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.

Active energy compression scheme is presently being investigated for future laser-plasma accelerators. This method enables generating laser-plasma accelerator electron beams with a small, $\sim {10}^{-5}$, relative slice energy spread. When modulated by a laser pulse, such beams can produce coherent radiation at very high, $\sim 100$th harmonics of the modulation laser wavelength, which are hard to access by conventional techniques. The scheme has a potential of providing additional capabilities for future plasma-based facilities by generating stable, tunable, narrow-band radiation.

Magnetized liner inertial fusion (MagLIF) is an attractive concept for producing thermonuclear fusion reactions. The MagLIF platform involves the operation of Helmholtz coils to apply a 15 Tesla axial magnetic field to the load region, where a cylindrical, fuel-filled metal liner is imploded by a $\sim 20\mathrm{MA}$ current pulse. The fringe field from these coils extends into the transmission line that delivers the current to the target. We investigated the extent to which this applied field disturbs the nominal power flow within that transmission line. A simplified model of the geometry shows that adding the applied magnetic field results in magnetic field lines that connect the cathode to the anode, suggesting electrons may not be magnetically insulated in this region. Particle-in-cell simulations indicated the addition of the applied magnetic field would not significantly impact the current delivery to the load. Velocimetry was used to experimentally assess the current delivery with and without the applied magnetic field. We find no measurable effects of the applied field on current delivery in the configuration investigated in this study.

The feasibility of gamma-ray light sources based on the channeling phenomenon of ultrarelativistic electrons and positrons in oriented single crystals is demonstrated using rigorous numerical modeling. Case studies are presented for 10 GeV and sub-GeV $e^{-}/e^{+}$ beams incident on ${10}^{-1}-{10}^0\mathrm{mm}$ thick diamond and silicon crystals. It is shown that for moderate values of the beam average current ($\lesssim 10\mathrm{\mu }\mathrm{A}$), the average photon flux in the energy range of ${10}^0–{10}^2\mathrm{MeV}$ emitted within the ${10}^1–{10}^3\mathrm{\mu }\mathrm{rad}$ cone and 1% bandwidth can be on the level of ${10}^{10}\mathrm{photon}/\mathrm{s}$ for electrons and ${10}^{10}–{10}^{12}\mathrm{photon}/\mathrm{s}$ for positrons. These values are higher than the fluxes available at modern laser-Compton gamma-ray light sources.

High-frequency longitudinal impedances, particularly at wavenumbers $k\gg 1/{\sigma }_z$ (where ${\sigma }_z$ is the bunch length), can drive microwave instability and potentially limit the performance of modern low-emittance electron storage rings. In such rings, the coherent synchrotron radiation (CSR) emerges as a prominent contributor to these high-frequency impedances. This paper undertakes a systematic investigation into the effects of CSR on electron rings, utilizing the fourth-generation storage ring light source Elettra 2.0 and the low-energy ring of the SuperKEKB $e^{+}{e}^{-}$ circular collider as illustrative examples. Our work revisits theories of microwave instability driven by CSR impedance, extending the analysis to encompass other high-frequency impedances, such as resistive wall and coherent wiggler radiation. Through instability analysis and numerical simulations conducted on the two aforementioned rings, the study explored the impact of high-frequency impedances and their combined effects with broadband impedances caused by discontinuities in vacuum chambers.

Polarized x rays or gamma rays generated from inverse Compton scattering (ICS) sources are high-brightness, quasimonochromatic, and polarization-tunable and are widely applied in medical imaging, material science, and nuclear physics research. In this paper, to simulate ICS between electron beams and polarized laser beams, a Monte Carlo simulation code has been developed and implemented based on the geant4 toolkit. This code considers the polarization as well as the bandwidth of incident laser beams. The code is benchmarked by comparing its simulation results with those from the well-known Monte Carlo tool called cain and experimental results. The comparisons show that this code accurately reproduces the scattered photons, demonstrating its potential for various ICS-based applications.

The magnetic field optimization of RE-Ba-Cu-O (REBCO, $\mathrm{RE}=\text{rare}$ earth) bulk superconducting undulators is a fundamental step toward their implementation in an accelerator driven photon source, like a synchrotron or a free electron laser. In this article, we propose a sorting algorithm to reduce the undulator’s phase error based on the reconstruction of the trapped current inside the bulks of a staggered array undulator. The results obtained with a YBCO short prototype field-cooled down to 10 K in a 10 T magnetic field are reported. Finally, its performance is critically discussed in light of 2D magnetic field maps of its individual components, obtained at ${\mathrm{LN}}_2$ after the magnetization tests.

The Proton Power Upgrade initiative at Oak Ridge National Lab’s Spallation Neutron Source aims to greatly boost the capability of proton beam power. This upgrade involves the integration of seven additional cryomodules, each housing four six-cell high-beta ($\beta =0.81$) superconducting radio frequency cavities. These cavities, manufactured and processed by Research Instruments in Germany, underwent meticulous treatment, including electropolishing as the bulk and final vital step. Upon delivery to Jefferson Lab, a total of 28 cavities for seven cryomodules, along with an additional four cavities for a spare cryomodule, underwent thorough vertical qualification tests, meeting the required specifications. Following cavity tanking and subsequent rf testing, the assembly of eight cryomodules was successfully executed, with all 32 cavities demonstrating compliance with acceptance criteria. The transportation of the cryomodule to SNS for high-power testing in the tunnel yielded exceptional results. Notably, the performance surpassed specifications, in terms of quality factor and accelerating gradient.