The Future Circular Collider for hadrons (FCC-hh) is the proposed high-energy frontier particle collider, which is expected to enable proton-proton collisions at a center-of-mass energy of 100 TeV. The specifications for the FCC-hh injection kicker system are very challenging. To provide fast magnetic field rise and fall times, and low ripple during the flat-top, ferrite loaded transmission line type magnets will be used. To limit the beam coupling impedance, a suitable beam screen will be placed in the aperture of each kicker magnet. Due to issues associated with the “conventional” beam screen design, used in the injection kickers magnets of the Large Hadron Collider (LHC), such as high voltage (HV) breakdowns and yoke heating problems, we have developed a novel concept of a helical beam screen. The fundamental advantage of the new design, in comparison to the conventional beam screen, is a significant reduction of the maximum voltage induced on the screen conductors, thus a greatly reduced probability of an electrical breakdown of the beam screen. In addition, the longitudinal beam coupling impedance of the helical beam screen is optimized to reduce power deposition in the kicker magnet. Furthermore, the helical beam screen allows a reduction of the maximum transverse beam coupling impedance of the kicker system. Detailed numerical simulations, theoretical studies and experimental results demonstrate that a new design is a promising solution not only for the FCC-hh, but also for existing machines.

It is well known that the tuning of a long radio-frequency quadrupole accelerator is demanding. This study investigated how to realize efficient long RFQ accelerators with multiple shorter and independent cavities. From the rf point of view, the use of shorter cavities has many advantages, e.g., enlarged mode separation and simplified rf tuning, but the beam matching between cavities will become an issue, especially at high current and low energy. Taking a more than 9 m long, high current machine as an example, this paper presents the design concepts and methods leading to good beam quality and high beam transmission throughout a multistage RFQ accelerator.

The use of optical transition radiation (OTR) monitors is a standard technique to measure transverse beam profiles at many electron accelerators. With modern accelerator technology it is possible to produce and accelerate even ultrashort electron bunches with sub–femtosecond duration. Such bunches interacting with the OTR target generate coherent optical transition radiation (COTR). For the COTR case, a reconstruction of the bunch profile from a recorded image using a conventional optical scheme is a task with inconclusive solution. In this paper an approach is proposed which is based on the strict propagation of COTR fields through a focusing lens. As result a linear dependence of the measured rms image size on the bunch size is obtained.

Halo diffusion measurements at the CERN Large Hadron Collider (LHC) were conducted with beams for physics at 6.5 TeV by means of collimator scans, carried out between 2016 and 2018. From the time evolution of the beam losses recorded during a collimator scan, in which collimator jaws are moved in steps toward the beam core cutting beam tails, one can extract information on the halo diffusion and its population as a function of the transverse amplitude. In this paper, results of the first scans performed at different beam intensities for both planes and both beams using the primary collimators of the betatron-cleaning system are presented. The scans were performed with squeezed optics and colliding beams after a few hours of regular physics production, during so-called end-of-fill measurements. Beam losses are measured with ionization chambers close to the collimators, which enable 1 and 100 Hz acquisitions, as well as diamond beam loss monitors, which enable turn-by-turn and bunch-by-bunch acquisitions. Parametric fits of a diffusion model are applied to the time profile of losses, for both total and individual bunch intensity. The analysis of the measurements performed in various conditions was used to estimate the diffusion coefficient as a function of the transverse amplitude and the population of LHC beam tails.

Incoherent Cherenkov diffraction radiation was recently produced in the Cornell electron storage ring using counterpropagating beams (electrons and positrons) passing in the close vicinity of a dielectric made of fused silica. We present in this paper a collection of the experimental investigations that were performed on Cherenkov diffraction radiation in both the infrared and the visible range. Measurements were performed using an optical system functioning either in imaging conditions or in far field conditions to retrieve the angular distribution of the radiation. Polarization studies were also performed and showed that, when selecting the appropriate polarization, the beam size can be measured accurately. This study opens the path for new applications in noninvasive beam diagnostic for highly relativistic charged particle beams.

The resonant properties of the impedance of a two-layer circular metal-dielectric waveguide are investigated by taking into account the finite conductivity of the metallic pipe and the dissipation of the radiated energy in the dielectric layer. The dispersion relations for azimuthally symmetric monopole TM modes are analyzed. It is shown that, for an outer metallic layer with finite conductivity and a thin internal dielectric cover, the waveguide has only a single slowly propagating ${\mathrm{TM}}_{01}$ mode. The point charge longitudinal wake potentials are evaluated for thin and thick internal layers.

