Particle-in-cell simulations are among the most essential tools for the modeling and optimization of laser-plasma accelerators, since they reproduce the physics from first principles. However, the high computational cost associated with them can severely limit the scope of parameter and design optimization studies. Here, we show that a multitask Bayesian optimization algorithm can be used to mitigate the need for such high-fidelity simulations by incorporating information from inexpensive evaluations of reduced physical models. In a proof-of-principle study, where a high-fidelity optimization with fbpic is assisted by reduced-model simulations with wake-t, the algorithm demonstrates an order-of-magnitude speedup. This opens a path for the cost-effective optimization of laser-plasma accelerators in large parameter spaces, an important step toward fulfilling the high beam quality requirements of future applications.

One of the fundamental challenges for future leptonic colliders and neutrino factories as well as for high-sensitivity studies of lepton universality is to design and construct new high-intensity sources of muons and positrons. The next-generation sources should increase the intensity of the presently operating ones by at least 3 orders of magnitude and include an important option of producing longitudinally polarized leptons. The main effort to achieve this goal has been focused, so far, on the proton-beam-driven muon sources and electron-beam-driven positron sources. In this paper, we present exploratory studies of an alternative scheme which is based on high-intensity megawatt-class photon beams. Such beams could be delivered in the future by the Gamma Factory (GF) project. One of the GF multiple goals is to increase the energy range (by more than one order of magnitude) and the intensity (by more than 6 orders of magnitude) of presently operating photon sources. Such a leap can be achieved by extending the present hadron-collider modus operandi of the LHC with a new GF-operation-mode, allowing the collisions of beams with laser pulses. The exploratory studies presented in this paper demonstrate that more than ${10}^{13}$ muons of both signs and more than ${10}^{16}$ electrons/positrons per second can be produced by a GF source.

Dynamic aperture (DA) is an important nonlinear property of a storage ring lattice, which has a dominant effect on beam injection efficiency and beam lifetime. Generally, minimizing both resonance driving terms (RDTs) and amplitude dependent tune shifts is an essential condition for enlarging the DA. In this paper, we study the correlation between the fluctuation of RDTs along the longitudinal position and the DA area with double-bend and multibend achromat lattices. The fluctuation of one RDT is quantitatively represented by the average RDT at the positions of nonlinear magnets. It is found that minimizing the RDT fluctuations is more effective than minimizing the commonly-used one-turn RDTs in enlarging the DA, and thus can serve as a very powerful indicator in the DA optimization. It is also found that minimizing low-order RDT fluctuations can also reduce both higher-order one-turn RDTs and higher-order RDT fluctuations, which also indicates that even higher-order RDTs can also be controlled due to the reduction of higher-order RDT fluctuations. This could be the underlying physics behind the strong correlation between minimizing the RDT fluctuations and enlarging the DA. And this also suggests that controlling lower-order RDT fluctuations in the DA optimization can avoid the need for calculating higher-order RDTs to a large extent, which are not only more computationally complicated but also more numerous. Besides, the effectiveness of controlling the RDT fluctuations in enlarging the DA confirms that the local cancellation of nonlinear effects used in some diffraction-limited storage ring lattices is more effective than the global cancellation.

The Future Circular Collider (FCC) $e^{+}{e}^{-}$ injector complex needs to produce and transport high-intensity $e^{+}$ and $e^{-}$ beams at a fast repetition rate for topping up at collision energy. Two options are considered for a preaccelerator ring, to be used for an intermediate accumulation and acceleration before the bunches are transferred to the high-energy booster. One option is a slightly modified Super Proton Synchrotron (SPS), and the other is designing a new ring. We explore the needs and parameters of the existing SPS as prebooster ring and also develop a conceptual design of an alternative accelerator ring. After establishing the basic parameters, we describe the optics design and layout and discuss the single-particle linear and nonlinear dynamics optimization, including magnetic and alignment errors. In addition, we present synchrotron radiation power studies and analytical estimates of various collective effects for both prebooster ring design options, including space charge, intrabeam scattering, longitudinal microwave instability, transverse mode coupling instability, ion effects, electron cloud, and coherent synchrotron radiation.

