The Neutrinos at the Main Injector (NuMI) facility at Fermilab delivers an intense neutrino beam for multiple experiments by producing pions that decay into neutrinos, muons, and other particles. Magnetic horns—the primary pion focusing elements in the NuMI beamline—exhibit predominantly linear optics, enabling a predictable relationship between the proton beam and the resulting pion and muon phase spaces. This study has two primary objectives: first, to evaluate and confirm the linearity of the horn focusing mechanism using analytical models and numerical simulations; and second, to demonstrate that key beam parameters—such as proton beam intensity, beam position on target, and horn current—can be extracted from muon monitor observations within this linear optics framework. Using a machine learning model trained on spill-by-spill muon monitor data, we infer the horn current with a precision of $\pm 0.05\%$, the beam intensity with $\pm 0.1\%$, and the beam position on target with $\pm 0.018\mathrm{mm}$ horizontally and $\pm 0.013\mathrm{mm}$ vertically. This approach provides a reliable cross-check of beam parameters, helping to reduce systematic uncertainties that are critical for future experiments such as the Deep Underground Neutrino Experiment, which will rely on the neutrino beam produced by the Long-Baseline Neutrino Facility.

The LHC beam dumps are responsible for the safe absorption of the Large Hadron Collider (LHC) particle beams. In 2018, the two 6.4-tonne beam dumps that had been in operation since the LHC’s startup in 2008 were removed and replaced with upgraded versions. Endoscopic inspections of these beam dumps and experimental high-intensity proton-beam irradiation of material samples raised concerns about the structural integrity of the carbon-based materials in their cores. It was therefore decided to undertake an accelerated project of dismantling and postirradiation examination of the removed dumps as part of a wider program of work to ensure the safe operation of the LHC beam dumps in the coming years. This paper describes the decommissioning process for the two beam dumps carried out at CERN between 2021 and 2023, covering the preparatory studies, practical challenges encountered, and solutions implemented. It details the establishment of an operational framework, including the preparation of the working environment, the development of a method for cutting the irradiated 12-mm-thick duplex stainless-steel vessel, and the cut sequencing. Additionally, the paper presents the findings derived from the postirradiation examination of the different carbon-based core materials subjected to deposited energy densities up to $1.5\mathrm{kJ}/\mathrm{g}$. The extruded graphite plates within the vessel exhibited a cracking pattern, which was likely due to the dynamic response of the device upon beam impact, and their retaining rings were found to be displaced. Despite minor signs of surface deterioration, the expanded graphite sheets were intact, and the isostatic graphite blocks showed no evidence of material degradation. These findings were consistent across both beam dumps. Finally, this manuscript discusses the implications of these results for the safe operation of the beam dumps during LHC Run 3 (2022–2026) and for the design and construction of Run 3 operational spares.

This paper presents a beam optics and coupling feedback system for storage rings during operation, integrating deep learning methods with turn-by-turn (TBT) beam position monitor data. A deep learning network has been applied to extract phase advance and betatron coupling information implicit in TBT data. Subsequently, these data were fed into a multilayer neural network for rapid modeling and correction. The simulation data and sampled spectrum data from the Shanghai Synchrotron Radiation Facility (SSRF) were used to create training datasets for the network. The efficacy of this feedback system has been successfully demonstrated at the SSRF, enabling continuous monitoring and correction during each injection cycle. This method reduces optical distortion to less than 1% after correction, effectively suppresses optical and coupling distortion caused by insertion device movements during storage ring operation, and ensures robust performance under various operational conditions.

The efficient tagging of individual muons, along with the stability of the involved ultrathin carbon foil, is critical for ensuring fast, reliable, and reproducible low-energy muon spin rotation measurements. At the Paul Scherrer Institute’s low-energy muon beamline, we developed a method to monitor the tagging efficiency of the beamline using routinely collected muon decay histograms. This method leverages background comparison before and after the arrival time of the muons at the sample to extract the tagging efficiency, eliminating the need for additional detectors or measurements. By analyzing data collected between 2018 and 2024, we confirm the method’s reliability and validate its results using independent reference measurements. Furthermore, we establish a correlation between the tagging efficiency and foil thickness by investigating the impact of contamination and outgassing on the carbon foil, as well as the restoration effects of laser cleaning. The findings underscore the importance of monitoring the carbon foil’s condition to maintain consistent beamline performance and reproducibility. This method offers a practical approach for detector efficiency monitoring, applicable to other beamlines employing similar setups.

