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

The Super Tau-Charm Facility (STCF), a proposed high-luminosity electron-positron collider in China, requires a beam current of 2 A to achieve its design luminosity. Such intense current induces severe coupled-bunch instability driven by the rf cavity accelerating modes. This paper presents a systematic analysis of these instabilities across the STCF operational energy range of 1–3.5 GeV. Without active control, the low-order coupled-bunch modes, particularly the mode $-1$, exhibit growth rates significantly exceeding the radiation-damping rate. We demonstrate that the instability can be effectively suppressed using only the proportional-integral feedback loop of the baseline low-level rf system. The key lies in setting a unit proportional gain and a loop delay of approximately half the revolution period. This configuration creates a notch-filter response that attenuates the most dangerous mode $-1$. By combining analytical calculations with particle-tracking simulations, we show that this simple and robust approach maintains beam stability for the baseline design parameters across all design energies, thereby enabling reliable high-current operation without requiring additional dedicated feedback measures, such as direct or one-turn-delay feedback. A stability assessment based on the Nyquist criterion confirms the closed-loop stability of the system and indicates sufficient gain margins. These results provide a reliable foundation for the STCF rf and feedback system design, enabling stable high-current operation under realistic constraints. The proposed approach and the associated analysis framework also offer a practical reference for other high-current storage rings facing similar instability challenges.

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

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

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

We present the baseline design of the electron ring collimation system for the Electron-Ion Collider (EIC) at Brookhaven National Laboratory (BNL). The system addresses beam losses in a high-current electron storage ring with superconducting (SC) final-focus magnets and sensitive detectors, where uncontrolled losses can generate heat loads, radiation, and detector backgrounds and damage. The proposed collimation insertion localizes halo particle losses through reducing interaction region beam losses from beam-gas and Touschek scattering by several orders of magnitude while keeping detector backgrounds and cryostat heat loads within acceptable limits. Multiturn particle-tracking simulations show that the collimators do not significantly impact machine acceptance or beam lifetime, and their positions and apertures can be reoptimized for future lattice configurations. Ongoing work includes incorporating crab cavities and solenoid fields into simulations, refining vacuum conditions, and optimizing collimator geometry and materials. This design establishes a robust baseline for the EIC electron ring collimation system and supports continued lattice optimization for machine operations.

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

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

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

High magnetic fields can increase the occurrence of vacuum arcing, suggesting that both electric and magnetic fields can play a role in the vacuum arcing process. The mechanism of vacuum arcing in high magnetic fields is believed to involve both the cathode and the anode, with the cathode serving as the originator of field-emitting nanoprotrusions or tips, while the anode serves a secondary role. Significant heating of the anode surface can be achieved by magnetic focusing of the emitted electron beam, leading to an increased heat flux due to greater current density. We simulated the emitted electron beam in different configurations of the electric and magnetic fields using the particle-in-cell and finite-element methods. The heating caused by the impacting electron beam was simulated for magnetic fields ranging from 0 to 30 T. The directions of the electric and magnetic fields were found to play a major role in the focusing of the electron beam. We found that a sufficient temperature increase on the anode surface for evaporation can be reached at magnetic fields on the order of 10–30 T, suggesting the possibility of plasma initiation on the anode side.

We experimentally demonstrate a method for reconstructing the transverse 4D phase space of an electron beam at the time of emission from downstream diagnostics of the 4D phase space. This method does not rely on detailed knowledge of the beamline transport, besides assuming that linearity and symplecticity are satisfied. We apply this method to measure the transverse position and momentum phase space of electrons emitted from a spatially structured alkali-antimonide cathode. This method can uncover local correlations between emission location and momentum spread. We formulate this method analytically and investigate resolution limits.

The injection of positrons in a plasma wakefield has long been challenging due to the demand for an appropriate field structure. Here, we propose a novel approach whereby a positron source is positioned at the front of a hollow, positively charged driver. This configuration allows the plasma wakefield to continuously catch up with the positrons, enabling self-adaptive capture into the accelerating and focusing phase. Our analytical model demonstrates that positrons with varying angles, energies, and initial positions can be effectively injected into similar phases for further acceleration. In addition, introducing a transversely asymmetric driver can substantially enhance the energy gain without affecting the injection. Three-dimensional quasistatic PIC simulations show that injected positrons with a charge of 12 pC can be accelerated to energies exceeding 14 GeV using a 10 GeV hollow positron driver, corresponding to an acceleration gradient of $16\mathrm{GV}/\mathrm{m}$. These results provide a proof-of-principle demonstration for a new positron injection and acceleration scheme in plasma wakefields.

