We present a novel and flexible method to generate two-color x-ray free-electron-laser (FEL) pulses with variable color separation and time delay using a double-slotted foil on a beam with a large energy chirp. The color separation and time delay between pulses can be set independently by tuning the separation between the slots and the longitudinal dispersion of a magnetic chicane. This method presents some advantages in comparison to previous proposals; it is compact, no large hardware component is needed, and no modifications of the machine settings in the FEL injector are required. Start-to-end numerical simulations for the SwissFEL case show the validity and tunability of our method and its application range: $-60$ to $+80\mathrm{fs}$ time delay and 0 to 3% color separation between the two pulses for a central wavelength of 0.1 nm.

We use particle-in-cell (PIC) simulations to study the effects of variations of the incoming 400 GeV proton bunch parameters on the amplitude and phase of the wakefields resulting from a seeded self-modulation (SSM) process. We find that these effects are largest during the growth of the SSM, i.e., over the first five to six meters of plasma with an electron density of $7\times {10}^{14}{\mathrm{cm}}^{-3}$. However, for variations of any single parameter by $\pm 5\%$, effects after the SSM saturation point are small. In particular, the phase variations correspond to much less than a quarter wakefield period, making deterministic injection of electrons (or positrons) into the accelerating and focusing phase of the wakefields in principle possible. We use the wakefields from the simulations and a simple test electron model to estimate the same effects on the maximum final energies of electrons injected along the plasma, which are found to be below the initial variations of $\pm 5\%$. This analysis includes the dephasing of the electrons with respect to the wakefields that is expected during the growth of the SSM. Based on a PIC simulation, we also determine the injection position along the bunch and along the plasma leading to the largest energy gain. For the parameters taken here (ratio of peak beam density to plasma density $n_{b0}/n_0\approx 0.003$), we find that the optimum position along the proton bunch is at $\xi \approx -1.5{\sigma }_{zb}$, and that the optimal range for injection along the plasma (for a highest final energy of $\sim 1.6\mathrm{GeV}$ after 10 m) is 5–6 m.

The low energy RHIC electron cooler (LEReC) is currently under commissioning at BNL to improve RHIC luminosity for heavy ion beam energies below $10\mathrm{GeV}/\mathrm{nucleon}$. The linac of LEReC consists of a dc photoemission gun, one 704 MHz superconducting radio frequency booster cavity, and three normal conducting cavities. It is designed to deliver a 1.6–2.6 MeV electron beam, with peak-to-peak momentum spread $dp/p$ of less than $\pm 7\times {10}^{-4}$. Two of the three normal conducting cavities will be used in LEReC for energy spread correction: a single-cell 704 MHz cavity for energy dechirping and a three-cell 2.1 GHz third harmonic cavity for rf curvature correction. In this paper, we present the designs and rf test results of these two cavities.

Short dense electron bunches produced by modern photoinjectors are attractive from the viewpoint of the realization of powerful and effective sources of subterahertz radiation based on the spontaneous coherent mechanism of emission. This type of emission is realized if the effective phase size of the bunch with respect to the radiated wave is small enough. Therefore, the repulsion of particles caused by a strong Coulomb field inside the dense electron bunch strictly limits the duration of the radiation process due to the increase in the bunch length. We show that this problem can be solved by using the cyclotron mechanism of the spontaneous radiation due to the effect of compensation of the Coulomb repulsion in the phase space.

A new, hybrid design is proposed to eliminate the main systematic errors in the frozen spin, storage ring measurement of the proton electric dipole moment. In this design, electric bending plates steer the particles, and magnetic focusing replaces electric. The magnetic focusing should permit simultaneous clockwise and counterclockwise storage to cancel systematic errors related to the out-of-plane dipole electric field. Errors related to the quadrupole electric fields can be eliminated by successive runs of magnetic focusing with different strengths.

The ${\mathrm{CsK}}_2\mathrm{Sb}$ photocathode is capable of generating a high-intensity and low-emittance electron beam with visible laser light. In this study, we examined ${\mathrm{CsK}}_2\mathrm{Sb}$ photocathode evaporation on $n$- and $p$-type Si(100), GaAs(100), and Si(111) substrates, and compared their cathode performance. We found that the quantum efficiency of the $p$-type substrate was superior to that of the $n$-type substrate for the same substrate material and surface orientation. We show that this can be qualitatively analyzed by an energy-band model for the semiconductor(metal)-semiconductor junction between ${\mathrm{CsK}}_2\mathrm{Sb}$ and the substrate. We also evaporated the ${\mathrm{Cs}}_3\mathrm{Sb}$ cathode on $n$- and $p$-type Si(100) and GaAs(100) substrates, and confirmed that the cathode performance of all substrates was consistent with the model results. This finding indicates the possibility of improving the thin-film cathode performance by revisiting the semiconductor(metal)-semiconductor junction between the cathode and the substrate, which improves the understanding of high-performance photocathodes.

