Numerical optimization algorithms are already established tools to increase and stabilize the performance of particle accelerators. These algorithms have many advantages, are available out of the box, and can be adapted to a wide range of optimization problems in accelerator operation. The next boost in efficiency is expected to come from reinforcement learning algorithms that learn the optimal policy for a certain control problem and hence, once trained, can do without the time-consuming exploration phase needed for numerical optimizers. To investigate this approach, continuous model-free reinforcement learning with up to 16 degrees of freedom was developed and successfully tested at various facilities at CERN. The approach and algorithms used are discussed and the results obtained for trajectory steering at the AWAKE electron line and LINAC4 are presented. The necessary next steps, such as uncertainty aware model-based approaches, and the potential for future applications at particle accelerators are addressed.

The first electron cooling with rf-accelerated electron bunches was recently demonstrated at the low energy RHIC electron cooler (LEReC) at BNL. Successful cooling requires that the electrons in the cooling section have a small angular spread and are well aligned with respect to the copropagating ions. LEReC puts into practice a nonmagnetized cooling of the ions at Lorentz factors of $\gamma =4.1$ and 4.9. Hence, unlike in previous coolers, in which the transverse electron dynamics is constrained by longitudinal solenoid fields, the ion-electron focusing and steering strongly contribute to the average angular spread of the electron beam. In this paper we discuss the factors that affect the electron angles and describe the process of tuning the electron beam to maximize the cooling of ion bunches in RHIC.

Equipartitioning is an efficient beam-dynamics design scheme also for radio frequency quadrupole (RFQ). To realize an equipartitioned RFQ, it is essential to reproduce the nonuniform intervane voltage profile required from the beam dynamics. We developed a cavity design and tuning method to this end. The dimensions of a voltage-ramped four-vane cavity were determined by a three-dimensional rf simulation. To produce the desired vane voltage profile, the longitudinal variation of the cross-sectional shape was adjusted using the least-squares method based on the simulated responses of the vane-base width to the quadrupole-mode field. In the low-level tuning, slug-tuner positions were determined based also on the least-squares solutions obtained from the simulated tuner responses to the field profile. Finally, the deviation of the quadrupole-mode field was within 1.5% from the design profile, and the mixed dipole modes were 1% of the quadrupole-mode field after only two iterations of tuner adjustment; they are sufficiently smaller than the requirement of 2%.

Propagation distances of intense laser pulses and high-charge electron beams through the plasma are, respectively, limited by diffraction and self-deceleration. This imposes severe constraints on the performance of the two major advanced accelerator concepts: laser and plasma wakefield accelerators. Using numerical simulations, we demonstrate that when the two beams copropagate in the plasma, they can interact synergistically and extend each other’s travel distances. The key interactions responsible for the synergy are found to be laser channeling by the electron bunch, and direct laser acceleration of the bunch electrons by the laser pulse. Remarkably, the amount of energy transferred from the laser pulse to the plasma can be increased by several times by the guiding electron bunch despite its small energy content. Implications of such synergistic interactions for the high-gradient acceleration of externally injected witness charges are discussed, and a new concept of a laser-pulse and electron-bunch plasma accelerator is formulated.

High brightness electron beams are critical for x-ray free-electron laser performance. To achieve higher peak current, strong longitudinal compression often produces electron beams with peaks in the head and/or tail of the current profile. These current horns are formed after bunch compression due to nonlinear correlations in the longitudinal phase space and the higher order optics of the compressor. It has been suggested that this higher order compression can be corrected by inserting an octupole magnet near the center of a bunch compressor. However, this scheme provides a correlated transverse kick leading to growth of the projected emittance. We present here a method whereby octupole magnets are inserted into two sequential bunch compressors. By tuning a $\pi$ betatron phase advance between the two octupoles, the correlated transverse kick from the first octupole can be corrected by the second, while providing a cummulative adjustment of the higher order compression.

We report on systematic and high-precision measurements of the evolution of electron beams in a laser-wakefield accelerator (LWFA). Utilizing shock-front injection, a technique providing stable, tunable and high-quality electron bunches, acceleration and deceleration of few-MeV quasimonoenergetic beams were measured with cutting-edge technology sub-5-fs and 8-fs laser pulses. We explain the observations with dephasing, an effect that fundamentally limits the performance of LWFAs. Typical density dependent electron energy evolution with $57–300\mu \mathrm{m}$ dephasing length and 6–20 MeV peak energy was observed and is well described by a parabolic fit. This is a promising electron source for time-resolved few-fs electron diffraction.