We propose a dual-pulse micronozzle acceleration scheme that enables phase-locked acceleration of laser-driven protons, mitigating the conventional trade-off between maximum energy and laser-to-proton conversion efficiency. By introducing a delay-tuned synchronization window, a compact proton front generated by a shaping prepulse is injected into, and remains copropagating with, a quasistatic axial electric field driven by a delayed main pulse in a micronozzle cavity. This phase locking maintains the relative phase between the proton bunch and the accelerating field over an extended interaction length and duration, thereby suppressing thermal debunching and prolonging the effective acceleration stage. At main-pulse intensities of the order of ${10}^{21}\mathrm{W}/{\mathrm{cm}}^2$, sub-GeV-class proton cutoffs are achieved with a total laser-to-proton conversion efficiency of $\sim 20\%$. Notably, the efficiency of the application-relevant high-energy component exceeds $\sim 13\%$ for protons with energies $E_p>100\mathrm{MeV}$, indicating preferential energy loading into a compact, directed proton population rather than quasithermal sheath expansion. Comparative simulations with an unconfined dual-pulse hydrogen-rod target demonstrate that this performance gain arises from the combined effects of temporal synchronization and geometric confinement, which sustain a long-lived axial accelerating channel advected downstream with the proton front. An analytical model for the synchronization condition is developed and validated against the simulation data. Three-dimensional particle-in-cell simulations support that the phase-locking mechanism and the associated spectral hardening are preserved in 3D slit-nozzle geometries, with the confined target yielding cutoff energies $\sim 60\%$ higher than an unconfined hydrogen rod under identical laser conditions. These results identify phase-locked acceleration in a confined laser-driven structure as a practical design principle for compact, high-yield sub-GeV-class proton drivers, with direct relevance to secondary-particle sources such as pion and muon production, accelerator-driven systems, and laser-based neutron sources.

In this study, a waveguide-type radio frequency (rf) ceramic window with a simple structure was developed for vacuum-sealing rf cavities up to 250 kW in the synchrotron storage ring of a fourth-generation light source. The rf window was designed for easy and quick replacement in the event of ceramic failure. It was assembled with low dielectric loss tangent ($\tan \delta$) alumina ceramic in a copper frame for mounting between waveguide flanges. The ceramic surface was coated with titanium nitride (TiN) to suppress multipactor discharge. However, two problems occurred during the high-power rf operation test: overheating of the ceramic due to an unexpectedly high $\tan \delta$ and multipactor discharge on the ceramic surface. To eliminate high-$\tan \delta$ alumina before welding the window, screening methods were developed to remove such alumina. To suppress the discharge, a magnetic field was applied to the ceramic plate, which was coated with diamondlike carbon (DLC), a material with a lower secondary electron yield (SEY) than TiN. After these countermeasures were implemented, an operation at 250 kW rf was maintained for more than a week without interruption.

Flash photography is developing in the direction of multiframe and high precision, requiring accelerators to provide multipulse and small-spot electron beams. This imposes increasingly higher demands on the design of conversion targets. High-speed moving targets offer an effective solution to the problem of beam-target damage. Among them, electromagnetically driven high-speed moving targets represent the most promising technical approach. This paper first introduces the conceptual design of the electromagnetically driven target, then uses dynamics software to analyze the stress conditions of the target under strong impact, provides the maximum acceleration of the electromagnetic rail gun limited by the mechanical properties of the target material, and finally presents the preliminary experimental results. Under the drive of the existing electromagnetic rail gun in the laboratory, the target sheet achieved a motion speed of $1.7\mathrm{km}/\mathrm{s}$. The novel high-speed moving target technology introduced in this paper is expected to completely solve the beam-target interaction damage problem in multipulsed conversion targets and meet the new demands of flash photography development.

A proof-of-concept permanent magnet spiral inflector was constructed and used to successfully inject and accelerate protons in the Solid Pole Cyclotron 2 at iThemba LABS. This article describes the design, construction, and testing of the inflector and discusses its application to other cyclotrons.

