Cooling of beams circulating in storage rings is critical for many applications including particle colliders and synchrotron light sources. A method enabling unprecedented beam-cooling rates, optical stochastic cooling (OSC), was recently demonstrated in the Integrable Optics Test Accelerator (IOTA) electron storage ring at Fermilab [J. Jarvis et al., Nature (London) 608, 287 (2022)]. This paper describes the numerical implementation of the OSC process in the particle-tracking program elegant and discusses the validation of the developed model with available experimental data. The model is also employed to highlight some features associated with different modes of operation of OSC. The developed simulation tool should be valuable in guiding future configurations of optical stochastic cooling and, more broadly, modeling self-field-based beam manipulations.

In this paper, we applied an approximate entropy (ApEn) analysis to nonlinear beam dynamics. Due to the presence of strong nonlinear magnets, the degree of chaos in beam motion gradually increases proportionally with its amplitude. Such chaos can be quantitatively characterized with ApEn of beam turn-by-turn readings. ApEn analysis is a technique used to quantify the amount of regularity and the unpredictability of fluctuations with respect to time-series data. The ApEn, when applied as a chaos indicator, can then be used for nonlinear lattice optimization and analysis. The National Synchrotron Light Source II (NSLS-II) electron storage ring lattice was used as an example of a real-world application of our technique.

When computing the space-charge tune shift for a relativistic bunched beam within a cylindrical chamber, mirror currents for a coasting beam, initially introduced to replace the chamber wall, are employed. Subsequently, the obtained result is extended to encompass the bunched beam, taking into account the bunching factor which quantifies the distribution of bunches around the accelerator ring. In the process of derivation, the terms that characterize the bunch length are intuitively integrated into the formula. As a result, the validity of this approach has never been established. This study provides the derivation of the space-charge tune shift formula for both relativistic and nonrelativistic bunched beams right from the outset, employing the Green function formalism. Subsequently, it is compared with the earlier formula derived using mirror currents.

The hybrid multibend achromat (HMBA) lattice concept is adopted in some diffraction-limited storage ring designs, which can permit relatively large on-momentum dynamic aperture and relatively weak sextupoles. In a typical HMBA lattice, the main arc section is constrained by the transverse phase advances making $-I$ transformation for sextupole cancellation. In this paper, a new HMBA lattice concept with sextupole cancellation across a straight section is proposed, where $-I$ is made between adjacent dispersion bumps of two lattice cells. This makes the main arc section free of the phase advance constraint, and as a result, the number of bending magnets (bends) in the lattice cell and the cell tunes can be easily changed, thus providing more choices for lattice design. To achieve the large phase advances required for $-I$ in this new concept, the split bend is used as the matching bend, which is a bend split into two pieces with a quadrupole in between. The split bend also serves to reduce the emittance, and the large phase advances also give low beta functions in the straight section enhancing the insertion device brightness. Besides, for a given emittance goal, this new HMBA lattice can have fewer bends than the typical HMBA lattice due to stronger focusing in bend unit cells, which is beneficial for saving space and suppressing intrabeam scattering effect. Two lattices are given as examples to demonstrate this new concept and show its linear and nonlinear properties and further extension is also discussed.

The High Luminosity (HL) upgrade of the Large Hadron Collider (LHC) will increase the peak luminosity at the experiments by more than a factor of 5 with respect to the LHC design value. To achieve this goal, among the upgrade of several beam and machine parameters, the beam intensity will nearly double with respect to the operational LHC value, and the transverse beam emittance will decrease by 50% compared to the LHC design value. Past operational experience showed that coherent beam instabilities may occur for low, positive values of chromaticity, and a higher tune spread than predicted from simulations is required from the dedicated octupole magnets to provide enough Landau damping. With the HL-LHC brighter beams, stability margins will become tighter, and coherent instabilities become stronger if no dedicated mitigation measures are taken. An impedance reduction plan is therefore taking place targeting the collimation system, and the main contributor to the transverse beam coupling impedance at the flattop energy. New collimators with lower resistivity materials will replace the current LHC ones. In this work, we assess the benefits of this impedance reduction with respect to the transverse mode coupling instability threshold. This study quantifies the discrepancy between measured and predicted beam stability thresholds at low chromaticity. It also probes the expected gain of the impedance reduction plan of HL-LHC.

A 10-kW-class high-power extreme ultraviolet light source is required in mass microfabrication technologies, and a free-electron laser based on an energy recovery linac (ERL) is an attractive candidate. The design beam current is 9.75 mA, and the beam energy is 800 MeV. The cavity design is required to achieve a target gradient of $12.5\mathrm{MV}/\mathrm{m}$ with a high yield and efficient suppression of radiofrequency power losses through higher-order modes (HOMs). The HOMs generated in the cavity pass through the end cells and are damped by the beam-tube dampers. The cavity was designed mainly to reduce the heat generated by the monopole modes. Further, the transverse shunt impedance of dipole modes must be smaller than the specification related to the beam breakup effect. Considering the cavity design, the end cell was optimized by selecting the appropriate frequency obtained using the tuning curve method. The shape of the HOM damper was designed using the complex permittivity data obtained from the S-parameter method with sample pieces. This cavity design was confirmed to have a sufficient margin of accelerating gradient and adequately damped HOMs for the 10-mA-class ERL operation.

