We examine through numerical calculation the collision of counter-propagating trains of optically spaced electron or positron microbunches in a 1 TeV collider scenario for a dielectric laser accelerator. A time-dependent envelope equation is derived for arbitrary number of bunches in the classical limit, with inclusion of the radiation reaction force. Example parameters are examined based on a constrained luminosity relation that takes into account the bunch charge for optimal efficiency, material damage limits, and power constraints. We find that for initially identical counter-propagating Gaussian bunch trains, the periodic temporal structure leads to a peak in luminosity with number of bunches. For longer bunch trains, the enhancement then decreases inversely with number of bunches. The corresponding fractional energy loss of the beam is found to be of order 1.75%, which is reduced to 0.35% when the nonlinear radial dependence of the transverse force is included, with an average beamstrahlung parameter of 0.075, an important result considering that beamstrahlung losses are a critical concern for future TeV colliders.

Laser-plasma accelerators (LPAs) now routinely produce electron beams with GeV energies over acceleration lengths on the order of a few centimeters. This capability and the demonstration of multistage coupling make LPAs promising for numerous applications. However, beam quality preservation in multistage accelerators remains an obstacle because of the need to separate the laser pulse from the electron beam. Plasma mirrors can be used to redirect the laser pulse, but their substrate thickness threatens to substantially degrade the electron beam emittance. Ultrathin ($\sim 20\mathrm{nm}$) liquid crystal (LC) plasma mirrors are an excellent candidate to address this challenge. This work investigates the robustness of thin LC plasma mirrors in the presence of capillary discharge plasma and an auxiliary heater laser. We find they can be operated $\sim 10\mathrm{cm}$ from the capillary exit when a heater laser is used. We then performed a normalized emittance measurement enabled using a 20 nm LC plasma mirror to protect electron beam optics after the LPA. The emittance contribution from scattering through the plasma mirror is calculated to be of order 100 nm, much less than the measured emittance of $4.0\mu \mathrm{m}$. Finally, we develop a model to calculate plasma mirror performance based on the laser polarization and intensity, and plasma mirror thickness.

Active longitudinal beam optics can help free-electron laser facilities achieve cutting edge performance by optimizing the beam to produce multicolor pulses, suppress caustics, or support attosecond lasing. As the next generation of superconducting accelerators comes online, there is a need to find new elements which can both operate at high-beam power and which offer multiplexing capabilities at MHz repetition rate. Laser heater shaping promises to satisfy both criteria by imparting a programmable slice-energy spread on a shot-by-shot basis. We use a simple kinetic analysis to show how control of the slice energy spread translates into control of the bunch current profile, and then we present a collection of start-to-end simulations at LCLS-II in order to illustrate the technique.

In this work, we perform particle-in-cell simulations to investigate the electron bunch quality using a chirped laser pulse in a laser wakefield acceleration mechanism. Particular attention is devoted to the electron beam quality improvement using a chirped laser pulse through group delay dispersion. By adjusting the value of group delay dispersion, the electron beam quality was controlled and improved in the range: charge (31–325 pC), energy (240–485 MeV), energy spread (2.1%–12.5%), and bunch duration (3.3–17 fs). The electron bunch charge highly depends on the chirping of the laser pulse. We anticipate that the ultrashort sub-GeV electron bunch generated in the laser wakefield acceleration mechanism using chirped laser pulses may be crucial in developing free-electron laser-based compact radiation sources.

Breakdowns (BDs) may occur in high-voltage applications even in ultrahigh vacuum conditions. Previously, we showed that it is important to pay attention to the post-BD voltage recovery in order to limit the appearance of secondary BDs associated with the primary ones. This can improve the overall efficiency of the high-voltage device. In this study, we focus on the optimization of the linear post-BD voltage recovery, with the principle aim of alleviating the problem of the secondary BDs. We investigate voltage recovery scenarios with different starting voltages and slopes of linear voltage increase by using a pulsed dc system. We find that a higher number of pulses during the voltage recovery produces fewer secondary BDs and a lower overall BD rate. Lowering the number of pulses led to more dramatic voltage recovery resulting in higher BD rates. A steeper voltage increase rate led to a more localized occurrence of the secondary BDs near the end of the voltage recovery period. It was also found that the peak BD probability is regularly observed around 1 s after the end of the ramping period and that its value decreases exponentially with the amount of energy put into the system during the ramping. The value also decays exponentially with a half-life of ($1.4\pm 0.3$) ms if the voltage only increased between the voltage recovery steps.

