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Friday 18 September

Accelerator Physics · Cross-listed from physics.ins-det

A compact fibre-coupled active beamstop for fast time-resolved monitoring of synchrotron X-ray beams

Friedjof Tellkamp, David von Stetten and Martin Kollewe

We have developed a compact fibre-coupled active beamstop for simultaneous interception and time-resolved monitoring of the direct X-ray beam in synchrotron diffraction experiments. The complete beamstop tip has a maximum transverse dimension of 1 mm and incorporates a 700 $\mu$m-wide Ce:YAG scintillator. The scintillator converts the intercepted X-rays into visible light, which is transported through a multimode optical fibre to a silicon photodiode at the bottom of the blade that is connected to a dedicated fast amplifier electronics. Tests with a pulsed laser yielded a system decay time of 69.4 ns, consistent with reported luminescence decay times for Ce:YAG. Measurements at the P14.EH2 end station of PETRA III demonstrated the detection of individual bunch signals separated by 192 ns in the 40-bunch operating mode. The active beamstop also provided an in-situ timing signal for synchronising an experimental sequence with the approximately 190 $\mu$s transmission window of a mechanical X-ray chopper. The device therefore combines a compact obstruction of the direct beam with fast monitoring of X-ray timing and flux during diffraction data acquisition.
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Thursday 17 September

Accelerator Physics

Reconstruction of Beam Transverse Parameters in the Fermilab Side-Coupled Linac Using a Normalized Coordinate Framework

E. V. Chen, J-P. Carneiro, R. V. Sharankova et al. (+1 more)

Quadrupole scans are a commonly used tool for beam second moment reconstruction. Limitations in the strength of the magnets and layout of the beamline elements frequently preclude simple quadrupole-drift-detector scans from collecting sufficient data for reconstruction. Using a normalized coordinate framework, we characterize the prerequisites for a robust simple quadrupole scan and expand these prerequisites to reconstruction from more complex optics. The beam second moments are investigated at two locations in the Fermilab Side-Coupled Linac under simple and complex optics, using this framework to maximize information gained from wire scanner profile measurements.
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All authors E. V. Chen, J-P. Carneiro, R. V. Sharankova, A. V. Shemyakin

Wednesday 16 September

Accelerator Physics

Coherent collective amplification of terahertz microbunching seeded by laser frequency beating in relativistic electron beams

Wencai Cheng, Yin Kang, Kaiqing Zhang et al. (+5 more)

High-power, continuously tunable terahertz sources based on free-electron lasers require precise control of electron-beam microstructures. Quantitative prediction of the modulation amplitude across a broad frequency range remains challenging because the laser-induced distribution and its collective evolution must be treated together. To describe this coupled evolution, a nonlinear model is developed for microbunching seeded by a frequency-beating laser heater. The non-Gaussian heater-exit distribution is obtained by optical phase averaging and propagated through multistage compression in six-dimensional phase space. Source-induced correlations are retained, with space charge, coherent synchrotron radiation, and radio-frequency wakefields evaluated self-consistently. The wavelength-dependent bunching response is thereby connected to the laser beat frequency, laser power, and compression partition. The short-wavelength double peak is found to be governed mainly by longitudinal space charge, while bunching near a selected wavelength can be enhanced by redistributing compression at fixed total compression. The predicted source modulation and downstream response are benchmarked against Elegant and IMPACT-Z, respectively. Experimental measurements of the wavelength-dependent bunching factor at different laser pulse energies are found to agree well with the trends predicted by theory and simulation. The resulting framework provides a computationally efficient and predictive route to optimizing electron-beam microbunching for tunable, high-power terahertz generation at free-electron-laser facilities.
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All authors Wencai Cheng, Yin Kang, Kaiqing Zhang, Zhen Wang, Duan Gu, Guangling Chen, Chao Feng, Haixiao Deng

Accelerator Physics

Heavy ion driven plasma wakefield acceleration with drift-like phase-shift acceleration scheme

Li Jiangdong, Xia Guoxing, Liu Jie et al. (+2 more)

Traditional plasma density modulation schemes can extend the dephasing length but suffer from wakefield degradation as plasma density increases. To address this limitation, a drift-like phase-shift acceleration scheme is proposed, introducing drift sections between plasma cavities to enable controlled phase shifting and avoid dephasing. Idealized simulation results show that electrons are accelerated from 16 MeV to 700 MeV within 1.65 m with an energy spread of 5%. Furthermore, segmented simulations introducing realistic active plasma lenses verify the practical feasibility of this scheme, electrons are accelerated up to 720.5 MeV with a reduced energy spread of 3.3% within 1.67 m. The plasma wakefield maintains a robust amplitude throughout the propagation, suggesting the potential for continued acceleration beyond the simulated region. This scheme demonstrates a promising way to enable the witness beam to gain energy continuously from the tail to the head of the driver beam and provides a new approach for generating high-energy, high-quality beams in heavy ion driven plasma wakefield acceleration.
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All authors Li Jiangdong, Xia Guoxing, Liu Jie, Li Guangxian, Yang Jiancheng

Accelerator Physics

Ultralow Mean Transverse Energy and High Quantum Efficiency Cryogenic Bialkali Photocathode for MHz-Repetition-Rate Electron Sources

