In this paper we describe a new microbunching instability occurring in charged particle beams propagating along a straight trajectory. The nature of these exponentially growing plasma oscillations gave the reason for its name: plasma-cascade instability. Such instability can strongly amplify longitudinal microbunching originating from the beam’s shot noise, even to the point of saturation. Resulting random density and energy microstructures can drastically reduce beam quality. Conversely, such instability can drive novel high-power sources of broadband radiation or can be used as a broadband amplifier. We discovered this phenomenon in a search for such amplifier in the coherent electron cooling scheme [Phys. Rev. Lett. 102, 114801 (2009)] without separation of electron and hadron beams. In this paper we present a brief analytical theory of this new phenomenon, detailed numerical studies, the results of experimental demonstration as well as control of the longitudinal plasma-cascade instability.

In this paper we compare two concepts of a preaccelerator for the energy-recovery linac MESA: a hybrid injector that combines normal conducting and superconducting technology and a fully normal conducting injector. The particle source delivers polarized electrons at 100 keV. Thus the first accelerating section has to be of a graded-$\beta$ kind. The graded-$\beta$ section is designed with a final energy exceeding 1 MeV so TESLA-type nine-cell resonators can be used as superconducting cavities. The final energy of the preaccelerator shall be 5 MeV to avoid neutron production in the beam dump of the energy-recovery linac.

Reaching high energies with a plasma-wakefield accelerator may require use of multiple stages. This coupling of beams from one stage to another can be nontrivial due to a combination of high divergence and non-negligible energy spreads. In addition, a number of other requirements must be met, such as space-efficient in- and out-coupling of the driver, matching of beta functions, dispersion cancellation, and ensuring synchronization and alignment to within extremely tight tolerances. Satisfying all these requirements takes up space between the stages, which can significantly reduce the effective gradient of the accelerator. We review the challenges, the technical requirements, and some currently proposed methods for solving (or circumventing) the staging problem.

Impedance-driven (but not only) coherent beam instabilities are usually studied analytically with the linearized Vlasov equation, ending up with an eigenvalue system to solve. The eigenvalues describe the beam oscillation mode-frequency shifts, leading in particular to intensity thresholds defined by the longitudinal mode coupling instability in the longitudinal plane and by the transverse mode coupling instability in the transverse plane in the absence of chromaticity. This can be directly compared to measurements in particular for the lowest modes and in the absence of tune spread. In the presence of nonlinearities or when higher-order modes are involved, this becomes quite difficult, if not impossible, and the coupling between the modes cannot be directly measured (or simulated) anymore. Another important observable is the intrabunch motion, which can be also accessed analytically thanks to the eigenvectors. To the author’s knowledge, until now, the intrabunch signal has only been explained theoretically for independent longitudinal or transverse beam oscillation modes, i.e., when the bunch intensity is sufficiently low compared to the mode coupling threshold. It was never explained theoretically in detail when two (or more) modes are involved. For instance, no answers were already given to these questions: is (are) there some fixed point(s) when the transverse mode coupling instability starts? If yes, where is it (are they)? And what happens in the presence of mode decoupling? Any number of modes can be treated with the general approach discussed in this paper, which is based on the galactic Vlasov solver (which was previously successfully benchmarked against the pyheadtail macroparticle tracking code as concerns the beam oscillation mode-frequency shifts). However, to be able to clearly see what happens when the bunch intensity is increased, the simple case of two modes is discussed in detail. The purpose of this paper is to describe the different regimes, below, at, above the transverse mode coupling instability and also after the mode decoupling (as it happens sometimes), using a simple analytical model (where two modes are considered together), which helps to really understand what happens at each step. Better characterizing an instability is the first step before trying to find appropriate mitigation measures and push the performance of a particle accelerator. The evolution of the intrabunch motion with intensity is a fundamental observable with high-intensity high-brightness beams.