Several synchrotron light sources are currently designing a lattice upgrade to reach low (sub-nm) emittances, which inevitably entails a significant beam lifetime reduction. In view of the ALBA lattice upgrade, it was decided to evaluate different methods using the current ALBA storage ring to reach large betatron coupling as a way to increase the lifetime. In a first attempt, coupling was introduced by means of static skew quadrupolar magnets and by tuning the working point onto the resonance $Q_x=Q_y$, but in a realistic scenario, some kind of tune feedback is required to counteract the unavoidable tune drifts and fluctuations that would drive the system out of resonance. Furthermore, the condition $Q_x=Q_y$ constrains the linear optics resulting in an important lack of flexibility. Therefore a second method is proposed, based on the excitation of the coupling resonance with an ac skew quadrupole driven at the frequency $f_{\mathrm{rev}}(Q_x-Q_y)$. In this case, we used the existing four-electrode tune excitation stripline recabled as a skew quadrupole. A fast tune tracking system was implemented to drive the skew quadrupole exactly on the resonance despite the tune fluctuations. This work goes over the collected results and experiences, aiming to put into light pitfalls and limits of the application of coupling to achieve round beams in a synchrotron light source.

This paper presents the lattice design and optics ramping strategy for the cooling section of the hadron storage ring at the electron-ion collider. The main challenge is that available tuning knobs exceed beam-optics constraints. Independently optimized injection and top-energy optics often yield disconnected solutions, making interpolation impossible. To address this, we propose two new methods. The first is an intermediate-penalty scheme that ensures ramping path continuity by penalizing constraint violations at intermediate states. The second is a continuation-based approach that adapts high-energy optics to low energy, guided by an adaptive weighting scheme to balance injection and ramping constraints. The solutions meet all beam dynamics and hardware limits. The two methods offer a general strategy for ramping in systems where the solution space is under-constrained and the starting and target configurations are far apart.

Achieving strong focusing of MeV electron beams is a critical requirement for advanced beam applications such as compact laboratory x-ray sources, high gradient accelerators, and ultrafast electron scattering instrumentation. To address these needs, a compact radially magnetized permanent magnetic solenoid (PMS) has been designed, fabricated, and tested. The solenoid provides a compact and inexpensive solution for delivering high axial magnetic fields (1 T) to focus MeV electron beams. Field characterization of the solenoid demonstrates good agreement with analytical models, validating the PMS design. The electron beam test employs a high-brightness photoinjector to study the focusing properties of the PMS. The results indicate a focal length of less than 10 cm and a significant reduction in beam size with small spherical aberrations. Two application cases are evaluated: angular magnification in ultrafast electron diffraction setups and strong focusing for Compton scattering or other microfocus uses.

Recent work introduced a systematic method for designing so-called azimuthally modulated rf cavities that support transverse magnetic modes composed of user-desired multipoles, enabling precision control of the magnitude and orientation of multipolar components in rf cavity design. This paper extends this method to practical implementation by deriving the multipolar expansion of the longitudinal electric field in such rf cavities with beam pipes, as well as the momentum change of ultrarelativistic particles traversing these modes. The derived equations explicitly show the radial variation of the change in longitudinal and transverse momentum follows a polynomial rather than Bessel-function relationship. The expression for the longitudinal electric field is then compared to a field map obtained from the 3D electromagnetic simulation of an azimuthally modulated cavity designed to support a mode composed of monopole, dipole, and quadrupole components. Beam dynamics studies are presented to assess the derived expressions for the change in momentum, including the effects of relaxing the ultrarelativistic assumption. Finally, two example applications are presented: the first demonstrates the removal of unwanted transverse multipoles to create a multipole-free accelerating structure with a single-port coupler, whereas the second illustrates the synthesis of desired multipoles to create an rf cavity that transforms the transverse distribution of a beam from Gaussian to uniform.

