GaAs photocathodes in dc high-voltage photoguns are highly susceptible to ion back-bombardment, which reduces the photocathode quantum efficiency and limits the useful operating lifetime for producing polarized electron beams. This paper demonstrates that applying a small positive bias to the photogun anode can significantly suppress ion back-bombardment and increase charge lifetime. This technique was studied extensively using the Continuous Electron Beam Accelerator Facility photogun, where highly polarized electron beams created using a strained-superlattice $\mathrm{GaAs}/\mathrm{GaAsP}$ photocathode were used and charge lifetimes improved by almost a factor of 2. A new simulation code ionator was developed to model ion production and tracking in order to better understand and explain the factors that led to the performance improvement. Results of the experiments and simulations are discussed in detail.

In this paper, we present detailed simulations with asymmetric initial beam settings in the context of the proposed Future Circular Collider $e^{+}{e}^{-}$ (FCC-ee) using the xsuite framework. We compare simulated equilibrium bunch sizes and luminosities against an already existing analytical model, which shows remarkably good agreement for realistic small perturbations. We investigate the longitudinal top-up injection, the currently preferred injection scheme for the FCC-ee, using self-consistent simulations featuring beam-beam collisions with beamstrahlung and the injection process for the first time. We present and assess the sensitivity and required precision of the nominal beam parameters in a potential real-life operation by providing first estimates of the tolerances in the initial asymmetry of several machine parameters, with respect to the 3D flip-flop mechanism, obtained from parameter scan simulations.

Several analytical formulas have been developed to calculate the Touschek lifetime of beam in electron storage rings under different assumptions. Among them, Piwinski’s formula is the most general and precise for unpolarized beams, widely applied in the mainstream accelerator simulation software. Lee’s formula is conventionally adopted to evaluate the influence of beam polarization on the Touschek lifetime, but it is less general than Piwinski’s formula when applied to unpolarized beams. In this paper, a new formula for the Touschek lifetime of ultrarelativistic polarized beams is derived in the theoretical framework of Piwinski. Its applications to several lattices of electron storage rings reveal that the influence of beam polarization on the Touschek lifetime is weaker than that estimated with Lee’s formula especially when the vertical emittance is large.

Harmonic up-conversion schemes represent the most successful approach in externally seeded free-electron lasers (FELs) for obtaining radiation with laserlike properties at short wavelengths. They are, however, limited in the shortest achievable wavelengths, as the harmonic conversion efficiency decreases with increasing harmonic number, even for echo-enabled harmonic generation (EEHG). In the following, we introduce a novel externally seeded cascaded FEL scheme based on two EEHG stages, capable of obtaining unprecedented bunching fraction that is independent on the final FEL wavelength. The second stage can be operated both in conventional harmonic up-conversion but also in down-conversion, i.e., going toward (slightly) longer wavelengths. Higher bunching enables emission of laserlike radiation in the soft x-rays with large output power in a compact scheme. Its performance at 4 nm and shorter is investigated, demonstrating the ability to achieve multi-GW level, fully coherent pulses with laserlike properties and paving the way for externally seeded radiation at 1 nm and beyond.

While plasma-based accelerators have the potential to positively impact a broad range of research topics, a route to application will only be possible through improved understanding of their stability. We present experimental results of a laser wakefield accelerator in the nonlinear regime in a helium gas jet target with a density transition produced by a razor blade in the flow. Modifications to the target setup are correlated with variations in the plasma density profile diagnosed via interferometry and the shot-to-shot variations of the density profile for nominally equal conditions are characterized. Through an in-depth sensitivity study using particle-in-cell simulations, the effects of changes in the plasma density profile on the accelerated electron beams are investigated. The results suggest that blade motion is more detrimental to stability than gas pressure fluctuations, and that early focusing of the laser may reduce the deleterious effects of such density fluctuations.