D. Wang, S. Liu, J. Liu et al. (+15 more)

Simultaneously achieving high quantum efficiency (QE), ultralow mean transverse energy (MTE), and robust long-term operation under conditions relevant to continuous-wave (CW) X-ray free-electron lasers (XFELs) remains a central challenge for semiconductor photocathodes. This challenge arises from the trade-off between QE and MTE, as well as the difficulty of maintaining stable operation in high-field CW electron guns. Here we demonstrate a cryogenic K2CsSb photocathode that simultaneously achieves high QE, ultralow MTE, and robust long-term operation in a CW gun under XFEL-relevant operating conditions. Under cryogenic operation, the photocathode achieves an MTE of 50 meV while sustaining a QE of 5.4%. Milliampere-level CW current, including operation at 5 mA, was demonstrated together with an approximately 20-day operational history. The observations are consistent with improved carrier survival and/or surface escape in photocathodes prepared using the optimized recipe. These results show that the practical QE-MTE trade-off can be substantially mitigated in cryogenic bialkali photocathodes and provide a practical pathway toward high-brightness electron sources for CW XFELs and energy-recovery linacs.
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All authors D. Wang, S. Liu, J. Liu, Y. Dai, Z. Hong, J. Wang, Y. Shi, M. Tai, L. Feng, H. Xu, L. Lin, F. Wang, F. Zhu, J. Hao, S. Quan, K. Liu, H. Xie, S. Huang

Tuesday 15 September

Accelerator Physics

First Cobotic-Assisted String Assembly of PIP-II SSR2 Cavities at Fermilab

M. Parise, T. Aiazzi and C. Narug

Achieving robotic-assisted string assembly has been a longstanding objective in superconducting cavity pro-duction to improve precision, repeatability, and through-put while reducing operator-dependent variability. At Fermilab, robotic systems have been successfully deployed for the installation of individual components on SSR2 cavities inside a cleanroom environment. The single-cavity installations validated the feasibility of robotic techniques and provided confidence to extend automation to the assembly of a full prototype cavity string. Leveraging lessons learned from earlier operations, the robotic system was adapted for the string assembly. This work describes the robotic-assisted assembly process in detail, including alignment strategies for coupler flanges, torque control for bellows connections, and mitigation of particulate generation in cleanroom conditions. Performance metrics of the cryomodule are presented, together with an analysis of improvements needed for robotic assembly. The effort also represents a broader R&D initiative at Fermilab to the long-term goal of scalable SRF cryomodule production.
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Accelerator Physics

PIP-II Project Progress and Challenges

O. Napoly

The Proton Improvement Plan-II (PIP-II) project is under construction to provide the Fermilab Accelerator Complex with a new H$^-$ superconducting radiofrequency (SRF) linac and beam transfer line delivering 800 MeV protons at Booster injection at double energy and double intensity (6.7x10$^{12}$ protons per 20 Hz pulse) in comparison with the present Linac. The design goals of PIP-II Linac will be described, including delivering 1.2 kW beam power on Long Baseline Neutrino Facility (LBNF) target, continuous wave (CW) operation compatibility and future upgrades. The progress of the Linac construction and the development of the Commissioning Plan will be presented. Finally, the challenges of operating the SRF Linac to drive the Fermilab Booster will be addressed.
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Accelerator Physics

Implementation of a 300 kA Pulsed Power Supply

C.C. Jensen, N. Gurley and H. Pfeffer

The Long Baseline Neutrino Facility (LBNF) will produce the world's most intense neutrino beam. While the magnetic horns are expected to be replaced, the LBNF horn power supply is expected to last the lifetime of the project, 30 years. Three series connected magnetic horns will require a 300kA, 800$\mu$s pulses at a rate of 1.4 pps to focus the beam. Prototyping proved the component choices and designs. Test results for a single cell and the full 32 cell system will be presented.
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Accelerator Physics

A 20kA $\pm$0.12% Pulsed Power Supply for the PIP-II ORBUMP Injection Dipole Magnets

H. Pfeffer, M. Davidson, D. West et al. (+3 more)

Fermilab has embarked on its next evolution for future neutrino experiments through the Proton Improvement Plan II (PIP-II). The injection system now requires an increase from a 15 Hz to a 20 Hz injection rate with approximately a 30% increase in the field strength from the original Orbital Bump (ORBUMP) dipole magnets in the Booster ring [1]. The four series connected dipoles will require 3 kV, 20 kA, 660 $\mu$s pulses at the aforementioned injection rate with a flatness of $\pm$0.12%. This is achieved through a dual-capacitor pulsed power supply (PPS) comprised of ten parallel cells and must last for 25 years. The supply utilizes high-voltage film caps for the rise and fall portion of the waveform; however, it switches over to a 6 F electrolytic capacitor bank for the flattop portion to meet flatness. The supply utilizes energy recovery to reduce high voltage charging supplies and required power to operate.
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All authors H. Pfeffer, M. Davidson, D. West, S. Garcia-Schiefelbein, G. Lolov, R. Rivera-Colon

