High-brightness, ultrahigh peak current electron beams are of significant interest to applications including high-energy colliders, advanced accelerator concepts, strong field quantum electrodynamics, and laboratory astrophysics. Despite such interest, compressing electron beams to attosecond pulse durations and mega-ampere peak currents while preserving beam quality remains a challenge. In this work, we examine the feasibility and challenges involved in generating such extreme beams using plasma-based compression. Using particle-in-cell and particle tracking simulations, we demonstrate that the large electric field gradients in plasma wakefields enable orders of magnitude higher compression than what can be achieved with conventional radiofrequency-based compressors. By adding energy chirps greater than $500\mathrm{MeV}/\mathrm{\mu }\mathrm{m}$, bunch durations approaching 10 nm and peak currents approaching 1 MA can be achieved. Scalings of beam properties with respect to accelerator and plasma parameters are explored with limitations on achievable final beam duration and peak current evaluated. Optimal beam and plasma conditions are investigated for different applications, with the goal of experimentally demonstrating this technique at existing accelerator facilities. Insights gained from this study will help design the next-generation of high-brightness beams for new frontiers in scientific research.

As the Fermilab accelerator complex prepares for higher-intensity operation under the Proton Improvement Plan II (PIP-II), addressing the negative impacts of third-order resonance becomes crucial for the Recycler Ring (RR). Minimizing fluctuations of third-order resonance driving terms (RDTs) around the ring allows one to enhance dynamic aperture and reduce resonance-driven losses. This work experimentally demonstrates a mitigation scheme using normal and skew sextupoles, optimized via the response matrix method, in order to manipulate and minimize RDTs for a hadron storage ring. Furthermore, the effectiveness of this approach for a bunched beam at high intensities is evaluated through dynamic and static tune scans. The results show that resonance compensation remains effective in the presence of space charge, reducing beam loss and emittance growth compared to uncompensated operation. These new measurements extend previous observations of third-order resonance effects in high-intensity bunched beams by explicitly quantifying beam loss, emittance growth, and tail population under conditions where the RDTs are minimized around the ring. Overall, these findings strengthen the applicability of resonance control in the RR and highlight its importance for reliable high-intensity operation in the PIP-II era.

Intense uncoupled beams are conventionally transported through periodic focusing lattices composed of linear optical elements that provide radial confinement against defocusing space-charge forces. In such systems, interplane coupling has traditionally been regarded as an undesirable perturbation. Recently, however, the concept of matched transport in periodic quadrupole focusing channels has been extended from uncoupled beams to those exhibiting significant correlations between the two transverse degrees of freedom [C. Xiao et al., Periodic four-dimensional solution for transport of intense and coupled coasting beams through quadrupole channels, Phys. Rev. Accel. Beams 27, 031602 (2024)]. Building on this development, the present work generalizes the full four-dimensional rms-moment matching formalism to periodic solenoid transport channels. A notable application of this approach, for instance, is the efficient transport of high-current, low-energy proton coasting beams.

X-ray free-electron lasers (XFELs) of high brightness have opened new opportunities for exploring ultrafast dynamical processes in matter, enabling imaging and movies of single molecules and particles at atomic resolution. In this paper, we present a straightforward method for multiframe diffraction imaging, using the whole electron beam to generate four-color XFEL pulses with adjustable wavelength separation and time delay. The optical klystron scheme is introduced to enhance the FEL intensity and reduce the total length of the undulators. The time delay is tuned via a magnetic chicane between the undulators for different colors. Using parameters of SHINE, start-to-end simulations demonstrate the effectiveness and tunability of our method, achieving representative results such as time delays of hundreds of femtoseconds and four-color XFEL pulses spanning 1.8 to 2.7 nm with 0.3 nm intervals. The proposed scheme enables the recording of multiframe diffraction images in a single exposure, providing a new perspective for ultrafast molecular and atomic dynamics studies.

