arXiv subjects

Haixiao Deng

Publications and source records attributed to Haixiao Deng.

At least 19 recordsLinked to original sources

arXiv2026

Towards Cavity-Based X-ray Free-Electron Lasers: Milestones and Challenges

Cavity-based X-ray free-electron lasers (CBXFELs) could advance X-ray science by delivering fully coherent radiation with high spectral brightness and enhanced pulse control, addressing the limitations of conventional self-amplified spontaneous emission paradigms. Recent experiments at the European XFEL have demonstrated spectral narrowing and proof-of-concept multi-pass amplification, marking the first experimental signature of the tightly coupled three-body interaction between relativistic electron beams, FEL gain dynamics, and high-finesse Bragg cavities. This review consolidates advances in CBXFEL theory built on this framework, global R$\&$D efforts, and outstanding technical challenges arising from the coupled system. CBXFELs support applications in X-ray spectroscopy, quantum optics, and precision metrology. Future directions prioritize CBXFEL stabilization, X-ray comb generation, and applications in coherent quantum control of nuclear transitions.

physics.acc-ph↗

arXiv2026

Advanced Externally Seeded FEL Schemes for High-Repetition-Rate Operation at SHINE

Externally seeded free-electron lasers (FELs) are promising approaches for generating fully coherent soft-X-ray radiation. Their extension to shorter wavelengths and MHz-level repetition rates is, however, constrained by the limited availability of high-repetition-rate seed lasers with sufficient energy modulation. Recent self-amplification and direct-amplification experiments at the Shanghai Soft X-ray FEL facility have significantly relaxed the peak-power requirement for high-gain harmonic generation (HGHG) and opened a practical path toward echo-enabled harmonic generation (EEHG). Using the SHINE bypass line, three compatible high-repetition-rate seeded-FEL configurations are explored: self-modulation cascaded HGHG, self-modulation EEHG, and direct-amplification-driven EEHG. Numerical simulations indicate that these schemes can provide flexible routes toward MHz-level operation with harmonic generation beyond the 30th order. A common modulator-chicane layout is proposed to preserve compatibility among the candidate modes and to support future optimization and experimental implementation at SHINE.

physics.acc-ph↗

arXiv2026

High-power attosecond X-ray free-electron lasers: physics and design strategy

Attosecond pulses from X-ray free-electron laser (XFEL) have opened new opportunities for probing ultrafast electronic dynamics on the Angstrom--attosecond spatiotemporal scale. Most attosecond XFEL concepts rely on generating an ultrashort high-current spike through either external laser modulation or accelerator-based beam manipulation. Despite their different implementations, these approaches share the same essential physics, namely that the XFEL amplification is confined to a short effective lasing window within the electron beam. However, existing studies are often scheme-specific and do not yet provide a unified quantitative picture of how fundamental electron-beam properties constrain high-power attosecond performance. In this work, we investigate the general physics and scheme-independent requirements for generating high-power attosecond X-ray pulses from a short current spike. From the perspective of post-saturation superradiant evolution, we show that the effective lasing length of the electron beam governs both the attainable peak power and the pulse duration. We further examine the distinct roles of slice energy spread, slice emittance, energy chirp, undulator tapering, and transverse beam tilt. Our results reveal the trade-off between peak power, pulse shortening, and single-spike probability, and provide facility-independent guidelines for optimizing electron-beam phase-space manipulation toward terawatt-class attosecond XFEL operation.

physics.acc-ph↗

arXiv2026

U-Net-based surrogate modeling for attosecond X-ray free-electron lasers

Attosecond X-ray pulse generation in modern X-ray free-electron lasers relies on strongly compressed, precisely tailored electron bunches, making accurate diagnostics and control of the longitudinal phase space (LPS) essential. In the self-chirping scheme, collective effects in the linac generate a strong energy chirp that is converted into high peak current through pre-undulator compression, enabling isolated attosecond pulse generation. Reliable operation of this scheme depends on precise LPS control and fast diagnostics. In this work, we present a U-Net-based neural network surrogate that predicts two-dimensional LPS distributions directly from accelerator settings. The model exhibits excellent agreement with start-to-end simulation results. These results demonstrate the potential of neural network surrogates to facilitate real-time tuning and control in attosecond X-ray pulse generation.

physics.acc-ph↗

arXiv2026

High-power beyond extreme ultraviolet FEL radiation with flexible polarization at SHINE

