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Physics Physics Accelerator Physics

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Thursday 3 September

Accelerator Physics

Effects of the Direct Space Charge on Transverse Coupled-Bunch Instabilities

Chao Li

The ultralow transverse emittance of diffraction-limited electron storage rings leads to high beam densities, making direct space-charge effects increasingly relevant despite their relativistic suppression at high energy. In this work, we investigate the impact of direct space charge on transverse coupled-bunch instabilities, using PETRA-IV as a representative example. A Vlasov solver, multi-bunch particle tracking, and analytical calculations in normalized phase space are combined to study the coupled-bunch mode spectrum, the nonlinear incoherent tune spread, and the associated Landau damping. Particle-tracking simulations show that the space-charge-induced tune spread can provide sufficient Landau damping to stabilize higher-order head-tail modes. Landau stability diagrams constructed from the analytical tune spread of a frozen bi-Gaussian beam predict the stabilization of these higher-order modes, in agreement with the tracking simulations. These results demonstrate the important role of direct space charge in transverse coupled-bunch dynamics. The analysis framework developed here can also be extended to collective-instability studies in both electron and hadron accelerators.
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Accelerator Physics · Cross-listed from physics.plasm-ph

Dependence of self-injected bunch parameters on the plasma density gradient and laser pulse amplitude at LWFA in a conical plasma channel

D. S. Bondar, V. I. Maslov and I. N. Onishchenko

Laser wakefield acceleration (LWFA) is an advanced method of high gradient acceleration of charged particles with wakefield excited in plasma by laser pulse. A distinctive feature of this method is the ability to create and accelerate so-called self-injected bunches with unique parameters - charge, energy, emittance, and geometric dimensions - without the need for an external injector of the required bunches for their subsequent acceleration to higher energies. Self-injected bunches emerge due to the plasma electrons trapping by the excited wakefield (self-injection phenomenon). For many applications, self-injected bunches can be used directly in relevant experiments. In this paper the dependence of the self-injected bunch parameters on the laser pulse amplitude and longitudinal gradient of plasma density in tapered plasma channel is investigated using numerical simulation with the WarpX code. At the laser amplitude $a_0=E_0(m_{e0}c\omega/e)^{-1}=3.6$ in a conical channel with the radius decreasing from $4.0\,c/\omega_{pe}$ to $2.16\,c/\omega_{pe}$ and with the plasma density increasing linearly from $n_e=2.61\cdot10^{19}$ cm$^{-3}$ at the channel entrance to $3n_e$ at the exit, a self-injected bunch is obtained with the charge $32.1\,\mu$C/m, the mean longitudinal momentum $111.9\,m_ec$, the length $5.10\,\mu$m, the area $5.3\,\mu$m$^2$, the transverse emittance $1.6\cdot10^{-2}$ mm$\cdot$mrad.
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Wednesday 2 September

Accelerator Physics

Controlling a digital twin of the CEBAF injector from EPICS

Jude Doredant and Volker Ziemann

The Experimental and Instrumental Control System EPICS is used to control magnets and to read data from diagnostic devices in the CEBAF accelerator. We describe a prototype system where an EPICS installation, local on a laptop or Raspberry Pi, controls beam optics simulations. They can be running on the same computer or on a separate computer connected via the network. The prototype focuses on modeling and controlling the moderately complex injector beam line of CEBAF, which only comprises a few dozen magnets and beam monitors.
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Accelerator Physics

Rapid Mapping of Photocathode Quantum Efficiency: A Magnetized Electron Beam Imaging Approach

Yihan Liu, Lianmin Zheng and Yingchao Du

Quantum efficiency (QE) is a key property of photocathodes, and its uniformity is essential for producing high-brightness electron beams. Cathode imaging provides an in-situ and real-time approach for QE mapping, but in RF guns, a high charge per bunch is often needed to obtain a sufficient signal-to-noise ratio. Under such conditions, space charge effects can significantly degrade the imaging resolution and may even make point-to-point cathode imaging ineffective. In this paper, we propose a novel cathode imaging method based on a magnetized electron beam. Its feasibility is examined through theoretical analysis and beam dynamics simulations. The results show that the proposed method enables point-to-point cathode imaging in the ten picocoulomb charge regime. For a 10 pC, 3ps beam with a cathode magnetic field of 1200 Gauss, simulations indicate an imaging resolution of 11 um, representing nearly an order-of-magnitude improvement over the non-magnetized beam method.
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Monday 31 August

Accelerator Physics

A particle-in-cell model of beam dynamics in a dielectric wall accelerator

Christopher M. Lund, Paul M. Jung, Jamiel Nasser et al. (+6 more)

