We analytically examine several important characteristics of coherent radiation emitted from a relativistic electron beam in a helical undulator. The goal is to highlight specific attributes relevant to the production of bright x-rays with angular momentum in a free-electron laser (FEL) afterburner. The far-field complex electric field distribution is derived, and for small forward emission angles, it is confirmed that the spin and orbital components of angular momentum in the radiation are separable, additive, and aligned with the handedness of the electron motion. The coherent energy radiated at harmonics $h$ from a bunched electron beam is then calculated, and used to evaluate the emission energy from a FEL afterburner configuration. Results show that an afterburner undulator with $N_u\gg 1$ periods and of sufficient strength will produce harmonic pulses with energy $\sim 1/N_u^{h-1}$ times that of the fundamental, depending on the Fresnel number $N=k{\sigma }_x^2/L_u$ of the radiating electron beam.

The physics program at the future electron ion collider (EIC) calls for polarized neutron beams at high energies. The best neutron carriers are ${}^3\mathrm{He}$ nuclei and deuterons. Both neutron carriers are expected to be utilized by the spin physics program in the EIC. Owing to the small magnetic moment anomaly of a deuteron particle, significantly higher magnetic fields are required for spin rotation, so that full Siberian snakes are not feasible. However, the resonance strengths are weak, and the number of resonances is small. It is possible to deal with individual resonances using conventional methods such as a betatron tune jump and vertical ac dipole for intrinsic depolarizing resonances, and a weak partial snake for imperfection resonances. This study employed a realistic magnet ramp rate and parameters (both transverse and longitudinal emittances, as well as realistic lattices) to show that it is possible to accelerate polarized deuterons beyond 100 GeV/n. This paper summarizes the details of the feasibility of a polarized deuteron beam for the future EIC at Brookhaven National Laboratory.

Understanding the microscopic phenomena behind vacuum arc ignition and generation is crucial for being able to control the breakdown rate, thus improving the effectiveness of many high-voltage applications where frequent breakdowns limit the operation. In this work, statistical properties of various aspects of breakdown, such as the number of pulses between breakdowns, breakdown locations and crater sizes are studied independently with almost identical pulsed dc systems at the University of Helsinki and in CERN. In high-gradient experiments, copper electrodes with parallel plate capacitor geometry, undergo thousands of breakdowns. The results support the classification of the events into primary and secondary breakdowns, based on the distance and number of pulses between two breakdowns. Primary events follow a power law on the log–log scale with the slope $\alpha \approx 1.30$, while the secondaries are highly dependent on the pulsing parameters.

Neutron bang times $t_{\text{bang}}$ and mean neutron speeds $\overline{u}$ have been measured at the $Z$ facility for a series of $D_2$-filled targets, in magnetized liner inertial fusion experiments. Measurements were made by a novel neutron time-of-flight (nTOF) diagnostic, adapted for use at this facility, and consisted of detecting the neutron times in flight at seven independent scintillator–photomultiplier tube detectors (channels), located on three noncoplanar lines of sight, with distances to the neutron source varying between 690 and 2510 cm. The nTOF signals were analyzed by identifying fiducials on the detector traces to quantify the time in flight to each distance, using a nonrelativistic model for a uniformly thermalized, Maxwellian plasma distribution. The measured neutron arrival times were then linearly regressed on distance with the bang time and mean speed estimated from the fit parameters. A particular shot, 2584, is analyzed here to illustrate the method and the issues encountered in these measurements. On this particular shot, six usable channel traces were obtained. The standard errors of the parameter fits were as follows: $t_{\text{bang}}=3102.95\pm 0.97\mathrm{ns}$ (standard error) with six nTOF traces on the system clock and $\overline{u}=2.1524\pm 0.0032\mathrm{cm}/\mathrm{ns}$ (standard error), from which the mean, nonrelativistic, kinetic energy $\overline{E}$ of the neutrons was $2.4216\pm 0.0144\mathrm{MeV}$ (standard error). The estimates of $\overline{u}$ and $\overline{E}$ here agree within 1% of the published values for the $D{(d,n)}^3\mathrm{He}$ reaction. Hence, these measurements are consistent with the production of a thermalized, Maxwellian D-D fusion plasma in this experiment. The source duration was estimated to be $3.25\pm 0.84\mathrm{ns}$ (standard error) from six pulse-width measurements.

