Physical Review Accelerators and Beams

Better optimizers are not what automated accelerator tuning needs most; smaller problems are. Experiments at TRIUMF’s ISAC postaccelerator demonstrate that a validated beam-physics model can directly determine the machine optics, removing those parameters from black-box optimization altogether. The lower-dimensional problem that remains converges faster and more reliably, illustrating a broader principle: model what can be predicted from physics and optimize only what cannot.

At SuperKEKB, the nanobeam collision scheme and high beam currents result in short beam lifetimes, making efficient top-up injection essential for sustaining luminosity. We apply Bayesian optimization to improve the injection efficiency during physics data taking while imposing operational constraints to avoid beam aborts. Beam tests demonstrated improvements in injection efficiency of up to 32%, showing that automated optimization can be safely integrated into collider operation.

Wakes driven by transversely asymmetric electron beams in hollow plasma channels offer a promising route to high-quality positron acceleration, yet the conditions for sustaining long-lived, positron-favorable wakes remain to be clarified. Using fully three-dimensional particle-in-cell simulations, we identify two distinct driver evolution scenarios: quadrupole polarity reversal and continuous penetration of the driver into the plasma wall. We reveal the physical mechanisms governing this evolution and establish simple criteria for stable wake formation. The results provide practical guidance for sustaining long-lived quasi-steady wakes and enabling high-quality positron acceleration.

Inverse Compton scattering produces X-rays by colliding a laser pulse with relativistic electrons. Conventional head-on geometries require low beam energies, causing wide emission cones that limit brightness. A shallow, nearly co-propagating crossing angle lets a higher-energy beam deliver the same photon energy with far tighter collimation, yet this regime had not been explored in practice. The authors derive the geometric scaling laws of shallow-angle scattering, confirm them experimentally at a 5.8° crossing angle, and use the validated framework to evaluate future high-brightness radiation sources across diverse accelerator platforms.

This paper introduces a novel concept to reduce the longitudinal emittance in a low momentum compaction factor (low-alpha) storage ring, enabling the stable storage of electron bunches shorter than 100 nm. This design strategy can be applied to any quasi-isochronous storage ring to achieve very high radiation power through the longitudinal coherence of the emitted radiation. This paper addresses the nonlinear dynamics and aperture optimization for this type of storage ring. An optimal design example based on our analysis is presented. Single-particle tracking results demonstrate that an electron beam with an equilibrium root-mean-square bunch length of approximately 80 nm can be maintained in this ring, featuring a dynamic aperture larger than 1 mm.

We propose creative applications of axially magnetized permanent magnet rings for low-energy (keV) electron optics with an emphasis on preventing apparent emittance growth in a magnetic field, ease of integration, and minimization of parasitic aberrations. Extensive analysis of tolerances, and preliminary magnetic field measurements indicate that ‘extraordinary performance’ can be achieved using off-the-shelf ‘fridge’ magnets. Our design philosophy offers a different perspective on the otherwise arduous challenges of mechanical alignment of accelerator components.

Modern accelerators generate machine states that are high-dimensional and tightly coupled, making single channel monitoring an incomplete picture of true operating conditions. Building on graph neural network embeddings of the CEBAF injector, this work shows that 14 months of operational history collapses into a small number of persistent, physically interpretable regimes within a 16-dimensional learned space. The embedding supports stability baselining, outlier screening, and historical case-based retrieval, and a controlled beam study confirms it tracks deliberate reconfiguration coherently — demonstrating a practical operational reference space for holistic machine state monitoring.

A crabbing orbit is typically produced with crab cavities, but the same effect can arise through other mechanisms, including dispersion near an accelerating RF cavity or beam coupling impedance. We report the first experimental observation of crabbing induced by beam-beam interactions in the LHC. The measured displacement is approximately 10 m at one rms bunch length, in agreement with analytical predictions and numerical simulations. Wideband beam-position monitors resolve this signal, which is an order of magnitude smaller than the residual crabbing expected in the HL-LHC, validating a key diagnostic for future operation.

