The dynamics of electrode heating, sheath flow, and contaminant plasma evolution in Sandia National Laboratories’ high-power $Z$ accelerator is studied in a series of 2D relativistic particle-in-cell simulations. These dynamics can lead to the shunting of current before reaching the $Z$ pinch load, thus degrading load performance. Previous work has focused on current diverted in the upstream magnetically insulated transmission lines (MITLs) and post-hole convolute regions of $Z$. In these regions, losses were found to scale strongly with load impedance as well as the system vacuum and were calculated to be as high as 1–2 MA. Downstream from the convolute region in $Z$, current measurement is problematic, leading to a lack of understanding of the loss mechanisms in the small radius ($<3\mathrm{cm}$) MITL feeding the load. In this paper, we present the first ever 2D fully electromagnetic, fully kinetic simulations of plasma evolution and current shunting in the inner MITL region of $Z$. This region is defined by a radially converging MITL, which is a feature common to MA-scale $Z$ pinch accelerators. The electrodes in this region are rapidly heated via mainly Ohmic or skin depth heating. Plasmas quickly form, and surface contaminants are liberated as the temperatures exceed 700 K. Instabilities lead to a rapid plasma density fill of the inner MITL and subsequent current loss. The instability growth is likely due to the resistivity of the magnetized electrode plasma. The plasma, after exceeding ${10}^{15}{\mathrm{cm}}^{-3}$ density, leads to an additional 1–2 MA current loss in the inner MITL region.

The measurement of the betatron tunes in a circular accelerator is of paramount importance due to their impact on beam dynamics. The resolution of the these measurements, when using turn-by-turn (TBT) data from beam position monitors, is greatly limited by the available number of turns in the signal. Because of decoherence from finite chromaticity and/or amplitude detuning, the transverse betatron oscillations appear to be damped in the TBT signal. On the other hand, an adequate number of samples is needed, if precise and accurate tune measurements are desired. In this paper, a method is presented that allows for very precise tune measurements within a very small number of turns. The theoretical foundation of this method is presented with results from numerical and tracking simulations and experimental TBT data which are recorded at electron and proton circular accelerators.

The design of a compact crab cavity using Nb/Cu technology is presented. The cavity shape is based on a ridged waveguide resonator with wide-open apertures to provide access to the inner surface of the cavity and facilitate coating. It also provides natural damping for higher-order modes (HOMs) and comparatively low longitudinal and transverse impedances. A first prototype is being fabricated, originally within the HL-LHC framework and now for the Future Circular Collider study. The optimized shape is characterized and compared with respect to peak surface field balance, multipolar momentum components, and longitudinal and transverse impedances. Further investigations include the identification of multipacting barriers, the frequency sensitivity to pressure fluctuations in the helium bath, the radio frequency (rf) design of a fundamental mode coupler, and considerations of HOM damping. Along these topics, several methods have been established serving a wider range of applications, in particular, for the rf loss evaluation, the postprocessing of multipacting simulations, and a detailed error analysis of the pressure sensitivity simulations.

Superconducting niobium film cavities deposited on copper substrates ($\mathrm{Nb}/\mathrm{Cu}$) have suffered from strong field-dependent surface resistance, often referred to as the $Q$-slope problem, since their invention. We argue that the $Q$-slope may not be an intrinsic problem, but rather originates from a combination of factors which can be revealed in appropriate environmental conditions. In this study, extrinsic effects were carefully minimized in a series of experiments on a seamless cavity. The origin of the $Q$-slope in low frequency cavities is traced back to two contributions with different temperature and magnetic field dependences. The first component of $Q$-slope, affecting the residual resistance, is caused by trapped magnetic flux which is normally suppressed by a magnetic shield for bulk niobium cavities. The second, temperature dependent component of $Q$-slope, is similar to the medium-field $Q$-slope which is well known in bulk niobium cavities. These results are compared with theoretical models and possible future studies are proposed.

A 100 TeV center-of-mass energy frontier proton collider, in a new tunnel of 80–100 km circumference, is a central part of CERN’s Future Circular Colliders (FCC) design study. The designs of the injection and extraction systems of the FCC are initially based upon the parameters of the injection and extraction systems of the Large Hadron Collider and a preliminary study of the FCC beam optics and lattice. The injection and, in particular, the extraction systems of the FCC have to be highly reliable. In order to achieve high reliability, solid state switches will be used for the generators of the injection and extraction systems. This paper discusses topologies of these kicker systems, which are presently under consideration.

