We present a beam-dynamics study of the SACLA DC thermionic-cathode electron gun and a roadmap to 0.15 µm normalized emittance for 0.5 MeV, 500 pC bunches. Using analytical and numerical methods, we quantify emittance growth from four dominant sources: (1) cathode image charges, (2) nonlinear space charge, (3) gun-field aberrations, and (4) chromatic aberrations in a collimating solenoid. The analysis gives a closed-form expression for the net correlated emittance growth and reveals an ‘’interference term’’ that increases the net emittance when space-charge and solenoid effects act together.

Calculating Coherent Synchrotron Radiation (CSR) effects is one of the most computationally demanding tasks in accelerator physics. We address this using machine learning models—convolutional neural networks, including U-Nets, and a latent conditional diffusion model—trained on physics-based simulations of electron bunches in steady-state circular orbits. These models rapidly predict 3D CSR wakefields, achieving up to 1000× speedups, generalize well to varied distributions while maintaining reliable precision and efficiency for accelerator applications.

Free Electron Lasers (FELs) are remarkable sources of radiation with a wide range of tunability. However, their efficiency is typically only a few percent. Due to the scarcity of high-power THz sources, we are researching a method to address this challenge: Tapering Enhanced Superradiance (TES). This approach could pave the way for a new generation of THz sources based on FELs. This study presents a design for a wave-guided planar undulator as part of a compact FEL system, aimed at enhancing radiation output compared to standard superradiant emission schemes, and significantly more than conventional FEL operations. This breakthrough may usher in a new era of high-power THz radiation sources.

The simultaneous delivery of helium-4 and carbon-12 ions is proposed as a promising tool for treatment monitoring in carbon ion radiotherapy. This paper reports on an injection scheme that allows to generate such mixed species ion beams by means of sequentially injecting the different ion species into a synchrotron. The implementation at the MedAustron therapy accelerator complex in Austria marks the first time a mixed ion beam was generated using this double injection scheme, and the first time that a mixed helium-4 and carbon-12 beam was accelerated in an ion therapy facility.

Recent experiments show that the use of short, nanosecond-long radiofrequency (rf) pulses can achieve accelerating fields approaching 400 MV/m at X-band, while understanding of breakdown physics in the short-pulse regime remains limited. This work presents analytical and numerical simulations of dark current dynamics in X-band photogun cavities driven by nanosecond-long rf pulses, covering mechanisms such as field emission, multipacting, and plasma formation. The results reveal the advantages of using short rf pulses to reduce dark current and mitigate breakdown, offering a path toward a new class of compact accelerators with enhanced performance and reduced susceptibility to breakdown.

Radiotherapy with very high energy electron (VHEE) beams offers deep penetration and reduced sensitivity to tissue inhomogeneities compared to conventional x-rays or hadrons. Laser wakefield accelerators (LWFAs) provide a promising solution with their high gradients enabling tabletop setups, yet comprehensive dosimetry studies for LWFA-based VHEE prototypes have been lacking. Here, we characterized the 3D dose distribution of a stable VHEE beam generated by a 1 Hz LWFA prototype shaped to an 8 mm uniform field, and performed the multi-field irradiation from 20 angles creating a 16 mm diameter dose plateau at depth with 2.8% uniformity, advancing LWFA-VHEE toward clinical tumor targeting.

Accurate modeling of positron sources is essential for next-generation high-luminosity lepton colliders. This study presents the first experimentally validated start-to-end simulation framework for positron production and capture, benchmarked against beam measurements at SuperKEKB, the world’s most intense positron source. Combining Geant4 with RF-Track, the model successfully reproduces measured positron yield and parameter scans. Additionally, detailed benchmarking was conducted with other simulation tools, including EGS5, GPT, and ASTRA. The work establishes a robust and reliable framework for designing and optimizing future high-intensity positron sources such as the FCC-ee.

Superconducting radio-frequency (SRF) cavities, the heart of modern particle accelerators, are prone to performance-limiting ponderomotive instabilities. While the theory behind these instabilities is established, comprehensive experimental validation under active feedback control has been lacking. This work bridges that gap by developing a precise analytical model to predict the instability thresholds. We present the first direct experimental verification of the theory on the CAFE2 superconducting linac, demonstrating excellent agreement and providing practical guidance for ensuring stable accelerator operation.

