The crab-waist scheme becomes the baseline for future circular $e^{+}{e}^{-}$ colliders, where a large Piwinski angle is a must. It has been first found that a strong coherent head-tail instability in the vertical direction may be induced by the interplay of beam-beam interaction and ring impedance below the conventional transverse mode-coupling instability threshold. The collision would be stable only considering pure beam-beam interaction. That is to say, the new instability is a combined effect of beam-beam interaction and ring impedance. The instability can limit the performance of future colliders based on the crab-waist scheme. Mode analysis has been done to help understand the physics. Mitigation schemes are also presented and verified by analysis and simulation.

We report the high-gradient acceleration of electrons driven by a subnanosecond pulse of Ka-band Cherenkov superradiance (SR). The experiments are carried out in a combined “generator-accelerator” scheme, powered by two electron beams from a coaxial explosive-emission graphite cathode. The outer tubular beam ($\approx 300\mathrm{keV}$; $\approx 2.3\mathrm{kA}$) propagates along the periodic slow-wave structure (SWS) and generates a backward moving gigawatt-level SR pulse, which pumps a low-Q “pill-box” resonator located at the SWS input. The inner paraxial test beam ($\approx 250\mathrm{keV}$; $\approx 150\mathrm{A}$) passes through a hole in the resonator wall and is accelerated in extreme SR fields exceeding $500\mathrm{MV}/\mathrm{m}$. The energy of accelerated electrons is estimated by measuring the test beam current after it passes through aluminum filters (foils) that absorb low-energy electrons. It is shown that the test beam contains fractions with a maximum energy of 1.25 MeV, which, taking into account the pill-box length of 4 mm, corresponds to the extremely high averaged accelerating gradient of $250\mathrm{MV}/\mathrm{m}$.

We show how to construct an accelerator lattice cell that minimizes all intrinsic spin resonances. We then apply this approach to various toy electron and proton accelerators, considering, AGS-booster, AGS-like and CEPC/FCC-ee, and FCC-hh-like rings.

Magnetic field errors pose a limitation in the performance of synchrotrons, as they excite nonsystematic resonances, reduce dynamic aperture, and may result in beam loss. Their effect can be compensated by assuming knowledge of their location and strength. Established identification procedures are based on orbit response matrices or resonance driving terms. While they sequentially build a field error model for subsequent accelerator sections, a method detecting field errors in parallel could save valuable beam time. We introduce deep Lie map networks, which enable the construction of an accelerator model including multipole components for the magnetic field errors by linking charged particle dynamics with machine learning methodology in a data-driven approach. Based on simulated beam position monitor readings for the example case of SIS18 at GSI, we demonstrate inference of location and strengths of gradient and sextupole errors for all accelerator sections in parallel. The obtained refined accelerator model may support setup of corrector magnets in operation to allow more precise control over tunes, chromaticities, and resonance compensation.

Multibend magnetic bunch compressors are commonly used to produce beams with high peak current in modern accelerators that call for high-brightness beams. If not well controlled, the coherent synchrotron radiation (CSR) in the dipoles of magnetic bunch compressors can severely affect the produced beam qualities, e.g., leading to emittance dilution. In contrast to the case without bunch compression, suppression of the CSR-induced emittance growth with bunch compression can be more challenging mainly due to the difficulties of analyzing the net CSR effect that involves a varying bunch length. In this paper, with the aid of the CSR point-kick model, theoretical CSR cancelation conditions of a double-bend achromat (DBA) with bunch compression are obtained. Such conditions point to an asymmetric DBA design, having dipoles of different bending angles or different radii or both and satisfying a certain transfer matrix relation. Taking a DBA with dipoles of different bending angles as an example, it is demonstrated that the CSR-induced emittance growth can be significantly suppressed if the proposed conditions are satisfied. As a numerical demonstration of application, we implement multiple DBA cells basically satisfying the proposed conditions in a 180° arc compressor, showing promising performance in suppressing the CSR-induced emittance growth. This study suggests a new appealing way to design practical arc compressors and is promising to serve as a starting point for the design and optimization of CSR-immune arc transfer lines in energy recovery linacs and other modern recirculation accelerators.

Acceleration of polarized electron (positron) beams in a booster synchrotron may suffer from depolarization due to crossings of many spin depolarization resonances and this could limit its applications. We have studied the structure of spin depolarization resonances of a 100-km-scale booster lattice of the Circular Electron Positron Collider (CEPC). The lattice has eight arc regions with hundreds of FODO cells, interleaved with straight sections, leading to a high periodicity. Our analysis shows the contributions to the strength of intrinsic and imperfection spin resonances add up coherently near the superstrong resonances beyond 120 GeV but mostly cancel out and result in generally weak resonance strengths at lower beam energies. Detailed simulations confirm that beam polarization can be mostly maintained in the fast acceleration to 45.6 and 80 GeV but that severe depolarization may occur at even higher energies. This study suggests the possibility of acceleration of polarized electron (positron) beams to ultrahigh beam energies without the help of Siberian snakes and supports injecting highly polarized beams into the collider rings as an attractive solution for resonant depolarization measurements and longitudinally polarized colliding beam experiments for future 100-km-scale circular $e^{+}{e}^{-}$ colliders.

We improved a previously proposed method of using closed-orbit modulation for linear optics and coupling correction. Instead of fitting individual closed orbits, the improved method decomposes the orbit oscillation data into two orthogonal modes and fits the amplitudes of the modes at all beam position monitors. While the original method is limited to process around tens to a hundred orbits, the improved method can process thousands of orbits, which are easily available when alternating-current waveforms are applied to the two modulating correctors. The method has been experimentally demonstrated on the National Synchrotron Light Source (NSLS)-II storage ring.

Microscopic, or short-wavelength, instabilities are known for a drastic reduction of the beam quality and strong amplification of the noise in a beam. Space charge and coherent synchrotron radiation are known to be the leading causes of such instabilities. In this paper, we present a rigorous 3D theory of such instabilities driven by the space-charge forces. We define the condition when our theory is applicable to an arbitrary accelerator system with 3D coupling. Finally, we derive a linear integral equation describing such instability and identify conditions when it can be reduced to an ordinary second-order differential equation.

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.