To analyze nonlinear dynamic systems, we developed a new technique based on the square matrix method. We propose this technique called the “convergence map” for generating particle stability diagrams similar to the frequency maps widely used in accelerator physics to estimate dynamic aperture. The convergence map provides similar information as the frequency map but in a much shorter computing time. The dynamic equation can be rewritten in terms of action-angle variables provided by the square matrix derived from the accelerator lattice. The convergence map is obtained by solving the exact nonlinear equation iteratively by the perturbation method using Fourier transform and studying convergence numerically. When the iteration is convergent, the solution is expressed as a quasiperiodic analytical function as a highly accurate approximation and hence the motion is stable. The border of stable motion determines the dynamical aperture. As an example, we applied the new method to the nonlinear optimization of the NSLS-II storage ring and demonstrated a dynamic aperture comparable to or larger than the nominal one obtained by particle tracking. The computation speed of the convergence map is 30 to 300 times faster than the speed of the particle tracking, depending on the size of the ring lattice (number of superperiods). The computation speed ratio is larger for complex lattices with low symmetry, such as particle colliders.

Metallic coatings on the inner surfaces of the vacuum chambers are widely used in accelerators but the impact of the coating resistivity on the beam coupling impedance and beam dynamics is often not sufficiently evaluated. In this paper, it is shown that, for the usual coating thickness, the electrical resistivity of the coating can have a strong impact on the impedance and on the microwave instability threshold. In particular, using different nonevaporable getters coating as examples, the regime in which a coating with a lower resistivity produces a larger impedance than that with a higher resistivity is investigated.

The contribution of coherent wiggler radiation (CWR) to the microwave instability threshold in wiggler-dominated storage rings such as damping rings for colliders is discussed in detail. Three different coherent wiggler radiation impedance models are considered: the free-space steady-state model, the parallel-plates shielding steady-state model, and the rectangular-chamber shielding model. The field dynamics of CWR are compared, showing that the broad-band unshielded CWR becomes dominated by resonant structures when chamber shielding is considered. To suppress the narrow-band impedance in damping wigglers with chamber shielding, we propose employing a detuned damping wiggler. A new, simple, analytical method of solving the dispersion relation and detecting the CWR-driven microwave instability threshold is presented. The theory is compared with the numerical simulations of a Vlasov-Fokker-Planck solver for the Electron Ion Collider backup storage ring cooler and confirms that the microwave instability threshold gets higher for negative momentum compaction.

Successful operation of large-scale particle accelerators depends on the precise correction of magnet field or alignment errors present in the machine. In the Large Hadron Collider (LHC), transverse linear coupling has been shown to have a significant impact on the beam dynamics. However, current measurement methods are not sufficient for precise local coupling measurement at the interaction point. In this paper, an approach to determine interaction region local coupling corrections with a rigid waist shift based on correlated global variables such as the closest tune approach $|C^{-}|$ is presented. The validity of the method is demonstrated through simulations and experimental measurements taken during the LHC Run 3 commissioning in 2022, where determined corrections were applied and led to a measured luminosity increase of 9.7% and 3.5% at the ATLAS and CMS detectors, respectively.

A linear Breit-Wheeler module for the code geant4 has been developed. This allows signal-to-noise ratio calculations of linear Breit-Wheeler detection experiments to be performed within a single framework. The interaction between two photon sources is modeled by treating one as a static field, then photons from the second source are sampled and tracked through the field. To increase the efficiency of the module, we have used a Gaussian process regression, which can lead to an increase in the calculation rate by a factor of up to 1000. To demonstrate the capabilities of this module, we use it to perform a parameter scan, modeling an experiment based on that recently reported by Kettle et al. [New J. Phys. 23, 115006 (2021)]. We show that colliding 50-fs duration $\gamma$ rays, produced through bremsstrahlung emission of a 100 pC, 2-GeV laser wakefield accelerator beam, with a 50-ps x-ray field, generated by a germanium burn-through foil heated to temperatures $>150\mathrm{eV}$, this experiment is capable of producing $>1$ Breit-Wheeler pair per shot.

