In the framework of the LHC injectors upgrade project, the CERN proton synchrotron underwent hardware changes during the long shutdown 2. These changes required an update of the CERN proton synchrotron impedance model and an assessment of their impact on beam quality. In the following, the updated CERN proton synchrotron impedance model is presented and then validated with beam-based measurements, comparing impedance-induced tune shifts and instability growth rates with simulations performed with a macroparticle tracking code, pyheadtail, which includes the effects of beam-coupling impedance, betatron motion, nonlinear synchrotron motion, and space charge. The interplay between first- and second-order chromaticity, space-charge forces, and their effect on beam observables is thus studied in detail. Their respective impacts have been assessed and benchmarked with measurements to validate the PS transverse impedance model.

Recently, the generation of ultrashort x-ray free-electron laser (XFEL) pulses of less than 1 fs has been reported using additional electron bunch compression in front of undulators. In this bunch compression, a self-modulated energy chirp provided by space charge force is used. In the present paper, we discuss an optimal bunch compression scheme using a two-Double Bend Achromat (DBA) dogleg and an electron bunch having the energy chirp which is reversed with respect to that used for a conventional magnetic chicane bunch compressor. After giving analytical expressions of $R_{56}$ and $T_{566}$ of an ordinary DBA section, a method for evaluating $R_{56}$ and $T_{566}$ is presented for varied bending angle cases to achieve additional bunch compression. When changing the bending angles for controlling $R_{56}$ and $T_{566}$, the electron beam no longer passes the magnetic center of focusing quadrupole magnets of the DBA section, but the electron beam trajectory and transverse envelope remain the same downstream of the two-DBA dogleg, including the undulators. The analytical framework proposed in this paper would be useful to design and finely tune the bunch compression in front of the undulators to generate ultrashort XFEL pulses.

Studies of plasma processing for reduction of field emission in quarter-wave resonator (QWR) cryomodules are underway at the Facility for Rare Isotope Beams (FRIB), where a total of 104 QWRs are presently in operation. Driving the plasma with a higher-order mode allows for less mismatch at the fundamental power coupler and higher plasma density. The first plasma processing trial for FRIB QWRs in a cryomodule was conducted in January 2024. In-bunker cold tests of the cryomodule showed a significant reduction in field emission x-rays after plasma processing.

We present simulations of the acceleration and compression of electron bunches for megaelectronvolt ultrafast electron diffraction (MeV-UED) experiments planned at the National Centre for Nuclear Research in Poland (NCBJ). This compact MeV-UED facility, currently under construction, will consist of an electron photoinjector, a superconducting accelerating HZDR/RI-type cryomodule housing two TESLA cavities, and experimental chambers. Simulations demonstrated the feasibility of producing short electron bunches with a duration of a few femtoseconds for electron beam energies ranging from 2 to 5 MeV.

Highly overmoded and periodically loaded structures, such as the iris-line waveguide, offer an attractive solution for the efficient transportation of diffraction-prone THz pulses over long distances (hundreds of meters). This paper presents the full-scatter field theory that allows us to analytically derive all the spectral (modal) coefficients at the discontinuities of the iris line. The spectral analysis uses vector fields, superseding scalar field descriptions, to account for diffraction loss, polarization effects, and ohmic loss on practical conductive surfaces. An advanced application of Lorentz’s reciprocity theory, using a generalized guided-field configuration, is developed to reduce the complexity of the mode-matching problem over nonuniform sections. The used technique is quite general and applies to a wide class of structures, as it only assumes a paraxial incidence (i.e., a parabolic wave equation) along the axis of the structure. It removes the traditional assumption of very thin screens, allowing for the study of thicker screens in the high-frequency limit, while formulating the problem efficiently by scattering matrices whose coefficients are found analytically. The theory agrees with and expands previously established techniques, including Vainstein’s asymptotic limit and the forward-scatter approximation. The used formulation also facilitates accurate visualization of the transient regime at the entrance of the structure and how it evolves to reach steady state.