The generation of high-power radiation in the terahertz (THz) regime using free-electron lasers (FELs) is challenging due to strong diffraction and pronounced slippage effects. These constraints often limit the achievable pulse duration and peak power in conventional single-pass THz FEL configurations. In this work, we investigate an unseeded optical klystron FEL concept tailored for the THz regime. Using time-dependent three-dimensional simulations for resonant wavelengths of 10, 30, and $100\mathrm{\mu }\mathrm{m}$, we demonstrate that this approach enables the generation of coherent ultrashort THz pulses with durations of subpicoseconds (FWHM) and peak powers in the multihundred-megawatt range at wavelengths of 10 and $30\mathrm{\mu }\mathrm{m}$. To address the slippage challenge, we propose and numerically demonstrate a novel chicane-embedded optical delay scheme, which restores phase alignment between the radiation and microbunched electrons. Simulations confirm that careful tuning of the dispersive strengths allows staged amplification, preserving beam quality and reaching multimegawatt output power. These results highlight the potential of THz-tailored optical klystrons to generate compact, short, and high-intensity THz pulses, and lay the groundwork for future experimental studies and facility implementation.

Broad-bandwidth x-ray free-electron lasers (FELs) are critical scientific tools across diverse disciplines such as biology and chemistry. While substantial efforts have been devoted to achieving FEL bandwidth broadening, existing methods face limitations. This paper proposes a simple yet effective scheme for spectral broadening that utilizes a plasma wakefield accelerator (PWFA) to impart a large energy chirp to an electron beam. A specifically designed transport system then selectively injects the portion of the beam meeting the lasing requirements into the undulators for FEL generation. A start-to-end simulation demonstrates that the scheme can effectively manipulate the longitudinal and transverse phase space of the PWFA electron beam that subsequently generates a high-brightness x-ray FEL exhibiting a full-width bandwidth reaching 20%.

The SPARC_LAB facility, operating at the LNF-INFN laboratories in Frascati, has been devoted over the past years to R&D related to accelerators, advanced diagnostics, and generation of radiation ranging from THz to extreme UV light. The facility has recently been significantly upgraded and moved toward user-oriented applications, especially after the approval of two additional projects with the goal to develop radiation sources up to the soft x-ray range and in view of the EuPRAXIA project, whose preparatory phase is ongoing with the support of the European Commission. This paper summarizes the electron beam and radiation parameters and describes the layout and main features of the upgraded facility in view of future user-oriented experimental activities and applications.

In this paper, we investigate methods for suppressing transverse single-bunch instability. A theoretical framework incorporating longitudinal impedance, feedback, and chromaticity is presented. Our results show that the combined action of these effects effectively suppresses the transverse single-bunch instability. The instability threshold can be raised by a factor of 2 or more relative to $I_{\text{tmci}}$, defined as the threshold under transverse impedance only. The key stabilization mechanism arises from two effects: feedback and positive chromaticity cause the instability to be dominated by nonzero-order azimuthal modes, while longitudinal impedance introduces a significant tune spread in these modes. This stabilization mechanism, however, is ineffective in the negative chromaticity regime. The theory is applied to three electron storage rings: SuperKEKB, CEPC, and HEPS. Both the analysis and simulation lead to consistent conclusions.

The design of transverse electric (TE) mode cavities offers new opportunities for beam manipulation and beam-quality improvement. We present a field-driven methodology for synthesizing multipolar TE-mode cavities by superimposing cylindrical standing-wave solutions and enforcing perfect electric conductor boundary conditions through sequential quadratic programming. This procedure produces hybrid cavity geometries that are validated against eigenmode simulations in cst, with examples including ${\mathrm{TE}}_{\{0,2\}1}$ and ${\mathrm{TE}}_{\{2,4\}1}$ hybrid modes. The results demonstrate that prescribed multipolar content can be realized with high modal purity and initial superposition of the multipolar components is preserved. While the present study focuses on methodology and validation, the approach provides the foundation for future applications such as integrated emittance correction and chromaticity compensation.