A high-repetition-rate pulsed muon source operating at approximately 50 kHz holds the potential to improve the sensitivity of various particle physics and materials science experiments involving muons. In this article, we propose utilizing the high-repetition-rate pulsed electron beam at the SHINE facility to generate a surface muon beam. Our simulation studies indicate that an 8 GeV, 100 pC charge pulsed electron beam impinging on a copper target can produce up to $2\times {10}^3$ muons per pulse. Beamline optimization results demonstrate that approximately 60 surface muons per electron bunch can be efficiently transported to the end of the beamline. This translates to a surface muon rate of $3\times {10}^6{\mathrm{\mu }}^{+}/\mathrm{s}$ when the pulsed electron beam is operated at 50 kHz, which is comparable to existing muon facilities. This high-repetition-rate pulsed muon beam, with its ideal time structure, represents a unique and pioneering effort once constructed. It serves as a model for building cost-effective muon sources at existing electron machines with GeV electron energies. In addition to the typical challenges encountered in conventional muon beamlines, such as the installation and construction of the target station and beamline, the removal of substantial quantities of positrons is also a major challenge. A potential solution to this issue is also discussed.

Twiss parameterization proposed by Courant and Snyder is a keystone of accelerator physics. Several authors have extended it later to the case of the fully coupled betatron motion and synchrobetatron motion of a coasting beam. A few proposals have been made as well how to construct a parameterization which is fully symmetric with respect to all three spatial degrees of freedom. The key point for each of them is to find a similarity transformation which turns the transport matrix into a block-diagonal form with $2\times 2$ cells. In the present work, this approach has been considered in detail. It is based on the idea initially proposed by Mais and Ripken for transport matrix eigenvalue calculation only. It shows how to introduce a symmetric Twiss-like parameterization and an emittance concept for an arbitrary symplectic $6\times 6$ matrix. The resulting parameterization can be reduced to the well-known Lebedev-Bogacz parameterization in case of $4\times 4$ matrix. Special attention has been paid to the intrinsic ambiguity existing in the Twiss parameterization of a coupled motion.

Vacuum breakdown in accelerator structures is a critical challenge that occurs under high electric fields. In environments subjected to hydrogen ion irradiation or high beam losses, such as in radio-frequency quadrupoles, residual hydrocarbons from the vacuum may result in carbon contamination of the metal surfaces from charged particle induced cracking. Under these conditions, it has been assessed that surface carbon contamination leads to a decrement of the surface electric field holding properties. This study extends the latest research by exploring the efficacy of oxygen plasma cleaning (OPC) on metal electrodes irradiated by low-energy hydrogen ion beam with the purpose of reducing surface carbon contamination. OPC treatment has been employed on different metals, namely, copper beryllium (CuBe), oxygen-free copper (Cu-OFE), and stainless steel (SS316LN). Treated electrodes have been tested for electric field performance in a dc pulsed system and results compared with nonirradiated electrodes and irradiated ones without OPC treatment. The study indicates a significant reduction in carbon contamination by OPC, enough to allow irradiated materials to achieve performances comparable with the electric field strength of raw surfaces. Moreover, it has been observed that stainless steel samples had some alteration on the surface chemistry that enhanced the material’s ability to sustain high electric fields while decreasing vacuum arcing events. Notably, OPC treated SS316LN electrodes surpassed the performance value of untreated ones, demonstrating the potential of plasma treatments in extending the operational performance of accelerator components.

In electron storage rings, it is possible to increase the bunch length by applying phase modulation to the radio-frequency accelerating field by choosing appropriate parameters for the modulation. Such a bunch lengthening effect improves beam parameters such as beam lifetime, which can help us achieve better beam stability. It is well known that the modulation frequency around the double synchrotron frequency is effective in lengthening the bunch. The dependence of bunch lengthening on modulation frequency, the so-called detuning property, has a peak around the double synchrotron frequency with a frequency width that depends on the modulation amplitude. Nonlinear effects due to phase modulation lead to a peak frequency shift in a negative direction from the double synchrotron frequency, accompanied by an asymmetric peak shape. Beam current also affects the properties of the detuning condition of such bunch lengthening. We investigated the detuning property using a theoretical model and systematic measurements at the electron storage ring Karlsruhe Research Accelerator, and we verified the qualitative agreement between the experiments and the theoretical model. The macroparticle simulations have confirmed the agreement.

