We report on experimental measurements of energy transfer efficiencies in a GeV-class laser wakefield accelerator. Both the transfer of energy from the laser to the plasma wakefield and from the plasma to the accelerated electron beam was diagnosed by simultaneous measurement of the deceleration of laser photons and the acceleration of electrons as a function of plasma length. The extraction efficiency, which we define as the ratio of the energy gained by the electron beam to the energy lost by the self-guided laser mode, was maximized at $19\pm 3\%$ by tuning the plasma density and length. The additional information provided by the octave-spanning laser spectrum measurement allows for independent optimization of the plasma efficiency terms, which is required for the key goal of improving the overall efficiency of laser wakefield accelerators.

Superconducting photoelectron injectors are promising for generating highly brilliant pulsed electron beams with high repetition rates and low emittances. Experiments such as ultrafast electron diffraction, experiments at the Terahertz scale, and energy recovery linac applications require such properties. However, optimizing the beam properties is challenging due to the high number of possible machine parameter combinations. This article shows the successful automated optimization of beam properties utilizing an already existing simulation model. To reduce the required computation time, we replace the costly simulation with a faster approximation with a neural network. For optimization, we propose a reinforcement learning approach leveraging the simple computation of the derivative of the approximation. We prove that our approach outperforms standard optimization methods for the required function evaluations given a defined minimum accuracy.

Photoinjectors are the main high brightness electron sources for x-ray free electron lasers (XFELs). Photoinjector emittance reduction is one of the key knobs for improving XFEL lasing, so precise emittance measurement is critical. It is well known that rms emittance is very sensitive to low intensity tails of particle distributions in the phase space, whose measurement depends on the signal to noise ratio and image processing procedures. Such sensitivities make the interpretations of beam transverse brightness challenging, leading to different emittance definitions to reduce the impact of tail particles. In this paper, transverse phase space is analyzed in action and phase coordinates for both analytical models and experiments, which gives a more intuitive way to calculate the beam core brightness.

In a recent paper, the adiabatic theory of Hamiltonian systems was successfully applied to study the crossing of the linear coupling resonance, $Q_x-Q_y=0$. A detailed explanation of the well-known phenomena that occur during the resonance-crossing process, such as emittance exchange and its dependence on the adiabaticity of the process, was obtained. In this paper, we consider the crossing of the resonance of nonlinear coupling $2Q_x-2Q_y=0$ using the same theoretical framework. We perform the analysis using a Hamiltonian model in which the nonlinear coupling resonance is excited and the corresponding dynamics is studied in detail, in particular looking at the phase-space topology and its evolution, in view of characterizing the emittance exchange phenomena. The theoretical results are then tested using a symplectic map. Thanks to this approach, scaling laws of general interest for applications are derived.

This paper presents a first experimental demonstration of a new nondestructive method for aperture measurements based on ac dipoles. In high intensity particle colliders, such as the CERN Large Hadron Collider (LHC), aperture measurements are crucial for a safe operation while optimizing the optics in order to reduce the size of the colliding beams and hence increase the luminosity. In the LHC, this type of measurements became mandatory during beam commissioning and the current method used is based on the destructive blowup of bunches using a transverse damper. The new method presented in this paper uses the ac-dipole excitation to generate adiabatic forced oscillations of the beam in order to create losses to identify the smallest aperture in the machine without blowing up the beam emittance. A precise and tuneable control of the oscillation amplitude enables the beams to be reused for several aperture measurements, as well as for other subsequent commissioning activities. Measurements performed with the new method are presented and compared with the current LHC transverse damper method for two different beam energies and two different operational optics.

A miniaturized Penning ion source, capable of generating a milliampere ${\mathrm{H}}^{+}$ beam with low anode power supply and low gas consumption, was produced and tested for neutron tubes. For a better understanding and optimization of the Penning ion source, magnetic field, beam extraction system, and plasma simulations were conducted to research the optimization of the Penning ion source. The distance between the two cathodes is optimized based on the plasma simulations. Based on the optimization of the magnetic field, extraction system, and the distance of the cathodes, the extraction current of the Penning source had been increased from 1.21 to 1.72 mA, and the ${\mathrm{H}}^{+}$ ratio had been increased from 36% to 42.3%.

