Heavy-ion storage rings with internal targets offer highly efficient production of rare events, but are severely limited by stochastic charge-state conversion at each target interaction, which drives rapid transverse emittance growth. We introduce a concept based on a scaling fixed-field alternating-gradient (FFA) lattice with an azimuthally modulated field index k to overcome this fundamental limitation. This approach is found to significantly suppress emittance growth, particularly in the horizontal plane. Six-dimensional beam-tracking simulations incorporating energy loss, scattering, and charge-state conversion are presented, demonstrating effectiveness under realistic conditions.

X-band high-gradient structures are critical enabling technology for compact linear accelerators and gamma-ray sources. We present tuning, conditioning, and performance of six identical XT72 structures for Tsinghua’s VIGAS gamma source. All reliably achieve 80 MV/m with breakdown rate below $1\times {10}^{-4}$ per pulse, revealing strong localization of breakdowns near high-field input coupler cells, a power-law decay of the normalized breakdown rate, and a dependence on input power waveform. These results provide key benchmarks for high-gradient structure conditioning and future compact facilities.

Emittance minimization in high-brightness photoinjectors is a key challenge for x-ray free-electron lasers. We demonstrate a data- and model-guided framework combining physics-informed laser shaping with inverse emittance modeling. The approach achieves ~20% emittance reduction and enables reliable, stable shaped electron bunches for sustained high-intensity FEL operation at the European XFEL.

A simple yet effective scheme for spectral broadening is proposed which utilizes a plasma wakefield accelerator (PWFA) to impart a large energy chirp to an electron beam and a specifically designed transport system then selectively injects the portion of the beam meeting the lasing requirements. A start-to-end simulation demonstrates that a high-brightness X-ray FEL exhibiting a full-width bandwidth reaching 20% can be generated which is approximately five times greater than what is achievable with conventional accelerator technology.

This paper presents a compact route to ultrashort, high-intensity THz light using unseeded optical-klystron free-electron lasers. Time-dependent 3D simulations at 10, 30, and 100 µm show how natural THz slippage can phase-align energy modulation, create strong microbunching, and drive coherent amplification without an external seed. The study demonstrates sub-picosecond pulses with multi-hundred-megawatt peak powers, examines dispersion, diffraction, and harmonic-bunching options, and introduces a chicane-embedded optical-delay scheme to restore beam-radiation overlap for staged amplification. These results point toward compact, powerful tunable THz FEL sources for science.

X-ray free-electron lasers provide new opportunities for probing ultrafast dynamical processes in matter. Here we present a four-color XFEL scheme for multiframe diffraction imaging, in which the whole electron beam is used to generate pulses with adjustable wavelength separation and controllable time delay. By introducing optical-klystron enhancement in split undulators, the scheme improves FEL intensity while reducing the total undulator length.

We present a method for suppressing transverse single-bunch instabilities, using the combined action of longitudinal impedance, feedback, and chromaticity. The key stabilization mechanism arises from two effects: feedback and positive chromaticity shift the dominant modes of instabilities, while longitudinal impedance introduces a tune spread in these modes. The theory is applied to three electron storage rings. Both the analysis and simulation lead to consistent conclusions.

Laser wakefield acceleration (LWFA) studies commonly assume an ideal Gaussian laser driver, although real high-power laser pulses often exhibit complex transverse intensity and phase structures. We combine experiments and particle‑in‑cell simulations to show that actual laser profiles significantly modify the wakefield evolution and electron injection process. The nonideal laser distribution lowers the self‑focused intensity and broadens the plasma wake sheath, delaying injection. Incorporating the reconstructed laser profile in simulations reproduces experimentally observed ~200 pC, ~200 MeV electron beams, revealing the critical role of realistic laser structures in high‑charge LWFA.

Laser wakefield acceleration offers an exciting route to the generation of bright, high-energy electron bunches, but these electrons are generated with large divergence and must be captured well. GeV-scale electrons need strong quadrupoles. In this paper we propose a Focusing-Defocusing (FODO) array of equal-strength Permanent-Magnet Quadrupoles (PMQs) to gradually capture such bunches. A modular array of fixed-strength PMQs can be readily tuned for different electron energies by adding or removing magnets; we have developed a practical arrangement for the 1 GeV electrons expected at the UK Extreme Photonics Application Center (EPAC) facility, and have constructed novel 350 T/m capture PMQs.

