Laser wakefield acceleration (LWFA) may enable the next generation of TeV-scale lepton colliders. Reaching such energies will likely require multiple LWFA stages to overcome limitations on the energy gain achievable in a single stage. The use of stages, however, introduces challenges such as alignment, adiabatic matching between stages, and a lower average accelerating gradient. Here, we propose a discrete flying focus that can deliver higher energy gain in a single stage, thereby reducing the number of stages required for a target energy. A sequence of laser pulses with staggered focal points and delays drives a plasma wave in which an electron beam experiences a near-constant accelerating gradient over distances beyond those attainable with a conventional pulse. Simulations demonstrate that a discrete flying focus with a total energy of 150 J can transfer 40 GeV per electron to a 50-pC beam in a single 30-cm stage, corresponding to 50 dephasing lengths.

Laser-plasma accelerators (LPAs) enable compact, bright x-ray sources, but their practical application demands a significant reduction of beam instabilities that originate from drive laser fluctuations. Moreover, the complexity of the laser-plasma interaction makes it difficult to disentangle and quantify the impact of individual parameters on machine performance. To address this challenge, we develop a data-driven modeling strategy and apply it to an extensive dataset collected during a daylong operation of the LPA-based x-ray source LUX. By making use of an orthogonal-distance-based training objective, our approach reduces bias originating from measurement errors, which allows us to estimate the functional dependencies between laser, accelerated electrons, and generated undulator radiation. In addition, we demonstrate accurate prediction of the x-ray spectrum based on noninvasive measurements, showcasing the potential of our approach for building virtual diagnostics.

The loss of Landau damping (LLD) can lead to beam quality degradation in single- and multi-bunch beams in synchrotrons. In the longitudinal plane, the LLD threshold is increased by reducing the beam-coupling impedance, applying controlled emittance blowup, or employing higher-harmonic rf systems. This work focused on the significant threshold modifications when moving from the single- to the double-harmonic rf system case, depending on the relative phase, the rf voltages, and harmonic ratios. Moreover, the impact of the impedance cutoff frequency is discussed. Special attention is given to the configuration where both rf systems are in phase bunch shorting mode (BSM) or in counterphase bunch lengthening mode at the bunch position. Although in BSM, an approximate analytical expression can still be derived, numerical solutions of the linearized Vlasov equation are required in most cases. For the first time, we demonstrate that nonmonotonic amplitude dependence of the synchrotron frequency does not always lead to a vanishing LLD threshold. In BSM, this nonmonotonicity also results in counterintuitive beam evolution after a phase perturbation. The theoretical and semianalytical predictions are validated by an unambiguous beam measurement technique in the Proton Synchrotron and Super Proton Synchrotron at CERN and supported by macroparticle tracking simulations.

Space charge compensation (SCC) plays a crucial and foundational role in the transport of low-energy ion beams through residual gas, as it effectively mitigates beam divergence caused by space charge forces. At high residual gas pressures, the space charge compensation degree (SCCD) can exceed 100%, a situation referred to as overcompensation. In this study, a model for secondary particle production and tracking has been integrated into a beam optics calculation code to investigate the overcompensation process during the transport of a negative ion beam through residual hydrogen. The impact of secondary electrons, often overlooked, is examined in detail. The findings indicate that electrons predominantly accumulate in the overcompensation region, effectively mitigating overcompensation by leveling the potential and reducing the SCCD. Based on the electron growth rate, the accumulation process is categorized into three stages. Results pertaining to beam parameters reveal that electrons can affect the growth of beam emittance by altering the transverse potential. This research contributes to enhancing our understanding of SCC processes in the low-energy beam transport domain and improving the precision of future beam optics simulations.

The space-charge tune shift is a critical issue in circular proton, heavy-ion, and also low energy electron storage accelerators. Recently, some special electron rings with energies of several hundred MeV, nanometer-scale bunch lengths, and micrometer-scale bunch spacings have been proposed. In these novel rings, a significant longitudinal space-charge tune shift could be predicted by conventional methods. However, these methods may not be suitable when the bunches are extremely short. To address this issue, we derive a new set of formulas to calculate the longitudinal space-charge tune shift, which is applicable to arbitrary bunch lengths. These formulas incorporate the multibunch effects on the longitudinal space-charge tune shift, which have not been included in previous analyses of longitudinal space-charge tune shift calculations. The calculation results indicate that the tune shift predicted by our formulas is noticeably smaller than that obtained using the classical method. Moreover, our analysis demonstrates that the multibunch effect is not dominant in the longitudinal direction, even when the bunch spacing is extremely small.