Linac-based free-electron lasers (FELs) feature high brightness, narrow bandwidth, controllable polarization, and wide wavelength tunability. With the rapid development of superconducting radio-frequency technology, linacs can now operate at MHz-level repetition rates, enabling FELs with both high repetition rates and high average power. Beyond extreme ultraviolet (BEUV) radiation is of great interest for scientific research and industrial applications, especially for next-generation lithography. Owing to the main design parameters of SHINE, the generation of BEUV radiation is a natural capability of the facility. The BEUV characteristics at SHINE are investigated and its achievable performance as a high-average-power light source is evaluated. By applying undulator tapering to enhance the energy extraction efficiency, kilowatt-level BEUV radiation with controllable polarization is shown to be achievable. These results demonstrate that SHINE can provide a high-performance BEUV source, offering a realistic pathway toward a high-average-power light source for next-generation high-resolution lithography.

physics.acc-ph↗

arXiv2025

Theoretical and experimental studies of energy modulation to demodulation in seeded free-electron lasers

Laser manipulation plays a critical role in precisely tailoring relativistic electron beams through energy modulation, enabling the generation of coherent, intense, and ultrashort radiation in accelerator-based light sources such as synchrotron radiation facilities and free-electron lasers (FELs). However, laser-induced energy modulation inevitably degrades electron beam quality by increasing the energy spread, thereby limiting high-repetition-rate operation. Here, we investigate energy modulation and demodulation in a seeded FEL using two modulators separated by a tunable phase shifter. Analytical analysis and three-dimensional simulations show that a $\pi$ phase delay can nearly reverse the laser-beam interaction and substantially suppress the residual modulation. Diagnostics based on coherent undulator radiation and time-resolved measurements are established to characterize weak residual modulation, and a dedicated demodulation undulator is designed for controlled studies. Preliminary experiments performed at the Shanghai soft X-ray FEL facility using the existing seeding beamline demonstrate laser-induced energy-modulation suppression. Together with the analytical and numerical studies, these results establish a practical framework for investigating the transition from energy modulation to demodulation in seeded FELs, with potential applications in high-repetition-rate, fully coherent X-ray sources with improved preservation of electron beam quality.

physics.acc-ph↗

arXiv2025

NeutrSHINE: a high repetition rate ultrafast neutron source driven by SHINE electron beam

Neutrons serve as unique probes for exploring the microscopic structure of matter, with the performance of a neutron source fundamentally governing the depth of scientific exploration and the breadth of industrial applicability. To address application demands including nuclear data measurement in the ultra-high-energy region, fundamental particle physics research, highly efficient non-destructive neutron testing, and extreme environment simulation, an ultrafast neutron source driven by the 8 GeV electron beam from the Shanghai high-repetition-rate extreme light facility (SHINE) was conceptually proposed, named NeutrSHINE. Using multidisciplinary simulation tools, key neutronic parameters, thermal behavior of high-power neutron targets, and the factors affecting the time resolution of the source were analyzed. The results affirm the technical feasibility and promising application prospects of the NeutrSHINE concept.

physics.acc-ph↗

arXiv2025

Spectral Control of a Cavity-Based X-ray Free-Electron Laser via Active Mode Locking

Precise spectral control in the hard X-ray regime remains a long-standing challenge that limits applications in atomic-scale science and ultrafast spectroscopy. We present an actively mode-locked cavity-based X-ray free-electron laser that achieves deterministic spectral programmability with phase-locked pulse trains and comb-like spectra, by coherently modulating the electron-beam energy. Three-dimensional time-dependent simulations predict \SI{700}{\micro\joule} total energy, \SI{30}{\giga\watt} peak power, and frequency-comb spacing of \SI{1.55}{\electronvolt} set by the modulation frequency. We further develop selective single-line amplification via undulator tapering and absolute frequency positioning through modulation-laser tuning with better than $2 \times 10^{-5}$ relative precision. Importantly, stable mode-locked operation persists under >80\% peak-to-peak cavity-reflectivity variations, substantially relaxing requirements on X-ray optics. These results establish active mode locking as a practical route to fully coherent, spectrally agile hard X-ray sources and enable new opportunities in time-resolved core-level spectroscopy, X-ray quantum optics, and precision metrology.

physics.acc-ph↗

arXiv2025

Multi-color XFEL pulses with variable color separation and time delay for multi-frame diffraction imaging

