arXiv subjects

Tianlong He

Publications and source records attributed to Tianlong He.

11 recordsLinked to original sources

arXiv2026

Transverse coupled-bunch instabilities driven by high-order modes near the coupling resonance

Betatron coupling near the difference resonance has been explored and adopted in several fourth-generation storage rings, yet its influence on high-order-mode-driven transverse coupled-bunch instabilities has not been systematically established. Using the Hefei Advanced Light Facility (HALF) as an example, we investigate this effect through theoretical analysis and macroparticle tracking simulations. The theoretical predictions agree well with macroparticle tracking simulations in both the weak- and strong-instability regimes, validating the description over the parameter ranges considered. For the vertical HOMs studied with HALF parameters, suitable betatron coupling effectively suppresses the instability, offering potential benefits for transverse stability control.

physics.acc-ph↗

arXiv2026

Particle swarm optimization fitting of long-range wake potentials for trapped-mode parameter characterization in the HALF storage ring

Accurate extraction of trapped-mode impedance parameters of complex storage ring components is essential for assessing their impact on coupled-bunch instabilities. This paper proposes a parameter extraction method based on particle swarm optimization. By constructing a multi-resonator fitting model, trapped-mode parameters are extracted from partially decayed long-range wake potentials. Benchmark validation using a cylindrical pillbox cavity demonstrates that the proposed method yields results consistent with those obtained from the CST eigenmode solver and the existing differential evolution method for both longitudinal and transverse cases, while significantly reducing the computational cost. The method is further applied to three critical components of the Hefei Advanced Light Facility storage ring, demonstrating its applicability to complex structures.

physics.acc-ph↗

arXiv2026

A Realistic Proportional-Integral RF Feedback Model for Longitudinal Beam Dynamics Simulation

Modern fourth-generation storage ring light sources predominantly utilize digital I/Q-based proportional-integral (PI) feedback for their radio-frequency (RF) systems. This paper introduces a dedicated PI feedback model implemented in the STABLE tracking code to enable accurate and fast longitudinal beam dynamics simulations. The model's key innovation lies in its treatment of the continuous generator current, which is discretized into electron-bunch-like charge pulses, while the cavity voltage is refreshed on an RF-cycle basis. This methodology offers a more physically accurate model of the beam-cavity-feedback coupling, providing a versatile tool for precise longitudinal beam dynamics studies in single- and multi-rf configurations.

physics.acc-ph↗

arXiv2025

Crab-waist interaction region design and integration for the Super Tau-Charm Facility

The Super Tau-Charm Facility (STCF) is a new-generation $e^+e^-$ collider proposed in China, designed to operate in the center-of-mass (CoM) energy range of 2-7 GeV. To achieve the design luminosity exceeding 5*10^34 cm^-2s^-1 at the optimal CoM energy of 4 GeV, a large crossing angle combined with the crab-waist correction scheme is adopted. However, this scheme introduces strong nonlinearities in the interaction region (IR) due to the extremely low vertical beta function of beta_y* <=1 mm, which significantly limits dynamic and momentum apertures of the collider ring. This paper presents a comprehensive modular optics design that addresses these challenges through several key features: 1) local chromaticity correction up to third order to enhance momentum bandwidth; 2) exact -I transformation between chromatic sextupole pairs for nonlinear cancellation; 3) minimization of the dispersion invariant along the IR to improve local momentum acceptance; 4) optimized beta functions at crab sextupole locations to reduce strength requirements and associated nonlinearities. Resonance driving terms analysis confirms effective suppression of geometric aberrations while preserving the intended crab-waist effects. When integrated into the collider ring, the design achieves a Touschek lifetime exceeding 300 s at beam energy of 2 GeV, meeting STCF requirements. The impact of fringe fields from superconducting quadrupoles is mitigated using octupole correctors, and detector solenoid effects are fully suppressed via local anti-solenoid compensation. Furthermore, the defined machine-detector interface layout ensures minimal synchrotron radiation background at the IP beryllium chamber, while ultra-high vacuum conditions are required to suppress beam-gas background. This IR design represents the current optimal solution for STCF and has been incorporated into the project's conceptual design report.

