In this paper we present a novel approach to free electron laser (FEL) simulations based on the decomposition of the electromagnetic field in a finite number of radiation modes. The evolution of each mode amplitude is simply determined by energy conservation. The code is developed as an expansion of the general particle tracer framework and adds important capabilities to the suite of well-established numerical simulations already available to the FEL community. The approach is not based on the period average approximation and can handle long-wavelength waveguide FELs as it is possible to include the dispersion effects of the boundaries. Furthermore, it correctly simulates lower charge systems where both transverse and longitudinal space charge forces play a significant role in the dynamics. For free-space FEL interactions, a source dependent expansion approximation can be used to limit the number of transverse modes required to model the field profile and speed up the calculation of the system’s evolution. Three examples are studied in detail including a single pass FEL amplifier, the high efficiency TESSA266 scenario, and a THz waveguide FEL operating in the zero-slippage regime.

A novel mechanism for injection, emittance selection, and postacceleration for laser wakefield electron acceleration is identified and described. It is shown that a laser prepulse can create an ionized plasma filament through multiphoton ionization and this heats the electrons and ions, driving an ellipsoidal blast-wave aligned with the laser-axis. The subsequent high-intensity laser-pulse generates a plasma wakefield which, on entering the leading edge of the blast-wave structure, encounters a sharp reduction in electron density, causing density down-ramp electron injection. The injected electrons are accelerated to $\sim 2\mathrm{MeV}$ within the blast-wave. After the main laser-pulse has propagated past the blast-wave, it drives a secondary wakefield within the homogenous background plasma. On exiting the blast-wave structure, the preaccelerated electrons encounter these secondary wakefields, are retrapped, and accelerated to higher energies. Due to the longitudinal extent of the blast-wave, only those electrons with small transverse velocity are retrapped, leading to the potential for the generation of electron bunches with reduced transverse size and emittance.

In this work, we present a comprehensive theoretical description of the Lorentz force in dual-grating dielectric laser accelerators (DLA). Here we examine the dual-grating DLA under arbitrary illumination conditions, both single-side and dual-side drive. We improve upon previous descriptions of these forces by providing a unified description of all possible operating modes of these devices, and classify the modes into three categories. In particular, we predict the existence of the previously observed “line modes” and a novel “elliptical mode”, provide an experimental demonstration of this new mode, and suggest a possible application for it in improving the resolution of attosecond-scale streak cameras. Finally, we examine several complementary methods of tuning a dual-grating DLA to a desired operating mode—particularly dual-drive phase rotation and translational offset of the grating teeth.

Vacuum electrical breakdowns, also known as vacuum arcs, are a limiting factor in many devices that are based on application of high electric fields near their component surfaces. Understanding processes that lead to breakdown events may help mitigate their appearance and suggest ways for improving operational efficiency of power-consuming devices. Stability of surface performance at a given value of the electric field is affected by the conditioning state, i.e., how long the surface was exposed to this field. Hence, optimization of the surface conditioning procedure can significantly speed up the preparatory steps for high-voltage applications. In this article, we use pulsed dc systems to optimize the surface conditioning procedure of copper electrodes, focusing on the effects of voltage recovery after breakdowns and variable repetition rates as well as long waiting times between pulsing runs. Despite the differences in the experimental scales, ranging from ${10}^{-4}\mathrm{s}$ between pulses up to pulsing breaks of ${10}^5\mathrm{s}$, the experiments show that the longer the idle time between the pulses, the more probable it is that the next pulse produces a breakdown. We also notice that secondary breakdowns, i.e., those which correlate with the previous ones, take place mainly during the voltage recovery stage. We link these events with deposition of residual atoms from vacuum on the electrode surfaces. Minimizing the number of pauses during the voltage recovery stage reduces power losses due to secondary breakdown events, improving efficiency of the surface conditioning.

Generation of extremely bright, coherent synchrotron radiation in a short wavelength range is of remarkable interest in the synchrotron light source community. In this paper, a novel technique is adopted to produce coherent radiation, which uses an angular dispersion mechanism to enhance the microbunching of electrons by introducing a slight energy modulation in the long straight section of the storage ring. For this purpose, a lattice design of an extreme low emittance storage ring with 3.5 GeV energy is presented in this paper, which employs a higher-order achromat concept. We show the design results of the multibend achromat lattice with an emittance of 20 pm rad and a circumference of 900 m. Numerical simulation results demonstrate that this angular dispersion-induced microbunching scheme can be used to generate coherent radiation near the soft x-ray region, the repetition rate can reach 10 kHz, and the spectral brightness can reach $2.3\times {10}^{24}\mathrm{photons}{\mathrm{s}}^{-1}{\mathrm{mm}}^{-2}{\mathrm{mrad}}^{-2}$ ${(0.1\%\mathrm{bandwidth})}^{-1}$.