X-ray free-electron lasers (XFELs) of high brightness have opened new opportunities for exploring ultrafast dynamical processes in matter, enabling imaging and movies of single molecules and particles at atomic resolution. In this paper, we present a straightforward method for multi-frame diffraction imaging, using the same electron beam to generate four-color XFEL pulses with adjustable wavelength separation and time delay. The optical klystron scheme is introduced to enhance FEL intensity and reduce the total length of undulators. The time delay is tuned via a magnetic chicane between the undulators with various colors. Using parameters of SHINE, start-to-end simulations demonstrate the effectiveness and tunability of our method, achieving representative results such as time delays of hundreds of femtoseconds and four-color XFEL pulses spanning 1.8 to 2.7 nm with 0.3 nm intervals. The proposed scheme enables the recording of multi-frame diffraction images in a single exposure, providing a new perspective for ultrafast molecular and atomic dynamics studies.

physics.acc-ph↗

arXiv2025

Spatiotemporal shaping of attosecond X-rays with time-dependent orbital angular momentum

Attosecond X-ray pulses are indispensable for exploring ultrafast phenomena in matter on Angstrom and attosecond scales. Here we propose a new method to realize spatiotemporal shaping of attosecond X-rays through temporal control of the orbital angular momentum mode content using an X-ray free-electron laser. The method exploits transverse-mode-dependent frequency pulling in a deliberately detuned second stage, together with slippage between the seed and the amplified radiation. Three-dimensional simulations show a double-spike waveform in which the two spikes carry different dominant topological charges. The spike separation is tunable and can reach several hundred attoseconds. This provides a source-level route to spatiotemporally structured attosecond X-rays with controllable temporal structure and topological mode content.

physics.acc-ph↗

arXiv2025

AttoSHINE: Generation of continuous-wave terawatt-scale attosecond X-ray pulses at SHINE

Attosecond X-ray pulses are a critical tool for tracking ultrafast electron dynamics in condensed matter, molecular systems, and strongly correlated materials. Recent breakthroughs have pushed X-ray free electron lasers (XFELs) into the attosecond domain, significantly surpassing their previous femtosecond capabilities. Building on these advancements, this work investigates the potential of the Shanghai High Repetition Rate XFEL and Extreme Light Facility (SHINE), China's first continuous-wave (CW) XFEL, to generate intense attosecond X-ray pulses, thereby offering transformative capabilities for X-ray science. Through comprehensive start-to-end simulations, we show that SHINE is capable of producing hard X-ray pulses with peak powers reaching the terawatt-scale and average pulse durations of approximately 300 as. This is achieved using a self-chirping scheme within the existing machine configuration, requiring no additional hardware. Our findings demonstrate that CW XFELs can generate intense attosecond X-ray pulses at megahertz repetition rates, opening new opportunities for real-time studies of electronic dynamics in complex systems.

physics.acc-ph↗

arXiv2025

Generation of high-power attosecond x-ray FEL pulses carrying orbital angular momentum

X-ray beams carrying orbital angular momentum (OAM), are emerging as a powerful tool to probe matter. Recently, a method called self-seeded FEL with OAM (SSOAM) has been proposed to generate high-power x-ray OAM pulses, which places the traditional optical elements in the linear regime of the FEL amplification process before saturation to reduce the thermal load of the optical element. In this work, we propose to utilize the SSOAM scheme to produce attosecond x-ray vortices with high intensity. Numerical simulations demonstrate the x-ray OAM pulses with peak powers of more than one hundred gigawatts and a pulse duration of the order of hundred attoseconds can be achieved using the proposed method.

physics.acc-ph↗

arXiv2025

Multi-beam-energy control unit based on triple bend achromats

X-ray free electron lasers (XFELs) are the new generation of particle accelerator-based light sources, capable of producing tunable, high-power X-ray pulses that are increasingly vital across various scientific disciplines. Recently, continuous-wave (CW) XFELs driven by superconducting linear accelerators have garnered significant attention due to their ability to enhance availability by supporting multiple undulator lines simultaneously. However, different undulator lines typically require distinct electron beam qualities, particularly varying electron beam energy to achieve a wide range of photon energy tunability. Consequently, precise bunch-to-bunch control of electron beam energy is essential. A double-bend achromat based electron beam delay system has been proposed to enable multi-beam energy operations in CW-XFELs. In this paper, we introduce a novel delay system comprising four triple-bend achromats (TBAs). Based on parameters of the Shanghai High-Repetition-Rate XFEL and Extreme Light Facility, start-to-end simulations demonstrate that the TBA-based delay system achieves better electron beam qualities while providing a wide beam energy tuning range.

