At THz frequencies, fabricating a helically corrugated waveguide (HCW) by using a thin tap drill is relatively easier than machining an axially corrugated waveguide (ACW) for a cylindrical slow-wave free-electron laser (FEL). Our study shows that an ACW FEL is superior to an HCW FEL without a solenoid field, because the TM-TE mode coupling in an HCW is a radiation loss that increases the oscillation threshold of the FEL. However, with no solenoid field, self-starting cyclotron resonance maser in an HCW can still occur at THz frequencies, provided the FEL gain is sufficiently high.

Coherent synchrotron radiation (CSR) is detrimental for echo-enabled harmonic generation (EEHG) and can be quantified by one of various known CSR models, which implement different boundary conditions. We compare a number of relevant models for the upcoming EEHG experiment at the FLASH facility and integrate them into particle tracking simulations to estimate bandwidth of the CSR-affected EEHG bunching spectrum. We find that effects imposed by the boundary conditions (chicane chamber) can be important in a typical FLASH parameter space. The results are also generalized to electron beams more typical for other facilities.

A theoretical and numerical comparison of radiation from relativistic electrons in magnetic undulators and ion channels is presented. The focus is on high K/γ0​ scenarios, where deviations are revealed due to differences in magnetostatic versus electrostatic oscillations. A unified framework allows for direct comparison of the two systems, highlighting unique features in ion channel radiation for high K/γ0​. Additionally, a new transverse orbit precession effect in ion channels is identified, influencing radiation and beam dynamics, with novel insights for potential experimental applications.

During the 2023 LHC run, an unexpected vacuum module failure was traced to beam-induced heating in a sliding RF contact, triggered by field leakage and mechanical nonconformities. This study presents a detailed analysis of the failure mechanism, including vacuum and impedance evaluations, and highlights the critical role of fine design details in high-intensity beam environments. The findings inform mitigation strategies for high-luminosity LHC and offer valuable insights for future accelerator designs.

The Electron-Ion Collider (EIC), to be built at Brookhaven National Laboratory, will achieve a maximum peak luminosity of 10${}^{34}$ cm${}^{-2}$ s${}^{-1}$ for proton and electron collisions. To achieve this luminosity the EIC requires a transverse emittance ratio of about 11:1 for the proton beam. We have carried out a series of beam experiments in the Relativistic Heavy Ion Collider (RHIC) since 2017 and demonstrated a transverse emittance ratio of 11:1 with gold ion beams in 2023. This article presents more details and data analysis of these experiments, along with analytical estimates and numerical multi-particle simulations, and provides an outlook for future EIC coupling compensation.

Realizing a muon collider will require development of new accelerator technologies such as the rectilinear cooling system. This must shrink the volume of the beam by more than five orders of magnitude in six-dimensional phase space, from a melon-sized muon cloud to a laser-like particle beam. We describe improvements to the design of the cooling system. Our optimization used machine learning with a differential evolution algorithm, combined with human insight. We achieved a factor of 4 improvement in phase space volume occupied by the beam compared to the previous design. This improvement is crucial to obtain the highest luminosity beam at the highest energies.

Variable polarization is a critical frontier in the development of the next-generation transverse deflecting structure (TDS), which has advanced applications in proton therapy and particle accelerators. The first two-mode transverse deflecting structure (TTDS) has been fabricated in SSRF/SXFEL for beam tests to verify its ability to provide deflecting force in any polarization direction. The measurement method for TTDS in low-power tests is refined from the bead-pull measurement for a single-mode TDS. Feasibility was confirmed from the tuning experience. The knowledge gained from the low-power tests will also aid in rf design and measurement of other two-mode accelerating structures.

In accelerator storage rings, the performance of orbit feedback systems is susceptible to noise, which remains a key factor limiting improvements in orbit stability. This study develops a hybrid noise suppression framework by integrating the conventional Kalman filter algorithm with state-of-the-art machine learning-based feedback methods. The proposed approach was successfully validated in the electron storage ring of BEPCII (The Upgrade project of Beijing Electron Positron Collider), demonstrating a further improvement in orbit stability.

Plasma wakefield accelerators (PWFA) promise compact, high-gradient acceleration, but precise beam characterization remains a key challenge. This work introduces a novel framework that combines the Compton spectrometer with maximum likelihood estimation and machine learning to reconstruct key beam parameters from betatron radiation spectra. Decoding the energy and angular spectra of betatron radiation is essential for diagnosing high-energy electron beams in PWFA. Our methods enable accurate sub-micron beam diagnostics in extreme high-field environments like FACET-II, providing a critical advance for next-generation beam-plasma interaction studies.

