Radio frequency knock out resonant slow extraction is a standard method for extracting stored particle beams from synchrotrons by transverse excitation. Excitation signals comprising many betatron sidebands have been shown to reduce intensity fluctuations of the extracted beam spill and are used at several facilities. In this contribution, the effect of individual sidebands at different harmonics on the spill quality is systematically studied using efficient noise excitation signals fully covering the respective sidebands. Particle tracking simulations show a clear correlation between the chosen excitation frequency and the spill quality. This relation is attributed to the spectrum of particle motion shortly before their extraction, and an empirical relation between the spectrum, the excitation frequency, and the resulting spill quality is given. Experiments show an additional reduction of pileup when using higher excitation frequencies. These insights have implications for an optimal design of excitation waveforms and hardware for knock out extraction systems.

Electron beam ion source (EBIS) is capable of generating highly charged ions by ionizing neutral gas atoms using a high-energy electron beam. The generated ions are confined axially and radially by electric and magnetic fields within the breeding region until they are extracted by lowering the axial potential barrier. However, in various EBIS devices, significant ion heating has been observed, which can lead to stagnation in the ionization process, particularly for heavier ions. This stagnation imposes a practical limit on the maximum achievable charge state. To investigate plasma heating in an EBIS, the role of plasma instabilities induced by a Gaussian electron beam in a magnetically confined plasma is examined in detail. The resulting growth rates and dispersion relations confirm the formation of instabilities in the EBIS devices, which are dominant in the form of lower hybrid waves and may be responsible for the limited performance of EBIS in terms of generating fully stripped or highly charged ions. The resulting instability frequencies from the theoretical calculations lie close to ion plasma frequency, i.e., in the MHz range.

A klystron is a microwave vacuum electronic device that converts the kinetic energy of an electron beam into microwave signal energy based on the principle of velocity modulation. Owing to its high peak power, high efficiency, and high gain, the klystron has been widely used in scientific research, national defense, and industrial applications. The interaction between the electron beam and the radio-frequency (rf) field within the resonant cavities, known as the beam-wave interaction, is the core physical process governing klystron operation. A detailed understanding of this process, together with accurate numerical simulation using computational solvers, is essential for optimizing device performance and shortening the development cycle. This paper reviews the large-signal beam-wave interaction solvers commonly used in klystron design and development in mainland China, along with several internationally recognized simulation codes. The discussed solvers cover a range of computational dimensions from one-dimensional (1D) to three-dimensional (3D). Finally, the challenges and future prospects for the development of klystron beam-wave interaction solvers are discussed.

High-flux, high-energy muon beams hold transformative potential for particle physics, yet rapid acceleration to relativistic energies in plasma wakefields is limited by dephasing arising from their larger mass and lower velocity. Here, we propose a method for longitudinal injection and stable acceleration of low-energy muons using a plasma density up-ramp to separate the deceleration and acceleration regions. The nonlinear wakefield configuration required for muon injection was obtained analytically using Hamiltonian dynamics, and particle-in-cell (PIC) simulations showed that the initial energy and emittance conditions of injectable muons can be tuned. Driven by an ultraintense 23 GeV electron beam, muons with an initial energy of 300 MeV attain a final energy of 40 GeV, with an 18.85% energy spread and a normalized emittance of 8.86 mm mrad. These findings provide a feasible pathway toward the development of compact muon colliders, with potential to advance particle physics and enable the exploration of phenomena beyond the Standard Model.

The pepper-pot method is a widely used technique, originally proposed for measuring the emittance of space-charge-dominated electron beams from radio-frequency photoinjectors. With recent advances in producing high-brightness electron beams via laser-wakefield acceleration (LWFA), the method has also been applied to evaluate emittance in this new regime. Here, the limitations of this method for measuring the emittance of LWFA electron beams are investigated, particularly in parameter regimes where the true beam emittance is overestimated. Conducting an experiment at the JETi200 laser system, we measured an upper bound for the geometric beam emittance of $(26.2\pm 7.3)\mathrm{\mu }\mathrm{m}\text{mrad}$ using the pepper-pot method. This result is consistent with geant4 Monte Carlo simulations of the pepper-pot diagnostic, with an input beam–emittance parameter that matches both PIC simulations of the laser–plasma accelerator and an independent measurement using the transient optical grating method.

