arXiv subjects

Yu-Tong Li

Publications and source records attributed to Yu-Tong Li.

At least 19 recordsLinked to original sources

arXiv2025

Photometric and Spectroscopic Studies of Four Low Mass-ratio Contact Binaries with Period Longer than 0.7 days

Photometric and spectroscopic studies of four long-period low mass-ratio contact binaries, V0508 And, V0844 Aur, V0699 Cep, and NSVS 6259046, are performed. V0508 And, V0844 Aur, and V0699 Cep are found to be A-type low-mass-ratio medium-contact binaries, while NSVS 6259046 is found to be an A-type deep-contact binary. O - C analysis indicates no long-term variation in V0844 Aur. However, the orbital periods of the other three targets are increasing. We conclude that V0844 Aur, V0699 Cep and NSVS 6259046 are magnetically active, as evidenced by the presence and variable nature of the O'Connell effect in these systems. By analyzing the LAMOST spectroscopic data, we find out that NSVS 6259046 and V0508 And exhibit no chromospheric activity on the dates the LAMOST spectra were taken, while the low signal-to-noise ratio in LAMOST data for V0844 Aur prevents us from obtaining reliable results. We discover that V0699 Cep is an early-type contact binary with chromospheric activity. Their initial masses and ages are calculated. All four systems are determined to be currently stable. We collect 217 contact binaries with both spectroscopic and photometric observations, and compare the differences between short-period and long-period systems in terms of mass-luminosity relation and mass-radius relation, using 0.7 days as the period boundary.

astro-ph.SR↗

arXiv2025

All-Optical Generation of Dense, Multi-GeV, Longitudinally-Polarized Positron Beams

The production of high-yield, longitudinally polarized positron beams represents an outstanding challenge in advanced accelerator science. Laser-driven schemes offer a compact alternative but typically yield only transverse polarization, or require pre-polarized electron beams, and struggle to efficiently accelerate positrons to high energies. Here, we introduce an all-optical scheme that overcomes these limitations by integrating positron generation, acceleration, and spin manipulation in a unified framework. Through a head-on collision between an ultraintense, circularly polarized laser pulse and a counterpropagating unpolarized electron beam, we drive a robust QED cascade. The nonlinear Breit-Wheeler process within the cascade produces positrons that are born directly within the strong laser field. Crucially, these positrons are instantaneously captured and accelerated to multi-GeV energies (up to $\sim$9 GeV) via a direct laser acceleration mechanism, while their spins are simultaneously rotated to longitudinal alignment by the field dynamics. Our Monte-Carlo simulations confirm the simultaneous achievement of a high positron yield ($\sim$20 $e^+/e^-$), a high average longitudinal polarization ($\sim$50\%), and GeV-scale energies. This all-optical source, feasible at upcoming ultraintense laser facilities, presents a compact and efficient solution for applications in collider physics and fundamental high-energy experiments.

physics.acc-ph↗

arXiv2025

Detecting Linear Breit-Wheeler Signals with a Laser-Foil Setup

As a fundamental QED process, linear Breit-Wheeler (LBW) pair production predicted 90 years ago has not yet been demonstrated in experiments with real photons. Here, we propose an experimentally advantageous scheme to detect the LBW signal by irradiating a foil target with a single 10 PW-level laser. Our integrated QED particle-in-cell simulations demonstrate that the LBW signal can be explicitly distinguished from the Bethe-Heitler (BH) signal by comparing positron energy spectra behind the target at varying target thicknesses. The LBW positrons are created at the front of the target and subsequently experience both laser vacuum acceleration and sheath field acceleration to gain high energies, while BH positrons, originating within the target bulk, are only subjected to sheath field acceleration. As a result, the invariance of the high-energy tail of positron spectra with respect to the target thickness serves as a distinct signature of the LBW process. Notably, this scheme remains viable even when the BH yield dominates over the LBW yield.