We propose a modified maximum-entropy (MENT) algorithm for six-dimensional phase space tomography. The algorithm uses particle sampling and low-dimensional density estimation to approximate large sets of high-dimensional integrals in the original MENT formulation. We implement this approach using Markov Chain Monte Carlo (MCMC) sampling techniques and demonstrate convergence of six-dimensional MENT on both synthetic and measured data.

Developing multifrequency acceleration systems is crucial for optimizing beam dynamics in high-brightness particle beams, particularly for minimizing energy spread, manipulating bunch length, and facilitating beam bunching. This paper introduces a novel bimodal radio frequency (rf) cavity design that integrates both fundamental and harmonic modes within a single structure, offering enhanced spatial efficiency and improved operational flexibility for multifrequency rf systems. Unlike traditional designs, our cavity enables independent and simultaneous operation of each mode, along with effective suppression of higher-order modes (HOMs), which is critical for high-current beam stability. This breakthrough addresses significant challenges in bimodal cavity technology, such as intermode tuning coupling, rf-driving coupling, and effective HOM damping, providing essential solutions for high-current beam applications. Our work opens new research avenues in advanced rf cavity design, with significant implications for the future of high-brightness beam technologies, including advanced accelerator designs and improved beam quality.

This paper presents a radio frequency (rf) control system with attosecond resolution based on a carrier suppression interferometer operating a superconducting cavity at the Cryo Module Test Bench (CMTB). This novel application of the carrier suppression detector extends conventional heterodyne methods and improves the residual jitter of the regulated rf field in the cavity by more than one order of magnitude. The cavity operated at 1.3 GHz with a gradient of $8\mathrm{MV}/\mathrm{m}$ and a loaded quality factor of ${10}^7$. The setup achieved out-of-loop phase noise detection values of $\mathcal{L}=-180\mathrm{dBc}/\mathrm{Hz}$ at 10 kHz and $\mathcal{L}=-165\mathrm{dBc}/\mathrm{Hz}$ at 100 Hz with a time resolution of 189 as within an offset frequency range from 10 Hz to 1 MHz. The phase noise budget of subcomponents such as in-loop and out-of-loop detectors, high-power drive, microphonics, and the reference source is reported. The facility rf reference phase noise in the offset frequency range from 1 to 100 kHz is identified as the key noise contributor. Furthermore, the narrow-band cavity reduces the phase jitter experienced by the beam to just 116 as. The presented research combining conventional receivers with carrier suppression detectors in continuous wave operation is a key milestone toward attosecond resolution, in particular relevant for pump-probe experiments in free-electron laser facilities.

Symplectic simulation of space-charge effects is crucial for the design and operation of high-intensity particle accelerators. Traditional methods for simulating these effects are often computationally expensive, resulting in significant overhead. In this work, we introduce a generative model based on a U-Net architecture within a generative adversarial network framework to efficiently simulate space-charge effects. The model is trained to predict the transverse multiparticle space-charge Hamiltonian, which can be physically computed using a gridless spectral method. The one-step symplectic transverse transfer map for the particles is then obtained by differentiating the predicted Hamiltonian. Benchmarking results demonstrate that this generative model achieves an order of magnitude higher computational efficiency compared to the spectral method, providing a highly efficient alternative for simulating space-charge effects with a large number of particles. By maintaining symplecticity, the model effectively preserves the phase-space structure and mitigates nonphysical errors in long-term simulations. This model has been integrated into jutrack, a novel autodifferentiable accelerator modeling code developed in the julia programming language.

We present a method to construct high-order polynomial approximate invariants (AI) for nonintegrable Hamiltonian dynamical systems and apply it to a modern ring-based particle accelerator. Taking advantage of a special property of one-turn transformation maps expressed as square matrices, AIs can be constructed order by order iteratively. Evaluating AI with simulation data, we observe that AI’s fluctuation is actually a measure of chaos. Through minimizing the fluctuations, the stable region of long-term motions, i.e., the dynamic aperture of the accelerator, could be enlarged.