Laser surface structuring has proven to be an effective technique for achieving a copper surface with secondary electron yield (SEY) values close to or below unity. However, the attributes that minimize SEY, such as moderately deep grooves and redeposited nanoparticles, may lead to undesirable consequences, including increased radio frequency surface resistance. This investigation systematically examined data about different cleaning procedures designed to eliminate redeposited adsorbed particulates. Various analysis techniques were used iteratively after each consecutive cleaning step, providing insights into the evolving surface characteristics. The collected experimental results identified distinct impacts of microgrooves, groove orientation, and associated particulates on secondary electron yield and surface resistance. Exposing the crests while retaining high particulate coverage in the grooves leads to reduced SEY values and surface resistance, suggesting that the tips of the grooves exert a more significant influence on surface current density than the groove depth. At the same time, nanoparticles in the grooves have a more significant impact on SEY values than the exposed tips at the surface.

The observation of transverse Schottky spectra provides insight into intrabunch particle dynamics and serves in numerous facilities to derive important beam and machine parameters, such as the betatron tune, first-order chromaticity, and transverse emittance. The original theory confirming the validity of these observations has been, however, developed by taking strong assumptions on the particle motion in both the longitudinal and transverse planes. In this work, we loosen these assumptions, by proving that the relationships linking the spectra with transverse emittance and betatron tune remain universally unchanged regardless of the shape of the rf waveform and chromaticity up to any order, while for the first-order chromaticity, we derive compact formulas for error estimates. The derived theory is supported by macroparticle simulations and applied to typical beam and machine parameters in the Large Hadron Collider.

Double-aperture young interferometry is widely used in accelerators to provide a one-dimensional beam measurement. We improve this technique by combining and further developing techniques of nonredundant, two-dimensional, aperture masking, and self-calibration from astronomy. Using visible synchrotron radiation, tests at the ALBA synchrotron show that this method provides an accurate two-dimensional beam transverse characterization, even from a single 1 ms interferogram. The nonredundancy of the aperture mask in the technique enables it to be resistant to spatial phase fluctuations that might be introduced by vibration of optical components or in the laboratory atmosphere.

In the case of a single-digit picosecond bunch length, synchrotron light sources produce intense coherent radiation up to the THz range. The reduction of the bunch length by lowering the momentum compaction factor (low-$\alpha$) gives rise to the microbunching instability, which is on the one hand a crucial roadblock in the x-ray range during to the resulting effective bunch lengthening, but on the other hand also an opportunity for the generation of intense THz radiation if it can be controlled appropriately. In the KIT storage ring KARA (Karlsruhe Research Accelerator), two parallel plates with periodic rectangular corrugations are planned to be installed as a proof of principle in an electron storage ring. These plates create an additional longitudinal impedance based on their geometry, which can affect the beam dynamics. The resulting impedance manipulation will be used to study and control the longitudinal electron beam dynamics and the emitted coherent synchrotron radiation. This paper presents the results of systematic studies in simulation of the impact of additional corrugated plate impedances on the longitudinal beam dynamics using the example of the KARA storage ring. If the periodicity of the wake function of the corrugated plates matches the size of the substructures in the longitudinal bunch profile, the instability threshold can be effectively manipulated. This extends intense THz radiation to different beam current regimes.

This work shows the results obtained with a novel modified active disturbance rejection control (MADRC) algorithm when controlling the detuning of a 9-cell TESLA superconducting radio frequency cavity. This type of control is essential in SRF cavities with high loaded quality factors since they are extremely sensitive to detuning due to their extremely narrow bandwidth. The modification of the active disturbance rejection control is based on loop shaping techniques and confers ease of design and implementation, as well as an improved behavior in presence of delay. To begin with, a theoretical design of the controller based on an experimental transfer function of the cavity is carried out and its performance is compared with that of a proportional integral control. In this preliminary test, the simulation results indicate a remarkable performance superiority of the MADRC. Later, the controller is tested on a TESLA cavity validating both the MADRC algorithm and its design methodology, and achieving a peak detuning reduction of 63%, as well as an rms reduction of 80% with respect to the open loop.

To satisfy user demands for a high-brightness, high-capacity synchrotron light source, a novel multibend achromat (MBA) lattice structure has been developed for the Diamond-II storage ring upgrade. This lattice combines the high-dispersion locations characteristic of hybrid MBA lattices with newly introduced midcell straight sections to allow extra insertion devices to be installed. The resulting structure is named the modified hybrid six bend achromat (MH6BA) lattice. When adapted to fit in the existing 560.6 m Diamond storage ring tunnel, it reaches a natural emittance of 162 pm rad at 3.5 GeV. The characteristics of the MH6BA lattice, the methodology used to optimize the linear and nonlinear beam dynamics, and the lattice performance in a range of conditions are discussed.