Plasma based acceleration (PBA) is being considered for a next generation linear collider (LC). In typical PBA-LC designs, the extreme beam parameters are expected to trigger background ion motion, which can lead to longitudinally varying nonlinear focusing forces and result in emittance growth of the beam. While various schemes have been proposed to mitigate this at low beam energies, a solution to minimize the emittance growth in the later high energy stages of a multistage electron acceleration arm is yet to be found. In this paper, we propose to use an adiabatic plasma density ramp as a matching section that is able to match the witness electron beam to the low-density plasma entrance, where the beam initially has a large matched spot size so the ion motion effects are relatively small. As the beam propagates in the plasma density upramp (downramp), it is adiabatically focused (defocused) and its distribution maintains an equilibrium distribution throughout the entire process even when severe ion collapse has occurred. Simulation results from qpad show that within a single acceleration stage, this concept can limit the projected emittance growth to only $\sim 2\%$ for a 25 GeV, 100 nm normalized emittance witness beam and $\sim 20\%$ for a 100 GeV, 100 nm normalized emittance witness beam.

The ultrafast electron beam diffraction technology, which involves an electron beam at the level of several megaelectron volts, is crucial for studying ultrafast dynamic processes at the atomic level. To discern these processes at a temporal resolution of a few femtoseconds, the demand to focus on electron beams with femtosecond or even subfemtosecond width and time jitter is increasing. In this paper, we investigate the dynamics of electron beams as they interact with lasers, present an explanation of the mechanism that results in time jitter, and propose a method to mitigate or possibly eliminate the time jitter by inducing a specific energy chirp in the electron beam through the use of $X$-band photocathode rf gun. The feasibility of this method is confirmed by implementing bunch compression with two radially polarized laser pulses. Simulation results demonstrate that the time jitter can be reduced to less than 1 fs when the amplitude jitter of the rf gun is within $\pm 0.05\%$ and the phase jitter is within $\pm 0.5°$. Based on our findings, we anticipate that this approach will substantially advance the field of ultrafast science and technology.

We investigate electrostatic traps as a novel source of positron beams for accelerator physics applications. Penning-Malmberg (PM) traps are commonly employed in low-energy antimatter experiments. Positrons contained in the trap are cooled to room temperature or below. We calculate the thermal emittance of the positrons in the trap and show that it is comparable to or better than the performance of state-of-the-art photocathode guns. We propose a compact positron source comprised of a PM trap, electrostatic compressor, and rf accelerator that can be built and operated at a fraction of the cost and size of traditional target-based positron sources, albeit at a reduced repetition rate and with intrinsic angular momentum. We model the acceleration of a positron bunch up to an energy of 17.6 MeV with a final thermal emittance of $0.60\mathrm{\mu }\mathrm{m}\mathrm{rad}$ and bunch length of $190\mathrm{\mu }\mathrm{m}$. This system may be useful for accelerator physics studies, such as investigations of flat-beam sources for linear colliders and positron plasma wakefield acceleration.

Amplitude detuning measurements in the LHC have shown that a significant amount of detuning is generated via feed-down from high-order field errors in the magnets of the experiment insertion regions. This undesired effect can be detrimental to the luminosity production of the machine and is expected to be enhanced in the upcoming high-luminosity upgrade of the collider, the HL-LHC. In this study, we investigate the high-order errors in detail, performing amplitude detuning measurements during the commissioning of the LHC Run 3 and establish operational corrections via feed-down, using for the first time the dodecapole correctors in the insertion region.

We explore the performance of a recursive least-squares algorithm to determine the bandwidth ${\omega }_{12}$ and the detuning $\mathrm{\Delta }\omega$ of a superconducting cavity. We base the simulations on the parameters of the European Spallation Source double-spoke cavities. Expressions for the signal-to-noise ratio of derived parameters are given to explore the applicability of the algorithm to other configurations.

Institute of Modern Physics (IMP), Chinese Academy of Sciences, is the first domestic institute that widely applied solid-state power amplifier (SSPA) to linear accelerator project from the C-ADS project started since 2011. Until now, the same design has already been considered for two large-scale scientific facilities in Guangdong Province of southeast China, China initiative Accelerator Driven System and High Intensity Heavy-ion Accelerator Facility. In a recent decade of accelerator project construction, a large amount of valuable experience in solid-state power source design build and operation was accumulated with the operation and upgrade of several projects. Furthermore, following the successful development of the IMP SSPA, a new “standardization” design concept was put forward after many modifications and improvements, which means all designs meet the stringent system requirements: high performance, low cost, high scalability, high reliability, and ease of maintenance need. The new design and technology can be developed for this accelerator solution, such as the high-performance power amplifier, “hot swap” technology, and special combiners, which are all new methods for long-term stable operation and maintenance on-site, especially the processing failure without rf power interrupting. This paper will detail the development of “standardization” design and its technical characteristics.