The performance of the bunch lengthening using harmonic cavities is critically affected by the fluctuations of their RF voltages, which are caused by gaps in the bunch filling pattern. To mitigate such a transient beam-loading (TBL) effect, a compensation method using a broadband cavity has been proposed. This paper reports on the conceptual design of a broadband kicker cavity and performance tests of the low-power model cavity that can be used to compensate for the TBL effect. The actual operation and effectiveness of the kicker cavity in the storage ring are also discussed.

We present a noninterferometric method for measuring transverse electron beam size at free-electron lasers using synchrotron radiation intensity autocorrelation. Unlike wire scanners that yield only projected sizes, our technique enables cell-by-cell diagnostics along the undulator using existing hardware: a commissioning monochromator and an X-ray detector. The method extracts averaged slice sizes from noisy data with signal-to-noise ratios as low as 0.9. We validated the approach by introducing a controlled lattice mismatch, inducing sinusoidal beam size variation along the undulator, clearly reproduced in measurements.

The sequence formed by the tune resonances up to order $n$ exactly coincides with the Farey sequence, $F_n$. In circular accelerators it is important to place the tunes in regions free of resonances. From experience we know that the largest resonance-free gaps happen next to low order resonances. We provide a formal mathematical demonstration of this statement. We present a connection between the resonance sequence and one of the most important unsolved problems in mathematics, the Riemann Hypothesis (RH). If RH is true it implies that the resonance sequence is significantly more regularly spaced than if it was built at random. A new analytical estimate of the number of resonance lines in the two dimensional tune diagram is derived.

We calculated the intra-beam scattering (IBS) growth rates by extending the Bjorken and Mtingwa (B-M) model for a steady-state microbunching storage ring in which the electron bunch length can not be regarded as constant. An analytical method is proposed to predict the equilibrium emittance with IBS involved by an approximation for planar storage rings, while the iteration method was widely used to calculate the equilibrium emittance in electron rings. With the analytical method to predict equilibrium emittance, we are able to optimize the IBS effect by directly setting the equilibrium emittance as an objective.

Harmonic cavities are critical for enhancing beam stability in synchrotron light sources and future colliders. However, Transverse Mode Coupling Instability (TMCI) in arbitrary double RF systems remains a challenge. Most Vlasov solvers for TMCI are limited to single RF configurations. There is still no general Vlasov solver for TMCI analysis under arbitrary double RF settings, particularly under over-stretching conditions (characterized by two Stable Fixed Points, SFPs). We have developed a novel Vlasov solver for arbitrary double RF settings. Validated through good agreement with simulations, this solver proves reliable for TMCI analysis under different double RF configurations.

An implementation of a surface-impedance boundary condition in finite-difference time-domain electromagnetic codes making use of locally conformal (i.e. “partially filled”) cells for the modeling of finite-resistivity conducting surfaces is described. The model, which obviates the need for spatial resolution of the skin depth, is demonstrated by performing several test problems involving objects with finite conductivity in both relatively simple and more complex geometries.

Nonlinear space-charge is significant in high-intensity accelerators and has been widely studied using multi-particle tracking methods. We present a novel 2.5-dimensional symplectic space-charge solver designed for long beam bunches. A semi-analytical expression for a transverse Gaussian density distribution under open boundary conditions in a straight system is derived for arbitrary distributions in open space, and within rectangular and round conducting pipes. Extension of this solver to circular accelerator systems is discussed. We find that the fast 2.5-dimensional solver can be a good approximation to the fully three-dimensional solver for long bunches in large circular accelerators.

Optimization of beamlines usually relies on gradient‑free or finite‑difference methods, whose computational cost scales poorly with the number of parameters. “Differentiable” codes can scale much better, but theoretical and empirical analysis of their performance has largely been absent from the literature. We built a differentiable code, benchmarked its gradients against finite differences on a space-charged FODO cell and a future CERN beamline, and then integrated those gradients into gradient‑based optimizers, showing clear advantages over gradient‑free.

Laser-driven ion beams possess unique features like ultra-short pulses and high particle fluxes that enable the delivery of ultra-high dose rates relevant for biomedical applications such as FLASH radiotherapy. However, their broad energy spectra and large angular divergence remain major barriers to clinical integration. Here, we demonstrate through numerical simulations the feasibility of using a high-current two-solenoid system to collect and focus laser-driven proton beams, providing a promising beam-transport concept for future proton therapy applications.