Seeded free electron lasers (FELs) demonstrate a good performance and are successfully used in different user experiments in extreme ultraviolet and soft x-ray regimes. In this paper, a simple modification of the seeding scenario is proposed relying on the generation of two closely spaced bunches with very different properties: a low-current seeding bunch and a high-current bunch that amplifies coherent radiation, produced by the seeding bunch. This approach eliminates different limitations and mitigates some harmful effects in the standard scenario. In particular, one can generate very high harmonic numbers with a moderate laser power in a simple high-gain harmonic generation (HGHG) scheme. Alternatively, in case of moderate harmonic numbers, one can strongly reduce the required laser power thus simplifying the design of high repetition rate seeded FELs. An influence of beam dynamics effects (like nonlinearities of the longitudinal phase space of electron beams, coherent synchrotron radiation, longitudinal space charge, geometrical wakefields, and microbunching instabilities) on properties of the output radiation (spectrum broadening, pedestals, and stability) can be to a large extent reduced in the proposed scheme. In this paper, we illustrate the operation of the two-bunch seeding scheme in HGHG configuration with realistic start-to-end simulations for the soft x-ray user facility free electron laser in Hamburg. We show that nearly Fourier-limited multigigawatt pulses can be generated at 4 nm using the present compact design of the undulator system. With several thousand pulses per second, this can be a unique source for photon science.

High-frequency driven resonant spin rotators are routinely used as standard instruments in polarization experiments in particle and nuclear physics. Maintaining the continuous exact parametric spin resonance condition of the equality of the spin rotator and the spin precession frequency during operation is one of the challenges. We present a detailed analytical description of the effects of detuning the exact spin resonance on the precessing vertical and in-plane components of the polarization. An important part of the formalism presented here is the consideration of experimentally relevant spin decoherence effects. Within the developed formalism, we address the impact of feedback via pilot-bunch-based comagnetometry on continuous spin flips and on the related interpretation of charged-particle electric dipole moment searches using storage rings. We propose a spin-flip-based tomography of the longitudinal profile of polarization in a bunch, which is important for the evaluation of the polarization-dependent luminosity in collider experiments. We emphasize the potential importance of the previously unexplored phase of the horizontal polarization of the envelope as an indicator of the stability of radio-frequency-driven spin rotations in storage rings and as a testing ground for spin decoherence mechanisms.

In this paper, we propose a dielectric laser acceleration scheme based on the inverse Cherenkov effect, which uses the principle of alternating phase focusing to accelerate, confine, and bunch the subrelativistic electron beam over an extended distance. In this scheme, the reference electron is always synchronized with the acceleration wave and is thus continuously accelerated. Electrons around it are alternately subjected to focusing and defocusing forces by alternately jumping its phase. Optimizing the focusing period can achieve stable beam confinement. Simulations show that a 100 keV electron bunch can be coherently accelerated with the average acceleration gradient of $220.8\mathrm{MV}/\mathrm{m}$, and it can travel nearly $200\mathrm{\mu }\mathrm{m}$ without loss in a channel only 500 nm wide. This approach is scalable, potentially enabling the development of millimeter-scale MeV particle accelerators for ground-breaking use in medical, scientific, and materials research.

The rf system of China Spallation Neutron Source (CSNS) Rapid Cycling Synchrotron (RCS) utilizes vacuum tubes to drive the ferrite-loaded cavities. Ensuring stable tube operation in the rf system is crucial for the acceleration of high-intensity beams, as any instability may impede further increases in beam power. Therefore, the operation of the vacuum tube under heavy beam loading has attracted significant attention, particularly in the case of CSNS-II RCS, where the circulating beam current is expected to reach up to 15.25 A, imposing a substantial burden on the vacuum tube for beam loading compensation. Thus, a comprehensive analysis of tube operation is imperative. However, current methods for the tube operation analysis are developed based on wideband rf cavities, which are inadequate when the load of the tube is a narrowband ferrite-loaded cavity equipped with a tuning system. The presence of a tuning system renders the modeling method for wideband rf cavities inapplicable for ferrite-loaded cavities. Therefore, the development of a new method is necessary. This paper introduces a novel method for analyzing operation of vacuum tube driving a tuned cavity. The effectiveness of the method is validated by comparing the analysis results with measurements under beam loading conditions. Furthermore, an estimation of tube operation under 500 kW beam loading for CSNS-II is provided.

