The demand for high-power, narrowband, and tunable THz radiation sources is steadily increasing. Smith-Purcell radiation offers a compact method for resonantly generating narrowband THz sources with notable advantages over other approaches, though the power output still requires improvement. This work introduces a novel 3D emission-collection concept that enhances the efficiency of THz radiators, achieving an enhancement of over an order of magnitude in coherent radiation power compared to the 2D approach. Systematic simulations reveal that the single-layer helically rotated structure, optimized through inverse design, emits stronger THz radiation than other reported circumferentially closed gratings, achieving millijoule radiated energy. Helix gratings exhibit significant potential for developing accelerator-based high-power and tunable THz radiators and are easy to manufacture. Some methods concerning optimizing beam-grating properties are employed to improve the radiated THz intensity. These results provide new insights into developing high-power THz radiators, which may provide an alternative THz source for free-electron laser facilities.

Superconducting magnets with noncircular apertures are important for handling unconventional beam profiles and specialized accelerator applications. This paper presents an analytical framework for designing superconducting accelerator magnets with quasipolygonal apertures, aimed at generating precise circular field harmonics. In part 1, we explore the relationship between current distributions on quasipolygonal formers and their corresponding magnetic field harmonics. By employing conformal mapping techniques, we establish a connection between the design of quasipolygonal bore magnets and traditional circular bore configurations, facilitating the simplification of complex mathematical formulations. Part 2 applies the derived current distributions to the canted cosine-theta (CCT) coil magnet concept, focusing on designing analytic winding schemes that generate single or mixed circular harmonics within quasipolygonal apertures. This work not only advances the design of superconducting magnets but also broadens the scope of CCT technology to accommodate more complex geometries.

An important upgrade has been deployed for the collimation system of the Large Hadron Collider (LHC) for lead-ion beams that are already planned to reach their high-luminosity target intensity upgrade in the ongoing LHC Run 3 (2022–2026). While certain effects like e-cloud, beam-beam, impedance, injection, and dump protection are relaxed with ion beams, halo collimation becomes an increasing challenge, as the conventional multistage collimation system is about two orders of magnitude less efficient than that for proton beams. Ion fragments scattered out of the collimators in the betatron cleaning insertion risk quenching the cold dipole magnets downstream and may represent performance limitations. Planar channeling in bent crystals has been proven effective for high-energy heavy ions. A crystal collimation system was integrated into the LHC as a baseline solution for collimation in the High-Luminosity LHC (HL-LHC) era. In this paper, simulation and measurement results demonstrating the observation of channeling of heavy-ion beams and improvement in collimation cleaning in the multi-TeV energy regime are presented. The efficiency of the collimation scheme foreseen for the HL-LHC is presented. The highlights of the collimation performance in 2023, when crystal collimation was successfully deployed operationally for the first time, are also presented.

An inductive voltage adder named “Mi2” has been built at Commissariat à l’Énergie Atomique et aux Energies Alternatives to produce two intense pulsed electron beams of 2 kA, 700 keV, and 80 ns full width at half maximum from a cold velvet cathode. In a coupling of experimental and simulation studies, we aim to evaluate the emittance value of those beams. This study focuses on the first beam modelization. Multidimensional particle-in-cell simulation models are used to demonstrate the necessity of taking into account the discrete nature of the electronic emission in order to model a beam consistent with the experimental studies. This study also shows the influence of the emissive surface geometry on the beam current density and emittance and provides a numerical model matching the first beam structure.

Ultrashort broadband THz radiation is an essential tool for investigating nonlinear phenomena and other frontier scientific research. Here, we propose a novel scheme that leverages the nonlinear longitudinal space-charge oscillation process to amplify the chirped density modulation amplitude of the electron beam, which is then utilized to generate ultrashort THz radiation. In this scheme, by controlling the charge density during the beam transport process, the initial chirped density modulation is converted into current peaks with higher amplitude, thereby enabling subsequent ultrashort broadband THz radiation in an optimally tapered undulator. We analyze the key parameters affecting nonlinear longitudinal oscillations of the electron beam with initial chirped density modulation. Furthermore, simulation results verify the feasibility of enhancing the chirped density modulation via the space-charge oscillation. It shows the generation of a microbunched electron beam with a peak current exceeding 370 A, while maintaining the chirped characteristics of the density modulation. Consequently, ultrashort broadband radiation with a full width at half maximum pulse duration of 250 fs and a frequency range covering 1.5 to 5 THz can be produced in the subsequent undulator.

