The CERN Proton Synchrotron (PS) routinely crosses transition energy at around 6 GeV in order to accelerate protons that are injected in the Super Proton Synchrotron (SPS) or transferred to users of fixed target experiments. Depending on the beam parameters and intensity, a fast vertical coherent instability occurs during transition crossing. The instability, characterized by beam losses and a frequency spectrum in the range of 500–900 MHz, represents an important intensity limitation for the neutron time-of-flight (nTOF) beam, and in general could represent a bottleneck for future high intensity beams. In order to better understand the nature and the source of the instability and to find possible mitigations, a dedicated measurement campaign took place. Parallel to the measurements, beam dynamics simulations have been performed to study the observed instability. In particular, single bunch effects have been simulated using the PS transverse beam coupling impedance model developed over recent years. In this paper we present the measurements results along with the obtained instability thresholds. Different beam configurations and stabilizing effects, such as the gamma jump scheme and the octupole-induced tune spread, are also considered. The measurements results are compared with simulations.

We present a theory of coupled-bunch longitudinal instabilities that is based on the coupled set of Vlasov equations governing the particle distribution function, and can be applied to arbitrary longitudinal potentials. We find that the coupled-bunch growth rate is given by a dispersion relation that is parametrized by the eigenvalues of the linear (harmonic) coupled-bunch matrix problem. Our theory therefore treats the wakefield-driven source of the instability and the effect of Landau damping together and on equal footing, and also indicates that the stabilizing effects of Landau damping can approximately be compared to the instability growth rates in a harmonic potential that has the same bunch length. We then apply the theory to a weakly nonlinear oscillator and to a quartic potential that is relevant for ultralow emittance storage rings that employ bunch-lengthening systems. In the latter case we find that the theory compares quite favorably with particle tracking simulations for the parameters of the planned upgrade of the Advanced Photon Source.

Formal and experimental studies of Fabry-Perot cavities operating in burst mode are conducted in the context of Compton scattering. A dedicated optimization procedure for their use in this context is described for the first time, to the best of our knowledge. Provided that the number of bunches in the electron-beam train and their time separation are known, the parameters related to the optical system are chosen to maximize the ratio of the total intracavity laser-beam energy used for collisions to the total input laser beam seeded into the cavity operated in burst mode. A good qualitative agreement is obtained between the model and the experimental data. It is shown by studying the upgrade of an existing machine, STAR. Here a new optical system with increased performance without a large increase in complexity is obtained. This result paves the way towards future designs of high-flux, high-quality, low cost and compact x-ray facilities based on Compton scattering.

Advances in numerical methods for free-electron-laser (FEL) simulation under wiggler period averaging (WPA) are presented. First, WPA is generalized using the perturbative Lie map method. The conventional WPA is identified as the leading order contribution. Next, the shot-noise model under WPA is improved along with a particle migration scheme across the numerical mesh. The artificial shot noise arising from particle migration is suppressed. The improved model also allows using arbitrary mesh size, slippage resolution, and integration step size. These advances will improve modeling of longitudinal beam profile evolution for fast FEL simulation.

In radioactive ion beam experiments, beams containing isomers can be of interest in probing nuclear structure and informing astrophysical reaction rates. While the production of mixed in-flight ground state and isomer beams using nucleon transfer can be generally understood through distorted wave Born approximation methodology, low-spin isomer production via fast fragmentation is relatively unstudied. To attain a practical understanding of low-spin isomer production using fast fragmentation beams, a test case of ${}^{38}\mathrm{K}/{}^{38m}\mathrm{K}$ was studied at the National Superconducting Cyclotron Laboratory’s ReAccelerated Beam facility. Starting from lise++ predictions, the fragmentation momentum distribution was sampled to determine isomer production. In addition, the effects of the gas stopper gradient and charge breeding times were examined. In the case of ${}^{38}\mathrm{K}$, isomer production peaks at $\sim 57\%$. This maximum is observed just off the lise++ predicted optimum magnetic rigidity, with only small losses in beam intensity within a few percent of this optimum rigidity setting. Control of the isomer fraction was also achieved through the modification of charge breeding times. Fast fragmentation appears to be a feasible method for production of low-spin isomeric beams, but additional study is necessary to better describe the mechanism involved.