Imposing a density modulation on an electron beam may improve the brightness of a Thomson source by orders of magnitude via superradiant emission. In this paper, we analytically and numerically analyze electron beam modulation via the ponderomotive force due to the copropagating beat wave formed by two laser pulses at different frequencies. We show that energy modulation favorably scales with electron beam energy but is limited by the interaction length imposed by the finite waist of the laser pulses. Additionally, the effect of initial emittance and energy spread on the quality of microbunching is studied. Finally, we propose a superradiant extreme ultraviolet Thomson source based on ponderomotive bunching.

Pulsed power generators create high-energy-density conditions by rapidly delivering an immense pulse of electrical current to a compact imploding load. Accurately measuring the shape and amplitude of this load current pulse is essential to understanding the behavior of all pulsed power experiments. At the Z Pulsed Power Facility, the closest-in load current measurements are provided by velocimetry techniques such as VISAR (velocity interferometer system for any reflector) and PDV (photonic Doppler velocimetry). Here, fiber-coupled interferometers measure the velocity history of an exploding metallic flyer plate that is embedded in the vertical walls of the current return can. The flyer plate is driven outward by the magnetic pressure from the load current such that magnetohydrodynamic modeling can be used to determine the load current waveform from the measured velocity history. In this paper, we present the first load current velocimetry measurements to be made from the horizontal top flyer plate that carries current radially inward from the return can to the load. These spatially resolved measurements, which span $R=5–9\mathrm{mm}$, are enabled by a transformative new velocimetry diagnostic—a line-imaging velocity interferometer called Z Line VISAR (ZLV)—whose optical performance overcomes the measurement challenges presented by the steep velocity gradients encountered on the top flyer plate. To validate ZLV’s capabilities, a 14-MA, 100-ns experiment was conducted to losslessly couple current up the return can and radially inward across the top flyer plate. Comparisons between the ZLV data obtained from this experiment and two-dimensional magnetohydrodynamic simulations driven with the current measured on the return can indicate that the current delivery across the top flyer plate is indeed lossless to within the few-percent uncertainty of the ZLV data. Given that the current coupling is lossless, the experimental results are used to demonstrate that one-dimensional current unfold techniques can be applied to generate a radially resolved load current map from the ZLV velocity data. This analysis provides a template for how to use the ZLV diagnostic to determine the efficacy of current delivery in future experiments where losses may occur in close proximity to the load.

This paper presents a quantitative description of field emission in superconducting rf cavities. The goal is to fully reconstruct the physics involved in the phenomenon, from electron current generation at the emitter to the radiation pattern measured by the x-ray detector. The field emission process in the cavity was reconstructed using CST Particle Studio, and then the generation of x-ray radiation and its propagation toward the detectors were modeled analytically. The entire model was then applied to the specific case of the European Spallation Source medium beta superconducting cavities, and the theoretical predictions were cross-checked with experimental data collected during cavity vertical tests.

Applications of laser-plasma accelerators (LPA) require independent control of electron beam parameters. However, due to the complex coupling of the many variables governing the laser-plasma interaction, precisely tuning these parameters based on simple scalings is often impossible or at least suboptimal. Here, we apply multiobjective Bayesian optimization to derive optimal tuning curves for LPAs, both in simulations and experiments. For electron energies between 150 and 250 MeV, we demonstrate tuning of the charge over a range of nearly 100 pC, while preserving optimal beam loading conditions with energy spreads below 5%. The derived tuning curves can explain the sometimes counterintuitive interplay between laser and plasma control variables that is necessary to find the best trade-off between competing beam properties.

A novel method of measuring an electron beam’s energy spectrum and divergence using Compton-scattered x rays created by colliding a laser with an electron beam has been developed and tested using the compact laser-Compton x-ray source at LLNL. The method only requires an x-ray imaging device and a filter material whose $K$-edge energy matches that of the Compton-scattered x rays. $K$-edge filtering of energy-angle correlated Compton x rays causes large variations in intensity over the viewing angle. These intensity variations contain information about the electron beam’s properties. By using simulation tools and adjusting the beam parameters to match the shape of the acquired image, the electron beam can be characterized. As a demonstration of this technique, a $75\text{−}\mathrm{\mu }\mathrm{m}$ Sn foil was used to filter 30-keV Compton x rays created from Compton scattering of a $532\text{−}\mathrm{\mu }\mathrm{m}$ laser beam by a 30-MeV electron beam. The measured parameters were mean energy $E=28.51\pm 0.06\mathrm{M}\mathrm{eV}$, energy spread ${\sigma }_E<0.3\%$, and beam divergence ${\sigma }_{\theta }=1.8\pm 0.1\mathrm{m}\mathrm{rad}$.

