The most significant advances in accelerator-based light sources (i.e., x-ray free electron lasers) are driven by the production of a kiloampere-level peak current. As a prerequisite to achieve the desired high peak current, multistage symmetric C-chicane compressors are widely utilized due to their simple geometry and effectiveness. However, during the bunch compression process, coherent synchrotron radiation (CSR) effects may lead to transverse emittance dilution, eventually hindering progress toward achieving a higher peak current. Therefore, suppressing or even canceling the CSR effects in chicane compressors is essential. In this paper, we consider a general model of a four-bend chicane and conduct an explicit point-kick analysis of the coherent synchrotron radiation (CSR) effects, based on which we derive the conditions for completely canceling the steady-state CSR (ss-CSR) kicks using analytical formulas. These results yield two novel ss-CSR-immune chicanes that feature nonsymmetric C-shape and S-shape layouts, respectively. We also demonstrate their excellent emittance preservation capabilities with particle tracking simulations. By utilizing the proposed chicanes instead of the widely used C-chicane, the CSR-induced emittance growth can be reduced by about 1 order of magnitude in an almost identical bunch compression scenario.

This paper presents the concept and design of a novel accelerator magnet consisting of two concentric rings of permanent magnets arranged in a Halbach layout. By replacing one component, the device can be converted to any transverse multipole configuration. By the individual rotation of the two rings, both the strength and the direction of the field pattern can be continuously tuned in a wide range, including the special case of polarity reversal. The tuning range can be changed by a parametric change of the design. The device is split into two halves and can be easily opened to allow for its installation and removal around an existing beampipe. The performance of a quadrupole demonstrator prototype is presented, including measurement results of field quality, integrated gradient and hysteresis, and discussion of sensitivity to the remanent magnetization of the permanent magnets.

The Diamond-II storage ring upgrade will provide users with 1–2 orders of magnitude increase in brightness over the existing Diamond facility, as well as double the number of straight sections to allow for the installation of additional beamlines. A key performance requirement for the upgrade will be to provide a quasitransparent top-up injection scheme. The ring was originally designed to use a single-bunch aperture sharing injection scheme, in which short stripline kickers are used to kick the injected bunch into the storage ring’s dynamic aperture, while the kick remains weak enough to avoid displacing the existing stored bunch outside the acceptance. This scheme has been modified to implement a kick-and-cancel method to place the stored bunch back on axis and leave only the injected bunch to oscillate. The kicker power supplies are thus required to provide a double pulse with a few-microsecond pulse spacing. This new method is expected to significantly improve the transparency of the injection process and reduce the recovery time for the targeted bunch, along with minimizing the transverse wakefield effects and any interactions with the transverse multibunch feedback and harmonic cavity.

The room temperature continuous wave (cw) very-high-frequency (VHF) gun is one of the candidates for the electron gun of the high-repetition-rate free-electron lasers (FELs). The VHF gun operates with a cathode gradient of $\sim 20\mathrm{MV}/\mathrm{m}$ and an accelerating voltage of $\sim 750\text{kV}$. The gun dark current emission leads to beam loss along the FEL machine, therefore, it is a critical parameter for the performance of the cw gun. In this paper, we present a systematic study of the dark current reduction of the VHF gun, including cathode region optimizations, dark current tracking simulations, and measurements. Over-inserted cathode plugs were tested in two VHF guns of different acceleration gap sizes, and both demonstrated significant dark current reduction ratios of more than two orders of magnitude.

We present a new method for minimizing the effects of radiative depolarization in electron storage rings by use of a minimal number of special vertical orbit bumps. The bumps can be used to minimize the effects of radiative depolarization while simultaneously maintaining other common benefits of vertical orbits, e.g., transverse coupling and vertical dispersion control. Because simultaneously optimizing the large number of vertical correctors in a ring is operationally infeasible, we use dimensionality reduction to define a minimal number of the most effective groups of vertical correctors that can be optimized during operation, motivating the name “Best Adjustment Groups for ELectron Spin” (BAGELS). The method is streamlined by using suitable “basis bumps” instead of all individual vertical correctors. We define three types of basis bumps for different purposes: (i) generates no delocalized transverse coupling nor delocalized vertical dispersion, (ii) generates no delocalized vertical dispersion, and (iii) generates no delocalized transverse coupling. BAGELS has been essential in the design of the Electron Storage Ring (ESR) of the Electron-Ion Collider (EIC) and will be beneficial for any polarized electron ring, including FCC-ee. HERA and LEP would have likely benefitted as well. We use BAGELS to significantly increase polarization in the 18 GeV EIC-ESR, beyond what is achievable with conventional methods; in the 1-IP lattice, we nearly double the asymptotic polarization, and in the 2-IP lattice, we more than triple the asymptotic polarization. We also use BAGELS to construct knobs that can be used for global coupling correction, and knobs that generate vertical emittance for beam size matching, all while having minimal impacts on the polarization and orbit/optics.