Accelerator Physics

Ionization Profile Monitors for the IOTA Storage Ring at Fermilab

R. Thurman-Keup, N. Banerjee, M. Bressler et al. (+8 more)

Ionization profile monitors (IPM) are used in many accelerator laboratories around the world for non-destructive profile measurements of high-energy particle beams. They have been used in nearly all the synchrotrons built at Fermilab and are presently used in the Main Injector (MI), Recycler (RR), and Booster. This paper will present the current status of the design of a new pair of IPMs to be installed in the Integrable Optics Test Accelerator (IOTA), an experimental proton storage ring with a beam energy of 2.5 MeV. The design incorporates aspects of several IPMs previously or currently used at FNAL. Similar to the Booster IPM, ions are collected rather than electrons, thus requiring no external magnetic field. To increase the signal, each IPM uses two microchannel plates in the chevron configuration as is done in the MI/RR IPMs, and due to the ring's ultrahigh vacuum conditions, the local vacuum pressure is increased with a calibrated leak, akin to the former Tevatron IPMs, to increase the ionization yield per bunch. Data acquisition includes upgraded preamplifiers and leveraging of the digitizer design of the beam position monitor system already in use at IOTA. In addition to the mechanical and electronic designs, simulation results of the effects of the electric fields and injected gas on the circulating beam will also be presented.
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All authors R. Thurman-Keup, N. Banerjee, M. Bressler, K. Carlson, D. Edstrom, B. Fellenz, M. Mwaniki, H. Piekarz, A. Romanov, A. Semenov, V. Shiltsev

Monday 14 September

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

Direct observation of electron shedding from a laser-plasma accelerator

Sheroy Tata, Salome Benracassa, Heychal Davidovich et al. (+6 more)

Laser-plasma accelerators have demonstrated the ability to produce ultrashort relativistic electron bunches with peak currents suitable for compact light sources, ultrafast diffraction, and strong-field studies. However, their performance critically depends on preserving the longitudinal phase-space density of the beam as it exits the plasma accelerator. Here, we report the first direct observation of a previously unresolved process in which a highly charged electron bunch undergoes significant longitudinal expansion and progressively loses electrons during extraction from a laser-driven wakefield accelerator, a phenomenon we refer to as electron shedding. Using femtosecond relativistic electron microscopy, we tracked the evolution of the beam far beyond the accelerator exit and observed the bunch stretching over many plasma wavelengths before shedding electrons during propagation. Particle-in-cell simulations reproduce the observed behavior and reveal that it originates from a combination of effects when a high-charge-density beam exits the accelerator. These dynamics redistribute energy from the beam head into a low-energy tail, thereby reducing the useful peak charge density and ultimately decreasing the efficiency of the laser-plasma accelerator. Our results provide new insight into beam extraction and phase-space evolution in laser plasma accelerators and highlight the importance of controlling these collective effects for future applications.
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All authors Sheroy Tata, Salome Benracassa, Heychal Davidovich, Aaron Liberman, Anton Golovanov, Eitan Levine, Yang Wan, Eyal Kroupp, Victor Malka

Accelerator Physics

Simulation study of accelerator-based muography using the GeV-scale forward muon component at SHINE

Jiangtao Wang, Yinghe Wang, Si Chen et al. (+7 more)

Muography exploits the penetrating power of muons to image the interior of large dense objects, but cosmic-ray sources provide only ~1 {cm}^{-2} {min}^{-1} predominantly from above, limiting imaging speed and accessible geometries. Electron-driven muon production has recently been demonstrated with laser-wakefield accelerators, but shot-to-shot fluctuations hinder systematic studies required for quantitative accelerator muography. Using Geant4 Monte Carlo simulations, we model the full experimental setup at Shaft 2 of the Shanghai High repetition rate XFEL and Extreme light facility (SHINE), from a 3 GeV, 50 pC, 50 Hz commissioning electron beam interacting with a muon target through 25 m of beamline structures and a 3 m-thick concrete isolation wall. Approximately 0.28 effective reconstructed single-muon events per bunch, with residual kinetic energies below 1.2 GeV after traversing the wall, reach the downstream muography test area (~14 s^{-1}). The wall absorbs most charged background particles below ~1 GeV, while residual neutrons can be discriminated by their characteristic detector energy deposition. Scattering-tomography simulations show that $3\times 10^{5}$ effective muon events, accumulated in approximately 6 h at the commissioning rate, yield a Structural Similarity Index above 0.9, demonstrating the feasibility of quantitative accelerator-based muography using SHINE's electron-driven GeV-scale forward muon source. At the 8 GeV/50 kHz benchmark, the projected muon intensity exceeds $5 \times 10^{4}$ {\mu}/s, corresponding conservatively to approximately one muon per bunch at the detector. This exceeds the cosmic-ray flux by orders of magnitude, while SHINE's superconducting linac provides a stable, controlled platform for developing electron-on-target muography.
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All authors Jiangtao Wang, Yinghe Wang, Si Chen, Rongbing Deng, Kim Siang Khaw, Jun Kai Ng, Qisheng Tang, Dong Wang, Wenzhen Xu, Jia Cheng Yap

Accelerator Physics

High-Intense Gamma-Ray Emission from a Crystalline Undulator with Realistic Bending Profiles