The successful operation of the future Electron-Ion Collider is contingent on maintaining high hadron beam polarization up to 275 GeV. The Hadron Storage Ring lattice, however, features a symmetry-breaking interaction region that excites strong, nonsystematic spin resonances, posing a significant threat to polarization preservation. This paper systematically investigates two complementary strategies to ensure high polarization transmission. The first method involves optimizing the vertical betatron phase advance between Siberian snakes to orchestrate a cancellation of depolarizing kicks across the ring. We demonstrate through simulations that both dynamic and fixed-optics solutions based on this principle can successfully preserve polarization through the strongest resonances. The second, more powerful approach involves optimizing the snake rotation axes to suppress resonance driving terms at their source. We revisit established symmetric configurations, such as the Lee-Courant schemes, and introduce a novel, highly symmetric “Doubly Lee-Courant” (DLC) scheme, which enforces a local $\pi$ spin phase advance across every consecutive pair of snakes. Our analysis reveals a clear performance hierarchy, with the DLC configuration providing an exceptionally robust and energy-insensitive baseline for polarization preservation. We conclude that a hybrid strategy, using a DLC snake scheme as a symmetric foundation and betatron phase tuning for fine corrections, offers the most effective path forward for the EIC and future high-energy polarized-beam facilities.

Medium-temperature baking, denoting an in-vacuum heat treatment of superconducting rf cavities in the $220–350°\mathrm{C}$ range, has recently revealed significant beneficial effects on cavity performance by increasing its intrinsic quality factor. The baking process reduces the thickness of the niobium oxide surface layer and diffuses oxygen into the bulk material. Generally, heat treatment is performed in a vacuum furnace, which, however, has some disadvantages, including risk of contamination in the furnace and subsequent handling or later regrowth of the oxide layer. To eliminate these issues, ideally the heat treatment should be carried out within the module (“in situ”) with the cavity assembled in the cold string and thereby avoiding the need for an external vacuum furnace treatment. First, proof-of-principle experiments have demonstrated the feasibility of in situ treatment but required the installation of heat tape and temperature controllers. In this paper, we explore an alternative approach by heating the cavity with rf power in a cavity mode, thereby predominantly using the existing rf infrastructure and no need for extra ancillaries. Nevertheless, several challenges need to be addressed, including the need for sufficient rf coupling, providing homogeneous power deposition (especially in multicell cavities), and the drift of both the resonance frequency and the mode’s field pattern with temperature. The results from the rf heating experiments on a TESLA 9-cell cavity at HZB’s HoBiCaT facility are presented in this paper, including simultaneous bead-pull measurements to monitor the field profile of the different modes. In addition, a numerical model was developed to calculate the heat deposition profile. Benchmarking the model with the experimental results allowed for the successful determination of its free parameters. This model can help to predict the rf heating effects under various conditions to guide the implementation of an effective rf heating “recipe.” The model was used to demonstrate the feasibility of a heating scheme alternating between two modes to reach a sufficiently high and homogeneous temperature profile as required for mid-T baking.

Transparent top-up injection is important for maintaining photon beam stability and brightness in next-generation light sources. In traditional off-axis injection based on pulsed orbit bumps employing dipole kicker magnets, the injection transparency relies on the precise matching of magnetic fields to ensure a closed local orbit bump throughout the pulse duration. Similarly, alternative off-axis injection based on nonlinear kickers necessitates rigorous control of the magnetic field near the stored beam orbit. A key challenge common to both is mitigating the distortion of the pulsed kicker magnetic field caused by eddy currents in the metallic coating of the ceramic vacuum chamber, while simultaneously minimizing beam coupling impedance and heat load. This paper addresses this issue through the design of a ceramic vacuum chamber aimed at next-generation light sources, tailored to the requirements of the SPring-8 upgrade, SPring-8-II. We prototyped a ceramic vacuum chamber equipped with a strip-patterned titanium coating on its inner surface. Experimental characterization demonstrated that field deviations caused by eddy currents in the coating can be reduced to approximately 0.1% of the peak value throughout a pulse duration as short as $4\mathrm{\mu }\mathrm{s}$. These results satisfy the design requirements for beam injection in SPring-8-II.

Attosecond electron beams are essential for investigating ultrafast structural and electronic dynamics in matter with atomic-scale resolution. We propose a novel method that enables robust attosecond-level electron bunch compression. This method employs THz-driven linear energy chirping and multidimensional phase-space manipulation, effectively compressing the electron bunch and suppressing its arrival timing jitter. Implemented in an MeV ultrafast electron diffraction beamline, this method compresses a 3.18 MeV electron beam from an initial duration of 747 fs (rms) to 810 as (rms) while retaining 6 fC of charge, with 886 as (rms) arrival-time jitter. This approach enables unprecedented timing resolution in ultrafast sciences and offers significant potential for other accelerator applications involving attosecond-scale electron beams.