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.

An integrated detection system, consists of a thin diamond sensor incorporated into a compact signal loop, coupled in turn to an application-specific integrated circuit readout chip, has been tested for the first time, using ps-duration electron pulses from the Next Linear Collider Test Accelerator at the SLAC National Accelerator Laboratory. For readout frame rates as high as 120 kHz, the detection system is shown to achieve a linear dynamic range of ${10}^3$ and a bandwidth of 4–5 GHz, with no ringing seen on the tail of the output pulse. These results represent the best-performing multi-GHz ionizing particle detection system achieved to date.

Beam losses in high-intensity proton accelerators are largely caused by space-charge effects, particularly in the low energy beam transport (LEBT) region, where the beam remains nonrelativistic. While space-charge compensation (SCC) through residual gas ionization is a widely used mitigation technique, detailed time-resolved studies combining both simulations and direct experimental measurements are scarce. This work presents a comprehensive investigation of passive SCC in the Low Energy High Intensity Proton Accelerator LEBT using both Particle-in-cell/Monte Carlo Collision simulations and dedicated diagnostics. Detailed simulations using cst particle studio are carried out to investigate how compensating gas type, gas pressure, external magnetic field, and beam parameters affect the space-charge compensation time and degree, as well as the resulting variations in beam potential, emittance, and transverse size. Experimentally, the SCC dynamics are characterized using a diagnostic suite that features a retarding field analyzer (RFA) and our novel, indigenously developed particle monitor probe (PMP). The PMP is applied here to estimate SCC time from the transient ionization electron current at the beam periphery, while the RFA determines the local SCC degree. These measurements provide detailed information on the variation of SCC parameters with gas type, pressure, and solenoidal magnetic field. The combined simulation and experimental results provide new physical insights into the dynamics of SCC, particularly regarding the nonmonotonic effect of magnetic fields, and validate a diagnostic methodology applicable to other high-intensity proton injectors.

Generating two-color x-ray pulses is crucial for the research of ultrafast dynamics. The development of x-ray free electron lasers (XFELs) has enabled the production of high-power ultrashort two-color x-ray pulses. In this work, we propose a two-color XFEL scheme based on a self-seeding configuration. A chirped x-ray pulse is first generated by a chirped electron beam in the first undulator. Then the chirped electron beam passes through a chicane, and the chirped x-ray pulse passes through a multilayer-based two-color filter to produce a two-color seed. Finally, this seed is amplified to saturation by the electron beam in the second undulator, yielding ultrashort two-color x-ray pulses with spectral-temporal separation. This scheme offers a novel approach for producing ultrashort two-color XFEL pulses.

The future circular electron-positron collider will provide collisions at four interaction points (IPs) along a 91-km ring, with beam energies ranging from 45.6 GeV (Z pole) to 182.5 GeV ($\mathrm{t}\overline{\mathrm{t}}$ threshold). Several processes, induced by both the collisions at the IPs and the circulating beams, contribute to the radiation environment of the experimental insertion regions, exposing the detector, machine equipment, and electronics. Quantifying these radiation fields and their effects is essential to guide design strategies. In this work, the Monte Carlo code fluka is employed to simulate the propagation of radiation showers from both IP-driven and beam-driven sources, including radiative Bhabha scattering, incoherent pair creation, beamstrahlung, synchrotron radiation, and beam-gas interactions. Power deposition, equivalent fluences, and total ionizing dose are estimated to assess the exposure of sensitive components and to characterize tunnel radiation levels at the Z-pole and $\mathrm{t}\overline{\mathrm{t}}$-threshold modes. The results show that incoherent pairs dominate the detector environment, particularly in the tracker, while radiative Bhabha losses in the superconducting final-focus quadrupoles (FFQs) require shielding to ensure safe magnet operation. Tunnel radiation levels exceed the tolerance of standard equipment at both operational modes, being dominated by radiative Bhabha and collimation losses at the Z pole, and by synchrotron radiation at the $\mathrm{t}\overline{\mathrm{t}}$ threshold. While a tungsten layer is shown to effectively protect the FFQs, the high radiation levels elsewhere indicate the need for additional shielding in dipoles and drifts, as well as dedicated absorbers downstream of the IPs and collimators.

