions that are critically dependent on the interaction length. The theoretical framework seamlessly extends to dual-frequency fields through the equivalent beat wavelength concept, establishing the foundation for optimized electron beam-compression designs. Comprehensive simulations including parameter optimization and full particle tracking with space charge confirm the theoretical predictions. These results provide both a theoretical foundation and practical design principles for the generation of ultrashort relativistic electron beams in advanced applications.

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.