A novel radio frequency quadrupole (RFQ) resonant structure with bent vanes is proposed at the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). Compared with the traditional four-vane type RFQs, the bent-vane type structure is more compact at low frequency with significant reduction in the cross section size, and is capable of using a simple cooling system due to its rotationally symmetric structure. An 81.25 MHz proof-of-principle cavity is now under development at IMP. The beam dynamics design, electromagnetic optimizations with multipacting simulations, multiphysics analyses, and mechanical design of this novel bent-vane type RFQ (BV-RFQ) will be presented in this paper.

We present the observation and the detailed investigation of coherent Cherenkov diffraction radiation (CChDR) in terms of spectral-angular characteristics. Electromagnetic simulations have been performed to optimize the design of a prismatic dielectric radiator and the performance of a detection system with the aim of providing longitudinal beam diagnostics. Successful experimental validations have been organized on the CLEAR and the CLARA facilities based at CERN and Daresbury laboratory respectively. With ps to sub-ps long electron bunches, the emitted radiation spectra extend up to the THz frequency range. Bunch length measurements based on CChDR have been compared to longitudinal bunch profiles obtained using a radio frequency deflecting cavity or coherent transition radiation (CTR). The retrieval of the temporal profile of both Gaussian and non-Gaussian bunches has also been demonstrated. The proposed detection scheme paves the way to a new kind of beam instrumentation, simple and compact for monitoring short bunches of charged particles, particularly well-adapted to novel accelerator technologies, such as dielectric and plasma accelerators. Finally, CChDR could be used for generating intense THz radiation pulses at the MW level in existing radiation facilities, providing broader opportunities for the user community.

GaAs-based photocathodes are the state-of-the-art in the production of highly spin-polarized electron beams for accelerator and microscopy applications. While various novel structures of GaAs have been shown to increase the degree of polarization and quantum efficiency, all GaAs-based photocathodes require activation to negative electron affinity (NEA) to operate at the photon energies where the highest spin polarization is achieved. In this work, we report on NEA activation of bulk GaAs performed using Sb-Cs-O. We show this activation layer to be more robust with respect to traditional Cs and O layer in terms of charge extraction lifetime. This new activation layer is shown to improve the dark lifetime up to one order of magnitude and the charge extraction lifetime up to a factor of about 60, when compared with the traditional Cs-O activating layer. Trade-offs with other relevant parameters like quantum efficiency and electron spin polarization are also discussed.

Slow extraction of a quasicontinuous flux of high-energy protons is an important requirement for many high-energy physics experiments. This extraction type is associated with an unavoidable beam loss due to scattering on the thin septum element. The energy deposition of scattering products and resulting activation place performance limits on existing and planned high-power, high-energy fixed-target proton facilities. In the $400\mathrm{GeV}/c$ Super Proton Synchrotron (SPS) at CERN, a diffuser (or prescatterer), comprising an array of dense wires or ribbon located upstream of the electrostatic septum, has been designed to reduce absolute losses on the septum wires. As part of a concerted effort to investigate loss reduction techniques in the SPS in view of new physics experiments, the diffuser concept was explored in numerical simulation and analytically. A prototype device has been designed, built, installed, and tested in the SPS to prove the feasibility and quantify the performance reach. In this paper, the diffuser concept is briefly recalled and design considerations for the SPS use case are presented, with the analytical considerations and simulation studies for the optimization of the material and geometry. The device design is described, and the experimental results with a beam are presented and analyzed. The results are discussed, and an outlook is given for the operational feasibility and maximum obtainable performance gain. Conclusions are drawn on the implications for the application of the concept.

The superconducting Darmstadt linear electron accelerator (S-DALINAC) has been operated as an energy recovery linac (ERL) for the first time. The S-DALINAC is a recirculating superconducting radio-frequency (SRF) accelerator and had been upgraded with an additional recirculation beamline. It features a path length adjustment system that provides a freedom of choice of 360° for the rf phase difference between the electron bunches recirculated through the new beamline and the phase of the accelerating ${\mathrm{TM}}_{010}$ mode of the oscillating electromagnetic field in the SRF cavities of the accelerator. A choice of around 180° for this phase difference results in a deceleration of the recirculated beam and a corresponding transfer of the particles’ kinetic energy back to the energy of the electromagnetic field in the cavities. The main components relevant for ERL operation are described and data of the first operation as an SRF-ERL are presented.