physics.acc-ph↗

arXiv2025

Conceptual Design Report of Super Tau-Charm Facility: The Accelerator

Electron-positron colliders operating in the GeV region of center-of-mass energies or the Tau-Charm energy region, have been proven to enable competitive frontier research, due to its several unique features. With the progress of high energy physics in the last two decades, a new-generation Tau-Charm factory, Super Tau Charm Facility (STCF) has been actively promoting by the particle physics community in China. STCF holds great potential to address fundamental questions such as the essence of color confinement and the matter-antimatter asymmetry in the universe in the next decades. The main design goals of STCF are with a center-of-mass energy ranging from 2 to 7 GeV and a peak luminosity surpassing 5*10^34 cm^-2s^-1 that is optimized at a center-of-mass energy of 4 GeV, which is about 50 times that of the currently operating Tau-Charm factory - BEPCII. The STCF accelerator is composed of two main parts: a double-ring collider with the crab-waist collision scheme and an injector that provides top-up injections for both electron and positron beams. As a typical third-generation electron-positron circular collider, the STCF accelerator faces many challenges in both accelerator physics and technology. In this paper, the conceptual design of the STCF accelerator complex is presented, including the ongoing efforts and plans for technological R&D, as well as the required infrastructure. The STCF project aims to secure support from the Chinese central government for its construction during the 15th Five-Year Plan (2026-2030) in China.

physics.acc-ph↗

arXiv2025

Impact of coherent wiggler radiation impedance in Tau-Charm factories

Coherent synchrotron radiation (CSR) has long been recognized as a significant source of longitudinal impedance driving microwave instability in electron storage rings. In the pursuit of higher luminosity, next-generation circular $e^+e^-$ colliders operating in the few-GeV energy range, such as B-factories and Tau-Charm factories, are being designed with low-emittance beams and high beam currents. Damping wigglers are commonly introduced to reduce damping times and control beam emittance. In this study, we systematically investigate the impact of coherent wiggler radiation (CWR), a specific form of CSR generated within wigglers, on beam stability in Tau-Charm factories. We revisit the threshold conditions for CWR-induced microwave instability and evaluate its effects under realistic lattice configurations of collider rings. Furthermore, we examine theoretical models of longitudinal CWR impedance and identify improved formulations that better capture its influence. As an illustrative example, the developed CWR impedance models are applied to simulate beam stability in the Super Tau-Charm Facility currently under design in China.

physics.acc-ph↗

arXiv2025

Optics design of the Super Tau-Charm Facility collider rings

The Super Tau-Charm Facility (STCF), China's next-generation electron-positron collider, targets an unprecedented luminosity exceeding 5x10^34 cm^-2 s^-1 at a center-of-mass energy of 4 GeV. The implementation of a submillimeter vertical beta function at interaction point (< 1 mm) and crab-waist collision scheme in this low-energy regime introduces critical challenges through severe nonlinear effects that constrain dynamic aperture and degrade Touschek lifetime. To address these constraints, we propose a novel quasi-two-fold symmetric lattice design integrating several synergistic features: Linear optics optimization minimizing the H-invariant around the ring to maximize local momentum acceptance (LMA); Up to third-order of local chromaticity correction in the interaction region combined with second-order achromatic arc optics, enhancing off-momentum beam dynamics; Configured FODO arc structure with interleaved sextupole groups satisfying -I transformation, suppressing third-order geometric aberrations while optimizing Montague function distributions; Advanced final focus system integrating chromatic sextupoles, crab sextupoles, and strategically positioned octupoles to counteract final quadrupole fringe fields. Furthermore, we develop a multi-objective genetic algorithm using the in-house toolkit PAMKIT to simultaneously optimize 46 sextupole families, maximizing both dynamic aperture and momentum bandwidth. Optics performance is evaluated under error conditions with appropriate corrections, ensuring robust beam dynamics.

physics.acc-ph↗

arXiv2025

Phantom Domain Finite Element Method: A novel approach for heterogeneous materials

