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Chuanxiang Tang

Publications and source records attributed to Chuanxiang Tang.

At least 19 recordsLinked to original sources
arXiv2026

Weak-Strong Steady-State Microbunching Accelerator Light Source

We propose a phase space manipulation involving one energy modulation sandwiched by two dispersion sections which converts a bunched particle beam or bunch train to ultra-high-harmonic density modulation, while the energy modulation in principle can be arbitrarily weak. The same scheme can also be used for energy bunching, creating energy levels in a bunched beam. We further propose a mechanism invoking three laser modulators in a storage ring to longitudinally focus the electron beam both weakly and strongly, such that a microbunch train and its high-density-harmonics or energy bunching form and sustain turn-by-turn. We call this mechanism weak-strong steady-state microbunching (Weak-Strong SSMB). The longitudinal beta function can vary by seven orders of magnitude along such a ring, with the minimal value squeezed to 10 nm. An example application of Weak-Strong SSMB for kW coherent EUV radiation is presented. Extension to X-ray can be anticipated. An energy-leveled electron beam enables $\gamma$-ray frequency comb production. The ideas can be scaled to wavelengths like RF and THz, for bunch length and energy spread control, ultrashort X-ray and coherent THz generation. Our work establishes a new paradigm for longitudinal dynamics study, accelerator light source development, and opens great potential for accelerator physics and technology.

physics.acc-ph↗
arXiv2026

Echo Enhanced Strong Focusing for Coherent Short-Wavelength Radiation

Storage-ring-based fully coherent light sources, including steady-state microbunching (SSMB), as well as compact seeded FELs driven by laser plasma accelerators, typically have relatively large intrinsic energy spreads. Extending the spectral reach of these facilities toward the X-ray regime represents a major challenge, as existing seeded schemes require rather extreme parameters to generate appreciable microbunching at high harmonics. In this Letter, we propose an echo enhanced strong focusing scheme that employs transverse-longitudinal coupling together with the beam echo effect to simultaneously resolve the energy spread bottleneck and enable efficient high-harmonic generation. This approach substantially relaxes the requirements on both the intrinsic energy spread and the transverse emittance, paving the way for soft X-ray production using relatively weak laser modulation. Based on this scheme, we further present an SSMB storage ring capable of generating kW-level average power 6.7 nm soft X-ray radiation.

physics.acc-ph↗
arXiv2025

Terahertz-Driven Nano-tip Field-Emission Electron Gun and Cascaded Acceleration

This paper reports two versions of terahertz (THz)-driven nanotip field-emission electron guns: single-layer reflective guns (SLRGs) and double-layer reflective guns (DLRGs). Both guns use nanotip emitters and accelerate electrons through the electric field of the THz wave. SLRGs employ a reflective structure to superimpose the initial and subsequent half-cycles of the THz electric field, enhancing the field amplitude and acceleration efficiency. Experiments have demonstrated that SLRGs achieve higher acceleration efficiency than single-layer nonreflective guns (SLNRGs) for identical THz input energies. This constitutes direct experimental verification of the efficacy of the reflective structure. Theoretically, SLRGs operating in single-feed mode can match the acceleration efficiency of dual-feed SLNRGs while reducing operational complexity. DLRGs demonstrate THz-driven cascaded electron acceleration through precise scanning of the delay between two incident THz beams. This represents a direct experimental demonstration of cascaded acceleration in THz-driven electron sources. The experimental results of DLRGs align closely with the results of electron dynamics predicted by simulations. This establishes the foundation for developing multilayer high-acceleration-efficiency THz-driven high-energy electron guns. The ability to manipulate the THz for each layer individually holds promising potential for improving the beam quality of THz electron guns.

physics.acc-ph↗
arXiv2025

Sub-5-fs compression and synchronization of relativistic electron bunches enabled by a high-gradient $\alpha$-magnet and low-jitter photoinjector

Generating high-brightness relativistic electron bunches with few-femtosecond duration, while simultaneously achieving few-fs synchronization with ultrafast lasers, remains an outstanding challenge at the frontier of accelerator physics and ultrafast science. In this Letter, we present the beam physics and experimental demonstration of a new method that, for the first time, enables simultaneous control of bunch duration and synchronization with few-fs precision. Timing stabilization is achieved using a tailored high-gradient $\alpha$-magnet that optimizes the correlation between time of flight and momentum, together with a photocathode RF gun designed to suppress the effect of RF-to-laser timing jitter. Compression is realized by manipulating the time-momentum correlation in phase space, primarily through space-charge effects. Sub-5-fs rms bunch duration and synchronization are demonstrated. This method establishes a new regime in electron bunch control, unlocking new capabilities for ultrafast beam physics and applications.