Symplecticity of transfer maps is important for reliable evaluation of space-charge dominated beams in accelerators. Unfortunately, most simulation codes that include collective effects, such as space charge, do not use canonical phase-space variables and therefore are not symplectic in the presence of electromagnetic fields. In this paper, we present a numerical method to extract local linear symplectic transfer maps using particle tracking simulation code impact-t for space-charge dominated beams. We demonstrate this method for the photoinjector (113 MHz SRF gun) section of the Coherent electron Cooling (CeC) Proof of Principle (POP) experiment.

The continuous wavelength tunability of free-electron lasers (FELs) offers significant potential for research across various scientific fields. The Dalian Coherent Light Source (DCLS), when operating in the high-gain harmonic generation (HGHG) mode, has demonstrated wavelength tuning capability within the vacuum ultraviolet (VUV) range of 50–150 nm. To address diverse experimental demands, it is imperative to have the capability of fast wavelength switching and scanning. This study presents three wavelength tuning strategies experimentally implemented at DCLS, each characterized by specific tuning ranges and resolutions: (i) full-range continuous wavelength switching over 50–150 nm, typically requiring more than 2 h; (ii) wide-range coarse wavelength scanning with approximately 20% variation within several minutes; and (iii) narrow-range fine wavelength scanning with a range of about 0.44% and a scanning resolution of 0.01% in less than 1 min. Each strategy has its own advantages and limitations, and their combination significantly enhances the flexibility of wavelength manipulation at DCLS, accommodating diverse user requirements.

We present the detection of directional muon beams produced using a PW laser facility at the Lawrence Berkeley National Laboratory. The muon source is a multi-GeV electron beam generated in a $30\mathrm{cm}$ laser-plasma accelerator interacting with a high-$Z$ converter target. The GeV photons resulting from the interaction are converted into a high-flux, directional muon beam via pair production. By employing scintillators to capture delayed events, we were able to identify the produced muons and characterize the source. Using theoretical knowledge of the muon production process combined with simulations that are in excellent agreement with the experiments, we demonstrate that laser-plasma accelerators have the capability of generating electron beams with characteristics suitable to produce GeV-scale muons that offer unique advantages with respect to the cosmic background. Laser-plasma-accelerator-based muon sources can therefore enhance muon imaging applications thanks to their compactness, directionality, and high yields, which reduce the exposure time by orders of magnitude compared to cosmic ray muons. Using the geant4-based simulation code we developed to gain insight into the experimental results, we can design future experiments and applications based on LPA-generated muons.

This paper develops a homogenized harmonic balance finite element method (HomHBFEM) to predict the dynamic behavior of magnets with fast excitation cycles, including eddy current and skin effects. A homogenization technique for laminated yokes avoids resolving the individual laminates and the skin depth in the finite element (FE) mesh. Instead, the yoke is represented by a bulk surrogate material with frequency-dependent parameters. The ferromagnetic saturation of the yoke at higher excitation currents is tackled by a harmonic balance method, which accounts for a coupled set of frequency components. Thereby, a computationally expensive time stepping of the eddy-current field problem and a convolution of the homogenized yoke model are avoided. The HomHBFEM enables, for the first time, nonlinear simulations of fast corrector magnets, which are embedded in a fast orbit feedback system to counteract orbit disturbances over a broad frequency spectrum, and thus guarantee stable light-source operation. The results show the impact of the nonlinearity on the phase lag and the field attenuation, as well as the eddy current losses at frequencies up to several tens of kilohertz. The numerical validation for a C-dipole magnet example shows that the HomHBFEM achieves a sufficient accuracy at an affordable computational effort, with simulation times of a few hours. In comparison, standard three-dimensional transient FE simulations need to resolve the lamination thickness and the skin depth in space and the largest relevant frequency in time, which leads to a 2 to 3 orders of magnitude larger mesh and prohibitive computational effort, with simulation times of a few weeks on a contemporary computer server.

Beam halo is one of the most crucial issues limiting the machine performance and causing radioactivation in high-intensity accelerators. Understanding halo formation is vital for high-luminosity colliders, advanced light sources, and spallation neutron sources to effectively mitigate beam loss through machine optimizations and tailored collimation systems. This article details numerical and experimental investigations of transverse and longitudinal halos at the KEK Accelerator Test Facility. Halo distributions are modeled incorporating operational beam parameters, synchrotron radiation, beam-gas scattering, and Touschek-scattering processes. A newly developed yttrium aluminum garnet/optical transition radiation monitor, positioned at the extraction line, enables direct imaging of transverse and longitudinal halo distributions. The general consistency between predictions and observations in various conditions indicates that the Touschek scattering is the dominant mechanism forming the horizontal and momentum halos.