The technique of microbunched electron cooling (MBEC) is a coherent cooling scheme with possible applications in high-energy hadron and electron-ion machines. In our previous work we analyzed the cooling of the hadron energy spread and transverse emittance using a one-dimensional (1D) technique that tracked the microscopic fluctuations in the hadron and electron beams. However, in order to obtain analytical expressions for our key quantities, we limited ourselves to calculating and optimizing only the initial values of the cooling rates. In this paper, we extend our approach so that it properly addresses the issue of the long-term, dynamic evolution of the hadron beam. In order to do so, it becomes necessary to consider the synchrotron motion of the hadron beam, in conjunction with the effects of diffusion and intrabeam scattering (IBS). With these modifications, our formalism allows us to develop a simple numerical tool that can effectively model the final state of hadron beam after many passages through the MBEC cooler.

Protons and heavy-ion beams at unprecedented energies are brought into collisions in the CERN Large Hadron Collider (LHC) for high-energy experiments. The LHC multistage collimation system is designed to provide protection against regular and abnormal losses in order to reduce the risk of quenches of the superconducting magnets as well as keeping background in the experiments under control. Compared to protons, beam collimation in the heavy-ion runs is more challenging despite the lower stored beam energies, because the efficiency of cleaning with heavy ions has been observed to be 2 orders of magnitude worse. This is due to the differences in the interaction mechanisms between the beams and the collimators. Ion beams experience fragmentation and electromagnetic dissociation at the collimators that result in a substantial flux of off-rigidity particles that escape the collimation system. These out-scattered nuclei might be lost around the ring, eventually imposing a limit on the maximum achievable stored beam energy. The more stringent limit comes from potential quenches of superconducting magnets. Accurate simulation tools are crucial in order to understand and control these losses. A new simulation framework has been developed for heavy-ion collimation based on the coupling of the SixTrack tracking code, which has been extended to track arbitrary heavy-ion species, and the fluka Monte Carlo code that models the electromagnetic and nuclear interactions of the heavy ions with the nuclei of the collimator material. In this paper, the functionality of the new simulation tool is described. Furthermore, SixTrack-fluka coupling simulations are presented and compared with measurements done with ${{}^{208}\mathrm{Pb}}^{82+}$ ions in the LHC. The agreement between simulations and measurements is discussed and the results are used to understand and optimize losses. The simulation tool is also applied to predict the performance of the collimation system for the high-luminosity LHC. Based on the simulation results and the experience gained in past heavy-ion runs, some conclusions are presented.

Hybrid-implicit particle-in-cell (PIC) algorithms permit the simulation of complex problems involving both kinetic and fluid plasma regimes over large spatial and temporal scales. Fluid electrons can be computationally fast where and when fluid assumptions are valid. Additional flexibility is obtained if discrete PIC macroparticles, with velocities advanced by either fluid or kinetic equations, are permitted to dynamically migrate between the two descriptions based on phase space criteria. Ideally, these migrations result in energetic particles treated kinetically and dense thermal plasma particles as a fluid. With an energy-conserving particle advance, resolution of the plasma Debye length is not required for numerical accuracy or stability. For pulsed-power applications, the simulation time step is usually constrained by the electron cyclotron frequency, not the more restrictive plasma frequency. A new implicit technique permits accurate particle orbits even at highly underresolved cyclotron frequencies. Thus, greater temporal and spatial scales can be accurately modeled relative to conventional PIC techniques. In this paper, we describe the hybrid PIC technique and fully electromagnetic, hybrid simulations of plasma evolution and current shunting in an idealized accelerator designed for driving a $Z$-pinch load. The dynamics of electrode heating, electron transport, and surface contaminant evolution are studied in a series of relativistic hybrid-implicit PIC simulations. These dynamics can lead to the shunting of current before reaching the $Z$-pinch load, thus degrading load performance. Examining two previously published power flow problems, we compare results from fully kinetic, multifluid, and hybrid kinetic-fluid simulations and discuss the computational performance of these three options. The key thrust of the work is to identify possible computational acceleration, through hybrid methods, required for accelerator understanding and design.