physics.acc-ph↗
arXiv2025

Observation and Interpretation of Field Emission Saturation Induced by an Ultra-fast Intense Terahertz Field

Field emission under ultra-fast intense terahertz fields provides a promising approach for generating electron bunches with ultrashort pulse duration and high charge densities. It is generally believed that the field emission current described by traditional field emission theory increases dramatically with the applied electric field. However, we conducted extensive field emission experiments using quasi-single-cycle strong-field terahertz radiation at various energy levels and different temperatures and observed an intriguing phenomenon where the emitted charge reached saturation. A novel model is proposed to interpret this phenomenon, which considers the contribution of surface valence electrons and the dynamic replenishment of free electrons from the bulk to the surface. The experimentally observed convex relationship between the emitted charge and terahertz energy is consistent with the model prediction, unlike the concave relationship derived from the traditional field emission formula. In addition, another observed counter-intuitive phenomenon, the inverse correlation between the cathode temperature and saturated emission charge, is also well interpreted by the model. This work offers comprehensive insights into field emission dynamics under ultra-fast intense fields, paving the way for generating electron bunches with unprecedented temporal resolution.

physics.acc-ph↗
arXiv2024

Steady-State Micro-Bunching based on Transverse-Longitudinal Coupling

In this paper, three specific scenarios of a novel accelerator light source mechanism called steady-state micro-bunching (SSMB) have been studied, i.e., longitudinal weak focusing, longitudinal strong focusing and generalized longitudinal strong focusing (GLSF). At present, GLSF is the most promising among them in realizing high-power short-wavelength coherent radiation with a mild requirement on the modulation laser power. Its essence is to exploit the ultrasmall natural vertical emittance of an electron beam in a planar storage ring for efficient microbunching formation, like a partial transverse-longitudinal emittance exchange at the optical laser wavelength range. Based on indepth investigation of related beam physics, a solution of a GLSF SSMB storage ring which can deliver 1 kW-average-power EUV light is presented. The work in this paper, such as the generalized Courant-Snyder formalism, the analysis of theoretical minimum emittances, transverse-longitudinal coupling dynamics, and the derivation of bunching factor and modulation strengths for laser-induced microbunching schemes, is expected to be useful not only for the development of SSMB but also for future accelerator light sources in general that demand increasingly precise electron beam phase space manipulations.

physics.acc-ph↗
arXiv2022

Towards a compact all optical terahertz-driven electron source at Tsinghua University

We propose a physical design of a compact all optical terahertz (THz)-driven electron source. The 300 mm accelerator beamline, powered by Joule level laser system, is easily to be integrated to tabletop scale. A dual-feed THz-driven electron gun with an exponential impedance, a tapered dielectric loaded cylindrical waveguide, THz-driven bunch compressors and permanent magnet solenoids (PMS) have been designed and optimized. Dynamics simulations show that the electron source can deliver a 19 fC, 3 MeV electron beams with a normalized transverse emittance of 0.079 π.mm.mrad. A minimum relative energy spread of 0.04% or a minimum root-mean-square bunch length of 6.1 fs can be achieved by adjusting the beam shaping line. Sensitivity analysis shows that the THz-driven electron source can effectively work under a 1.5% energy jitter of the THz power system. Simulated diffraction pattern up to the fourth order of an aluminum sample based on the beamline can be clearly distinguished. A prototype THz gun has beam fabricated and is now under testing, more results will be reported in future works.

physics.acc-ph↗
arXiv2022

Advanced RF Structures for Wakefield Acceleration and High-Gradient Research

Structure wakefield acceleration (SWFA) is one of the most promising AAC schemes in several recent strategic reports, including DOE's 2016 AAC Roadmap, report on the Advanced and Novel Accelerators for High Energy Physics Roadmap (ANAR), and report on Accelerator and Beam Physics Research Goals and Opportunities. SWFA aims to raise the gradient beyond the limits of conventional radiofrequency (RF) accelerator technology, and thus the RF to beam energy efficiency, by reducing RF breakdowns from confining the microwave energy in a short (on the order of about 10 ns) and intense pulse excited by a drive beam. We envision that the following research topics, within the scope of AF7, are of great interest in the next decade: advanced wakefield structures, terahertz and sub-terahertz (THz) structures, and RF breakdown physics. Research on SWFA in the above directions would directly contribute to long-term large-scale applications, including AAC-based linear colliders and compact light sources. There is also potentially a strong synergy between SWFA and other AAC concepts, when structures are combined with plasmas into hybrid AAC schemes. Research on novel structures is at the core of advancing SWFA, and is critical to future AAC-based linear colliders; at the same, it has a strong synergy with other directions, such as cavity designs, high-power microwave systems and sources, and compact light sources.