This article investigates electron bunch density fluctuations in the $1-10\mathrm{\mu }\mathrm{m}$ wavelength range, focusing on their impact on coherent electron cooling (CEC) in hadron storage rings. In this study, we compare the shot-noise model with experimental observations using bandwidth-filtered near-infrared optical transition radiation (OTR) photodiode signals, where the transverse bunch size is much larger than the OTR wavelength of interest. The relativistic electron bunch ($\gamma \approx 50$) parameters are close to those proposed for the coherent electron cooler in the electron-ion collider (EIC) project. Preliminary feasibility experiments were conducted, and the noise factors are presented, supported by particle tracking. No major density fluctuations or effective cooling rate decrease were revealed for EIC CEC design parameters. Additionally, longitudinal-space-charge-induced microbunching for the chicane-compressed bunch was observed with coherent OTR enhancements up to 100 times, providing further calibration of the measurement method.

The next-generation Compton gamma-ray sources (CGSs) based on storage rings require a high-power, well-focused laser beam as a photon driver, which can be realized using a Fabry-Perot cavity (FPC). In this work, we reexamine the behavior and performance of a two-mirror, nearly concentric resonator by introducing a new figure of merit representing the cavity’s proximity to instability. This figure of merit is used to analyze various aspects of the cavity design, including misalignment, beam coupling limitations, and beam size scaling. We then examine several factors affecting the gamma-ray flux, such as the interaction area, crossing angle for collision, frequency matching between the electron and laser beams, and intrabeam scattering effects. Using an example CGS, we demonstrate how to make improved design choices for a two-mirror resonator to enhance the gamma-ray beam flux. This work shows that simple two-mirror Fabry-Perot cavities are well suited as the laser driver for the next-generation storage ring-based CGS, while offering superior advantages in gamma-ray beam polarization control compared to more complex four-mirror, nonplanar resonators.

The paper evaluates the thermal regime of a cryogenically cooled copper photocathode integrated into a continuous-wave superconducting radio-frequency injector cavity with direct thermal contact. Such a photoinjector layout is being developed at DESY and has recently demonstrated a record-high $50\mathrm{MV}/\mathrm{m}$ axial electric field in radio-frequency tests, marking an important milestone. To address the thermal effect of the picosecond excitation laser, we first develop a two-temperature model to describe the temperature of the emitting surface at cryogenic temperatures and solve it numerically. Subsequently, we present a one-temperature model of the bulk photocathode coupled with an electromagnetic model of the injector cavity. For the current injector design, we predict a negligible impact of the laser on the intrinsic quality factor of the cavity, identifying instead the cryogenic stability of the copper cathode as the primary operational limit. To overcome cooling challenges, we propose an improved configuration of the cathode plug. For the proposed geometry, the multiphysics analysis confirms stable performance at a nominal 2 W laser power, sufficient for 100 pC beams at 1 MHz under optimistic quantum efficiency assumptions. Operation at higher laser loads will benefit from further dedicated cryogenic analysis.

Laser wakefield acceleration (LWFA) has enabled generation of high-energy electron beams and radiation sources, which are of great importance in high-energy density physics. However, identifying the optimal laser-plasma parameters—particularly variations in laser pulse duration and focal spot size—is crucial for producing not only high-charge, high-energy electron beams but also high-brightness, high-flux betatron radiation. In this study, we investigated the optimum laser parameters for generating high-charge electron beams and betatron radiation in subcritical density plasmas (SCDP). Three-dimensional particle-in-cell (3D-PIC) simulations show that a 550 TW laser pulse can generate electron beams with charges of several tens of nanocoulombs (nC) and energies up to 500 MeV. Furthermore, these electron beams produce high-brightness x-ray radiation, achieving a peak brightness of $1.6\times {10}^{22}\mathrm{photons}/(\mathrm{s}{\mathrm{mm}}^2{\mathrm{mrad}}^2$ 0.1% BW) at 10 keV, with a total photon number reaching $4\times {10}^{12}$ (energy $>$1 keV).

Radio frequency (rf) photoguns are widely used in modern particle accelerator facilities, particularly in free electron lasers (FELs), as they provide very low-emittance, high-brightness electron bunches. Presently, the rf technology mostly used for photoguns is the S-band ($\sim 3$ GHz), with typical cathode peak fields of 80–120 MV/m and maximum repetition rates of $\sim 100$ Hz. According to beam dynamics studies, the higher the peak electric fields on the cathode, the better the quality of the beam emerging from the gun. In this respect, S-band rf guns represent the state-of-the-art, both in terms of reachable cathode peak fields and repetition rates. An innovative C-band rf gun, operating at 5.712 GHz and aiming at reaching cathode peak fields larger than 160 MV/m, with repetition rates exceeding 400 Hz, has been designed, realized, and high-power tested. Its device consists of a 2.5-cell standing wave (SW) cavity with a four-port mode launcher whose overcoupled design allows it to operate with short rf pulses (300 ns). Its realization has been based on the new brazing-free technology developed and successfully tested at the Frascati Laboratories of the National Institute of Nuclear Physics (INFN-LNF, Italy). In the paper, we illustrate the design and realization performed by INFN-LNF, and its high-power rf test, which was conducted at the C-band test facility at the Paul Scherrer Institute (Switzerland).