The data produced at the particle physics experiments at the Large Hadron Collider (LHC) contain not only the signals from the collisions, but also a background component from proton losses around the accelerator. Understanding, identifying and possibly mitigating this machine-induced background is essential for an efficient data taking, especially for some new physics searches. Among the sources of background are hadronic and electromagnetic showers from proton losses on nearby collimators due to beam-halo cleaning. In this article, the first dedicated LHC measurements of this type of background are presented. Controlled losses of a low-intensity beam on collimators were induced, while monitoring the backgrounds in the ATLAS detector. The results show a clear correlation between the experimental backgrounds and the setting of the tertiary collimators (TCTs). Furthermore, the results are used to show that during normal LHC physics operation the beam halo contributes to the total beam-induced background at the level of a percent or less. A second measurement, where the collimator positions are tightened during physics operation, confirms this finding by setting a limit of about 10% to the contribution from all losses on the TCTs, i.e. the sum of beam halo and elastic beam-gas scattering around the ring. Dedicated simulations of the halo-related background are presented and good agreement with data is demonstrated. These simulations provide information about features that are not experimentally accessible, like correlations between backgrounds and the distributions of proton impacts on the collimators. The results provide vital information about the dependence between background and collimator settings, which is of central importance when optimizing the LHC optics for maximum peak luminosity.

X-ray free-electron lasers (XFELs) are cutting-edge scientific instruments for a wide range of disciplines. Conventionally, the narrow bandwidth is pursued in an XFEL. However, in recent years, the large-bandwidth XFEL operation schemes are proposed for x-ray spectroscopy and x-ray crystallography, in which overcompression is a promising scheme to produce broad-bandwidth XFEL pulses through increasing the electron beam energy chirp. In this paper, combining with the beam yaw correction to overcome the transverse slice misalignment caused by the coherent synchrotron radiation, finding out the overcompression working point of the linac is treated as a many-objective (having four or more objectives) optimization problem, thus the nondominated sorting genetic algorithm III (NSGA-III) is applied to the beam dynamic optimization for the first time. Start-to-end simulations demonstrate a full bandwidth of 4.79% for Shanghai soft x-ray free-electron laser user facility. Furthermore, the NSGA-III is found to be very efficient by providing a set of reference points with preferences. This paves the way for applying this algorithm to online optimization of a real machine.

In more than four decades, particle acceleration by plasma wakefield has demonstrated its feasibility and efficiency. This acceleration technique is now starting to be planned for providing high-quality beams to well-defined user communities. High beam energy is also considered by piling successive plasma acceleration stages. In this context, avoiding beam degradation, on top of all emittance degradation, is the main concern when transferring the accelerated beam to the users or to the following acceleration stage. After examining the behavior of the trace and the phase emittances when crossing through a conventional transfer line, we are able to determine the criteria to be achieved in the plasma ramps so as to minimize emittance growth. Then the optimal density profile is studied for these ramps at the entrance and exit of a plasma stage accelerating electrons from the energy of 150 MeV to 5 GeV. Finally, the design of an optimal transfer line allows showing that the emittance growth can be contained to less than 10% in realistic conditions when transferring a beam to a free-electron laser.

Field emission is one of the most serious issues in the degradation of superconducting cavity performance. However, surveying field emission sources during the cavity performance test (called the “vertical test”) and cryomodule operation is difficult. Therefore, to precisely investigate electron emissions from the field emission source, we developed a diagnostic system for the energy recovery linac 1.3 GHz nine-cell superconducting cavity for both vertical tests and the cryomodule operation. The developed system is comprised of two types of sensors: a carbon sensor and a Si $p$-$i$-$n$ diode, that measure the temperature rise and the radiation produced by electron emissions, respectively. Rotating the sensor array around the cavity axis in the vertical test provides detailed information on the entire cavity surface. The precise x-ray mapping profile measured by the system in the vertical test enables identification of the local emission source. This paper describes how the field emission source is identified in the vertical test from the standpoint of not only experimental results obtained with the newly developed rotating mapping system but also the detailed approach based on precise simulations. In addition, field emission studies with the optimized instrumentation have been extended to cryomodule tests, both standalone and in the beam line. These developments allow us to monitor changes in the cavity field emission signatures introduced by cryomodule assembly and beam operation.

During the summer and fall of 2018 the Cornell High Energy Synchrotron Source (CHESS) is undergoing an upgrade to increase high-energy flux for x-ray users. The upgrade requires replacing one-sixth of the Cornell Electron Storage Ring (CESR), inverting the polarity of half of the CHESS beam lines, and switching to single-beam on-axis operation. The new sextant is comprised of six double-bend achromats (DBAs) with combined-function dipole-quadrupoles. Although the DBA design is widely utilized and well understood, the constraints for the CESR modifications make the CHESS-U lattice unique. This paper describes the design objectives, constraints, and implementation for the CESR accelerator upgrade for CHESS-U.