Novel schemes for generating ultralow emittance electron beams have been developed in past years and promise compact particle sources with excellent beam quality suitable for future high-energy physics experiments and free-electron lasers. Current methods for the characterization of low emittance electron beams such as pepperpot measurements or beam focus scanning are limited in their capability to resolve emittances in the sub 0.1 mm mrad regime. Here we propose a novel, highly sensitive method for the single shot characterization of the beam waist and emittance using interfering laser beams. In this scheme, two laser pulses are focused under an angle creating a gratinglike interference pattern. When the electron beam interacts with the structured laser field, the phase space of the electron beam becomes modulated by the laser ponderomotive force and results in a modulated beam profile after further electron beam phase advance, which allows for the characterization of ultralow emittance beams. 2D PIC simulations show the effectiveness of the technique for normalized emittances in the range of ${\epsilon }_n=[0.01,1]\mathrm{mm}\mathrm{mrad}$.

Undulator radiation from synchrotron light sources must be transported down a beam line from the source to the sample. A partially coherent photon beam may be represented in phase space using a Wigner function, and its transport may use some similar techniques that are familiar in particle beam transport. We describe this process in the case that the beam line is composed of linear focusing and defocusing sections as well as apertures. We present a compact representation of the beam line map involving linear transformations and convolutions. We create a $1:1$ imaging system with a single slit on the image plane and observe the radiation downstream to it. We propagate a Gaussian beam and undulator radiation down this sample beam line, drawing parameters from current and future ultra low emittance light sources. We derive an analytic expression for the partially coherent Gaussian case including passage through a single slit aperture. We benchmark the Wigner function calculation against the analytical expression and a partially coherent calculation in the synchrotron radiation workshop (SRW) code.

We introduce a diagnostic for measuring the transverse phase space (TPS) and transverse second moments of a particle beam in a single shot. The diagnostic consists of a slit followed by a simple projection beam line to project the TPS onto an imaging screen. The beam line is composed of a skew quadrupole magnet followed by a normal quadrupole magnet with an imaging screen at the end. The skew quadrupole generates a coupling between the initial $x$ and the final $y$, while the normal quadrupole removes the correlation between the initial $x$ and the initial $x^{\prime }$. Thus, the initial $(x,x^{\prime })$ is mapped onto the final $(x,y)$. The slit is needed before the projection beam line to remove the initial $(y,y^{\prime })$ contribution from the final $(x,y)$ image on the screen. We present simulation studies and a demonstration of the diagnostic with an experiment performed at Argonne Wakefield Accelerator facility. We also discuss the method’s limitations.

Modern synchrotron light source storage rings, such as the Swiss Light Source upgrade (SLS 2.0), use multibend achromats in their arc segments to achieve unprecedented brilliance. This performance comes at the cost of increased focusing requirements, which in turn require stronger sextupole and higher-order multipole fields for compensation of their effects on particles with energy deviation and lead to a considerable decrease in the dynamic aperture and/or energy acceptance. In this paper, to increase these two quantities, a multiobjective genetic algorithm (MOGA) is combined with a modified version of the well-known tracking code tracy. As a first approach, a massively parallel implementation of a MOGA is used. Compared to a manually obtained solution this approach yields very good results. However, it requires a long computation time. As a second approach, a surrogate model based on artificial neural networks is used in the optimization. This improves the computation time, but the quality of the results deteriorates beyond that of the manually obtained solution. As a third approach, the surrogate model is retrained during the optimization. This ensures a solution quality comparable to the one obtained with the first approach while also providing an order of magnitude speedup. Finally, good candidate solutions for SLS 2.0 are shown and further analyzed.

A new method to generate short wavelength free-electron laser output with modulated polarization at attosecond timescales is presented. Simulations demonstrate polarization switching timescales that are four orders of magnitude faster than the current state of the art and, at x-ray wavelengths, approaching the atomic unit of time of approximately 24 attoseconds. Such polarization control has significant potential in the study of ultrafast atomic and molecular processes. The output alternates between either orthogonal linear or circularly polarized light without the need for any polarizing optical elements. This facilitates operation at the high brightness x-ray wavelengths associated with FELs. As the method uses an afterburner configuration it would be relatively easy to install at existing FEL facilities, greatly expanding their research capability.