The fluctuation in electron time of flight during lattice transport is crucial for various applications, such as ultrafast electron diffraction, optical stochastic cooling in storage rings, and sustaining steady-state microbunching in storage rings. A major issue impacting the time-of-flight fluctuation of the electron beam during transport is the lattice’s nonlinear effects. In this study, we introduce a design concept aimed at reducing the electron beam’s time-of-flight fluctuation in an isochronous lattice up to the subattosecond level. This approach employs a high-order achromat lattice that effectively eliminates time-of-flight contributions arising from low-order nonlinearities. We also present an optimized design example based on this concept, achieving approximately 0.3 attoseconds root-mean-square time-of-flight fluctuation (equivalent to 0.1 nm) with specific beam emittance values of ${\epsilon }_x\sim 1\mathrm{nm},{\epsilon }_y\sim 1\mathrm{nm},$ and ${\sigma }_{\delta }\sim 1\times {10}^{-3}$.

This work introduces a reinforcement learning algorithm designed for real-time luminosity optimization in collider experiments. The neural network architecture is selected from multiple candidates through systematic evaluation of their training performance. Prior to training, the input data undergo multistage preprocessing before being fed into the neural network. Our approach combines off-line pretraining on historical accelerator data with online fine-tuning during operation. By processing accelerator measurements over multisecond timescales, the reinforcement learning model dynamically adjusts the magnetic structure to maintain luminosity stability under changing beam conditions. The autonomous nature of the method eliminates the need for manual intervention, enhancing both operational efficiency and beam stability in long-term operation. Experimental validation on the VEPP-4M collider demonstrates the feasibility of the approach and provides a foundation for future development and deployment in accelerator systems.

Addressing the demands for compactness and tunability in magnet systems for fourth-generation light sources, this paper presents the design and development of a novel tunable permanent quadrupole magnet (PQM) prototype. The prototype achieves a maximum field gradient of $65\mathrm{T}/\mathrm{m}$ and demonstrates a continuous tuning range of $39\mathrm{T}/\mathrm{m}$. Within the effective aperture, normalized multipole field components are maintained below $5\times {10}^{-4}$. Experimental validation confirms that the prototype meets the engineering design requirements for the Hefei Advanced Light Facility. Compared to conventional electromagnetic quadrupoles, this PQM features a compact structure, with negligible power consumption and high magnetic field quality, representing a promising technology for advanced synchrotron radiation light sources and free-electron lasers.

The performance degradation caused by carbon contaminants has become one of the significant obstacles to the long-term, high-gradient, stable operation of the SRF cavities. This paper investigates the sources and effects of different forms of carbon contamination and recovery strategies through experiments on niobium samples and a 162.5 MHz half-wave SRF cavity. Our findings reveal that adventitious carbon and oxides are the primary chemical compounds on the niobium surface, with the work function decreasing linearly as carbon content increases. Additionally, it was observed that carbides can form through the chemical deposition of residual carbon-containing gas in a vacuum under the action of argon ions. The adsorption and deposition of carbon-containing gas molecules reduce surface work function, thereby lowering the field emission onset and quench point of the cavity. However, the combination of thermal cycling and rf conditioning facilitates the desorption of adsorbed gas molecules, leading to a full recovery of cavity performance. In contrast, carbon deposition forms strong chemical bonds with the cavity surface, necessitating more intensive removal techniques. We demonstrate that plasma cleaning effectively eliminates carbon contamination and restores cavity performance. Inspired by the formation of NbC in niobium sample experiments, we propose that, during long-term SRF cavity operation, carbides primarily originate from the chemical deposition of carbon-containing gas molecules adsorbed on the surface in the presence of energy-carrying particles. These insights deepen our understanding of carbon contaminant-induced cavity performance degradation and recovery, providing a basis for selecting suitable online performance recovery methods to maintain the cavity’s stable, high-gradient operation.