The ongoing trend towards synchrotron light storage rings with ultralow emittance lattices leads to greater challenges to achieve beam stability, sufficient Touschek lifetime, low heating of machine components, and conservation of the emittance at high bunch charge. One solution to meet these challenges is to lengthen the electron bunches with harmonic cavities. Many upgrade proposals therefore include harmonic cavities to enhance the machine performance. This is also the case for the MAX IV 3 GeV storage ring, which employs passive third harmonic cavities to achieve up to five times bunch lengthening. Unfortunately, the performance of the harmonic cavities is reduced if a gap in the fill pattern is required. In this paper, the effect on synchronous phase and bunch length due to a gap in the fill pattern for rings with passive harmonic cavities is calculated in a self-consistent way including the bunch form factor. The aim is to achieve faster simulation of various schemes for compensating a gap compared to multiparticle tracking. A new semianalytical method based on an iterative matrix formulation is presented, as well as a single-particle tracking code including the bunch form factor. The results from these methods are compared to both results from a multiparticle tracking code and measurements at the MAX IV 3 GeV storage ring. The importance of including the bunch form factor in simulations is evaluated and discussed.

Electron accelerators and synchrotrons can be operated to provide short emission pulses due to longitudinally compressed or substructured electron bunches. Above a threshold current, the high charge density leads to the microbunching instability and the formation of substructures on the bunch shape. These time-varying substructures on bunches of picoseconds-long duration lead to bursts of coherent synchrotron radiation in the terahertz frequency range. Therefore, the spectral information in this range contains valuable information about the bunch length, shape and substructures. Based on the KAPTURE readout system, a 4-channel single-shot THz spectrometer capable of recording 500 million spectra per second and streaming readout is presented. First measurements of time-resolved spectra are compared to simulation results of the Inovesa Vlasov-Fokker-Planck solver. The presented results lead to a better understanding of the bursting dynamics especially above the micro-bunching instability threshold.

Future light sources such as synchrotron radiation sources driven by an energy recovery linac, free electron lasers, or THz radiation sources have in common that they require injectors, which provide high-brilliance, high-current electron beams in almost continuous operation. Thus, the development of appropriate highly brilliant electron sources is of key importance. With its superconducting radio-frequency photo-injector (SRF gun) the Helmholtz-Zentrum Dresden-Rossendorf provided a promising approach for this key component, which has since been adopted in other laboratories. Nevertheless, some limitations occur caused by electron multipacting, which should be suppressed in order to further improve the gun. In this contribution, we present a detailed analysis of multipacting in the critical area of the SRF gun and different suppression techniques for it. The analytical predictions on the threshold for multipacting are qualitatively comparable with numerical simulation results and experimental data. Finally, we present specific surface structuring as an effective method to mitigate the multipacting phenomenon from the photocathode channel.

We have developed an intrinsic resonance free circular electron accelerator. This lattice could be placed in the existing RHIC tunnel and accelerate electrons from 400 MeV to 18 GeV avoiding all major polarization loss usual in such machines.

We introduce a perturbation-theory, mode-analysis method for longitudinal multibunch instabilities driven by the higher harmonic cavity (HHC) fundamental mode. The method, based on the exact solution of the unperturbed particle motion in the rf bucket and suitable for modeling the effect of cavities with general settings, is applied to study the feasibility of reutilizing the existing Advanced Light Source (ALS) HHCs in the ALS Upgrade (ALS-U). We find that with ALS cavities the ALS-U would be susceptible to a fast $\ell =1$ mode instability. Interestingly, the instability is driven by the imaginary rather than the real part of the cavity fundamental-mode impedance.

