Coherent effects in the ionization loss of ultrarelativistic electron ensembles (bunches) are studied on the basis of perturbation theory of quantum mechanics. These effects originate from the interference of the electrons’ proper electric fields with each other. General expressions for the ionization loss in this case, as well as their applicability conditions, are derived. It is shown that for ensembles of sufficiently small spatial size, achievable in modern accelerator facilities, the discussed effects lead to an increase of the ionization loss by several orders of magnitude compared to the result predicted by the Bethe-Bloch formula. In this case, the effective mean ionization potentials of the atomic shells depend not only on the structure of the atomic levels, but on the ensemble form factors as well. The numerical calculations are made for hydrogen atoms, taking into account the density effect in the ionization loss. The ionization loss due to resonant excitation and ionization of atoms by ensembles with periodical modulation of the particle density (realized at free-electron lasers) is investigated.

We have established and also implemented a nondestructive online monitoring system for measuring the stripper foil degradation, such as foil thinning and pinhole outbreak, for the first time during beam operation in a high-intensity proton accelerator. We aimed to achieve a realistic and longer lifetime of a stripper foil by ensuring proper uses and determining its end of usefulness without any failure. A stripper foil is used for negative hydrogen (${\mathrm{H}}^{-}$) stripping to proton (p) for multiturn charge-exchange injection in high-intensity proton accelerators. A longer foil lifetime is expected, while foil failure during operation should be avoided, as it reduces the accelerator availability and also has serious issues for regular accelerator maintenance. A proper use of the foil should also be ensured to minimize the replacement of the foil magazine, as it involves unhealthy radiation exposure to the workers. We have measured the partially stripped ${\mathrm{H}}^0$ and unstripped ${\mathrm{H}}^{-}$ charge fractions of the injection ${\mathrm{H}}^{-}$ beam out of the stripper foil to understand details of foil degradation, such as foil thinning and pinhole formation due to high-intensity beam irradiation, which are believed to be foil breaking signals. We used two independent beam monitoring devices and precisely measured both ${\mathrm{H}}^0$ and unstripped ${\mathrm{H}}^{-}$ charge fractions by each monitor. As a result, we obtained a detail of foil degradation during operation to determine a realistic end of its usefulness by successfully achieving a record of nearly 2 yr of operation with a single foil without any failures. The detailed measurement result of the foil degradation also gives strong feedback for producing stronger and durable stripper foils. The present research was done by using simple and ordinary beam diagnostic devices including a nondestructive one, which can be easily applied to overcome the stripper foil issues in any similar existing and next-generation further higher-intensity accelerators.

We investigate the dynamics of electrons counterpropagating along a radially polarized optical Bessel beam (OBB). (i) It is shown that a significant fraction of the electrons can be transversally trapped by the OBB even in the case of “unmatched” injection. Moreover, (ii) these transversally trapped particles (TTPs) can be transported without loss along many thousands of wavelengths. As long as there is full longitudinal overlap between the electrons and laser pulse, this transport distance is limited only by the length of the OBB region. (iii) The unique profile of the transverse field components facilitates guiding either azimuthally symmetric pencil beams or annular beams. Space charge tends to totally suppress the annular beams, and it reduces the amount of charge trapped on axis for pencil beams. (iv) Assessment of the emittance of the TTPs alone reveals typical values of 10–50 pm. In fact, our simulations indicate if we trace the emittance of those particles that are trapped from the input to the output of the OBB, we find that this emittance is conserved. (v) We developed an analytic model whereby we average over the fast oscillation associated with the counterpropagating electrons and OBB. The resulting Hamiltonian has a Bessel potential $J_1^2(u)$, which, when operated in the linear regime near equilibrium, causes rotation of the phase space. A Kapchinskij-Vladimirskij beam-envelope equation is derived including space-charge and emittance effects. Relying on conservation of the longitudinal canonical momentum, the energy spread in the interaction region is determined in terms of the OBB intensity and the electron energy.

For more than a decade free-electron lasers (FELs) have been in operation, providing scientists from many disciplines with the benefits of ultrashort, nearly transversely coherent radiation pulses with wavelengths down to the Ångstrom range. If no further techniques are applied, the FEL will only amplify radiation from the stochastic distributed electron density in the electron bunch. Contemporary developments aim at producing stable and single-mode radiation by preparing an electron bunch with favorable longitudinal electron density distributions using magnets and conventional laser pulses (seed), hence the name “seeding.” In recent years, short wavelength FELs at high electron beam energies and high repetition rates were proposed and built. At those repetition rates, an external seed with sufficient power to manipulate the electron beam cannot be provided by present state-of-the-art laser systems, thus no external seeding scheme could be applied yet. In this paper, we present ways to seed FELs to generate short wavelength radiation at high repetition rates, making use of tested electron beam manipulation schemes. For our parameter study, we used the parameters of FLASH, the free-electron laser in Hamburg. First simulations are presented, showing the feasibility of the method proposed.