The plasma photocathode has previously been proposed as a source of ultrahigh-brightness electron bunches within plasma accelerators. Here, the scheme is extended by using a radially-polarized ionizing laser pulse to generate high-charge, high-brightness electron bunches with symmetric transverse emittance. Efficient start-to-end modeling of the scheme, from ionization and trapping until drive bunch depletion, enables a multiobjective Bayesian optimization routine to be performed to understand the performance of the radially-polarized plasma photocathode, quantify the stability of the scheme, and explore the fundamental relation between the witness bunch charge and its emittance. Comparison of plasma photocathodes driven by radially- and linearly-polarized laser pulses shows that the former yields higher-brightness electron bunches when operating in the optimally-loaded regime.

For fourth-generation synchrotron light sources, the triple rf system has been proposed to achieve further bunch lengthening compared to the commonly used double rf system, as well as to meet specific requirements for longitudinal injection. In a previous study for the double rf system [T. He et al., Periodic transient beam loading effect with passive harmonic cavities in electron storage rings, Phys. Rev. Accel. Beams 25, 024401 (2022)], it was shown that the periodic transient beam loading (PTBL) effect, also referred to as mode 1 instability, could limit the maximum bunch lengthening. Intuitively, in the triple rf system, the harmonic cavity (HC) operating at a higher harmonic of the fundamental rf frequency is expected to help mitigate the mode 1 instability driven by the lower-order HC, as the two HCs are tuned in opposite directions. However, contrary to this intuition, both tracking simulations and semianalytical calculations presented in this paper show that the two HCs of the triple rf system together actually strengthen the PTBL effect. To better understand this unexpected behavior, we introduce a novel concept of steady-state response matrix of cavity voltage, which simplifies the semianalytical algorithm and directly links cavity voltage perturbations to the complex bunch form factor perturbations. Additionally, we propose a modified semianalytical method that efficiently and accurately determines the PTBL threshold for the triple rf system. Using the parameters of the Hefei Advanced Light Facility storage ring as a case study, we apply this modified method to explore the relationship between the threshold $R/Q$ values of the two HCs under different bunch lengthening scenarios. All results are well validated through tracking simulations. Our findings show that the $R/Q$ values of the two HCs must be sufficiently low to avoid the PTBL effect under the desired optimum bunch lengthening condition. This result provides valuable guidance for the design of the triple rf system.

In a superconducting linear accelerators (linacs), accurately monitoring beam dynamics is essential for minimizing beam losses and ensuring stable operations. However, destructive diagnostics must be avoided in superconducting sections to prevent the occurrence of particulates and outgassing, rendering direct measurements of the beam envelope particularly challenging. This study presents a nondestructive method that uses beam position monitors (BPMs) to estimate the transverse beam envelope based on measurements of the quadrupole moment of the beam distribution. Although this concept was originally proposed in the 1980s, its application, especially to hadron beams, has been limited because of low signal sensitivity and the accuracy constraints associated with conventional BPM geometries. To overcome these challenges, we employed $\cos 2\theta$-type BPMs, which offer improved sensitivity to quadrupole components and are well suited for low-$\beta$ heavy ion beams. This method was applied to the heavy ion beams in the superconducting RIKEN linac (SRILAC), for which data from eight BPMs were combined with transfer matrix calculations and supplemental wire scanner data. The resulting beam envelope estimates exhibited good agreement with conventional quadrupole scan results, demonstrating the feasibility of this technique for routine, nondestructive beam monitoring in superconducting accelerator sections.

High-repetition-rate free-electron laser (FEL) facilities such as LCLS-II rely on continuous-wave (cw) injectors to deliver high-quality electron beams to multiple undulator lines simultaneously. Ensuring injector reliability under nonideal conditions is critical for sustained user operations. In this work, we investigate the fault tolerance of a representative cw photoinjector by analyzing its performance under various degraded scenarios, including reduced gun energy, radio-frequency (rf) cavity failures, solenoid field constraints, and early-stage energy gain requirements. Using multiobjective optimization with a genetic algorithm framework (Xopt), we assess achievable beam quality—focusing on transverse emittance and bunch length—across a range of fault conditions. Our results reveal that while certain failures (e.g., buncher off) significantly impact emittance, coordinated adjustments to laser parameters, rf phases, and cavity gradients can still partially recover performance. The injector demonstrates resilience to a variety of perturbations, confirming its robustness for high-demand cw FEL operations.