In state of the art photoinjector electron sources, thermal emittance from photoemission dominates the final injector emittance. Therefore, low thermal emittance cathode developments and diagnostics are very important. Conventional thermal emittance measurements for the high gradient gun are time-consuming and thus thermal emittance is not measured as frequently as quantum efficiency during the lifetime of photocathodes, although both are important properties for the photoinjector optimizations. In this paper, a single shot measurement of photoemission transverse momentum, i.e., thermal emittance per rms laser spot size, is proposed for photocathode rf guns. By tuning the gun solenoid focusing, the electrons’ transverse momenta at the cathode are imaged to a downstream screen, which enables a single shot measurement of both the rms value and the detailed spectra of the photoelectrons’ transverse momenta. Both simulations and proof of principle experiments are reported.

Accurate measurements of optics functions, $\beta$, $\alpha$, D, are essential for proper operation of a synchrotron both for machine protection and for performance. $\beta$ functions can be obtained from different observables: phase and amplitude of the transverse betatron oscillations and change of the tune by modulating the current of quadrupoles (K-modulation). Reconstruction of $\beta$ function using the betatron phase, in combination with K-modulation, has been the main measurement approach in the LHC IRs. Nonetheless, challenges have appeared in the $\beta$ calculation using these two techniques when aiming for smaller ${\beta }^{*}$, i.e., $\beta$-values at the interaction point (IP) in the LHC and the HL-LHC. The third $\beta$ measurement technique, based on the beam position monitor (BPM) signal amplitude, has not been used as widely in the LHC as the other methods since it requires accurate BPM gain calibration. This paper presents the development of a technique based on optics measurements to calibrate accurately LHC IR BPMs, allowing to use of the amplitude information in the measurement of IR $\beta$ functions.

Results on fabrication and experimental characterization of wire scanners (WS) with submicrometer spatial resolution are presented. Independently fabricated at PSI and FERMI by means of nanolithography, the proposed WS solutions consist of 900 and 800 nm wide free-standing stripes ensuring a geometric resolution of about 250 nm. The nanofabricated WS were tested successfully at SwissFEL where low-charge electron beams with a vertical size of 400–500 nm were characterized. Further experimental tests at 200 pC confirmed the resilience to the heat-loading of the structures. With respect to conventional WS consisting of a metallic wire stretched onto a frame, the nanofabricated WS allow for an improvement of the spatial resolution from the micrometer to the submicrometer level as well as of beam invasiveness thanks to an equivalent reduction of the impact surface of the scanning stripe. The present work represents an important milestone toward the goal—that PSI and FERMI are pursuing—to realize a standard WS solution with minimal invasiveness and submicrometer resolution.

EuroCirCol is a conceptual design study of a post-LHC, Future Circular Hadron Collider (FCC-hh) with 50 TeV of beam energy and 100 km long, which aims to expand the current energy and luminosity frontiers. The vacuum chamber of the FCC-hh will have to cope with unprecedented levels of synchrotron radiation power for proton colliders, dealing simultaneously with a tighter magnet aperture. Considering that the high radiation power and photon flux will release larger amounts of gas into the system, the difficulty to meet the vacuum specifications increases substantially compared with the LHC. This paper presents a study on the beam induced vacuum effects for the FCC-hh novel conditions, the different phenomena which, owing to the presence of the beam, have an impact on the accelerator’s vacuum level. It is concluded that thanks to the adopted mitigation measures the proposed vacuum system shall be adequate, allowing us to reach $\le 1\times {10}^{15}{\mathrm{H}}_{2eq}/{\mathrm{m}}^3$ with baseline beam parameters within the first months of conditioning.

The free-electron laser process extracts energy from a relativistic electron beam to create high-power, coherent x-ray radiation. The energy loss leaves an imprint of the radiation on the longitudinal energy-time phase-space of the electron beam. At the Linac Coherent Light Source, the X-band transverse deflecting mode cavity measures the longitudinal phase-space, and an x-ray temporal power profile is predicted according to time slices difference between lasing-off and lasing-on measurements. However, the algorithm cannot include physical effects such as slippage in deep saturation and introduces errors from mismatches between the lasing-off and lasing-on measurements. Instead, considering the two-dimensional phase-space as a spatial image, we use a computer vision algorithm to predict the power profile. We simulate thousands of pairs of electron beams and x-ray power profiles to train convolutional neural networks, and test models on both simulated and experimental data. We demonstrate significant improvement compared with the traditional algorithm for a range of conditions.