Originating from the stochastic nature of the photon emission process, the longitudinal coordinates of electrons diffuse 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 effect limiting the lowest bunch length realizable in an electron storage ring from single-particle dynamics perspective. Energy spread diverges when bunch length is pushed to the limit given by this diffusion. Here we report the first experimental evidence supporting the existence of this effect. The measurements confirm the breakdown of the classical formula of bunch length and energy spread in an electron storage ring with globally small and locally large phase slippage.

Advanced spectroscopy experiments and new physics experiments with positronium atoms in vacuum will benefit from positronium production in an environment free of magnetic and electrostatic fields. Here, we present a novel scheme for generating a bunched positron beam. The positron bunches are prebunched before extraction from a buffer-gas trap, nonadiabatically extracted from a 700 G magnetic field, energy elevated up to 20 kV, and bunched on a target in a free field. According to simulations of the system, 60% of cooled positrons in the buffer-gas trap are extracted and focused on the target in a time spread of 2.5 ns full width tenth maximum (FWTM) and a spot of about 4 mm FWTM for positron implantation energy higher than 3 keV. These performance numbers are achieved in the same apparatus through a combination of several innovative beam manipulations.

In this work, nondestructive virtual diagnostics are applied to retrieve the electron beam time of arrival and energy in a relativistic ultrafast electron diffraction (UED) beamline using independently measured machine parameters. This technique has the potential to improve the temporal resolution of pump and probe UED scans. Fluctuations in time of arrival have multiple components, including a shot-to-shot jitter and a long-term drift which can be separately addressed by closed loop feedback systems. A linear-regression-based model is used to fit the beam energy and time of arrival and is shown to be able to predict accurate behavior for both long- and short-time scales. More advanced time-series analysis based on machine learning techniques can be applied to improve this prediction further.

The European Synchrotron Radiation Facility (ESRF) storage ring (SR) has been dismantled and replaced by the Extremely Brilliant Source (EBS) which has now been commissioned. This new fourth generation light source has much smaller vacuum chambers than its predecessor, which necessitates a careful optimization of the vacuum systems and an accurate impedance model to be able to deliver all beam modes at the design current and predict future machine performance in the presence of strong collective effects. This paper will report on the first beam based measurements with a single bunch in order to characterize the short range wakefield model of the EBS SR and to make a first comparison with predictions. The results from transverse instability thresholds and tune shift measurements will be presented, as well as bunch length and phase variation with the current. Microwave instability threshold measurements have shown some discrepancy with predictions, which could arise from welding defects in the machining of the chambers. These discrepancies and other possible causes will be discussed.

Charge exchange injection is the standard mechanism used to accumulate short, intense pulses of proton beams from an ${\mathrm{H}}^{-}$ injector into a synchrotron. Historically, this process has relied on injection foils to remove the two electrons from the ${\mathrm{H}}^{-}$, a technique that has been successfully employed for beam powers up to 1.4 MW. However, such foils are known to sublimate beyond a threshold beam power density, requiring the development of another stripping technology that does not have the same limitation. This work reports on the experimental development of laser assisted charge exchange (LACE) as a mechanism for replacing the foils. In the present work, a method of laser assisted charge exchange that is scalable to full duty factor operation is experimentally demonstrated. The method, termed “sequential resonance excitation,” relies on a two-step quantum excitation of the electron from the ground state to the excited state. Compared with previously reported LACE experiments that utilized a single-step excitation, this method significantly reduces the required peak laser power, allowing for scalability to millisecond-long ${\mathrm{H}}^{-}$ pulses with conventional laser technology.