R. Negrello, L. Malagutti, L. Bandiera et al. (+11 more)

The development of compact and intense $\gamma$-ray sources in the MeV energy range remains a significant frontier in radiation physics, with profound implications for nuclear physics, medicine and applied science. In this work, we present a comprehensive numerical investigation of the photon emission probability and brilliance of a Crystalline Undulator (CU) based on a periodically bent Si(110) crystal. Our approach integrates realistic deformation profiles obtained via Finite Element Method simulations of a realistic sample, where bending is induced by patterned Si$_3$N$_4$ surface stressors. Relativistic molecular dynamics simulations, performed using the MBN Explorer software package, consider a 10 GeV positron beam consistent with the foreseen FACET-II facility specifications. Our results reveal a distinct undulator radiation peak in the 1.6-2.1 MeV range, well-separated from the broader channeling radiation background. We show that for an optimal aperture angle of $1/2\gamma$, the source reaches a maximum peak brilliance of about $5\times10^{22}$~photons/s/mm$^2$/mrad$^2$/0.1%~BW. This performance is highly competitive with large-scale Gamma-Beam Systems and exceeds that of Inverse-Compton Scattering sources, confirming the potential of crystalline undulators as high-brilliance, compact light sources for the MeV domain.
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All authors R. Negrello, L. Malagutti, L. Bandiera, N. Canale, F. Cescato, P. Fedeli, V. Guidi, A. Mazzolari, G. Paternó, J. Reyes Garrido, M. Romagnoni, A. Sytov, A. V. Korol, A. V. Solov'yov

Friday 11 September

Accelerator Physics

Damping Ring Optimization in Natural Emittance and IBS Dominated Regimes

Matthew Andorf, Nigel Lockyer, Emilio Nanni et al. (+2 more)

Linear electron-positron colliders enable precision measurements of Standard Model observables and beyond. Modern linear colliders need damping rings wherein the emittance of the source is damped and asymmetrized. A previous work [Phys. Rev. ST Accel. Beams 4, 021001 (2001)] delineated a systematic procedure for damping ring design. In this paper, we apply those principles, and frame damping ring design as a constrained numerical minimization problem when including intrabeam scattering. We use a multi-objective genetic optimization to map the transition from natural- emittance-dominated to intrabeam-scattering-dominated regimes. We compare to analytic results where possible to good agreement. The analysis is applied to parameters for the C$^{3}$ collider concept, but the procedure is fully generalizable to other linac parameters.
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All authors Matthew Andorf, Nigel Lockyer, Emilio Nanni, Caterina Vernieri, Jared Maxson

Accelerator Physics

Progress in Muon Cooling and Moderation Techniques

Ce Zhang, Kim Siang Khaw and Chris Rogers

Muon beams are essential tools for a wide range of applications such as the search for rare muon decays and muon spectroscopy. However, conventional muon beams produced from pion decay have large emittance and broad momentum spread. Conventional beam-cooling techniques cannot improve the beam quality significantly owing to the 2.2 $\mu$s muon mean lifetime. This has motivated the development of dedicated muon beam cooling and moderation methods. This review surveys the principal approaches, including ionisation cooling for high-brightness muon beams, low-energy muon moderation and frictional cooling for slow muon production, and laser ionisation of thermal muonium for ultra-low-emittance muon sources. The challenges associated with producing slow negative muons and emerging concepts based on cyclotron trapping and muon-catalysed fusion are also discussed. These complementary techniques offer promising pathways towards next-generation muon facilities and precision experiments.
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Thursday 10 September

Accelerator Physics

Continuous-Wave Operation at 30 MV/m of a Superconducting Radio-Frequency Gun Cryomodule with an Exchangeable Cathode Plug

T. Konomi, T. Xu, Y. Choi et al. (+30 more)

A prototype 185.7 MHz superconducting radio-frequency (SRF) electron gun cryomodule has been developed for the proposed LCLS-II-HE Low Emittance Injector at SLAC. It has an in-situ ultra-high-vacuum exchangeable cathode plug system compatible with semiconductor photocathodes. With a copper plug, the gun achieved continuous-wave RF operation at a cathode field of 30 MV/m. No measurable degradation was observed after plug removal and reinsertion with the cavity at 4.4 K. This result is a significant improvement over past continuous-wave SRF gun systems with exchangeable plugs.
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All authors T. Konomi, T. Xu, Y. Choi, K. Elliott, B. Gower, B. Tousignant, J. Wenstrom, A. Taylor, C. Compton, W. Chang, X. Du, W. Hartung, S. H. Kim, S. Miller, D. Morris, M. Patil, L. Popielarski, J. Wei, Z. Yin, J. Smedley, J. Maniscalco, C. Adolphsen, M. Murphy, D. A. Gonnella, M. P. Kelly, T. Petersen, A. Arnold, S. Gatzmaga, R. Steinbrück, P. Murcek, R. Xiang, J. Teichert, J. Lewellen

Accelerator Physics

Physics-Informed Drift Diagnosis for Laser-Plasma Accelerator Operations

Ou Labun, Calin Hojbota, Mara Klebonas et al. (+4 more)