Dielectric wall accelerator (DWA) technology has been proposed as a compact, cost-effective alternative to rf accelerators for proton therapy, but its beam dynamics and practical feasibility remain relatively unexplored. In this work, we derive a three-dimensional, time-dependent axisymmetric electromagnetic field model from a prescribed on-wall excitation and implement it as stacked external field elements in the particle-in-cell code Warp. In the absence of experimental DWA beam-transport data, the implementation is cross-checked against a previously developed linear optics model in TRANSOPTR using deliberately idealized beam conditions. Strong agreement is observed between the two models in this regime. The models are then compared for larger transverse and longitudinal emittances, bunch charges up to 1x10^8e, and beam parameters representative of a low-energy proton source. The PIC simulations remain consistent with the linear predictions over much of the investigated range, while also identifying wall interactions, longitudinal phase-space distortions, and space-charge induced aberrations. The PIC model represents an intermediate step between linear optics and combined, geometry-specific electromagnetic and particle-transport simulations. It resolves particle-level transport and beam self-fields while retaining an analytical field description that can be varied without committing to a particular DWA structure. More detailed effects may be introduced through externally generated field data or empirical corrections, including models of cell-to-cell coupling, providing a practical basis for increasingly realistic DWA field and beamline studies.
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All authors Christopher M. Lund, Paul M. Jung, Jamiel Nasser, Morgan J. Maher, Julien Bancheri, Chau Giang Bui, Thomas Planche, Rick Baartman, Jan Seuntjens

Accelerator Physics · Cross-listed from cond-mat.mtrl-sci

Cryogenic Enhancement of Electron Spin Polarization from a Strained GaAs/GaAsP Superlattice Photocathode

Matt Grau, Colin Kirk, Greg Blume et al. (+7 more)

We report electron spin polarization of 95.0 +/- 0.8 (stat) +/- 2.4 (sys)% from a strained GaAs/GaAsP superlattice photocathode grown by metalorganic chemical vapor deposition (MOCVD) and cooled to dry-ice temperature (195 K). We achieved this polarization with 97.8% circularly polarized excitation light and a quantum efficiency of 0.7% at the peak polarization wavelength. This measurement exceeds the values of previously reported GaAs-based photocathode polarizations, which have clustered near 92% for two decades. We vary the temperature of the cathode and measure the polarization and quantum-efficiency spectra at 295 K, 273 K, 195 K, and 77 K. The polarization rises from 91.2(1)% at 295 K to its maximum at 195 K, while the spectral peak shifts from 775 nm to 739 nm (78 meV) over the full temperature range, tracking the widening band gap. The spectra probe two depolarization mechanisms: a thermalized transport channel that is suppressed on cooling, and energy-dependent hot-electron relaxation that persists and sets the low-temperature saturation. The polarization recovers after a full cooling and warmup cycle and is stable while the quantum efficiency decays, which disfavors surface energy filtering as the origin of the gain. These results indicate that modest cooling to 195 K, for which dry ice suffices, is a practical route to higher-polarization GaAs-based electron sources.
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All authors Matt Grau, Colin Kirk, Greg Blume, John Hill, Sushil Poudel, Alimohammed Kachwala, Marcy Stutzman, Joseph Michael Grames, Sylvain Marsillac, Matt Poelker

Accelerator Physics

Modernization and Statistical Validation of a Multilayer TRIM.SP Code for Low-Energy Muon and Ion Implantation

Zaher Salman, Ryan M. L. McFadden and Thomas Prokscha

We describe the staged modernization of a local multilayer TRIM.SP implementation, denoted TRIM.SP-NL. The work included behavior-preserving refactoring, correction of localized bookkeeping and initialization defects, support for more than five elements per layer, reproducible build modes, and a runtime-selectable random-number-generator (RNG) interface. The RANLUX RNG remains the default reference backend, while PCG32 and xoshiro256** provide faster alternatives without changing the input-deck format. The code validation combined exact regression testing with statistical comparisons over 13 representative target multilayer configurations, comprising 143 energy-configuration points per backend and 429 simulations in total. Across the test suite, PCG32 and xoshiro256** reduced runtime by factors of 1.98 and 2.01, respectively. The implanted, backscattered, and transmitted fractions, as well as the mean implantation depths, range straggling, and representative depth profiles showed no systematic RNG dependence. Together with a Node.js-based graphical interface for setup, scans, and result collection, the Fortran engine forms the open-source TRIM.SP-NL Workbench.
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Accelerator Physics

Magnetizing nonlinear plasma wakefields for positron acceleration

Yung-Kun Liu, Pisin Chen, Ching-En Lin et al. (+2 more)

It is known that only a narrow plasma wakefield sliver in the electron-beam-driven blowout regime suits positron acceleration. Using 3D simulations, we show that matching the cyclotron frequency $\omega_{c}$ with the plasma frequency $\omega_{p}$ forms a stable electron column on axis, expanding the suitable phase space for positron acceleration sizably. For a plasma density $n_p = 10^{16}\text{ cm}^{-3}$ in a 35 T field, the interval expands 4.3 times, and a witness positron beam gains 100–150 MeV over 6 cm ($1.6$–$2.5$~GeV/m) with a 92% capture rate.
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All authors Yung-Kun Liu, Pisin Chen, Ching-En Lin, Spencer Gessner, Bernhard Hidding