A compact energy-recovery linac (cERL) has been under development at KEK to drive key accelerator technologies, demonstrate energy recovery under various conditions, and support beam applications. cERL began beam operations in 2013 to create stable, low-emittance, energy-recovered beams. The project focused on gradually increasing the current transported in a stepwise fashion while maintaining low beam losses at each step. However, the energy recovery of high-current beams is an issue that needs to be addressed. In this study, we propose a method for high-current beam tuning of approximately 1 mA under energy-recovery conditions with extremely small beam loss. We prepared a collimator and a local fast beam loss monitor to reduce the beam loss. We report successful results at approximately 1 mA, demonstrating 100.0% energy recovery with extremely small beam loss. The radiofrequency (rf) amplitude and phase varied by less than 0.02% and 0.02°, respectively. The study findings are expected to provide insights into steady beam operation for high-brightness beams and high-efficiency energy-recovery operations required for future extreme ultraviolet free-electron lasers.

Laser-plasma accelerators (LPAs) offer an attractive alternative to conventional accelerators for the development of compact electron sources and next-generation light sources. Due to orders-of-magnitude larger accelerating gradients, LPAs enable the acceleration of high-brightness electron beams to ultrarelativistic energies in millimeter- to centimeter-scale distances. However, LPA stability is limited by shot-to-shot fluctuations of the driving laser system. Specifically, fluctuations in the final-focus longitudinal position result in correlated instability in LPA electron beam qualities, including total beam charge, average beam energy, and energy spread. We demonstrate active stabilization of the longitudinal focal position for a 100 TW-class laser system. This repetition-rate scalable stabilization system leverages noninvasive wave front monitoring of a copropagating, unamplified kHz pulse train to guide corrective adjustments to the focal position of a 1 Hz amplified drive laser via an upstream telescope on millisecond timescales. In this demonstration, the approach limits standard deviation fluctuations of the amplified drive beam’s longitudinal focal position to $\pm 0.22\mathrm{mm}$ (representing at least a 53% reduction), achieved with a correction bandwidth that reaches the Nyquist frequency limit. Consequently, we observe marked improvements in both long-term and shot-to-shot LPA stability in terms of charge and spectrum.

In low-energy beam transport of pulsed, high-perveance ion beams operating under neutralized space-charge conditions, understanding the time required for the compensation process to develop—known as the build-up time—is critically important for ensuring efficient beam transmission and maintaining beam quality. This study presents a comprehensive investigation into the dynamics and optimization of space-charge compensation (SCC) in the low-energy beam transport line of a high-perveance proton beam, integrating numerical simulations with time-resolved experimental measurements. A detailed 3D numerical simulation has been developed, which takes into account both secondary electron and ion dynamics to evaluate the temporal evolution of the beam potential and its dependence on various parameters. Additionally, experimental measurements complement the numerical results, providing validation of SCC dynamics. These measurements analyze the effects of injected gas properties, pressure levels, and beam current on build-up time, while also considering nonlinear space-charge effects that influence SCC efficiency and the beam’s transverse emittance evolution. A correlation between SCC and radio-frequency quadrupole matching conditions is also examined, demonstrating the necessity of adjusting solenoid magnet settings to maintain optimal transmission efficiency.

Beams designated for the LHC are injected into the SPS in multiple batches. Given the tight spacing of 200 ns between these batches, the injection kickers have to be precisely synchronized with the injected beam to minimize injection oscillations. Due to machine drift, the optimal settings for the kickers vary. This paper presents an active controller trained by reinforcement learning that counteracts the machine drifts by adjusting the settings. The agent was exclusively trained in a simulation environment and directly transferred to the accelerator. Although its results are slightly worse than those obtained by an explicit numerical optimizer, the BOBYQA algorithm, the agent attains these results much faster since it requires far less computation.

The focus-scan scheme has been adopted to diagnose the emittance of the electron beams produced from laser wakefield accelerators. The spot size of the electron beam has an energy dependence after propagating through a beamline consisting of two or more quadrupoles, which can be used to fit the beam emittance and the Courant-Snyder (C-S) parameters. We use the concept of the principal planes to unify the analysis of the focus-scan scheme. The fitting procedures and the effects of the length variation between the source, magnets, and screen on the diagnostic are presented. Particle-in-cell simulations using plasmas with different downramp lengths are conducted to study their effects on the measurements of the emittance and C-S parameters. Our analysis provides a unified understanding of the focus-scan scheme and can be applied to beamlines consisting of more quadrupoles.