The all-optical synchronization systems used in various x-ray free-electron lasers (XFEL) such as the European XFEL usually include bunch arrival-time monitors (BAM), which observe the transient fields of passing electron bunches through their pickups. The extracted signal is used to modulate the amplitude of a reference laser pulse in a Mach-Zehnder type electro-optical modulator. The laser pulse is typically much shorter than the voltage signal and interacts only with an instantaneous value of the applied signal. With the emerging demand for future experiments with short FEL shots, fs precision is required for the synchronization systems even with 1 pC bunches. Since the sensitivity of the BAM depends in particular on the slope of the bipolar signal at the zero crossing and thus, also on the bunch charge, a redesign with the aim of a significant increase by optimized geometry and bandwidth is inevitable. In this contribution the theoretical foundations of the pickup signal are aggregated and treated with a focus on short bunches as well as a general formulation. The analytical treatment reveals design limitations and suggests several approaches for further development. Based on these considerations a new pickup concept with possible advantages in manufacturing and temporal resolution is simulated and its performance is compared to the previous concept. The design offers good potential and a significant improvement of slope and voltage is found. Nevertheless the target set for 1 pC operation was not fully reached yet and further optimization is necessary. The observed improvement is mainly achieved by the reduced distance to the beam and a higher bandwidth.

Virtual diagnostic (VD) is a computational tool based on deep learning that can be used to predict a diagnostic output. VDs are especially useful in systems where measuring the output is invasive, limited, costly or runs the risk of altering the output. Given a prediction, it is necessary to relay how reliable that prediction is, i.e., quantify the uncertainty of the prediction. In this paper, we use ensemble methods and quantile regression neural networks to explore different ways of creating and analyzing prediction’s uncertainty on experimental data from the Linac Coherent Light Source at SLAC National Lab. We aim to accurately and confidently predict the current profile or longitudinal phase space images of the electron beam. The ability to make informed decisions under uncertainty is crucial for reliable deployment of deep learning tools on safety-critical systems as particle accelerators.

High fidelity charged particle beam dynamics, especially if collisional, require specialized numerical methods. The challenges include the ability to deal with very large particle numbers, long-range electromagnetic forces, and vast spatial and timescales. We developed a novel algorithm to address these challenges. The main characteristics of the algorithm are Strang splitting to separate near and far forces, the fast multipole method to lower the computational cost of the far region, and the Simò integrator to capture all close encounters efficiently in the near region. The algorithm is fully adaptive both in space and time, while maintaining symplecticity to machine precision. Its performance is illustrated with two challenging examples from nonlinear multiparticle beam dynamics, including the first electron cooling simulations based on first principles.

High-dimensional optimization is a critical challenge for operating large-scale scientific facilities. We apply a physics-informed Gaussian process (GP) optimizer to tune a complex system. Typical GP models learn from past observations to make predictions, but this reduces their applicability to systems where there is limited relevant archive data. Instead, here we use a fast approximate model from physics simulations to design the GP model. The GP is then employed to make inferences from sequential online observations in order to optimize the system. Simulation and experimental studies were carried out to demonstrate the method for online control of a storage ring. Our method is a simple prescription to construct a custom GP model, including correlations between the high-dimensional input space, while encoding the physical response of a system. The ability to inform the machine-learning model with physics, without relying on the availability and range of prior data, may have wide applications in science.

This work proposes a novel longitudinal phase-space reconstruction method for hadron machines. The proposed method is based on a Kalman filter and can therefore provide real-time estimates of the phase-space reconstruction. The main input for the method is real-time measurements of the longitudinal bunch profile. Beam conditions in the LHC are used throughout this work as examples of the applicability and practical implementation of such a method. Longitudinal phase-space reconstructions obtained with the proposed method are compared with a tomographic-based technique using experimentally logged data from the LHC wall current monitors.