The Facility for Rare Isotope Beams driver linac was designed for acceleration of multiple-charge-state beams after the stripping at ion beam energies of 17 to $20\mathrm{MeV}\mathrm{/u}$ depending on the ion species. The linac includes a 180° achromatic bend to select multiple charge states for further acceleration and to dump unwanted charge states after the first 45° magnet. Two rf rebunchers between the stripper and the linac segment 2 (LS2) are required to minimize the effective emittance growth of the multiple-charge-state beams and provide matching to the LS2. Recent studies have shown that the best choice for these two rebunchers is a room-temperature (RT) accelerating structure capable of providing robust operation in the presence of the stripped heavy ion beam contaminants. The latter can result in uncontrolled losses of contaminant ions after the passage off the stripper. Therefore, using a superconducting (SC) rebuncher after the stripper is not rational due to possible contamination and performance degradation of SC cavities over a long period of operation. For the beam bunching in the energy range from 13 to $22\mathrm{MeV}\mathrm{/u}$, two 161 MHz rebunchers based on interdigital H-type (IH) structure were developed, built, installed and commissioned with beam at the FRIB linac. This is the first experimental demonstration for application of a cw RT drift tube accelerating structure in this energy range.

High-fidelity physics simulations are powerful tools in the design and optimization of charged particle accelerators. However, the computational burden of these simulations often limits their use in practice for design optimization and experiment planning. It also precludes their use as on-line models tied directly to accelerator operation. We introduce an approach based on machine learning to create nonlinear, fast-executing surrogate models that are informed by a sparse sampling of the physics simulation. The models are $\mathcal{O}({10}^6)–\mathcal{O}({10}^7)$ times more computationally efficient to execute. We also demonstrate that these models can be reliably used with multiobjective optimization to obtain orders-of-magnitude speedup in initial design studies and experiment planning. For example, we required 132 times fewer simulation evaluations to obtain an equivalent solution for our main test case, and initial studies suggest that between 330–550 times fewer simulation evaluations are needed when using an iterative retraining process. Our approach enables new ways for high-fidelity particle accelerator simulations to be used, at comparatively little computational cost.

As the beams at the interaction points (IPs) of circular colliders are pushed toward smaller sizes, the correction of the magnetic field errors in high-$\beta$ regions become increasingly important, but also challenging. This paper presents an algorithm developed to compute automatically local corrections. This algorithm has been successfully used in the LHC and in simulations of the HL-LHC to establish tolerances for the magnetic errors. The limitations of the current ${\beta }^{*}$ measurement technique (K-modulation) are studied, together with alternative techniques for ${\beta }^{*}$ control: computing the minimum $\beta$ near the IP using the betatron phase measured with new instrumentation and locating the beam waist via luminosity scans. This push toward smaller beam sizes also requires large $\beta$-functions in the arcs that enhance local errors currently negligible. Experimental results of a way of correcting this new type of local errors using orbit bumps in sextupoles is also presented. These studies forecast a drastic change in the LHC commissioning strategy to be applied in the HL-LHC for needing luminosity measurements in intermediate stages.

The linac of the European project Extreme Light Infrastructure-Nuclear Physics (ELI-NP) foresees the use of 12 traveling wave $C$-band accelerating structures. The cavities are 1.8 m long, quasiconstant gradient, and have a field phase advance per cell of $2\pi /3$. They operate at 100 Hz repetition rate, and, because of the multibunch operation, they have been designed with a dipole higher-order mode (HOM) damping system to avoid beam breakup. The structures have symmetric input and output couplers and integrate, in each cell, a damping system based on silicon carbide (SiC) rf absorbers coupled to each cell through waveguides. An optimization of the electromagnetic and mechanical design has been done to simplify the fabrication and to reduce the costs. The cavities have been fabricated, and the first full-scale prototype has been also successfully tested at the nominal gradient of $33\mathrm{MV}\mathrm{/m}$, repetition rate of 100 Hz, and pulse length of 820 ns. It represents, to our knowledge, the first full-scale linac structure with HOM damping waveguides and SiC absorbers tested at this high gradient. In the paper, we illustrate the realization process of such a complicated device together with the low and high power test results.