Local chromatic correction optics are proposed for the Future Circular $e^{+}-e^{-}$ Collider. These new optics assume an identical layout of the magnets at all operation energies and include a new design for the arcs, for the straight sections, and for the final focus. The arcs design is a step forward from the classic FODO lattice that achieves near cancellation up to the fourth order of chromatic and geometric aberrations. Straight sections and the final focus benefit from the application of transparency conditions and are thus minimally impacting the global beam dynamics. The final focus design includes both vertical and horizontal chromaticity correction sections, crab sextupoles, and additional specific sextupoles and decapoles for the optimization of the dynamics off-energy. Decapoles are very effective in mitigating the reduction of dynamic aperture due to synchrotron radiation. Nonlinear magnet settings are further optimized with multiobjective algorithms. The overall properties of the new optics proposed are analyzed in the presence of errors and compared to other optics designs.

The maximum achievable beam current in an energy recovery linac (ERL) is often constrained by beam breakup (BBU) instability. Our previous research highlighted that filling patterns have a substantial impact on BBU instabilities in multipass ERLs. In this study, we extend our investigation to the eight-cavity model of the Powerful ERL for Experiment (PERLE). We evaluate its requirements for damping cavity higher order modes (HOMs) and propose optimal filling patterns and bunch timing strategies. Our findings reveal a significant new insight: while filling patterns are crucial, the timing of bunches also plays a critical role in mitigating HOM beam loading and BBU instability. This previously underestimated factor is essential for effective BBU control. We estimated the PERLE threshold current using both analytical and numerical models, incorporating the designed PERLE HOM dampers. During manufacturing, HOM frequencies are expected to vary slightly. Our study found no significant difference in BBU suppression for relative rms frequency jitters of 0.001, 0.002, and 0.005 for the same HOM. Introducing a jitter of 0.001 into our models, we found that the dampers effectively suppressed BBU instability, achieving a threshold current an order of magnitude higher than the design requirement. Our results offer new insights into ERL BBU beam dynamics and have important implications for the design of future ERLs.

A key challenge in particle accelerators is to achieve high peak intensity. Space charge is particularly strong at lower energy such as during injection and typically limits achievable peak intensity. The beam stacking technique can overcome this limitation by accumulating a beam at high energy where space charge is weaker. In beam stacking, a bunch of particles is injected and accelerated to high energy. This bunch continues to circulate, while a second and subsequent bunches are accelerated to merge into the first. It also allows the user cycle and acceleration cycles to be separated which is often valuable. Beam stacking is not possible in a time varying magnetic field, but a fixed field machine such as an fixed field alternating gradient accelerator (FFA) does not sweep the magnetic field. In this paper, we describe experimental demonstration of beam stacking of two beams at KURNS FFA in Kyoto University. The momentum spread and intensity of the beam was analyzed by study of the Schottky signal, demonstrating stacking with only a slight increase of momentum spread of the combined beams. The intensity of the first beam was, however, significantly reduced. rf knockout is the suspected source of the beam loss.

The recently installed internal gas target at LHCb presents exceptional opportunities for an extensive physics program for heavy-ion, hadron, spin, and astroparticle physics. A storage cell placed in the LHC primary vacuum, an advanced Gas Feed System, the availability of multi-TeV proton and ion beams, and the recent upgrade of the LHCb detector make this project unique worldwide. In this paper, we outline the main components of the system, the physics prospects it offers, and the hardware challenges encountered during its implementation. The commissioning phase has yielded promising results, demonstrating that fixed-target collisions can occur concurrently with the collider mode without compromising efficient data acquisition and high-quality reconstruction of beam-gas and beam-beam interactions.

Recent $e^{+}{e}^{-}$ colliders operate with very low emittance and high current. The space-charge tune shift is not negligible in tau-charm and B factories. The vertical space-charge tune shift is on the order of 0.02–0.1 and is comparable to the synchrotron tune and the beam-beam tune shift. The beam-beam mode coupling instability has been studied with and without vacuum environment impedance. The mode coupling is influenced by the space-charge force, which exhibits similar characteristics to the beam-beam force. The combined effect of beam-beam, space charge, and vacuum environment impedance is discussed within the framework of the mode coupling theory. Strong-strong beam-beam simulation, considering the space-charge force and the impedance, are conducted to validate the theory and to study general cases such as asymmetric colliders.

