arXiv subjects

Hui-Chun Wu

Publications and source records attributed to Hui-Chun Wu.

10 recordsLinked to original sources

arXiv2025

Circular polarization of fast radio bursts by asymmetric erosion in longitudinally magnetized plasma

Magnetars are likely to be the origin of all fast radio bursts. The recent detection of circularly polarized bursts suggests that they might be generated deep inside magnetar magnetospheres. However, the mechanism behind the circular polarization remains uncertain. Here, we study the propagation of an intense radio pulse in a longitudinally magnetized electron-dominant plasma by particle-in-cell simulations. When the field strength of the radio pulse exceeds the background magnetic field, it can excite a nonlinear plasma wakefield and continually erodes due to energy transfer to the wake. Along the magnetic field, the plasma wakefield launched by the right-circularly polarized pulse is much stronger and more nonlinear than that by the left-circularly polarized pulse. Hence, the erosion rates of the two circularly polarized modes are significantly different. We discover that this asymmetric erosion can generate circularly polarized modes from a linearly polarized pulse at relativistic intensities, even when the cyclotron frequency is much higher than the radio frequency. Finally, we present a proof-of-principle simulation to demonstrate the generation of circularly polarization by this magneto-induced asymmetric erosion in the nonuniform environment of magnetar magnetospheres.

astro-ph.HE↗

arXiv2024

Mechanism of circular polarization in giant pulses and fast radio bursts

Some giant pulses and fast radio bursts exhibit notable circular polarization, which remains unexplained and carries significant implications for their emission mechanisms. In this study, we identify multiple nanoshot pairs uniformly spaced by approximately 21 $μ$s within a giant pulse emitted by the Crab pulsar. Among these pairs, a subset displays left-hand and right-hand circular polarization in two distinct nanoshots. We propose that the occurrence of such nanoshot pairs with dual circular polarizations arises from the fragmentation of a linearly-polarized nanoshot along the magnetic field lines under the extreme Faraday effect, leveraging highly-asymmetrical pair plasma and the ultra-intense field of nanoshots. The asymmetry in pair plasmas is likely linked to discharge activities in pulsars. Moreover, the intense field of nanoshots induces cyclotron resonance within the magnetosphere, effectively slowing down the propagation velocity of the circularly polarized mode. Our findings suggest that Crab giant pulses composing nanoshots originate in its polar cap region and escape the magnetosphere along the polar magnetic field. This mechanism can also elucidate the origin of circular polarization in some fast radio bursts and thus lends support to their magnetospheric origin.

physics.plasm-ph↗

arXiv2024

Compression and acceleration of ions by ultra-short ultra-intense azimuthally-polarized light

An efficient plasma compression scheme by azimuthally-polarized (AP) light is proposed. An AP light possesses a donut-like intensity pattern, enabling it to compress and accelerate ions toward the optical axis across a wide range of parameters. When the light intensity reaches the relativistic regime of $10^{18}$ $\mathrm{W}/\mathrm{cm}^{2}$, and the plasma density is below the critical density, protons can be compressed and accelerated by the toroidal soliton formed by the light. The expansion process of the soliton can be well described by the snow-plow model. Three-dimensional (3D) particle-in-cell (PIC) simulations show that within the soliton regime, despite the ion density surpassing ten times of the critical density, their energy is relatively low for efficient neutron production. When the light intensity increases to $10^{22}$ $\mathrm{W}/\mathrm{cm}^{2}$, and the plasma density is tens of the critical density, deuterium ions can be compressed to thousands of the critical density and meanwhile accelerated to the MeV level by a tightly-focused AP light during the hole-boring (HB) process. This process is far more dramatic compared to the soliton regime, and can produce up to $10^{4}$ neutrons in a few light cycles. Moreover, in the subsequent beam-target stage, neutron yield is assessed to reach over $10^{8}$. Finally, we present a comparison with the results by a radially-polarized (RP) light to examine the influence of light polarization.

physics.plasm-ph↗

arXiv2022

Upper Field Strength Limit of Fast Radio Bursts

Fast radio bursts (FRBs) are cosmological radio transients with unclear generation mechanism. Known characteristics such as their luminosity, duration, spectrum and repetition rate, etc. suggest that FRBs are powerful coherent radio signals at GHz frequencies, but the status of FRBs near source remain unknown. As an extreme astronomical event, FRBs should be accompanied by energy -- comparable or even more powerful x/γ-ray counterparts. Here, particle-in-cell simulations of ultra-strong GHz radio pulse interaction with GeV photons show that at 3*10^12V/cm field-strengths, quantum cascade can generate dense pair plasmas, which greatly dampen the radio pulse. Thus, in the presence of GeV photons in the source region, GHz radio pulses stronger than 3*10^12V/cm cannot escape. This result indicates an upper field-strength limit of FRB at the source.