Polarized electron beams are essential for precision measurements at future storage-ring colliders, but conventional self-polarization can take many hours. We model repeated interactions with intense laser pulses and embed the resulting spin and energy kicks in six-dimensional BMAD tracking of an FCC-ee Z-pole bunch. At a beam-preserving operating point, the simulated polarization reaches 0.69% after 100,000 turns with no macroparticle loss, identifying both the promise and practical laser requirements of multishot polarization.

Extending electron cooling to very high energies can benefit significantly from redistribution of cooling between longitudinal and transverse degrees of freedom. Existing analytical treatments generally rely on a linear, small-amplitude approximation to the friction force, limiting their applicability to realistic cooler designs. We derive exact closed-form cooling rates with dispersive redistribution, expressed through Carlson elliptic integrals and reduced to elementary functions for a key special case. The results enable rapid and accurate optimization of high-energy electron coolers.

Quantifying the ion fraction of mixed helium-carbon ion beams is essential for developing such beams for online range monitoring in ion beam therapy. Until now, it could only be obtained from invasive measurements after extraction. We introduce a novel noninvasive method that infers the mixing ratio directly in the synchrotron from RF regulation loop corrections caused by small rigidity offsets between the two ion species. Validated by simulations and measurements, it has become an established diagnostic tool for mixed-beam accelerator and medical physics research and could support the clinical translation of mixed-ion beam therapy by providing a basis for a mixing-ratio interlock system.

Ultrashort electron beams from plasma accelerators enable a regime in which undulator radiation can be controlled on the scale of individual optical cycles. Simulations show that tailoring the undulator magnetic field can directly shape the emitted waveform, producing frequency-chirped pulses and cycle-to-cycle changes in polarization. This capability may open new opportunities for probing ultrafast dynamics and polarization-sensitive applications.

Laser modulation can improve the limited longitudinal coherence of modern storage rings, but most implementations use each region of an electron bunch only once per turn to generate coherent light. We propose multiple echo-enabled harmonic generation (multi-EEHG), which repeatedly uses the same region within one turn to deliver coherent light at different wavelengths to multiple beamlines. Simulations predict up to one billion photons per pulse with few-meV bandwidth without a monochromator. This approach makes more efficient use of stored electron bunches, enabling coherent light delivery to multiple experimental stations.

This work presents a new method for estimating the threshold current related to beam breakup (BBU) instability, utilizing a physics-inspired neural network. The proposed approach is significant for the development of efficient future particle accelerators. A multi-layered bidirectional long short-term memory (BiLSTM) neural network is used to analyze how the geometrical tolerances affect the BBU threshold current. The findings indicate that a physically meaningful feature significantly influences the predictive performance and generalization of the proposed BiLSTM-based neural network model.

We have demonstrated a new 11.424 GHz SLED-type RF pulse compressor capable of powering high-gradient X-band devices with pulse lengths around 20 ns. Our compact pulse compressor uses spherical cavities supporting axially symmetric TE modes with minimal electric fields on cavity surfaces, intended to improve high-power robustness versus existing compact spherical SLEDs using a TE dipole mode. Built and high-power tested at SLAC, it demonstrated a peak power of 317 MW over a FWHM of 27.1 ns. This is the highest peak power achieved from a single-stage compact SLED RF pulse compressor at this frequency, providing a viable route to the high-gradient, short pulse regime for RF accelerating structures.

High-energy accelerator facilities require efficient beam interception without sacrificing experimental uptime or floor space. This study presents a compact, wall-embedded switchable beam dump that integrates both beam-pass and full-interception states inside a single vacuum vessel to enable fast operational switching. By synchronously rotating dynamic shielding units by 90°, the system isolates the beam upstream while providing safe personnel access to downstream experimental areas. Radiation-transport and vacuum simulations demonstrate compliance with stringent safety criteria, offering a scalable design to enhance operational flexibility in complex beamlines.

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.