Normal-conducting radio frequency cavities are essential in particle accelerators, but their operational stability is often compromised by thermal-induced resonant frequency detuning during high-power operation. This paper introduces a digital low-level radio frequency system that performs autonomous frequency tracking and compensation in a standalone configuration. Specifically, during powering up and rf conditioning, the tracking operates without dependence on external instrumentation. The system employs a vector modulator driven by two orthogonal analog sinusoids of equal amplitude to modulate the reference signal, achieving an adjustable output bandwidth of $\pm 1.56\mathrm{MHz}$ with a precision of 95.3 Hz entirely within the LLRF framework. A phase-locked loop controller embedded in the system dynamically synchronizes the rf frequency to the cavity resonance, while a real-time spectrum analyzer implemented in the FPGA monitors frequency deviations in a closed-loop manner. Experimental results verify that the system can resolve spectral components within a 1.56-MHz bandwidth and generate arbitrary-frequency rf signals over a 3-MHz span using only its integrated LLRF resources. In high-power tests, the embedded PLL-based tracking maintained the reflected power at 8.5% of the incident power under thermal detuning exceeding 200 kHz, thereby ensuring operational stability through a unified LLRF-based approach.

Bayesian optimization (BO) has recently emerged as a powerful approach for on-line beam tuning, and it is rapidly gaining adoption across accelerator facilities due to its flexibility and efficiency in handling complex optimization tasks. At the Facility for Rare Isotope Beams, rapid and reliable tuning is essential to support the delivery of diverse ion species. To improve the practicality of BO in this setting, we implemented several enhancements, including scalarized composite objective construction for multicriteria optimization, asynchronous evaluation for better resource utilization, prior-mean-assisted optimization to accelerate convergence, and a local search strategy for rapid completion of the task. We present the details of these methods, discuss challenges-encountered, and share our experience applying them to specific beam-tuning tasks.

The electron-ion collider (EIC), to be constructed at Brookhaven National Laboratory, will collide polarized high-energy electron beams with hadron beams, achieving luminosities of up to $1.0\times {10}^{34}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$ in the center-of-mass energy range of 20–140 GeV. To reach such high luminosity, the EIC will employ small, flat beams at the interaction point. According to the design of the EIC hadron storage ring (HSR), hadron beams with a large transverse emittance ratio of 11:1 will be generated at the injection energy using an electron cooling technique and then accelerated to high energies for collisions. Accelerating hadron beams with such a large emittance ratio had never been demonstrated elsewhere—until our recent beam experiment at the relativistic heavy ion collider (RHIC). In this experiment, we successfully generated a large transverse emittance ratio of 13:1 with a gold-ion beam at 31 GeV/nucleon using stochastic cooling. We then accelerated this beam, with a transverse emittance ratio of 11:1, from 31 to 100 GeV/nucleon. Thanks to RHIC’s high-performance orbit, tune, and decoupling feedback systems, the large emittance ratio was well maintained throughout the 5-min-long acceleration process. This experiment fully validated the EIC/HSR design assumptions—namely, that large-emittance-ratio hadron beams can be generated at injection energy and then accelerated to high energies for collisions.

We show that the emittance of fourth-generation 6 GeV machines such as PETRA IV is close to what is theoretically achievable due to beam intensity limitations from space charge and intrabeam scattering. By investigating these limitations, in particular their scaling with the bare lattice emittance and the beam energy, we argue that achieving further significant emittance reduction is only possible by increasing the beam energy. We show further that electron energy increase is also the most efficient path toward higher radiation brightness.

