The room temperature continuous wave (cw) very-high-frequency (VHF) gun is one of the candidates for the electron gun of the high-repetition-rate free-electron lasers (FELs). The VHF gun operates with a cathode gradient of $\sim 20\mathrm{MV}/\mathrm{m}$ and an accelerating voltage of $\sim 750\text{kV}$. The gun dark current emission leads to beam loss along the FEL machine, therefore, it is a critical parameter for the performance of the cw gun. In this paper, we present a systematic study of the dark current reduction of the VHF gun, including cathode region optimizations, dark current tracking simulations, and measurements. Over-inserted cathode plugs were tested in two VHF guns of different acceleration gap sizes, and both demonstrated significant dark current reduction ratios of more than two orders of magnitude.

The collimators in the SuperKEKB rings are indispensable for reducing beam background noise in the Belle II particle detectors and protecting essential components, such as the final focusing superconducting quadrupole magnets, from beams that may deviate into abnormal orbits. This study developed a new robust collimator using a carbon-fiber composite (CFC) as the head material to receive a beam kicked by the accidental firing of the injection kicker. Simulation results confirmed that the new CFC collimator suppressed the temperature rise in the collimator head caused by a beam hit through accidental firing of an injection kicker more effectively than the conventional tungsten collimator. Additionally, radiation damage to the particle detector was reduced to a sufficiently low level with the new CFC collimator. When CFC was adopted as the head material, the head length was longer, and thus the impedance was higher than that of tungsten. Calculations confirmed that using CFC as the head material of the D06H3 horizontal collimator did not result in beam instability for a head length of up to 160 mm. Further, these calculations also verified that the temperature of the surface of collimator jaws irradiated by synchrotron radiation remained within $100°\mathrm{C}$, despite the steeper taper angle owing to the 160 mm head length. Initial copper and CFC bonding tests revealed cracks in the CFC caused by stress owing to the difference in thermal expansion. In subsequent bonding tests, slits were made in the copper to alleviate this stress and prevent the formation of cracks in the CFC. A sample with copper slits exhibited no cracks in the CFC. Based on these measurements and calculations, we manufactured a collimator using CFC as the head material and installed it within the SuperKEKB LER ring as a countermeasure against accidental kicker fires. The installed collimators were also evaluated for issues resulting from factors such as beam impedance, the ability to protect the Belle II particle detectors during accidental fires, and the ability to cool the high-intensity synchrotron radiation surface. All evaluated items indicated no major issues.

In this paper, we present a novel scheme for the controlled generation of tunable narrowband $\gamma$-ray radiation by ultrarelativistic positron beams inside acoustically driven periodically bent crystals. A novel acoustic crystalline undulator is presented, in which the excitation of a silicon single crystal along the (100) planar direction by a piezoelectric transducer periodically modulates the crystal lattice in the [100] axial direction. An ultrarelativistic positron beam is directed diagonally into the crystal and propagates along the (110) planes. The lattice modulation forces the positrons to follow periodic trajectories, resulting in the emission of undulator radiation in the MeV range. A computational methodology for the design and development of such acoustically based light sources is presented together with the results of simulations demonstrating the favorable properties of the proposed technology. The longitudinal acoustic strains induced in the crystal by high-frequency piezoelectric elements are calculated by finite element simulations. The resulting bending profiles of the deformed crystal planes are used as geometrical conditions for the relativistic molecular dynamics simulations that calculate the positron trajectories and the spectral distribution of the emitted radiation. The results show a strong enhancement of the emitted radiation within a narrow spectral band defined by the bending period, demonstrating the feasibility and potential of the proposed technology.

A measurable chaos indicator is used as the online optimization objective in tuning a complicated nonlinear system—the National Synchrotron Light Source-II storage ring. Through analyzing the Shannon entropy in measured Poincaré maps, not only can the commonly used nonlinear characterizations be extracted, but more importantly, the chaos can be quantified and then used for an online regularization of these maps. The method itself is general and applicable to other tunable nonlinear systems as well.

We present an unsupervised learning framework for detecting anomalous superconducting radio-frequency (SRF) cavity behavior at the Continuous Electron Beam Accelerator Facility (CEBAF), emphasizing its initial performance and effectiveness. Key to the system’s success was the development of data acquisition systems (DAQs) that capture fast-sampled, information-rich signals, essential for detecting transient effects. The approach involves creating daily cavity-specific models using principal component analysis to handle variations in rf signal behavior and mitigate performance degradation from data drift. This unsupervised method eliminates the need for expensive labeling by continuously updating models with recent data. Deployed and operational for 3 months before a scheduled shutdown, the system successfully identified several issues with DAQ signals, confirming its effectiveness. Despite access to only a fraction of CEBAF’s SRF cavity signals, the framework efficiently detected several instances requiring intervention, demonstrating a significant improvement over traditional, labor-intensive methods of manual plot inspection.