Particle accelerators have played a pivotal role in scientific research, medical applications, and industrial processes. However, conventional radio-frequency accelerators face limitations due to their large size, high cost, and dependence on external microwave sources. This paper presents a novel linear accelerator design that integrates a high-power microwave source directly into the accelerator structure, eliminating the need for external drivers and enabling a more compact, cost-effective, and simplified system. The proposed design employs an X-band backward wave oscillator driven by a 50 keV hollow continuous electron beam. The generated microwave power is coupled into a central X-band standing-wave acceleration structure via a radial coupler. Simulations demonstrate that a witness beam passing through this structure can be accelerated from 50 keV to over 4 MeV across 10 acceleration cells. This work serves as a foundational study on integrated accelerator concepts, offering a pathway toward more efficient, scalable, and versatile particle acceleration systems.

At the 3-GeV rapid cycling synchrotron (RCS) of the Japan Proton Accelerator Research Complex, we have minimized the beam loss at high-intensity beam operation up to the designed beam power of 1 MW. This was accomplished through systematic numerical simulations and experimental studies conducted over 2 years from 2020 and 2022. We first focused on mitigating uncontrolled beam losses originating from the foil scattering of the circulating beam, which results in high residual radiation levels, particularly in the injection area. These uncontrolled beam losses were well mitigated by reducing the stripper foil size and optimizing the injection beam size. Subsequently, we attempted to minimize the space charge effects and the associated beam losses at 1 MW by implementing several measures, such as refining both longitudinal and transverse injection paintings, correcting betatron resonances, and optimizing betatron tunes at injection. Consequently, we reduced beam losses throughout the RCS, including the collimator area, by 80% as compared to those observed at 1-MW trial operation in 2020. The residual beam loss at 1 MW is now estimated to be well below ${10}^{-3}$, which is primarily attributed to residual foil scattering-induced beam losses. Notably, beam losses in areas other than the collimator section are uncontrolled and required to be well minimized to ensure stable operation and regular accelerator maintenance. We have considerably reduced the machine activation, achieving a 1-order-of-magnitude decrease at the collimator compared to that in 2020. This improvement has enabled the RCS to maintain more than 98% availability for user operation recently. The simulation and measurement results, demonstrating good agreement, provide a detailed understanding of the beam loss mechanism at each step and guide the implementation of effective optimizations.

Recent studies have actively explored the feasibility of a novel storage ring light source based on the steady-state microbunching (SSMB) mechanism. Many interesting investigations have been limited to single-particle or pure-optics effects. In SSMB storage rings, laser modulators, consisting of an undulator and a copropagating laser beam, are utilized to manipulate the longitudinal bunch length at the radiator position. When the electron bunch traverses the modulator undulator, it emits coherent undulator radiation around the resonant wavelength with respect to the modulation laser wavelength. To increase the intensity of the modulation laser, an enhancement cavity can be used. Within the enhancement cavity, the modulation laser field is confined in a closed path, allowing the intracavity power to be accumulated and enhanced. Within this cavity, not only is there an accumulated modulation laser from external injection, but also coherent undulator radiations—both of which influence the dynamics of the electron beam on subsequent turns. In this paper, we develop a simple model to analyze the electron beam dynamics in laser modulator cavities, including the effects of finite loss due to mirrors in the cavity and the externally injected laser power required to compensate for this energy dissipation, as well as the dynamical effects of coherent undulator radiation accumulated in the cavity over multiple turns. This model also allows the estimatation of the radiation characteristics in the radiator of the SSMB storage ring. Our method combines the beamline transfer matrices with a low-gain free-electron-laser oscillator model. The advantage of this approach lies in its ability to provide quick estimates for the aforementioned dynamical effects. We then discuss the feasibility of three recently proposed scenarios, namely, the amplifier SSMB, the frequency-beating SSMB, and the harmonic SSMB, in the presence of the aforementioned dynamical effects. The analysis indicates that all three scenarios remain feasible under the current preliminary design parameters.