Friday 28 August

Accelerator Physics

Macroscopic Classical and Quantum Models of Inverse Compton Scattering

Emerson Rogers

Inverse Compton sources — in which a relativistic electron beam scatters a laser pulse to produce tunable, collimated, high-energy radiation — have emerged as promising compact radiation sources, with applications in nuclear photonics, medical imaging, and nanoscale metrology. As laser intensities increase and electron energies grow, the interaction enters the radiation reaction regime, where the energy radiated by the electron becomes a significant fraction of its kinetic energy. Predicting the scattered electron energy spectrum from first principles has remained an unsolved problem. Existing models are either classical equations of motion requiring large-scale multiparticle simulation, or quantum models relying on approximations of uncertain validity with the same simulation burden. None directly produce a spectral prediction. This dissertation presents a novel framework that resolves this deficiency. The laser pulse is represented as a coherent quantum state of the electromagnetic field and the electron beam as a statistical quantum state encoding its momentum distribution — the natural realization of particle-field scattering within quantum electrodynamics. For a Gaussian laser pulse, a closed-form analytic expression for the scattered spectrum is derived requiring no simulation, no approximation of the laser field profile, and no large particle ensembles. The classical limit recovers Landau-Lifshitz dynamics exactly, establishing the coherent-state model as the quantum electrodynamic realization of classical radiation reaction theory. The framework is validated against existing experimental data, and fundamental structural limitations of all current modeling approaches are identified and analyzed.
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Accelerator Physics · Cross-listed from cond-mat.supr-con

A Three-Fluid Model of Dissipation at Surfaces in Superconducting Radiofrequency Cavities

Matthias Liepe Michelle M. Kelley, Sean Deyo, Nathan Sitaraman et al. (+4 more)

Experiments on superconducting cavities have found that under large RF fields the quality factor can improve with increasing field amplitude, a so-called “anti-Q slope.” We numerically solve the Bogoliubov-de Gennes equations at a superconducting surface in a parallel magnetic field, finding at large fields there are surface quasiparticle states with energies below the bulk superconducting gap that emerge and disappear as the field cycles. Modifying the standard two-fluid model, we introduce a “three”-fluid model where we partition the normal fluid to consider continuum and surface quasiparticle states separately. We compute dissipation in a semi-classical theory of conductivity, where we provide physical estimates of elastic scattering times of Bogoliubov quasiparticles with point-like impurities having potential strengths informed from complementary ab initio calculations of impurities in bulk niobium. We show, in this simple yet effective framework, how the relative scattering rates of surface and continuum quasiparticle states can play a role in producing an anti-Q slope while demonstrating how this model naturally includes a mechanism for turning the anti-Q slope on and off.
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All authors Matthias Liepe Michelle M. Kelley, Sean Deyo, Nathan Sitaraman, Danilo B. Liarte, Tomás Arias, James P. Sethna, Thomas Oseroff

Accelerator Physics

Wakefield-Dressed Relativistic Vortex Electrons in Plasma Accelerators

Zhigang Bu, Lingang Zhang and Liangliang Ji

Plasma wakefield acceleration is usually regarded as a classical mechanism for producing high- energy charged-particle beams. Here we show that an axisymmetric plasma wakefield can also act as a moving quantum structure that supports relativistic vortex electron states. Starting from the Dirac equation, we derive the electron spinor eigenstates with definite total angular momentum in an ideal bubble-regime wakefield. The transverse focusing field confines and quantizes the electron transverse motion into Laguerre-Gaussian vortex modes, while the longitudinal electric field acceler- ates the electron without destroying the symmetry-protected angular momenta. We further analyze the localized and off-axis vortex electron wave-packets, and non-ideal wakefield perturbations, and identify the conditions for preserving electron-vortex-state purity. These results suggest plasma wakefield as a route toward high-energy vortex electrons and extend plasma-based acceleration from classical beam dynamics to quantum-state control of relativistic particles.
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Accelerator Physics · Cross-listed from physics.ins-det

Full-field fluorescence computed tomography (F3CT) using a calibrated virtual cone-beam pinhole geometry

Thomas Zillhardt, Yunhui Chen, Alexander Rack et al. (+4 more)

We present F3CT, a synchrotron-based hyperspectral full-field fluorescence computed tomography technique that avoids raster scanning by combining a pinhole aperture with an energy-resolving 2D detector. A virtual cone-beam model and two-stage calibration-reconstruction workflow enable 3D elemental mapping under full-field illumination. The method is demonstrated on biological and geological specimens, resolving silver-stain distributions in zebrafish tissue and high-energy fluorescence signatures in rock cores. Phase-contrast tomograms acquired sequentially under the same experimental geometry provide co-registered structural context. F3CT provides a high-throughput route to 3D XRF imaging and establishes a foundation for correlated structural and chemical tomography, with potential for future in-situ and operando implementations.
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All authors Thomas Zillhardt, Yunhui Chen, Alexander Rack, Matthew Veale, Matt Wilson, Philip J. Withers, Nicola Viganò

