Radio frequency (rf) bunchers are often utilized to control a longitudinal bunch length in low- and medium-energy transport lines. However, nonlinear rf fields in the bunchers deform a longitudinal phase space distribution, which causes emittance growth leading to beam quality degradation and unexpected beam losses. This paper describes a decrease in the longitudinal rms emittance in a high-intensity deuteron linac, where the longitudinal phase space deformed by a nonlinear rf field in the buncher is self-linearized by a longitudinal nonlinear space charge force. To quantitatively explain this phenomenon, we develop two formulas to express deformations of phase space and forces acting on the beam. We perform particle tracking simulations with the tracewin code using a 125 mA, 5 MeV deuteron beam at the Linear IFMIF Prototype Accelerator (LIPAc) and validate the two formulas by applying them to LIPAc. The quantitative evaluations of the deformations with the formulas lead to a deeper understanding of beam physics and are also expected to be useful in mitigating phase space deformation.

Beam loss monitors (BLMs) and beam current monitors (BCMs) are ubiquitous at particle accelerators around the world. These simple devices provide noninvasive high-level beam measurements but give no insight into the detailed 6D $(x,y,z,p_x,p_y,p_z)$ beam phase-space distributions or dynamics. We show that generative conditional latent diffusion models can learn intricate patterns to solve the extreme inverse problem of mapping waveforms of tens of BLMs or BCMs along an accelerator to detailed 2D projections of a charged particle beam’s 6D phase-space density. This transformational method can be used at any particle accelerator to transform simple noninvasive devices into detailed beam phase-space diagnostics. We demonstrate this concept via multiparticle simulations of the high-intensity beam in the kilometer-long Los Alamos Neutron Science Center linear proton accelerator.

The High Energy Photon Source (HEPS) Linac is a normal-conducting electron linear accelerator capable of producing high bunch charge beam. Its bunching system includes two subharmonic bunchers (SHBs), one prebuncher, one buncher, and one accelerating structure. The SHB is commonly used in the low-energy section for longitudinal bunching to increase the bunch charge. The phase and voltage of the SHB are crucial for obtaining a high-quality electron beam in Linac. This paper presents a method for calibrating the phase and voltage of the SHB using the time-of-flight technique. Experiments were conducted at the HEPS Linac using two beam position monitors. According to the simulations, we found that bunch length and space charge effect have a significant impact on the results, and we propose corrective measures to address them. To enhance experimental efficiency, we propose a data processing method called the truncated averaging technique that eliminates the need for corrective measures, which has been validated through both simulations and experiments, yielding outstanding results. This paper presents detailed insights into both the simulation and experimental procedures.

To investigate the internal spin structure of nucleons and explore fundamental phenomena such as parity and time-reversal symmetry violations, polarized deuteron beams are pivotal because they serve as a unique alternative to polarized neutron beams and allow for the exploration of tensor polarization—a critical feature of spin-1 systems that offers distinct insights beyond vector polarization. This study advances the understanding of tensor-polarized deuteron beam dynamics in synchrotrons, with direct relevance to the Electron-Ion Collider in China (EicC). By deriving spin tensor transfer matrices and validating their accuracy through numerical simulations, we demonstrate the tensor polarization evolution in bending magnets, solenoids, and quadrupoles. The analysis of depolarizing resonances during acceleration reveals negligible polarization losses under the EicC’s design parameters, ensuring the feasibility of high-precision spin experiments without dedicated polarization maintenance. Furthermore, the planar-constrained translational dynamics of spin tensors, fundamentally distinct from the rotational mechanisms governing spin vectors, offer novel strategies for spin manipulation in future accelerators. These results provide theoretical and computational foundations for spin tensor control in the EicC and broader applications in spin-correlation experiments.

As the ring-based light source community is moving toward fourth-generation light sources, many facilities plan to upgrade their boosters in parallel to meet the more demanding beam properties for the storage ring, especially in terms of a much lower emittance. Concerns over collective effects have, therefore, risen, particularly in the transverse planes, since the vacuum chamber dimensions tend to be reduced as a way to achieve a stronger focusing force on the beam. In this article, we present numerical studies on transverse beam instabilities, both in the single- and multibunch regimes, in the SOLEIL II booster as an example of a low-emittance booster. We show that Landau damping is an efficient mechanism for suppressing both transverse single-bunch and coupled-bunch instabilities. We also prove that the damping in the longitudinal plane can diffuse to the transverse plane and limit the transverse emittance growth. Moreover, we have discovered that the beam can exhibit sawtooth instability at high energy and that broadband impedance is one of the key factors in suppressing transverse coupled-bunch instability.

We use Bayesian optimization in combination with three-dimensional particle-in-cell simulations to determine the optimal laser and plasma parameters that, for a given laser pulse energy, maximize the cut-off energy of an electron beam produced by laser wakefield accelerators. We assume a Gaussian laser driver with matched spot size and amplitude and investigate both self-guiding in a uniform-density plasma and guiding in a preformed plasma channel with matched radius. To interpret the simulation results quantitatively, we derive novel analytical expressions for the maximum electron energy and the corresponding acceleration distance, accounting for the effects of laser diffraction and energy depletion. Based on the results obtained, we discuss the potential scalability of the optimal input (plasma density, pulse duration, amplitude, spot size, and channel radius) and output (electron energy, electron charge, acceleration length, and acceleration efficiency) parameters to laser systems of arbitrary energy.

High-power proton linear accelerators have important applications in both scientific research and industry. However, the operation of such accelerators with megawatt-level beam power is expensive and limits the broad availability of these facilities. In this Letter, we propose a novel energy recovery proton linear accelerator in which the final GeV-level proton beam is reinjected into the linear accelerator from the accelerator exit and is decelerated in the same accelerator down to about 2 MeV, close to the initial beam energy. This substantially reduces the power consumption of the proton linear accelerator and also avoids the need for a high-power beam dump. We demonstrate this concept through self-consistent simulations and show that the beam-beam effect between the forward-accelerating beam and the backward-decelerating beam would not be a limiting factor for the proposed concept. A potential application of this concept to an electron-ion collider based on energy recovery electron and ion accelerators is discussed.

We show that an anomalous pinch can occur in ultrarelativistic electron-electron or positron-positron beam interaction, caused by the combined interplay of collective beam motion (disruption) and strong-field quantum electrodynamics (SF-QED). The locally created electron-positron pairs, from SF-QED effects, screen the self-fields of the beams and can invert the polarity of the Lorentz force, resulting in a pinch of the beams. A theoretical model predicts the pinch condition and is confirmed by first-principles three-dimensional particle-in-cell simulations. This anomalous pinch causes density compression, increases the collision luminosity, and amplifies the local magnetic fields and the quantum parameter of the beam particles by several orders of magnitude.

Laser-plasma accelerators present a promising alternative to conventional accelerators. To fully exploit the extreme amplitudes of the plasma fields and produce high-quality beams, precise control over electron injection into the accelerating structure is required, along with effective laser pulse guiding to extend the acceleration length. Recent studies have demonstrated efficient guiding and acceleration using hydrodynamic optically field-ionized plasma channels. This guiding technique has also been combined with controlled electron injection to produce high-quality electron beams at the GeV level using a 50 TW laser. The present work extends these results to higher laser power, demonstrating the generation of quasimonoenergetic electron beams with peak energies exceeding 2 GeV, for a PW-class laser.

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.