In the context of the large hadron collider (LHC) luminosity upgrade, rf crab cavities (CCs) are key components for achieving the target integrated luminosity. Due to the increased bunch intensity required to achieve this goal, the collider will operate with a large crossing angle scheme to alleviate the long-range beam-beam effects. The CCs will counteract the geometric luminosity reduction factor resulting from the large crossing angle. Amplitude and phase noise injected from the CC low-level rf control are known to induce transverse bunch emittance growth. This study presents measurements performed in the CERN Super Proton Synchrotron (SPS), where two CCs were installed in 2023 for testing purposes. Only one cavity was used for this experiment. A particular focus is given to emittance growth studies driven by amplitude noise: the measured emittance growth was found to be dependent on the amplitude detuning induced by the SPS octupoles, although no dependence was predicted by the available theories and models. A new lattice model is presented that is able to reproduce both the previous measurements with phase noise and these new ones. The second part of the study is dedicated to the beam distribution evolution: the Beam Synchrotron Radiation Telescope (BSRT) measurements were able to capture the evolution of the tails of the bunch over time. Under the effect of amplitude noise, the tail contribution is reduced; in the absence of noise, it remains constant. To understand this effect, an analytical and numerical framework is used to quantify the mechanism of diffusion of the beam profile. The results of a full simulation on the SPS lattice with heavy tailed distribution evolution are then presented and discussed.

Nanocrystalline soft magnetic alloy (MA) cores are essential magnetic loading components in proton/heavy-ion synchrotron rf cavities. While conventional MA cores fabricated from $16\mathrm{\mu }\mathrm{m}$ ribbons offer reliable performance, further enhancing their shunt impedance and thermal stability to meet the demands of next-generation high-power synchrotrons presents significant challenges. Although the use of thinner ribbons (e.g., $13\mathrm{\mu }\mathrm{m}$) can improve high-frequency magnetic properties, their widespread application is hindered by manufacturing difficulties and reduced packing factor consistency. To address this, we propose an embedded MA core design that strategically integrates $13\mathrm{\mu }\mathrm{m}$ ribbons in specific radial regions alongside normal $16\mathrm{\mu }\mathrm{m}$ ribbons. This approach not only increases shunt impedance but also optimizes the electromagnetic field distribution, thereby reducing peak surface temperature and improving thermal stability. This paper first elucidates the physical mechanism through which the embedded structure improves temperature uniformity. Additionally, a two-dimensional heat transfer numerical model is developed and experimentally validated to assess the thermal performance of the embedded MA core under actual operating conditions. Experiments are conducted and the results demonstrate the embedded core’s peak surface temperature is reduced by $2.5°\mathrm{C}$ compared to a normal MA core. Finally, parameter optimization of the embedded structure is conducted, demonstrating that optimizing the radial position, volume ratio, and thickness of thinner ribbons can achieve a temperature reduction of approximately $6°\mathrm{C}$ and a 12% impedance increase. The findings of this study provide a theoretical foundation for the performance optimization of MA cores in high-power synchrotrons.

During accelerator operation, the beam trajectory in free-electron laser (FEL) facilities frequently drifts from its predefined path due to machine instabilities and environmental factors, thereby degrading beam quality. In this work, we propose an online trajectory correction method based on dictionary learning, which significantly reduces data requirements and allows real-time, continuous updates of the response matrix. Simulation studies under static and drifting conditions demonstrate that the dictionary learning method achieves a root-mean-square error of $0.2\mathrm{m}/\mathrm{rad}$ or lower for response matrix elements, using only $\sim 1000$ trajectory-excitation data pairs. Experimental validation at the Shanghai Soft X-ray Free Electron Laser (SXFEL) further demonstrates its robustness, achieving a correction precision better than 0.01 mm and confirming its suitability for online implementation in advanced XFEL facilities.

This paper presents a high-power beam test of the first IH-type linac structure additively manufactured from pure copper performed in May 2025. The 433 MHz cavity with a total length of 20 cm has been tested up to 25 kW at a duty cycle of 2%, which corresponds to a cavity voltage of 0.86 MV with peak fields at the drift tubes reaching up to $68\mathrm{MV}/\mathrm{m}$ without rf breakdown at full power. A 1.4 MeV proton beam from a Van de Graaff accelerator has been accelerated up to 2.212 MeV, demonstrating the effective acceleration voltage of 0.817 MV at full power. These high-power beam tests show performance on par with conventionally manufactured H-mode structures, paving the way for future linac structures benefiting from the advantages of additive manufacturing.

