In order to reduce the beam density efficiently for a safe beam abort, we analytically studied the motion of an aborted electron beam undergoing a sinusoidal kick by a beam shaker working at a constant frequency. Since the rf power is switched off, the betatron tune changes gradually with time due to chromatic effects. Chromatic aberration, together with a finite energy spread, enhances the dilution effect of the beam density, while the change in betatron tune with time causes a phase slippage that suppresses the growth of oscillation amplitude by the beam shaker. In order to treat such a situation properly, we formulated the motion of an aborted electron beam, taking into account nonlinear chromatic effects, under the adiabatic condition that the change in betatron tune is much slower than the betatron oscillation. In addition, by considering the case of a linearly varying betatron tune, it is shown that the response of an aborted electron beam can be interpreted as a superposition of waves, i.e., diffraction of waves. Our investigation not only provides a criteria for the determination of shaker’s frequency for a safe beam abort but also has applications to resonance crossing phenomena.

BLUE is a four-cavity linear transformer driver system in the Plasma, Pulsed Power, and Microwave Laboratory at the University of Michigan. Each BLUE cavity can store up to 2 kJ of electrical energy and provide an open-circuit voltage of 200 kV when using the maximum bipolar charge voltage of $\pm 100\mathrm{kV}$. The impedance of each cavity is approximately $0.5\mathrm{\Omega }$, and the rise time of the current pulse into a reasonably matched load is $\sim 100\mathrm{ns}$. The system will be used to study high-energy-density $Z$-pinch plasmas and high-power radiation sources. The facility has been equipped to handle intense bursts of x rays, nuclear fusion neutrons, and high-power microwaves (HPM). This paper reports on the construction and performance of the first (prototype) BLUE cavity and its auxiliary systems, as well as preliminary experiments using the cavity to drive resistive loads and a magnetically insulated line oscillator (MILO) HPM device. In these initial single cavity, BLUE-driven MILO experiments, 1.19 GHz microwave oscillations were successfully generated.

This paper continues the work of two previous treatments of bunch lengthening by a passive harmonic cavity in an electron storage ring. Such cavities, intended to reduce the effect of Touschek scattering, are a feature of fourth generation synchrotron light sources. The charge densities in the equilibrium state are given by solutions of coupled Haïssinski equations, which are nonlinear integral equations. If the only wake fields are from cavity resonators, the unknowns can be the Fourier transforms of bunch densities at the resonator frequencies. The solution scheme based on this choice of unknowns proved to be deficient at the design current when multiple resonators were included. Here we return to the conventional formulation of Haïssinski equations in coordinate space, the unknowns being charge densities at mesh points on a fine grid. This system would be awkward to solve by the Newton method used previously, because the Jacobian matrix is very large. Here a new solution is described, which is both Jacobian-free and much simpler. It is based on an elementary fixed point iteration, accelerated by Anderson’s method. The scheme is notably fast and robust, accommodating even the case of extreme overstretching at current far beyond the design value. The Anderson method is promising for many problems in accelerator theory and beyond, since it is quite simple and can be used to attack all kinds of nonlinear and linear integral and differential equations. Results are presented for ALS-U with updated design parameters. The model includes harmonic and main rf cavities, compensation of beam loading of the main cavity by adjustment of the generator voltage, and a realistic short range wake field (rather than the broadband resonator wake invoked previously).

We describe a method for precisely regulating the gradient magnet power supply (GMPS) at the Fermilab Booster accelerator complex using a neural network trained via reinforcement learning. We demonstrate preliminary results by training a surrogate machine-learning model on real accelerator data to emulate the GMPS, and using this surrogate model in turn to train the neural network for its regulation task. We additionally show how the neural networks to be deployed for control purposes may be compiled to execute on field-programmable gate arrays (FPGAs), and show the first machine-learning based control algorithm implemented on an FPGA for controls at the Fermilab accelerator complex. As there are no surprise latencies on an FPGA, this capability is important for operational stability in complicated environments such as an accelerator facility.

A framework for integrating transfer matrices with particle-in-cell simulations is developed for TeV staging of plasma wakefield accelerators. Using nonlinear transfer matrices in terms up to ninth order in normalized energy spread $\sqrt{⟨\delta {\gamma }^2⟩}$ and deriving a compact expression for the chromatic emittance growth in terms of the nonlinear matrix, plasma wakefield accelerating stages simulated using the three-dimensional particle-in-cell framework osiris 4.0 were combined to model acceleration of an electron beam from 10 GeV to 1 TeV in 85 plasma stages of meter scale length with long density ramps and connected by simple focusing lenses. In this calculation, we find that for initial relative energy spreads below ${10}^{-3}$, energy-spread growth below ${10}^{-5}$ of the energy gain per stage and normalized emittance below mm-mrad, the chromatic emittance growth can be minimal. The technique developed here may be useful for plasma collider design, and potentially could be expanded to encompass nonlinear wake structures and include other degrees of freedom such as lepton spin.