Aviation cargo unit load device (ULD) containers are typically much smaller than standard shipping containers, with a volume of around $1{\mathrm{m}}^3$. Standard 3–6 MeV x-ray screening linacs have too much energy to obtain sufficient contrast when inspecting ULDs, hence a lower 1 MeV linac is required. In order to obtain a small physical footprint, which can be adapted to mobile platform applications, a compact design is required, hence $X$-band radio-frequency technology is the ideal solution. A prototype 1.45 MeV linac cavity optimized for this application has been designed by Lancaster University and Science and Technology Facilities Council (STFC), manufactured by Comeb (Italy) and tested at Daresbury Laboratory using an e2v magnetron, modulator, and electron gun. The cavity is a bi-periodic $\pi /2$ structure, with beam-pipe aperture coupling to simplify the manufacture at the expense of shunt impedance, while keeping the transverse size as small as possible. The design, manufacture, and testing of this linac structure is presented. In order to optimize the image it is necessary to be able to modify the energy of the linac. It can be changed by altering the rf power from the magnetron but this also varies the magnetron frequency. By varying the beam current from 0–70 mA the beam energy varied from 1.45 to 1.2 MeV. This allows fast energy variation by altering the focus electrode bias voltage on the electron gun while keeping the dose rate constant by varying the repetition frequency. Varying the beam energy by varying the rf power and by varying the beam current are both studied experimentally. The momentum spread on the electron beam was between 1% and 5% depending on the beam current of 0–70 mA

This paper describes the physical design, rf measurement, and tuning result of a 325 MHz four-vane radio frequency quadrupole (RFQ) with a ramped inter-vane voltage. The 3-meter-long RFQ will accelerate the negative hydrogen ion beam from 50 keV to 3 MeV with a peak current of 5 mA, a pulse width of $10–40\mu \mathrm{s}$, and a maximum repetition rate of 0.5 Hz. During tuning we reveal that the coupling loop of the coaxial coupler adopted for this RFQ enhances the interference between the dipole field distribution and the coupling coefficient. Therefore, an iteration method is adopted to simultaneously achieve the desired field distribution and coupling coefficient. After tuning, the relative error between the measured field distribution and the designed curve for the operating quadrupole mode is within $\pm 2.7\%$, and the dipole components are about $\pm 1.9\%$ of the quadrupole mode. The measured coupling coefficient of the rf power coupler equals 1.03, with the desired value of 1.04.

The physics involving the emission of coherent undulator radiation from a bunch of correlated electrons wiggling inside an undulator has been very well established in the frequency domain. With the aim of describing the process of emission of coherent undulator radiation in time domain while incorporating the effect of hyperbolic field profile of the undulator and various electron beam parameters, we present a time-domain analysis which can serve as a complimentary formulation to the frequency-domain analysis of the coherent undulator radiation. The formulation requires revisiting the equation of motion of an electron moving inside a planar undulator. An electron beam with substantial spread along the direction of the magnetic field will undergo betatron oscillations due to the hyperbolic profile of the undulator fields. We show that the betatron motion can be conveniently expressed in terms of Mathieu functions. The new set of equations of motion have been used to study the effect of betatron motion on the emitted undulator radiation. The single-particle time-domain analysis is then easily extended to multiparticle systems by correctly accounting for the phase-difference between the individual electromagnetic waves emitted by different electrons occupying different phase-space in the bunch.

In order to handle the extremely high stored energy in future proton-proton colliders, an extremely high-efficiency collimation system is required for safe operation. At the LHC, the major limiting locations in terms of particle losses on superconducting (SC) magnets are the dispersion suppressors downstream of the transverse collimation insertion. These losses are due to the protons experiencing single diffractive interactions in the primary collimators. How to solve this problem is very important for future proton-proton colliders, such as the Future Circular Hadron-Hadron Collider and the Super Proton-Proton Collider. In this article, a novel method is proposed, which arranges both the transverse and momentum collimation in the same long straight section. In this way, additional absorbers between the two cleaning hierarchies can clean those particles related to the single diffractive effect, with the downstream momentum collimation system intercepting any further leakage. The effectiveness of the method has been confirmed by multiparticle simulations. In addition, SC quadrupoles with special designs such as an enlarged aperture and good shielding are adopted to enhance the phase advance in the transverse collimation section so that tertiary collimators can be arranged to clean off the tertiary halo which emerges from the secondary collimators and improve the collimation efficiency. With one more collimation stage in the transverse collimation, the beam losses in both the momentum collimation section and the experimental regions can be largely reduced. Multiparticle simulation results with the merlin code confirm the effectiveness of the collimation method. At last, we provide a protection scheme of the SC magnets in the collimation section. The fluka simulations show that, by adding some special protective collimators in front of the magnets, the maximum power deposition in the SC coils is reduced dramatically, which is proven to be valid for protecting the SC magnets from quenching.

