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.

With the rapid development of laser plasma accelerator, multiple functional beamlines have been designed to transport such pulsed particle beam with large energy spread and divergence angle. Accordingly, the demand for optimal design and online feedback of laser beamlines has also been put forward. We have developed a graphics processing unit–accelerated multiparticle beam transport simulation algorithm based on the open source project “HPSim” to meet the above need. The algorithm implements a series of functions such as the simulation of solenoids, the simulation of exponentially decreasing energy spectrum beams, and cross-programming and cross-language calls. In terms of computational efficiency, the algorithm is approximately 20 times more efficient than multiparticle simulation program tracewin, meeting the simulation time-consumption requirement of a hundred milliseconds for virtual accelerator software. The algorithm’s monoenergetic and exponentially decreasing energy spectrum beam simulation capabilities are verified in the beamline design for CLAPA-I and CLAPA-II at Peking University. The algorithm can be applied not only to virtual accelerator for real-time beam transport simulations but also to fast and efficient beamline magnets optimization design.

Steady-state microbunching (SSMB) storage rings are an appealing option for high-power extreme ultraviolet (EUV) light sources. In this paper, we propose a generalized longitudinal strong focusing (GLSF) scheme to boost the average power of short-wavelength radiation from an SSMB source with a technical demand within the present reach. This scheme employs transverse-longitudinal coupling dynamics to produce steady-state ultrashort bunches and reduces the required modulation laser power significantly by exploiting the low vertical beam emittance in a planar ring. Linear beam dynamics are studied in detail, including bunch compression, modulation cancellation, and vertical-longitudinal decoupling. An instance of a linear lattice layout is given, and kW-level quasicontinuous-wave 13.5-nm EUV radiation can be achieved in a GLSF SSMB storage ring with 1-MW modulation laser power. We believe that the GLSF scheme can also be used in conventional storage rings for various purposes concerning beam manipulation.

This study investigated an unknown instability in the crab-crossing scheme, which is caused by the interaction between beam loading on crab cavities and the coherent beam-beam force at the interaction point (IP). We developed an analysis method by formulating the transfer functions of the beam loading on the crab cavities and the related rf control loops, considering the beam-beam force on the colliding beams. The analysis was first applied to simple cases with no control loops to demonstrate the intrinsic stability of crab cavities under the beam loading and the beam-beam force, which corresponds to the Robinson stability for accelerating cavities. Furthermore, analysis with the control loops was applied to analyze unexpected oscillation phenomena observed in the crab-crossing operation in KEKB. The results obtained from the analysis were consistent with the observed phenomena at the following points: First, a time-domain simulation based on this analysis reproduced the observed coherent oscillation. Second, the relative orbit displacement between the colliding beams estimated in the analysis of the measured oscillation amplitude was consistent with the saturation effect of the beam-beam force at the IP. Third, rf-related parameter dependencies of stability obtained in the analysis were consistent with those in the measurements. The mechanism investigated in this study explained its essential function in the instability observed during the KEKB crab-crossing operation.

This paper discusses the trends and tradeoffs between transverse ${\sigma }_x$ and longitudinal ${\sigma }_z$ bunch dimensions, rf injector gradient, bunch charge, and intrinsic electron mean transverse energy (MTE), where all can be chosen to be independent, and the resulting effects on emittance and transverse brightness. Using a practical example of a quarter wave normal conducting photoinjector, it is computationally found that regardless of MTE and bunch charge, there is a universal relation between the gradient $E$ and the aspect ratio of the bunch (${\sigma }_x/{\sigma }_z$) leading to the highest brightness. This computational result is understood using an analytical formalism consisting of Kim’s emittance formulation and a two-dimensional space charge model. The results, obtained computationally and interpreted in a robust physical framework, could therefore provide the basis for an express mapping approach for emittance forecasting when used with practical injector system design requirements and limitations.

Beamstrahlung is a dominant effect in the beam dynamics of the high luminosity next-generation lepton collider FCC-ee. We characterize the beamstrahlung radiation for the beam parameters at the four working energies, from the Z-pole to the $t\overline{t}$ threshold, and present the effect of this radiation in the machine-detector-interface region. We discuss the conceptual need for a photon dump due to the high power produced, which is in the order of hundreds of kilowatts. We also discuss the detector induced backgrounds due to the incoherent $e^{+}{e}^{-}$ pairs produced by the interaction of the beamstrahlung photons at the interaction point.

In recent $e^{+}{e}^{-}$ colliders, a collision scheme with a large crossing angle has been designed and adopted. In two colliding beams, mode coupling between beam-beam oscillation modes is discussed in this paper. The typical $\sigma$ and $\pi$ modes are extended to include longitudinal motion. The tune shifts of the $l=0$ and $l=\pm 1$ modes are presented explicitly. The behavior of beam-beam modes is analyzed for different Piwinski angles. The instability due to the vacuum pipe impedance is discussed in terms of mode coupling between these beam-beam modes. Beam-beam simulations based on the strong-strong model have shown that a vertical beam-beam instability is induced by crosstalk between the beam-beam collision and the impedance. It is shown that the mode coupling theory for colliding beams reproduces the results of the beam-beam simulation. The impact of hourglass effects on the instability is also studied by simulation.