physics.acc-ph↗

arXiv2025

Cavity-based compact light source for extreme ultraviolet lithography

A critical technology for high-volume manufacturing of nanoscale integrated circuits is a high-power extreme ultraviolet (EUV) light source. Over the past decades, laser-produced plasma (LPP) sources have been actively utilized in this field. However, current LPP light sources may provide insufficient average power to enable future manufacturing at the 3 nm node and below. In this context,accelerator-based light sources are being considered as promising tools for EUV lithography. This paper proposes a regenerative amplifier free-electron laser EUV source with harmonic lasing, drivenby a superconducting energy-recovery linac (ERL). By utilizing the nth harmonic, the required electron beam energy is reduced to 1/sqrt(n) of that in conventional schemes. The proposed configuration, employing an electron beam energy of approximately 0.33 GeV with a short-period (16 mm) undulator, is estimated to provide an average EUV power of about 2 kW. This approach significantly reduces the required electron energy and facility size relative to other accelerator-based proposals,thereby offering new possibilities for constructing high-power EUV sources with low-energy ERLs.

physics.acc-ph↗

arXiv2024

Space Charge-Induced Emittance Growth in the Downstream Section of ERL Injectors

The injector for ERL-FEL has been widely researched. Unlike traditional linacs, the bunch in the injector for ERLs requires additional deflection and matching section at lower energies. It makes the bunch more susceptible to the effects of the Space Charge. This will lead to a degradation in beam quality. In this paper, we comprehensively analyze the impact of space charge on ERL-injector and propose new design concepts to further maintain beam quality.

physics.acc-ph↗

arXiv2024

The design of high-brightness ERL-FEL injector based on VHF electron gun

In the past decade, the fourth-generation light source based on the combination of Energy Recovery Linac (ERL) and Free-Electron Laser (FEL) using superconducting linear accelerators has garnered significant attention. It holds immense potential, particularly in generating high-power Extreme Ultraviolet (EUV) light sources. This article primarily focuses on the physical design of an injector for ERL-FEL, based on a Very High Frequency (VHF) electron gun with a charge of 100 pC. The beam energy is accelerated to 10 MeV using 3-cell superconducting cavity. The optimization of beam parameters is conducted through employment of BMad and ASTRA simulations, incorporating the concept of Merger optimization. The beam emittance is less than 0.6 mm mrad, and the peak current at the injector exit exceeds 18 A. We present a new method to evaluate the Longitudinal Space Charge (LSC) effects in merger sections, which can be readily applied in design work. Furthermore, we introduce a novel type of merger. The performance of this new merger is comparable to the previously known optimum, the zigzag merger, offering a potential alternative solution for injectors in ERLs.

physics.acc-ph↗

arXiv2024

Flexible multi-bunch-length operation for continuous-wave x-ray free-electron lasers

The X-ray free-electron lasers (XFELs) are cutting-edge instruments pivotal in a broad range of fields, providing high-power X-ray pulses with durations spanning from femtoseconds to attoseconds. One of the critical challenges in XFEL facilities is the simultaneous accommodation of diverse requirements for XFEL operation modes and photon properties across different undulator lines. This paper proposes a dipole-kicker combination in the bunch compressors to vary the electron bunch length for the continuous-wave XFEL facilities driven by a superconducting linac. This method enables optimization of the electron bunch length on a per-bunch basis, tailored to each specific needs of each undulator. Through start-to-end simulations based on the parameters of the Shanghai high-repetition-rate XFEL and extreme light facility, we demonstrate the feasibility of this technique. The results show its effectiveness in enabling simultaneous operations of self-amplified spontaneous emission and externally seeded FEL across different undulator lines, ensuring optimal electron bunch compression for each undulator line.

physics.acc-ph↗

arXiv2023

Coupler RF kick and emittance optimization of the SHINE injector

Coupler RF kick due to the asymmetric structure caused by the coupler, is more likely to lead to emittance growth in the SHINE injector with low beam energy. The calculation of coupler RF kick and resulting emittance dilution has been studied in detail in the literature. In this paper, a novel approach is provided that a lossy material is placed on the surface of the superconducting cavity to approximate the Q0 of the TESLA cavity, and a frequency solver of CST is used to simulate the electromagnetic field distribution, which is used to calculate coupler RF kick, and calibrated against the results of CST Particle Tracking Studio with a good agreement. In order to minimize the emittance growth of SHINE injector, a 1.3 GHz symmetric twin-coupler cavity is adoped in the single-cavity cryomodule, and the rotational angle and permutation of the 8 cavities in the 8-cavities cryomodule is optimized. Ultimately, the optimized emittance is lower than the design parameter.

physics.acc-ph↗