Laser-plasma accelerators (LPAs) sustain accelerating gradients of order $100\,\mathrm{GV/m}$, but routine operation remains difficult: electron beam metrics drift over an operating shift, and the root physical cause is often invisible to the available diagnostics. We formulate LPA operation as a latent state-space model in which three effective interaction-point variables, the normalized laser amplitude $a_0$, the normalized plasma electron density $\tilde n_e$ and the residual pulse chirp $\mathcal{C}$, are inferred from routine electron beam observations by an extended Kalman filter. The emission model, which maps the latent state to the diagnostics, is kept structurally separate from the {transition} model, which describes how the latent state evolves between shots. The separation supports diagnosis in two stages, one asking which latent variable moved and one asking what moved it. The implemented emission model is a toy model, yielding an expected performance in line with current facilities and using 3D blow-out regime dependencies where relevant. We conduct synthetic sessions to test the effectiveness of the detection and attribution protocols, finding that attribution is limited by excitation rather than by shot count or diagnostic resolution. Because the construction needs only a set of physical latent variables, an emission model and a family of hardware-derived transition models, it transfers to other drift-prone subsystems. We argue that the accuracy of the whole procedure is limited by the emission model rather than by the inference method.
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All authors Ou Labun, Calin Hojbota, Mara Klebonas, Mike Downer, Rafal Zgadzaj, Phil Franke, Lance Labun

Accelerator Physics

Coupling periodic-cell and finite-bunch dynamics for structured photocathodes

Dmitry Bazyl and Igor Zagorodnov

Patterning a photocathode with submicrometre features can enhance nonlinear photoemission by concentrating the optical field, but the surface geometry can also increase the transverse momentum spread of the emitted electrons. Resolving nanoscale surface fields across an injector-scale illuminated area within a full rf gun simulation is computationally demanding. To couple these scales, we developed an approach which combines a self-consistent finite flat-cathode calculation with the particle-resolved difference between matched structured and flat periodic calculations. The finite calculation determines the macroscopic bunch evolution and space-charge field. The periodic difference determines the local change caused by the surface. For a finite but relatively small test problem, comparison with a fully resolved finite-array WarpX calculation gives differences of 1.3% in rms energy spread and less than 0.1% in projected normalized emittance. We then apply the method to representative FEL photoinjector parameters with a 100 pC emitted source distributed over more than half a million periods, and track the composed bunch through an L-band rf gun and solenoid. The initial difference between the projected horizontal and vertical emittances becomes much smaller after rf acceleration and solenoid focusing. At 1.52 m downstream of the cathode, the projected emittances are nearly equal and exceed those of the matched flat-cathode reference by less than 2% in both planes. The central-slice emittances exceed the reference values by approximately 3% horizontally and 5% vertically.
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Wednesday 9 September

Accelerator Physics

Turn-by-turn Tune Analysis Using Adaptive BPM Ensembles in the Fermilab Mu2e Delivery Ring*

D. Steinkamp

The Mu2e experiment at Fermilab requires stable resonant slow extraction from the Delivery Ring, making reliable tune monitoring an important operational diagnostic. This work investigates BPM-based tune-candidate extraction using synchronized turn-by-turn position data distributed across multiple digitizers. Each spill contains approximately 50,000 turns from many BPMs in both transverse planes, enabling spectral analysis of tune-like structure. The analysis captures coherent spill snapshots, verifies synchronization using stream timestamps, and computes tune candidates in configurable horizontal and vertical tune bands. Rather than relying on a single BPM or fixed BPM list, it evaluates BPM quality on a spill-by-spill basis and selects small adaptive BPM ensembles. A multi- spill study shows that tune observability is distributed and dynamic rather than concentrated in one globally optimal BPM. Adaptive ensembles improve tune-candidate quality compared with single-BPM selections, with the clearest results in the vertical plane. The horizontal plane shows useful ranking structure but weaker visibility under present thresholds. Direct evaluation of fixed global BPM sets shows that static selections do not reproduce dynamic per-spill performance. These results motivate an adaptive BPM-ensemble approach for Delivery Ring tune analysis using selected BPM subsets, confidence metrics, and quality flags rather than a single preferred BPM or fixed BPM list.
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Tuesday 8 September

Accelerator Physics

Flexible Spectral-Normalized Neural Gaussian Process for Dynamic Aperture Prediction

Yousra El-Bachir, Frederik Van der Veken, Davide di Croce et al. (+4 more)

We address the challenge of scalable uncertainty quantification in large-scale scientific applications, where complex state-of-the-art machine learning methods are often computationally infeasible. Our primary contribution is a simple yet effective empirical Bayes method for automatically tuning the hyperparameters of a flexible, heteroscedastic Spectral-normalized Neural Gaussian Process. This approach retains the expressiveness and uncertainty-awareness of semi-Bayesian neural models while significantly reducing the computational burden by integrating hyperparameter learning directly into the training loop. We demonstrate the practical impact of our method on the task of estimating the dynamic aperture in circular particle accelerators, a fundamental problem in high-energy physics colliders and storage rings, using simulation data from the case of the Large Hadron Collider at CERN. Traditional approaches to DA estimation require extensive particle-tracking simulations, which are prohibitively time-consuming and resource-intensive. Our results show that the proposed method achieves competitive predictive performance and well-calibrated uncertainty estimates at much lower computational cost than state-of-the-art approaches. We stress that, beyond this application, the proposed empirical Bayes framework offers a general solution for training heteroscedastic neural models in situations where manual hyperparameter tuning is impractical. Accordingly, we anticipate that this framework can be applied to other domains that encounter comparable computational limitations.
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All authors Yousra El-Bachir, Frederik Van der Veken, Davide di Croce, Carlo Emilio Montanari, Massimo Giovannozzi, Ekaterina Krymova, Tatiana Pieloni

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

Demonstration of a 10-metre-long discharge plasma source for plasma wakefield acceleration