We propose a new approach for a high-density free-neutron target, primarily aimed at nuclear astrophysics reaction studies in inverse kinematics with radioactive ions circulating in a storage ring. The target concept integrates four key subsystems: a neutron production source driven by a supercompact cyclotron utilizing ${}^9\mathrm{Be}$($p,xn$) reactions, an optimized moderator/reflector assembly using either heavy water or beryllium oxide with a graphite reflector shell to thermalize fast neutrons, a cryogenic liquid hydrogen moderator to maximize thermal neutron density in the interaction region, and beam pipe geometries that enable neutron-ion interactions while maintaining vacuum conditions for ion circulation. This integrated approach focuses on feasibility by incorporating readily available technologies. Using a commercial supercompact cyclotron delivering a proton beam of $130\mathrm{\mu }\mathrm{A}$, the design achieves thermal neutron areal densities of $\sim 3.4\times {10}^6\mathrm{n}/{\mathrm{cm}}^2$ for a proof-of-concept demonstrator at the CRYRING ion-storage ring at GSI Darmstadt. This autonomous accelerator-target assembly design enables deployment at both in-flight and ISOL facilities to exploit their complementary production mechanisms. Potential upgrades based on higher-energy and/or higher-current cyclotrons will enable an increase in areal density to $\sim {10}^9\mathrm{n}/{\mathrm{cm}}^2$. In combination with a customized low-energy storage ring and a radioactive ion-beam facility, the proposed solution could deliver luminosities above ${10}^{23}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, thereby enabling neutron capture measurements of $\sim \mathrm{mb}$ cross sections within a few days of experiment. The proposed system represents a significant milestone toward enabling large neutron-capture surveys on short-lived nuclei, thereby opening a new avenue for understanding the synthesis of heavy elements in our universe.

Six X-band accelerating structures with identical radio frequency (rf) designs have been developed and fabricated for the Very Compact Inverse Compton Scattering Gamma-Ray Source (VIGAS) project at Tsinghua University, representing the first large-scale deployment of such devices with consistent rf performance in China. This article presents the tuning, conditioning, and high-power performance results of these structures, with emphasis on in-depth analysis of conditioning processes and rf breakdown phenomena. The six structures achieved an accelerating gradient of $80\mathrm{MV}/\mathrm{m}$ with a breakdown rate of no more than $1\times {10}^{-4}$ per pulse after systematic conditioning. Our study reveals several critical aspects of rf breakdown behavior in X-band high-gradient accelerating structures. Key observations include the localization of breakdown events in regions of high electric field, particularly in cells adjacent to the input coupler; a power-law dependence of the normalized breakdown rate on cumulative pulse count, with notable variation in fitting parameters across different structures; and a two-exponential statistical distribution characterizing breakdown intervals. These findings offer important implications for optimizing conditioning strategies for high-gradient rf structures. Furthermore, the analysis of performance variations among nominally identical structures uncovers underlying physical mechanisms governing breakdown, establishing a useful benchmark for the development of future compact accelerator facilities.

The design of the Compact Linear Collider (CLIC) main beam injector complex linacs necessitates high-gradient, traveling-wave accelerating structures engineered to withstand substantial beam-loading effects due to the elevated beam current and demanding acceleration requirements of CLIC operations. In this work, we present a comprehensive design and optimization study of 2 GHz traveling-wave structures tailored to these stringent requirements. The electron and positron linacs accelerate the beams up to 2.8 GeV with a nominal bunch charge of 1 nC, followed by the booster linac, which increases the energy to 9 GeV with a nominal bunch charge of 0.83 nC. Each bunch train consists of 352 bunches, requiring careful management of beam dynamics and wakefield effects to ensure stable operation. Through detailed analytical modeling and extensive parameter sweeps, we optimized the iris geometry of the accelerating structures to enhance shunt impedance, reduce surface electric fields, and suppress long-range wakefields via detuning strategies, thereby minimizing undesired beam-cavity interactions. Given the high beam current, we analyzed beam-loading effects and implemented compensation techniques to minimize bunch-to-bunch energy spread, supporting reliable, high-efficiency acceleration essential for CLIC operations. This study advances the development of high-performance accelerating structures that operate with high beam currents, which is crucial for achieving CLIC goals.