SACLA is the world’s most compact hard x-ray free-electron laser and has been successfully and reliably operated for over a decade. To meet increasing user demands, an upgrade of the SACLA linear accelerator is planned in order to achieve a kHz-level repetition rate and enhanced beam brightness. A key component of this upgrade is a new injector, based on the existing pulsed dc gun with a thermionic cathode, which has demonstrated high beam quality, excellent stability, and simple maintenance. In this work, we present an in-depth beam dynamics study of a highly promising new SACLA injector architecture. Multistage velocity bunching, combined with one stage of magnetic compression and careful control of emittance growth, allows for up to 3 orders of magnitude of bunch compression while keeping the emittance growth at the submicron level. Our optimization approach utilizes a genetic algorithm coupled to 3D space-charge-dominated beam tracking to refine beam performance and identify optimal injector parameters. Thus, we demonstrate that the proposed injector can deliver electron beams with bunch durations and emittances comparable to those produced by the state-of-the-art $C$-band injector based on an rf photocathode gun [A. Giribono et al., Phys. Rev. Accel. Beams 26, 083402 (2023)]. Additionally, our extensive beam dynamics analysis of the dc electron gun (see part 2) provides valuable insights applicable to low-voltage continuous-wave very high frequency guns and superconducting radio-frequency guns.

This paper presents two conceptual low-energy injector configurations optimized for high-brightness, high-repetition-rate XFELs. The designs were evaluated at a nominal bunch charge of 150 pC using multiobjective optimization techniques and further assessed through start-to-end simulations incorporating collective effects to determine their suitability for delivering high-brightness beams under FEL-relevant conditions. One of the injector design configurations employs a hybrid rf and magnetic compression scheme, while the second relies solely on two-stage rf compression. Start-to-end simulations through the main linac, including collective effects, were performed to assess the suitability of each design for meeting XFEL requirements. The results highlight the critical role of longitudinal charge distribution symmetry in maintaining beam quality during acceleration and compression, and demonstrate that appropriately balancing compressibility, emittance preservation, and phase space linearity is essential for meeting the stringent demands of XFEL operation.

In storage ring-based synchrotron radiation light sources, in-vacuum undulators (IVUs) are widely used considering their advantages of short period length and high field quality. The impedance of the IVU and heat load deposition due to the beam passage are among the main concerns during the design and operation of the IVU. In this work, both transverse and longitudinal trapped modes are investigated, and novel mitigation strategies to reduce the impedance are proposed. Additionally, the longitudinal broadband impedance increase introduced by the bulge flexible taper transitions is presented. To obtain comprehensive and reliable information on the impedance, laboratory bench measurements are performed to identify the longitudinal and transverse impedance of the IVU experimentally. A reasonably good agreement was reached between the numerical predictions and the measurements.

Beam loss is the most critical topic in the high-power accelerator community. During high-power proton beam commissioning of Chinese ADS Front end, unexpected irradiation dose was detected opposite to the proton beam deflection direction. This indicates that there are parasitic ${\mathrm{H}}^{-}$ particles inside the proton beam, which were first observed in the linac facility. In the paper, regarding this beam loss phenomenon, a qualitative analysis was carried out through nuclide analysis and was based on the theoretical foundation of molecular science. Moreover, a quantitative analysis was conducted through beam experiments, and finally verified this beam loss mechanism, which needs to be considered in the future design of high-intensity and high-power proton accelerators.

Beam injection is one of the most crucial issues for next-generation light sources such as diffraction-limited storage rings. We propose a novel transparent off-axis beam injection scheme for low-emittance storage rings of future light sources. The scheme aims to achieve efficient beam injection into a narrow dynamic aperture with small injection amplitude, while minimizing perturbation to the stored beam with transparent beam injection. An in-vacuum thin eddy-current septum magnet is one of the key components of the scheme to achieve the small injection amplitude. A prototype of the magnet with an accommodating ultrahigh vacuum (UHV) chamber was designed and fabricated. Magnet performance of the prototype demonstrated that an injection amplitude as small as 3 mm is feasible, fulfilling the tight constraint of injection amplitude in off-axis beam injection for next-generation light sources. The prototype achieved a UHV pressure of better than $1\times {10}^{-7}\mathrm{Pa}$, ensuring the vacuum performance required for stable operation in storage rings.

A detailed, step-by-step design methodology of a ferroelectric fast reactive tuner (FE-FRT) capable of modulating Mega VAR reactive powers on a submicrosecond timescale is given. Closed expressions of values for all the components of the tuner are detailed, and tuner performance optimization is addressed, resulting in a figure of merit measure of FE-FRT tuner performance and use case applicability over a wide range of rf frequencies and reactive power levels. This enables addressing feasibility and rapid assessment of design parameters given an FE-FRT tuning scenario defined by required tuning range, cavity operating frequency, and cavity stored energy.