C. Amoedo, N. Lopes, N. E. Torrado et al. (+13 more)

The AWAKE experiment at CERN uses high-energy proton bunches to drive plasma wakefields capable of accelerating electron bunches up to hundreds of GeV in a single plasma. Realising this potential requires a plasma source that can extend to tens of metres while maintaining high longitudinal uniformity and reproducibility. Towards this goal, we report on the first experimental characterisation and beam-based validation of a 10-metre-long pulsed-DC discharge plasma source (DPS) used as an alternative to the laser-ionised rubidium vapour plasma source (VPS). The DPS reliably delivered plasma densities in three noble gases (He, Ar, Xe) at electron densities between 10$^{14}$ and 10$^{15}$ cm$^{-3}$. Proton bunch self-modulation was observed with all three gases, and the resulting modulation frequency was used as a beam-based diagnostic of the plasma density, showing good agreement with laboratory interferometry measurements. The event-to-event modulation frequency was demonstrated to be reproducible to within 0.63%, comparable to that of the VPS. These results establish the DPS as a valid alternative plasma source for beam-driven wakefield experiments, and support its further development towards longer plasma lengths. The present design is tailored to AWAKE requirements, but the underlying discharge concept provides a route to long, reproducible plasma sources with straightforward density control through discharge-beam timing for other beam-plasma experiments.
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All authors C. Amoedo, N. Lopes, N. E. Torrado, F. Silva, P. Muggli, L. Verra, M. Turner, G. Zevi Della Porta, M. Bergamaschi, A. Clairembaud, J. Mezger, F. Pannell, N. Z. van Gils, E. Gschwendtner, A. Sublet, the AWAKE Collaboration

Monday 7 September

Accelerator Physics

Design and optimization of the NSLS-II lattice with complex bend replacement

Minghao Song, Yoshiteru Hidaka, Bernard Kosciuk et al. (+4 more)

The demand for brighter photon beams has driven the development of diffraction-limited storage rings with ultralow electron beam emittance. A complex bend (CB) lattice was recently proposed as an approach for the future NSLS-II upgrade. As a step toward this upgrade, NSLS-II plans to replace two existing electromagnetic dipoles with permanent-magnet CBs, providing the first experimental demonstration of the CB concept in a storage ring. This paper presents the linear optics design, nonlinear optimization, and machine error evaluation of the modified NSLS-II lattice. The optimized lattice provides sufficient dynamic and momentum apertures for off-axis injection and routine operation and can be corrected with the existing NSLS-II correction scheme. Scraper measurements near the planned CB installation location validate the beam aperture requirements used in the CB design.
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All authors Minghao Song, Yoshiteru Hidaka, Bernard Kosciuk, Patrick N'Gotta, Sushil Sharma, Timur Shaftan, Guimei Wang

Accelerator Physics

Higher-order space-charge stability in anisotropic beams: Vlasov-Poisson derivation, refined dispersion relations, and stability charts

Abhishek Pathak

The Hofmann stability chart is used to screen working points in space-charge-dominated linacs. We identify two errors in its published higher-order dispersion relations: missing $(1\mp2\hat\eta^2/\alpha)$ factors in the third-order $S^4$ coupling residues, and a sign error in the stated isotropic reduction of the fourth-order relation. Both corrections follow from Hofmann's Vlasov-Poisson equations without fitted parameters. They reproduce coherent tune-shift coefficients in the author's later monograph that the printed forms miss by 24% and 127%. Mode-resolved figures from a published application agree with the corrected relations and reject the printed forms, indicating an inconsistency between the 1998 equations and the calculations underlying those tested figures. We quantify the effect on the non-oscillatory stability chart. Inside the adopted $S^2\le10$ comparison domain, printed and corrected forms disagree on 0.73-2.11% of cells, with no preferred direction. Among excluded cells, disagreement reaches 22%, and the printed relation over-predicts instability at every sampled anisotropy. This concentration may help explain why the errors persisted, although it does not establish their historical cause. For PIP-II, the corrected chart flags four of thirty-two evaluable periods, including one on a third-order odd branch missed by a second-order screen. This count covers non-oscillatory modes only and remains conditional on an unresolved factor-five disagreement between two codes on transverse emittance growth.
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Sunday 6 September

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

Magnetic island structures in relativistic laser-driven plasma channels

Dongchi Cai, Zheng Gong, Guanqi Qiu et al. (+3 more)

We develop a theoretical model for self-generated magnetic islands in relativistic laser-driven channels in near-critical-density plasmas. The islands arise from the nonlinear superposition of the quasi-static magnetic fields generated by the longitudinal channel current $j_x$ and the laser-front driven transverse current $j_y$. By deriving the critical conditions among laser depletion, transversely symmetric channel formation, and magnetic-island formation, we identify the laser-plasma parameter window in which the magnetic island structures can exist. Within this window, the balance between the laser ponderomotive force and the charge-separation force, expressed through an effec tive electron density $n_\mathrm{eff}$, determines the transverse island width $H$, whereas the mismatch between the laser group and phase velocities determines the longitudinal period $L$. Large-scale particle-in-cell simulations over a broad range of laser intensities and plasma densities validate the resulting scaling laws. The model turns the island geometry from a qualitative feature of the channel field into a predictable quantity, providing a basis for tailoring electron transport, particle acceleration, high-energy radiation, and novel fusion ignition schemes in relativistic laser-plasma interactions.
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All authors Dongchi Cai, Zheng Gong, Guanqi Qiu, Deji Liu, Yinren Shou, Xueqing Yan