astro-ph.HE↗

arXiv2011

JPIC & How to make a PIC code

Author developed the parallel fully kinetic particle-in-cell (PIC) code JPIC based on updated and advanced algorithms (e.g. numerical-dispersion-free electromagnetic field solver) for simulating laser plasma interactions. Basic technical points and hints of PIC programming and parallel programming by message passing interface (MPI) are reviewed. Most of contents come from Author's notes when writing up JPIC and experiences when using the code to solve different problems. Enough "how-to-do-it" information should help a new beginner to effectively build up his/her own PIC code. General advices on how to use a PIC code are also given.

physics.plasm-ph↗

arXiv2010

Laser-like X-ray Sources Based on Optical Reflection from Relativistic Electron Mirror

A novel scheme is proposed to generate uniform relativistic electron layers for coherent Thomson backscattering. A few-cycle laser pulse is used to produce the electron layer from an ultra-thin solid foil. The key element of the new scheme is an additional foil that reflects the drive laser pulse, but lets the electrons pass almost unperturbed. It is shown by analytic theory and by 2D-PIC simulation that the electrons, after interacting with both drive and reflected laser pulse, form a very uniform flyer freely cruising with high relativistic gamma-factor exactly in drive laser direction (no transverse momentum). It backscatters probe light with a full Doppler shift factor of 4*gamma^2. The reflectivity and its decay due to layer expansion is discussed.

physics.plasm-ph↗

arXiv2008

Coherent Thomson backscattering from laser-driven relativistic ultra-thin electron layers

The generation of laser-driven dense relativistic electron layers from ultra-thin foils and their use for coherent Thomson backscattering is discussed, applying analytic theory and one-dimensional particle-in-cell simulation. The blow-out regime is explored in which all foil electrons are separated from ions by direct laser action. The electrons follow the light wave close to its leading front. Single electron solutions are applied to initial acceleration, phase switching, and second-stage boosting. Coherently reflected light shows Doppler-shifted spectra, chirped over several octaves. The Doppler shift is found to be proportional to γ_x^2=1/(1-β_x^2), where β_x is the electron velocity component in normal direction of the electron layer which is also the direction of the driving laser pulse. Due to transverse electron momentum p_y, the Doppler shift by 4*γ_x^2=4*γ^2/(1+(p_y/mc)^2) ~= 2*γis significantly smaller than full shift of 4*γ^2. Methods to turn p_y -> 0 and to recover the full Doppler shift are proposed and verified by 1D-PIC simulation. These methods open new ways to design intense single attosecond pulses.

physics.plasm-ph↗

arXiv2008

The reflectivity of relativistic ultra-thin electron layers

The coherent reflectivity of a dense, relativistic, ultra-thin electron layer is derived analytically for an obliquely incident probe beam. Results are obtained by two-fold Lorentz transformation. For the analytical treatment, a plane uniform electron layer is considered. All electrons move with uniform velocity under an angle to the normal direction of the plane; such electron motion corresponds to laser acceleration by direct action of the laser fields, as it is described in a companion paper. Electron density is chosen high enough to ensure that many electrons reside in a volume λ_R^3, where λ_R is the wavelength of the reflected light in the rest frame of the layer. Under these conditions, the probe light is back-scattered coherently and is directed close to the layer normal rather than the direction of electron velocity. An important consequence is that the Doppler shift is governed by γ_x=(1-(V_x/c)^2)^{-1/2} derived from the electron velocity component V_x in normal direction rather than the full γ-factor of the layer electrons.

physics.plasm-ph↗

arXiv2008

Acceleration of ultra-thin electron layer. Analytical treatment compared with 1D-PIC simulation

In this paper, we apply an analytical model [V.V. Kulagin et al., Phys. Plasmas 14,113101 (2007)] to describe the acceleration of an ultra-thin electron layer by a schematic single-cycle laser pulse and compare with one-dimensional particle-in-cell (1D-PIC) simulations. This is in the context of creating a relativistic mirror for coherent backscattering and supplements two related papers in this EPJD volume. The model is shown to reproduce the 1D-PIC results almost quantitatively for the short time of a few laser periods sufficient for the backscattering of ultra-short probe pulses.

physics.plasm-ph↗

arXiv2007

Single-cycle megawatt terahertz pulse generation from a wavelength-scale plasma oscillator driven by ultrashort laser pulses

The tremendous applications of terahertz (THz) spectroscopy and imaging require THz sources in different parameters. We propose a novel scheme to generate single-cycle powerful THz pulses by ultrashort intense laser pulses incident obliquely on a tenuous plasma slab of few THz wavelengths in thickness. This is made possible by driving a large amplitude electron plasma wave in the plasma slab, thus producing a net transient current at the plasma surfaces. Theory and simulations show that such a THz source is capable of providing megawatt power and field strength of MV/cm, which may open up new horizons for nonlinear THz science and applications.

physics.plasm-ph↗