physics.plasm-ph↗

arXiv2024

Achieving High Polarization of Photons Emitted by Unpolarized Electrons in Ultrastrong Laser Fields

Nonlinear Compton scattering driven by ultraintense lasers presents a promising avenue for enhancing the photon energy, brilliance, and setup compactness of $γ$-ray sources. However, a significant challenge lies in achieving a high polarization degree with commonly generated unpolarized electrons, thus addressing a longstanding puzzle in the field. Here we investigate the polarization dynamics of photons emitted by an unpolarized electron beam interacting with a counter-propagating ultraintense laser pulse numerically, and propose a novel method to generate highly polarized $γ$ rays via nonlinear Compton scattering with the aid of vacuum dichroism effect. Our simulations reveal that high-brilliance $γ$ rays with polarization beyond 90\% are feasible in a single-shot interaction, rivaling the highest achieved by any $γ$-ray sources to date, based on a developed Monte Carlo method incorporating polarization-resolved tree processes of nonlinear Compton scattering and Breit-Wheeler pair production and one-loop vacuum polarization. This generation method showcases an extraordinary ultra-high polarization degree and a user-friendly all-optical experimental setup, while harnessing the high photon energy and brilliance characteristic of nonlinear Compton scattering sources, thus making it of great potential for experimental applications.

physics.plasm-ph↗

arXiv2024

Electromagnetically Induced Transparency in Strongly Relativistic Regime

Stable transport of laser beams in highly over-dense plasmas is of significance in fast ignition of inertial confinement fusion, relativistic electrons generation, and powerful electromagnetic emission, but hard to realize. Early in 1996, Harris proposed an electromagnetically induced transparency (EIT) mechanism, analogous to the concept in atom physics, to transport a low-frequency (LF) laser in over-dense plasmas aided by a high-frequency pump laser. However, subsequent investigations show that EIT cannot occur in real plasmas with boundaries. Here, our particle-in-cell simulations show that EIT can occur in strongly-relativistic regime and results in stable propagation of a LF laser in bounded plasmas with tens of its critical density. A relativistic three-wave coupling model is developed, and the criteria and frequency passband for EIT occurrence are presented. The passband is sufficiently wide in strongly-relativistic regime, allowing EIT to work sustainably. Nevertheless, it is narrowed to nearly an isolated point in weakly-relativistic regime, which can explain the quenching of EIT in bounded plasmas found in previous investigations.

physics.plasm-ph↗

arXiv2023

Control of energy spectra and enhancement of energy conversion of fast electrons generated by dual-color picosecond lasers

After the successful fusion ignition at National Ignition Facility, seeking for a high-gain fusion scheme becomes the next hot-spot in inertial confinement fusion community. Fast ignition provides an alternative due to its potential to reduce the energy of driven lasers and achieve higher target gain, whose core step is generating fast electron beam using picosecond lasers. The properties of the electron beam, such as energy spectra, determines the succeed heating process and the yield. In this study, with 2D particle-in-cell simulations, we find that the energy transfer ratio would significantly increase, and the energy spectra would be lifted up in the dual-color injecting scheme, where an extra weak low-frequency laser is injected with the main pulse. These phenomena are attributed to the longitudinal electric field modulation in low-density region and electromagnetically-induced-transparency process in the microchannels formed near the critical surface. Our results can be applied to fast ignition schemes and experimental designs of super-hot electron generation with different pulse durations. For example, in the DCI project with a fast ignition scheme, such a dual-color intense picosecond laser facility is under construction to control the fast electron generation and enhance the laser transparency.

physics.plasm-ph↗

arXiv2023

Steady regime of radiation pressure acceleration with foil thickness adjustable within micrometers under 10-100 PW laser