Laser wakefield acceleration (LWFA) can produce relativistic electron beams and various secondary particles in centimeter-long plasmas, making it a valuable particle source with important applications in many disciplines. In this work, we examine the effects of nonideal transverse intensity and phase distribution of laser pulses on LWFA through both experimental measurements and particle-in-cell simulations. The complex transverse profile of the 75 TW laser pulses reduces the self-focused intensity in the plasma compared with a transversely Gaussian laser pulse. Furthermore, the sheath structure of the nonlinear plasma wake excited by realistic laser pulses is wider and more complicated than that of a Gaussian laser pulse. These features hinder electron injection. As the laser pulse propagates through the plasma, its intensity profile gradually becomes elliptical and drives a plasma wake with a sharp sheath near the azimuths of the major axis, leading to electron injection. When using a realistic laser profile in simulations, both the charge and energy of injected electrons closely match experimental results ($\sim 200\mathrm{pC}$ of charge and $\sim 200\mathrm{MeV}$ peak energy), whereas the Gaussian laser simulations produce much higher charge ($\sim 500\mathrm{pC}$). Our findings reveal differences in injection dynamics between LWFA driven by nonideal laser pulses and those driven by Gaussian pulses and are useful for applications of LWFA that demand high-charge electron beams.

The survival of charged particles in synchrotrons requires avoiding setting the beam on machine resonances, the most dangerous of which are the integer and half-integer. Nevertheless, operationally, the transverse tunes may change dynamically, crossing these resonances and resulting in unwanted beam quality degradation and beam loss. For high-intensity beams, the process of resonance crossing is even more critical, as space charge, in addition to the incoherent effects, may also generate coherent effects. The interplay of the speed of resonance crossing, the space charge incoherent tune spread, and the half-integer resonance width is fundamental for the beam behavior. In this paper, we study the crossing of the half-integer resonance $2Q_y=9$ in the CERN Proton Synchrotron Booster (PSB) for a range of parameters using a coasting beam. We take advantage of the newly commissioned PSB to investigate the beam response experimentally. The experimental findings are also discussed with the support of simulations.

Bunch shaping in electron linear accelerators is of critical importance, as it enables the realization of prescribed projection profiles along specific coordinates of the bunch distribution for linac-based x-ray free-electron laser (XFEL) applications. It is evident that temporally flattop electron bunches yield more uniform slice properties, reduced emittance growth, and enhanced FEL gain compared to conventional Gaussian profiles. In this work, we present a data- and model-guided framework that consists of a physics-informed model and an inverse emittance model for minimizing the bunch emittance through laser pulse shaping and the control of laser and accelerator parameters in high-brightness photoinjectors. With a temporal flattop laser shape selected, a flat slice emittance distribution along the bunch has been experimentally obtained at the European XFEL. A measured central slice emittance of about $\text{0.4\hspace{0.17em}\hspace{0.17em}}\mathrm{\mu }\mathrm{m}$ at 250 pC is achieved using a 20 ps flattop laser pulse at the photocathode. Here we present the results of a commissioning experiment, marking a practical application of temporally flattop-shaped electron bunches for stable photon delivery in a user experiment week at the facility. Within limited tuning time, a pulse energy of 1.7 mJ has been achieved at photon energies of 7.1 and 7.6 keV for the two hard x-ray undulator beamlines, surpassing the user requirements. Such a data- and model-driven framework opens up new opportunities for end-to-end optimization of machine parameters, thereby enhancing the capabilities of large-scale FEL facilities.

Enabled by progress in superconducting technology, several continuous wave linear accelerators are foreseen in the next decade. For these machines, it is of crucial importance to track the main cavity parameters, such as the resonator bandwidth and detuning. The bandwidth yields information on the superconducting state of the cavity. The detuning should be minimized to limit the required power for operation of the cavity. The estimation of these parameters is commonly implemented in the digital electronics of the low-level rf control system to minimize the computation delay. In this proceedings paper, we present a way to compute the bandwidth and detuning using a Luenberger observer. In contrast to previous methods, the state observer yields estimations at the native control system sample rate without explicitly filtering the input signals. Additionally, the error convergence properties of the estimations can be controlled intuitively by adjusting gain parameters. Implementation considerations and test results for the derived observer are presented in the manuscript.