The space-charge driven envelope instability can be of great danger in high intensity accelerators. Linear accelerators were designed to avoid this instability by keeping the zero current phase advance per lattice period below 90 degrees. In this paper, we studied the acceleration effects on the instability in a periodic solenoid and radio-frequency (rf) focusing channel and a periodic quadrupole and rf focusing channel using a three-dimensional envelope model and self-consistent macroparticle simulations. Our results suggest that the envelope instability might be dramatically mitigated with a reasonable accelerating gradient in both channels. This suggests that the zero current phase advance without acceleration might be above 90 degrees in linear accelerators where the accelerating gradient is sufficiently high.

rf photoguns find several types of applications as high brightness electron sources for free-electron lasers, energy recovery linacs, Compton and Thomson sources, and high-energy linear colliders. The high peak current and low transverse emittance of the generated beam are obtained with the combination of a high peak electric field ($>100\mathrm{MV}/\mathrm{m}$) at the cathode surface, a proper choice of the solenoid field around, or immediately after, the gun, and special fabrication and treatments of the cathode itself. On the other hand, to increase the average electron current, a high repetition rate ($>100\mathrm{Hz}$) and/or a multibunch rf gun have to be developed. These types of devices are, in general, fabricated by brazing processes of copper machined parts. The brazing processes require a large vacuum furnace, are very expensive, and pose a not negligible risk of failure. A new fabrication technique for this type of structure has been recently developed and implemented at the Laboratories of Frascati of the National Institute of Nuclear Physics (INFN-LNF, Italy) and already applied to an rf gun now operating at a relatively low cathode peak field and low repetition rate [D. Alesini et al., Phys. Rev. Accel. Beams 18, 092001 (2015)]. It is based on the use of special rf-vacuum gaskets that allow a brazing-free realization process. The $S$-band gun of the Extreme Light Infrastructure-Nuclear Physics Gamma Beam System, under construction in Magurele (Bucharest, Romania), has been realized with this new technique and represents a further and fundamental step toward the consolidation of this technology for high gradient particle accelerator fabrication. It operates at 100 Hz with a $120\mathrm{MV}/\mathrm{m}$ cathode peak field and $1.5\text{−}\mu \mathrm{s}$-long rf pulses to house the 32 bunches necessary to reach the target gamma flux. High gradient tests, performed at full power and a full repetition rate, have shown the extremely good performances of the structure in terms of the breakdown rate and conditioning time and definitively demonstrated the reliability and suitability of such fabrication process for high gradient structure realization. In this paper, we report and discuss the electromagnetic and thermomechanical design, the realization process, and all the experimental results at low and high power at a full repetition rate.

High quantum efficiency photocathodes are mandatory for the operation of photoinjector driven electron accelerators with high average current and high brightness beams. Photocathodes based on bi-alkali antimonides, e.g., ${\mathrm{CsK}}_2\mathrm{Sb}$, exhibit high quantum efficiencies for visible light and can be operated close to the photoemission threshold, thus they are suitable candidates to provide high current and low emittance electron beams. In this paper, a codeposition procedure of K and Cs on Sb resulting in high quantum efficiency photocathodes is presented and compared to the sequential growth procedure that was established for photomultiplier and accelerator applications. In-situ x-ray photoelectron spectroscopy is applied to gain insights into the reaction pathway of antimony with alkali metals, and to optimize the growth process of ${\mathrm{CsK}}_2\mathrm{Sb}$ on Mo. It has been found that the average stoichiometry of the samples is similar after both procedures. The study also presents the behavior of the photocurrent at cryogenic temperatures, the influence of cooling and warmup cycles on the photocathode lifetime and our experience with storage and transport. This work demonstrates that our codeposition growth procedure reproducibly delivers high quantum efficiency photocathodes, and that their quantum efficiency, when excited with green photons, is not influenced by cryogenic temperatures.