Accelerator Physics · Cross-listed from hep-ph

Future of Artificial Intelligence for Science in Japan 2024 Community Report

Yoshitaka Itow, Jia Liu, Hirokazu Maesaka et al. (+16 more)

This white paper summarizes scientific challenges and AI/ML research opportunities identified through the FAIRS Japan 2024 unconference process. The discussion focuses on three major physics domains: accelerator physics, cosmology and astrophysics, and neutrino physics. Although each domain has distinct scientific goals and experimental constraints, several common technical themes emerge: high-dimensional reconstruction, fast and accurate simulation, uncertainty propagation, simulation-to-data mismatch, anomaly detection, real-time decision-making, and shared infrastructure.
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All authors Yoshitaka Itow, Jia Liu, Hirokazu Maesaka, Vinicius Mikuni, Nhat-Minh Nguyen, Hironao Miyatake, Atsushi J. Nishizawa, Patrick de Perio, Daniel Ratner, Kazuhiro Terao, Leander Thiele, Omar Alterkait, Francois Drielsma, Rocio Garcia, Masako Iwasaki, Ahsani Hafizhu Shali, Federica Tarsitano, Takahiro Terada, Junjie Xia

Thursday 27 August

Accelerator Physics

Large Magnet Structures for Fusion Technology: Lessons from ITER

Neil Mitchell

Over a period of 40 years, the ITER project has provided many examples in large magnet structures, from pre-concept to design to manufacture to delivery and assembly, from which lessons can be learned for the future. This presentation is divided into three parts. The first, 'Making and Assembling Large (steel) Cryogenic Structures for Magnets' is based on ITER Coil Structure experience, particularly the Central Solenoid (CS) and Toroidal Field (TF) structures and particularly on large 316LN forgings weighing about 50t individually, requiring then high accuracy machining and deep (up to 0.3m) welds with controlled distortion. New materials development is often proposed. This is easy on a laboratory scale, but often not transferable to an industrial large scale. Examples are provided. The issues to be considered (and solved) are large scale quality (and repair of defects), joining, tolerances and assembly. The second 'Designing Large Steel Support Structures for Magnets' is concerned with structural design codes & criteria, and modern analysis, especially of imperfect, but manufacturable structures. Points that are considered are typical critical issues with cyclically loaded thick components, analysis of partial penetration welds and other welds not permitted under normal American Society of Mechanical Engineers (ASME) welding procedures (including fillet welds and/or those volumetrically not inspectable by Non-Destructive Examinations (NDE)) and allowing for tolerance compensation (i.e. designing with overmetal or designing for shimming). The third 'Avoiding or Reducing Large Steel Support Structures for Magnets' shows how it is possible to choose other routes to include structural materials in magnets, for example (as in ITER TF coils) by using plates to contain the conductor, or by using a thick conductor jacket to at least share the loads with massive steel cases.
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Accelerator Physics

Muon acceleration at J-PARC

Shusei Kamioka, Masato Kimura and Yuga Nakazawa

Muon acceleration is a key technology for producing low-emittance muon beams over a wide energy range. Various acceleration schemes have been proposed for applications ranging from low-energy $\mu$SR and precision particle-physics measurements to neutrino factories and muon colliders. Experimental demonstrations of muon acceleration, however, have so far been limited. At J-PARC, a positive-muon accelerator based on the production and acceleration of ultraslow muons is being developed, and in 2024 the first RF acceleration of positive muons was demonstrated. In this review, we provide a brief overview of muon-acceleration methods and related experiments, and then review the acceleration method, current status, and future prospects at J-PARC.
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Accelerator Physics

Machine Learning Based ROI Segmentation for Beam Imaging Diagnostics at Accelerators

Prachiti Sujit Chandratreya, Frank Mayet, Sergey Tomin et al. (+1 more)

The European XFEL accelerator produces high-brightness X-ray pulses using relativistic electron beams. Beam characterization is performed at multiple diagnostic stations using scintillator screens, where beam images are analyzed to extract key parameters such as emittance, energy spread, and current profile. Accurate detection of the region of interest in these images is essential for reliable beam diagnostics and stable accelerator operation. Conventional methods, such as bounding-box-based beam localization, can become less reliable for complex or non-ideal beam profiles, such as low-intensity signals, tilted or streaked beams, and multiple beam structures. In this work, we develop machine learning-based approaches for region of interest detection directly from beam images. These ML methods adapt to variations in beam shape and intensity and enable more precise, pixel-level identification of beam regions. Experimental results demonstrate improved robustness and accuracy in challenging conditions compared to traditional techniques. The proposed methodology is validated at the European XFEL and is broadly applicable to image-based beam diagnostics across accelerator facilities.
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All authors Prachiti Sujit Chandratreya, Frank Mayet, Sergey Tomin, Jitendra Kumar