Accelerator Physics

Incoherent scattering of highly ultrarelativistic channeled particles on electrons

Victor V. Tikhomirov

This article systematically examines the effects that must be taken into account when modeling the dechanneling of highly ultrarelativistic charged particles caused by incoherent scattering by electrons of crystal atoms. Changes in the particle electromagnetic field that occur in condensed matter at highly ultrarelativistic energies are taken into account. The excitation of both collectivized electrons and inner-shell electrons of atoms is considered, taking into account thermal vibrations and the inhomogeneous periodic distribution of atoms in crystals. Differences in the calculations of stopping power and mean square scattering angle by electron of channeled particles are described. The need to separate both of them into diffuse processes, accompanied by the transmission of a transverse momentum which does not exceed the transverse momentum of the channeled particles, and instantaneous dechanneling processes with a greater transmitted momentum, the variation range of which reaches several orders of magnitude, is substantiated. The main result of the present work is expressions for the mean square projected scattering angles per unit length in planes parallel and perpendicular to the atomic plane, which also allow for the introduction of effective minimum momenta for incoherent scattering. Expressions of both types take into consideration completely the nonlocal effects of momentum transfer from crystal electrons to particles moving along classical trajectories, and allow for the formulation of simple methods for modeling the incoherent scattering of ultrarelativistic positively charged particles.
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Accelerator Physics

Preliminary design of continuous wave Low-Level RF systems for S3FEL

Z. Y. Zhang, J. F. Zhu, H. L. Ding et al. (+4 more)

In the Shenzhen Superconducting Soft X-ray Free Electron Laser (S3FEL), Continuous Wave (CW) Low-Level Radio Frequency (LLRF) systems perform critical functions including adjusting the power coupling of accelerator cavities, regulating the amplitude and phase of the RF field, and maintaining the resonance frequency and phase of the cavities. These functions are essential to ensure the electron beam operates at the accelerating phase. Within S3FEL, each superconducting cavity is driven by a solid-state amplifier (SSA), with each SSA paired with a dedicated LLRF system. Based on the distinct acceleration cavities employed, the CW LLRF systems for S3FEL are categorized into four types: 1. Primary accelerator LLRF systems (superconducting, 1.3 GHz; quantity: 168), 2. Harmonic cavity LLRF systems (superconducting, 3.9 GHz; quantity: 16), 3. VHF electron gun LLRF systems (room temperature, 216 MHz; quantity: 4), 4. Buncher LLRF systems (room temperature, 1.3 GHz; quantity: 2). These four LLRF system categories exhibit differing requirements for RF field and acceleration cavity control. This report presents the preliminary design schemes for these four types of CW LLRF systems.
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All authors Z. Y. Zhang, J. F. Zhu, H. L. Ding, J. W. Han, W. Li, J. Y. Yang, W. Q. Zhang

Friday 4 September

Accelerator Physics

Indirect Integration of Longitudinal and Transverse Wake Potentials for Unequal Beam Pipes and Arbitrary Beam Velocity

Igor Zagorodnov and Dmitry Bazyl

Indirect integration replaces the long uniform beam-pipe parts of a wakefield calculation by field problems in the pipe cross sections. Earlier ultrarelativistic methods were developed mainly for the longitudinal wake, whereas the transverse wake was usually obtained from the Panofsky–Wenzel theorem. We derive indirect formulas that complete a transverse Lorentz-force integral already accumulated in a time-domain calculation. At $\beta=1$, each semi-infinite tail is found from a Dirichlet Poisson problem driven by $E_z$ and a Neumann Poisson problem driven by $cB_z$, describing the TM and TE contributions, respectively. We obtain both a fixed-time moving-window representation and a fixed-plane time-history representation for equal or unequal input and output pipes. The method is then extended to a rigid bunch moving with constant velocity $0<\beta c<c$. The longitudinal correction satisfies an anisotropic elliptic equation in $(x,y,s)$ and provides an additional source for the transverse TM problem. In a two-port finite-reference convention, the complete fields, including space charge, are integrated directly between two fixed planes, while the semi-infinite tails are calculated after subtraction of the stationary field in each pipe. The resulting Panofsky–Wenzel relation contains the difference of the stationary transverse electric fields at the two ports. Numerical tests for an unequal rectangular step-out at $\beta=1$ and $\beta=0.8$ confirm the transverse indirect integration and the unequal-pipe boundary term.
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Thursday 3 September

Accelerator Physics · Cross-listed from physics.optics

Nonlinear Photoemission for Bright Beams in X-Band Photoinjectors

Casey Timms, Lucas Malin, Kevin Eckrosh et al. (+15 more)

