In advanced accelerator-based light sources and colliders, bunch compressors like arc-type (DBA) and linear-type (chicane) are widely used to generate high-quality electron beams with kiloampere (kA)-level peak currents. However, a serious problem in increasing the peak current even higher is the significant degradation of beam quality caused by the Coherent Synchrotron Radiation (CSR) effect. To tackle this, we develop a new analytical model for CSR that can describe beam transport with varying bunch lengths, establish a practical framework for analyzing CSR in both DBA and chicane-type compressors, and design CSR-suppressed DBA compressors (arc-type) as well as non-symmetric C- and S-shaped chicanes (linear-type). General analytical conditions for CSR cancellation are derived for these designs. Simulations show that, with these new compressors, high beam quality can be maintained even when the peak current is increased up to 10 kA. This work provides important guidance for enhancing the performance of existing accelerator facilities, as well as for the development of next-generation accelerator-based light sources and colliders.

The mechanism of the steady-state microbunching (SSMB) storage ring is being actively investigated. In the conceptual design, a laser modulator used to modulate the electron beam include the co-propagating laser beam, undulator magnets and potential cavity mirrors, forming a laser modulator cavity. In this work the longitudinal single-bunch and multi-bunch collective dynamics are studied that may arise due to coherent undulator radiation, based on the macroparticle model. For multi-bunch multi-turn case, the dispersion equation is derived, and a detuning parameter is introduced to characterize the frequency deviation between the external laser and the resonant undulator radiation, and solve for the instability growth rates of different multibunch modes. When the detuning approaches a specific multi-bunch mode divided by the number of total microbunches, this instability mechanism tends to amplify that mode. Furthermore, possible mitigation effect of the potential well on the instability is discussed. This work may shed light on the underlying physical mechanisms of longitudinal collective beam dynamics in the laser cavity modulators of an SSMB storage ring.

Due to the unique role of terahertz (THz) radiation in the electromagnetic spectrum, it possesses significant scientific value and potential applications in fundamental science, biomedical research, spectroscopy, and etc. This paper proposes a novel mechanism for generating continuous kilowatt-level coherent terahertz radiation in steady-state microbunching storage rings, based on self-sustaining laser modulation processes. The analysis employs the transfer matrix method from accelerator physics, considering the dynamical evolution of electron beams during multiple passes through the laser modulator, as well as radiation damping and quantum excitation effects in the storage ring. Numerical tracking results demonstrate the feasibility of this mechanism. In a demonstrative case, we show that 1 kW continuous coherent radiation can be achieved at 5 THz frequency, corresponding to electric field strengths on the order of MV/m. Since this scheme is based on free electrons, its radiation output characteristics can be tuned over a broad frequency range of 1-10 THz, offering extremely high application value in scientific research.

The existing theoretical treatment of single-pass microbunching instability (MBI) typically assumes a coasting beam and adopts a linear framework, within which the microbunching gain may grow without bound. While the inclusion of intrabeam scattering (IBS) introduces damping effects that may suppress excessive gain, these models remain fundamentally linear and do not capture saturation behavior. In this work, we develop a quasi-linear theory of MBI based on the Vlasov equation, incorporating the evolution of beam energy spread induced by the instability itself. The quasi-linear formulation yields a set of coupled equations describing the evolution of the bunching factor and energy spread, still under the coasting beam approximation where different modulation wavelengths evolve independently. This approach provides a more realistic description of the nonlinear evolution of MBI and offers insight into its natural saturation mechanism.

Microbunching instability (MBI) driven by short-range wakefields in high-brightness electron beams has been an active area of research over the past decade. While most existing studies focus on single-pass or linear accelerators --- particularly few-dipole bunch compressor chicanes --- MBI studies in multi-bend transport lines has relied predominantly on time-consuming numerical simulations. In this work, we present a quick estimate for evaluating MBI gain in generic multi-bend beamlines, thereby avoiding computational costs. Starting from Volterra integral equation governing the bunching factor, we first find the optimal wavelength and introduce physically motivated simplifications to derive the maximum gain. A gain spectrum is then constructed based on physical insights into MBI amplification mechanisms. The results show good agreement with detailed numerical calculations from Vlasov solver. The developed approach enables quick and reasonably accurate estimates of the MBI gain using only the lattice optics functions and the initial beam parameters, offering a practical tool for beamline design and mitigation of MBI.

Conventional theory of single-pass microbunching instability (MBI) is primarily based on the coasting-beam approximation, which assumes that the modulation wavelength is much shorter than the bunch length. However, in isochronous beamlines, the characteristic modulation wavelength may sometimes become comparable to the bunch length, rendering the coasting-beam assumption invalid. In this paper we develop a bunched-beam theory of MBI, starting from the linearized Vlasov equation, aiming to quantify the impact of finite bunch length on the evolution of density modulations. Our analysis reveals that the final MBI gain, or the amplified bunching factor, exhibits a dependence on the initial modulation phase, a feature absent in the existing coasting-beam model. The proposed bunched-beam formulation may offer additional physical insights into the underlying mechanism of MBI, particularly in regimes where the finite extent of the bunch plays a non-negligible role.

Ultrafast electron diffraction (UED) is a powerful technique for observing atomic-scale structural dynamics in materials. Electron beam parameters—beam size, divergence, energy spread, and bunch length—determine spatio-temporal resolution. Traditional diagnostic methods require complex instrumentation that cannot be integrated into routine workflows, particularly for high-repetition-rate facilities. We present a machine learning approach enabling comprehensive, non-invasive extraction of electron beam parameters directly from diffraction patterns. Deep neural networks trained on physics-based simulations decode signatures that beam parameters imprint on diffraction images. The method exploits distinct physical mechanisms: geometric effects from beam size, angular distortions from divergence, chromatic aberrations from energy spread, and temporal convolution from bunch length. This enables bunch length measurement without dedicated temporal diagnostics—traditionally one of the most challenging parameters to access non-invasively. The trained models can be deployed across UED facilities using standard imaging detectors, democratizing access to advanced diagnostics. This approach eliminates expensive specialized equipment and enables real-time beam monitoring and optimization, enhancing experimental throughput and data quality for ultrafast materials characterization.