We proposed and demonstrated a novel technique to measure time-resolved transverse emittances of the hydrogen ion (${\mathrm{H}}^{-}$) beam in a 1-GeV high-power accelerator. The measurement is performed in a nonintrusive manner by using a laser comb—laser pulses with controllable multilayer temporal structure. The technique has been applied to the transverse emittance measurement of a 1-GeV ${\mathrm{H}}^{-}$ beam in the Spallation Neutron Source high energy beam transport line. More than 20 time-resolved emittances have been simultaneously determined within a macropulse, a single minipulse, or a single bunch of the 1.4-MW neutron production ${\mathrm{H}}^{-}$ beam from a single measurement.

This paper describes a method for reducing the energy of the cyclotron motion for charged particles in coherent beams using a nonadiabatic magnetic field. For the purpose of reducing the cyclotron motion, a local magnetic field reduction of the main guiding field should be situated in the region of the descending phase of the beam oscillation. The required local magnetic field depression can be produced with a soft iron ring or with a magnet coil. The nonadiabatic element can be positioned on the descending part of any period of the beam oscillation where the beam still remains sufficiently coherent. The effect of reducing the cyclotron motion with such nonadiabatic magnetic field is independent of the electric field of the cathode-anode gap and seems to be a universal method for the cyclotron motion control in coherent beams. Therefore, it can be used for reducing the cyclotron motion of electron beams produced with different kinds of guns with different perveances and sizes. For instance, the method is capable of creating ripple-free, laminar beams even for magneto-immersed guns positioned in a magnetic field of only a few hundred Gs and with a cathode emission current density exceeding $30\mathrm{A}/{\mathrm{cm}}^2$. It can also be applied for guns producing tubular beams, as demonstrated by our simulations. The results of computer simulations are presented, which demonstrate the capability of effective cyclotron motion control with nonadiabatic magnetic field.

The multichannel switching mode of a controlled multigap switch is studied. The switch is designed for capacitive energy storage, with a charging voltage of up to 100 kV and an energy output time of about 100 ns. The interval between the high-voltage and low-voltage electrodes of this switch is divided into seven series-connected gaps using ball electrodes, and six parallel rows of these ball electrodes allow for multichannel switching. An optical system based on collimators and photodiodes was used to determine the number of channels in the switch gaps. Statistics on the number of channels were collected and the probability of ignition of parallel spark channels was calculated for each of the switch gaps under different operating conditions. We show that both the electrical isolation between the channels and the rate of rise of the triggering pulse significantly affect the number of parallel ignited channels and their distribution over consecutive gaps. At a high triggering voltage pulse rise rate ($\sim 800\mathrm{kV}/\mu \mathrm{s}$), a larger number of spark channels are ignited in the switch with electrical isolation between the channels and the characteristics of this switch are better. At a low triggering voltage pulse rise rate (less than $250\mathrm{kV}\mu \mathrm{s}$), better characteristics are realized in the switch without electrical isolation between the channels.

A semianalytical model has been developed to study a combined effect of beamstrahlung due to beam-beam interaction and beam coupling impedance in the future lepton colliders CEPC and FCC-ee. This model allows evaluating an impact of the coupling impedance on the bunch length and energy spread in collision. The model is benchmarked against numerical simulations. Analytical estimates for the supercolliders are presented.

At the Large Hadron Collider (LHC), the interplay between a series of effects, including intrabeam scattering (IBS), synchrotron radiation, longitudinal beam manipulations, two beam effects (beam-beam, e-cloud) and machine nonlinearities, can change the population of the core and tails and lead to non-Gaussian beam distributions, at different periods during the beam cycle. By employing generalized distribution functions, it can be demonstrated that the modified non-Gaussian beam profiles have an impact in the beam emittance evolution by itself and thereby to the collider luminosity. This paper focuses on the estimation of beam distribution modification and the resulting rms beam size due to the combined effect of IBS and synchrotron radiation by employing a Monte-Carlo simulation code which is able to track 3D generalized particle distributions (SIRE). The code is first benchmarked over classical semianalytical IBS theories and then compared with measurements from the LHC at injection and collision energies, including projections for the High-Luminosity LHC (HL-LHC) high brightness regime. The impact of the distribution shape on the evolution of the bunch characteristics and machine performance is finally addressed.