The two-temperature relativistic electron spectrum from a low-density ($3\times {10}^{17}{\mathrm{cm}}^{-3}$) self-modulated laser wakefield accelerator (SM-LWFA) is observed to transition between temperatures of $19\pm 0.65$ and $46\pm 2.45\mathrm{MeV}$ at an electron energy of about 100 MeV. When the electrons are dispersed orthogonally to the laser polarization, their spectrum above 60 MeV shows a forking structure characteristic of direct laser acceleration (DLA). Both the two-temperature distribution and the forking structure are reproduced in a quasi-3D osiris simulation of the interaction of the 1-ps, moderate-amplitude ($a_0=2.7$) laser pulse with the low-density plasma. Particle tracking shows that while the SM-LWFA mechanism dominates below 40 MeV, the highest-energy ($>60\mathrm{MeV}$) electrons gain most of their energy through DLA. By separating the simulated electric fields into modes, the DLA-dominated electrons are shown to lose significant energy to the longitudinal laser field from the tight focusing geometry, resulting in a more accurate measure of net DLA energy gain than previously possible.

Recently, the first direct measurement of a full 6D accelerator beam distribution was reported [B. Cathey, S. Cousineau, A. Aleksandrov, and A. Zhukov, Phys. Rev. Lett. 121, 064804 (2018)]. That work observed a correlation between energy and transverse coordinates, for which the energy distribution becomes hollowed near the transverse core. This hollowing is obscured when the 6D phase space is projected onto one- and two-dimensional axes. This article illustrates how a similar structure emerges from simulation of an initially uncorrelated, high density bunched beam as the result of velocity perturbation from initial nonlinear space charge forces. This phenomenon has not been widely recognized in accelerator systems, but parallels can be drawn to observations of laser-ionized nanoclusters and electron sources for diffraction. While this effect provides insight into the origin of the measured core correlation, it does not provide a complete description. A better reproduction of the measured structure can be obtained via self-consistent simulation through the radio-frequency quadrupole.

Two energy-varying beam extraction schemes for energy sweeping compact rapid cycling therapy are discussed. One is one-turn (fast) extraction of a ${\mathrm{C}}^{6+}$ bunch from a fast-cycling hadron driver accelerator assisted by an off-momentum bump orbit, in which a combination of a kicker and a septum magnet are fired at a desired timing (or energy). The other is energy sweeping extraction, in which a ${\mathrm{C}}^{5+}$ bunch delivered from the laser ablation ion source is injected and accelerated in the barrier bucket up to the final energy. A fraction of the trapped ${\mathrm{C}}^{5+}$ ions is continuously released from the bucket on the way to the final energy to form a coasting beam over an expected time (or energy) region, and the beam drifts inward in the large flat momentum dispersion region. The drifting ${\mathrm{C}}^{5+}$ beam hits the stripper carbon foil located at the front of the bending magnet and near the inner surface of the vacuum chamber in the large flat dispersion region and is converted to a ${\mathrm{C}}^{6+}$ beam. The combination of the enhanced bending radius and septum magnet allows easy ${\mathrm{C}}^{6+}$ beam extraction by the single septum magnet downstream.

Plasma wakefield dynamics over timescales up to 800 ps, approximately 100 plasma periods, are studied experimentally at the Advanced Wakefield Experiment (AWAKE). The development of the longitudinal wakefield amplitude driven by a self-modulated proton bunch is measured using the external injection of witness electrons that sample the fields. In simulation, resonant excitation of the wakefield causes plasma electron trajectory crossing, resulting in the development of a potential outside the plasma boundary as electrons are transversely ejected. Trends consistent with the presence of this potential are experimentally measured and their dependence on wakefield amplitude are studied via seed laser timing scans and electron injection delay scans.

The ATF2 beam line at KEK was built to validate the operating principle of a novel final-focus scheme devised to demagnify high-energy beams in future linear lepton colliders; to date vertical beam sizes as small as 41 nm have been demonstrated. However, this could only be achieved with an electron bunch intensity $\sim 10\%$ of nominal, and it has been found that wakefield effects limit the beam size for bunch charges approaching the design value of ${10}^{10}{e}^{-}$. We present studies of the impact of wakefields on the production of “nanobeams” at the ATF2. Wake potentials were evaluated for the ATF2 beam line elements and incorporated into a realistic transport simulation of the beam. The effects of both static (component misalignments and rolls, magnet strength errors and beam position monitor resolution) and dynamic (position and angle jitter) imperfections were included and their effects on the beam size evaluated. Mitigation techniques were developed and applied, including orbit correction, dispersion-free steering, wakefield-free steering, and interaction point tuning knobs. Explicit correction knobs to compensate for wakefield effects were studied and applied, and found to significantly decrease the intensity dependence of the beam size.