Accelerator Physics

Digital Signal Processing for Beam Instrumentation

Andrea Boccardi

These proceedings summarise a lecture on Digital Signal Processing (DSP) delivered at the CERN Accelerator School (CAS) on Beam Instrumentation (BI). In this context, a signal is any measurable quantity conveying information about the beam. A digital signal is obtained by discretising a continuous-time signal in both time (sampling) and amplitude (quantisation). Digital signal processing refers to the set of techniques used to analyse and manipulate such signals. Given the breadth of the field, this contribution focuses on fundamental DSP concepts and on techniques most commonly encountered in beam instrumentation applications.
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Wednesday 26 August

Accelerator Physics

A European high-energy heavy-ion facility for electronics irradiation based at CERN: Concept Design Report

Edgar Cristopher Cortés García, Ruben García Alia, Matthew Alexander Fraser et al. (+2 more)

This proposal presents a concept design for a European high-energy heavy-ion facility dedicated to electronics radiation-effects testing, directly addressing the critical challenges of global beam time shortages and energy limitations in existing facilities. Aligned with the HORIZON-CL4-2026-SPACE-03-85 call under Horizon Europe's Cluster 4 (Digital, Industry, and Space), the facility will enhance European sovereignty in space technology by providing approximately 1,900 additional hours of heavy-ion beam time annually–a 50% increase in European capacity–and extending the accessible ion energy range up to 100 MeV/nucleon, significantly improving penetration capabilities for state-of-the-art electronics testing. The facility will leverage CERN's Low Energy Ion Ring (LEIR) synchrotron, integrating a dedicated extraction system, transfer line, and user experimental area. The project timeline includes a technical design study, followed by infrastructure and hardware production, installation, commissioning, and user operations. Funding will be secured through a collaborative framework involving CERN and the European Union, ensuring sustainable deployment and long-term operational success.
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All authors Edgar Cristopher Cortés García, Ruben García Alia, Matthew Alexander Fraser, Maciej Slupecki, Markus Widorski

Accelerator Physics

Feasibility demonstration of a high-momentum, high-purity muon beamline at the J-PARC Hadron Experimental Facility

Kotaro Shirotori, Takaya Akaishi, Kazuya Aoki et al. (+16 more)

We have established and characterized a novel high-purity muon beam delivery scheme, designated as $\mu$20, at the high-momentum hadron beamline (B-line) of the J-PARC Hadron Experimental Facility. This scheme delivers tertiary muons produced via the in-flight backward decays of secondary pions in the straight section of the beamline, inherently suppressing parent hadron contamination. In a systematic performance evaluation, the muon beam achieved a standalone intensity of $1.7\times10{^3}$ particles/spill with a spatial spread (standard deviation, $\sigma$) of approximately 15 mm at the downstream detector position. A systematic attenuation analysis utilizing an iron absorber verified a high muon purity of 98.0$\pm$0.6% for the 3 GeV/$c$ muon beam mode. Crucially, this work marks the first successful feasibility study of a high-momentum and high-purity muon beamline operation at an experimental facility in Japan.
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All authors Kotaro Shirotori, Takaya Akaishi, Kazuya Aoki, Wen-Chen Chang, Ryotaro Honda, Yusuke Hori, Takatsugu Ishikawa, Nobuyuki Kobayashi, Ruri Kurasaki, Che-Sheng Lin, Yuhei Morino, Megumi Naruki, Hiroyuki Noumi, Kyoichiro Ozawa, Shinya Sawada, Ken Suzuki, Hitoshi Takahashi, Taiga Toda, Natsuki Tomida

Tuesday 25 August

Accelerator Physics

Enhancing 10 TeV $\gamma\gamma$-collider luminosity through scattering-laser wavelength selection in the presence of prolific electron-positron pair production

S. S. Bulanov, T. Barklow, C. Benedetti et al. (+8 more)

A $\gamma\gamma$-collider capable of reaching the 10 TeV parton-center-of-momentum (pCM) frontier of particle physics may enable the study of phenomena beyond the Standard Model. Based on compact linear wakefield accelerator technology, such a collider could be realized by Compton scattering multi-TeV lepton beams off moderate-intensity laser pulses close to the collider interaction point, producing the required $\gamma$-photons. It is shown that, for a wide range of scattering-laser wavelengths, $\gamma\gamma$-collisions at the interaction point can meet the luminosity requirements for novel particle physics studies, even in the presence of the prolific electron-positron pair production that accompanies the interaction of the scattering laser with the multi-TeV lepton beam. Notably, this pair production imposes a natural limit on the maximum achievable photon luminosity. Accounting for this limit and for the angular divergence of the Compton photons yields an enhanced $\gamma\gamma$-collider luminosity for 250 nm and 1.25 nm scattering lasers. Moreover, the secondary pairs can themselves be exploited in physics studies, since their luminosity is high enough to produce heavy particles at rates needed for discoveries well beyond the reach of existing colliders.
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All authors S. S. Bulanov, T. Barklow, C. Benedetti, A. Formenti, S. Gessner, R. Lehe, A. Rastogi, S. Pagan Griso, C. B. Schroeder, A. Schwartzman, J. Osterhoff

