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.

Recently, gradients on the order of $1\mathrm{GV}/\mathrm{m}$ have been obtained with single-cycle ($\sim 1\mathrm{ps}$) THz pulses produced by the conversion of high peak power laser radiation, in nonlinear crystals ($\sim 1\mathrm{mJ}$, 1 ps, up to 3% conversion efficiency). Such high intensity radiation can be utilized for charged particle acceleration. For efficient acceleration, a large number of accelerating cells have to be stacked together with delay lines in order to provide proper timing between accelerating fields and moving electrons. Additionally, THz pulses in individual cells need to be focused transversely to maximize the accelerating gradient. This focusing is currently done by the tapering of the waveguide. In this paper, we propose instead to use dielectric lenses for the robust focusing of THz pulses in accelerating cells. Each lens is made from the same material as the delay line in a monolithic unit. We have fabricated prototype units from quartz and silicon. Such an approach simplifies the fabrication and alignment of the multicell accelerating structure. We present a design in which a 100 microJoule THz pulse produces a 600 keV energy gain in a 5-mm long 10-cell accelerating structure for an ultrarelativistic electron. This approach can be extended to nonrelativistic particles.

Dielectric laser acceleration (DLA) achieves remarkable gradients from the optical near fields of a grating structure. Tilting the dielectric grating with respect to the electron beam leads to deflection forces, and the DLA structure can be utilized as a microchip undulator. We investigate the beam dynamics in such structures analytically and by numerical simulations. A crucial challenge is to keep the beam focused, especially in the direction of the narrow channel. An alternating phase focusing scheme is optimized for this purpose and matched lattice functions are obtained. We distinguish synchronous operation with phase jumps in the grating and asynchronous operation with a strictly periodic grating and well-designed synchronicity mismatch. Especially, the asynchronous DLA undulator is a promising approach, since a simple, commercially available grating suffices for the focusing lattice design. We pave the way toward experiments of radiation generation in these structures and provide estimates of the emitted radiation wavelength and power. The analytical models are validated by numerical simulations in the dedicated DLA simulation tool dlatrack6d and astra, where the underlying laser fields are computed by cst studio.

The recent achievements in the performance of superconducting rf (SRF) photoinjectors have opened a new era in the development of reliable high-brightness cw electron sources. While the SRF guns become one of the most promising technologies, the compatibility of the SRF environment with the complex photocathodes remains at the forefront of the modern accelerator science. The SRF cavities operate at cryogenic temperatures providing the ultrahigh vacuum environment highly beneficial for the photocathode performance. However, the necessity to keep the photocathodes at room temperature while being surrounded by the cavity walls that are kept at cryogenic temperatures creates an additional complexity for the SRF gun design and operation. The complex and volatile chemical compounds used for photocathodes have a high chance of contaminating the surfaces of the SRF cavity. When deposited, such compounds could create centers for cold electron emission which degrade the performance of the SRF guns. Such a circumstance would require the development of the in-situ processing techniques for restoring the SRF cavity performance. This paper presents the results of the successful implementation and application of the He conditioning method for cavity restoration using the existing SRF photoinjector at Brookhaven National Laboratory (BNL). The method has proven to be extremely effective and resulted in a dramatic improvement in the BNL gun performance.

The transverse emittance of a charged particle beam is an important figure of merit for many accelerator applications, such as ultrafast electron diffraction, free electron lasers, and the operation of new compact accelerator concepts in general. One of the easiest to implement methods to determine the transverse emittance is the phase advance scan method using a focusing element and a screen. This method has been shown to work well in the thermal regime. In the space charge dominated laminar flow regime, however, the scheme becomes difficult to apply because of the lack of a closed description of the beam envelope including space charge effects. Furthermore, certain mathematical, as well as beamline design criteria must be met in order to ensure accurate results. In this work, we show that it is possible to analyze phase advance scan data using a fully connected neural network (FCNN), even in setups, which do not meet these criteria. In a simulation study, we evaluate the performance of the FCNN by comparing it to a traditional fit routine based on the beam envelope equation. Subsequently, we use a pretrained FCNN to evaluate measured phase advance scan data, which ultimately yields much better agreement with numerical simulations. To tackle the confirmation bias problem, we employ additional mask-based emittance measurement techniques.