Recent studies of power flow and particle transport in multi-MA pulsed-power accelerators demonstrate that electrode plasmas may reduce accelerator efficiency by shunting current upstream from the load [Bennett et al., Phys. Rev. Accel. Beams 24, 060401 (2021)]. The detailed generation and evolution of these electrode plasmas are examined here using fully relativistic, Monte Carlo particle-in-cell (PIC) and magnetohydrodynamic (MHD) simulations over a range of peak currents (8–48 MA). The PIC calculations, informed by vacuum science, describe the electrode surface breakdown and particle transport prior to electrode melt. The MHD calculations show the bulk electrode evolution during melt. The physical description provided by this combined study begins with the rising local magnetic field that increases the local electrode surface temperature. This initiates the thermal desorption of contaminants from the electrode surface, with contributions from atoms outgassing from the bulk metal. The contaminants rapidly ionize forming a ${10}^{15}–{10}^{18}{\mathrm{cm}}^{-3}$ plasma that is effectively resistive while weakly collisional because it is created within, and rapidly penetrated by, a strong magnetic field ($>30\mathrm{T}$). Prior to melting, the density of this surface plasma is limited by the concentration of absorbed contaminants in the bulk ($\sim {10}^{19}{\mathrm{cm}}^{-3}$ for hydrogen), its diffusion, and ionization. Eventually, the melting electrodes form a conducting plasma (${10}^{21}–{10}^{23}{\mathrm{cm}}^{-3}$) that experiences $\mathbf{j}\times \mathbf{B}$ compression and a typical decaying magnetic diffusion profile. This physical sequence ignores the transport of collisional plasmas of ${10}^{19}{\mathrm{cm}}^{-3}$ which may arise from electrode defects and associated instabilities. Nonetheless, this picture of plasma formation and melt may be extrapolated to higher-energy pulsed-power systems.

The application of microfabrication techniques to beam physics responds to a need in the particle accelerator community for miniature, economical components that can be used in a range of diverse applications. In this work, we show the first reported results of relativistic (3.2 MeV kinetic energy) electron beam focusing using a microfabricated Panofsky-like quadrupole. The measurements revealed a spurious vertical steering dipole field which was corrected by asymmetrically exciting the currents in the device. The differences between experimental data and expected results are investigated with the help of radia simulations.

The implementation of concatenated superconducting rf cavities into high current storage rings raises the demand for appropriate beam pipe bellows. In order to compensate for length variations of the cavities due to thermal shrinkage and tuning, a mechanically soft element is required. It also should be of the least interaction with the beam to reduce the deposition of electromagnetic field energy, i.e., wakefields. Even in close neighborhood to high level cavity fields, parasitical Ohmic losses need to be minimized in order not to exceed cryogenic cooling capabilities. Overall construction length is an issue since real estate is strongly limited in between the existing magnetic lattice. The device needs to be compatible with cryogenic and ultrahigh vacuum conditions. The paper describes such a device as it was developed and tested in the framework of the Variable pulse length Storage Ring (VSR) Demo project, pursued at Helmholtz-Zentrum Berlin as a possible upgrade of the synchrotron light source BESSY II. It is denoted as Collimating Shielded Bellow, named such since it furthermore acts as an actively cooled synchrotron light collimator. The mechanical, electrodynamical, and thermal designs are presented. Particular attention is paid to the testing setup and the vacuum performance with the beam observed during extended testing periods in BESSY II.

Standard deep learning methods, such as Ensemble Models, Bayesian Neural Networks, and Quantile Regression Models provide estimates of prediction uncertainties for data-driven deep learning models. However, they can be limited in their applications due to their heavy memory, inference cost, and ability to properly capture out-of-distribution uncertainties. Additionally, some of these models require post-training calibration that limits their ability to be used for continuous learning applications. In this paper, we present a new approach to provide prediction with calibrated uncertainties that includes out-of-distribution contributions and compare it to standard methods on the Fermi National Accelerator Laboratory (FNAL) Booster accelerator complex.