Accelerator Physics

High-charge collimated and energy-selected laser-driven MeV electron beams produced by magnetic selection

I. Cohen, I. Slabu, Q. Peysson et al. (+16 more)

We have developed a compact passive energy-selector for MeV-range electrons produced by irradiating solid targets by ultra-intense short-pulse lasers. The device allows for generating electron beams with a variable energy spread over a broad range of energies, from tens of keV to tens of MeV. Here we have demonstrated its use by producing electrons from solid targets in the MeV range and with a ~10% bandwidth, thereby compensating the intrinsic broadband nature of the electrons produced from such source. Coupled with a pulsed magnetic field to further compensate the intrinsic large divergence of this source, it allows to produce a highly-collimated beam of narrow-band and ultra-fast electrons, suitable for a wide range of applications, e.g. radiation therapy or time-resolved electron probing.
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All authors I. Cohen, I. Slabu, Q. Peysson, S. Dorard, Y. Abe, J. Béard, T. Moraine, S.N. Chen, A. Chessa, K.Iida, P. Kempski, Y. Kuramitsu, H. Kusano, F. Nikaido, M. Ruszkowski, K. Sakai, N. Tamaki, O. Tesileanu, J. Fuchs

Monday 24 August

Accelerator Physics

White Paper on a Novel FFA-Based CEBAF Upgrade to 22 GeV

S. A. Bogacz, R. M. Bodenstein, K. E. Deitrick et al. (+11 more)

We present a conceptual design for a cost-effective upgrade of the Continuous Electron Beam Accelerator Facility (CEBAF) to 22 GeV using non-scaling fixed-field alternating-gradient (FFA) arcs built from Halbach-style permanent magnets. Building on the eight-pass energy-recovery demonstration at the Cornell–BNL CBETA test accelerator, the design replaces the highest-energy recirculation arcs with a pair of FFA arcs that simultaneously transport six passes spanning a factor-of-two momentum range. We describe the machine layout; including the injector, recirculating linacs, spreaders and recombiners, FFA arcs, splitters, transition and extraction regions; together with beam-dynamics validation studies covering emittance growth, synchrotron-radiation-driven depolarization, and orbit correction, and we summarize permanent-magnet design, prototyping, and radiation-resiliency results. This white paper documents the accelerator physics underpinning a staged path to 22 GeV that preserves CEBAF's multi-hall, high-luminosity, polarized-beam capabilities.
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All authors S. A. Bogacz, R. M. Bodenstein, K. E. Deitrick, B. R. Gamage, R. Kazimi, D. Khan, E. Nissen, S. Ogur, Y. Roblin, P. Rossi, J. S. Berg, S. J. Brooks, D. Trbojevic, V. Morozov

Sunday 23 August

Accelerator Physics

Stationary electron vortex states in a plasma bubble field

Hui-Dong Huang, Qi Meng, Zhi-Bin Wang et al. (+3 more)

Plasma wakefield accelerators (PWFAs) offer accelerating gradients of 10-100~GV/m and relativistically propagating plasma bubbles capable of confining charged particles. We study the stationary states of a vortex electron at the bubble center by solving the corresponding quasi-relativistic Schrödinger equation. Analytical solutions are obtained with Laguerre-Gaussian transverse modes and Hermite-Gaussian longitudinal envelopes. Comparing the resulting beam parameters with experimentally accessible vortex-electron bundles, we find that the transverse beam waist supported by the plasma bubble is comparable to that achieved by current electron-optical techniques. The longitudinal confinement further provides a favorable parameter regime for stable injection. Our results indicate the feasibility of maintaining localized vortex-electron states in a plasma-bubble wakefield and provide an analytical starting point for investigating their subsequent acceleration and stability.
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All authors Hui-Dong Huang, Qi Meng, Zhi-Bin Wang, Liang Lu, Jian Chen, Li-Ping Zou

Accelerator Physics · Cross-listed from physics.plasm-ph

Wavefront-Guided Electron Injection for Direct Laser Acceleration in Relativistic Laser-Driven Plasma Channel

Heng Wang, Liang Sheng, Long Zeng et al. (+2 more)

