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.

High-brightness, ultrahigh peak current electron beams are of significant interest to applications including high-energy colliders, advanced accelerator concepts, strong field quantum electrodynamics, and laboratory astrophysics. Despite such interest, compressing electron beams to attosecond pulse durations and mega-ampere peak currents while preserving beam quality remains a challenge. In this work, we examine the feasibility and challenges involved in generating such extreme beams using plasma-based compression. Using particle-in-cell and particle tracking simulations, we demonstrate that the large electric field gradients in plasma wakefields enable orders of magnitude higher compression than what can be achieved with conventional radiofrequency-based compressors. By adding energy chirps greater than $500\mathrm{MeV}/\mathrm{\mu }\mathrm{m}$, bunch durations approaching 10 nm and peak currents approaching 1 MA can be achieved. Scalings of beam properties with respect to accelerator and plasma parameters are explored with limitations on achievable final beam duration and peak current evaluated. Optimal beam and plasma conditions are investigated for different applications, with the goal of experimentally demonstrating this technique at existing accelerator facilities. Insights gained from this study will help design the next-generation of high-brightness beams for new frontiers in scientific research.

As the Fermilab accelerator complex prepares for higher-intensity operation under the Proton Improvement Plan II (PIP-II), addressing the negative impacts of third-order resonance becomes crucial for the Recycler Ring (RR). Minimizing fluctuations of third-order resonance driving terms (RDTs) around the ring allows one to enhance dynamic aperture and reduce resonance-driven losses. This work experimentally demonstrates a mitigation scheme using normal and skew sextupoles, optimized via the response matrix method, in order to manipulate and minimize RDTs for a hadron storage ring. Furthermore, the effectiveness of this approach for a bunched beam at high intensities is evaluated through dynamic and static tune scans. The results show that resonance compensation remains effective in the presence of space charge, reducing beam loss and emittance growth compared to uncompensated operation. These new measurements extend previous observations of third-order resonance effects in high-intensity bunched beams by explicitly quantifying beam loss, emittance growth, and tail population under conditions where the RDTs are minimized around the ring. Overall, these findings strengthen the applicability of resonance control in the RR and highlight its importance for reliable high-intensity operation in the PIP-II era.

Intense uncoupled beams are conventionally transported through periodic focusing lattices composed of linear optical elements that provide radial confinement against defocusing space-charge forces. In such systems, interplane coupling has traditionally been regarded as an undesirable perturbation. Recently, however, the concept of matched transport in periodic quadrupole focusing channels has been extended from uncoupled beams to those exhibiting significant correlations between the two transverse degrees of freedom [C. Xiao et al., Periodic four-dimensional solution for transport of intense and coupled coasting beams through quadrupole channels, Phys. Rev. Accel. Beams 27, 031602 (2024)]. Building on this development, the present work generalizes the full four-dimensional rms-moment matching formalism to periodic solenoid transport channels. A notable application of this approach, for instance, is the efficient transport of high-current, low-energy proton coasting beams.

X-ray free-electron lasers (XFELs) of high brightness have opened new opportunities for exploring ultrafast dynamical processes in matter, enabling imaging and movies of single molecules and particles at atomic resolution. In this paper, we present a straightforward method for multiframe diffraction imaging, using the whole electron beam to generate four-color XFEL pulses with adjustable wavelength separation and time delay. The optical klystron scheme is introduced to enhance the FEL intensity and reduce the total length of the undulators. The time delay is tuned via a magnetic chicane between the undulators for different colors. Using parameters of SHINE, start-to-end simulations demonstrate the effectiveness and tunability of our method, achieving representative results such as time delays of hundreds of femtoseconds and four-color XFEL pulses spanning 1.8 to 2.7 nm with 0.3 nm intervals. The proposed scheme enables the recording of multiframe diffraction images in a single exposure, providing a new perspective for ultrafast molecular and atomic dynamics studies.