In this paper we discuss a compact, laser-plasma-based scheme for the generation of positron beams suitable to be implemented in an all-optical setup. A laser-plasma-accelerated electron beam hits a solid target producing electron-positron pairs via bremsstrahlung. The back of the target serves as a plasma mirror to in-couple a laser pulse into a plasma stage located right after the mirror where the laser drives a plasma wave (or wakefield). By properly choosing the delay between the laser and the electron beam the positrons produced in the target can be trapped in the wakefield, where they are focused and accelerated during the transport, resulting in a collimated beam. This approach minimizes the ballistic propagation time and enhances the trapping efficiency. The system can be used as an injector of positron beams and has potential applications in the development of a future, compact, plasma-based electron-positron linear collider.

Beam-driven wakefield accelerators are foreseen to enable compact accelerator-based light sources and play a critical role in future linear-collider concepts. This class of wakefield acceleration has been extensively studied over the last four decades with a focus on demonstrating its ability to support high-accelerating gradient and, most recently, enhanced transformer ratios. Yet, the associated detrimental transverse wakefields have not been examined in as many details due to the limited diagnostics available. In this paper, we introduce a beam-based single-shot transverse-wakefield measurement technique. The approach employs a witness “hollow” electron beam to probe the wakefields generated by a drive bunch. We show how the transverse distortions of the hollow probe provide a direct measurement of the wakefield distribution within the area circumscribed by the probe. The ability to directly measure a full structure of the transverse wakefield could help to develop mitigation schemes and ultimately suppress the adverse beam-break-up instabilities. We discuss a practical implementation of the method and demonstrate its performance with the help of start-to-end simulations.

Niobium superconducting cavity performances are limited by the quench magnetic field of niobium. Maximum achievable accelerating field is estimated by simulation during the conception phase. The full geometry of the cavity is implemented but no roughness is taken into consideration. However, several experiences showed that the roughness can increase locally the magnetic field, leading to an effective peak magnetic field on the surface higher than expected by simulation. It can be at the origin of premature quench. The present article presents simulations of grains and defects with different sizes and its effect on the local magnetic peak field. The values implemented in the model are taken from actual surface measurements made by confocal microscopy on representative Nb samples prepared with usual surface treatments. Cut-out of fabrication defects and some thin films have been also explored. As expected, the mean roughness brought by usual chemical polishing as well as electropolishing surface treatment does not increase significantly the local field. But calculation based on actual values sometimes encountered in the surface measurement, or on specific defects, can reach very high field enhancement and can explain premature quench on some cavities within an otherwise good performing production batch.

Current experiments investigating radiation reaction employ high energy electron beams together with tightly focused laser pulses in order to reach the quantum regime, as expressed through the quantum nonlinearity parameter $\chi$. Such experiments are often complicated by the large number of latent variables, including the precise structure of the electron bunch. Here we examine a correlation between the electron spatial and energy distributions, called an energy chirp, investigate its significance to the laser-electron beam interaction and show that the resulting effect cannot be trivially ignored when analyzing current experiments. In particular, we show that the energy chirp has a large effect on the second central moment of the electron energy, but a lesser impact on the first electron energy moment or the photon critical energy. These results show the importance of improved characterization and control over electron bunch parameters on a shot-to-shot basis in such experiments.

Tsinghua University is currently developing a very-high-frequency electron gun that operates in continuous-wave (cw) mode at 216.667 MHz. This gun will serve as the beam source for the Shanghai high repetition rate XFEL and extreme light facility. The cavity profile has been optimized to minimize input power, peak surface electric field, and peak wall power density. The profile optimization also takes into account the suppression of multipacting effect. Additionally, thermal analysis has been performed to guide the design of water cooling channels in coordination with gun mechanical design. The fabrication of the gun has been completed, and the frequency and quality factor measured in cold test are in good agreement with simulation expectations. During high power conditioning, 75 kW cw radio frequency power was successfully fed into the gun, corresponding to a cathode gradient of $27\mathrm{MV}/\mathrm{m}$ and a gun voltage of 780 keV. This operating gradient is currently the highest in the world for room-temperature VHF guns. Under this condition, the maximum dark current collected by the Faraday cup at the gun exit was 376 nA when the strength of the gun solenoid was scanned. To measure and optimize the beam quality, a test beamline was constructed. After preliminary optimization, the 95% projected transverse emittance was 0.161 mm mrad for 10 pC bunches with a bunch length of 0.49 mm rms, 0.429 mm mrad for 50 pC bunches with a bunch length of 1.15 mm rms, and 0.853 mm mrad for 100 pC bunches with a bunch length of 1.44 mm rms.