Physical Review Accelerators and Beams

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

Beyond-niobium superconducting radio-frequency cavities often involve coating the cavity with a superconducting thin-film. Depositing on a non-trivial geometry, e.g. TESLA-type cavities, is likely to yield an inhomogeneous thickness distribution on the boundary. We simulate the effects thereof on the quality factor by modeling them as Gaussian random fields, and provide a general simulation scheme that is applicable to other scalar quantities on the boundary and quantities of interest.

Suppression of higher-order modes (HOMs) in high-current storage rings requires compact absorbers capable of withstanding severe continuous-wave thermal loads and accelerating-mode leakage. Here, an axially graded hybrid absorber combining high-thermal-conductivity silicon carbide with dispersive ferrite is developed using an analytical transmission-line cascade model to optimize broadband impedance matching. When integrated into a 1.5 GHz $T{M}_{020}$-mode cavity, this arrangement eliminates low-frequency absorption dips, achieving strong HOM damping ($Q_{ext}<150$) while reducing peak ferrite thermal stress by 46.2% under multi-kilowatt continuous loads.

We demonstrate spatial conditioning of microcoulomb-class electron beams using electromagnetic solenoids. By applying ~10T solenoidal fields, we focused the electron beam onto a detection plane and measured a fluence enhancement factor of 6 over unconditioned electron beams. The chromatic focusing of the solenoids improved the resolution of electron beam radiographs produced using the focused electron beam. We also demonstrate a method using angled electromagnetic solenoids to simultaneously condition LPA electron beam spatial and spectral properties. This enables the spatial separation of a narrow-energy-spread population of the electron beam from the bulk of the electron beam.

Accelerator electrodes must be conditioned, trained by millions of pulses, before they hold their design field, and what the training changes in the metal has remained hidden. Under a sloped anode, one copper electrode saw fields from 80 MV/m down to nearly zero. Where the field was high, the lattice beneath the surface is measurably more disordered, like metal that has been flexed far below its strength for a very long time. The electrode seems to remember its field history in the arrangement of its dislocations.

This paper reports experimental results demonstrating 100 kW continuous-wave (cw) RF power transmission through a newly developed 1.3 GHz input power coupler, representing the highest cw power demonstrated with an L-band input coupler to date. The high-power test was performed at room temperature, with 100 kW transmitted continuously for 10 hours without any interlock. The power transmission wasn’t limited by the coupler, and the measured temperature rise across the coupler remained below 20 °C, demonstrating its suitability for high-power cw operation.

Flat beams can improve collider luminosity, but they cannot be formed and transported at low-energy due to destructive space-charge forces. Circular modes address this challenge by retaining unequal intrinsic emittances within a round transverse beam profile. This work establishes lattice conditions for preserving these modes during transport and acceleration, supported by analytical space-charge models and particle-in-cell simulations. The results show how intrinsic flatness can survive strong space charge at low-energy and be recovered as a physically flat beam downstream, providing a framework for high-intensity hadron accelerators and future colliders.