We propose a novel algorithm to deal with the interaction of beam-cavity, which is convenient to be implemented on the platform of matlab supporting graphics process unit operations and can significantly improve the computational efficiency. Actually, this algorithm has been implemented and the corresponding code is named stable, which can be used to study the longitudinal beam dynamics of arbitrary filled bunches in electron storage rings. In this paper, the stable code is mainly used to evaluate the bunch lengthening produced by the passive higher harmonic cavity. We first carry out efficiency tests for stable and the results are satisfactory. We then benchmark the stable code against the semianalytical approach and elegant tracking for equilibrium and nonequilibrium cases, respectively, and the results are in good agreement. Furthermore, the stable code is applied to the case of Advanced Light Source upgrade (ALS-U) with distributed gaps, and the results firstly reveal a regular and periodic transient beam loading effect and thus show a nonequilibrium state for the overstretching case. As a second example, the stable code is also applied to the case of the Hefei advanced light facility (HALF) storage ring with uniformly distributed gaps, and the results show that the passive normal conducting harmonic cavities with a total $R/Q$ of $\sim 200\mathrm{\Omega }$ will cause severe transient effects, thus being not suitable for Hefei advanced light facility.

Impedance modeling for accelerator applications has improved over the years, largely as a result of advances in simulation capabilities. While this modeling has been successful in reproducing certain measurements, it is still a significant challenge to predict collective effects in real machines. In this paper, we review our approach to impedance modeling and the subsequent simulations of collective effects. We discuss the choice of the electrodynamics codes and the required computer power resources, modeling of the geometric, and resistive wall impedances, their comparison with analytical approaches, and their application for simulating of the collective effects with tracking and beam-induced heating.

There has been much research on longitudinal bunch splitting and coalescing for accelerator performance improvements in rf synchrotrons. In this paper, we report a scheme with several induction cells that goes beyond the limitation in which the phase drift speed of beam buckets must be much lower than that of the maximum off-momentum particles to minimize longitudinal emittance blowup. In principle, additional pulse acceleration voltages can be applied to produce a momentum jump of the beam and save beam manipulation time, which is crucial for fast-cycling synchrotrons with limited injection and extraction times. This paper compares the new and conventional schemes with experimental results. Finally, the beam behaviors are discussed with macroparticle simulations.

The Large Hadron electron Collider (LHeC) is a proposed future particle-physics project colliding 60 GeV electrons from a six-pass recirculating energy-recovery linac (ERL) with 7 TeV protons stored in the LHC. The ERL technology allows for much higher beam current and, therefore, higher luminosity than a traditional linac. The high-current, high-energy electron beam can also be used to drive a free electron laser (FEL). In this study, we investigate the performance of an LHeC-based FEL, operated in the self-amplified spontaneous emission mode using electron beams after one or two turns, with beam energies of, e.g., 10, 20, 30 and 40 GeV, and aim at producing x-ray pulses at wavelengths ranging from 8 to 0.5 Å. In addition, we explore a possible path to use the 40 GeV electron beam for generating photon pulses at much lower wavelengths, down to a few picometer. We demonstrate that such ERL-based high-energy FEL would have the potential to provide orders of magnitude higher average brilliance at Å wavelengths than any other FEL either existing or proposed. It might also allow a pioneering step into the picometer wavelength regime.

Electron beams to be accelerated in beam-driven plasma wakes are commonly formed by a photocathode and externally injected into the wakefield of a preceding bunch. Alternatively, using the plasma itself as a cathode offers the possibility of generating ultrashort, low-emittance beams by trapping and accelerating electrons from the ambient plasma background. Here, we present a beam-driven plasma cathode realized via laser-triggered density-downramp injection, showing stable beam formation over more than a thousand consecutive events with an injection probability of 95%. The plasma cathode is highly tunable, resulting in the injection of electron bunches of tens of pC of charge, energies of up to 79 MeV, and relative energy spreads as low as a few percent. The stability of the injected beams was sufficiently high to experimentally determine their normalized emittance of $9.3\mu \mathrm{m}$ rms with a multishot method.

We present numerical simulations and experimental results of the self-modulation of a long proton bunch in a plasma with linear density gradients along the beam path. Simulation results agree with the experimental results reported [F. Braunmller, T. Nechaeva et al. (AWAKE Collaboration), Phys. Rev. Lett. 125, 264801 (2020)]: with negative gradients, the charge of the modulated bunch is lower than with positive gradients. In addition, the bunch modulation frequency varies with gradient. Simulation results show that dephasing of the wakefields with respect to the relativistic protons along the plasma is the main cause for the loss of charge. The study of the modulation frequency reveals details about the evolution of the self-modulation process along the plasma. In particular for negative gradients, the modulation frequency across time-resolved images of the bunch indicates the position along the plasma where protons leave the wakefields. Simulations and experimental results are in excellent agreement.

