Dynamic Coulomb expansion of dense, nonuniform particle beamlets can lead to causticlike formations in the charge density profile of the resultant beam. Such features only occur for cold beams where thermal diffusion of the particles is minimized. These phenomena have been observed experimentally for select cases with ion beams but not yet for electron beams as the electron dynamics occur over far faster timescales. We simulate the interaction of electron bunches while varying the initial transverse temperature and density profiles to determine the thresholds that characterize this pattern formation. Furthermore, we extend the simulations to geometric arrangements that are more pertinent to the nanotip cathode application and consider the effects of asymmetries and the impact of a low-density halo on the overall process. The simulations are conducted using a previously documented high-fidelity algorithm for collisional particle dynamics. These studies identify the temperature and density thresholds required for caustic formation and provide guidance for optimizing the design of nanoscale structured cathodes. We also demonstrate that the correlation between the final density patterns and the underlying beam parameters enables a virtual diagnostic tool for measuring ultralow emittance beams solely based on a transverse charge density profile.

Radio frequency (rf) control loops are vital components for the optimal operation of rf systems in synchrotron particle accelerators. Modeling these systems in macroparticle tracking simulations provides improved understanding of the complex interaction between the control loops, the beam-induced voltages in the rf cavities, and the circulating beam. This paper details an implementation of local and global rf control loops in the beam longitudinal dynamics (blond) simulation suite, which allow for the coupling of the two systems in a macroparticle tracking code for the first time. First, benchmarks of the local control loops in the CERN Super Proton Synchrotron (SPS) and Large Hadron Collider (LHC) are given. The implementation coupling the local and global loops is then tested in the SPS with momentum shifts at constant magnetic field and through the damping of phase offsets at injection into the LHC. Both test cases show the expected behavior and validate the model in blond. Finally, the SPS simulation model, which now is able to combine beam-induced voltages, global control loops and local ones, is benchmarked against measurements at the bunch-to-bucket transfer into the real accelerator. The simulations, using the coupled implementation of the loops, are able to reproduce bunch-by-bunch beam parameters at flat bottom as well as uncaptured beam generated at injection.

A development effort to improve the performance of superconducting radio-frequency half-wave resonators (SRF HWRs) is underway at the Facility for Rare Isotope Beams (FRIB), where 220 such resonators are in operation. Our goal was to achieve an intrinsic quality factor ($Q_0$) of $\ge 2\times {10}^{10}$ at an accelerating gradient ($E_{\mathrm{acc}}$) of $12\mathrm{MV}/\mathrm{m}$. FRIB production resonators were prepared with buffered chemical polishing. First trials of electropolishing (EP) and post-EP low-temperature baking of FRIB HWRs allowed us to reach higher gradient (15 MV/m, limited by quench) with a higher quality factor at high gradient, but $Q_0$ was still below our goal. Trapped magnetic flux during the Dewar test was found to be a source of $Q_0$ reduction. Three strategies were used to reduce the trapped flux: (i) adding a local magnetic shield (LMGS) to supplement the “global” magnetic shield around the Dewar for reduction of the ambient magnetic field; (ii) performing a “uniform cooldown” (UC) to reduce the thermoelectric currents; and (iii) using a compensation coil to further reduce the ambient field with active field cancellation (AFC). The LMGS improved the $Q_0$, but not enough to reach our goal. With UC and AFC, we exceeded our goal, reaching $Q_0=2.8\times {10}^{10}$ at $E_{\mathrm{acc}}=12\mathrm{MV}/\mathrm{m}$.

Transverse beam profile monitoring in high-radiation areas is challenging due to camera degradation. A proposed solution employs a multimode fiber (MMF) to relay optical signals from the radiation zone to a shielded area, where a standard CMOS camera can operate safely. However, MMF transmission introduces significant distortions, producing complex speckle patterns at the output. We present a machine-learning–based method to reconstruct the transverse beam distributions from these patterns. The model was trained solely on synthetic data generated via stochastic Gaussian mixture simulations, in which the samples were displayed on a laser-illuminated digital micromirror device (DMD) and relayed through a 5-m MMF. The same setup was used for testing, with beam images on a scintillating screen from CERN’s CLEAR facility replayed on the DMD. The model achieved a 1.49% mean relative root mean square error across four key beam parameters—77% lower than the 6.59% baseline, demonstrating its potential for diagnostics in radiation-constrained environments.

The design of radio frequency (rf) cavities in particle accelerators constitutes a multiobjective optimization problem (MOOP) characterized by strict equality constraints and computationally expensive evaluations. Here, a multiobjective Bayesian optimization (MOBO) framework is applied to rf cavity design for the first time to efficiently identify high-quality Pareto-optimal solutions under limited simulation budgets. To maintain high sample efficiency of the MOBO framework in problems with extremely strict equality constraints, two constraint-handling strategies are investigated: a penalty-based scalar acquisition function (sAF) strategy and a novel two-stage acquisition strategy (tAS). The tAS divides the sampling process into two stages. In the first stage, a vectorized acquisition function (vAF) is combined with a multiobjective evolutionary algorithm (MOEA) to efficiently search for candidate points in the full high-dimensional objective space. In the second stage, a sAF is employed to select the most promising point from candidate points for actual evaluation. Results on a test problem and the design of a practical 499.65 MHz rf cavity show that tAS outperforms penalty-based sAF in handling equality constraints within the MOBO framework. Furthermore, by using the MOBO framework with tAS, high-quality Pareto-optimal solutions are obtained within 1000 simulations, demonstrating a significant advantage of efficiency over conventional rf cavity optimization algorithms. Additionally, tAS allows for the flexible adjustment of MOEA, vAF, and sAF configurations, thereby providing flexible adaptation to other MOOPs in particle accelerators.