It has been known that a high-brightness electron beam emits broadband synchrotron radiation when traversing a curved orbit. The radiation reaction at wavelengths comparable to the bunch length or to the wavelength of a phase space modulated beam may lead to collective instabilities. In this paper, we investigate a potential single-pass instability mechanism of coherent-radiation-induced longitudinal multibunch beam breakup (BBU) instability in the presence of a closely spaced microbunch train in an undulator, particularly when the microbunch spacing is close to the resonant wavelength of the undulator. This problem is formulated based on the macroparticle model together with the slippage constraint on the beam-wave interaction. The set of coupled differential equations for individual microbunches can be solved analytically for the first few microbunches with linearization of the coherent radiation wakefield, and numerically in general nonlinear cases for unequal spacing or nonuniform filling charges. The underlying mechanisms, including the slippage effect, the potential-well effect leading to extra focusing, dependence of microbunch spacing and filling patterns, are discussed. The analysis is then applied to the recently proposed steady-state microbunching (SSMB) mechanism with two examples serving for the high average coherent radiation power sources for extreme ultraviolet (EUV) and infrared wavelength regions. For the specific scenario considered in this paper, it is found that when the microbunch spacing is close to the fundamental resonant wavelength, the coherent radiation can provide extra longitudinal focusing for the individual microbunches, leading to more stable multibunch oscillations. For the preliminary nominal SSMB designs with the average beam current of 1 A, our studies show that the single-pass longitudinal BBU instability should not be a severe issue.

With the advent of increased computational resources and improved algorithms, machine learning-based models are being increasingly applied to complex problems in particle accelerators. However, such data-driven models may provide overly confident predictions with unknown errors and uncertainties. For reliable deployment of machine learning models in high-regret and safety-critical systems such as particle accelerators, estimates of prediction uncertainty are needed along with accurate point predictions. In this investigation, we evaluate Bayesian neural networks (BNN) as an approach that can provide accurate predictions along with reliably quantified uncertainties for particle accelerator problems, and compare their performance with bootstrapped ensembles of neural networks. We select three accelerator setups for this evaluation: a storage ring, a photoinjector, and a linac. The problems span different data volumes and dimensionalities (e.g., scalar predictions as well as image outputs). It is found that BNN provide accurate predictions of the mean along with reliable estimates of predictive uncertainty across the test cases. In this vein, BNN may offer an attractive alternative to deterministic deep learning tools to generate accurate predictions with quantified uncertainties in particle accelerator applications.

As the next generation of light sources is pushing toward high-brightness storage rings with ultralow emittance, the control of the beam nonlinear dynamics becomes increasingly challenging. Nonlinear perturbations, arising from sextupole cross talks, set the limit to the achievable dynamic performance of the machine. Octupoles can be an efficient mean to tackle the remaining resonant driving terms generated to the second order by the lattice sextupoles. However, they have been used sparingly in light sources because of a lack of control on higher order effects. In this paper, we discuss the optimal positioning of octupoles, with respect to the sextupoles present in the lattice, for a local and simultaneous compensation of all fourth order on-momentum phase and amplitude dependent driving terms, using only three families of octupoles. In addition, higher order geometrical terms are also minimized, including among others, second-order tune shift with amplitude. This study is a continuation of past research made on the optimal use of octupoles for the operation of future light sources. The correction method was built on observations made on a simple model, then applied to a realistic low-emittance lattice, designed in the framework of the upgrade of the National Synchrotron Light Source II, to demonstrate its potential.

The full-energy beam injection system from SACLA to the SPring-8 storage ring has been developed. Low-emittance beam injection is required for the future upgrade of SPring-8 due to its small injection beam aperture. The electron beam injection from an X-ray free-electron laser linear accelerator not only satisfies the emittance requirement but also it confers a cost benefit compared to a conventional dedicated injector system. In SPring-8, the SACLA linear accelerator is now delivering X-ray free-electron laser photon beams and serving as a full-time injector of SPring-8 in parallel.