A dual-energy electron storage ring is a novel concept initially proposed to cool hadron beams at high energies. The design consists of two closed rings operating at significantly different energies: the low-energy ring and the high-energy ring. These two rings are connected by an energy recovery linac (ERL) that provides the necessary energy difference. The ERL features superconducting radio-frequency (SRF) cavities that first accelerate the beam from the low energy $E_L$ to the high energy $E_H$ and then decelerate the beam from $E_H$ to $E_L$ in the next pass. The different SRF cavities in the ERL section can be adjusted based on the applications. In this paper, we present a possible layout of a dual-energy electron storage ring. The preliminary optics of the ring is designed to optimize chromaticity correction, dynamic aperture, momentum aperture, beam lifetime, radiation damping, and intrabeam scattering effects. The primary focus of this paper is on the stability conditions and beam dynamics studies associated with this storage ring.

Next-generation accelerator concepts, which hinge on the precise shaping of beam distributions, demand equally precise diagnostic methods capable of reconstructing beam distributions within six-dimensional position-momentum spaces. However, the characterization of intricate features within six-dimensional beam distributions using current diagnostic techniques necessitates a substantial number of measurements, using many hours of valuable beam time. Novel phase space reconstruction techniques are needed to reduce the number of measurements required to reconstruct detailed, high-dimensional beam features in order to resolve complex beam phenomena and as a feedback in precision beam shaping applications. In this study, we present a novel approach to reconstructing detailed six-dimensional phase space distributions from experimental measurements using generative machine learning and differentiable beam dynamics simulations. We demonstrate that this approach can be used to resolve six-dimensional phase space distributions from scratch, using basic beam manipulations and as few as 20 two-dimensional measurements of the beam profile. We also demonstrate an application of the reconstruction method in an experimental setting at the Argonne Wakefield Accelerator, where it is able to reconstruct the beam distribution and accurately predict previously unseen measurements $75\times$ faster than previous methods.

In high intensity and high energy colliders, such as the CERN Large Hadron Collider (LHC) and its future high-luminosity upgrade, interactions between the two beams around the different interaction points impose machine performance limitations. In fact, their effect reduces the beam lifetime, and therefore, the collider’s luminosity reach. Those interactions are called beam-beam long-range (BBLR) interactions, and a possible mitigation of their effect using dc wires was proposed for the first time in the early 2000’s. This solution is currently being studied as an option for enhancing the HL-LHC performance. In 2017 and 2018, four demonstrators of wire compensators have been installed in the LHC. A 2-yearlong experimental campaign followed in order to validate the possibility to mitigate the BBLR interactions in the LHC. During this campaign, a proof-of-concept was completed and motivated an additional set of experiments, successfully demonstrating the mitigation of BBLR interactions effects in beam conditions compatible with the operational configuration. This paper reports in detail the preparation of the experimental campaign, including the corresponding tracking simulations and the obtained results, and draws some perspectives for the future.

This study developed a pressure-anomaly detection system utilizing machine learning for the vacuum system of the SuperKEKB accelerator. The system identified abnormal pressure behaviors among approximately 600 vacuum gauges before triggering the conventional alarm system, facilitating the early implementation of countermeasures and minimizing potential vacuum issues. By comparing the recent pressure behaviors of each vacuum gauge with the previous behaviors, the program detected anomalies using the decision boundary of a feed-forward neural network previously trained on actual abnormal behaviors. Realistic regression models for pressure data curves enabled a reasonable prediction of the causes of anomalies. The program, implemented in python, has been operational since April 2024. Although based on a rudimentary machine-learning concept, the developed anomaly detection system is beneficial for ensuring the stable operation of large-scale machines, including accelerators, and is helpful in designing systems for fault detection.

The Facility for Rare Isotope Beams (FRIB) is a major nuclear physics facility for research with fast, stopped, and reaccelerated beams that was successfully commissioned in May 2022. A key capability of FRIB is the production of an acceleration of the uranium beam, but this capability requires the facility to work at the design limits of the lowest charge-to-mass ratio and the highest power density on the beam intercepting devices. This paper presents techniques for overcoming the significant challenges in accelerating the uranium beam, culminating in the demonstration of 10.4 kW on target, and the discovery of three new isotopes. The high-power uranium beam enabled us to produce and identify ${}^{88}\mathrm{G}\mathrm{a}$, ${}^{93}\mathrm{A}\mathrm{s}$, and ${}^{96}\mathrm{S}\mathrm{e}$, within the first 24 h of operation. The successful uranium operation at FRIB sets a new record for accelerated uranium beam power above 10 kW and opens a new avenue of research with rare isotopes.