Allison-type emittance scanners are widely used to measure projected 2D phase space distributions of low energy beams. This paper extends the conventional data analysis model to introduce three significant corrections that commonly arise in the pursuit of high resolution measurements. First, effective longitudinal asymmetry in the E-dipole placement (typically resulting from directional choice of relief cuts in thick slit-plates) causes deviation from the ideal voltage-to-angle conversion relation. Second, finite slit thickness generates variation in weights of data points that should be compensated. Third, when the interval between data points is smaller than the device resolution (ordinary in the angular data accumulation), a detailed account of the phase space region contributing to each data point can be used to resolve the beam distribution more accurately. These findings are illustrated by simulations with numerically generated phase space distributions. The improved model is applied to experimental measurements of an Ar ion beam with an Allison scanner operating at the front-end of the Facility for Rare Isotope Beams (FRIB) at Michigan State University. Results show that the improved model obtains better agreement among a set of measurements and modifies beam moments significantly (can be $\sim 10\%$ relative to conventional methods, with larger deviations at increasing angular divergence), thus rendering the corrections important for accurate high resolution phase-space characterizations. Python code tools that implement the improved analysis described are made available. These tools are readily applicable to any Allison scanner given a specification of the device geometry and scan ranges associated with each measurement.

A compact laser plasma accelerator (CLAPA) that can stably produce and transport proton ions with different energies less than 10 MeV, $<1\%$ energy spread, several to tens of pC charge, is demonstrated. The high current proton beam with continuous energy spectrum and a large divergence angle is generated by using a high contrast laser and micron thickness targets, which later is collected, analyzed and refocused by an image-relaying beam line using a combination of quadrupole and bending electromagnets. It eliminates the inherent defects of the laser-driven beams, realizes precise manipulation of the proton beams with reliability, availability, maintainability and inspectability (RAMI), and takes the first step towards applications of this new generation of accelerator. With the development of high-rep rate Petawatt (PW) laser technology, we can now envision a new generation of accelerator for many applications in the near future soon.

The usage of numerical models to study the evolution of particle beams is an essential step in the design process of particle accelerators. However, uncertainties of input quantities such as the beam energy and magnetic field lead to simulation results that do not fully agree with measurements; hence, the final machine will behave slightly differently than the simulations. In the case of cyclotrons, such discrepancies affect the overall turn pattern or may even alter the number of turns in the machine. Inaccuracies at the PSI Ring cyclotron facility that may harm the isochronism are compensated by additional magnetic fields provided by 18 trim coils. These are often absent from simulations or their implementation is very simplistic. In this paper, a newly developed realistic trim coil model within the particle accelerator framework OPAL is presented that was used to match the turn pattern of the PSI Ring cyclotron. Because of the high-dimensional search space consisting of 48 design variables (simulation input parameters) and 182 objectives (i.e., turns), the simulation and measurement cannot be matched in a straightforward manner. Instead, an evolutionary multiobjective optimization with a population size of more than 8000 individuals per generation together with a local search approach were applied that reduced the maximum absolute error to 4.5 mm over all 182 turns.

A hard x-ray self-seeding (HXRSS) setup will soon be available at the European X-ray Free Electron Laser (XFEL). The availability of high repetition rate x-ray pulses poses novel challenges in the setup development, compared to the choices made at other facilities, mainly crystal heat-load and radiation-damage issues. However, high-repetition rate is expected to allow for unprecedented output characteristics. A two-chicane HXRSS setup is found to be optimal for the European XFEL. In this paper we discuss the physical choices peculiar to that facility and simulations done, which allow us to fix the parameter for the setup design.

The recently constructed H4-VLE beam line, a tertiary extension branch of the existing H4 beam line in the CERN North Area, was commissioned in October 2018. The beam line was designed with the purpose of providing very low energy (VLE) hadrons and positrons to the NP-04 experiment, in the momentum range of $1–7\mathrm{GeV}/c$. The production of these low-energy particles is achieved with a mixed hadron (pions, kaons, protons), $80\mathrm{GeV}/c$ secondary beam impinging on a thick target. The H4-VLE beam line has been instrumented with prototype scintillating fiber detectors providing the beam profile, intensity, and time-of-flight measurement of the beam particles, that, together with Cherenkov threshold counters, permit an event-by-event particle identification over the entire momentum spectrum. In this paper, we present detailed results of the beam line performance and the measured beam composition, as well as the comparison of these measurements with simulations performed during the design phase using fluka and geant-4-based Monte-Carlo codes.

The application of the Chao-Raimondi emittance adapter to the SOLEIL storage ring is studied theoretically in terms of linear and nonlinear beam dynamics. This unique scheme recently proposed elsewhere using a pair of emittance adapters and a solenoid in between them allows a local conversion of an electron beam in a third generation light source to an ultralow emittance beam of the fourth generation. The study of potential benefits of the resultant synchrotron radiation is of crucial importance. Electron beam properties and magnetic fields inside the undulator solenoid are treated as inputs of the investigations. Characteristics of photon beams emitted by a round and small electron beam in an undulator are studied in detail on the basis of the near-field wave propagation method. The photon flux density gain, which is a figure of merit along with transverse coherence, is evaluated at the sample of two hard and soft x-ray beam lines existing at SOLEIL.