The onset of nonlinear effects, such as ponderomotive broadening, increases the radiation bandwidth and thereby places a stringent limitation on the laser intensity used in inverse Compton sources. Recently, we have shown that a judicious longitudinal laser frequency modulation (“chirping”) can perfectly compensate for this ponderomotive broadening and restore the narrow band property of scattered radiation in the Thomson regime, when electron recoil during the collision with the laser can be neglected. Consequently, using QED, the laser chirping has been extended to the Compton regime, where electron recoil is properly accounted for. Here we present a new, semiclassical model for computation of scattered spectra in the Compton regime. We also derive a comprehensive generalization of the expressions for chirping prescription for linearly polarized laser pulses in 1D plane-wave approximation with arbitrary shapes and arbitrary scattering angle in the Compton regime. We use these new expressions to show that the higher-order harmonics in sources with high laser fields and high electron beam energies (nonlinear Compton regime) will be nonlinearly redshifted when compared to those with lower beam energies (Thomson regime). The chirping prescription will act to correct ponderomotive broadening in very high harmonics.

We review the theory of optical klystrons, with its applications for self-amplified spontaneous emission free electron lasers in mind. We show that previous theories miss terms in the power gain factor that cannot be neglected, and we illustrate differences between the previously known analytical expressions, new ones found in this paper, and numerical calculations. We then consider the use of optical klystrons for electron energy-spread and radiation coherence-time diagnostics purposes.

High radio-frequency (rf) losses measured for cavities in original Continuous Electron Beam Accelerator Facility (CEBAF) cryomodules, compared to the losses measured in single-cavity tests, have been a long-standing issue related to their performance. We summarize experimental evidence of increased rf losses in CEBAF cavities arising from the fundamental power coupler cold window and waveguide, respectively. Cryogenic rf tests were done on cavities tested in vertical cryostats as well as inside cryomodules in the accelerator tunnel. The cold window metallization losses were assessed by combining numerical results with measured data obtained with an existing cryogenic waveguide resonator setup. The results showed that the cold window metallization losses can increase the cavity rf heat load at 2.07 K by up to 86%, depending on the standing-wave pattern in the fundamental power coupler waveguide, and that such losses are reduced if the distance between the waveguide and the cavity cells is increased.

Originating from the stochastic nature of the photon emission process, the diffusion of the electron longitudinal coordinate exists even if the global phase slippage of a storage ring is zero, as we cannot zero all the local phase slippages simultaneously. This quantum diffusion is viewed as the most fundamental limit of the lowest bunch length realizable in an electron storage ring from the single-particle dynamics perspective. Here, we present an analysis of this effect using the Courant-Snyder parametrization in the longitudinal dimension. Analytical formulas for the longitudinal emittance, energy spread, and bunch length are derived. The same formalism is used to discuss the application of multiple radio frequency systems for longitudinal strong focusing. The presented work is expected to be useful in the development of novel light source mechanisms like steady-state microbunching, where a short bunch length or small longitudinal emittance is needed.

In circular accelerators, crossing the linear coupling resonance induces the exchange of the transverse emittances, provided the process is adiabatic. This has been considered in some previous works, where the description of the phenomenon has been laid down, and more recently, where a possible explanation of the numerical results has been proposed. In this paper, we introduce a theoretical framework to analyze the crossing process, based on the theory of adiabatic invariance of Hamiltonian mechanics, which explains in detail various features of the emittance exchange process.

Low longitudinal emittance means small energy spread and short bunch length, which makes high-power short-wavelength coherent radiation possible. In a storage ring, due to quantum excitation, the equilibrium longitudinal emittance is mainly the overall contribution of all bend-related elements, such as bends, undulators and laser modulators. By introducing longitudinal Twiss function and analyzing the 6D one-turn map in 3D Twiss form, the longitudinal emittance contribution of all these elements is theoretically studied in this paper, and a general method for minimizing the storage ring equilibrium longitudinal emittance is proposed. An integrated optimization of longitudinal beta function shows, the longitudinal emittance scales as the third power of bending angle, and can be as low as subpicometer with a beam energy of 400 MeV in an ultimate state.

We describe the development and operation of a regenerative, cryogenically-formed discharge waveguide formed by freezing nitrous oxide gas onto the inner wall of a sapphire capillary. We demonstrate a technique for varying the channel diameter in situ and present guiding of low-power laser pulses through 6-cm long waveguides with channel diameters of 0.7, 0.8, and 1 mm. Measurements and simulations of the output laser fluence showed that the matched spot size could be adjusted with the thickness of the solid nitrous oxide layer.