The chirp-taper free-electron laser (FEL) scheme has been proposed to generate subfemtosecond broadband pulses using laser-modulated electron bunches in x-ray FELs. In this paper, we extend the chirp-taper concept to the postsaturation superradiant regime by combining an energy-modulated beam with a sinelike undulator taper. We show that subfemtosecond duration pulses with peak powers approaching a terawatt may be obtained with typical x-ray FEL parameters such as those at the Linac Coherent Light Source. The method shown here can produce a stable source of attosecond-scale, high-power x-ray pulses suitable for probing electronic dynamics within molecules.

In 2016 the luminosity reach of the Large Hadron Collider (LHC) was increased by reducing the $\beta$-function in the main collision points below the design value to ${\beta }^{*}=40\mathrm{cm}$. This was possible thanks to a specially matched betatron phase advance between the extraction kickers and some sensitive machine elements that would otherwise risk to be damaged by miskicked beam in case of an asynchronous beam dump. This method imposed the demand to guarantee the phase advance always stays within an acceptable tolerance, including operational actions like tune adjustments. Therefore, a new interlock system on the quadrupole magnet currents was put in place to safeguard the stability of the phase advance. This paper describes the technical implementation of this power-converter interlock (PcInterlock) and the strategies used to derive appropriated tolerances to allow sufficient protection without risking false beam dump triggers. The experience with the new PcInterlock settings in 2016-18 are discussed.

The new generation of storage-ring based synchrotron-radiation light sources imposes challenges on off-axis injection schemes owing to small dynamic apertures. In many upgrade projects of existing light sources, including the Swiss Light Source (SLS) upgrade, it is planned to reuse their injector booster synchrotrons with little to no modifications, although their emittances may not be suitable for the upgraded storage ring. A reduction of injection beam emittance relaxes the requirements for injection devices and machine aperture. Since the beam emittance in an electron synchrotron is much smaller in the vertical plane than in the horizontal, where injections are normally implemented, a significant improvement can be made by reducing the horizontal emittance by exchanging the transverse emittances through coupling resonance crossing. We find a meaningful scaling parameter for the quality of the emittance exchange, showing the relationship between coupling strength and crossing speed. Radiative effects are added to investigate the consequences for electron synchrotrons. Finally, we present the first experimental demonstration of transverse emittance exchange in the SLS booster synchrotron.

We present here analytical formulas derived for sector coils with 2N poles, an iron screen and optional iron poles. These formulas can be used to produce optimized conceptual electromagnetic designs of superconducting cosine-theta corrector magnets with 2N poles. The main guidelines of the corresponding algorithm, i.e., design algorithm for sextupoles and higher, which has been used to produce valid conceptual electromagnetic designs of the Future Circular Collider sextupole and octupole magnets, are also presented. Generic conceptual design studies performed with DASH and DASH2in1 (for nested corrector magnets configurations) are also shown and their outcome is discussed.

SuperKEKB is an asymmetric energy electron positrion collider currently under commissioning in Japan. It aims to achieve a record high luminosity of $8\times {10}^{35}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, for which accurate control of ${\beta }_y^{*}$ is needed. The advanced final focus system is also relevant for studies related to future linear colliders, which similarly need to achieve very small beam sizes. To work for SuperKEKB, the $K$ modulation technique is generalized to allow known quadrupole fields between the modulated magnets and the interaction point. Initial measurements taken in HER agree with simulations, and show that $K$ modulation is suitable for measuring the minimum of the $\beta$ function at the interaction point within 1%, however it is too uncertain to be used for measuring the displacement of the beam waist away from the interaction point. In addition, tune shift equations for a modulated quadrupole are derived without assuming a thin lens perturbation, giving a simple method to calculate the tune shift to second order in the quadrupole strength modulation.