Shanghai Synchrotron Radiation Facility (SSRF)/Shanghai soft X-ray FEL facility (SXFEL) has developed an advanced transverse deflecting structure two-mode transverse deflecting structure (TTDS) to deflect beams at any angle. The rf design of the TTDS uses a two-mode structure utilizing both the ${\mathrm{H}\mathrm{E}\mathrm{M}}_{11}$ and ${\mathrm{H}\mathrm{E}\mathrm{M}}_{12}$ modes for simultaneous vertical and horizontal deflections, operating at 2856 and 5712 MHz, respectively. The design was published in Physical Review Accelerators and Beams in April 2024. A TTDS prototype with 14 regular cells and 2 coupler cells was fabricated at SSRF/SXFEL. Because of the differences in electromagnetic field distributions between the two modes of the TTDS, the nonresonant perturbation measurement was studied by taking the coexisting two modes in mind. Based on an independent tuning process between the modes, a sequential tuning process was designed and established. Using the existing measurement system in the Microwave Laboratory at SSRF, all necessary measurements and tuning have been completed, and the TTDS prototype is ready for high-power tests.

A fundamental comparison between a magnetic undulator and an ion channel, or betatron, radiation from relativistic electrons is presented. While conventional theories nominally range from the undulator ($K<1$) to the wiggler ($K>1$) regime, they are only applicable for sufficiently large Lorentz factors (${\gamma }_0\gg K$). They therefore do not account for high $K/{\gamma }_0$ cases, for which we show that particle trajectories and radiation characteristics strongly deviate from the linear predictions in both magnetic undulators and ion channels. This problem arises from the fundamental differences between a magnetostatically and electrostatically induced oscillation. A reformulation of both the ion channel betatron wavelength and amplitude, as well as the same parameters in a magnetic undulator, permits us to compare cases with equivalent oscillation period and amplitude in the two different scenarios. The notable differences in spectral features of the two radiation mechanisms can then be addressed via numerical simulations of single particle as well as full beam dynamics. Additionally, we identify and quantify a novel transverse orbit precession effect in ion channels for particles with initial angular momentum relative to the device axis. This effect, which is significant in cases of strong transverse kinetic energy oscillations, alters both the radiation divergence and the beam emittance. In this paper, we present this new theoretical framework and compare its results with numerical simulation applied to realizable experimental tests of such radiation sources.

The muon collider has the potential to be a powerful tool for the exploration of frontiers in particle physics. In order to reach high luminosity, the 6D emittance of the muon beam needs to be reduced by several orders of magnitude. The cooling process for a muon collider involves two parts: initial six-dimensional cooling and final transverse cooling. This paper focuses on the former and proposes a conceptual design of the rectilinear cooling channel with additional dipole magnets. We first introduce a general method for designing the rectilinear cooling channel. Subsequently, we apply this method to develop two rectilinear cooling channels before and after a bunch merge system. Furthermore, we investigate the impact on cooling performance of employing $\pi$-mode rf cavities and the effect of errors in the magnetic and rf fields.

Coherent synchrotron radiation (CSR) is known for its detrimental effects on the performance of free electron lasers (FEL). While it is commonly considered a source of beam quality degradation in linacs, in externally seeded FEL schemes, CSR is known to spoil the bandwidth of the FEL radiation. Investigating CSR’s effect on seeded FELs requires a realistic estimation of CSR-induced energy modulations, known as wakes. Several models exist for calculating CSR wakes, differing in their treatment of transient and shielding effects. This work applies various models to compute CSR wakes in a magnetic chicane. In particular, it highlights the importance of the chicane chamber shielding for relatively long electron bunches used in seeded FELs. We use the practical example of the echo-enabled harmonic generation (EEHG) seeding technique to establish that proper consideration of transient and shielding effects is essential for estimating the impact of CSR on seeded FEL performance. This corrected estimate is vital for reaching the near-Fourier limit bandwidth of EEHG by the currently available methods.

The photon flux resulting from high-energy electron beam interactions with high-field systems, such as those found in the upcoming FACET-II experiments at the SLAC National Accelerator Laboratory, yields deep insight into the electron beam’s underlying dynamics during the interaction. However, extracting this information is an intricate process. To demonstrate how to approach this challenge using modern methods, this paper utilizes simulated data that models plasma wakefield acceleration-derived betatron radiation in experiments to determine reliable methods of reconstructing key beam and beam-plasma interaction properties. For betatron radiation measurements, translating the observed $200\mathrm{keV}$ to $30\mathrm{MeV}$ photon double-differential energy-angle spectra obtained from an advanced Compton spectrometer requires testing multiple methods to optimize the pipeline from its response to incident electron beam information. The paper compares maximum likelihood estimation and machine learning to refine the translation of photon spectra into precise electron beam metrics, such as spot size, energy, and emittance, enhancing the understanding of beam behavior within these dense, high-field environments. We also introduce machine learning and the expected maximization algorithm to reconstruct the primary photon spectrum, employing a multilayer neural network for regression analysis of the energy and angle spectra. With appropriate modifications, the advanced methods reproduce relevant incident beam parameters with high accuracy, even for beam sizes in the $<10\mathrm{\mu }\mathrm{m}$ range. This capacity is critical to understanding intense beam propagation and its optimization in plasma.

