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.

When using the harmonic cavity to stretch the beam in electron storage rings, we expect the beam can be lengthened effectively and also stably. Semianalytical approaches can be used to obtain the equilibrium distribution but generally failed in the prediction of its stability. In this work, a novel perturbation method is proposed to check whether the calculated equilibrium solution can survive from the $l=1$ mode perturbation. For the case of uniform beam filling, we derive a simple formula that can determine the threshold of $l=1$ mode instability under specific parameters, which is well verified with tracking simulation for the Hefei Advanced Light Facility storage ring.

We report x-ray emission caused by betatron motion of electrons in a laser wakefield accelerator driven by high intensity ($\sim 4\times {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$) laser pulse interaction with a 4-mm long helium gas-jet and study the role of collective oscillation of electrons as well as plasma density. The collective oscillations of electrons, inferred from the observed wavy pattern in the electron beam spectrum (up to 300 MeV) led to enhance peak x-ray flux which was found to be further enhanced at higher plasma densities. A similar correlation between the enhancement in x-ray emission and collective oscillation was also observed in nitrogen gas target. Possible physical pictures for the above observations are also suggested and discussed. For the helium gas jet target, the typically observed x-ray source brightness is $\ge 1\times {10}^{20}\mathrm{photons}/\mathrm{s}{\mathrm{mm}}^2{\mathrm{mrad}}^2$ in 0.1% BW, with a critical energy of $35\pm 5\mathrm{keV}$. We have also demonstrated the application of the betatron x-ray source in high-resolution imaging.

Plasma accelerators sustain extreme field gradients and potentially enable future compact linear colliders. Although tremendous progress has been achieved in accelerating electron beams in a plasma accelerator, positron acceleration with collider-relevant parameters is challenging. A recently proposed positron acceleration scheme relying on the wake generated by an electron drive beam in a plasma column has been shown to be able to accelerate positron witness beams with low emittance and low energy spread. However, since this scheme relies on cylindrical symmetry, it is possibly prone to transverse instabilities that could lead, ultimately, to beam breakup. In this article, we show that the witness beam itself is subject to various damping mechanisms and, therefore, this positron acceleration scheme is inherently stable toward the misalignment of the drive and witness beams. This enables stable, high-quality plasma-based positron acceleration.

The 1.4-MW Spallation Neutron Source (SNS) currently operates as the world’s highest time-average power spallation source and uses mercury as the spallation target material. The mercury circulates through a stainless-steel target vessel and heat exchanger loop that removes the heat deposited from the incident proton beam in the target. The mercury vessel is part of the fixed but replaceable target module. The target module is not safety credited in the SNS design, but reliable long life is vital for meeting operational goals and minimizing cost and waste. SNS is a short-pulse ($\sim 0.7\mu \mathrm{s}$) neutron source with a repetition rate of 60 Hz with 1.0 GeV protons onto the target. The short-pulse operation introduces two mechanisms that threaten the longevity of the mercury vessel. The pulsed target heating is essentially isochoric, which results in a distributed pressure field in the mercury with each pulse. After each pulse, the pressure propagates, interacts with, and strains the stainless-steel vessel. For intermittent times, this pressure field leads to mercury cavitation. The first mechanism of concern for the target lifetime is high-cycle fatigue to the stainless-steel vessel and the second is cavitation erosion. At the onset of the SNS project, the R&D program sought to address the need for an accurate and practical method to estimate the fatigue life of the mercury vessel. Around 2000, the R&D team conducted a series of in-beam mercury target experiments at the Weapons Neutron Research Facility at Los Alamos Neutron Science Center. These experiments successfully produced a small database of pulse response strain data. The data were used to develop and benchmark an explicit dynamic finite element analysis technique for estimating target fatigue life. SNS is now a mature facility with about 15 years of operation and 29 SNS target modules consumed. Since 2015, strain measurements have been collected from actual targets, and abundant target vessel strain data are now available in greater detail than was possible in 2000. Variances in pulse response data have been characterized using tolerance interval analysis to benchmark the simulation technique more thoroughly. The simulated results were found to match measurements generally well, but the method is always conservative. A potential adjustment to the cavitation threshold parameter value was suggested. Variances in the pulse response data are moderate, which is largely explained by the fact that data were acquired in 60-Hz burst mode and by the lasting vibration of the vessel between pulses.

Proton beams with up to 100 pC bunch charge, 0.48 MeV cutoff energy, and divergence as low as 3° were generated from solid targets at kHz repetition rate by a few-mJ femtosecond laser under controlled plasma conditions. The beam spatial profile was measured using a small aperture scanning time-of-flight detector. Detailed parametric studies were performed by varying the surface plasma scale length from 8 to 80 nm and the laser pulse duration from 4 fs to 1.5 ps. Numerical simulations are in good agreement with observations and, together with an in-depth theoretical analysis of the acceleration mechanism, indicate that high repetition rate femtosecond laser technology could be used to produce few-MeV proton beams for applications.