High-gain free-electron lasers (FELs) are driven by short, high-charge density electron beams as only produced at dedicated single pass or recirculating linear accelerators. We describe new conceptual, technical, and modeling solutions to produce subpicosecond, up to $\sim 100\mu \mathrm{J}$-energy extreme ultra-violet and soft x-ray FEL pulses at high- and tunable repetition rates, from diffraction-limited storage ring light source. In contrast to previously proposed schemes, we show that lasing can be simultaneous to the standard multibunch radiation emission from short insertion devices, and that it can be obtained with limited impact on the storage ring infrastructure. By virtue of the high-average power but moderate pulse energy, the storage ring-driven high-gain FEL would open the door to unprecedented accuracy in time-resolved spectroscopic analysis of matter in the linear response regime, in addition to inelastic scattering experiments.

A two-dimensional theory of coherent synchrotron radiation for a bending magnet with finite length is developed as an extension to the one-dimensional theory. On axis, the transient longitudinal wakes agree exactly to those in the one-dimensional theory. Off axis, the deviations are small but noticeable. The theory includes the transient centrifugal and centripetal forces after electron bunch enters and then exits the magnet. However, the transverse wakes are an order of magnitude smaller than the longitudinal ones.

A new model for the description of beam instabilities in synchrotrons featuring wakefields and space charge forces is proposed, using the circulant matrix approach. The predictions of this model are discussed in light of past ones, with a particular emphasis on the possible mitigation of the transverse mode coupling instability by space charge forces. The existence of transient amplification in spite of the absence of unstable eigenvalues in configuration featuring strong space charge forces is also addressed. It is shown that this behavior can be recovered when considering an airbag distribution. Yet when considering a more realistic Gaussian distribution, the radial modes lead to other types of mode coupling instabilities. The predictions of the new model are then compared to results of an experiment conducted at the CERN Super Proton Synchrotron, showing a reasonable agreement.

In modern high-brightness electron accelerators, the goal of preserving the beam quality from the source has long been pursued to ensure the fulfillment of their scientific missions. However, when delivering the electron beam through a beam transfer line, which may consist of bending magnets, the performance of a high-brightness electron beam may be degraded by the coherent synchrotron radiation (CSR) and the residual longitudinal dispersion. It is of critical importance to preserve the transverse emittance and control the bunch length variation as well as maintain the longitudinal phase space distribution in such a beam transfer line. For a multipass beam transfer line, the achromatic cell designs with stable optics would bring great convenience. In this study, for the periodic stable symmetric triple-bend achromat designs with identical dipoles, it is discovered that the CSR-induced emittance growth and the longitudinal dispersion up to high orders can be minimized simultaneously when a certain transfer matrix relation is satisfied; to be specific, the matrix entries between the first two dipoles should follow $[m_{11},m_{21}({\mathrm{m}}^{-1})]\approx (-2,0)$. Numerical optimizations demonstrate that the normalized transverse emittance, the bunch length, and the longitudinal phase space distribution are well preserved. In addition, such a design strategy shows the potential merit of effectively mitigating the microbunching instability.

Kinetic simulations of Sandia National Laboratories’ $Z$ machine are conducted to understand particle transport in the highly magnetized environment of a multi-MA accelerator. Joule heating leads to the rapid formation of electrode surface plasmas. These plasmas are implicated in reducing accelerator efficiency by diverting current away from the load [M.R. Gomez et al., Phys. Rev. Accel. Beams 20, 010401 (2017), N. Bennett et al., Phys. Rev. Accel. Beams 22, 120401 (2019)]. The fully-relativistic, electromagnetic simulations presented in this paper show that particles emitted in a space-charge-limited manner, in the absence of plasma, are magnetically insulated. However, in the presence of plasma, particles are transported across the magnetic field in spite of being only weakly collisional. The simulated cross-gap currents are well-approximated by the Hall current in the generalized Ohm’s law. The Hall conductivities are calculated using the simulated particle densities and energies, and the parameters that increase the Hall current are related to transmission line inductance. Analogous to the generalized Ohm’s law, we extend the derivation of the magnetized diffusion coefficients to include the coupling of perpendicular components. These yield a Hall diffusion rate, which is equivalent to the empirical Bohm diffusion.