We investigate electron injection into direct laser acceleration (DLA) in relativistic laser-driven plasma channels using particle-in-cell simulations. We identify and characterize a wavefront-guided injection mechanism, in which electrons are continuously fed into the plasma channel through the density pile-up layer at the laser-pulse front. Phase-space analysis reveals a localized injectable region within the pile-up layer, indicating that only a selected subset of electrons satisfies the conditions required for the subsequent direct laser acceleration. This mechanism provides a physical interpretation for the high-charge capability of DLA by explaining how electrons are continuously supplied to the accelerating channel. Beyond this continuous supply process, the injection dynamics are further modulated by the periodic variation of the carrier phase at the laser-pulse front. The spatial locations of injected electrons are found to be closely associated with magnetic-island structures formed under laser-phase modulation, suggesting that the laser wavefront not only supplies electrons but also organizes their entry into the accelerating channel. These findings advance the physical understanding of energetic-electron generation in relativistic laser-driven subcritical-density plasma channels and are relevant to the development of compact DLA-based particle and radiation sources.
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All authors Heng Wang, Liang Sheng, Long Zeng, Yang Li, Zheng Gong

Friday 21 August

Accelerator Physics

Flow-based surrogate models for particle tracking

Matthias Remta, Yann Dutheil and Francesco Velotti

Particle tracking is a fundamental tool for particle-accelerator design and optimisation. Conventional tracking routines provide high accuracy but are computationally demanding, especially when simulating large particle ensembles or long time spans. As a result, optimising moderate- to high-dimensional parameter spaces is challenging, and real-time surrogate models remain out of reach for many applications. This contribution introduces a surrogate-modelling approach based on conditional flow matching (CFM). A CFM model is trained on tracking simulations of CERN's Proton Synchrotron (PS) over a 10-dimensional parameter space. The trained model reproduces final phase-space distributions with a median squared maximum mean discrepancy MMD$^2$ of $3\times 10^{-4}$ and mean inference time of 0.04 s, a speed-up of three orders of magnitude over conventional tracking. To capture distribution-dependent dynamics that vanilla CFM cannot represent, we extend the model with cross-attention over the initial particle ensemble (Cross-Attention-CFM) and demonstrate that this extension recovers performance on a space-charge benchmark in the PS, where vanilla CFM degrades. Finally, we introduce Hybrid-CFM, in which a small number of conventionally-tracked particles are used to inform the model. On the same 10-dimensional PS task, Hybrid-CFM with 100 auxiliary particles trained on 200 distributions matches the vanilla CFM trained on 1500, and improves the worst-case (90th-percentile) MMD$^2$ by roughly a factor of four, substantially reducing the upfront cost of building a surrogate.
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Thursday 20 August

Accelerator Physics

Energy spectrums of slowed neutrons in heterogeneous uranium-carbon and thorium-carbon fission media

Vyacheslav Lavrukhin, Serhiy Chernezhenko, Mykhailo Shcherbyna et al. (+2 more)

Using the Geant4 and OpenMC Monte Carlo codes, the energy spectra of moderated neutrons were studied in homogeneous uranium dicarbide and uranium dioxide media, as well as in heterogeneous uranium and carbon and thorium and carbon channel type fission structures with various compositions and lattice parameters. In homogeneous uranium dicarbide the spectral maximum lies in the from 20 to 50 keV range, which is important for the design of a prototype fast single channel reactor operating in the traveling wave fission mode with a soft fast neutron spectrum. A heterogeneous thorium and carbon medium that forms a thermal neutron spectrum has been identified, enabling the realization of a traveling wave mode of neutron nuclear fission on thermal neutrons. By varying channel lattice parameters and introducing burnable absorbers (Cd, In), the possibility of forming a super thermal neutron spectrum (from 1 to 20 eV) in a heterogeneous uranium and carbon medium was demonstrated. The results are useful for the development of nuclear transmutation reactors and next generation reactors operating in the traveling wave mode.
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All authors Vyacheslav Lavrukhin, Serhiy Chernezhenko, Mykhailo Shcherbyna, Viktor Tarasov, Edgar Zhelezko

Accelerator Physics

Electron energy gain in a dielectric laser accelerator as a function of the base angle of a triangular grating structure

O.O. Svystunov, A.V. Vasyliev, I.V. Beznosenko et al. (+2 more)

Dielectric laser accelerators (DLAs) represent a compact and cost-effective alternative to conventional RF accelerators. Despite the various grating geometries already studied, a comprehensive investigation of triangular profiles, particularly the effect of the base angle of the saw-tooth grating profile, remains insufficient. This paper presents the results of numerical particle-in-cell (PIC) simulations of electron acceleration in DLAs based on double gratings with a triangular profile. One of the gratings, onto which the laser beam is incident, is transparent, while the second grating was either transparent or reflective for the laser pulse. The geometry of the first grating was fixed. A systematic study was conducted on the influence of the base angle of the second grating ($\alpha = 5^\circ - 44^\circ$), its spatial orientation, the presence of a reflective gold coating, and the shape of the incident laser pulse (plane wave versus Gaussian profile) on the acceleration rate. The results reveal a complex interplay between these parameters. For an electron beam with an initial energy of 10 MeV, a maximum accelerating rate of 345 MeV/m was achieved for the structure with a left-handed reflective grating and a base angle of $\alpha = 10^\circ$ when excited by a Gaussian pulse. Under plane-wave excitation, the left-handed reflective grating with $\alpha = 25^\circ$ provided a rate of 325 MeV/m. It is shown that for single-bunch injection, nearly the maximum energy gain can be attained within a temporal window of 0.5 fs. The obtained results provide a quantitative basis for optimizing DLA designs with triangular gratings, highlighting the significant role of the saw-tooth profile base angle and the grating type in achieving high rates for next-generation accelerators.
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All authors O.O. Svystunov, A.V. Vasyliev, I.V. Beznosenko, R.A. Melnichuk, G.V. Sotnikov