The UK Extreme Photonics Applications Centre (EPAC) intends to flexibly provide laser-accelerated GeV-scale electrons to a wide array of experiments and like other such facilities must efficiently capture them from a laser-wakefield acceleration (LWFA) source. Here we describe a unique, modular beam capture optics system based on a FODO array of Halbach permanent-magnet quadrupoles and show how that modular arrangement can be used in different configurations to provide energy-selected and focused bunches for different purposes. We present an engineering concept for the beamline that incorporates diagnostics and drive laser extraction and describe the effect of field errors and misalignments and their mitigation.

In heavy-ion accelerators used for radioactive isotope production, the accelerated beam is typically directed onto a target and then discarded in a beam dump. To make more efficient use of the beam, recycling of the beam passed through the target is proposed in the framework of the so-called energy recovery internal target (ERIT). In the ERIT system, the target is irradiated by a circulating beam, while the energy lost in the target is recovered using rf cavities. So far, such a system has been realized only for proton beams. Here, an ERIT system for heavy-ion beams is demonstrated for the first time. A major challenge is the circulation of ions with multiple atomic charge states. After passing through the target, ions rapidly reach an equilibrated charge-state distribution, independent of the initial charge state. This stochastic charge-state conversion (SCSC) leads to rapid beam-emittance growth. To mitigate this effect, we develop a method to match the closed orbits and betatron functions of different charge states at the target location, based on a scaling fixed-field alternating-gradient (FFA) lattice. We present the design of such an FFA ring and show, through 6D beam-tracking simulations, that the transverse emittance growth induced by SCSC can be significantly reduced over a practical number of turns, whereas longitudinal emittance growth remains significant and is discussed separately.

A basic performance test of a novel ion source is carried out to attain an extremely narrow beam for pinpoint irradiation of single ions onto an intended target. In principle, the normalized root-mean-squared emittance of the beam can reach a sub-femtometer level, which makes a transverse spot size of nanometer order available. The system is based on a compact linear Paul trap equipped with a Doppler-cooling device. The trap is the most popular, economical type consisting simply of a few metal plates and rods. Among several possible choices of ion species, we picked ${{}^{40}\mathrm{Ca}}^{+}$ for the present investigation. A small number of ${{}^{40}\mathrm{Ca}}^{+}$ ions are first laser cooled to near absolute zero where they form a spatially ordered configuration called a Coulomb crystal. The axial-potential well provided by dc bias voltages on two planar electrodes is then deformed gradually to eject the ions from the trap along the design orbit. We demonstrate that it is possible to launch the ultracold ions one by one at arbitrary timing with no serious radio-frequency heating. These ions are accelerated to 1 keV immediately after the extraction. The effectiveness of a few different ion-ejection schemes is experimentally verified and supported by three-dimensional molecular dynamics simulations.

Double-rf systems have become a standard solution in advanced storage ring light sources for beam acceleration and bunch lengthening. In this context, a compact single-cell normal-conducting bimodal rf cavity is proposed, in which the ${\mathrm{TM}}_{010}$ accelerating mode and the ${\mathrm{TM}}_{020}$ harmonic mode operate simultaneously within a single cavity. To suppress harmonic power leakage through the fundamental-mode coupler, a low-pass filter is introduced. In addition, tunable band-stop filters are employed to provide selective isolation of the operating modes and to enable efficient extraction and damping of parasitic higher-order modes. Based on this concept, a complete rf design of a single-cell bimodal cavity with a fundamental frequency of 500 MHz and a corresponding third-harmonic frequency of 1.5 GHz is presented, and key issues including frequency tuning, shunt impedance optimization, and higher-order-mode damping are systematically investigated. The results show that, under the condition of frequency matching between the two operating modes, their frequencies and coupling strengths can be adjusted with a high degree of independence. With higher-order modes effectively suppressed, the shunt impedances of the ${\mathrm{TM}}_{010}$ and ${\mathrm{TM}}_{020}$ modes reach $4.74$ and $1.2\mathrm{M\Omega }$, respectively, corresponding to reductions of only 0.35% and 1.25% compared with the undamped case. These results validate a general design approach toward compact bimodal cavities for storage-ring double-rf systems. The mechanical design has been completed, and fabrication is currently in progress.

