Physical Review Accelerators and Beams

The storage ring proton Electric Dipole Moment (sr-pEDM) experiment uses a novel symmetric-hybrid frozen-spin storage ring with radial electric bending and alternate magnetic focusing to minimize systematics, enabling precise measurement of the vertical rotation of the polarization of polarized stored proton beams. We present a detailed analysis of second-order effects arising from the interplay of stray radial magnetic fields with electric quadrupole and sextupole fields, and show how the ring’s symmetries—beam reversal and magnetic quadrupole polarity switching—suppress these effects below the sr-pEDM experiment’s targeted sensitivity.

Scaling fixed-field alternating gradient (FFA) accelerators offer strong potential for high-intensity proton drivers but face complex nonlinear dynamics under tight spatial limits. Through large-sample tracking of over 360,000 configurations under engineering errors, a 300–600 MeV scaling-FFA lattice is established for the CSNS Phase-II upgrade. A spiral FD doublet lattice (N=16) is identified as the optimal baseline, mitigating parameter bottlenecks and expanding dynamic aperture. Machine learning feature attribution further reveals the critical role of vertical edge focusing in beam survivability.

We propose a versatile electron-optical system featuring a semi-immersed electron gun and a magnetic coil that generates a nonadiabatic magnetic field for an electron-beam ion source (EBIS). The system is capable of producing electron beams over a wide operating range, with beam currents of 0.2–3.0 A, current densities of 200–1900 A/cm², and electron energies of 2–20 keV. This flexible design is ideally suited for carbon-ion cancer therapy accelerator facilities, charge breeding of radioactive ions, and other accelerator systems requiring highly charged ions.

Sudden beam loss (SBL)—the loss of much of the stored beam within a few turns—has limited SuperKEKB’s luminosity for nearly five years, damaging collimators and the Belle II detector. Reproduction experiments and laboratory analysis pin SBL on dust–beam interactions. Such interactions occur at many accelerators, but SuperKEKB’s tiny vertical aperture (nano-beam scheme) and high stored current make the damage uniquely severe. One dust source was vacuum sealant degraded into black stains; removing them sharply reduced SBL. Yet because discharges also produce dust, SBL cannot be fully eliminated in high-current, nano-beam machines—a key lesson for future colliders like FCC-ee and CEPC.

Coherent terahertz generation in storage rings is typically limited in repetition rate by the time required for radiation damping to restore the electron beam. We propose an active demodulation scheme that uses a phase-reversed laser interaction to rapidly compensate the induced energy modulation after THz emission. Simulations for HLS-II indicate that coherent 10-THz radiation could be generated at repetition rates on the order of 100 kHz.

Laser Wakefield Acceleration offers the potential for more compact and cost-effective electron accelerators. Significant efforts are being made to improve electron beam quality for various applications, particularly through the design of suitable gas targets and optimization of laser and target parameters. The present study investigates LWFA in a specially designed capillary gas-cell setup for generating high-quality, GeV-scale electron beams using combined hydrodynamic and Particle-In-Cell (PIC) simulations. The results reveal the impact of tailored gas-density profiles on LWFA and beam quality, with multiple injection mechanisms identified and analyzed.

A new type of fast pulse generator which utilizes wideband RF amplifiers and a new chopper chamber with high-impedance strip-line electrodes has been developed to reliably and accurately cut out a short bunch from the long-pulsed beam emitted from a single-crystal CeB${}_6$ hot cathode at the X-ray free-electron laser (XFEL) facility SACLA. Using these apparatuses, we succeeded in forming a nanosecond multi-bunch structure in the thermionic electron beam and generating a double-bunch XFEL pulse with a total pulse energy of 1mJ at 10 keV. The results will provide new possibilities for XFEL beams: higher intensity, higher coherence, and higher pulse repetition rate.

This paper proposes a novel emittance compensation scheme for the HALF diffraction-limited storage ring based on the quantum excitation effect. The proposed quantum-excitation-based scheme achieves emittance compensation comparable to the usually used radiation-damping-based scheme, even though the compensation wiggler is significantly shorter. It also yields significantly smaller energy spread variations and thus very small effective emittance variations at dispersive straight sections, with the effective emittance considered basically compensated.

We present a more accurate formula for calculating the intra-beam scattering (IBS)-induced path length deviation of an electron beam traversing the lattice, which can aid in lattice optimization. Additionally, we have developed a tool to simulate IBS-kick effects on beam dynamics. The tool can directly compute the IBS equilibrium emittance in storage rings and efficiently evaluate the associated path length deviations for an electron beam traversing a lattice. These capabilities are important for applications such as optical stochastic cooling and steady-state microbunching, which strictly require the isochronicity of lattice.

Precise measurements of the muon anomalous magnetic moment motivate the development of a low-emittance muon beam at J-PARC using rf linacs. Positive muons have so far been accelerated only through the radio-frequency quadrupole (RFQ) stage. We report the first full-size 324-MHz interdigital H-mode drift tube linac dedicated to muon acceleration. Designed to follow the RFQ and accelerate muons from 0.34 to 4.26 MeV, it uses a cost-effective three-piece copper cavity with monolithic drift tubes. Low-power tuning reduced the peak-to-peak on-axis field error to 1.4%. The cavity operated stably at the nominal peak power of 390 kW and reached an accelerating field 10% above the design value.