The beam-beam interaction is one of the most critical factors determining the luminosity performance of colliders. As a circular collider utilizing the crab-waist scheme, multiple factors, such as beam-beam, crab waist, impedances, etc., interact to determine the luminosity of SuperKEKB. The interplay of these factors makes it challenging to predict luminosity via simulations. This paper presents recent advances in understanding the luminosity performance of SuperKEKB from beam-beam simulations and experiments. The key aspects affecting the luminosity of SuperKEKB, as well as the areas where further research is needed, are highlighted.

Undulator tapering based on a prebunched electron beam and a strong seed laser has been demonstrated as a promising method to greatly enhance free-electron laser (FEL) efficiency and suppress sideband instability. Different from the former works that introduce an ideal bunching or a seed laser or a self-seeding mechanism, in this paper, we propose to enhance FEL efficiency by optimizing the tapering strategy with a prebunched electron beam from echo-enabled harmonic generation (EEHG). The simulation results demonstrate that the efficiency enhancement can be effectively maintained in a quite long undulator, in which the whole FEL efficiency is improved by more than 2 orders and a clear center spectrum inherited from EEHG can be preserved. Benefitting from a stable initial phase space with an appropriate tapering strategy, the nature of stabilities can be well preserved at both time and frequency domain. Through jumping the resonant phase, an effective sideband suppression can be realized and the stability of output power is improved. In this scheme, the inhomogeneous shot noise and microbunching instability also have an important impact on efficiency enhancement and output stability. It shows a different instability feature from the first stage to the following tapered undulator in comparison to the self-seeding tapered scheme.

Regulating the arrival time of electron bunches is a crucial step to improve the temporal resolution of accelerator-based pump-probe experiments. In this regard, an electron beam regulation method called beam-based feedback, has been shown to work well for stabilizing longitudinal beam properties on pulsed accelerator machines. Essentially, the method resembles a typical design of a proportional regulator, where the plant is represented by an electron beam response matrix, and where the inversion of such matrix produces the regulator. In recent years, however, linear accelerators that operate in a continuous-wave mode have received increasing attention. One of the key features of such machines is the improved statistics of measured data, which enables a high-resolution spectral analysis of the noise acting on the electron beam. This new insight allows us to reinterpret the electron beam regulation as a disturbance rejection goal, where the disturbance is based on measured frequency data. In this work, we show that the proportional beam-based feedback method has a principal performance limitation that becomes apparent by analyzing continuous-wave data. To improve this situation, we propose a regulator design that incorporates a dynamical disturbance model formulated in the context of the so-called ${\mathcal{H}}_2$ mixed-sensitivity problem. The designed regulator demonstrated excellent agreement between the model and measurements carried out at the continuous-wave linear accelerator ELBE and showed a potential to improve the proportional regulator approach.

Short-pulse two-beam acceleration (TBA) is a promising advanced accelerator concept that may meet the luminosity and cost requirements of future large-scale linear colliders. Accelerating structures with high group velocity (on the order of 0.1c) are required in this scheme to reduce the filling time so as to efficiently accelerate the main beam with multiple bunches. In conventional accelerating structures such as metallic disk-loaded and dielectric-loaded, high group velocity usually implies low shunt impedance, which limits the rf-to-main-beam efficiency and causes a bottleneck in the overall wall-plug-to-main-beam efficiency. A dielectric disk accelerator (DDA) is a hybrid structure, which has geometry similar to metallic disk-loaded accelerators, but replaces the metal disks with dielectric disks to form the cells. Such a configuration could achieve simultaneous high group velocity and high shunt impedance, making a DDA a very attractive candidate for the short-pulse TBA approach by significantly improving power efficiency. In this manuscript, we present the start-to-end development of two $X$-band single-cell DDA prototypes, including the rf and mechanical design, fabrication, cold test, high power beam test, and post-test inspection. Of the two, a clamped structure performed better, achieving $102\mathrm{MV}/\mathrm{m}$ accelerating gradient from 320-MW input power.