In this paper, we introduce the Phantom Domain Finite Element Method (PDFEM), a novel computational approach tailored for the efficient analysis of heterogeneous and composite materials. Inspired by fictitious domain methods, this method employs a structured mesh to discretize the entire material domain while utilizing separate, independent meshes for the inclusions. These inclusion meshes are coupled to the structured mesh via a substitution matrix, enabling them to act as phantom meshes that do not directly contribute to the final system of equations. This framework offers significant advantages, including enhanced flexibility in handling complex inclusion geometries and improved computational efficiency. To assess the accuracy and robustness of the proposed method, numerical experiments are conducted on structures containing inclusions of various geometries. In order to emphasize the efficiency of the PDFEM method, a numerical simulation is presented to highlight its advantages in the case of long natural fibers, such as flax and linen. These simulations are compared against FEM calculations, demonstrating the efficiency of PDFEM. Indeed, meshing such fine structures requires an extremely high number of elements, and in some cases, meshing becomes particularly challenging due to the complexity of the geometries.

math.NA↗

arXiv2023

Analytic formulas for the D-mode Robinson instability

The passive superconducting harmonic cavity (PSHC) scheme is adopted by several existing and future synchrotron light source storage rings, as it has a relatively smaller R/Q and a relatively larger quality factor (Q), which can effectively reduce the beam-loading effect and suppress the mode-one instability. Based on the mode-zero Robinson instability equation of uniformly filled rigid bunches and a search algorithm for minimum, we have revealed that the PSHC fundamental mode with a large loaded-Q possibly triggers the D-mode Robinson instability [T. He, et al., Mode-zero Robinson instability in the presence of passive superconducting harmonic cavities, PRAB 26, 064403 (2023)]. This D-mode Robinson instability is unique because it is anti-damped by the radiation-damping effect. In this paper, analytical formulas for the frequency and growth rate of the D-mode Robinson instability are derived with several appropriate approximations. These analytical formulas will facilitate analyzing and understanding the D-mode Robinson instability. Most importantly, useful formulas for the D-mode threshold detuning calculation have finally been found.

physics.acc-ph↗

arXiv2023

Impact of short-range wakefields from radio-frequency cavity resonant modes on bunch lengthening

The high-performance operation of fourth-generation synchrotron light sources critically depends on harmonic cavities (HCs) to alleviate statistical collective effects through bunch lengthening. Active HCs are preferred over passive ones for achieving theoretically optimum bunch lengthening in Timing-mode operation, which is characterized by low average current and high bunch charge. This advantage stems from their ability to control cavity voltages via generator current. However, this study reveals a previously overlooked limitation: the detrimental impact of short-range wakefields from RF cavity resonant modes on bunch lengthening at high bunch charge. Using Hefei Advanced Light Facility parameters, we demonstrate that these wakefields can significantly degrade bunch lengthening. Further analysis with PETRA-IV parameters reveals that fine-tuning the HC voltage can optimize bunch lengthening. Notably, under specific HC settings, there exists two equilibrium bunch distributions. Our findings highlight the critical influence of RF cavity short-range wakefields on beam dynamics at high bunch charge, emphasizing their essential inclusion in the evaluation and optimization of HC performance for new-generation synchrotron light sources.

physics.acc-ph↗

arXiv2023

Terahertz scale microbunching instability driven by nonevaporable getter coating resistive-wall impedance

Non-evaporable getter (NEG) coating is widely required in the next generation of light sources and circular $e^+e^-$ colliders for small vacuum pipes to improve the vacuum level, which, however, also enhances the high-frequency resistive-wall impedance and often generates a resonator-like peak in the terahertz frequency region. In this paper, we will use the parameters of the planned Hefei Advanced Light Facility (HALF) storage ring to study the impact of NEG coating resistive-wall impedance on the longitudinal microwave instability via particle tracking simulation. Using different NEG coating parameters (resistivity and thickness) as examples, we find that the impedance with a narrow and strong peak in the high frequency region can cause micro-bunching instability, which has a low instability threshold current and contributes to a large energy spread widening above the threshold. In order to obtain a convergent simulation of the beam dynamics, one must properly resolve such a peak. The coating with a lower resistivity has a much less sharp peak in its impedance spectrum, which is helpful to suppress the micro-bunching instability and in return contributes to a weaker microwave instability.

physics.acc-ph↗