The generation of high-power radiation in the terahertz (THz) regime using free-electron lasers (FELs) is challenging due to strong diffraction and pronounced slippage effects. These constraints often limit the achievable pulse duration and peak power in conventional single-pass THz FEL configurations. In this work, we investigate an unseeded optical klystron FEL concept tailored for the THz regime. Using time-dependent three-dimensional simulations for resonant wavelengths of 10, 30, and $100\mathrm{\mu }\mathrm{m}$, we demonstrate that this approach enables the generation of coherent ultrashort THz pulses with durations of subpicoseconds (FWHM) and peak powers in the multihundred-megawatt range at wavelengths of 10 and $30\mathrm{\mu }\mathrm{m}$. To address the slippage challenge, we propose and numerically demonstrate a novel chicane-embedded optical delay scheme, which restores phase alignment between the radiation and microbunched electrons. Simulations confirm that careful tuning of the dispersive strengths allows staged amplification, preserving beam quality and reaching multimegawatt output power. These results highlight the potential of THz-tailored optical klystrons to generate compact, short, and high-intensity THz pulses, and lay the groundwork for future experimental studies and facility implementation.

Broad-bandwidth x-ray free-electron lasers (FELs) are critical scientific tools across diverse disciplines such as biology and chemistry. While substantial efforts have been devoted to achieving FEL bandwidth broadening, existing methods face limitations. This paper proposes a simple yet effective scheme for spectral broadening that utilizes a plasma wakefield accelerator (PWFA) to impart a large energy chirp to an electron beam. A specifically designed transport system then selectively injects the portion of the beam meeting the lasing requirements into the undulators for FEL generation. A start-to-end simulation demonstrates that the scheme can effectively manipulate the longitudinal and transverse phase space of the PWFA electron beam that subsequently generates a high-brightness x-ray FEL exhibiting a full-width bandwidth reaching 20%.

The SPARC_LAB facility, operating at the LNF-INFN laboratories in Frascati, has been devoted over the past years to R&D related to accelerators, advanced diagnostics, and generation of radiation ranging from THz to extreme UV light. The facility has recently been significantly upgraded and moved toward user-oriented applications, especially after the approval of two additional projects with the goal to develop radiation sources up to the soft x-ray range and in view of the EuPRAXIA project, whose preparatory phase is ongoing with the support of the European Commission. This paper summarizes the electron beam and radiation parameters and describes the layout and main features of the upgraded facility in view of future user-oriented experimental activities and applications.

In this paper, we investigate methods for suppressing transverse single-bunch instability. A theoretical framework incorporating longitudinal impedance, feedback, and chromaticity is presented. Our results show that the combined action of these effects effectively suppresses the transverse single-bunch instability. The instability threshold can be raised by a factor of 2 or more relative to $I_{\text{tmci}}$, defined as the threshold under transverse impedance only. The key stabilization mechanism arises from two effects: feedback and positive chromaticity cause the instability to be dominated by nonzero-order azimuthal modes, while longitudinal impedance introduces a significant tune spread in these modes. This stabilization mechanism, however, is ineffective in the negative chromaticity regime. The theory is applied to three electron storage rings: SuperKEKB, CEPC, and HEPS. Both the analysis and simulation lead to consistent conclusions.

The design of transverse electric (TE) mode cavities offers new opportunities for beam manipulation and beam-quality improvement. We present a field-driven methodology for synthesizing multipolar TE-mode cavities by superimposing cylindrical standing-wave solutions and enforcing perfect electric conductor boundary conditions through sequential quadratic programming. This procedure produces hybrid cavity geometries that are validated against eigenmode simulations in cst, with examples including ${\mathrm{TE}}_{\{0,2\}1}$ and ${\mathrm{TE}}_{\{2,4\}1}$ hybrid modes. The results demonstrate that prescribed multipolar content can be realized with high modal purity and initial superposition of the multipolar components is preserved. While the present study focuses on methodology and validation, the approach provides the foundation for future applications such as integrated emittance correction and chromaticity compensation.