Microbunched electron cooling (MBEC, Ratner, Phys. Rev. Lett. 111, 084802 (2013)), a special category of coherent electron cooling, has been under consideration as a candidate method for the strong hadron cooling component of the Electron Ion Collider (EIC) at BNL. Most of the theoretical treatments of this scheme so far have been based on simplified models for the key space charge effect in which electrons and/or hadrons are approximated by charged disks. Although such disk-based models provide a flexible tool that captures a good part of the underlying physics, they lack the fidelity and robustness of a rigorous analysis that explicitly treats the hadron and cooler electron beams as collections of point charges. In this paper, we present such a rigorous, three-dimensional (3D) model, both from a theory-based and a simulation perspective. Specifically, we calculate the generalized wakefield (or Green’s function) of the cooling system—a crucial quantity that determines the overall performance—paying special attention to the dependence of the wake on the hadron transverse positions. Additionally, our treatment takes into account previously neglected 3D effects that can cause damping of the plasma oscillations in the amplification section of the cooler, such as focusing and the angular spread of the electron beam.

We present a theoretical framework for analyzing longitudinal coupled-bunch instabilities in double-rf systems with even filling patterns, accounting for potential-well distortion and multiple azimuthal modes. The linearized Vlasov equation is solved in the frequency domain for an arbitrary rf potential to derive the Lebedev equation. We unified different formulations, obtaining results from recent publications as particular cases. Applications to Robinson dipole-quadrupole mode coupling and the periodic transient beam loading (PTBL)/mode-1 instability are presented. Notably, for the first time, theoretical predictions of the mode-1 thresholds show excellent agreement with experimental data. The analysis reveals that the PTBL instability is triggered when coherent focusing is lost for the dipole motion of the coupled-bunch mode-1. We also confirm that this instability is dependent on azimuthal mode interactions and resistant to Landau damping, providing new insights into its mechanism. The methods are implemented in the open-source package pycolleff, offering a useful semianalytical tool for studying instabilities in electron storage rings with harmonic cavities.

Accurate characterization of radiation pulse profiles is crucial for optimizing beam quality and enhancing experimental outcomes in free electron laser (FEL) research. In this paper, we present a unique approach that employs machine learning techniques for real-time virtual diagnostics of FEL radiation pulses. Our simple artificial intelligence (AI)-based diagnostic tool utilizes longitudinal phase space data obtained from the X-band transverse deflecting structure to reconstruct the temporal profile of FEL pulses in real time. Unlike traditional single-shot methods, this AI-driven solution provides a noninvasive, highly efficient alternative for pulse characterization. By leveraging state-of-the-art machine learning models, our method facilitates precise, single-shot measurements of FEL pulse power, offering significant advantages for FEL science research. This work outlines the conceptual framework, methodology, and validation results of our virtual diagnostic tool, demonstrating its potential to significantly impact FEL research.

Hadrontherapy utilizes accelerated ion beams to deliver precise and effective cancer treatments, depositing energy at specific depths while sparing healthy tissues. Gantries are large rotating structures that direct beams at various angles around the patient, improving dose conformity to tumors and the robustness of the treatment plan while minimizing radiation exposure to surrounding organs. While proton gantries are widespread and commercially available, the diffusion of carbon ion gantries is limited due to more demanding technological challenges and financial investments. This paper presents the magnetic design of the scanning magnets for the EuroSIG (European Superconducting Ion Gantry) project, aimed at developing a novel superconducting gantry for hadrontherapy. The chosen layout foresees the positioning of the scanning magnets downstream of the last bending section. This simplifies the production of the superconducting dipole but increases the complexity of the scanning system in terms of integrated fields, bending angles, and ramp rates. To address this challenge, analytical models and optimization tools were employed to achieve balanced configurations for the scanning magnets, improving compatibility with power converters. Three-dimensional numerical simulations were implemented to evaluate and improve the steady-state field quality of horizontal and vertical scanning magnets. Particle tracking was used to validate the effective beam deflection in both magnets and assess beam position errors at the isocenter. Furthermore, quasistatic simulations were carried out to evaluate integral field errors due to the hysteretic behavior of the yoke alloy. Continuing the analysis of dynamic phenomena, the generation of eddy currents in the laminated yoke was investigated, highlighting the delay of the generated field with respect to the imposed current profile. The studies reported in this manuscript confirmed the design’s feasibility, posing a solid base for the construction of the first scanning magnet prototype.

