Space charge compensation (SCC) is prevalent and fundamental in the transport of low-energy ion beams through residual gas. At high residual gas pressures, overcompensation may occur. In this paper, we incorporated a model for secondary particle production and tracking into a beam optics calculation code to investigate the overcompensation process as a negative ion beam traverses residual hydrogen. The impact of secondary electrons, often overlooked, is examined in detail. This work serves to refine our comprehension of SCC processes within the realm of low-energy beam transport and promises to enhance the accuracy of future beam optics simulations.

We have successfully applied autoresonance to beam extraction for the first time, enabling millisecond spill in compact synchrotrons crucial for carbon-ion FLASH radiotherapy. This novel technique uses a single-cycle frequency sweeping excitation to simultaneously drive the entire beam into resonance, overcoming the speed limit of conventional methods. It requires only an additional octupole magnet. Our research includes a detailed analysis of the autoresonance threshold in extraction, with simulations that closely align with theoretical predictions.

Free-electron lasers (FELs) require flexible and efficient wavelength control to support diverse scientific experiments, particularly in high-gain harmonic generation (HGHG) FELs. This work presents three complementary FEL wavelength tuning strategies experimentally implemented at the Dalian Coherent Light Source (DCLS): (i) full-range continuous wavelength switching over 50–150\,nm, (ii) wide-range coarse wavelength scanning within several minutes, and (iii) narrow-range fine wavelength scanning with a resolution of 0.01\%. Collectively, these methods significantly enhance wavelength manipulation capability at DCLS and provide a valuable reference for other HGHG-based FEL facilities.

Beam halo in high-intensity accelerators has long been recognized as limiting performance due to effects like beam-gas scattering (BGS), nonlinearities, and collective interactions, with prior ATF studies identifying elastic BGS as dominant for vertical halos. This work integrates synchrotron radiation, BGS, and Touschek processes into detailed simulations of the KEK ATF damping ring, matched against direct observations using a high-dynamic-range YAG/OTR monitor. The key finding is that Touschek scattering predominantly forms horizontal and momentum halos, confirmed by consistent predictions and measurements under varying gas pressures and beam intensities with a dynamic range of ~10${}^5$.

Coherent wiggler radiation (CWR) can play a significant role in the collective dynamics of low-energy, high-current electron–positron colliders. This work develops analytical models to quantify CWR impedance and evaluate its impact on beam stability including its interplay with coherent synchrotron radiation (CSR) impedance. The results offer practical guidance for wiggler design and beam-stability optimization in new-generation tau–charm factories and other electron rings employing damping wigglers.

This study presents a comprehensive investigation of intra-bunch dynamics in a high-intensity proton synchrotron, where strong space-charge forces and large chromatic phase coexist, through theoretical analysis, simulations, and experiments at the J-PARC Main Ring. Simulations supported by analytic foundations and experimental results, reveal how space charge modifies the decoherence and recoherence of intra-bunch motion, as well as the importance of indirect space-charge effects to suppress the beam instabilities. This integrated approach deepens the understanding of beam instabilities and provides a basis for future control strategies in MW-class proton drivers.

This work advances the plasma photocathode concept — a promising source of ultra-high-brightness electron bunches within plasma accelerators — by introducing a radially polarized ionizing laser pulse. We combine start-to-end simulations with multi-objective Bayesian optimization to demonstrate that our approach can generate high-charge beams with symmetric emittance, and we explore the fundamental trade-off between bunch charge and beam quality. The study shows that, compared to conventional linearly polarized pulses, radially polarized pulses can generate higher brightness bunches in the high charge, optimally loaded regime.

Laser–plasma accelerators (LPA) enable compact radiation sources, but their broader use in applications depends on advancing machine performance. The acceleration process is governed by a complex interplay of fluctuating parameters, which makes it difficult both to trace instabilities and to uncover the relationships that determine beam quality. We present a data-driven methodology to model these dynamics, designed to mitigate bias introduced by measurement errors. Applying it to data from a day-long operation of the LPA-driven x-ray source LUX, we recover parameter dependencies and demonstrate virtual diagnostics of the x-ray spectrum that could support future applications.

Laser wakefield acceleration may enable the next generation of linear colliders, but is limited by the challenge of staging. In this work, we propose a discrete flying focus that can deliver higher energy gain in a single plasma stage than a conventional laser pulse, 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. Particle-in-cell simulations demonstrate the effectiveness of this method.

Very high energy electron (VHEE) beams have been explored as a radiotherapy modality offering advantageous dose distribution, deep penetration and potential of ultra-high dose-rate, but compact delivery from laser-wakefield accelerators (LWFAs) remains hindered by large energy spreads. This work proposes a simple scheme using only two dipole magnets to guide LWFA-based VHEE beams along varied trajectories, achieving a concentrated dose peak up to 20 cm deep in a water phantom while minimizing entrance dose. Robust to energy spreads and enabling precise control, the approach supports uniform dose profiles via weighted superpositions, paving the way for compact, clinical LWFA-VHEE systems.

The FCC-ee injector complex requires a high-energy linac capable of accelerating beams to 20 GeV with excellent stability and efficiency. We present a comprehensive RF design and optimization of traveling-wave structures operating at 2.8 GHz, including beam-loading compensation, wakefield suppression, and pulse compression. The resulting design achieves high power efficiency, minimal energy spread, and robust thermal-mechanical stability, providing a reliable foundation for the FCC-ee injector.

