The $\overline{\mathrm{P}}\mathrm{ANDA}$ experiment at the High Energy Storage Ring (HESR) of the Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, will be a unique setup to study the strong interaction in antiproton-proton collisions. The combination of a windowless cluster-jet target and a high-intensity antiproton beam, cooled by stochastic cooling down to momentum spreads of $4\times {10}^{-5}$ in the presence of the target, is ideal to study, e.g., the line shape of exotic candidates in the charmonium spectrum, such as the ${\chi }_{c1}(3872)$. For first studies on the performance of the cluster-jet target and the stochastic cooling, both were installed at the COoler SYnchrotron (COSY) at the Forschungszentrum Jülich, Germany. Detailed studies on the beam quality for different target settings with different densities were performed and will be presented in this paper. The results show that the cluster-jet target is able to provide the required hydrogen target thickness of more than $2\times {10}^{15}{\mathrm{atoms}/\mathrm{cm}}^2$ for the $\overline{\mathrm{P}}\mathrm{ANDA}$ experiment and that the stochastic cooling is able to efficiently cool the beam in presence of such a target thickness, although the system is not yet complete compared to the setup foreseen for the HESR. The presented results strengthen the theoretical understanding of the beam-target interaction and stochastic cooling performance and will provide important input for operation later at the HESR.

The nonultrarelativistic wake function theory is not only applicable to proton or heavy ion accelerators, but with the emergence of new accelerator physics, it has been found to be equally applicable to electron accelerators such as the steady-state microbunching (SSMB) storage rings. However, existing results are still far from practical applications. In this paper, building on previous work, we further generalize the previous point source model into a more universal ring model. When considering the nonultrarelativistic limit, we discover a completely new lowest-order transverse wakefield and estimate its intensity to be comparable to the dipole field under certain circumstances. Moreover, by separating the space charge component, we propose a numerical algorithm that can compute the resistive wall field more efficiently. Finally, we calculate the parasitic loss and kick angle using SSMB parameters as simple applications.

The performance of x-ray free-electron lasers (XFELs) strongly depends on the longitudinal emittance of the electron beam. We show that by reducing the injector drive laser pulse duration, the electron beam peak current can be increased by a factor of 2 or more, resulting in a significant improvement in longitudinal emittance. Comprehensive simulations and measurements carried out at SwissFEL confirm the expected trade-off of moderately higher transverse emittance. This simple approach can be readily adapted to other XFEL facilities.

We analyze beam-induced depolarizing effects in the hydrogen jet target (HJET) at the relativistic heavy ion collider (RHIC), which has been used for absolute hadron beam polarimetry and shall be employed at the electron-ion collider (EIC). The EIC’s higher bunch repetition frequencies and shorter bunch durations shift beam harmonics to frequencies that can resonantly drive hyperfine transitions in hydrogen, threatening to depolarize the target atoms. Using frequency-domain analysis of beam harmonics and hyperfine transition frequencies, we establish a photon emission threshold above which beam-induced fields are too weak to cause significant depolarization. For EIC injection (23.5 GeV) and flattop (275 GeV), beam-induced depolarization through the bunch structure renders operation at the current RHIC magnetic guide field at the target ($B_0=120\mathrm{mT}$) untenable. Increasing the magnetic guide field at the target to $B_0\approx 400\mathrm{mT}$ moves all hyperfine transition frequencies to at least 3 times the cutoff frequency, ensuring reliable absolute beam polarimetry with the required 1% precision at the EIC.

In this paper, we present a design of a high-efficiency high-gain free-electron laser oscillator based on the use of a strongly tapered undulator for extracting energy from high-brightness electron beams. We provide an analytical model of the setup followed by numerical simulations for lasing at the wavelength of 13.5 nm. We discuss the optimization of the system in steady state and the conditions necessary for the pass-per-pass buildup of the power from shot noise level. We propose the use of fast phase shifters as a way to accelerate the buildup. Finally, we present time-dependent simulations of the oscillator and discuss the role of spectral filtering. The optimized working point yields a total energy conversion efficiency from the electron beam to output radiation above 1% at the wavelength of 13.5 nm.

The dynamic vacuum effect is the primary limitation of beam intensity in high-intensity heavy-ion synchrotrons. Although the dynamic vacuum effect in the arc section is mitigated effectively by the beam collimators, the gas desorption induced by injection beam loss occurring in the injection electrostatic septum (ESi) is still an unresolved issue. In addition, the gas desorption is enhanced due to some special mechanisms, including the ultrafast beam loss process, multiple traversals of wire surfaces, and high-voltage breakdown. Mitigating the dynamic vacuum effect in the ESi is a critical approach to achieve a higher beam intensity. Therefore, a novel ES collimator is proposed to reduce beam deposition on the septum. Dynamic simulation of the dual-plane painting injection of BRing reveals the excellent performance of this scheme by a tenfold reduction in beam loss. The features associated with collimation efficiency and structure optimization are analyzed in detail.

Automatic differentiation is a powerful tool for computing derivatives of simulation results with respect to given parameters. In this Letter, we have applied this tool to investigate the instability of a dynamical system governed by 21 ordinary differential equations. This second-order instability (named envelope instability) is driven by space-charge effects and has a significant impact on the operational regimes of particle accelerators. Our study delves into the three-dimensional envelope instability, incorporating both transverse and longitudinal coupling. Conventionally, analyzing this complex system would necessitate solving 441 ordinary differential equations, which is computationally intractable. However, by employing automatic differentiation, we were able to track only 21 equations. This approach allowed us to uncover an additional instability stopband, which arises from space-charge-induced coupling and has not been reported in previous studies. This research highlights the significant advantages of automatic differentiation in analyzing complicated dynamical systems involving a large number of ordinary differential equations.

Integrating a high-power microwave source with an accelerating structure into a single device is a key strategy for developing compact, even portable, high-energy particle accelerators, which hold significant promise for scientific, medical, and industrial applications. To achieve this objective, we propose integrating an X-band coaxial transit-time oscillator (CTTO) microwave source with an X-band accelerating structure, thereby drastically minimizing the total volume of particle accelerators. Leveraging the CTTO’s advantages, low operating voltage, stable frequency, high power, and high efficiency, along with the high acceleration gradient of X-band accelerators, our design demonstrates that a CTTO operating at 50 kV (generating 7.8 MW of radio-frequency power at 9.3 GHz) can accelerate an electron beam from 50 keV to 10.96 MeV over a 320 mm length.

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).