The vacuum chambers for diffraction limited storage ring differ from the previous storage rings generation in three main aspects: the cross-section dimension, which is divided by a factor of 2 or more to fit smaller magnet apertures; the material, which includes much more copper for heat dissipation and to limit resistive wakefields; the coating of the inner surface with a nonevaporable getter (NEG) to ensure good pumping level despite low conductance. This paper gives a detailed description of the vacuum chambers’ design used in the arc section of SLS 2.0, starting with conceptual design choices based among others on synchrotron radiation heat and wakefield considerations. The particular case of the main bending magnet vacuum chamber is explained in detail from design to manufacturing, including the development of an appropriate NEG coating procedure. Finally, the overall assembly of a 17 m long arc, its activation to reach pressure in the ${10}^{-11}\mathrm{mbar}$ range followed by transport and installation into the magnets is presented. This validates the choice of ex situ activation for an arc vessel made out of copper, with wall thicknesses as small as 1 mm and with less than 0.5 mm clearance to magnet poles.

The Linac for Image Guided Hadron Therapy (LIGHT) system has been developed as the first commercial high-frequency linac-based proton therapy accelerator system. As part of the development process and of the technical validation of the concept, a reduced energy prototype linac (including all the key functional aspects of the full energy accelerator) has been designed and tested in parallel with the preparation work for a full-scale system. The LIGHT prototype construction, installation, and commissioning with beam allowed the identification of technical gaps and design improvements before the transfer to a medical device implementation. This paper gives a comprehensive summary of the design and integration aspects and presents for the first time the results achieved with the LIGHT prototype that was installed and commissioned on CERN premises in the years 2015–2019.

Shanghai Synchrotron Radiation Facility/Shanghai Soft X-ray FEL Facility is currently developing an advanced variable polarization transverse deflecting structure TTDS (two-mode transverse deflecting structure) using a dual-mode rf structure concept. Driven by two different rf power sources, this novel TDS works using both the ${\mathrm{HEM}}_{11}$ and ${\mathrm{HEM}}_{12}$ modes for simultaneous vertical and horizontal deflections, consequently, it can provide a time-varying polarization of the electrical field at ultrafast speeds. It is capable of producing circular and elliptical polarizations as well as flexible vector combinations through amplitude and phase modulation from a low-level rf system. The work presented in this paper is focused on the analysis and design of the variable polarization TDS, consisting of dual-mode cells and two dual-mode couplers. Designing and optimizing for dual-mode design and optimization is complex; consequently, an advanced optimization procedure based on neural networks and multiobjective algorithms has been developed. This improves the accuracy and efficiency of the rf structure design process. Through iterations, the dual-mode cells in the final design are optimized for high impedance and other rf performance criteria for both the ${\mathrm{HEM}}_{11}$ and ${\mathrm{HEM}}_{12}$ modes. The two couplers for rf power input and output are also optimized. Based on the optimized design and rf sensitivity analysis, the mechanical design has been completed and is now ready for manufacture.

We investigate the use of tapered rectangular dielectric-lined waveguides (DLWs) for the acceleration of low-energy, subrelativistic, electron bunches by the interaction with multicycle narrowband terahertz (THz) pulses. A key challenge exists in this subrelativistic regime; the electron velocity changes significantly as energy is gained. To keep electrons in the accelerating phase, the phase velocity must also be increased to match. We present simulations which demonstrate that the dielectric thickness can be kept constant and the width of the dielectric lining can be tapered along the direction of travel to vary the phase velocity, an approach only possible by the use of a rectangular waveguide geometry. The properties of tapered DLWs are discussed and following this, a design process is presented to demonstrate that the way this tapering can be optimized for different pulse and beam parameters. The minimum accelerating gradient for electron bunch capture is derived and compared to simulations. As examples of this design process, designs are considered based on considerations of the THz source, incoming electron beam, and manufacturing tolerances. A maximum THz pulse energy of $22.5\mathrm{\mu }\mathrm{J}$ in the DLW was considered, which represents what is readily achievable using mJ-level regenerative amplifier laser systems together with optical-to-terahertz conversion in lithium niobate crystals. This will be more than double the energy of a 100 keV electron beam, increasing it to 205 keV. We describe the optimization process and present a detailed exploration of the beam dynamics, discussing how the performance will further improve with compressed bunches.

The compact linear accelerator for research and applications (CLARA) is a 250 MeV ultrabright electron beam test facility at STFC Daresbury Laboratory. A user beamline has been designed to maximize the exploitation of CLARA in a variety of fields, including novel acceleration and new modalities of radiotherapy. In this paper, we present the specification and design of this beamline for full energy beam exploitation. We outline the key elements that will provide users access to ultrashort, low emittance electron bunches in two large experiment chambers. The results of start-to-end simulations are reported that verify the expected beam parameters delivered to these chambers. Key technical systems are detailed, including those which facilitate a combination of electron bunches with high-power laser pulses.