We consider a 3D electron bunch at the cathode, modelled via photoemission, with realistic (asymmetric) spatial and temporal distributions for accelerator beam dynamics studies at the European X-ray Free-Electron Laser (European XFEL). A series of measurements are performed for low energy beams in the European XFEL injector. Using modeled 3D electron bunches, beam dynamics simulations, enabled by a three-dimensional (3D) space-charge solver with valid image-charge calculation on the photocathode plane, have shown improved agreements on measured beam properties (e.g., charge, bunch length, and shape) over a large range of variable machine parameters (e.g., cathode drive laser pulse energy, rf gun phase) for injector operation. In addition, the beam dynamics close to the cathode in a strongly space-charge dominated regime is studied in the rf gun. In light of the choice of a suitable machine working point along a so-called emission curve, the impact of the space-charge effect on the longitudinal length and shape of the produced electron bunch is analyzed in comparison to the measurements downstream the injector exit.

The compact linear accelerator for research and applications (CLARA) is an ultrabright electron beam test facility being developed at STFC Daresbury Laboratory. The ultimate aim of CLARA is to test advanced free electron laser (FEL) schemes that can later be implemented on existing and future short-wavelength FELs. In addition, CLARA is a unique facility to provide a high-quality electron beam to test novel concepts and ideas in a wide range of disciplines and to function as a technology demonstrator for a future United Kingdom x-ray FEL facility. CLARA is being built in three phases; the first phase, or front end (FE), comprises an S-band rf photoinjector, a linac, and an S-bend merging with the existing versatile electron linear accelerator beam line; the second phase will complete the acceleration to full beam energy of 250 MeV and also incorporate a separate beam line for use of electrons at 250 MeV; and the third phase will include the FEL section. The CLARA FE was commissioned during 2018, and the facility was later made available for user experiments. Significant advancements have been made in developing high-level software and a simulation framework for start-to-end simulations. The high-level software has been successfully used for unmanned rf conditioning and for characterization of the electron beam. This paper describes the design of the CLARA FE, performance of technical systems, high-level software developments, preliminary results of measured beam parameters, and plans for improvements and upgrades.

Linear transformer drivers can be used as fast pulsed-power generators to produce and study matter under extreme conditions, with densities larger than 10 times that of solids. While its scientific investigations can start in a laboratory, it will eventually require a source of x rays that can be provided only by specialized light source facilities. When relocation becomes a necessity, a compact, modular design is clearly advantageous. This paper shows how nested transmission lines can reduce considerably the footprint of MA-class pulsed-power generators based on linear transformer driver technology.

We present measurements of two-color x-ray free electron laser (FEL) pulses generated with a novel scheme utilizing a sextupole magnet. The sextupole, in combination with a standard orbit control tool, is used to suppress the radiation from the bunch core, while keeping the head and the tail of the beam lasing, each at a different photon energy. The method is simple, cost-effective and applicable at any repetition rate. We demonstrate the tunability of the scheme and discuss its advantages and practical limitations.

The High-Luminosity LHC Project aims to increase the integrated luminosity that will be collected by the Large Hadron Collider for the needs of the high energy physics frontier by the end of its Run 3 by more than a factor ten. This will require doubling the beam intensity, and in order to ensure coherent stability until the brighter beams are put in collision, the transverse impedance of the machine has to be reduced. As the major portion of the ring impedance is generated by its collimation system, several low resistivity jaw materials have been considered to lower the collimator impedance and a special collimator has been built and installed in the machine to study their effect. In order to assess the performance of each material we performed a series of tune shift measurements with LHC beams. The results show a significant reduction of the resistive wall tune shift with novel materials, in good agreement with the impedance model and the bench impedance and resistivity measurements. The largest improvement is obtained with a molybdenum coating of a molybdenum-graphite jaw. This coating, applied to the most critical collimators, is estimated to lower the machine impedance by up to 30% and the stabilizing Landau octupole threshold by up to 240 A after accounting for uncertainties of the model and other destabilising effects. A half of the overall improvement can be obtained by coating the jaws of a subset of 4 out of 11 collimators identified as the highest contributors to machine impedance. This subset of low-impedance collimators is being installed during the Long Shutdown 2 in 2019-2020.