For future linear colliders, a nanometer-scale beam size at the interaction point (IP) is one of the most challenging technical aspects. To explore the feasibility of a final focus system with a high chromaticity level, comparable to that of the Compact Linear Collider, the ultralow ${\beta }^{*}$ optics has been proposed and tuned at the KEK Accelerator Test Facility 2. In this paper, the recent experimental results are presented, which demonstrate the capability of achieving and stabilizing a vertical average beam size of 60 nm and below at the virtual IP. The observed vertical beam size is about 20 nm above the numerical predictions in the presence of static and dynamic imperfections. We interpret this discrepancy as beam size growth due to multipole fields, beam jitters and wakefield effects, and diagnostic errors.

Reducing the intrinsic emittance of photocathodes is one of the most promising routes to improving the brightness of electron sources. However, when emittance growth occurs during beam transport (for example, due to space charge), it is possible that this emittance growth overwhelms the contribution of the photocathode, and, thus, in this case source emittance improvements are not beneficial. Using multiobjective genetic optimization, we investigate the role intrinsic emittance plays in determining the final emittance of several space-charge-dominated photoinjectors, including those for high-repetition-rate free electron lasers and ultrafast electron diffraction. We introduce a new metric to predict the scale of photocathode emittance improvements that remain beneficial and explain how additional tuning is required to take full advantage of new photocathode technologies. Additionally, we determine the scale of emittance growth due to point-to-point Coulomb interactions with a fast tree-based space-charge solver. Our results show that, in the realistic high-brightness photoinjector applications under study, the reduction of thermal emittance to values as low as $50\mathrm{pm}/\mu \mathrm{m}$ (1 meV mean transverse energy) remains a viable option for the improvement of beam brightness.

Recirculating energy recovery linacs are a promising technology for delivering high power particle beams ($\sim \mathrm{GW}$) while only requiring low power ($\sim \mathrm{kW}$) rf sources. This is achieved by decelerating the used bunches and using the energy they deposit in the accelerating structures to accelerate new bunches. We present studies of the impact of the bunch packet filling pattern on the performance of the accelerating rf system. We perform rf beam loading simulations under various noise levels and beam loading phases with different injection schemes. We also present a mathematical description of the rf system during the beam loading, which can identify optimal beam filling patterns under different conditions. The results of these studies have major implications for design constraints for future energy recovery linacs, by providing a quantitative metric for different machine designs and topologies.

The Landau damping mechanism stabilizes the impedance driven head-tail modes in hadron synchrotrons. Because of the frequent coherent beam instabilities observed in the Large Hadron Collider in 2012, the Landau damping of the beams was doubted. Therefore, a transverse beam transfer function (BTF) system was installed in order to probe the transverse Landau damping of the beams and compare the measurements with expectations. The results of the BTF measurements acquired during several experiments in the LHC are summarized including different machine configurations. The method developed for the data analysis is described, and the comparison between the models and the measurements is also presented.

We propose and we numerically design a working point for beam driven plasma wakefield acceleration that allows us to preserve the witness quality. This working point is a new scheme where a low density driver generates a wakefield in linear regime and the witness transverse evolution is dominated by the beam loading effect. We derived optimal matching conditions to prevent the phase space degradation by means of a transverse envelope equation.

In synchrotrons, nonuniform fill patterns, which give rise to beam phase transients and a spread in synchrotron tune between bunches, have been observed to damp longitudinal coupled-bunch instabilities driven by higher-order modes in rf cavities. The transients are especially large in the presence of Landau cavities, which are used commonly in storage-ring light sources and particularly in the new generation of diffraction-limited storage rings. A method has recently been devised to predict the beam transient including complex form factors for the different bunches. This has now been extended to accurately predict the growth-rates and oscillation frequencies of coupled-bunch modes for arbitrary fill patterns, taking the individual complex form factors and equilibrium phases of the different bunches into account. In this paper, the extended method is presented and the theory is outlined. For a case with significant transient beam loading, predictions of the resulting beam transient and bunch profiles are compared to measurements. Predictions of coupled-bunch mode behavior are then benchmarked against results from the macroparticle tracking code mbtrack with good agreement. Finally, the method is used to predict the behavior of coupled-bunch modes as a function of the fields in passive Landau cavities.

In the frame of the Tulip project, an application for accelerator technology to radiation therapy, supported by a CERN Knowledge Technology Fund, the design, manufacture and test of a compact, fast trimmable iron cobalt (FeCo) accelerator dipole magnet prototype has been performed and completed at CERN. The aim of this project is to illustrate the benefits of a FeCo magnet for medical gantries in which weight and size are critical. The prototype is a 250 mm long dipole providing a central field of up to 1.8 T, the field can be trimmed in the percent range at frequencies of up to 200 Hz. The experimental results clearly demonstrate that FeCo magnets can be interesting alternatives for medical gantries because of their linear behavior up to high fields and their compact nature.