High-frequency linear accelerators are very suitable for carbon ion therapy, thanks to the reduced operational costs and the high beam quality with respect to synchrotrons, which are presently the only available technology for this application. In the framework of the development of a new linac for carbon ion therapy, this article describes the design of a compact 750 MHz radio frequency quadrupole (RFQ) with trapezoidal vanes. A new semianalytic approach to design the trapezoidal-vane RFQ is introduced together with the relevant beam dynamics properties. The RFQ is split into two decoupled rf cavities, both of which make use of a novel dipole detuning technique by means of length adjustment. The splitting is described both from the rf and the beam dynamics point of view. The paper concludes with the rf design of the full structure, including maximum surface field and thermal studies.

In order to minimize the surface resistance in superconducting cavities, a deeper understanding of residual resistance due to trapped magnetic flux is necessary. For that purpose, a combined temperature and magnetic field mapping system is employed to map magnetic flux trapped in a superconducting cavity, and the related increase in surface resistance. By cooling down a 1.3 GHz TESLA single cell cavity several times with externally applied static magnetic fields with different orientations with respect to the cavity, a statement can be made about how the angle between the applied magnetic field and the cavity’s surface affects flux trapping, and surface resistance. For example, a significantly higher increase in surface resistance is observed when the applied magnetic field is perpendicular to the cavity’s surface compared to when it is parallel.

A destabilizing effect of the detuning impedance has been recently observed in simulations of the CERN Proton Synchrotron (PS) at the injection energy: while without the detuning impedance the instability is faster in the vertical plane as expected (due to the elliptical shape of the vacuum chamber), with detuning impedance the instability appears to be faster in the horizontal plane. In order to understand the detuning impedance destabilizing effect, we study the collective behavior for the simpler case of a coasting beam with PS-like parameters and a simplified impedance model. The analysis, carried out from both numerical and theoretical points of view, highlights a new destabilizing mechanism related to the coupling of slow and fast waves.

High intensity beams may suffer from serious beam quality degradation if the focusing scheme allows for occurrence of resonances or instabilities. For transverse focusing a commonly accepted and respected lattice design rule is to choose the phase advance per structure period below the 90° resonant stop band, which was implicitly applied to the longitudinal phase advance the same way. A recent study pointed out that for lattice structures with more than one rf gap per period the 90° restriction needs not to be applied the same way to the longitudinal focusing as to the transverse one [I. Hofmann and Oliver Boine-Frankenheim, Phys. Rev. Lett. 118, 114803 (2017)], thus offering more design flexibility. The present paper is motivated by an interest to accelerate intense proton beams above longitudinal 90° in the new poststripper drift tube linac of the GSI universal linear accelerator by using the rf power supplies optimized for the much stiffer heavy ions. We confirm that a strictly periodic prolongation of the first cavity could allow acceleration above longitudinal 90° with only minor beam quality degradation. However, the combination of a high longitudinal phase advance with intercavity sections breaking the periodicity shows that matching challenges—rather than resonances—determine emittance growth. In the present case it is found tolerable for up to three such sections. This confirms the validity of the principle and at the same time its limits under practical conditions.

Micrometer-sized dust particles present in the Large Hadron Collider (LHC) beam pipe are believed to have caused many thousands of sporadic beam loss events around the LHC. These so-called unidentified falling objects (UFOs) have been under continuous study since the start of high intensity beam operation in the LHC due to their impact on the LHC availability: 139 beam dumps and 12 magnet quenches during Run II (2015–2018) alone. To mitigate the impact of UFOs on future accelerators such as the High Luminosity LHC and the Future Circular Collider, it is fundamental to foster a better understanding of these beam loss events. In this paper, key observations made since the start of LHC operation are summarized and the prevailing UFO hypothesis is confronted by a compilation of observations acquired during Run II. In particular, it is shown that UFOs must carry an initial negative charge before entering the proton beam, or that they are by some other means accelerated toward the beam not only by gravity. A simulation model for the dynamics of the dust particles and their interaction with the beam was developed over the years. This model is improved and validated by measurements. It is however also shown that a subset of observed beam losses, which contain a positive time profile skewness, cannot be explained by it.