In SuperKEKB high-current beam operation, achieving the stability of the zero mode associated with the accelerating mode of the rf system is an important concern because of heavy beam loading. The issue is a more serious problem in the high-energy ring (HER) compared to the low-energy ring (LER) because of the high impedance of superconducting cavities operating in the HER. As a countermeasure, the rf system is equipped with a direct rf feedback system (DRFB) to effectively reduce the impedance of the cavities and a zero mode oscillation damper. The effect of these mitigations on improving the stability while coexisting with other control loops was quantitatively studied using two independent methods, namely, analysis with a characteristic equation and simulation in the time domain. For the analysis, we extended the transfer functions in the Pedersen model to include the DRFB to analyze the rf system with beam loading, while coexisting with the other control loops such as amplitude and phase controls. The consistency between both methods was confirmed by comparing the results obtained from them for different cases. Furthermore, simulation results were compared with measurements conducted during beam operation; they were consistent with each other. Having confirmed the validity of these methods, they were applied to extensively study the stability for future higher beam current operations in SuperKEKB. Particularly, several operation parameter sets, including possible system modifications, were identified for the HER and LER with considerably good margins to compensate for possible deficiencies in the performance of cavities or control functions at higher beam currents. The results can be used as guidelines for future beam operations with beam current increasing in a stepwise manner.

Dust grains interacting with the beam of particle accelerators are believed to be the cause of several detrimental effects such as beam losses, emittance growth, pressure bursts, and even quenches of superconducting magnets. Experimental observations suggest that these grains are positively charged in electron storage rings and negatively charged in the Large Hadron Collider (LHC). In this paper, the charging mechanisms for dust grains in the LHC are discussed and a possible explanation for the observed polarity is presented. Electron collection, secondary electron emission, and photoelectric emission are considered because of the presence of electron clouds and synchrotron radiation. It is found that the same mechanisms can explain both the positive grain polarity observed in electron storage rings and the negative polarity in the LHC. As a consequence of the charge acquired, the possibility of grains orbiting the beam is discussed. The orbital dynamics in a logarithmic potential is analyzed and critical parameters for describing such orbits are introduced. Finally, LHC beam losses attributed to beam-dust interactions with multiple loss peaks are presented. It is shown that they have an amplitude and a peak separation consistent with what can be expected for orbiting grains.

The Advanced Wakefield (AWAKE) Experiment is a proof-of-principle experiment demonstrating the acceleration of electron beams via proton-driven plasma wakefield acceleration. AWAKE Run 2 aims to build on the results of Run 1 by achieving higher energies with an improved beam quality. As part of the upgrade to Run 2, the existing proton and electron beamlines will be adapted and a second plasma cell and new 150-MeV electron beamline will be added. The specification for this new 150-MeV beamline will be challenging as it will be required to inject electron bunches with micron-level beam size and stability into the second plasma cell while being subject to tight spatial constraints. In this paper, we describe the techniques used (e.g., numerical optimizers and genetic algorithms) to produce the design of this electron line. We present a comparison of the methods used in this paper with other optimization algorithms commonly used within accelerator physics. Operational techniques are also studied including steering and alignment methods utilizing numerical optimizers and beam measurement techniques employing neural networks. We compare the performance of algorithms for online optimization and beam-based alignment in terms of their efficiency and effectiveness.

We study short-range geometric wakefields of a normal conducting third harmonic cavity, developed by ALBA, which is currently under investigation for the BESSY II main storage ring upgrade. As is commonly done to enhance the shunt impedance, the single cell cavity shape includes smooth nose-cone transitions to the outside beam pipe, which significantly affects the wakefields. Utilizing the general approach developed by the earlier work [B. Podobedov and G. Stupakov, Phys. Rev. ST Accel. Beams 16, 024401 (2013)], a model of point-charge wakefield for cavities of this type is suggested. The wakefield model consists of two parts: the singular part, inherently difficult for numerical codes, is found analytically, and the nonsingular part, which could be retrieved from relatively nondemanding numerical wakefield computations. Based on this hybrid approach, the wakefields due to arbitrary short bunches can be readily obtained both efficiently and accurately. The model was cross-checked against direct numerical simulations and a good agreement was found within the entire range of bunch lengths relevant to potential applications, from sub-millimeter down to $1\text{−}\mu \mathrm{m}\mathrm{rms}$. We also observed that the wakefield model of a single cavity applies equally well to a small number of coupled cavities after some straightforward parameter scaling.

Experimental results, supported by precise modeling, demonstrate optimization of a plasma-based injector with intermediate laser pulse energy ($<1\mathrm{J}$), corresponding to a normalized vector potential $a_0=2.15$, using ionization injection in a tailored plasma density profile. An increase in electron bunch quality and energy is achieved experimentally with the extension of the density downramp at the plasma exit. Optimization of the focal position of the laser pulse in the tailored plasma density profile is shown to efficiently reduce electron bunch angular deviation, leading to a better alignment of the electron bunch with the laser axis. Single peak electron spectra are produced in a previously unexplored regime by combining an early focal position and adaptive optic control of the laser wavefront by optimizing the symmetry of the prefocal laser energy distribution. Experimental results have been validated through particle-in-cell simulations using realistic laser energy, phase distribution, and temporal envelope, allowing for accurate predictions of difficult to model parameters, such as total charge and spatial properties of the electron bunches, opening the way for more accurate modeling for the design of plasma-based accelerators.