Synchronizing free-electron laser pulses for time-resolved experiments is necessary as an unsynchronized system reduces the effective repetition rate of the experiment. Additionally, free-electron laser longitudinal coherence and spectral brightness are limited by electron beam energy spread. In this paper, a variable bunch compressor design and compression schemes were developed to: reduce klystron-induced arrival time jitter by 85% while preserving peak current and transverse emittance and to minimize electron beam correlated energy spread for potential self-seeding or HB-SASE modes.

The success of x-ray free-electron lasers (XFELs) in recent years has greatly advanced many scientific fields. However, most of these XFELs suffer from a low longitudinal coherence and instabilities due to the stochastic start-up process. A cavity-based x-ray free-electron laser (CBXFEL) is a possible future direction in the development of fully coherent and stable x-ray sources. One of the challenges of a CBXFEL is the requirement for the $\mathrm{\mu }\mathrm{m}\text{−}\mathrm{sized}$ electron bunches to fully overlap in three dimensions with the circulating $\mathrm{\mu }\mathrm{m}\text{−}\mathrm{sized}$ x-ray pulses in an x-ray cavity tens or hundreds of meters long. We present here the development of an x-ray diagnostics system for accurate alignment of x-ray beams in the CBXFEL cavity. The system was developed within the framework of a CBXFEL research and development collaboration among Argonne National Laboratory, SLAC National Accelerator Laboratory, and Spring-8. All the designed diagnostic components have been fully characterized at the Advanced Photon Source to demonstrate a capability for sub-$\mathrm{\mu }\mathrm{rad}$ angular and $\mathrm{\mu }\mathrm{m}$ spatial alignment of the CBXFEL cavity.

Due to scattering, particles in a ring may have large momentum deviations, which will change their dynamics, potentially resulting in particle losses. Optimization of off-momentum nonlinear dynamics is, therefore, important for increasing beam lifetime. When analyzing off-momentum dynamics using the approach of resonance driving terms (RDTs), chromatic RDTs are usually used, which denote the deviations from on-momentum dynamics. In this paper, instead of using chromatic RDTs, we propose using off-momentum RDTs to analyze off-momentum dynamics, which is a more direct way. As in the case of on-momentum dynamics, the nonlinear driving terms for off-momentum particles include off-momentum RDTs and off-momentum amplitude-dependent tune shifts, which are calculated based on off-momentum linear optics. Similar to the analysis of on-momentum dynamics in our previous work [B. Wei et al., Phys. Rev. Accel. Beams 26, 084001 (2023)], we study the fluctuation or variation of off-momentum RDTs along the longitudinal position, which is very effective for analyzing and optimizing off-momentum dynamic apertures (DAs). Similar results are obtained for the off-momentum case: (i) minimizing the element-by-element average of the RDTs is more effective than minimizing one-turn RDTs in enlarging DAs and (ii) minimizing the average of lower-order RDTs helps reduce higher-order RDTs.

At the 3 GeV ring of the MAX IV Laboratory, a fourth-generation ring-based synchrotron light source, an asymmetric influence of the sign of the amplitude-dependent tune shift (ADTS) on the transverse mode-coupling instability has been observed. Measurements of the instability, in dedicated single-bunch experiments at low chromaticity, revealed a significant dependence of the dynamics of the instability above threshold on the sign of the ADTS. While for a negative sign of the ADTS, the crossing of the instability does not lead to a loss of beam current, a positive sign results in the loss of 40% or more of the beam current at the threshold. In order to investigate the observed asymmetry, the systematic measurement of beam dynamics above the threshold has been conducted in combination with particle tracking simulations with mbtrack2 and theoretical calculations of the Landau damping due to the ADTS. The findings point toward an influence of the Landau damping in combination with the low synchrotron frequency, which indicates that this effect could become relevant in the future low-emittance electron storage rings.