Longitudinal bunch shaping is gaining increasing attention due to the large impact it has on modern accelerators. The existing methods for controlling the density profile fall into three categories: laser shaping, emittance exchange, or correlations between the energy and one of the other coordinates. These methods require either (i) dispersive elements (e.g., dipole magnet), which generate coherent synchrotron radiation, or (ii) shaping near the cathode, which can distort the profile due to space-charge effects. These collective effects significantly decrease shaping quality as the bunch charge increases. None of the existing methods are currently capable of shaping high-charge bunches. In this paper, we introduce a transverse deflecting cavity-based shaping method to avoid the impact of collective effects on the shaping process. In this method, a transverse deflecting cavity introduces the $z-x$ correlation, and a transverse mask tailors the longitudinal density profile. This method provides high-quality shaping regardless of charge level and preserves the beam quality.

Energy recovery linear accelerators (ERLs) rely upon single-axis superconducting radio frequency (SRF) cavities to be an efficient source of relativistic electrons for high energy and nuclear physics. SRF cavities are also considered relevant for next-generation photon factories and radio-isotope production facilities. The ultimate energy recovery capability for the accelerator would be the ability to operate with a sufficiently spent beam and decrease the energy of the beam before the beam dump to a value lower than the beam injection energy. This is especially important for high current accelerators, where the beam injection energy could be as high as several MeV, and systems where partial beam loss can be expected. The efficient operation of energy recovery in linear accelerators is adversely affected by the typical degradation of the beam quality and current loss. This hinders the application of the ERLs in research and industry, and enabling the use of spent (partially lost current), degraded beams should broaden their application. We suggest that the use of the asymmetric operating mode observed in the dual-axis asymmetric cavities will enable such ultimate capabilities. We discuss the advantages of the application of fields of different amplitude along the cavity accelerating and decelerating axes and demonstrate that the fields can be tuned separately in each axis of the dual-axis cavity. The design of such a cavity and ways to optimize the energy recovery of a spent electron beam by tuning the dual-axis asymmetric SRF cavity are discussed.

This paper presents a novel approach for constructing neural networks which model charged particle beam dynamics. In our approach, the Taylor maps arising in the representation of dynamics are mapped onto the weights of a polynomial neural network. The resulting network approximates the dynamical system with perfect accuracy prior to training and provides a possibility to tune the network weights on additional experimental data. We propose a symplectic regularization approach for such polynomial neural networks that always restricts the trained model to Hamiltonian systems and significantly improves the training procedure. The proposed networks can be used for beam dynamics simulations or for fine-tuning of beam optics models with experimental data. The structure of the network allows for the modeling of large accelerators with a large number of magnets. We demonstrate our approach on the examples of the existing PETRA III and the planned PETRA IV storage rings at DESY.

The extraction efficiency is one of the key parameters of an oscillator-type free electron laser (FEL). An extraction efficiency higher than 5% was observed with the midinfrared (MIR) FEL at Kyoto University using a dynamic cavity desynchronization method. The observed efficiency is the highest extraction efficiency realized in MIR-FEL oscillators driven by normal conducting linacs. A numerical simulation was performed, and the evolution of the electron energy distribution in the macropulse was compared with the experimental result. The details of the evolution of the electron energy distribution were investigated to reveal the physics of this high-efficiency lasing.

The study of collective effects in circular accelerators can be tackled by solving numerically the Vlasov equation or by using tracking codes. The two methods are obtained with different approaches: Vlasov solvers consider a continuous distribution function and describe the beam with coherent oscillation modes in frequency domain (ending up usually with an eigenvalue system to solve), while tracking codes use macroparticles and wakefields in time domain. In this paper we present two Vlasov solvers for the study of collective effects (from impedances/wakefields only) which evaluate the frequency shift of coherent oscillation modes and possible mode coupling instability in the single-bunch case for both longitudinal and transverse planes. In the longitudinal plane the Vlasov solver also takes into account the potential well distortion due to the wakefields under some conditions. In parallel to this theoretical approach, tracking codes, which include collective effects, have been used as benchmark. In particular, starting from their results, we also propose a new method to study the frequency shift of coherent modes and compare it with the output of the Vlasov solvers.

Closed orbit feedback (COFB) systems at synchrotrons used for the global orbit correction rely on the pseudoinversion of the orbit response matrix (ORM). A mismatch between the model ORM used in the controller and the actual machine ORM can affect the performance of the feedback system. In this paper, the typical sources of such model mismatch such as acceleration ramp ORM variation, intensity-dependent tune shift and beta beating are considered in comprehensive simulation studies. Their effect on the performance and the stability margins are investigated for both the slow and fast regimes of a COFB system operation. The spectral radius stability condition is utilized instead of the small gain theorem to arrive at the theoretical limits of COFB stability. Comparisons with simulations for SIS18 of GSI and experiments at the cooler synchrotron (COSY) in the Forschungzentrum Jülich (FZJ) are presented.