Physical Review Accelerators and Beams

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.

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.

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.

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.

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.

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.

Steady-state microbunching promises intense coherent light from storage rings, but radiation trapped in the laser modulator can destabilize the electron beam. Using optical cavity mode decomposition, this study tracks the evolution of the radiation field to predict single-bunch instability growth. The analysis reveals how cavity geometry, optical losses, and beam tune shape resonances, providing a framework for choosing operating conditions that reduce the instability.

Coherent synchrotron radiation (CSR) can degrade beam quality when intense electron bunches are deflected. This work derives conditions for suppressing CSR kicks in both dipole magnets and subsequent drift sections and applies them to symmetric and antisymmetric quadruple-bend-achromat. Tracking simulations demonstrate emittance preservation for multikiloampere bunches, including transport through cascaded cells for large-angle deflection.

In electron cooling devices, at low energies the electron beam is immersed in a continuous solenoid, which provides a cold and tightly focused beam in the cooling and transport sections. At relativistic energies, the adiabatic criterion generally cannot be fulfilled, so alternative electron beam transport is needed. Several methods exist for reducing transverse momentum: reduction in the amplitude of the impact at each perturbation; compensating for the influence of edge fields at the entrance and exit of the magnetic element; and adjusting the Larmour phase in the transport channel to minimize the sum of all local perturbations. A realizable strategy may involve using all of these methods.

Different attosecond XFEL schemes based on SASE can deliver distinct combinations of peak power, pulse duration, and single-spike probability. We show that these differences can be understood from the effective lasing window, which sets how far the radiation evolves into the post-saturation superradiant regime. The current-spike length sets the available window for this evolution, while peak current, energy spread, emittance, energy chirp, and transverse tilt further modify the pulse properties. These results provide a physical picture and practical guidance for electron-beam manipulation and the design of terawatt-class attosecond XFELs.

In the SuperKEKB superconducting (SC) cavities, cylindrical ferrite dampers have been installed for higher-order-mode (HOM) damping. While the HOM power increases as ramping up beam current, we observed that its increasing rate depends on the bunch fill pattern. To investigate this, wakefield simulations were conducted. HOM power is generally evaluated from the coupling impedance, which provides only the total beam power loss. When multiple absorbers are present, the absorbed power of each absorber remains unknown. Then, we introduce power calculation methods that utilize the local electromagnetic fields. In this paper, the theory and application to the SC cavity are presented.

Plasmas offer an opportunity to shrink the next generation of particle accelerators with their high accelerating gradients. As we push to higher energy, the alignment of the witness bunch in the wake becomes of critical importance. In the quasilinear regime of beam-driven plasma wakefield acceleration, the transverse motion of a misaligned witness can be quantified and predicted, and a predictive model for emittance preservation developed as a result. This model can then be applied to inform alignment tolerances for future accelerators.

Corrugated wakefield structures have enabled advanced XFEL pulse control at low repetition rates of about 100 Hz. We report the first pulsed megahertz demonstration at the European XFEL, combining an L-shaped structure with low-beta optics and a collimator. Hard x-ray spectra at 7 keV indicate an approximate fourfold pulse shortening, and trains of 100 pulses at 1.125 MHz show uniform properties. Neutron dose measurements verify that beam-loss mitigation keeps the radiation level at the undulator entrance within safe limits.

Arclike bunch compressors can produce ultrashort electron bunches, but coherent synchrotron radiation (CSR) can degrade beam quality during compression. We develop an analytical model incorporating steady-state and drift CSR to guide compressor design and longitudinal matching. For Gaussian bunches, theoretical calculations provide a preliminary assessment of the longitudinal impact of CSR during compression, including changes in bunch length and peak current. Simulations demonstrate that a two-stage arclike compressor designed using the theory developed in this work can compress a 100 pC bunch to an rms duration of about 50 fs with negligible emittance growth.

Maintaining vertical emittance stability is essential for modern high-brightness synchrotron light sources. Here, we present a real-time deep reinforcement learning correction framework using a Dueling Deep Q-Network and curriculum learning to dynamically adjust skew quadrupoles. $In$ $situ$ experimental validation at the Shanghai Synchrotron Radiation Facility demonstrates beam-size stabilization at the sub-micrometer fluctuation level, proving the practical feasibility of online reinforcement learning during active user beam delivery.