In recent electron-positron colliders designed and operated with very low emittance and high current, the energy density of the beam has increased significantly compared to earlier designs. Under these conditions, interactions between the beam and residual materials within the beam pipe exert considerable mutual influence. The residual materials are heated by the beam, leading to evaporation, charging, and possible fission, eventually resulting in a plasma state. Conversely, the beam undergoes emittance growth due to electromagnetic interactions with the resulting plasma. We investigate the effects of such dust-induced plasma on the beam through numerical simulations of these processes.

In the context of the HITRIplus project and EuroSIG international collaboration, a carbon-ion gantry is being designed for possible installation at the National Center for Oncological Hadrontherapy (CNAO), Pavia, Italy. The design is based on superconducting (SC) magnet technology for the dipoles, in order to reduce gantry dimensions and weight. Unlike normal-conducting magnets, SC magnets are sensitive to local beam losses, which can lead to local energy deposition and potential quenches, thus inducing considerable gantry downtime. This contribution presents exploratory studies of energy deposition in the SC dipole magnets currently foreseen for the gantry. The studies are carried out by means of Monte Carlo simulations, using the fluka code to characterize ruling factors such as beam spot size and impact angle. In this exploratory phase, simulations were carried out assuming simplified beam impact conditions.

The $\overline{\mathrm{P}}\mathrm{ANDA}$ experiment at the High Energy Storage Ring (HESR) of the Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, will be a unique setup to study the strong interaction in antiproton-proton collisions. The combination of a windowless cluster-jet target and a high-intensity antiproton beam, cooled by stochastic cooling down to momentum spreads of $4\times {10}^{-5}$ in the presence of the target, is ideal to study, e.g., the line shape of exotic candidates in the charmonium spectrum, such as the ${\chi }_{c1}(3872)$. For first studies on the performance of the cluster-jet target and the stochastic cooling, both were installed at the COoler SYnchrotron (COSY) at the Forschungszentrum Jülich, Germany. Detailed studies on the beam quality for different target settings with different densities were performed and will be presented in this paper. The results show that the cluster-jet target is able to provide the required hydrogen target thickness of more than $2\times {10}^{15}{\mathrm{atoms}/\mathrm{cm}}^2$ for the $\overline{\mathrm{P}}\mathrm{ANDA}$ experiment and that the stochastic cooling is able to efficiently cool the beam in presence of such a target thickness, although the system is not yet complete compared to the setup foreseen for the HESR. The presented results strengthen the theoretical understanding of the beam-target interaction and stochastic cooling performance and will provide important input for operation later at the HESR.

The nonultrarelativistic wake function theory is not only applicable to proton or heavy ion accelerators, but with the emergence of new accelerator physics, it has been found to be equally applicable to electron accelerators such as the steady-state microbunching (SSMB) storage rings. However, existing results are still far from practical applications. In this paper, building on previous work, we further generalize the previous point source model into a more universal ring model. When considering the nonultrarelativistic limit, we discover a completely new lowest-order transverse wakefield and estimate its intensity to be comparable to the dipole field under certain circumstances. Moreover, by separating the space charge component, we propose a numerical algorithm that can compute the resistive wall field more efficiently. Finally, we calculate the parasitic loss and kick angle using SSMB parameters as simple applications.

The performance of x-ray free-electron lasers (XFELs) strongly depends on the longitudinal emittance of the electron beam. We show that by reducing the injector drive laser pulse duration, the electron beam peak current can be increased by a factor of 2 or more, resulting in a significant improvement in longitudinal emittance. Comprehensive simulations and measurements carried out at SwissFEL confirm the expected trade-off of moderately higher transverse emittance. This simple approach can be readily adapted to other XFEL facilities.