We present the first rf studies of the medium-$\beta$ superconducting radio frequency (SRF) elliptical cavities designed for Michigan State University’s Facility for Rare Isotope Beams (FRIB) energy upgrade linac. The proposed energy upgrade for this continuous-wave (CW) superconducting linac will double the final beam energy from 200 to $400\mathrm{MeV}/\mathrm{u}$ for the heaviest uranium ions within the 80 m of space available in the FRIB tunnel. Two prototype ${\beta }_{\mathrm{opt}}=0.65644\mathrm{MHz}$ five-cell elliptical SRF cavities were fabricated and tested to validate the novel cavity design with three conventional rf recipes: (1) Electropolishing (EP-only), (2) $\mathrm{EP}+48\mathrm{h}$ $120°\mathrm{C}$ bake ($\mathrm{EP}+\text{baking}$), and (3) Buffered chemical $\text{Polishing}+48\mathrm{h}$ $120°\mathrm{C}$ bake ($\mathrm{BCP}+\text{baking}$). The EP-only recipe achieved a 2 K quality factor ($Q_0$) of $2.3\times {10}^{10}$ at the FRIB energy upgrade design accelerating gradient ($E_{\text{acc}}$) of $17.5\mathrm{MV}/\mathrm{m}$, and $Q_0$ of $1.2\times {10}^{10}$ at a maximum gradient of $26\mathrm{MV}/\mathrm{m}$, where the gradient was ultimately limited by the available rf amplifier power available for this test. These results validate the potential of the novel 644 MHz medium-$\beta$ cavity design and motivate its use in future high-${\mathrm{Q}}_0$ development work. The multipacting band, which exists at $E_{\mathrm{acc}}\approx 10\mathrm{MV}/\mathrm{m}$, was completely conditioned in the continuous-wave mode. We also observed that combining the 48 h $120°\mathrm{C}$ baking treatment with EP did not improve the EP-only cavity performance at 2 K due to increased residual resistance ($R_0$) and increased medium-field Q-slope. $\mathrm{BCP}+\text{baking}$ was also found to produce lower ${\mathrm{Q}}_0$ in this cavity due to increased medium-field Q-slope. The mechanical modes of this cavity were measured at room temperature, verifying that the quality factor of the dominant “accordion” mechanical mode these medium-$\beta$ cavities are particularly vulnerable to is close to that of the 1.3 GHz TESLA cavities. Thus, this mode arising from the novel geometry of these cavities is shown to not be of excessive concern for resonance control.

The design and test results of a pulsed power-driven X-ray source which is named Hawkeye-I are described. The Hawkeye-I is developed to execute flash X-ray radiography. It is a six-stage induction voltage adder and terminated by a positive polarity rod-pinch diode. The six stages are identical and the prime energy of each stage is provided by a tesla transformer elevating the output voltage from a discharge brick which consists of two $4\mu \mathrm{F}$ capacitors and a three-electrode gas switch. Comparing to the induction voltage adders driven by Marx generators, the numbers of gas switches and capacitors in the energy storage sections of Hawkeye-I decrease significantly. The Hawkeye-I can produce a maximum output voltage of about 4.2 MV. The corresponding X-ray source spot size and dose at 1 m are 1.4 mm and 17.9 rad, respectively. Fifty consecutive shots have been conducted to evaluate the reliability of the machine and 98% of the shots are successful. Furthermore, as the energy propagating in the accelerator is mainly determined by seven laser-triggered gas switches, the Hawkeye-I is capable of controlling the timing of the X-ray pulse accurately.

Shortening the pulse length of input power is a possible approach to raise the accelerating gradient limit of normal conducting structures. We present a novel design of a parallel-coupled structure that operates at ultrashort input power pulses. The principle of the design is to shorten the filling time of the whole structure by feeding the cells individually so that the duration of power transmission between cells can be saved. An X-band (11.994 GHz) parallel-coupled structure design with a 10 ns pulse length is presented in this work. Previous high-power tests of X-band structure prototypes show that a gradient of $120\mathrm{MV}/\mathrm{m}$ could be achieved for a 200 ns pulse length. Based on the law requiring that $E^{30}{t}^5=\text{constant}$, this 10-ns-structure design should be able to reach a gradient of $200\mathrm{MV}/\mathrm{m}$. A detailed circuit model and real-time electromagnetic field simulation methods for designing the parallel-coupling structure are also presented and discussed in this paper.