The higher efficiency of superconducting radio-frequency (SRF) cavities compared to normal-conducting ones enables the development of high-energy continuous-wave linear accelerators (linacs). Recent progress in the development of high-quality ${\mathrm{Nb}}_3\mathrm{Sn}$ film coatings along with the availability of cryocoolers with high cooling capacity at 4 K makes it feasible to operate SRF cavities cooled by thermal conduction at relevant accelerating gradients for use in accelerators. A possible use of conduction-cooled SRF linacs is for environmental applications, requiring electron beams with energy of 1–10 MeV and 1 MW of power. We have designed a 915 MHz SRF linac for such an application and developed a prototype single-cell cavity to prove the proposed design by operating it with cryocoolers at the accelerating gradient required for 1 MeV energy gain. The cavity has a $\sim 3\mathrm{\mu }\mathrm{m}$ thick ${\mathrm{Nb}}_3\mathrm{Sn}$ film on the inner surface, deposited on a $\sim 4\mathrm{mm}$ thick bulk Nb substrate and a bulk $\sim 7\mathrm{mm}$ thick Cu outer shell with three Cu attachment tabs. The cavity was tested up to a peak surface magnetic field of 53 mT in liquid He at 4.3 K. A horizontal test cryostat was designed and built to test the cavity cooled with three Gifford-McMahon cryocoolers. The rf tests of the conduction-cooled cavity, performed at General Atomics, achieved a peak surface magnetic field of 50 mT and stable operation was possible with up to 18.5 W of rf heat load. The peak frequency shift due to microphonics was 23 Hz. These results represent the highest peak surface magnetic field achieved in a conduction-cooled SRF cavity to date and meet the requirements for a 1 MeV energy gain.

The accurate calculation of the beam coupling impedance for particle accelerators is necessary to carefully assess the machine stability against impedance-driven collective effects. A first order evaluation of the beam coupling impedance is often done by means of analytical formulas and/or 2D numerical codes. The infinite length approximation is often used to simplify the calculation of the beam coupling impedance of accelerator elements. This is expected to be a reasonable assumption for devices whose length is greater than the transverse dimension but may be a less accurate approximation for segmented devices. In this work, we present the application of the mode matching method to the calculation of the transverse dipolar impedance of a cylindrical cavity loaded with a toroidal insert. By choosing different insert electromagnetic properties (permittivity, permeability, and conductivity) and dimensions, the model can represent a beam pipe, a thin insert, a lossy cavity, or a collimator for which the effect of the finite length is investigated. The method is successfully benchmarked against available analytical formulas, field-matching codes, and 3D commercial solvers. The proposed model allows for performing wide parametric scans and reaching accurate results, therefore becoming an essential tool for the impedance evaluation of accelerator devices.

Recently, reinforcement learning (RL) algorithms have been applied to a wide range of control problems in accelerator commissioning. In order to achieve efficient and fast control, these algorithms need to be highly efficient, so as to minimize the online training time. In this paper, we incorporated the beam position monitor trend into the observation space of the twin delayed deep deterministic policy gradient (TD3) algorithm and trained two different structure agents, one based on physical prior knowledge and the other using the original TD3 network architecture. Both of the agents exhibit strong robustness in the simulated environment. The effectiveness of the agent based on physical prior knowledge has been validated in a real accelerator. Results show that the agent can overcome the difference between simulated and real accelerator environments. Once the training is completed in the simulated environment, the agent can be directly applied to the real accelerator without any online training process. The RL agent is deployed to the medium energy beam transport section of China Accelerator Facility for Superheavy Elements. Fast and automatic orbit correction is being tested with up to ten degrees of freedom. The experimental results show that the agents can correct the orbit to within 1 mm. Moreover, due to the strong robustness of the agent, when a trained agent is applied to different lattices of different particles, the orbit correction can still be completed. Since there are no online data collection and training processes, all online corrections are done within 30 s. This paper shows that, as long as the robustness of the RL algorithm is sufficient, the offline learning agents can be directly applied to online correction, which will greatly improve the efficiency of orbit correction. Such an approach to RL may find promising applications in other areas of accelerator commissioning.

We propose and demonstrate a nonintrusive diagnostic technique for measuring longitudinal bunch profiles of an operational high-power hydrogen ion (${\mathrm{H}}^{-}$) beam in the Spallation Neutron Source (SNS) linear accelerator at Oak Ridge National Laboratory (ORNL). The technique consists of a laser wire formed by a picosecond pulsed laser beam and a virtual slit created by controlling the magnetic field of the electron detector. The virtual slit eliminates the influence of the transverse beam profile and size on the longitudinal bunch profile measurement. The effectiveness of the diagnostics is investigated first in numerical simulations followed by experimental demonstrations performed on the neutron production (1.4 MW on target) ${\mathrm{H}}^{-}$ beam at the SNS superconducting linac. The results from the present measurement method are verified by an independent measurement approach.