The Shanghai high repetition rate XFEL and extreme light facility linear accelerator employs two 3.9 GHz third harmonic cryomodules. The 3.9 GHz fundamental power coupler is one of the key components of the cryomodule and is designed to handle 2 kW of continuous-wave rf power, meeting operational requirements with a beam current of 0.3 mA and cavity gradient of $13.1\mathrm{MV}/\mathrm{m}$. A key innovation of the 3.9 GHz coupler is the incorporation of an adjustable antenna insertion depth, which enables online external quality factor ($Q_{\mathrm{ext}}$) tuning and eliminates the need for a three-stub tuner, reducing system complexity and cost. Two 3.9 GHz coupler prototypes were fabricated and underwent comprehensive rf testing. The couplers achieved excellent performance with an insertion loss of $-0.16\mathrm{dB}$ and return loss of $-43.22\mathrm{dB}$ at 3.9 GHz after structural optimization. High-power rf conditioning demonstrated stable operation at 2 kW in traveling wave mode and 1 kW in standing wave mode. Sixteen production 3.9 GHz couplers were manufactured and integrated into two third harmonic cryomodules, which achieved maximum total cavity voltages of 63.6 and 59.8 MV, respectively, with average usable accelerating gradients of $23.0$ and $21.6\mathrm{MV}/\mathrm{m}$. The cryomodules have been installed in the tunnel and initial beam commissioning has been completed successfully.

The proposed proton electric dipole moment (pEDM) experiment at Brookhaven National Laboratory (BNL), to be built inside the alternating gradient synchrotron (AGS) tunnel, aims to measure the proton’s electric dipole moment with a sensitivity of ${10}^{-29}\mathit{e}\mathrm{cm}$. This paper presents the design of the injection line from the AGS booster to the pEDM storage ring, utilizing portions of the existing booster-to-AGS (BtA) transfer line. Building on the symmetric-hybrid lattice design [, Comprehensive symmetric-hybrid ring design for a proton-EDM experiment at below ${10}^{-29}\mathit{e}\mathrm{cm}$, Phys. Rev. D 105, 032001 (2022)], our study emphasizes rigorous optics matching, detailed particle and spin tracking, and systematic error mitigation essential for achieving a target sensitivity of ${10}^{-29}\mathit{e}\mathrm{cm}$. This design preserves the proton’s vertical spin orientation within $\pm 20\mathrm{mrad}$, a critical requirement for the pEDM measurement. Particle and spin tracking simulations using the ray-tracing code Zgoubi [F. Méot, Zgoubi users’ guide, Technical Report, Brookhaven National Laboratory (BNL), Relativistic Heavy Ion Collider (RHIC), Upton, NY, 2012] validate the design’s performance, demonstrating its feasibility for this precision experiment. The simulation results demonstrate that both clockwise (CW) and counterclockwise (CCW) injection lines meet the stringent beam envelope and polarization requirements.

We analyze the radiation of a charged particle bunch passing through two grids parallel to each other (the double grid screen). The grid is composed of thin conductors that form square cells and have galvanic contact at the cross points. It is assumed that the wavelengths under consideration are much greater than the cell size, and the latter is much greater than the wire thickness. In this approximation, the structure can be described analytically by the averaged boundary conditions. Asymptotic analysis of the electromagnetic field shows that the bunch generates cylindrical and spherical waves. In particular, it is demonstrated that the cylindrical ones are dominant in the area between the grids. It is important to note that the structure under consideration can be practically transparent to a bunch, which is quite promising, in particular, for developing new methods of nondestructive bunch diagnostics.