Synchrotron radiation (SR) originated at superconducting bending magnets is known to be at the origin of several beam detrimental effects related to vacuum instabilities. One of the major challenges in the design of the vacuum beam pipes of high-energy hadron colliders is the SR coping strategy. In the case of the future circular hadron collider (FCC-hh), a Cu-coated beam screen (BS) operating in the range of 40–60 K has been designed with the aim of protecting the superconducting magnet cold bores from direct synchrotron irradiation. In order to experimentally study the FCC-hh BS vacuum and cryogenic performance, two sample prototypes were manufactured and installed in the beam screen test-bench experiment (BESTEX) at the Karlsruhe Research Accelerator (KARA) at the Karlsruhe Institute of Technology (KIT). The emitted SR has a critical energy of 6.2 keV, very similar to the 4.6 keV of FCC-hh. Irradiation at both room (RT) and cryogenic (77 K) temperatures showed a significant reduction of the molecular photostimulated desorption yields ($\eta$) of the FCC-hh beam screen compared to those of Cu samples. A first approximation of $\eta$ and its evolution with the photon dose accumulated on the FCC-hh BS prototype at 77 K allows to estimate that a machine conditioning period of $\sim 1.2$ months would be needed to reduce the photostimulated molecular density at the necessary levels to ensure a 100 h beam lifetime at nominal FCC-hh operation.

The present state of progress in laser wakefield acceleration encourages considering it as a practical alternative to conventional particle accelerators. A promising application would be to use a laser-plasma accelerator as an injector for a synchrotron light source. Yet, the energy spread and jitter of the laser-plasma beam pose a significant difficulty for an efficient injection. In this paper, we propose a design of a prototype injector to deliver 500 MeV low-intensity electron bunches to the DESY-II electron synchrotron. The design utilizes presently available conventional accelerator technology, such as a chicane and an X-band radio frequency cavity, to reduce the energy spread and jitter of the electron beam down to a sub-per-mille level.

A new $X$-band high-power rotating mode SLAC energy doubler (SLED)-type rf pulse compressor is proposed. It is based on a novel cavity type, a single open bowl-shaped energy storage cavity with high-quality factor and compact size, which is coupled to the waveguide using a compact rotating mode launcher. The novel cavity type is applied to the rf pulse compression system of the main linac rf module of the klystron-based option of the Compact Linear Collider. Quasispherical rotating modes of ${\mathrm{TE}}_{1,2,4}$ and ${\mathrm{TE}}_{1,2,13}$ are proposed for the correction cavity and storage cavity of the rf pulse compression system, respectively. The storage cavity working at ${\mathrm{TE}}_{1,2,13}$ has a quality factor above 240 000 and a diameter less than 33 cm. The design of the pulse compressor and in particular of the high-Q cavity will be presented in detail.

Electron cooling is a fundamental process to guarantee the beam quality in low energy antimatter facilities. In extra low energy antiproton, the electron cooler reduces the emittance blowup of the antiproton beam and thus delivers a focused and bright beam to the experiments at the unprecedentedly low kinetic energy of 100 keV. In order to achieve a cold beam at this low energy, the electron gun of the cooler must emit a monoenergetic and relatively intense electron beam. An optimization of the extra low energy antiproton electron cooler gun involving a cold cathode is studied, with the aim of investigating the feasibility of using carbon nanotubes (CNTs) as cold electron field emitters. CNTs are considered among the most promising field emitting material. However, stability data for emission operation over hundreds of hours, as well as lifetime and conditioning process studies to ensure optimal performance, are still incomplete or missing, especially if the purpose is to use them in operation in a machine such as extra low energy antiproton. This manuscript reports experiments that characterize these properties and ascertain whether CNTs are reliable enough to be used as cold electron field emitters for many hundreds of hours.