We present a comprehensive study unveiling advancements in $\alpha$ particle spectra manipulation that can be achieved through the implementation of a helical coil target with tube (HCT). Leveraging particle-in-cell simulations, we demonstrate the ability to control the energy distribution of $\alpha$ particle bunches within a narrow range, down to a few MeV. This development marks the first instance of successful ion energy manipulation facilitated by the HCT configuration. Importantly, our investigations reveal a significant enhancement in radioisotope production, with yields ranging from 10 to 3000 times greater with an HCT than without. These findings underscore the transformative potential of the HCT approach in improving $\alpha$ particle production in the context of laser-plasma acceleration and its consequential impact on radioisotope production for diverse applications. We also investigate the production of radioisotopes using proton acceleration with the HCT configuration and demonstrate that the yield can be increased by a factor of 30.

Superconducting, curved magnets can reduce accelerator footprints by producing strong fields ($>3\mathrm{T}$) and reducing the total number of magnets through their capability for combined-function multipolar fields, making them an attractive choice for applications such as heavy ion therapy. There exists the problem that the effect of strongly curved harmonics and fringe fields on compact accelerator beam dynamics is not well represented: existing approaches use integrated cylindrical multipoles to describe and model the fields for beam dynamics studies, which are invalid in curved coordinate systems and assume individual errors cancel out over the full machine. In the modeling of these machines, the effects of strongly curved harmonics and fringe fields on compact accelerator beam dynamics need to be properly included. An alternative approach must be introduced for capturing off-axis fields in a strongly curved magnet, which may affect long-term beam stability in a compact accelerator. In this article, we investigate the impacts of deploying a curved canted-cosine-theta (CCT) superconducting magnet in a synchrotron for the first time. We develop a method to analyze and characterize the 3D curved fields of an electromagnetic model of a CCT developed for the main bending magnets of a compact (27 m circumference) carbon ion therapy synchrotron, designed within the Heavy Ion Therapy Research Integration Plus European project (HITRIPlus), and the CERN Next Ion Medical Machine Study (NIMMS). The fields are modelled in the compact synchrotron in mad-x/ptc to study their effects on beam dynamics and long-term beam stability. Results indicate that the synchrotron is able to operate with the presence of the magnetic field gradients, with considerable improvement to the long-term beam stability in the machine after tuning the higher order field gradients. The insights gained through the methods presented allow the optimisation of both magnet and synchrotron designs, with the potential to impact the operational performance of future ion therapy facilities.

CUT18, a cryogenic permanent magnet undulator at the Taiwan Photon Source, represents a significant advancement in undulator technology, offering enhanced flexibility, sustainability, and reliability. It uses a cooling method based on the latent heat of vaporization of liquid nitrogen (${\mathrm{LN}}_2$), utilizing the facility’s existing ${\mathrm{LN}}_2$ supply line to improve resource efficiency and reduce operational costs. The ${\mathrm{LN}}_2$ evaporative cooling system delivers exceptional thermal management, with cooling margins far exceeding the heat load at a beam current of 500 mA, ensuring stable operation even under high beam-induced heating conditions. In addition to this, the adoption of advanced NdFeB magnets, optimized with a 17° magnetization tilt, significantly enhances magnetic field performance and achieves a high deflection parameter ($\mathit{K}>2$).

Symplectic integrators play a pivotal role in the long-term tracking of charged particles within accelerators. To obtain symplectic maps in accurate simulations of single-particle trajectories, two key components are addressed: precise analytical expressions for arbitrary electromagnetic fields and a robust treatment of the equations of motion. In a source-free region, electromagnetic fields can be decomposed into harmonic functions, applicable to both scalar and vector potentials, encompassing both straight and curved reference trajectories. These harmonics are constructed according to boundary surface field data due to the uniqueness theorem. By finding generating functions to satisfy the Hamilton-Jacobi equation via a perturbative ansatz, we derive maps that are symplectic at any order of the expansion. This method yields a direct mapping from initial to final coordinates, bypassing the tracking of intermediate coordinates. Our developed particle-tracking algorithm translates the field harmonics into beam trajectories, offering a high-resolution integration method in accelerator simulations.

The SPIRAL2 superconducting linear accelerator (LINAC), which has been operational since 2019, employs superconducting, independently phased rf resonators to deliver a wide range of particle beams. Designed for flexibility in particle types, intensities, and energies, it utilizes superconducting quarter-wave resonators, the performance of which is critically dependent on the reliability of the cryogenic operation. This paper reviews the evolution of the SPIRAL2 cryogenic system from commissioning to routine operation, which was initially commissioned in 2017. It highlights the key challenges encountered, including thermoacoustic oscillations, thermal management, and the abnormal behavior of cavities. In addition, it explores the integration of thermodynamic modelling and machine learning techniques to enhance system control and diagnose problems. This work serves as a comprehensive resource for advancing cryogenic operations and the performance of superconducting LINACs.

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.