The surface resistance of copper samples with an amorphous carbon (a-C) coating or with laser surface structuring, the surface treatments of choice for electron cloud suppression in critical cryogenic sectors of the high-luminosity upgrade of the Large Hadron Collider (HL-LHC), has been measured for the first time at a cryogenic temperature using the quadrupole resonator at CERN. Three different frequencies of relevance for evaluating beam impedance effects, namely, 400, 800, and 1200 MHz, have been investigated. No significant increase in surface resistance is observed for the a-C coating, compared to plain copper. In the case of laser structuring, the surface resistance depends on the direction of the surface currents relative to the laser-engraved groove pattern. The increase is minimal for parallel patterns, but in the perpendicular case the surface resistance increases considerably. Radio frequency (rf) heating from wake losses would then also increase in the HL-LHC case; however, the reduction in the power deposited onto the cold surfaces thanks to electron cloud suppression would still outweigh this effect.

SuperKEKB is a very high luminosity $e^{+}{e}^{-}$ collider consisting of a low energy ring (LER) of positrons of 4 GeV and a high energy ring (HER) of electrons of 7 GeV. The high luminosity will be achieved thanks to the nanobeam scheme, where very strongly focused beams (${\sigma }_y=60\mathrm{nm}$) will collide at a large crossing angle of 83 mrad. To maintain high luminosity operation, fast luminosity monitoring is essential in the presence of dynamic instabilities, for feedback and optimization. Fast luminosity monitors consisting of a single crystalline diamond sensor (LAL), a Cherenkov and scintillator detector (KEK) were placed in both rings downstream of the IP (interaction point) to measure the signal from the radiative Bhabha process at zero photon scattering angle ($\sigma \approx 200\mathrm{mbarn}$). During single beam commissioning of SuperKEKB, the fast luminosity monitors were used as beam loss monitors to measure the single beam losses (beam-gas bremsstrahlung, Touschek scattering, Coulomb scattering). In this paper, a detailed description of the experimental setup, the data acquisition scheme, the data analysis and the comparison with the simulation for the diamond sensors, will be reported.

In the article of Migliorati, Aumon, Koukovini-Platia, Huschauer, Metral, Sterbini and Wang [Phys. Rev. Accel. Beams 21, 120101 (2018)], hereafter the Article and the Authors, the long-standing qualification of the PS instability as the transverse mode-coupling instability, or TMCI, is not mentioned, while a model of the beam breakup (BBU) is discussed. Do the observations and modeling of the Article refute the long-standing idea that the PS instability is the TMCI?

Studies of the crab cavities at KEKB revealed that the rf phase could shift by up to 50° within $\sim 50\mu \mathrm{s}$ during a quench; while the cavity voltage is still at approximately 75% of its nominal amplitude. If such a failure were to occur on the HL-LHC crab cavities, it is likely that the machine would sustain substantial damage to the beam line and surrounding infrastructure due to uncontrolled beam loss before the machine protection system could dump the beam. We have developed a low-level rf system model, including detuning mechanisms and beam loading, and use this to simulate the behavior of a crab cavity during a quench, modeling the low-level rf system, detuning mechanisms and beam loading. We supplement this with measurement data of the actual rf response of the proof of principle double-quarter wave crab cravity during a quench. Extrapolating these measurements to the HL-LHC, we show that Lorentz force detuning is the dominant effect leading to phase shifts in the crab cavity during quenches; rather than pressure detuning which is expected to be dominant for the KEKB crab cavities. The total frequency shift for the HL-LHC crab cavities during quenches is expected to be about 460 Hz, leading to a phase shift of no more than 3°. The results of the quench model are read into a particle tracking simulation, SixTrack, and used to determine the effect of quenches on the HL-LHC beam. The quench model has been benchmarked against the KEKB experimental measurements. In this paper we present the results of the simulations on a crab cavity failure for HL-LHC as well as for the SPS and show that beam loss is negligible when using a realistic low-level rf response.

The spin structure of the proton is one of the unsolved problems in physics. The Relativistic Heavy Ion Collider (RHIC) Spin Physics Program has been conducting research to investigate the spin structure by colliding high energy polarized proton beams. Maintaining high spin polarization while achieving high luminosity is critical for this program. Two of the challenges related to the program are to maintain spin polarization during spin flipping, a process designed to reduce systematic errors in spin asymmetry measurements, and during beam acceleration from the low energy source to the high energy collision experiment. Recently, a modification of the accelerator’s magnet configuration (optics) was developed for reducing the spin tune spread. With the new optics implemented, higher spin flipping efficiency was achieved at both injection and top beam energies. The spin tune spread was also minimized during beam acceleration and the transmission efficiency of spin polarization was evaluated. In this report the modifications needed for minimizing the spin tune spread and their impact on the global beam optics are described.