In the Z-pinch pulsed-power accelerators, multi-MA currents rapidly heat the electrodes above the threshold for plasma formation. The stability and dynamics of an electron sheath emitted from the highly magnetized electrode plasmas are investigated using semi-analytic theory and kinetic particle-in-cell simulation which show strong resistive plasma instability (RPI) growth. The nonlinear evolution of the RPI drives electron vortices and uncovers sufficient ion charge for plasma transport across the gap in several nanoseconds with plasma densities sufficient to generate published current losses via a Hall mechanism.

Electron cloud buildup limits the performance of high-intensity proton accelerators like the LHC, driving beam instabilities and cryogenic heat loads. Existing methods to measure the maximum secondary electron yield (δmax) typically require dedicated instrumentation and beam-time interruptions. This work introduces a novel in situ technique that extracts δmax directly from routine electron cloud current measurements during standard LHC operation, using the transition to a linear regime with the number of injected bunches. Applied to Vacuum Pilot Sector data, it enables noninvasive, long-term monitoring of vacuum-surface conditioning.

Adding a relativistic ionization front to electron beam seeding is an effective way to control the self-modulation of an entire proton bunch in plasma. By using the relativistic ionization front to truncate an available electron seed bunch, reproducible seeding of self-modulation is demonstrated at plasma densities seven times higher than previously achieved, while simultaneously increasing the seed wakefield amplitude. This opens a route to controlled, high-gradient plasma wakefield acceleration with long proton drivers.

Laser-induced energy modulation and demodulation play a critical role in advanced beam-manipulation schemes for coherent radiation generation, while residual energy modulation can degrade electron-beam quality and limit scheme performance. Here, we systematically investigate the transition from modulation to demodulation through theory and three-dimensional simulations, explore a route toward complete demodulation, and develop diagnostics for weak residual modulation, followed by preliminary experiments at SXFEL. This work reports the first experimental observation of energy demodulation in a seeded FEL and establishes diagnostics for resolving weak residual modulation.

Inverse Compton scattering (ICS) is a promising method for generating coherent and tunable x-rays in a compact setup. Optimizing ICS x-ray sources is often left to simulations, which can be time consuming and not always provide a physical understanding of the limitations. This paper presents a closed form analytical framework to optimize the output of an ICS x-ray source at arbitrary interaction angles between pulsed electron and laser beams. We find that a co-propagating, grazing angle geometry is especially useful for soft x-ray generation.

Free-electron laser oscillators exhibit rich dynamics that can strongly affect pulse stability and output power. Using a three-dimensional, time-dependent simulation framework for the University of Hawai‘i FEL oscillator, we characterize its saturation, temporal and spectral evolution, and response to cavity desynchronization. We find that modest desynchronization can enhance pulse energy, while larger detuning suppresses spiking and improves robustness to timing jitter, providing guidance for future operation of the facility.

This paper develops an efficient nonlinear optimization method that minimizes on- and off-momentum ring-averaged resonance driving terms, which is applied to the SOLEIL storage ring. One optimized solution achieves a dynamic aperture comparable to that obtained from tracking-based nonlinear optimization while improving beam lifetime, as experimentally validated. It is also found that reducing ring-averaged resonance driving terms helps optimize working point and control tune shifts with momentum.

The lattice design for the Super Tau-Charm Facility (STCF), a high-luminosity, low-energy electron-positron collider, is challenging. This work presents the design and nonlinear optimization of a one-fold STCF lattice and provides a framework for high-luminosity low-energy electron-positron colliders. The design procedure comprises lattice-independent global parameter optimization, interaction region design with local chromatic correction and crab-waist sextupoles, and full-ring nonlinear optimization with a two-stage strategy. The optimized lattice achieves target luminosity while maintaining sufficient dynamic aperture, momentum acceptance, and Touschek lifetime.

Enhanced SASE (ESASE) operation of an FEL uses a train of current spikes to locally enhance the electron beam current for improved FEL performance. However, these current spikes generate a strong space charge field, which builds up a linear chirp over each current spike. This effect must be compensated by a positive taper of the undulator field, but measurements at SwissFEL show a much stronger optimum taper than anticipated. Elongation of each current spike under its own space charge force spreads the already-induced microbunches of the FEL amplification apart, causing an adiabatic red shift in the emitted radiation. This effect is visible in the plot which shows matching simulations.

Multilayer NbTiN coatings are a promising route toward superconducting RF cavities operating beyond the limits of bulk niobium. By combining XRD, optical profilometry, AFM, UPS, and field-emission scanning microscopy, we show that high-temperature annealing substantially improves the microstructure and suppresses field emission, increasing the onset field from below 60 MV/m to about 200 MV/m. The results establish thermal annealing as a key processing step for enhancing the performance and reliability of NbTiN thin films for next-generation SRF cavities.