We revisit the nonlinear lattice design approach for the National Synchrotron Light Source II (NSLS-II) storage ring. By suppressing chaos, we identify alternative sextupole configurations to the original design, which relied on the conventional strategy of simultaneously minimizing resonance driving terms (RDTs) and amplitude-dependent detuning (ADD). These alternatives achieve comparable performance while requiring fewer sextupoles. A detailed comparison of two representative solutions is presented and supported by experimental validation. Our results show that the dynamic aperture correlates more strongly with global chaos than with individual RDTs, and that the importance of minimizing ADD may have been overstated in earlier design strategies.

The rapid advancement of laser compression techniques has brought increasing attention to few-cycle laser-plasma interactions. Through comprehensive particle-in-cell simulations, we demonstrate that compressing laser pulses to sub-10-femtosecond durations significantly enhances transient electrostatic shocks. Crucially, the plasma dynamics are strongly modulated by the laser carrier-envelope phase (${\mathrm{\Phi }}_{\text{CEP}}$). Our results reveal a pronounced cosinelike dependence of the proton cutoff energy on ${\mathrm{\Phi }}_{\text{CEP}}$, with certain values of ${\mathrm{\Phi }}_{\text{CEP}}$ yielding superior performance due to enhanced electron displacement and stronger charge-separation fields. Phase-space analysis uncovers expanding “source”-like electron orbits, whose gyration radii correlate with ${\mathrm{\Phi }}_{\text{CEP}}$-dependent energy transfer. We further develop an analytical model connecting the electrostatic shock strength, inferred from the proton energy, to the laser-plasma interaction parameters. This model is validated through extensive parameter scans over target density, thickness, laser intensity, and ${\mathrm{\Phi }}_{\text{CEP}}$. Our findings establish a predictive framework for ${\mathrm{\Phi }}_{\text{CEP}}$-controlled plasma dynamics in short-pulse laser interactions, providing insight into mechanisms relevant to laboratory high-energy proton sources and origin of energetic cosmic rays in astrophysical environments.

Pursuing on-chip particle accelerators has long been a focal point for researchers, and the inverse Smith-Purcell dielectric laser accelerator holds great promise as a viable option. However, achieving the cascade acceleration and focusing of subrelativistic electrons on a chip remains one of the most formidable challenges. In this paper, we propose a dynamic phase method to address this issue. This method takes advantage of the phase slippage between the electron and the acceleration wave, enabling transverse alternate focusing, defocusing, and continuous acceleration. Simulation results show that a 29.50 keV electron beam with a 20 eV spread can achieve an acceleration gradient of $39\mathrm{MV}/\mathrm{m}$ with a captured efficiency of 95% within a 120-μm distance in a 200-nm-wide channel. Compared with the alternating phase method, this approach streamlines the fabrication process by removing the need for incremental adjustments of the phase velocity at each accelerating stage to match the electron energy. We anticipate that applying the dynamic phase-driven approach will significantly propel the development of on-chip accelerators.

In this study, we employ hydroforming technology to develop a 1.3 GHz one-cell $\mathrm{Nb}/\mathrm{Cu}$ full-seamless cavity to reduce cost and improve performance. Superconducting radiofrequency (SRF) cavities made of pure niobium are expensive because of the material’s high cost and the complex manufacturing process, which involves electron beam welding. To overcome these challenges, extensive research has been conducted on using copper substrates coated with niobium, leveraging hydroforming to eliminate welding and seams, thereby reducing cost and simplifying production. The hydroforming process involves expanding oxygen-free copper tubes in two steps, followed by vacuum annealing to recover elongation. This method exhibited high productivity, with forming times of 51 and 78 s for the two steps. However, the process introduced surface roughness, compromising optimal rf performance. Following hydroforming, the cavities were coated with niobium via high-power impulse magnetron sputtering. This coating method ensured excellent film adhesion and can withstand high-pressure water rinsing. We conducted rf measurements of the cavities, achieving an accelerating gradient of $12\mathrm{MV}/\mathrm{m}$ at 4.2 K. The quality factor was slightly below requirements, attributed to surface roughness from the hydroforming process. The maximum accelerating gradient reached $15.7\mathrm{MV}/\mathrm{m}$ at 1.85 K with no field emission. We demonstrate the advantages of full-seamless cavities, including reduced manufacturing costs, high reproducibility, and the elimination of welding seams that can introduce defects. However, challenges such as surface roughness and its impact on the rf performance must still be addressed. This study makes a significant contribution to the development of cost-effective, high-performance SRF cavities for coating studies.

We report the first direct experimental observations of resonance island formation in a hybrid multibend achromat (HMBA) storage ring lattice. Using a low-emittance electron beam, we achieved high-precision measurements that resolve the onset, evolution, and transient dynamics of particle capture near resonance islands. These measurements are compared with simulation predictions, enabling a direct validation of HMBA lattice models. The experimental results also provide new insight into the capture and trapping mechanisms governing the island population, as well as beam lifetime in resonance islands. Together, these findings establish a detailed experimental basis for understanding nonlinear dynamics in next-generation HMBA-based synchrotron light sources.

Electron storage rings based on multibend-achromat (MBA) lattices can achieve exceptionally low natural emittances. At the Paul Scherrer Institute in Switzerland, the Swiss Light Source facility underwent an upgrade project, replacing its triple-bend achromat ring with a seven-bend achromat ring. Due to the compact nature of MBA lattices, magnetic fields can be influenced by neighboring magnets. These magnetic crosstalk effects turned out to be significant during the design phase: the integral fields of the magnets could be altered by several percent. We present how we incorporated compensations for these effects into the design and how we tuned the magnets prior to installation. The crosstalk compensations were validated through the accelerator commissioning.

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.