We consider the capability of flexible spin-transparent polarization control and manipulation in conventional synchrotrons at integer spin resonances by means of spin navigators. The latter are designed as a couple of small solenoids separated by a constant beam bend. We formulate the requirements to the navigator design considering the criteria for stability of the spin motion in the presence of synchrotron energy oscillations. We propose the design of a novel spin-flipping system free of resonant beam depolarization based on such a spin navigator. We discuss the possibilities of testing spin-flipping systems at an integer spin resonance with protons in the Nuclotron ring at JINR in Dubna, Russia, and with deuterons in the RHIC rings at BNL in Upton, New York. The results are relevant to the existing and future facilities where the spin transparency mode can be applied for polarization control.

In this paper, we describe a harmonic kicker system used in the beam exchange scheme for the circulator cooling ring (CCR) of the Jefferson Lab Electron-Ion Collider. By delivering an ultrafast deflecting kick, a kicker directs electron bunches selectively in/out of the CCR without degrading the beam dynamics of the CCR optimized for ion-beam cooling. We will discuss the design principle of the kicker system and demonstrate its performance with various numerical simulations. In particular, the degrading effects of realistic harmonic kicks on the beam dynamics, such as 3D kick field profiles interacting with the magnetized beam, are studied in detail with a scheme that keeps the cooling efficiency within allowable limits.

In order to improve machine performance, the optics parameters of the storage ring need to be measured accurately. The commonly used accelerator optics parameters, including momentum compaction factor and dispersion function, are key parameters that reflect the state of the machine. It is difficult to measure them with traditional methods during the user operation run. Based on a high-resolution bunch-by-bunch three-dimensional measurement system, a noninvasive measurement of momentum compaction factor and the dispersion function was performed at the Shanghai synchrotron radiation facility (SSRF). The dispersion function can be obtained purely from the direct observation results of the bunch-by bunch 3D position, based on the definition of the dispersion function during normal user operation run without perturbing the beam. The theoretical background and experimental results of the method are discussed in this paper. The measurement results are in good agreement with the design values.

Many future particle colliders require beam crabbing to recover geometric luminosity loss from the nonzero crossing angle at the interaction point (IP). A first demonstration experiment of crabbing with hadron beams was successfully carried out with high energy protons. This breakthrough result is fundamental to achieve the physics goals of the high luminosity LHC (HL-LHC) and the future circular collider (FCC). The expected peak luminosity gain (related to collision rate) is 65% for HL-LHC and even greater for the FCC. Novel beam physics experiments with proton beams in CERN’s Super Proton Synchrotron (SPS) were performed to demonstrate several critical aspects for the operation of crab cavities in the future HL-LHC including transparency with a pair of cavities, a full characterization of the cavity impedance with high beam currents, controlled emittance growth from crab cavity induced rf noise.

In preparation for a demonstration of optical stochastic cooling in Cornell Electron Storage Ring, we have developed a particle tracking simulation to study the relevant beam dynamics. Optical radiation emitted in the pickup undulator gives a momentum kick to that same particle in the kicker undulator. The optics of the electron bypass from pickup to kicker couples betatron amplitude and momentum offset to path length so that the momentum kick reduces emittance and momentum spread. Nearby electrons contribute an incoherent noise. Layout of the bypass line is presented that accommodates optics with a range of transverse and longitudinal cooling parameters. The simulation is used to determine cooling rates and their dependence on bunch and lattice parameters for bypass optics with distinct emittance and momentum acceptance.

Longitudinal collective modes of a bunched beam with a repulsive inductive impedance (the space charge below transition or the chamber inductance above it) are analytically described by means of reduction of the linearized Vlasov equation to a parameterless integral equation. For any multipolarity, the discrete part of the spectrum is found to consist of infinite number of modes with real tunes, which limit point is the incoherent zero-amplitude frequency. In other words, notwithstanding the rf bucket nonlinearity and potential well distortion, the Landau damping is lost. Hence, even a tiny coupled-bunch interaction makes the beam unstable; such growth rates for all the modes are analytically obtained for arbitrary multipolarity. In practice, however, the finite threshold of this loss of Landau damping is set either by the high-frequency impedance roll-off or intrabeam scattering. Above the threshold, growth of the leading collective mode should result in persistent nonlinear oscillations.