Quasi-monoenergetic GeV-scale protons are predicted to efficiently generate via radiation pressure acceleration (RPA) when the foil thickness is matched with the laser intensity, e.g., $L_{mat}$ at several nm to 100 nm with $10^{19}-10^{22} \rm ~W cm^{-2}$ available in laboratory. However, non-monoenergetic protons with much lower energies than prediction were usually observed in RPA experiments, because of too small foil thickness which is hard to bear insufficient laser contrast and foil surface roughness. Besides the technical problems, we here find that there is an upper-limit thickness $L_{up}$ derived from the requirement that the laser energy density should dominate over the ion source, and $L_{up}$ is lower than $ L_{mat}$ with the intensity below $10^{22} \rm~ W cm^{-2}$, which causes inefficient or unsteady RPA. As the intensity is enhanced to $\geq 10^{23} \rm ~W cm^{-2}$ provided by 10-100 PW laser facilities, $L_{up}$ can significantly exceed $L_{mat}$ and therefore RPA becomes efficient. In this regime, $L_{mat}$ acts as a lower-limit thickness for efficient RPA, so the matching thickness can be extended to a continuous range from $L_{mat}$ to $L_{up}$; the range can reach micrometers, within which foil thickness is adjustable. This makes RPA steady and meanwhile the above technical problems can be overcome. Particle-in-cell simulation shows that multi-GeV quasi-monoenergetic proton beams can be steadily generated and the fluctuation of the energy peaks and the energy conversation efficiency remains stable although the thickness is taken in a larger range with increasing intensity. This work predicts that near future RPA experiments with 10-100 PW facilities will enter a new regime with the adjustable and large-range foil thickness for steady acceleration.

physics.plasm-ph↗

arXiv2021

Dense Polarized Positrons from Laser-Irradiated Foil Targets in the QED Regime

Dense positrons are shown to be effectively generated from laser-solid interactions in the strong-field quantum electrodynamics (QED) regime. Whether these positrons are polarized has not yet been reported, limiting their potential applications. Here, by QED particle-in-cell simulations including electron-positron spin and photon polarization effects, we investigate a typical laser-solid setup that an ultraintense linearly polarized laser irradiates a foil target with $μ$m-scale-length preplasma. We find that once the positron yield becomes appreciable with the laser intensity exceeding $10^{24}~\rm W/\rm cm^2$, the positrons are obviously polarized. The polarized positrons can acquire $>30\%$ polarization degree and $>30$ nC charge with a flux of $10^{12}\,{\rm sr}^{-1}$. The polarization relies on the deflected angles and can reach 60\% at some angles and energies. The angularly-dependent polarization is attributed to the asymmetrical laser fields positrons undergo in the skin layer of overdense plasma, where the radiative spin-flip and radiation reaction play significant roles. The positron polarization is robust and could generally appear in future 100-PW-class laser-solid experiments for various applications.

physics.plasm-ph↗

arXiv2021

YUNIC: A Multi-Dimensional Particle-In-Cell Code for Laser-Plasma Interaction

For simulating laser-plasma interactions, we developed a parallel, multi-dimensional, fully relativistically particle-in-cell (PIC) code, named YUNIC. The core algorithm is introduced, including field solver, particle pusher, field interpolation, and current interpolation. In addition to the classical electromagnetic interaction in plasmas, nonlinear Compton scattering and nonlinear Breit-Wheeler pair production are also implemented based on Monte-Carlo methods to study quantum electrodynamics (QED) processes. We benchmark YUNIC against theories and other PIC codes through several typical cases. YUNIC can be applied in varieties of physical scenes, from relativistic laser-plasma interactions to astrophysical plasmas and strong-field QED physics.

physics.plasm-ph↗

arXiv2021

Generation of polarized positron beams via collisions of ultrarelativistic electron beams

