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.

A repetitive source of collimated, high-energy electrons is experimentally demonstrated using a 1.3 TW femtosecond laser pulse. The source is based on the efficient coupling of the pulse with an undercritical plasma slab formed by a nanosecond prepulse-induced breakdown of a thin liquid ethanol microjet. By optimizing the temporal delay between the prepulse and the main pulse, a quasi-Maxwellian electron beam is generated with a divergence below 0.1 rad, a detected energy of $\sim 25\mathrm{MeV}$, and a charge up to 0.25 nC ($3.8\mathrm{nC}/\mathrm{J}$) in the $>1\mathrm{MeV}$ range. Numerical simulations corroborate the experimental results and point to a complex acceleration mechanism involving direct laser acceleration and self-modulated laser wakefield acceleration within a bell-shaped plasma profile. This versatile approach highlights the particular advantages of liquid targets, including robustness, simplicity, and suitability for long-term operation in vacuum, for future high-repetition-rate laser-plasma applications.

This paper introduces the injector design for the Super Tau-Charm Facility (STCF), addressing the long-standing challenge of generating high-charge (8.5 nC) and high-brightness electron beams. The design pioneers a segmented optimization strategy that effectively decouples the mitigation of space-charge effects from the management of high-energy wakefields. Through multiobjective optimization in the low-energy section and sophisticated beam manipulation in the high-energy section utilizing S/X-band structures and a magnetic compressor, the injector delivers a 2 GeV beam with remarkable properties: a mere 0.22% rms energy spread and transverse normalized emittances of 6.3/5.9 mm mrad. Robustness is demonstrated through error analysis, showing emittance growth is suppressed via orbit correction and stringent alignment tolerances. Its defining innovation is the innate support for both 8.5 nC swap-out and 1.5 nC off-axis injection within a single lattice, seamlessly toggled without hardware intervention. This design provides a versatile and high-performance injection platform ready for the demands of the STCF.

Leveraging the interaction between the beam and the radio-frequency (rf) cavity to measure the beam phase reduces tuning time and mitigates slow drifts induced by ambient conditions. An online phase-measurement approach based on beam-induced rf transients was previously demonstrated at DESY using superconducting rf cavities in an electron accelerator under open-loop operation. However, open-loop operation is unsuitable for high-current proton or heavy-ion linacs due to elevated risks of beam loss and trips. This work proposes two novel schemes that operate under closed-loop conditions. The first, a virtual open-loop (VOL) method, solves the cavity difference equation to remove the contribution of feedback (and/or feedforward) control from the cavity field, thereby recovering the beam-loading signal under an equivalent open-loop condition from which the beam parameters are determined. The second, a steady-state (SS) method, determines the beam parameters (i.e., beam current and phase) when beam-loading and cavity-detuning compensation are sufficient and infers the beam phase and current by computing the vector difference of steady-state forward voltages with and without beam. Both approaches were systematically validated on the buncher cavity at the European Spallation Source (ESS), showing agreement with independent beam diagnostics, with phase error within $\pm 0.5°$ and current error within $\pm 4\%$. The methods enable real-time, accurate beam-phase measurement during closed-loop operation and are readily extensible to other high-power accelerator systems.

The collimator system is a significant source of impedance in ring colliders and can drive transverse mode coupling instabilities (TMCIs) through vertical dipolar wakefields. In this study, we investigate the impact of collimator geometry on wake potentials using electromagnetic simulations based on simplified and realistic models. Various taper shapes, jaw widths, and structural modifications such as hollowed and asymmetric configurations are evaluated with a focus on reducing the vertical dipolar kick factor. Three mitigation strategies are proposed and benchmarked using the pyheadtail tracking code, with the SuperKEKB Low Energy Ring used as a case study. Our results show that the implementation of hollowed exponential taper jaws, especially in one-sided layouts, leads to a substantial reduction in the total ${\beta }_{\mathrm{y}}$-weighted dipolar kick factor and increases the TMCI threshold by up to approximately 90%.

We present a photoemission model based on the Fowler-DuBridge equation that accounts for the dynamic modulation of the Schottky barrier during the emission of high-charge-density electron bunches. By incorporating the time-dependent effective electric field normal to the photocathode surface (including real-time emission-induced electric fields), our model captures the ultrafast transient modulation of the electron emission potential barrier that governs the instantaneous current density. This approach links single-electron photoemission effects with macroscopic collective charge effects, enabling accurate predictions of the time-dependent charge production for high-brightness photoinjectors. Furthermore, we validate the model by comparing it with the experimental results obtained with a high-gradient photoinjector equipped with ultraviolet pulse shaping capabilities. Our model highlights the importance of dynamic surface effects in shaping the initial phase space of electron bunches. The effects are particularly relevant for future lepton colliders where producing on-demand bunch charge with a tailored phase space is required, including the top-up injection scheme for CERN’s Future Circular Collider (FCC).