In current beam simulation software, there is no functionality specifically designed for the generation and tracking calculation of secondary particles produced during beam transport. These secondary particles are produced from collisions between the beam and background gas, as well as collisions with the electrodes. These processes can lead to beam particle loss, energy dissipation, electrode overheating, and device arcing issues. This paper addresses this challenge using Lua code based on simion. It enables the complete calculation of particle generation, transport, and loss processes. It overcomes four shortcomings of EAMCC and comsol, enabling it to simulate various beam types and processes beyond just the extraction of negative hydrogen ions. To demonstrate the application of this code, we investigated the neutral beam overdose issue in ion implanters, thermal deposition in large-scale ion sources, and arcing problems in electrostatic low-energy beam transport.

We present a new method for minimizing the effects of radiative depolarization in electron storage rings by use of a minimal number of special vertical orbit bumps. The bumps can be used to minimize the effects of radiative depolarization while simultaneously maintaining other common benefits of vertical orbits, e.g., transverse coupling and vertical dispersion control. Because simultaneously optimizing the large number of vertical correctors in a ring is operationally infeasible, we use dimensionality reduction to define a minimal number of the most effective groups of vertical correctors that can be optimized during operation, motivating the name “Best Adjustment Groups for ELectron Spin” (BAGELS). The method is streamlined by using suitable “basis bumps” instead of all individual vertical correctors. We define three types of basis bumps for different purposes: (i) generates no delocalized transverse coupling nor delocalized vertical dispersion, (ii) generates no delocalized vertical dispersion, and (iii) generates no delocalized transverse coupling. BAGELS has been essential in the design of the Electron Storage Ring (ESR) of the Electron-Ion Collider (EIC) and will be beneficial for any polarized electron ring, including FCC-ee. HERA and LEP would have likely benefitted as well. We use BAGELS to significantly increase polarization in the 18 GeV EIC-ESR, beyond what is achievable with conventional methods; in the 1-IP lattice, we nearly double the asymptotic polarization, and in the 2-IP lattice, we more than triple the asymptotic polarization. We also use BAGELS to construct knobs that can be used for global coupling correction, and knobs that generate vertical emittance for beam size matching, all while having minimal impacts on the polarization and orbit/optics.

A critical technology for high-volume manufacturing of nanoscale integrated circuits is a high-power extreme ultraviolet (EUV) light source. Over the past decades, laser-produced plasma (LPP) sources have been actively utilized in this field. However, current LPP light sources may provide insufficient average power to enable future manufacturing at the 3 nm node and below. In this context, accelerator-based light sources are considered to be promising tools for EUV lithography. This paper proposes a regenerative amplifier free-electron laser EUV source with harmonic lasing, driven by a superconducting energy-recovery linac (ERL). By utilizing the $n$th harmonic, the required electron beam energy is reduced to $1/\sqrt{n}$ of that in conventional schemes. The proposed configuration, employing an electron beam energy of approximately 0.33 GeV with a short-period (16 mm) undulator, is estimated to provide an average EUV power of about 2 kW. This approach significantly reduces the required electron energy and facility size relative to other accelerator-based proposals, thereby offering new possibilities for constructing high-power EUV sources with low-energy ERLs.

The concept of the coaxial cascade-line rectangular-pulse generator is put forward, the basic idea of which is to cascade two coaxial lines of equal impedance together and make one line turn around to let the two lines use a common middle tube. The obvious feature of this kind of generator is that it can form a rectangular pulse with a width of 4 times the electrical length of one line. Compared with the conventional single-tube generator, the total stored energy is doubled. Namely, the output pulse width of the coaxial cascade-line generator is doubled for the same output voltage, which is beneficial for miniaturization. The principle of the cascade pulse forming line is first introduced; then the coaxial cascade-line rectangular-pulse generator is suggested. The simulation results prove the feasibility of the cascade-line generators. Finally, the design of a Tesla-type generator with an output power of 20 GW and a pulse width of 40 ns is presented based on the coaxial cascade-line method.

Optimizing the injection process in particle accelerators is crucial for enhancing beam quality and operational efficiency. This paper presents a framework for utilizing reinforcement learning (RL) to optimize the injection process at accelerator facilities. By framing the optimization challenge as an RL problem, we developed an agent capable of dynamically aligning the beam’s transverse space with desired targets. Our methodology leverages the soft actor-critic algorithm, enhanced with domain randomization and dense neural networks, to train the agent in simulated environments with varying dynamics promoting it to learn a generalized robust policy. The agent was evaluated in live runs at the cooler synchrotron COSY and it has successfully optimized the beam cross section reaching human operator level but in notably less time. An empirical study further validated the importance of each architecture component in achieving a robust and generalized optimization strategy. The results demonstrate the potential of RL in automating and improving optimization tasks at particle acceleration facilities.