At KARA, the Karlsruhe Research Accelerator of the Karlsruhe Institute of Technology synchrotron, the so-called short-bunch operation mode allows the reduction of the bunch length down to a few picoseconds. The microbunching instability resulting from the high degree of longitudinal compression leads to fluctuations in the emitted terahertz radiation, referred to as bursting. For extremely compressed bunches at KARA, bursting occurs not only in one but in two different bunch-current ranges that are separated by a stable region. This work presents measurements of the thresholds and bursting frequencies in both regimes. Good agreement is found between the data and numerical solutions of the Vlasov-Fokker-Planck equation.

We consider the collective beam dynamics in the accelerator of the European X-ray Free Electron Laser for electron bunches with charges of 250 and 500 pC. The dynamics has been studied numerically with different computer programs, and we struggle to include properly the whole beam physics with collective effects. Several scenarios of the longitudinal dynamics are considered for the peak currents of 5 and 10 kA. A procedure for the choice of the machine parameters is described, and the tolerances of radio frequency parameters of accelerating modules along the accelerator are calculated. The bunch properties at the end of the accelerator before the undulator line are analyzed. A comparison between measurements and simulations for the beam energy loss due to wakefields is presented.

Recent studies of the performance of radio-frequency (rf) copper cavities operated at cryogenic temperatures have shown a dramatic increase in the maximum achievable surface electric field. We propose to exploit this development to enable a new generation of photoinjectors operated at cryogenic temperatures that may attain, through enhancement of the launch field at the photocathode, a significant increase in five-dimensional electron beam brightness. We present detailed studies of the beam dynamics associated with such a system, by examining an S-band photoinjector operated at $250\mathrm{MV}/\mathrm{m}$ peak electric field that reaches normalized emittances in the 40 nm-rad range at charges (100–200 pC) suitable for use in a hard x-ray free-electron laser (XFEL) scenario based on the LCLS. In this case, we show by start-to-end simulations that the properties of this source may give rise to high efficiency operation of an XFEL, and permit extension of the photon energy reach by an order of magnitude, to over 80 keV. The brightness needed for such XFELs is achieved through low source emittances in tandem with high current after compression. In the XFEL examples analyzed, the emittances during final compression are preserved using microbunching techniques. Extreme low emittance scenarios obtained at pC charge, appropriate for significantly extending temporal resolution limits of ultrafast electron diffraction and microscopy experiments, are also reviewed. While the increase in brightness in a cryogenic photoinjector is mainly due to the augmentation of the emission current density via field enhancement, further possible increases in performance arising from lowering the intrinsic cathode emittance in cryogenic operation are also analyzed. Issues in experimental implementation, including cavity optimization for lowering cryogenic thermal dissipation, external coupling, and cryocooler system, are discussed. We identify future directions in ultrahigh field cryogenic photoinjectors, including scaling to higher frequency, use of novel rf structures, and enabling of an extremely compact hard x-ray FEL.

Acceleration to high energies in electron-driven wakefield accelerators requires a carefully tailored drive beam. High driver currents can, however, modify the accelerating structure response, shifting the optimal driver profile. In this work we present a simple model to describe this nonlinear beamloading in hollow plasma channels. The increase in channel radius and the generation of a return current in the bulk plasma act to modify the resonant frequency and accelerator loss factor. Modeling these effects allows the optimal driver profile to be calculated.

A metallic metasurface-based laser-driven particle accelerator for subrelativistic particles is proposed and studied theoretically. The metasurface consists of a nonperiodic array of nanoslits which focuses the field of the driving laser, utilizing the phenomenon of extraordinary plasmonic transmission, to maximize the acceleration gradient. In order to account for the actual change in the particles’ velocity during their propagation through the structure, the separation between successive slits is not constant but rather optimized according to the expected trajectory of the particles. The metasurface laser accelerator (MLA) is designed for an ultrafast driving laser source operating at $2\mu \mathrm{m}$ wavelength. An approximate analytical model verified by particle tracking simulations predicts a net average acceleration with a normalized acceleration gradient of 1.34 times the incident laser field. Compared to other laser-driven accelerator designs, the MLA provides substantially higher efficiency, due to the field enhancement associated with nanoantennas, and relaxed fabrication challenges (especially for subrelativistic particles). It is found that the output particle beam is microbunched, suggesting the possibility of using a short MLA structure as a prebuncher to improve the initial capture efficiency in a subsequent longer MLA device. The impact of space-charge effects is also studied, and the loaded gradient and optimal bunch charge are estimated.