We present an analytic solution for conical superconducting accelerator magnets and apply it to canted-cosine-theta magnets, which can potentially be used for the interaction region of particle colliders and medical accelerators.

The work presents a new digital filter suitable for the construction of a single pick-up transverse feedback (TFB) with a larger tune acceptance and higher gain than can be achieved by the traditional Hilbert phase-shifter technique.

We propose methods to extract key parameters required for optimal feedback operation solely through active manipulations of the beam by the transverse feedback system and analysis of data acquired from it. They allow not only to shorten the TFB commissioning time from several shifts to less than a minute, but also regular, fully automated TFB system checks which can now be performed in case of suboptimal performance, saving significant amounts of machine time.

In laser wakefield accelerators, pulse-front tilt in a driving laser pulse can deviate both the laser and the electron beam. Here, we examine experimentally how this steering applies to the x-rays generated during acceleration. The correlation with the electron deflection is determined, enabling non-invasive active stabilization of either beam, without significant impact on key beam parameters such as energy, charge, and divergence.

The High-Luminosity LHC upgrade demands tighter control of beam instabilities through reduced machine impedance. We performed the first beam-based validation of the new low-impedance collimators installed during LS2, confirming a fivefold reduction in impedance compared to previous designs. These measurements, supported by detailed 3D electromagnetic simulations, demonstrate the effectiveness of Mo-coated MoGr collimators in ensuring beam stability for HL-LHC operation.

We used a laser–plasma accelerator to generate multi‑GeV electron beams that produced muons when stopped in a high‑Z target. We experimentally identified, for the first time in a laser-plasma accelerator‑based source, two distinct components of the muon flux: a directional, high‑energy beam from pair production and an isotropic, low‑energy background from meson decay. This result paves the way for compact, deployable muon sources for advanced imaging of large or concealed objects. Directional, high‑energy muons can penetrate dense materials, enabling noninvasive 3D imaging in geophysics, archaeology, and nuclear inspection.

In novel electron rings with extremely short bunch lengths, such as in steady-state microbunching (SSMB), conventional methods often predict a significant longitudinal space-charge tune shift. However, these approaches may not be applicable for extremely short bunches. We develop formulas to accurately calculate the longitudinal space-charge tune shift for arbitrary bunch lengths. Our findings indicate that, after eliminating invalid approximations, the longitudinal space-charge tune shift is considerably smaller than predicted by conventional methods. Our results demonstrate that the multibunch effect is not a dominant factor in the longitudinal direction, even for minimal bunch spacing.

For 4th-generation synchrotron light sources, triple RF systems have been proposed to achieve further bunch lengthening (BL) and meet specific longitudinal injection requirements. However, double RF system simulations and experiments confirm that periodic transient beam loading (PTBL) can limit maximum BL. This work improves tracking simulations and algorithms to systematically investigate PTBL in the triple RF system. Contrary to expectations that higher-order harmonic cavity (HC) would dampen PTBL driven by lower-order HC, we find that both HCs work together to enhance the PTBL effect. To avoid PTBL under optimal BL conditions, the R/Q values of both HCs must be sufficiently low.

Detailed understanding of the loss of Landau damping (LLD) is vital to predict beam quality degradation in hadron synchrotrons. This work explores the benefits of double-harmonic rf systems, as well as the impact of the beam coupling impedance parameters. An approximate analytical expression is presented when both RF systems are in phase. Other cases require numerical solutions of the linearized Vlasov equation. For the first time, we demonstrate that a non-monotonic synchrotron frequency distribution does not necessarily imply LLD. Theoretical and semi-analytical predictions are supported by macroparticle tracking and validated by beam-based measurements in the PS and SPS at CERN.

The erosion of Radio-Frequency Quadrupole (RFQ) electrodes due to sputtering under ion irradiation is examined through coupled particle tracking and SDTrimSP (Static-Dynamic Transport of Ions in Matter Sequential-Parallel processing) simulations. Model validation was conducted using quartz crystal microbalance (QCM) measurements with proton and argon beams. The findings demonstrate that heavy-ion irradiation induces significantly higher sputtering yields and erosion rates, potentially leading to critical frequency perturbations and performance degradation in high-current RFQs. These effects are critical in accelerators with stringent operational requirements and long lifetimes and should be taken into consideration in the design of the next generation RFQ cavities.

Efficiently simulating laminated magnets with fast excitation cycles is a long-standing challenge. Especially when a nonlinear magnetization curve must be considered, simulation times often become prohibitive. To address this problem, we introduce the homogenized harmonic balance finite element method (HomHBFEM), which combines a frequency-dependent homogenization of the yoke laminations with a harmonic balance method and thus drastically reduces the computational cost. Thereby, the HomHBFEM has allowed us, for the first time, to conduct nonlinear simulations of the fast orbit corrector magnets for the future light source PETRA IV at DESY.

Symplectic maps are essential in finding analytically tractable solutions to the three-dimensional Maxwell-Vlasov equations in the relativistic, space-charge dominated regime. However, start-to-end symplectic tracking codes are not readily available, especially for photo-injectors. In this paper, we present a symplectification algorithm that transforms mechanical into canonical coordinates, while accounting for external EM fields and self-field space charge forces. Demonstrated on the 113 MHz SRF photo-injector at BNL’s Coherent electron Cooling (CeC) experiment, this algorithm is broadly applicable and enables further analytical evaluation of microscopic instabilities.