A higher harmonic cavity (HHC) is popularly employed in synchrotron light storage rings to enhance the machine performance, which requires its fundamental mode resonant frequency to be tuned above the radio-frequency harmonic. However, this detuning is likely to cause Robinson instability. In this paper, we focus on a mode-zero Robinson instability driven by the fundamental mode of a passive superconducting harmonic cavity (PSHC). This instability oscillates slightly below the detuning frequency of PSHC and was recently observed in tracking simulations or experiments for several synchrotron light sources, but the underlying mechanisms have not been well understood. To investigate this instability, we modify the conventional Robinson instability equation with the inclusion of the damping effect. By solving directly this modified equation combined with performing macroparticle tracking simulation, it is found that this instability is largely dependent on the momentum compaction factor, the $Q$ value and detuning of PSHC, and even the radiation damping time. Most importantly, this instability can be significantly enhanced by a higher $Q$ of PSHC and a lower radiation damping time, which is completely contrary to the conventional Robinson instability.

We conduct a detailed measurement and analysis of a hadron beam in five-dimensional phase space at the Spallation Neutron Source Beam Test Facility. The measurement’s resolution and dynamic range are sufficient to image sharp, high-dimensional features in low-density regions of phase space. To facilitate the complex task of feature identification in the five-dimensional phase space, we develop several analysis and visualization techniques, including nonplanar slicing. We use these techniques to examine the transverse dependence of longitudinal hollowing and longitudinal dependence of transverse hollowing in the distribution. This analysis strengthens the claim that low-dimensional projections do not adequately characterize high-dimensional phase space distributions in low-energy hadron accelerators.

We present analytical and numerical studies of short-range wakefields in an L-shaped corrugated structure. This structure has variable streaking polarization and the quadrupole wakefield component is suppressed when the beam moves with equal distances to both plates. These features provide promising capability in multidimensional beam phase space diagnostic and fresh-slice applications. We first give zeroth-order analytical expressions of the longitudinal and the transverse wake functions using the conformal mapping method. Then we combine these results with the first-order approximate formulas of a single-plate structure to obtain more accurate analytical approximations of the wake functions of the L-shaped structure. In order to confirm the accuracy of the analytical model, we have developed a numerical method based on integral equations in the frequency domain for arbitrarily shaped waveguides with surface impedance boundary conditions.

Plasma waves can be used to create efficient and small-sized electron accelerators. Despite the already demonstrated success in reaching GeV-scale energy gain in the motion of electrons, laser-induced wakefields still suffer from the transverse spreading and the loss of synchronization with the accelerating particles. In this work, to tackle these problems, we propose using a two-wavelength structured laser beam with a designed angular dispersion and adjustable group velocity in free space to efficiently excite a nondiverging plasma wave. This approach allows one to generate an electron bunch with low transverse spread and reach high average acceleration gradients.

We revisit free-electron laser (FEL) equations in the presence of a planar undulator to investigate effects on longitudinal FEL dynamics upon seeding by a harmonic. For this study, we adopt slowly varying phase approximation, collective variables, and Klimontovich description of particle beams to explore the effects of harmonic seeding on low-gain saturation, low-gain amplification, and exponential growth for on-axis radiation emission. We also investigate the effects of frequency detuning and nonzero energy spread on power gain and growth rate for low-gain and high-gain FELs, respectively.

In the aspect of longitudinal beam bunching, the bunching strength can be controlled by the rf cavity phase and voltage. However, these machine parameters are different from those that interact with the beam itself. In order to gain control of the beam-cavity interaction, cavity calibration must be performed. Furthermore, it relies on fitting the beam energy gain versus cavity phase to a calibration function. Under the conventional assumption of relativistic beam conditions, the calibration function is a first harmonic sinusoidal function (a sinusoidal function with a period of $2\pi$). However, this expression is insufficient for a high-voltage bunching cavity. Due to beam acceleration inside the cavity, an energy bias and a second harmonic function should be included to modify the conventional calibration function, even for a relativistic electron beam. In this paper, we will derive this modification and provide a comparison to both the Coherent Electron Cooling Experiment and the impact-t simulation, respectively.