We present a framework with which to analyze the effects of magnetic fringe fields. The theory defines the fringe field to be the transition between two regions of nearly constant field, and can incorporate constant multipoles in the magnet body. We then analyze Cartesian dipoles and derive symplectic fringe field maps that are applicable to longitudinal and/or transverse gradient dipoles. We verify the fringe maps with tracking, and show how we incorporated the theory into the tracking code elegant. The resulting elements and several supporting scripts are now available for users, and we conclude with several predictions relevant to the APS Upgrade project.

The quality of electron beams produced from plasma-based accelerators, i.e., normalized brightness and energy spread, has made transformative progress in the past several decades in both simulation and experiment. Recently, full-scale particle-in-cell (PIC) simulations have shown that electron beams with unprecedented brightness (${10}^{20}–{10}^{21}\mathrm{A}/{\mathrm{m}}^2/{\mathrm{rad}}^2$) and 0.1–1 MeV energy spread can be produced through controlled injection in a slowly expanding bubble that arises when a particle beam or laser pulse propagates in density gradient, or when a particle beam self-focuses in uniform plasma or has a superluminal flying focus. However, in previous simulations of work on self-injection triggered by an evolving laser driver in a uniform plasma, the resulting beams did not exhibit comparable brightnesses and energy spreads. Here, we demonstrate through the use of large-scale high-fidelity PIC simulations that a slowly expanding bubble driven by a laser pulse in a uniform plasma can indeed produce self-injected electron beams with similar brightness and energy spreads as for an evolving bubble driven by an electron beam driver. We consider laser spot sizes roughly equal to the matched spot sizes in a uniform plasma and find that the evolution of the bubble occurs naturally through the evolution of the laser. The effects of the electron beam quality on the choice of physical as well as numerical parameters, e.g., grid sizes and field solvers used in the PIC simulations are presented. It is found that this original and simplest injection scheme can produce electron beams with beam quality exceeding that of the more recent concepts.

The combination of a transverse deflecting structure (TDS) and a downstream screen is a well-established setup for the characterization of particle bunch length, detailed current profile, or in conjunction with a dipole spectrometer, the full longitudinal phase space. In low-$\gamma$ regimes, the inevitable induced energy spread together with longitudinal dispersion during the drift through the TDS can alter the phase space of the particle bunch to be characterized. The consequential bunch lengthening can render measuring ultrashort bunches impossible, even for high available streaking powers and hence theoretical resolution. In this work, the resulting effective resolution of a TDS is estimated analytically and the optimal streaking voltage $V_{\mathrm{opt}}$ is determined. The findings are subsequently benchmarked using matrix-based particle tracking. Furthermore, additional transverse, as well as other resolution limiting effects of a realistic streaking setup are discussed. A potential mitigation scheme using an additional downstream focusing element is evaluated. The findings presented in this work can substantially broaden the usable beam energy range of existing TDS setups and overall support the design of new setups.

The phase, growth rate, and onset of long proton bunch self-modulation in plasma can be controlled by a preceding short charged particle bunch. In this paper, by analyzing the growth rates of the self-modulation obtained from particle-in-cell simulation results, we identify two modes of self-modulation, namely noise-seeded and externally seeded self-modulations, and investigate their onset timings. We find that a uniform onset of the self-modulation at each slice of the long proton bunch is crucial for fine-tuning its phase and amplitude. We then demonstrate that a low-energy and low-current electron seed bunch in overdense plasma generates near-axis radial wakefields similar to those observed in the blowout regime. Consequently, the resultant self-modulation is excited as a single mode simultaneously along the entire long proton bunch.

This numerical study is focused on electro-optic (EO) spatial decoding of transition radiation (TR) produced by a relativistic electron bunch passing through a metal foil. The calculations included the imaging of polychromatic transition radiation from an electron bunch and the process of EO spatial decoding. From an experimental perspective, a careful examination of the calculation approach of the data analysis is essential. Therefore, to thoroughly understand the process of signal generation and examine the possibility of adopting a less time-consuming treatment, comparative studies were conducted on detailed and simplified models of both transition radiation imaging and EO signal generation. All calculations are defined in SI units for the convenience of experimental measurements. For TR imaging, the results suggest that the simplified analytical model is sufficient to perform polychromatic calculations with considerable accuracy. For EO spatial decoding, we discussed the process of EO signal generation using 1D and 2D models. We found that the 1D model was sufficient for rapid data analysis. Furthermore, the temporal energy chirp was demonstrated to have a minimal impact on the shapes of EO signals. This numerical study could facilitate measurements of 3D electron charge density profiles in both laser wakefield acceleration and conventional accelerator research.