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.

For the development of new synchrotron light sources, new operation modes are considered. For a successful application of these schemes, however, an understanding of the effects involved is essential. The KIT storage ring, Karlsruhe Research Accelerator (KARA), allows the implementation of new operating modes, including tests using a variety of high-performance beam diagnostics devices. Negative momentum compaction optics has been established at KARA at various beam energies. Systematic studies of the resulting beam dynamics showed significant differences to the dynamics at equivalent positive momentum compaction factors. Moreover, a longitudinal instability occurs at low absolute values of the momentum compaction factor, caused by coherent synchrotron radiation, which is the dominant longitudinal impedance at KARA. This work outlines the status of the operation with negative momentum compaction factors at KARA and presents the current-dependent bunch length at negative momentum compaction. Furthermore, the observation of the instability and its dynamics, including significant deviations from threshold predictions, is shown and compared to the equivalent instability at positive momentum compaction.

Magnetically insulated transmission lines (MITLs) are critical components for efficiently transporting power from pulsed power sources to a diverse range of loads, including magneto-inertial confinement fusion and radiation production. This study investigates the behavior of planar MITLs, taking into account the source voltage and source and load impedances, using two-dimensional theory and simulations. Self-limited behavior, where the MITL operating impedance (voltage divided by current) is insensitive to the value of load impedance, is extended to small-to-large gap distance MITL transitions. Load-limited behavior, in which the load impedance is below the self-limited impedance, causes the MITL operating impedance to adjust to match the load impedance and can be extended to describe large-to-small transitions. In both cases, the operating impedance of the larger MITL section changes to match the self-limited operating impedance of the smaller section. This behavior extends to multiple transitions and to both low and high impedance loads. Assuming equilibration occurs (pulse duration larger than MITL transit time), the entire MITL operating impedance becomes the self-limited impedance of the smallest-gap MITL section or the load impedance, whichever is lower.

A novel cooling mechanism for heavy-ion beams, termed charge-exchange cooling (CEC), is proposed. Phase-space-dependent zoned charge exchange combined with charge-state-dependent closed-orbit distortions (CODs) enables rapid, autonomous, turn-by-turn contraction of the phase-space distribution for continuous-wave heavy-ion beams. The resulting dynamics forms a stochastic affine-map system with state-dependent switching and a net negative drift. CEC is implemented in the charge stripper ring 1 (CSR1), under development at the RIKEN RI Beam Factory, where zoned charge exchange and charge-state-specific CODs are readily incorporated. Proof-of-principle studies based on two- and multicharge-state models, together with realistic CSR1 simulations, demonstrate fast turn-by-turn cooling under practical machine conditions and establish CEC as a new beam-cooling principle with potential implications for future accelerator design and applications.

We consider the radiation emitted by a purely longitudinally accelerated relativistic electron bunch and find, contrary to the usual expectation of negligible emission, that for ultrashort compact bunches within reach of current experiments, a significant and measurable radiation pulse is produced. The underlying mechanism for the enhanced emission is coherence in the single-electron emission over $\mathcal{O}({10}^8)$ electrons. We derive analytic descriptions of the emission spectrum and energy for acceleration fields and bunch conditions encountered in rf-driven particle accelerators, and in the extreme high-gradient conditions of plasma-driven acceleration. Both the bunch duration and the duration of the acceleration have a role in describing a phase-matching limitation to the coherence, with the latter dependent on the acceleration field strength and the initial energy of the electrons. In extremely compact bunches, just beyond those currently realized in the laboratory, the radiation losses approach the Larmor radiation for a massively charged particle ($Q\sim {10}^{9}e$). In such conditions, the implied energy of the longitudinal acceleration radiation may approach the energy available from the electric field driving the acceleration, raising questions about a connection to mechanisms of field depletion.

High-quality and prebunched electron beams can produce coherent x-rays with high intensity and narrow bandwidth, which are essential for modern light sources. An all-optical scheme based on plasma-based acceleration for producing bright electron beams that are prebunched on the nanometer scale is proposed. By using a density modulation created by two low intensity counterpropagating lasers, the phase velocity of the plasma wake excited by an intense driver laser in a uniform plasma can be modulated at a frequency twice that of the colliding lasers, thus turning the injection on and off. The injected electrons are microbunched at the Doppler-shifted modulated wavelength, corresponding to the phase velocity of the gradually expanding wakefield. It is demonstrated that by controlling the properties of the drive and colliding lasers, beams with exotic prebunched structures can be produced, which may have critical applications in ultrafast high power x-rays. This extremely compact, all-optical scheme for producing ultrabright prebunched electron beams may therefore enable novel applications for ultrafast x-ray users and arouse general interest in various fields.