Plasma cleaning is a technique that can be applied in superconducting radio-frequency cavities in situ in cryomodules to decrease their level of field emission (FE). We developed the technique for Linac Coherent Light Source II (LCLS-II) cavities and we present in this paper the full development and application of plasma processing to the LCLS-II High Energy verification cryomodule (vCM). We validated our plasma processing procedure on the vCM, fully processing four out of eight cavities of this CM, demonstrating that cavity performance was preserved in terms of both accelerating field and quality factor. Applying plasma processing to this clean, record breaking cryomodule also showed that no contaminants were introduced in the string, maintaining the vCM FE-free up to the maximum field reached by each cavity. We also found that plasma processing eliminates multipacting (MP)-induced quenches that are frequently observed within the MP band field range. This suggests that plasma processing could be employed in situ in CMs to mitigate both FE and MP, significantly decreasing the testing time of cryomodules, the linac commissioning time and cost, and increasing the accelerator reliability.

We report experimental test-beam results on dielectric-loaded waveguide detectors that utilize microwave Cherenkov signals to time and characterize high energy particle showers. These results are used to validate models and produce high-fidelity simulations of timing plane systems that yield picosecond time tags and millimeter-level spatial coordinates for the shower centroid. These timing planes, based on the Askaryan effect in solid dielectrics, are most effective at the high center-of-momentum energies planned for the Future Circular Collider (FCC-hh). They will be of particular interest in the forward region due to their high radiation immunity. We use our beam test results and geant4 simulations to validate a hybrid microwave detector model, which is used to simulate a reference timing plane design for the FCC forward calorimeters. Our results indicate that 0.5–3 ps particle timing is possible for a wide range of collision products in the reference FCC hadron collider detector, even with current technology.

The Gamma Factory proof of principle experiment aims at colliding laser pulses with ultrarelativistic partially stripped ion beams at the CERN Super Proton Synchrotron. Its goals include the first demonstration of fast cooling of ultrarelativistic ion beams and opening up many possibilities for new physics measurements in various domains from atomic physics to particle physics. A high average-power, pulsed laser system delivering approximately 200 kW needs to be implemented for this aim. This is possible thanks to state-of-the-art optical systems that recently demonstrated similar performances in the laboratory environment. Challenges lie in the implementation of this kind of laser system in the harsh environment of hadronic machines including their robust and fully remote operation. The design of this laser system, involving a high quality factor enhancement cavity, is drawn and described in this article. Mitigation procedures are proposed to overcome limitations imposed by the occurrence of degenerate high-order mode at high average power in such optical resonators. We show that the operation at average power above 200 kW is feasible.

Diamagnetic loops (DML) can be used as a noninvasive method for the measurement of beam size in electron beam accelerators that use solenoidal magnetic transport. The loop fundamentally measures the magnetic flux excluded by a diamagnetic object. A comprehensive theory relates the rms beam radius to the excluded flux measured by the DML. We have built and calibrated a DML apparatus. Recently, this DML has been used to measure the size of the electron beam near the final focus of a flash-radiography linear induction accelerator. The results are in agreement with predictions from transport code simulations. In this article, we review and summarize the construction, calibration, and electron-beam testing of this DML.

We present high-resolution measurements of the electron beam energy spread at the SwissFEL injector as a function of the electron bunch charge, the optics, and the longitudinal dispersion of the lattice. The measured values are in general an order of magnitude higher than what is predicted by standard simulation codes. The measured dependences indicate that the energy spread blowup is caused primarily by intrabeam scattering and microbunching instabilities, effects not covered in the conventional modeling of radio-frequency injectors. We present further a numerical model that qualitatively reproduces the experimental data and an approach to mitigate the energy spread deterioration. The work underlines the importance of considering the energy spread in the optimization and design of high-brightness electron beam sources and the need to develop new models to adequately understand and simulate the observed physics effects.

The occurrence of vacuum arcs or radio frequency (rf) breakdowns is one of the most prevalent factors limiting the high-gradient performance of normal conducting rf cavities in particle accelerators. In this paper, we search for the existence of previously unrecognized features related to the incidence of rf breakdowns by applying a machine learning strategy to high-gradient cavity data from CERN’s test stand for the Compact Linear Collider (CLIC). By interpreting the parameters of the learned models with explainable artificial intelligence (AI), we reverse-engineer physical properties for deriving fast, reliable, and simple rule–based models. Based on 6 months of historical data and dedicated experiments, our models show fractions of data with a high influence on the occurrence of breakdowns. Specifically, it is shown that the field emitted current following an initial breakdown is closely related to the probability of another breakdown occurring shortly thereafter. Results also indicate that the cavity pressure should be monitored with increased temporal resolution in future experiments, to further explore the vacuum activity associated with breakdowns.