The SPARC LAB facility, operating at the LNF-INFN laboratories in Frascati in Italy, has been recently upgraded with the aim to enable user-oriented applications by offering high-brightness electron beams and ultra-short light pulses from THz up to the soft X-rays range. The paper summarizes the electron beams and radiation parameters and describes the layout and main infrastructures available to external users.

The Compact Linear Collider (CLIC) injector linacs must sustain high-gradient acceleration under strong beam loading and tight wakefield constraints. We present optimized 2 GHz traveling-wave structures combining iris tapering, RF pulse shaping, and a novel double-train RF operation scheme. Together, these enable stable acceleration with controlled wakefields and flat gradients across long bunch trains. The resulting design significantly reduces the required number of RF structures, klystrons, and peak RF power, yielding a more compact and energy-efficient injector complex compatible with future collider RF systems.

Transverse Electric (TE) RF cavities are important tools for beam manipulation and beam-quality improvement, but systematic methods for designing cavities with prescribed multipolar TE fields remain limited. This work presents a method for synthesizing such cavities by superimposing cylindrical standing-wave solutions and enforcing conducting boundary conditions through numerical optimization. CST eigenmode simulations validate the resulting hybrid geometries and show that the prescribed multipolar content is realized with high modal purity.

Ultrashort, high-current electron beams can drive advances in next-generation accelerators, ultrafast light sources, and attosecond science, yet conventional compression techniques are fundamentally limited by collective effects and beam degradation. Using simulations and analytical modeling, we demonstrate that plasma-based compression can achieve orders-of-magnitude stronger longitudinal compression while preserving beam quality. By imprinting energy chirps exceeding 500 MeV/μm onto an electron bunch, this approach can compress beams to durations approaching 10 nm and peak currents nearing 1 MA, enabling unprecedented beam brightness and new regimes of ultrafast beam-driven science.

This work addresses the question of how precisely one can control the orbit of a single ion. A unique ion source based on a simple radio-frequency quadrupole trap is employed to provide an extremely narrow beam that enables one to irradiate a target with nanometer precision. This novel source is equipped with a Doppler laser cooler to reduce the temperature of a trapped ion cloud to near absolute zero, where a phase transition to a Coulomb crystalline state occurs. We demonstrate experimentally and numerically that it is possible to extract these ultracold ions one by one from the trap at arbitrary timing and transport them precisely along the design orbit.

Third-order resonances limit performance in high-intensity bunched beams. We experimentally demonstrate mitigation in the Fermilab Recycler Ring by minimizing resonance driving terms using sextupole correction. Measurements with bunched beams across low and high intensities show reduced beam loss, emittance growth, and halo population. These results establish resonance control as a key tool for intensity-frontier accelerator operation.

Operation close to half-integer resonances can lead to significant beam quality degradation and beam loss in high-intensity/high-brightness synchrotrons. At the CERN Proton Synchrotron Booster, the controlled crossing of the $2Q_y=9$ half-integer resonance was experimentally investigated and benchmarked against six-dimensional self-consistent tracking simulations. The study demonstrates the role of incoherent space charge effects in shaping the beam profile evolution and beam losses. The observed dependence of beam losses on the resonance crossing rate, beam intensity, and resonance strength provides a basis for devising mitigation strategies.

The concept of matched transport in periodic quadrupole focusing channels has been extended from uncoupled beams to those exhibiting significant correlations between the two transverse degrees of freedom.

In this paper, several rms-moment-matched transport solutions exhibiting substantial interplane coupling are derived analytically for a periodic solenoid focusing channel with alternating polarities. Particle tracking simulations, performed along this channel using the particle-in-cell Beampath code, confirm that the proposed rms-moment matching formalism remains accurate and robust even in the presence of strong nonlinear space-charge forces.

A Luenberger state observer provides real-time estimates of detuning and half bandwidth in superconducting radio frequency (SRF) cavities, offering insight into parameters critical for safe and energy efficient operation. Based on a gain-scheduled linear cavity model with parameter-varying pole placement, it features stable and tunable estimation error dynamics. Differences from existing approaches are discussed and evaluated using experimental data, outlining the observers potential for future applications in resonance control and quench detection.