Space charge limits the performance of intense proton beams, but modeling it in scaling fixed-field accelerators is challenging because bunches have a long, arc-shaped geometry and closed orbits vary with energy. A new 2.5D method reconstructs local beam coordinates from a family of energy-dependent closed orbits, enabling slice-based space charge calculations for these bunches. Implemented in PyFFA and benchmarked against analytical predictions, the method provides an efficient tool for studying high-intensity beam dynamics in scaling FFAs.

The microbunching process in seeded free-electron lasers (FELs) can be numerically calculated based on the specific components and their parameters used. To facilitate the discovery and optimization of such FELs, in this paper we present an automated optimization method for the microbunching process in seeded FEL schemes. It is demonstrated that the method can automatically reproduce established seeded FELs, such as echo-enabled harmonic generation, eliminating the need for prior physical analysis. Moreover, the method yields new findings in seeded FELs that go beyond mere replication.

Computing the brilliance and coherence of synchrotron radiation requires a complicated numerical process based on the Wigner function representation, which defines the photon flux density in the 4-dimensional phase space. This process can be potentially time-consuming, especially in a multiple-beam-waist configuration. In this case a reliable result could be obtained only with a high computational cost by using Monte Carlo simulation. To overcome this difficulty, a numerical method called “quasi convolution” (QC) has been introduced, with which the computation time can be reduced by 2-4 orders of magnitude without sacrificing numerical accuracy.

At injection energy, high-order magnetic field errors limit how long the LHC beam survives, and the third-order chromaticity driven by decapolar fields has long disagreed with the magnetic model. Using forced oscillations from an AC dipole, the authors directly measure chromatic amplitude detuning and a decapolar resonance driving term for the first time. They trace the discrepancy mainly to the decay of decapolar fields in the main dipoles, and show that sextupoles and octupoles also drive decapole-like resonances. Corrections derived from this work are now used in LHC operation and improve beam lifetime.

Further beam-power upgrades beyond the current 1 MW operation of the J-PARC Main Ring Synchrotron (MR) require improved longitudinal matching between the 3 GeV Rapid-Cycling Synchrotron (RCS) and MR to mitigate space-charge driven beam loss after injection. To address this issue, we developed a bunch-lengthening method that exploits the unstable fixed point generated by a second-harmonic RF voltage to produce a stretched bunch before extraction. We also developed an optimization algorithm to efficiently determine the RF-voltage pattern. Beam experiments demonstrated bunch stretching and improved longitudinal matching, establishing a practical method for future high-intensity operation.

The Super Tau-Charm Facility (STCF) requires high-charge electron bunches with exceptionally low emittance and minimal energy spread. We present a comprehensive beam dynamics design of the STCF electron injector for its on-axis swap-out injection scheme, in which the bunching system is optimized using the Non-dominated Sorting Genetic Algorithm III (NSGA-III). By integrating systematic multi-objective optimization with high-fidelity simulations, our design delivers 8.5 nC bunches at 3.5 GeV, overcoming the intrinsic limitations of thermionic cathodes while maintaining their proven stability.

The High Energy Photon Source has achieved stable beam operation at 150 mA using five 166.6 MHz superconducting cavities equipped with on-cell fundamental power couplers. This paper presents the development and validation of these couplers, overcoming challenges in high-power handling, low cryogenic heat load, and cleanroom assembly compatibility. Iterative design refinements reduced the maximum coupler-cavity interface temperature from 15.4 K to 10.8 K and halved the average 4.2 K heat load to 1.5 W. The results established the highest beam-operation power reported for on-cell couplers in TEM-type superconducting cavities, advancing their application in high-current accelerators.

Harmonic cavities lengthen electron bunches in storage-ring light sources, but bunch gaps excite transient beam loading (TBL) that degrades this lengthening. While dedicated broadband cavities offer an established hardware solution, we explore an alternative using main RF cavity feedforward alone. By introducing the Gap-Induced Linear Decomposition (GILD) model, we reveal key physical insights that directly guide a Gap Constraint Relaxation (GCR) optimization under practical bandwidth limits. Simulations of the KEK light source and an SSMB ring demonstrate that comparable bunch lengthening is restored within realistic RF power budgets, offering a cost-effective scheme without extra hardware.

Conventional transverse RF deflectors require significant capital investment in a high-power rf source, waveguide systems, as well as installation space within existing accelerator infrastructure. In this paper, we present design criteria for an alternative method, in which the external high-power RF power part is replaced by beam-driven excitation of resonant cavity fields. A drive bunch is used to generate the streaking field experienced by a witness bunch one rf period later. Several modern FEL facilities already support two-bunch operation, opening the way for the realization of the concept using existing accelerator infrastructure.

By representing the long-range resistive-wall wakefield as a sum of exponential functions, we convert the history-dependent convolution into a recursive update that avoids storing the bunch-centroid history and is naturally suited to GPU acceleration. Implemented in the tracking code STABLE, this method maintains growth-rate accuracy while achieving at least a two-orders-of-magnitude speedup over the tracking code Pelegant. This enables systematic large-scale parameter scans, demonstrated for the Hefei Advanced Light Facility (HALF) by exploring the interplay among filling pattern, chromaticity, and harmonic-cavity bunch lengthening.