We developed a new robust collimator using a carbon fiber composite (CFC) as the head material to receive a beam kicked by the accidental firing of the injection kicker. Simulation results confirmed that the new CFC collimator suppressed temperature rise in the collimator head caused by beam hit through accidental firing of an injection kicker more effectively than the conventional tungsten collimator. We installed a collimator using CFC as the head material within the SuperKEKB. The installed collimators were evaluated for issues resulting from factors such as beam impedance and the ability to protect the particle detectors during accidental fires.

Beampipes are essential components of particle accelerators, acting as vacuum chambers that precisely guide particle beams at near-light speeds. These beampipes are designed with high precision to minimize interference between the beam/produced particles and the beampipe material to have a better signal-to-noise ratio. In this study, we have designed and developed beampipe material for low-energy accelerators operating at a few MeV to GeV energies, focusing on optimizing both radiation length and elastic modulus. Utilizing machine learning algorithms and thermodynamic simulations, we analyze Al-Ti-V alloys to achieve a higher value of the figure of merit (FoM).

Very high frequency (VHF) band normal conducting guns are used as electron sources for high-repetition-rate free electron lasers, including LCLS-II in the US and SHINE in China. Substantial dark current can lead to unwanted radiation losses in these guns. It is demonstrated that by slightly over-inserting a cathode plug into the gun, the dark current can be reduced from the microampere (μA) range to the nanoampere (nA) scale, without impacting beam performance. This straightforward approach could serve as a universal strategy for dark current suppression across various types of RF guns, extending beyond VHF guns.

Velvet is a commonly used material for cold cathodes, particularly because of its low emission threshold and fairly uniform beam emission. The emitted beam uniformity is particularly well shown by Cerenkov imaging. However, current electronic emission models do not allow to retrieve the experimentally measured emittances and there is no known relationship between a beam’s profile by imaging diagnosis and its emittance. In this paper, we propose a method to estimate the emittance of a beam from its current density, extracted from Cerenkov imaging, and demonstrate the discrete nature of the electronic emission from a cold velvet cathode.

Microbunching instability is one of the potential show-stoppers to longitudinal coherence at the Fourier limit level of linac-driven XUV free-electron lasers. Two semi-analytical models of the instability have been extended to include several beam collective effects, and successfully compared in a wide range of accelerator parameters. The inclusion of lower-order terms of the instability gain and intrabeam scattering has allowed the authors to reach agreement with published experimental observations, to date either in disagreement or in partial agreement with theory. The models have been applied to the design of a chicane-free laser heater for efficient suppression of the instability.

A proposal is presented for generating undulator-based ultrashort broadband terahertz (THz) radiation. The method harnesses the nonlinear longitudinal oscillations of an electron beam to form intense, chirped microbunches for THz generation. Compared to existing schemes, this approach eliminates the need for complex external modulators, potentially making such advanced THz sources more accessible.

The propagation of intense relativistic electron beams is often described using an envelope equation. Such equations are usually derived under the constraint that the beam is paraxial. However, in propagation through dense media scattering can cause non-paraxial divergence of the beam envelope. This article describes the development and application of an envelope equation to handle such situations, as well as that of a magnetically self-pinched beam.

Accelerator physics presents unique challenges for Natural Language Processing due to its highly specialized terminology and intricate concepts. This paper introduces AccPhysBERT, the first sentence embedding model fine-tuned specifically for accelerator physics, demonstrating superior performance over general-purpose models. We showcase example applications, such as semantic paper-reviewer matching and integration into Retrieval-Augmented Generation systems, highlighting its broader potential for efficient information retrieval and knowledge discovery within the field.

High intensity linear accelerator simulations require significant computational resources. The Advanced Virtual Accelerator Software (AVAS) introduces novel approaches to balance simulation accuracy and computational efficiency. By implementing a symmetry-based S-PICNIC algorithm that accelerates space-charge field calculations up to four times, and by using a combined approach that switches between time-based and position-based particle tracking, AVAS achieves reliable multi-particle simulations with over 100 million macro-particles.

Field emission and its radiological effects are a pernicious problem in SRF linacs. Researchers at Jefferson Lab leverage machine learning algorithms to redistribute SRF cavity gradients while minimizing radiation readings from detectors in the CEBAF south linac. This process combines limited active data collection with offline optimization of a surrogate model. During a proof-of-concept demonstration, significant reductions in radiation dose rates were achieved without lowering beam energy.