The ongoing development of low-cost and compact x-ray light sources is vital for increasing the number of experiments that can be performed at the atomic scale. One option for a compact and simple photoinjector is to operate in blow-out mode, in which a high-charge, short-bunch length electron beam self-expands by space-charge forces into a uniformly ellipsoidal shape. This mode necessitates a photoinjector whose initial electron pulse has a pulse duration on the order of $100$ fs. A photoinjector utilizing linear photoemission with an ultraviolet laser in this regime presents significant technical challenges, as many optical substrates degrade at these wavelengths. In this paper, we present an alternative design that uses nonlinear photoemission to produce electron beams with high charge and short bunch length in an X-band (9300 MHz) photoinjector. This photoinjector design produces an electron beam with a bunch charge of up to $200$ pC, an RMS bunch length of $291$ fs, a normalized transverse emittance of $1$ to $1.4$ mm$\cdot$mrad, and an energy spread of $10^{-4}$. This results in a beam brightness of $10^{18}$ A/m$^2$, which is bright enough for our beamline's purposes. We also present ongoing efforts are aimed at further increasing the electron beam brightness by using a spatial light modulator to shape the laser profile at the cathode.
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All authors Casey Timms, Lucas Malin, Kevin Eckrosh, Sean E. Tilton, Lidia C. Santander, Hyung Seo Lee, Rejul Jaswal, Alex Gardeck, Eric Everett, Alan Dupre, Mukhtar Hussain, Sami Tantawi, William Graves, Henrik Loos, Mark Holl, Arvinder Sandhu, Robert Kaindl, Samuel Teitelbaum

Accelerator Physics

HELIX: a hybrid envelope-multiparticle linac code with differentiable space-charge optimization

Abhishek Pathak

HELIX (Hybrid Envelope-multiparticle LInac eXplorer) is a Python hadron-linac framework developed at Fermilab for the PIP-II superconducting H- linac. It combines exact-gradient matching through a self-consistent nonlinear particle-in-cell (PIC) space-charge solve with the lattice-card workflow used for machine design. One TraceWin-format lattice drives an rms-envelope solver with 3-D linearized space charge, a multiparticle tracker with a 3-D FFT PIC solve, and linear matrix analysis over one data model, and the constraint cards that steer six classical matching algorithms also steer a differentiable seventh, which re-expresses a fixed-energy subset of the tracking-plus-PIC map as a single PyTorch graph and returns reverse-mode Jacobians exact to floating-point precision at a cost set by the number of constraints rather than of knobs. On a six-quadrupole manufactured verification problem the exact-Jacobian run reaches a tolerance-normalized residual of $3.2\times10^{-10}$, some 200 times below the finite-difference endpoint at the same forward-equivalent budget; the Jacobian cost is flat in the knob count, with the crossover between ten and twelve knobs and reverse mode 1.45 times faster at sixteen. Verification is hierarchical, from analytic envelope references through $10^{-9}$-$10^{-13}$ cross-implementation PIC parity to TraceWin benchmarks on the PIP-II low-energy, medium-energy and accelerating lines: with space charge on, the rms moments along the MEBT and half-wave-resonator line agree within 0.7%, and over the 186-m linac the energy agrees within 0.03% and transmission within the combined statistical uncertainty. HELIX also provides machine-learned field-map surrogates, Pareto exploration, error Monte Carlo with orbit correction, failure compensation, and RFQ transport, with NumPy, C++/OpenMP, CUDA and Metal backends.
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Accelerator Physics

Design optimization, commissioning, and uncertainty analysis of the quadrupole resonator system at Jefferson Lab for SRF material characterization

Mingqi Ge, Sarra Bira, Kristof Brunner et al. (+14 more)

A quadrupole resonator (QPR) provides a sample-based platform for characterizing materials for superconducting radio-frequency (SRF) applications under controlled field, frequency, and temperature conditions. This paper presents the design optimization, commissioning, and validation of the Jefferson Lab QPR system, including a quantitative assessment of measurement uncertainty. The resonator geometry was re-optimized from the CERN version-II design to improve quadrupole-mode separation and enable four usable modes at 400, 806, 1221, and 1640 MHz. The measurement system combines self-excited-loop RF operation, cable-loss-corrected power calibration, decay-based external-Q calibration, and RF-DC thermal-substitution calorimetry to determine the peak surface magnetic field Bpk and sample surface resistance Rs. Commissioning measurements on bulk Nb and Nb3Sn-Ta-Cu samples validated the system response over a broad range of frequency, temperature, and RF field. The extracted superconducting energy-gap parameters are consistent with the reported values for Nb and Nb3Sn, as well as with those obtained from single-cell cavity measurements. The commissioned system operates from 1.8 K to near the superconducting transition temperature of the sample, with accessible Bpk values from approximately 5 mT to a sample- and temperature-dependent heater-power-budget limit; a maximum field of 60 mT was demonstrated for bulk Nb at 400 MHz and 4 K. The combined relative standard uncertainties are 8.3% for Bpk and below 18% for Rs when r = PDC2/PDC1 is less than 0.9. The worst-case Bpk resolution at the 95% confidence level is approximately 1.35 mT, while the Rs resolution is below 1 nOhm at 10 mT and 2 K. These results establish the JLab QPR as a calibrated, multi-frequency platform with quantified measurement uncertainty for SRF material characterization.
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All authors Mingqi Ge, Sarra Bira, Kristof Brunner, Natalie Gale, Marco Garlasche, Valentin Giglia, Oleksandr Hryhorenko, Justin Kent, Peter Owen, Uttar Pudasaini, Guillaume Rosaz, Karol Scibor, Pramita Tiwari, Anne-Marie Valente-Feliciano, Lorena Vega Cid, Walter Venturini, Haipeng Wang