physics.acc-ph↗
arXiv2022

Continuous and Coordinated Efforts of Structure Wakefield Acceleration (SWFA) Development for an Energy Frontier Machine

Structure wakefield acceleration (SWFA) is well suited for the linear collider (LC) application due to its natural ability to accelerate positrons and preserve emittance. Under the SWFA roadmap, which was developed in response to Snowmass 2013 recommendations, four principal technologies: drive beam, main beam, wakefield structure, and LC facility design, have been investigated. The two SWFA schemes under development are the collinear wakefield accelerator (CWA), in which the drive and main beam follow the same path through a structure, and the two-beam accelerator (TBA), where the drive and main beam pass through different structures. To further advance the SWFA technology in the next decade, continuous and coordinated efforts must be carried out in a more synchronized way. This whitepaper is written to address the research needs in SWFA for preparation of Snowmass 2022.

physics.acc-ph↗
arXiv2020

Cascaded high-gradient terahertz-driven acceleration of relativistic electron beams

Terahertz (THz)-driven acceleration has recently emerged as a new route for delivering ultrashort bright electron beams efficiently, reliably, and in a compact setup. Many THz-driven acceleration related working schemes and key technologies have been successfully demonstrated and are continuously being improved to new limits. However, the achieved acceleration gradient and energy gain remain low, and the potential physics and technical challenges in the high field and high energy regime are still under-explored. Here we report a record energy gain of 170 keV in a single-stage configuration, and demonstrate the first cascaded acceleration of a relativistic beam with a 204 keV energy gain in a two-stages setup. Whole-bunch acceleration is accomplished with an average accelerating gradient of 85 MV/m and a peak THz electric field of 1.1 GV/m. This proof-of-principle result is a crucial advance in THz-driven acceleration with a major impact on future electron sources and related scientific discoveries.

physics.acc-ph↗
arXiv2020

Rapid thermal emittance and quantum efficiency mapping of a cesium telluride cathode in an rf photoinjector using multiple laser beamlets

Thermal emittance and quantum efficiency (QE) are key figures of merit of photocathodes, and their uniformity is critical to high-performance photoinjectors. Several QE mapping technologies have been successfully developed; however, there is still a dearth of information on thermal emittance maps. This is because of the extremely time-consuming procedure to gather measurements by scanning a small beam across the cathode with fine steps. To simplify the mapping procedure, and to reduce the time required to take measurements, we propose a new method that requires only a single scan of the solenoid current to simultaneously obtain thermal emittance and QE distribution by using a pattern beam with multiple beamlets. In this paper, its feasibility has been confirmed by both beam dynamics simulation and theoretical analysis. The method has been successfully demonstrated in a proof-of-principle experiment using an L-band radiofrequency photoinjector with a cesium telluride cathode. In the experiment, seven beamlets were generated from a microlens array system and their corresponding thermal emittance and QE varied from 0.93 to 1.14 $μ$m/mm and from 4.6 to 8.7%, respectively. We also discuss the limitations and future improvements of the method in this paper.

physics.acc-ph↗
arXiv2020

Single shot cathode transverse momentum imaging in high brightness photoinjectors

In state of the art photoinjector electron sources, thermal emittance from photoemission dominates the final injector emittance. Therefore, low thermal emittance cathode developments and diagnostics are very important. Conventional thermal emittance measurements for the high gradient gun are time-consuming and thus thermal emittance is not measured as frequently as quantum efficiency during the lifetime of photocathodes, although both are important properties for the photoinjector optimizations. In this paper, a single shot measurement of photoemission transverse momentum, i.e., thermal emittance per rms laser spot size, is proposed for photocathode RF guns. By tuning the gun solenoid focusing, the electrons transverse momenta at the cathode are imaged to a downstream screen, which enables a single shot measurement of both the rms value and the detailed spectra of the photoelectrons transverse momenta. Both simulations and proof of principle experiments are reported.

physics.acc-ph↗
arXiv2019

Visualizing the melting processes in ultrashort intense laser triggered gold mesh with high energy electron radiography