We analyze beam-induced depolarizing effects in the hydrogen jet target (HJET) at the relativistic heavy ion collider (RHIC), which has been used for absolute hadron beam polarimetry and shall be employed at the electron-ion collider (EIC). The EIC’s higher bunch repetition frequencies and shorter bunch durations shift beam harmonics to frequencies that can resonantly drive hyperfine transitions in hydrogen, threatening to depolarize the target atoms. Using frequency-domain analysis of beam harmonics and hyperfine transition frequencies, we establish a photon emission threshold above which beam-induced fields are too weak to cause significant depolarization. For EIC injection (23.5 GeV) and flattop (275 GeV), beam-induced depolarization through the bunch structure renders operation at the current RHIC magnetic guide field at the target ($B_0=120\mathrm{mT}$) untenable. Increasing the magnetic guide field at the target to $B_0\approx 400\mathrm{mT}$ moves all hyperfine transition frequencies to at least 3 times the cutoff frequency, ensuring reliable absolute beam polarimetry with the required 1% precision at the EIC.

In this paper, we present a design of a high-efficiency high-gain free-electron laser oscillator based on the use of a strongly tapered undulator for extracting energy from high-brightness electron beams. We provide an analytical model of the setup followed by numerical simulations for lasing at the wavelength of 13.5 nm. We discuss the optimization of the system in steady state and the conditions necessary for the pass-per-pass buildup of the power from shot noise level. We propose the use of fast phase shifters as a way to accelerate the buildup. Finally, we present time-dependent simulations of the oscillator and discuss the role of spectral filtering. The optimized working point yields a total energy conversion efficiency from the electron beam to output radiation above 1% at the wavelength of 13.5 nm.

The dynamic vacuum effect is the primary limitation of beam intensity in high-intensity heavy-ion synchrotrons. Although the dynamic vacuum effect in the arc section is mitigated effectively by the beam collimators, the gas desorption induced by injection beam loss occurring in the injection electrostatic septum (ESi) is still an unresolved issue. In addition, the gas desorption is enhanced due to some special mechanisms, including the ultrafast beam loss process, multiple traversals of wire surfaces, and high-voltage breakdown. Mitigating the dynamic vacuum effect in the ESi is a critical approach to achieve a higher beam intensity. Therefore, a novel ES collimator is proposed to reduce beam deposition on the septum. Dynamic simulation of the dual-plane painting injection of BRing reveals the excellent performance of this scheme by a tenfold reduction in beam loss. The features associated with collimation efficiency and structure optimization are analyzed in detail.

Automatic differentiation is a powerful tool for computing derivatives of simulation results with respect to given parameters. In this Letter, we have applied this tool to investigate the instability of a dynamical system governed by 21 ordinary differential equations. This second-order instability (named envelope instability) is driven by space-charge effects and has a significant impact on the operational regimes of particle accelerators. Our study delves into the three-dimensional envelope instability, incorporating both transverse and longitudinal coupling. Conventionally, analyzing this complex system would necessitate solving 441 ordinary differential equations, which is computationally intractable. However, by employing automatic differentiation, we were able to track only 21 equations. This approach allowed us to uncover an additional instability stopband, which arises from space-charge-induced coupling and has not been reported in previous studies. This research highlights the significant advantages of automatic differentiation in analyzing complicated dynamical systems involving a large number of ordinary differential equations.

Integrating a high-power microwave source with an accelerating structure into a single device is a key strategy for developing compact, even portable, high-energy particle accelerators, which hold significant promise for scientific, medical, and industrial applications. To achieve this objective, we propose integrating an X-band coaxial transit-time oscillator (CTTO) microwave source with an X-band accelerating structure, thereby drastically minimizing the total volume of particle accelerators. Leveraging the CTTO’s advantages, low operating voltage, stable frequency, high power, and high efficiency, along with the high acceleration gradient of X-band accelerators, our design demonstrates that a CTTO operating at 50 kV (generating 7.8 MW of radio-frequency power at 9.3 GHz) can accelerate an electron beam from 50 keV to 10.96 MeV over a 320 mm length.