Landau damping of coherent modes is strongly dependent on the exact shape of the particle bunches. One often assumes that the transverse distributions in high-energy hadron colliders can be approximated by Gaussian distributions, in acceptable agreement with measurements, but known to be only a first approximation. In this paper, it is investigated how a specific change of the transverse distributions can cause a loss of Landau damping. A mechanism is introduced where the coherent modes, which are excited by noise in the machine, act back on the individual particles through wakefields. The impact is modeled as a narrow diffusion in frequency space, and therefore also in action space due to amplitude dependent detuning, which leads to a local flattening of the distribution. This distribution evolution corresponds to the drilling of a borehole in the stability diagram, i.e. a local reduction of the imaginary part of the curve. Hence, initially stable regions are changed into unstable ones at the real frequencies of the coherent modes. To mitigate this instability mechanism, one must operate the machine with a stability margin of magnitude that depends on the noise amplitude and the coherent modes. In this model, the latency is defined as the time from the start of the noise excitation, on an initially Gaussian distributed bunch, to the bunch instability. The proposed model is found to agree with results in dedicated latency experiments performed in the LHC, where bunches eventually went unstable with more than twice the detuning strength required for the stabilization of a Gaussian distribution.

We develop the basic scaling laws for the design of a proof of principle two-beam accelerator based on a solid-state active medium (TBA-AM). The essence of the paradigm is to rely on a low-charge trigger bunch which will generate a weak intensity Cherenkov wake in a dielectric extraction structure. The wake consists of a discrete spectrum of eigenfrequencies, one of which is amplified by the active dielectric material. The exponential growth of the wake, behind the trigger bunch, continues as long as virtually all the energy from the medium is depleted. At this point, the electromagnetic field reaches saturation and thus its maximum value. Beyond this peak gradient, a trailing bunch may be placed and thus accelerated. Evidently, the major difference between the proposed paradigm and the existing ones is the energy source: in existing schemes the energy is stored in the driving bunch, whereas in ours, the required energy is stored in the active medium. Special attention is paid to three quantities which characterize the interacting mode: its initial amplitude generated by the trigger bunch, its exponential growth, and its saturation level. The latter indicates that accelerating gradients in excess of $1\mathrm{GV}/\mathrm{m}$ are feasible in a less than 10 cm long structure.

X-ray free-electron lasers based on superconducting accelerator technology deliver ultrashort photon pulses with unprecedented peak brilliance at high repetition rates. Continuous and noninvasive monitoring of the current profile of the electron bunches is essential for the operation and control of the accelerator. Longitudinal diagnostics based on coherent radiation have already shown their potential at various free-electron laser facilities, and the multi-GeV electron beams of x-ray free-electron lasers are powerful sources for the generation of broadband coherent diffraction radiation. We present noninvasive current profile measurements with a few femtoseconds resolution based on spectroscopy of coherent diffraction radiation in the frequency range 0.7–58 THz. The current profiles, reconstructed from the spectroscopic data with an advanced phase retrieval method, are compared with measurement results obtained with a transverse deflecting structure. For the first time, bunch-resolved current profiles have been recorded simultaneously to user operation at European XFEL for all bunches in the bunch train at MHz repetition rates.

Knowledge of the longitudinal beam parameters is important for understanding beam dynamics in any machine. In a linear accelerator (linac), to guarantee a stable motion in the longitudinal plane, the settings for the radio frequency cavities must be established by determining the phase and amplitude witnessed by the beam. In most modern hadron linacs, the requirements on phase and amplitude accuracy are 1° and 1% respectively, which is particularly challenging. Although systematic approaches for finding the phase and amplitude of cavities are available in many facilities with operational linacs, it is also vital to have a system in place when the standard time-of-flight based functionality is down or not yet available (as is often the case in the early stages of commissioning). Such a method should be relatively fast and simple, and should not depend on time-of-flight measurements or a rigorous phase calibration of the beam position monitors. This work presents a new phase- and amplitude-scan method which only requires information from transverse positions measured at beam position monitors downstream of the cavity being scanned. The scanning then relies on transverse trajectory correction, which has been developed extensively by the community. As a by-product, this method can also provide estimates for a cavity’s transverse misalignment. The method has been developed and tested for a single-cell cavity; the simulated effectiveness for a multicell cavity is also presented.

The Large Hadron Collider (LHC) is affected by the electron cloud (EC) phenomenon that can provoke beam instabilities, detrimental heat loads and pressure increases in the vacuum system. An innovative dedicated system called vacuum pilot sector (VPS) provides a continuous monitoring of the electron flux and of the pressure signals thanks to electron pickup and vacuum gauges. The VPS system is installed in a room temperature, field-free part of the LHC storage ring. Several technical surfaces, such as ex situ nonevaporable getter (NEG), amorphous carbon coating and copper, are simultaneously tested. The main outcomes of this study show that the EC signals have: (1) a linear dependence upon the number of bunches and upon the bunch population in the multipacting regime, (2) a multipacting threshold at a given bunch population, (3) a reduction under beam conditioning, (4) a strong dependence on the filling pattern and beam energy. The comparison between different surfaces shows that amorphous carbon coating reduces drastically the EC buildup, thanks to its low secondary electron yield (SEY) and photoelectron yield (PY), while copper and ex situ NEG coated surfaces suffer of EC multipacting, even after several months of operation. The multipacting rate coefficients are higher for copper than for ex situ NEG, as predicted from the SEY estimation. Other detailed experimental observations are discussed in this paper.