A novel scheme is proposed for generating a polarized positron beam via multiphoton Breit-Wheeler process during the collision of a 10 GeV, pC seeding electron beam with the other 1 GeV, nC driving electron beam. The driving beam provides the strong self-generated field, and a suitable transverse deviation distance between two beams enables the field experienced by the seeding beam to be unipolar, which is crucial for realizing the positron polarization. We employ the particle simulation with a Monte-Carlo method to calculate the spin- and polarization-resolved photon emission and electron-positron pair production in the local constant field approximation. Our simulation results show that a highly polarized positron beam with polarization above $40\%$ can be generated in several femtoseconds, which is robust with respect to parameters of two electron beams. Based on an analysis of the influence of $γ$-photon polarization on the polarized pair production, we find that a polarized seeding beam of the proper initial polarization can further improve the positron polarization to $60\%$.

physics.acc-ph↗

arXiv2021

Spin and polarization effects on the nonlinear Breit-Wheeler pair production in laser-plasma interaction

The spin effect of electrons/positrons ($e^-$/$e^+$) and polarization effect of $γ$ photons are investigated in the interaction of two counter-propagating linearly polarized 10-PW-class laser pulses with a thin foil target. The processes of nonlinear Compton scattering and nonlinear Breit-Wheeler pair production based on spin- and polarization-resolved probabilities are implemented into the particle-in-cell (PIC) algorithm by Monte Carlo methods. It is found from PIC simulations that the average degree of linear polarization of emitted $γ$ photons can exceed $50\%$. This polarization effect leads to reduced positron yield by about $10\%$. At some medium positron energies, the reduction can reach $20\%$. Furthermore, we also observe that the local spin polarization of $e^-$/$e^+$ leads to a slight decrease of the positron yield about $2\%$ and some anomalous phenomena about the positron spectrum and photon polarization at the high-energy range, due to spin-dependent photon emissions. Our results indicate that spin and polarization effects should be considered in calculating the pair production and laser-plasma interaction with the laser power of 10-PW class.

physics.plasm-ph↗

arXiv2020

Low-frequency whistler waves excited by relativistic laser pulses

It is shown by multi-dimensional particle-in-cell simulations that intense secondary whistler waves with special vortex-like field topology can be excited by a relativistic laser pulse in the highly magnetized, near-critical density plasma. Such whistler waves with lower frequencies obliquely propagate on both sides of the laser propagation axis. The energy conversion rate from laser to whistler waves can exceed 15%. Their dispersion relations and field polarization properties can be well explained by the linear cold-plasma model. The present work presents a new excitation mechanism of whistler modes extending to the relativistic regime and could also be applied in magnetically assisted fast ignition.

physics.plasm-ph↗

arXiv2020

Quantum-stochasticity-induced asymmetry in angular distribution of electrons in a quasi-classical regime

Impacts of quantum stochasticity on the dynamics of an ultra-relativistic electron beam head-on colliding with a linearly polarized ultra-intense laser pulse are theoretically investigated in a quasi-classical regime. Generally, the angular distribution of the electron beam keeps symmetrically in transverse directions in this regime, even under the ponderomotive force of the laser pulse. Here we show that when the initial angular divergence $Δθ_i \lesssim 10^{-6} a_0^2$ with $a_0$ being the normalized laser field amplitude, an asymmetric angular distribution of the electron beam arises due to the quantum stochasticity effect, via simulations employing Landau-Lifshitz, quantum-modified Landau-Lifshitz equations, and quantum stochastic radiation reaction form to describe the radiative electron dynamics respectively. The asymmetry is robust against a variety of laser and electron parameters, providing an experimentally detectable signature for the nature of quantum stochasticity of photon emission with laser and electron beams currently available.

physics.plasm-ph↗

arXiv2019

Steering of high energy electron beam in laser plasma accelerators

By using Dazzler system and tilting compressor grating, we provide an effective way of using the laser group delay dispersion (GDD) to continuously steer the high energy electron beam which is accelerated by asymmetric laser-wakefield. The deviation angle of electrons is as the same as the angular chirped laser pulse from its initial optical axis, which is determined by the laser pulse-front-tilt (PFT). This unique method can be continuously used to control over the pointing direction of electron-pulses to the requisite trajectories, especially for the alignment sensitive devices such as electron-positron collider or undulator. Besides, the effect of PFT on the qualities of electron beam has been investigated.