High energy electron radiography (HEER) is a promising tool for high energy density physics diagnostics, apart from other tools like X/γ ray shadowgraphy and high energy proton radiography. Impressive progresses have been made in development and application of HEER in past few years, and proved its potentials for high-resolution imaging of static opaque objects. By taking advantages of short pulse duration and tunable time structure of high energy electron probes, time-resolved imaging measurement of high energy density gold irradiated by ultrashort intense lasers has been performed. Phenomena of different time periods from picosecond to microsecond have been observed, thus proving feasibilities of this technique for imaging of static and dynamic objects.

physics.app-ph↗
arXiv2019

Experimental demonstration of the correction of coupled transverse dynamics aberration in an rf photoinjector

The production of electron bunches with low transverse emittance approaches the thermal emittance of the photocathode as various aberrations are corrected. Recently, the coupled transverse dynamics aberration was theoretically identified as a significant source of emittance growth and a corrector magnet was proposed for its elimination [D.H. Dowell, F. Zhou, and J. Schmerge, PRAB 21, 010101 (2018)]. This aberration arises when the beam acquires an asymmetric distribution that is then rotated with respect to the transverse reference axis thus introducing a correlation in the vertical and horizontal planes. The asymmetry is introduced by a weak quadrupole field in the rf gun or emittance compensation solenoid and the rotation is caused by the solenoid. This Letter presents an experimental study of the coupled transverse dynamics aberration in an rf photoinjector and demonstrates its elimination by a quadrupole corrector consisting of a normal and a skew quadrupole. The experimental results agree well with theoretical predictions and numerical simulations. The study also demonstrates the emittance of a low charge beam can be preserved during transportation at its thermal value, which was 1.05 mm mrad/mm, for the cesium telluride photocathode and 248 nm UV laser used.

physics.acc-ph↗
arXiv2018

Overestimation of thermal emittance in solenoid scans due to coupled transverse motion

The solenoid scan is a widely used method for the in-situ measurement of the thermal emittance in a photocathode gun. The popularity of this method is due to its simplicity and convenience since all rf photocathode guns are equipped with an emittance compensation solenoid. This paper shows that the solenoid scan measurement overestimates the thermal emittance in the ordinary measurement configuration due to a weak quadrupole field (present in either the rf gun or gun solenoid) followed by a rotation in the solenoid. This coupled transverse dynamics aberration introduces a correlation between the beam's horizontal and vertical motion leading to an increase in the measured 2D transverse emittance, thus the overestimation of the thermal emittance. This effect was systematically studied using both analytic expressions and numerical simulations. These studies were experimentally verified using an L-band 1.6-cell rf photocathode gun with a cesium telluride cathode, which shows a thermal emittance overestimation of 35% with a rms laser spot size of 2.7 mm. The paper concludes by showing that the accuracy of the solenoid scan can be improved by using a quadrupole magnet corrector, consisting of a pair of normal and skew quadrupole magnets.

physics.acc-ph↗
arXiv2017

Experimental Polarization Control of Thomson Scattering X/γ-ray Source

Thomson scattering of intense laser pulses from relativistic electrons allows us to generate high-brightness and tunable-polarization X/γ-ray pulses. This paper demonstrates the polarization control of the Thomson scattering source experimentally. The polarization of X/γ-ray is related to incident polarized laser beams, which is controlled by rotating a quarter-wave plate. In the experiment, the polarization of X-ray is determined by recording the spatial distribution of scattered photons, produced by X-ray irradiating on a target. According to modulation curves analysed from experiment results, the conclusion is that the polarization of Thomson scattering source is tunable and controllable. Meanwhile, stokes parameters of X/γ-ray whose energy varies between tens of keV and MeV are simulated. Considering the paraxial photons, it is showed that the polarization is nearly the same value in a wide range of X-ray energy according to simulation results.

physics.acc-ph↗
arXiv2017

Experiments on bright field and dark field high energy electron imaging with thick target material

Using a high energy electron beam for the imaging of high density matter with both high spatial-temporal and areal density resolution under extreme states of temperature and pressure is one of the critical challenges in high energy density physics . When a charged particle beam passes through an opaque target, the beam will be scattered with a distribution that depends on the thickness of the material. By collecting the scattered beam either near or off axis, so-called bright field or dark field images can be obtained. Here we report on an electron radiography experiment using 45 MeV electrons from an S-band photo-injector, where scattered electrons, after interacting with a sample, are collected and imaged by a quadrupole imaging system. We achieved a few micrometers (about 4 micrometers) spatial resolution and about 10 micrometers thickness resolution for a silicon target of 300-600 micron thickness. With addition of dark field images that are captured by selecting electrons with large scattering angle, we show that more useful information in determining external details such as outlines, boundaries and defects can be obtained.

physics.acc-ph↗