A new laser-driven proton therapy facility is being designed by Peking University. The protons will be produced by laser–plasma interaction, using a 2-PW laser to reach proton energies up to 100 MeV. We hope that the construction of this facility will promote the real-world applications of laser accelerators. Based on the experimental results and design experience of existing devices in Peking University, we propose a beam transmission system which is suitable for the beam produced by laser acceleration, and demonstrate its feasibility through theoretical simulation. It is designed with two transport lines to provide both horizontal and vertical irradiation modes. We have used a locally-achromatic design method with new canted-cosine-theta (CCT) magnets. These two measures allow us to mitigate the negative effects of large energy spread produced by laser-acceleration, and to reduce the overall weight of the vertical beamline. The beamline contains a complete energy selection system, which can reduce the energy spread of the laser-accelerated beam enough to meet the application requirements. The users can select the proton beam energy within the range 40–100 MeV, which is then transmitted through the rest of the beamline. A beam spot with diameter of less than 15 mm and energy spread of less than 5% can be provided at the horizontal and vertical irradiation targets.

Extension of stable longitudinal coherence from vacuum ultraviolet to x rays is highly sought after in the free-electron laser (FEL) community, but it is often prevented by bandwidth broadening originated in the electron beam microbunching instability. We demonstrate that a proper tuning of the linear optics before the beam enters the undulator mitigates the microbunching-induced sideband instability. The experiment was conducted at the Fermi FEL operated in echo-enabled harmonic generation mode, where the spectral brightness at 7 nm wavelength was doubled. The FEL performance is compared to nonoptimized optics solutions and characterized in terms of peak intensity and spectral bandwidth shot-to-shot stability. The technique has straightforward implementation, because it uses quadrupole magnets routinely adopted for beam transport, and it applies to any FEL architecture, so paving the way to the production of high-intensity Fourier-transform limited x-ray pulses in existing and planned FEL facilities.

As the highest power proton linear accelerator in continuous wave mode, China initiative Accelerator Driven sub-critical System linear accelerator will deliver 2.5 MW proton beam to the target inside the reactor. Vacuum transition from $5\times {10}^4\mathrm{Pa}$ to several ${10}^{-6}\mathrm{Pa}$ is required at the accelerator to target section. A vacuum differential system located at the transfer line is designed to meet the requirement of the windowless target and upstream collimators are considered for power loss control in this paper. A vacuum test based on vacuum transition verifies that about 10 orders of magnitude vacuum transition could be achieved as required, and gas scattering simulation shows that the power loss and beam size increase are at an acceptable level before the last magnet.

We propose a new type of axisymmetric dielectric target which effectively concentrates Cherenkov radiation (CR) generated in the bulk of the material into a small vicinity of a focus point. It can be called the “axicon-based concentrator for CR.” A theoretical investigation of radiation field produced by a charge moving through the discussed radiator is performed for the general case where a charge trajectory is shifted with respect to the structure axis. The idea of a dielectric target with a specific profile of the outer surface was presented and developed in our preceding papers. However, contrary to the previous configuration of such a target (which was investigated for both centered and shifted charge trajectory), the current version of the device allows the efficient concentration of CR energy from relativistic particles, making this device extremely prospective for various applications.

At the GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt, Germany, a prototype cryomodule (advanced demonstrator) for the superconducting (SC) continuous wave (CW) Helmholtz Linear Accelerator (HELIAC) is under construction. A transport line, comprising quadrupole lenses, rebuncher cavities, beam correctors, and adequate beam instrumentation has been built to deliver the beam from the GSI 1.4 MeV/u High Charge Injector (HLI) to the advanced demonstrator, which offers a test environment for SC CW multigap cavities. In order to achieve proper phase space matching, the beam from the HLI must be characterized in detail. In a dedicated machine experiment the bunch shape has been measured with a nondestructive bunch shape monitor (BSM). Therefore, different bunch projections were obtained by altering the voltage of two rebunchers. These measurements were combined with dedicated beam dynamics simulations using the particle tracking code dynamion. The longitudinal bunch shape and density distribution at the beginning of the matching line are fully characterized by a tomographic reconstruction method based on a non-negative least square minimization approach.