The Electron-Ion Collider (EIC), to be built at Brookhaven National Laboratory, will collide a polarized high-energy electron beam with polarized proton and light ion beams in the center-of-mass energy range of 20–140 GeV, achieving a maximum peak luminosity of up to $1\times {10}^{34}{\text{cm}}^{-2}{\mathrm{s}}^{-1}$ for the proton and electron collisions. To achieve such a high luminosity, the EIC will employ small, flat beams at the interaction point, which requires a transverse emittance ratio of about 11:1. Since 2017, we have carried out a series of beam experiments to test the feasibility of achieving such a large emittance ratio in the Relativistic Heavy Ion Collider (RHIC). Thanks to the existing stochastic cooling and decoupling feedback system in RHIC, we successfully demonstrated a large transverse emittance ratio of 11:1 with gold ion beams in 2023. In this article, we will present more details and data analysis for these experiments, along with analytical estimates and numerical multiparticle simulations, and provide an outlook for future EIC coupling compensation.

The Advanced Virtual Accelerator Software (avas) has been developed to simulate beam transport processes in high-intensity linear accelerators and has been used in the China Initiative Accelerator Driven System (CiADS) project. In the study of beam dynamics for high-intensity linear accelerators, 3D multiparticle simulations are required due to numerous nonlinear effects caused by strong space charge effects. However, the use of multiparticle simulation is often limited by the computational speed of the particle-in-cell (PIC) algorithm. To achieve fast large-scale multiparticle simulation, avas uses a parallel framework implemented in c++ and improves related algorithms. Specifically, the PIC algorithm has been modified based on symmetry, resulting in the S-PICNIC algorithm. Under the test conditions described in this paper, compared to the standard algorithm, the S-PICNIC algorithm can accelerate the process of solving the space charge field by 4 times while maintaining the simulation accuracy. In addition, avas uses a combined particle motion mode that switches between two modes (using either time or position as the independent variable) based on the element type. This adaptive switching effectively balances the accuracy and speed of numerical simulation. This method has enabled the fast simulation of over 100 million macroparticles. Furthermore, avas uses object-oriented programming methods, where objects are designed based on real physical objects. This design allows avas to preset operating parameters through numerical simulation. In the test, avas successfully preset the operating parameters of the superconducting section of the Chinese ADS front-end demonstration linear Accelerator, resulting in a negligible beam loss and a deviation between the energy set value and the actual measured value of about 0.5%. This paper introduces the structure of the program and related physical models. Benchmark results are also discussed to verify the reliability of the code.

Field emission can cause significant problems in superconducting radio-frequency linear accelerators (linacs). When cavity gradients are pushed higher, radiation levels within the linacs may rise exponentially, causing degradation of many nearby systems. This research aims to utilize machine learning with uncertainty quantification to predict radiation levels at multiple locations throughout the linacs and ultimately optimize cavity gradients to reduce field emission-induced radiation while maintaining the total linac energy gain necessary for the experimental physics program. The optimized solutions show over 40% reductions for both neutron and gamma radiation from the standard operational settings.

During the third run of the Large Hadron Collider in 2023, which had the highest intensity bunch population compared to previous runs, a beam dump incident with operational delays occurred due to increased losses attributed to pressure spikes within a warm vacuum sector. Subsequent inspections revealed localized annealing and plasticization of the tension spring in the sliding contact radio frequency finger of a 212 mm diameter vacuum module, alongside traces of vapor deposition on the various module components with the stainless steel spring material. A comprehensive analysis involving vacuum and beam impedance studies was conducted to investigate the triggering mechanisms behind the warm vacuum module failure. The findings indicate localized beam-induced heating, which could lead to the annealing of the spring with a consequent cascade of effects. Systematic studies of a similar possible failure mechanism for other sizes of vacuum modules were also conducted. Additionally, investigations into potential mitigation measures were performed.

The systematic comparison of two semianalytical models of microbunching instability affecting electron beams in single pass or recirculating linear accelerators is reported. The comparison is comprehensive of numerous features of the instability, such as low and high gain contributions, either linearized or at second order, coherent synchrotron and edge radiation, and Landau damping by transverse emittance and laser heater, investigated for single and double magnetic bunch length compression. The comparison is enriched by three different expressions—one of which is new—to calculate intrabeam scattering. The inclusion of this effect allows the recovery of agreement with published experimental observations, to date either in disagreement with theory or in partial agreement by virtue of blind fitting of parameters. The study is complemented by the evaluation of beam heating for oblique electron-laser interaction, this scheme being relevant for high repetition rate free-electron lasers, space constrained, or subject to perturbations by the laser heater chicane.