Accelerator Physics

Mutually phase-stable tunable attosecond soft X-ray attosecond pulses from a free-electron laser

River Robles, Kurtis Borne, Mathew Britton et al. (+16 more)

We demonstrate the production of mutually phase-stable attosecond X-ray pulse pairs with tunable relative time delays and phases in a cascaded X-ray free-electron laser. We showcase the method in an experiment at the LCLS-II, in which a shaped electron beam is used in a split undulator configuration to generate the two attosecond pulses. We achieve mutual phase stability by reusing microbunching generated in the first undulator in order to seed the FEL process in the second at a detuned frequency. We measure controllable temporal delays between the two pulses directly in the time domain using angular streaking of photoelectrons, with a step size of 250 attoseconds. We then show that the behavior of the X-ray spectrum is consistent with phase stability between the two pulses, with a relative phase that can be easily tuned using inter-undulator phase shifters. This method is particularly well-suited to few to ten eV energy separations and sub to few femtosecond time delays, which are ideal for experiments in the soft X-ray regime for pushing the limits of our models for molecular dynamics and exerting direct coherent control over quantum systems.
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All authors River Robles, Kurtis Borne, Mathew Britton, David Cesar, Paris Franz, Veronica Guo, Gabriel Just, Kirk A. Larsen, Randy Lemons, Vyacheslav Leshchenko, Xiang Li, Ming-Fu Lin, Razib Obaid, Nicholas Sudar, Jun Wang, Zhen Zhang, James Cryan, Taran Driver, Agostino Marinelli

Tuesday 18 August

Accelerator Physics

Machine learning assisted Bayesian calibration of an accelerator digital twin from orbit response data

Christopher Kelly, Kevin A. Brown, Georg H. Hoffstaetter et al. (+2 more)

Digital twins of particle accelerators are used to plan and control operations and to design data collection campaigns. Accurate modeling typically requires knowledge of quantities that are hard to measure directly, e.g., magnet alignments, transfer functions relating power supply currents to magnetic fields, magnet nonlinearities, and stray fields. In this work we introduce multiplicative parameters to the quadrupole transfer functions to parametrize these effects. We use Bayesian methods to probabilistically estimate these parameters and their uncertainties by calibrating the Bmad digital twin to beam measurements performed at the AGS Booster at Brookhaven National Laboratory. The inference is computationally accelerated using a machine learning emulator of the physical accelerator digital twin trained to a perturbed-parameter ensemble of Bmad simulations. The result is a joint posterior distribution over the parameters constrained by the data, taking into account beam monitor errors. Incorporating estimates of the parameters into the digital twin is shown to result in a significant improvement in the quality of the model and provides error bars on the model parameters and predictions.
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All authors Christopher Kelly, Kevin A. Brown, Georg H. Hoffstaetter, Weijian Lin, Nathan M. Urban

Accelerator Physics

Field deployment of a laser wakefield accelerator for on-site application

Bo Guo, Xiaonan Ning, Dexiang Liu et al. (+7 more)

Successive innovations in particle accelerators have continually expanded the frontiers of scientific discovery. Laser wakefield accelerators promise to transform science, medicine, and industry, yet moving them from laboratory demonstrations to reliable real-world operation has remained a central, long-standing challenge. Here we report a field-deployable system that produced 100-MeV-class electron beams with 1%-level energy stability during 72 hours of continuous operation and supported routine full-power use throughout a seven-month field trial in an industrial setting. Applied to in situ micro-nondestructive testing, the system generated tens-of-MeV bremsstrahlung X-rays that enabled three-dimensional microtomography of dense materials at sub-50-{\mu}m spatial resolution and revealed 100-{\mu}m-scale internal defects in large composite structures, extending the capabilities beyond those of existing high-energy X-ray sources. These results mark a transition of laser wakefield acceleration from laboratory proof of concept toward practical deployment in scientific and industrial applications.
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All authors Bo Guo, Xiaonan Ning, Dexiang Liu, Yue Ma, Weiwang Zeng, Mingyuan Wei, Shengtai Wei, Jianfei Hua, Yang Wan, Wei Lu