Optics measurements at the LHC are mainly based on turn-by-turn signal from hundreds of beam position monitors (BPMs). Faulty BPMs produce erroneous signal causing unreliable computation of optics functions. Therefore, detection of faulty BPMs prior to optics computation is crucial for adequate optics analysis. Most of the faults can be removed by applying traditional cleaning techniques. However, optics functions reconstructed from the cleaned turn-by-turn data systematically exhibit a few nonphysical values which indicate the presence of remaining faulty BPMs. A novel method based on the Isolation Forest algorithm has been developed and applied in LHC operation, allowing to significantly reduce the number of undetected faulty BPMs, thus improving the optics measurements. This report summarizes the operational results and discusses the evaluation of the developed method on simulations, including extensive studies and optimization of the preexisting cleaning technique and verification of a new method in terms of coupling measurement. The advantages of the chosen algorithm compared to some other unsupervised learning techniques are also discussed.

We present recent progress with our algorithm and its implementation called simpa described in a previous paper. The algorithm has a new and unique approach to long-term 4D tracking of charged particles in arbitrary static electromagnetic fields. Using the improvements described in this paper, we made frequency analysis and dynamic aperture studies in ELENA. The effect of the end fields and the perturbation introduced by the magnetic system of the electron cooler on dynamic aperture is shown. A special feature of this study is that we have not introduced any multipole errors into the model. The dynamic aperture calculated in this paper is the direct consequence of the geometry of the magnetic elements. Based on the results, we make a few suggestions to reduce the losses during the deceleration of the beam.

The building block of the linear transformer driver pulser, known as a brick, can be thought of as the smallest, independent unit from which the entire pulser is built. In a “standard” brick configuration, two capacitors and a spark-gap switch are connected in series and are oil insulated. Such a brick could be assembled, for example, from two 80-nF, 100-kV capacitors and a 200-kV spark-gap switch. A single brick in this configuration is capable of generating a current pulse with up to 50 kA of peak current and a rise time on the order of 100 ns, after the capacitors have been discharged at a combined output voltage of 200 kV (twice the bipolar charge voltage). By contrast, in a “dry” brick configuration, the two capacitors are placed in parallel and a multigap, multichannel “ball” switch follows them in series. The dry brick is epoxy insulated, and its slightly different configuration allows it to have a peak output current of up to 100 kA, after being discharged at a total output voltage of 100 kV (equal to the unipolar charge voltage). In this paper, we present how these two different brick topologies, either oil insulated (standard) or epoxy insulated (dry), affect a pulser’s output parameters, where the pulser has been constructed by connecting the outputs of $N$ bricks together in parallel. We find that in many cases, the use of a dry brick configuration results in more current being delivered to the load. We also find that if the time to peak current is not a critical parameter for the experiment, then the lower charging voltage across the switch and the absence of insulating oil make the dry brick an excellent alternative to the standard brick, especially for smaller research laboratories with limited maintenance staff.

Studies of possible energy upgrades of the Large Hadron Collider (LHC) aim at increasing the current nominal beam energy of 7 TeV in view of expanding the discovery potential and physics reach of the LHC. Some critical aspects of the feasibility of partial or full energy upgrades are studied here, together with novel mitigation measures. Higher beam energies can be realized by pushing the installed main dipoles to their limits or by replacing different fractions of installed magnets. Moreover, a revised lattice design for a full energy upgrade, also known as the high-energy LHC (HE-LHC), is presented. The studies reported focus on the linear optics design and on the lattice layout. In particular, an optics with 60° transverse phase advance in the regular arc cells, meant to alleviate the strength requirements for the main quadrupoles, has been designed in view of possible beam tests during the next LHC run 3. The methods developed in this article, for matching the ring geometry to a reference tunnel, for improving the beam aperture, and for minimizing the required field strengths for new high-energy LHC-like machines, will also be applicable to other future circular collider projects.

Energy-resolved secondary electron spectroscopy has been performed on air-exposed standard Cu samples and modified Cu surfaces that are tested and possibly applied to efficiently suppress electron cloud formation in the high-luminosity upgrade of the Large Hadron Collider at CERN. The Cu samples comprise pristine oxygen-free, carbon-coated and laser-structured surfaces, which were characterized prior to and after electron irradiation and rare-gas ion bombardment. Secondary-electron and reflected-electron yields measured with low charge dose of the samples exhibit a universal dependence on the energy of the primary impinging electrons. State-of-the-art models can successfully be used to describe the spectroscopic data. The supplied spectral dependence of electron emission and integrated electron yield as well as the derived parametrization can serve as a basis for forthcoming simulations of electron cloud formation and multipacting.