The Low Energy RHIC electron Cooling (LEReC) project at Brookhaven National Laboratory recently demonstrated for the first time cooling of hadron bunches with radio-frequency (rf) accelerated electron bunches. LEReC uses a high-voltage photoemission electron gun with stringent requirements for beam current, beam quality, and stability. The electron gun has a photocathode with a high-power fiber laser, and a novel cathode production, transport, and exchange system. It has been demonstrated that the high-voltage photoemission gun can continually produce a high-current electron beam with a beam quality suitable for electron cooling. We describe the operational experience with the LEReC dc photoemission gun in RHIC and discuss the important aspects needed to achieve the required beam current, beam quality, and stability.

Standard quadrupole beam-based alignment (BBA) techniques rely on orbit data and on the sequential variation of quadrupole and orbit corrector magnets (OCM). This results in time-consuming measurements of the order of several hours in most circular accelerators. Fast (10 kHz) beam position monitors (BPM) and OCMs with ac power supplies are routinely used in modern synchrotron light sources to drive fast orbit feedback systems. In this paper we show how they can be employed also to dramatically reduce the time for any quadrupole BBA to several minutes only, ensuring the same level of accuracy and precision. Moreover, conversely to the standard BBA, the new procedure accounts automatically for any level of betatron coupling, BPM roll and OCM tilt. In the case of the ALBA 3rd generation light source, the time for a complete measurement dropped from 5 hours to 10 minutes, a reduction by a factor 30. As further extension of this novel approach, an even faster skew quadrupole BBA was demonstrated in ALBA for the first time, taking advantage of the additional ac modulation of the skew quadrupole field. Results from this fully ac measurement are compared with those obtained via a traditional dc scan of the skew quadrupole.

Self-seeding is one of the most promising methods to improve the longitudinal coherence and spectral purity of free-electron lasers (FELs). Measurements of the multishot-averaged, soft x-ray self-seeding spectrum at the Linac Coherent Light Source (LCLS) FEL often have a pedestal-like distribution around the seeded wavelength. In this paper, a theoretical model based on the mechanism of spectral sideband generation is developed to explain the statistical behavior of the seed and pedestal. The model is in good agreement with statistical analysis of numerical FEL simulations and experimental measurements that show that pedestal fluctuations reflect the level of long-wavelength microbunching structures in the beam, and are driven by fluctuations of the seed.

It is known that high quality proton beams can be produced in the radiation pressure acceleration (RPA) by using a circularly polarized ultraintense super-Gaussian laser. However, a transverse mismatching phenomenon between the laser intensity profile and the particle spatial distribution appears in the later stage of RPA, which leads to a decompression of proton beam and broadening of the energy spectrum. To weaken this effect, a new scheme with an additional plasma channel located behind a thin hydrogen foil is proposed. It is found that a good local matching can be maintained when the laser pulse propagates in the channel, which contributes to a stable RPA for a longer time. Two-dimensional particle-in-cell simulations show that the proton beam has a peak energy of 2.0 GeV and energy spread of 13.8% at $t=300\mathrm{fs}$. With further acceleration until $t=500\mathrm{fs}$, a better quality beam with about 40% increase in peak energy and 26.2% improvement in energy conversion efficiency for high-energy protons ($\ge 1.5\mathrm{GeV}$) can be obtained finally. Meanwhile, the energy spread drops from 100% to 28.5%. This work may provide a more promising way to generate the high quality proton beam.

The ions acceleration through an fs laser irradiating a reduced graphene oxide foil in a target-normal-sheath-acceleration regime is investigated using a SiC detector connected in time-of-flight configuration. The experimental data indicated maximum proton energy of 1.8 MeV and a large number of carbon ions accelerated at different energies depending on their charge state. Particle-in-cell (PIC) simulations were applied to the studied target given the electron density as a function of the space and time and permitted one to evaluate the electrical field developed in the rear side of the foil driving the forward ion acceleration. The simulation indicates that the carbon ions are subjected to a lower acceleration with respect to protons, due to their slow velocity, depending on the charge-to-mass ratio. The latter does not permit carbon ions to be affected by the maximum electric field but a lower intensity due to the fast time decay of the electric field. Considering the angular emission of protons and the six carbon ions, the charge particles assume a Boltzmann energy distribution with a fixed cutoff at high energy, in agreement with the experimental measurements of ion energy.