physics.plasm-ph↗

arXiv2019

High spatial resolution diffraction diagnostics for intense terahertz sources

We propose a high resolution spatial diagnostic method via inserting a millimeter-gap grating into the collimated terahertz beam to monitor the minute variation of the terahertz beam in strong-field terahertz sources, which is difficult to be resolved in conventional terahertz imaging systems. To verify the method, we intentionally fabricate tiny variations of the terahertz beam through tuning the iris for the infrared pumping beam before the tilted pulse-front pumping (TPFP) setups. The phenomena can be well explained by the the theory based on tilted pulse front technique and terahertz diffraction. We believe our observation not only help further understand the mechanism of intense terahertz generation, but also may be useful for strong-field terahertz applications.

physics.optics↗

arXiv2019

Guided propagation of extremely intense lasers in plasma via ion motion

The upcoming $10-100$ petawatt laser facilities may deliver laser pulses with unprecedented intensity of $10^{22}-10^{25}\rm~W cm^{-2}$, which can trigger various nonlinear quantum electrodynamic processes in plasma. For effective laser plasma interactions at such high intensity levels, guided laser propagation is critical. However, this becomes impossible via usual plasma electron response to laser fields due to electron cavitation by the laser ponderomotive force. Here, we find that ion response to the laser fields may effectively guide laser propagation at such high intensity levels. The corresponding conditions of the required ion density distribution and laser power are presented and verified by three-dimensional particle-in-cell simulations. Our theory shall serve as a guide for future experimental design involving ultrahigh intensity lasers.

physics.plasm-ph↗

arXiv2019

Spin-polarization effects of an ultrarelativistic electron beam in an ultraintense two-color laser pulse

Spin-polarization effects of an ultrarelativistic electron beam head-on colliding with an ultraintense two-color laser pulse are investigated comprehensively in the quantum radiation-dominated regime. We employ a Monte Carlo method, derived from the recent work of [Phys. Rev. Lett. {\bf 122}, 154801 (2019)], to calculate the spin-resolved electron dynamics and photon emissions in the local constant field approximation. We find that electron radiation probabilities in adjacent half cycles of a two-color laser field are substantially asymmetric due to the asymmetric field strengths, and consequently, after interaction the electron beam can obtain a total polarization of about 11\% and a partial polarization of up to about 63\% because of radiative spin effects, with currently achievable laser facilities, which may be utilized in high-energy physics and nuclear physics. Moreover, the considered effects are shown to be crucially determined by the relative phase of the two-color laser field and robust with respect to other laser and electron beam parameters.

physics.plasm-ph↗

arXiv2018

Determining Carrier-Envelope Phase of Relativistic Laser Pulses via Electron Momentum Distribution

The impacts of the carrier-envelope phase (CEP) of a long relativistic tightly-focused laser pulse on the dynamics of a counter-propagating electron beam have been investigated in the, so-called, electron reflection regime, requiring the Lorentz factor of the electron $γ$ to be approximately two orders of magnitudes lower than the dimensionless laser field parameter $ξ$. The electrons are reflected at the rising edge of the laser pulse due to the ponderomotive force of the focused laser beam, and an asymmetric electron angular distribution emerges along the laser polarization direction, which sensitively depends on the CEP of the driving laser pulse for weak radiative stochastic effects. The CEP siganatures are observable at laser intensities of the order or larger than $10^{19}$ W/cm$^2$ and the pulse duration up to 10 cycles. The CEP detection resolution is proportional to the electron beam density and can achieve approximately $0.1^{\circ}$ at an electron density of about $10^{15}$ cm$^{-3}$. The method is applicable for currently available ultraintense laser facilities with the laser peak power from tens of terawatt to multi-petawatt region.

physics.optics↗