We propose a novel positron beam loading regime in a hollow plasma channel that can efficiently accelerate $e^{+}$ beam with a high gradient and narrow energy spread. In this regime, the $e^{+}$ beam coincides with the drive $e^{-}$ beam in time and space and their net current distribution determines the plasma wakefields. By precisely shaping the beam current profile and loading phase according to explicit expressions, three-dimensional particle-in-cell (PIC) simulations show that the acceleration for $e^{+}$ beam of $\sim \mathrm{nC}$ charge with $\sim \mathrm{GV}/\mathrm{m}$ gradient, $\lesssim 0.5\%$ induced energy spread, and $\sim 50\%$ energy transfer efficiency can be achieved simultaneously. Besides, only tailoring the current profile of the more tunable $e^{-}$ beam instead of the $e^{+}$ beam is enough to obtain such favorable results. A theoretical analysis considering both linear and nonlinear plasma responses in hollow plasma channels is proposed to quantify the beam loading effects. This theory agrees very well with the simulation results and verifies the robustness of this beam loading regime over a wide range of parameters.

We present the design of a high repetition rate MeV energy ultrafast electron diffraction instrument based on a dc photoelectron gun and an superconducting rf (SRF) linac with multiple independently controlled accelerating and bunching cavities. The design is based on the existing Cornell photoinjector, which can readily be applied to the presented findings. Using particle tracking simulations in conjunction with multiobjective genetic algorithm optimization, we explore the smallest bunch lengths, emittance, and probe spot sizes achievable. We present results for both stroboscopic conditions (with single electrons per pulse) and with ${10}^5\mathrm{electrons}/\mathrm{bunch}$ which may be suitable for single-shot diffraction images. In the stroboscopic case, the flexibility provided by the many-cavity bunching and acceleration allows for longitudinal phase space linearization without a higher harmonic field, providing sub-fs bunch lengths at the sample. Given low emittance photoemission conditions, these small bunch lengths can be maintained with probe transverse sizes at the single micron scale and below. In the case of ${10}^5$ electrons per pulse, we simulate state-of-the-art 5D brightness conditions: rms bunch lengths of 10 fs with 3-nm normalized emittances, while now permitting repetition rates as high as 1.3 GHz. Finally, to aid in the design of new SRF-based ultrafast electron diffraction machines, we simulate the trade-off between the number of cavities used and achievable bunch length and emittance.

The Target Absorbers for Neutrals (TANs) are located in a high intensity radiation environment inside the tunnel of the Large Hadron Collider (LHC). TANs are positioned about 140 m downstream from the beam interaction points. Seven 40-cm long fused silica rods with different dopant specifications were irradiated in the TAN by the Beam RAte of Neutrals (BRAN) detector group during $p+p$ data taking from 2016 to 2018 at the LHC. The peak dose delivered to the fused silica rods was 18 MGy. We report measurements of the ${}^{22}\mathrm{Na}$ activation of the fused silica rods carried out at the University of Illinois at Urbana-Champaign and Argonne National Laboratory. At the end of the irradiation campaign, the maximum ${}^{22}\mathrm{Na}$ activity observed was $A=21\mathrm{kBq}/\mathrm{c}{\mathrm{m}}^3$ corresponding to a density, $\rho =2.5\times {10}^{12}/\mathrm{c}{\mathrm{m}}^3$, of ${}^{22}\mathrm{Na}$ nuclei. fluka Monte Carlo simulations have been performed by the CERN fluka team to estimate ${}^{22}\mathrm{Na}$ activities for the irradiated BRAN rod samples. The simulations reproduce the ${}^{22}\mathrm{Na}$ activity profile measured along the rods, with a 35% underestimation of the experimental measurement results.

We report on the effects of transverse spatial misalignment on the performance and stability of x-ray free electron laser oscillators (XFELOs). For this study, we adopt the FEL driven paraxial resonator model in which the transverse profile of the radiation field is represented by Gauss-Hermite mode expansion. Then, we apply Gaussian optics for transporting the radiation field along with the misalignment in the optical cavity. We divide the misalignment into relevant categories of static and periodic types to understand their effects on FEL gain, lasing, and saturation. After that, we associate the first order moments of the radiation field to that of driven simple harmonic oscillator models to identify regions of instability. Throughout this report, we validate our models and analyses via theoretical calculations, analytical approximations, and simulations.

Coherent electron cooling is a promising technique to cool high-intensity hadron bunches by imprinting the noise in the hadron beam on a beam of electrons, amplifying the electron density modulations, and using them to apply cooling kicks to the hadrons. The typical size for these perturbations can be on the $\mu \mathrm{m}$ scale, allowing us to extend the reach of classical stochastic cooling by several orders of magnitude. However, it is crucial to ensure that the electron and hadron beams are longitudinally aligned within this same $\mu \mathrm{m}$ scale. In order to provide fast feedback for this process, we discuss the extension of signal suppression to coherent electron cooling and show in both theory and simulation that certain components of the spectral noise in the hadron beam will be predictably modified at the several percent levels, which may be detected by observations of the radiation of the hadron beam.

Four dipole magnetic chicanes are typically used in high-performance accelerators to compress electron bunches to their final peak currents but subject the beam to emittance dilution due to the coherent synchrotron radiation (CSR) effect. In this paper, we present a new magnetic bunch compressor geometry designed to locally suppress the CSR-induced emittance degradation called the five-bend chicane. We will include a simulation study comparing a five-bend chicane to the nominal four-bend chicane of the LCLS-II-HE’s second bunch compressor (BC2) beamline. The study will consist of emittance and energy spread growth due to CSR using a one-dimensional, elegant, and two-dimensional, csrtrack, CSR algorithm in free space, and the free-electron laser performance using genesis.