SuperKEKB collimator system protects the superconducting final focusing magnets (QC1 and QC2) from quenching caused by beam halo particles. For effective shielding, the minimum aperture of vertical collimators is set smaller than the physical aperture of QC1. However, such collimators become dominant sources of beam coupling impedance, which can limit the maximum beam current due to impedance-driven instabilities. To address this issue, a nonlinear collimator (NLC) was introduced in February 2024. NLC is designed to reduce both beam backgrounds and collimator impedance, thereby suppressing beam instabilities caused by short-range wakefields. In this paper, we describe the results on impedance effects, beam background reduction, and beam lifetime when the nonlinear collimation system is turned on and off. These results validate the NLC concept and offer a promising approach for future high-luminosity colliders.

Technical details of a recently proposed system for direct current (dc) induction acceleration are described. This dc induction acceleration system comprises two induction cells, each with two insulating gaps; one acts as an acceleration gap, and the other is placed on the outer cage of the induction cell to act as a resetting gap. Both gaps are bridged by diodes. The system is energized by a bipolar switching power supply. Continuous dc acceleration is limited by the saturation of magnetic materials distributed along the beam line, such as the beam-guiding magnets that act as effective absorbers of the magnetic flux that is released into the surrounding area upon resetting of the induction acceleration cell core. A secondary loop current (the primary loop current being the current that excites the magnetic core of the induction acceleration cell) necessarily flows on the metal beam-chamber surface distributed along the accelerator ring. This secondary loop current tends to degrade the resetting of the induction acceleration core and must be canceled by an artificially added tertiary loop current, whose loop passes symmetrically through the guiding magnets but bypasses the dc induction acceleration cells. In this way, resetting of the induction cores is secured at the expense of magnet saturation. dc induction acceleration can be performed via intermittent operation to release the absorbed extra flux. The effects of this externally absorbed flux on the beam orbit and optics are discussed. Applications of the dc induction acceleration are of practical interest. In addition, proposals are made for a dc induction microtron and a fixed field alternating gradient accelerator for heavy ions, which could replace existing electrostatic accelerators such as Van de Graaff or tandem accelerators, and an isochronous storage ring with dc induction acceleration for electrons/positrons, which could lead to the ultimate form in efficient continuous wave free-electron laser.

Since the emergence of laser-plasma accelerators (LPAs), substantial work has been dedicated to using LPAs to drive free-electron lasers (FELs) for a broad range of applications. Despite recent breakthroughs, which have proven the fundamental feasibility of operating FELs with an LPA source, stable FEL operation over multiple hours without operator input had yet to be achieved. In this work, we report significant improvements to the stability of a hundred terawatt laser system, resulting in successful demonstration of reliable, long-term operation of an LPA-driven FEL in the self-amplified spontaneous emission (SASE) regime at 420 nm. The LPA source delivered 100 MeV electron beams at 1 Hz with high stability over more than 10 h, enabling over 8 h of continuous FEL operation without operator input. The acquired data were subsequently used to investigate correlations between the measured undulator radiation and parameters of the drive laser, plasma source, and electron beam. The revealed connections between LPA and FEL performance gave important additional insights into ways to further improve and stabilize the system, thus demonstrating the capability of our setup to serve as a powerful platform for future studies of LPA-driven FEL operation. The one-of-a-kind integration of multiple stabilization concepts onto the LPA facility discussed in this manuscript yielded significant accelerator and light source improvements (with residual correlations suggesting even more is possible), which will positively impact LPA applications at large.

The compression of relativistic electron beams by laser fields requires an in-depth understanding of ponderomotive processes. However, existing theoretical frameworks fail to capture the full complexity of these modulation processes. This paper develops a covariant oscillation-center formulation of the ponderomotive four-force and derives a laboratory-frame closed-form expression for its longitudinal component evaluated on the axis of a tightly focused radially polarized beam. This force, commonly described through cycle averaging that neglects instantaneous field phases, has validity conditions that are critically dependent on the interaction length. The theoretical framework seamlessly extends to dual-frequency fields through the equivalent beat wavelength concept, establishing the foundation for optimized electron beam-compression designs. Comprehensive simulations including parameter optimization and full particle tracking with space charge confirm the theoretical predictions. These results provide both a theoretical foundation and practical design principles for the generation of ultrashort relativistic electron beams in advanced applications.