An envelope equation is used to explore the propagation of a focused electron beam through dense conductive media like partially ionized high-pressure gas, metal vapor plumes from beam-target interactions, or low-density solid-phase targets. Envelope equations have been a most useful tool for predicting experimental results of beam propagation in gas, and they are refined for the general problem of propagation through a dense medium. The envelope equation is modified to account for the large beam divergence resulting from multiple scattering. The envelope code results are in qualitative agreement with published experimental data.

Accelerator physics presents unique challenges for natural language processing (NLP) due to its specialized terminology and complex concepts. A key component in overcoming these challenges is the development of robust text embedding models that transform textual data into dense vector representations, facilitating efficient information retrieval and semantic understanding. In this work, we introduce AccPhysBERT, a sentence embedding model fine-tuned specifically for accelerator physics. Our model demonstrates superior performance across a range of downstream NLP tasks, surpassing existing models in capturing the domain-specific nuances of the field. We further showcase its practical applications, including semantic paper-reviewer matching and integration into retrieval-augmented generation systems, highlighting its potential to enhance information retrieval and knowledge discovery in accelerator physics.

We investigate a slow-wave THz free-electron laser consisting of axially or helically corrugated waveguides driven by an annular electron beam. When the device is driven by a 10-mA electron beam at 30 keV, we numerically demonstrated backward-wave oscillation at 0.2212 and 0.2651 THz in axially and helically corrugated waveguides, respectively, without a solenoid field within a submillimeter aperture. We developed a unified theoretical framework for the two types of waveguides and successfully explained the observed backward-wave resonances. In our theory, azimuthal-symmetry breaking in a helically corrugated waveguide couples the synchronous transverse-magnetic radiation mode to a transverse-electric mode, resulting in radiation loss for the oscillation. Therefore, this study concludes superior radiation performance for the axially corrugated waveguide as a slow-wave THz free-electron laser. Furthermore, we show the opportunity of exciting self-started cyclotron resonance maser oscillations at THz frequencies from a helically corrugated waveguide without a solenoid field.

The present investigation reports the design and development of an advanced material with a high figure of merit (FoM) for beampipe applications in particle accelerators by bringing synergy between computational and experimental approaches. Machine-learning algorithms have been used to predict the phase(s), low density, and high radiation length of the designed Al-Ti-V alloys. Al-Ti-V alloys with various compositions for single-phase and dual-phase mixtures, liquidus temperature, and density values were obtained using the latin hypercube sampling method in TC Python Thermo-Calc software. The obtained dataset was utilized to train the machine-learning algorithms. Classification algorithms such as XGBoost and regression models such as linear regression and random forest regressor have been used to compute the number of phases, radiation length, and density, respectively. The XGBoost algorithm shows an accuracy of 98%, the linear regression model shows an accuracy of 94%, and the random forest regressor model is accurate up to 99%. The developed Al-Ti-V alloys exhibit high radiation length as well as a good combination of high elastic modulus and toughness due to the synergistic effect of the presence of hard ${\mathrm{Al}}_3\mathrm{Ti}$ phase along with a minor volume fraction of FCC $(\mathrm{Al}{)}_{\mathrm{ss}}$ solid solution phase mixture. The comparison of our alloys, alloy-1 (${\mathrm{Al}}_{75.2}{\mathrm{Ti}}_{22.8}{\mathrm{V}}_2$) and alloy-2 (${\mathrm{Al}}_{89}{\mathrm{Ti}}_{10}{\mathrm{V}}_1$) shows an increase in the radiation length by 7 times and a decrease in the density by 2 to 3 times as compared to stainless steel 304, the preferred material for constructing beampipes in low-energy particle accelerators. Further, we experimentally verify the elastic modulus of the alloy-1 and compute the FoM equal to 0.416, which is better than other existing materials for beampipes in low-energy experiments.

Orbit stability is a critical performance metric for modern synchrotron radiation facilities and colliders, necessitating effective orbit correction and slow orbit feedback systems. In recent years, innovative orbit feedback methods leveraging neural networks have been progressively implemented, demonstrating improved performance. BEPCII has also attempted to implement an orbit feedback system based on neural networks. Furthermore, we have noted that orbit stability is significantly affected by noises. To address the impact of noises, we introduced the Kalman filtering method into the orbit feedback system and combined it with the existing neural network framework, creating a dual neural network orbit feedback system. This innovative approach has led to further improvements in overall orbit stability.