Efficient steering of particle beams in accelerators by means of bent crystals has been pursued over the last decades. Recent achievements in their operational deployment at CERN has resulted in a renewed interest in their exploitation for advanced accelerator and high energy physics concepts. Efforts to date have been focused on establishing planar channeling, given the intrinsic complexity of the process. However, the know-how developed with operation of bent crystals at the Large Hadron Collider has allowed this to be pushed a step further, demonstrating that axial channeling can also be used in the multi-TeV regime. Present results were achieved through a combination of mechanical crystal rotation and local orbit bump, proving that incorporating an additional degree of freedom in the design of future goniometers could enable the operational deployment of axial channeling, opening up the possibility for new and more efficient particle steering applications with bent crystals.

Laser-driven (LD) ion acceleration has been explored in a newly constructed short focal length laser beamline at the BELLA petawatt facility (interaction point 2, iP2). For applications utilizing such LD ion beams, a beam transport system is required, which for reasons of compactness be ideally contained within 3 m. While they are generated from a micron-scale source, large divergence and energy spread of LD ion beams present a unique challenge to transporting them compared to beams from conventional accelerators. This study gives an overview of proposed compact transport designs using permanent magnets satisfying different requirements depending on the application for the iP2 laser beamline such as radiation biology, material science, and high-energy density science. These designs are optimized for different parameters such as energy spread and peak proton density according to the application’s need. The various designs consist solely of permanent magnet elements, which can provide high magnetic field gradients on a small footprint. While the field strengths are fixed, we have shown that the beam size is able to be tuned effectively by varying the placement of the magnets. The performance of each design was evaluated based on high-order particle tracking simulations of typical LD proton beams. We also examine the ability of certain configurations to tune and select beam energies, critical for specific applications. A more detailed investigation was carried out for a design to deliver 10 MeV LD accelerated ions for radiation biology applications. With these transport system designs, the iP2 laser beamline is ready to house various application experiments.

We report on the operation of the first high peak and average power single-pass terahertz free-electron laser (THz FEL) based on a high-brightness photoinjector for developing a THz source for pump-probe experiments at advanced x-ray FELs (XFELs). Electron beams with $17\mathrm{MeV}/\mathrm{c}$ momentum and charge up to 2.4 nC were used to generate narrow-band 3 THz radiation with pulse energies of more than 0.1 mJ. The novelty of the realized method is that the undulator radiation by electron beams is much longer than the radiation wavelength, with FEL starting from coherent beam contribution rather than shot noise. This proved to be not a simple scaling of the XFEL mechanism due to the significant role of the bunching factor in the considered case.

Plasma can support extremely high-gradient strong electromagnetic fields, making it suitable not only for laser acceleration but also for short-distance manipulation of the charged particle beams, thereby enabling the miniaturization of particle accelerators. Straight capillary discharge plasma lenses have previously garnered widespread attention from researchers for their excellent focusing capabilities. We introduced the theory of using a curved capillary discharge plasma channel to deflect charged particle beams [Phys. Rev. Accel. Beams 24, 031301 (2021)] and, in this paper, present the first experimental demonstration of plasma-based deflection and focusing of laser-accelerated proton beams. Combined with a dipole magnet, the emittance after transmission was measured, revealing that magnetic field nonlinearity and multiple Coulomb scattering tend to increase the proton beam’s emittance. This work is of great significance for the development of compact beam transport systems.

In recent years, the field of positron acceleration in plasma wakefield accelerators has witnessed rapid theoretical advancements, and several effective schemes have been proposed. In these studies, longitudinally uniform plasma is taken into account, with the positron beam located at the start of the second wakefield bubble. The electron filament near the positron beam provides Coulomb attraction force for the guiding of the positron beam. However, the influence of vacuum-plasma transition on positron beams has not been considered yet, where wakefield bubble is larger than that in the density plateau region, and the positron beam is in the defocusing region. In this paper, we study the evolution of positron beam size in the vacuum-plasma transition region and find out that the transition is detrimental to low-energy positron beams. Wide drive beams with relatively large transverse sizes are proposed as a possible solution to this difficulty.

A new design technique combining the improved Kyhl method with the improved Kroll method has been developed to efficiently match couplers for traveling-wave (TW) accelerating structures [Z. Huang et al., Phys. Rev. Accel. Beams 27, 082001 (2024)]. However, such a new technique requires some complicated modifications for the TW deflecting structure operating at HEM mode. This paper proposes a more efficient and universal approach for designing couplers in any TW structures including accelerating and deflecting structures. This approach is based on a new criterion $|R_c|\to 0$ and an equivalent circuit model to facilitate fast and accurate optimizations. By using this new approach, the optimizations on couplers for any TW structures with arbitrary starting dimensions can converge in just several iterations, significantly simplifying the overall design process while maintaining a high accuracy.