We present a perturbative method for constructing approximate invariants of motion directly from the equations of discrete-time symplectic systems. This framework offers a natural nonlinear extension of the classic Courant-Snyder (CS) theory for systems with 1 degree of freedom—a foundational cornerstone in accelerator physics now spanning seven decades and historically focused on linear phenomena. The original CS formalism emerged under conditions where nonlinearities were weak, design goals favored linear motion, and analytical tools—such as the Kolmogorov-Arnold-Moser theory—had not yet been fully developed. While various normal-form methods have been proposed to treat near-integrable dynamics, the approach introduced here stands out for its conceptual transparency, minimal computational overhead, and direct applicability to realistic systems. We demonstrate its power and versatility by applying it to several operational accelerator configurations at the Fermi National Accelerator Laboratory (Fermilab), illustrating how the method enables fast, interpretable diagnostics of nonlinear behavior across a broad range of machine conditions.

This paper presents an alternative approach to designing multibend achromat (MBA) lattices with distributed sextupoles. It is based on the inherent structure of MBA lattices, which are composed of three smaller (recurring) substructures that can be optimized individually. These substructures consist of only a few elements; therefore, detailed parameter studies are feasible, and underlying principles can be revealed. The final linear lattice is a combination of the optimized substructures. Design decisions are mostly dictated by constraints, leaving limited alternatives under the given goals and boundary conditions. The linear lattice is constructed rather than optimized. Thus, it is justified to talk about a deterministic lattice design approach that provides a solid starting point for further fine-tuning and nonlinear optimization. The significance of time- and CPU-intensive optimization algorithms, such as multiobjective genetic algorithms, is much reduced. They can be used for fine-tuning the superperiod and for nonlinear optimization. Although the method optimizes the linear structure, it also considers nonlinear aspects such as chromatic sextupole strength and robustness against deviations from the design parameters. The approach was used to develop a lattice for the fourth-generation synchrotron light source BESSY III, planned at the Helmholtz-Zentrum Berlin. The design criteria and target values were taken from the BESSY III project. Different goals or boundary conditions may lead to alternative decisions or objectives to be considered. Implications of using this approach for higher energy or higher superperiodic lattices are briefly discussed. The resulting solution for BESSY III and initial nonlinear studies are presented.

The high-frequency, high-gradient, low-$\beta$ accelerator structure is a key technology enabling compact ion linacs for hadron therapy, garnering significant interest in cancer treatment research. Currently, multiple institutions globally are advancing this technology. However, the achievable accelerating gradient remains limited to approximately $50\mathrm{MV}/\mathrm{m}$ due to radio-frequency (rf) breakdown (BD) limitations. To enable higher accelerating gradients, researchers at the Institute of Modern Physics, Chinese Academy of Sciences, have proposed and designed a low-$\beta$ high-gradient cryogenic copper cavity targeting a theoretical accelerating gradient of $60\mathrm{MV}/\mathrm{m}$ with a specified breakdown rate ($\mathrm{BDR}\le {10}^{-6}\text{bpp}/\mathrm{m}$). This paper details the design, fabrication, and tuning methodology of a $\beta =0.3$ cryogenic traveling-wave high-gradient accelerating structure. Cell tests were performed to verify the cryogenic performance of the cavity. Upon cooling from room temperature to 77 K, the cavity’s quality factor (Q) increased by a factor of 2.9, while its resonant frequency shifted upward by 9.85 MHz. Subsequently, a 15-cell cryogenic traveling-wave structure was fabricated and tuned to meet stringent design specifications. The conduction cooling methodology was experimentally validated, achieving a stable cavity temperature of $77\pm 0.3\mathrm{K}$ within 5 h. This work establishes a framework for tuning cryogenic traveling-wave copper cavities and implementing conduction cooling in high-gradient systems.

Dynamic detuning, encompassing dynamic Lorentz force detuning (LFD) and microphonics, significantly restricts the operational stability of superconducting radio-frequency (SRF) cavity modules. This paper proposes a novel semianalytical method for calculating the dynamic detuning of SRF cavities, reducing computation time by several orders of magnitude compared to transient finite element analysis (fea). Leveraging this computational efficiency, genetic algorithms (GA) are employed to determine the optimal parameters for active compensation. This work provides a high-efficiency simulation framework for dynamic detuning research, with potential for expansion into integrated low-level radio frequency (LLRF) system models. In this study, the coupling between mechanical loads (including ground vibrations, pressure fluctuations, Lorentz forces, and fast tuning) and dynamic detuning is analyzed to develop a modified mechanical model for SRF cavities. A key advancement of this model is the incorporation of a correction term for high-frequency mechanical modes, which enhances simulation accuracy relative to conventional approaches that neglect them. The proposed method was first validated using a 1.3 GHz TESLA cavity, demonstrating excellent agreement with transient fea solvers and published experimental data. Subsequently, the method was applied to evaluate microphonics in the 499.8 MHz cavity of the Hefei Advanced Light Facility (HALF), specifically characterizing the impact of ground vibrations and pressure fluctuations. Regarding active compensation, an optimized half-sinusoidal pulse suppressed the dynamic LFD of the TESLA cavity from 220 Hz to less than 5 Hz during the beam flattop, and from 600 Hz to less than 40 Hz during the entire rf-on region. The global optimum for this three-parameter optimization problem was effectively identified within a broad search space. Furthermore, an optimized PI controller reduced microphonics of the HALF SRF module induced by stochastic ground vibrations from $\pm 400$ to $\pm 50\mathrm{Hz}$, thereby improving the open-loop phase and amplitude stability from $\pm 2°$ and $\pm 13\%$ to $\pm 0.2°$ and $\pm 1.3\%$, respectively.