Highly efficient superconducting radio-frequency cavities exhibit low heat loss and are used as components in accelerators and superconducting devices due to their high $Q$-values. The precise location of magnetic flux trapping in cavities is necessary to identify its effects on the performance of superconducting cavities. In this study, we report a new combined mapping system to measure the temperature and magnetic field on the equator. The proposed system comprehensively maps local magnetic field changes as magnetic flux trapping due to quenching. Our experimental results show that magnetic flux trapping due to quenching increases the local surface resistance of superconducting cavities at 2 K. Thus, the proposed system elucidates the relationship between local flux trapping due to quenching and surface resistance in superconducting cavities and highlights the effect of quenching on the surface resistance. This system can aid in the development of superconducting cavities with higher $Q$ values.

For the accelerator community and the vacuum scientists, the understanding of the beam interactions with a vacuum chamber is fundamental to provide solutions to mitigate pressure rises induced by electron, photon, and ion molecular desorption. Moreover, beam instabilities induced by ion and electron clouds must be investigated in order to find solutions to reduce them. This study presents in situ measurements of pressure evolutions and electrical currents performed during the LHC RUN II (2018). The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. These charged particles are accelerated away from the beam and reach the vacuum chamber wall, inducing, among other phenomena, stimulated desorption and secondary electron emission. Moreover, protons emit synchrotron radiations that also induce photodesorption and photoelectron production. Experimental measurements of the electrical signals recorded on copper electrodes were compared to calculations considering both the secondary electron yield of copper and the electron energy distribution. All measurements performed with the Vacuum Pilot Sector in the LHC ring show the importance of taking into account a large variety of phenomena in order to understand the pressure evolution in the LHC. Results show that the multipacting threshold, corresponding to an increase in the electron cloud density, strongly depends on the number of protons per bunch. Finally, the ion current was measured with a biased electrode lower than $-500\mathrm{V}$. It was much higher than expected, pointing its origin not only from simple beam-gas ionization but also from the ionization of the residual gas by the electron cloud.

A space charge algorithm is constructed that is a hybrid between envelope and multiparticle models. The transverse dynamics is simplified by tracking the transverse envelope of each macroparticle. The equations of motion are derived from the Hamiltonian-fluid formulation of the Vlasov Poisson system. A novel electrostatic self-field solver is derived that solves directly for the self-consistent on-axis potential and linear defocusing force, including longitudinal and beam pipe boundary conditions. The implementation of the algorithm, hyper3d, is presented. It is tested against the particle-in-cell code warp, using various configurations of a periodic focusing structure with rf cavities. The required number of macroparticles is reduced substantially compared to standard particle-tracking codes. This model is well adapted to cases where the transverse dynamics is linear and where the details of the longitudinal dynamics are the principal point of interest; an example is where a dc beam is converted to a bunched beam.

The main obstacle to the practical implementation of the dielectric wakefield acceleration (DWA) concept is the development of the beam breakup instability due to transverse dipole wakefields generated when the beam propagates off axis in the accelerating structure. One of the methods to suppress this instability is to elliptically shape the beam and accelerate it in a planar structure. Here, we report a detailed experimental investigation of the transverse dynamics of elliptical beams in planar dielectric structure with parameters mimicking future DWA modules. Both dipole and quadrupole wakefields’ effects on beam stability and projected emittance were studied. This study has highlighted the importance of counteracting quadrupole wakefields in future DWA implementations.

Radio-frequency (rf) photoemission electron guns operating at higher accelerating gradients offer a pathway to produce brighter electron bunches. Brighter beams are expected to revolutionize many areas of science: they could form the backbone of next-generation compact x-ray free-electron lasers or provide coherent ultrafast quantum electron probes. We report on the experimental demonstration of an rf photoemission electron source supporting an accelerating field close to $400\mathrm{MV}/\mathrm{m}$ at the photocathode surface without major rf breakdown or significant dark current over a 3-week experimental run. This unprecedented gradient was achieved by operating the gun in a new regime with short rf pulses, $\sim 9\mathrm{ns}$ $X$-band (11.7 GHz). The demonstrated paradigm provides a viable path to form relativistic electron beams with unprecedented brightness.