The high-brightness beams being produced in current and future accelerators present new machine protection concerns with the potential for high-energy-density (HED) conditions ($>100\mathrm{J}/{\mathrm{mm}}^3$) in beam-intercepting components. Simulating HED conditions in accelerators requires utilizing a suite of physics codes for particle dynamics, particle-matter interactions, and hydrodynamics. This paper describes a method of coupling the codes elegant, fluka, and flash to simulate the effects of a rapid beam loss in the advanced photon source storage ring and the resulting interaction of the beam and collimators. This paper expands previous work [J. Dooling et al., Collimator irradiation studies at the advanced photon source, in Proceedings of the IBIC-2023 (2023), pp. 245–249] by introducing a definition of the evolving geometry of the collimator surface as well as providing methods for simulating the absorption of synchrotron radiation and tracking shower particles produced during beam strikes. We demonstrate this framework by simulating machine conditions of the APS ring before and after its recent upgrade. Simulation results are compared with observed damage to collimators and test samples taken from the APS ring.

X-ray transition radiation emitted by high-energy electrons in a thin foil placed in a strong external magnetic field is considered. The modification of the radiation properties resulting from the bending of the particle’s trajectory in the magnetic field is investigated. It is shown that, due to the interference between transition and synchrotron radiation, the total radiation spectrum can significantly differ from the simple sum of the conventional spectra of these two types of emission. Both constructive and destructive interference can occur in this case. The dependence of the magnitude of this interference on the electron energy and the acceptance angle of the radiation detector is analyzed.

One of the primary issues in designing particle accelerators is the effects of energy losses and collective instabilities caused by the conductivity of the vacuum chamber. Due to its relevance, many authors have long focused on studying the coupling impedance of lossy pipes with various geometries and metals. Most studies were developed for relativistic beams and room-temperature conductivity. In recent years, there has been increasing interest in the high-frequency impedance behavior of ultrashort bunches, as in the case of free-electron lasers, and in the anomalous conductivity at cryogenic temperatures of the vacuum chambers. This work introduces a new unifying theoretical framework for analyzing electromagnetic interactions in cylindrical pipes excited by charged particles traveling on the axis. In this context, a key achievement is the derivation of a rigorous expression for the electrical conductivity, based on the Boltzmann theory of a Fermi gas, which forms the foundation for subsequent developments. New formulas for the surface impedance of cylindrical pipes are presented, from which a more general impedance expression is derived. Our approach captures the system’s behavior across various energy and frequency regimes. Eventually, the anomalous regime is studied rigorously, providing analytical solutions for the electromagnetic field at cryogenic temperatures. Indeed, new and exact expressions for the anomalous surface impedance for carriers’ reflection coefficients $p$ = 0 and $p$ = 1 are obtained, offering deeper insight into the electromagnetic properties of such systems under extreme conditions.

On-line monitoring of beam phase and energy gain in the cavity is of great importance for the stable operation of accelerators, especially in high-power scenarios. As cavities are typically calibrated with a low-current, short-pulse beam for safety purposes, the beam behavior may deviate from the calibrated case when running high-current beams, due to the strong space charge effect and other possible factors. Motivated by the high-power operation demand at European Spallation Source and the further understanding of beam-cavity interaction, methods for beam phase and energy gain on-line measurement have been developed. The beam phase was measured based on the steady-state cavity model, as demonstrated in the half wave resonator cavity at CAFE2. In this paper, this method was not only applied to a single-cell buncher cavity but also, for the first time, extended to the more complex low-energy multicell RFQ cavity for verification. Additionally, an energy-conservation-based method for measuring beam energy gain in a cavity was proposed and validated.

The brightness of an electron emitter is fundamentally limited by the intrinsic emittance of the electrons born at a cathode. Different electron-emission mechanisms from a cathode give different values of mean transverse energy for calculating the intrinsic emittance of emitted electrons. This study reports a theory for calculating the mean transverse energy of electrons generated at a cathode via mixed thermionic, multiphoton, and field emissions. The theory successfully reveals the details of thermionically assisted multiphoton emission from a copper cathode excited by an intense infrared laser. An extension of the theory to calculate the intrinsic emittance and brightness of an electron emitter is also presented.