[Show abstract][Hide abstract] ABSTRACT: Magnetic reconnection experiments in high-energy-density (HED) laser-produced plasmas have recently been conducted at the Shenguang-II (SG-II) facility. Two plasma bubbles and a 'frozen-in' magnetic field are generated by irradiating an Al foil using two laser beams. As the two bubbles with opposing magnetic fields expand and squeeze each other, magnetic reconnection occurs. In the experiments, three well-collimated high-speed electron jets are observed in the fanlike outflow region of the laser-driven magnetic reconnection. Based on two-dimensional (2D) particle-in-cell (PIC) simulations, we demonstrate that the three electron jets in the outflow region of laser-driven magnetic reconnection are super-Alfvénic, and their formation mechanism is also revealed in this paper. The two super-Alfvénic jets at the edge are formed by the outflow electrons, which move along magnetic field lines after they are accelerated in the vicinity of the X-line by the reconnection electric field. The super-Alfvénic jet at the center is formed by the electrons that come from the outside of the plasma bubbles. These electrons are reflected by the magnetic field in the pileup region and are meanwhile accelerated by the resulting electric field.
Full-text · Article · Aug 2014 · New Journal of Physics
[Show abstract][Hide abstract] ABSTRACT: Recently, magnetic reconnection has been realized in high-energy-density
laser-produced plasmas. Plasma bubbles with self-generated magnetic
fields are created by focusing laser beams to small-scale spots on a
foil. The bubbles expand into each other, which may then drive magnetic
reconnection. The reconnection experiment in laser-produced plasmas has
also been conducted at Shenguang-II (SG-II) laser facility, and the
existence of a plasmoid was identified in the experiment [Dong et al.,
Phys. Rev. Lett. 108, 215001 (2012)]. In this paper, by performing
two-dimensional (2-D) particle-in-cell simulations, we investigate such
a process of magnetic reconnection based on the experiment on SG-II
facility, and a possible explanation for the formation of the plasmoid
is proposed. The results show that before magnetic reconnection occurs,
the bubbles squeeze strongly each other and a very thin current sheet is
formed. The current sheet is unstable to the tearing mode instability,
and we can then observe the formation of plasmoid(s) in such a multiple
Full-text · Article · Nov 2013 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: The driving mechanism of solar flares and coronal mass ejections is a topic of ongoing debate, apart from the consensus that magnetic reconnection plays a key role during the impulsive process. While present solar research mostly depends on observations and theoretical models, laboratory experiments based on high-energy density facilities provide the third method for quantitatively comparing astrophysical observations and models with data achieved in experimental settings. In this article, we show laboratory modeling of solar flares and coronal mass ejections by constructing the magnetic reconnection system with two mutually approaching laser-produced plasmas circumfused of self-generated megagauss magnetic fields. Due to the Euler similarity between the laboratory and solar plasma systems, the present experiments demonstrate the morphological reproduction of flares and coronal mass ejections in solar observations in a scaled sense, and confirm the theory and model predictions about the current-sheet-born anomalous plasmoid as the initial stage of coronal mass ejections, and the behavior of moving-away plasmoid stretching the primary reconnected field lines into a secondary current sheet conjoined with two bright ridges identified as solar flares.
[Show abstract][Hide abstract] ABSTRACT: Magnetic reconnection is one of the most important processes in astrophysical, space, and laboratory plasmas, and magnetic island is an important feature in reconnection. Therefore, identifying the structures of magnetic island is crucial to improving our understanding of magnetic reconnection. Using two-dimensional (2-D) particle-in-cell (PIC) simulations, we demonstrate that the out-of-plane magnetic field has a dip in the center of magnetic island, which is formed during multiple X line guide field reconnection. Such structures are considered to be produced by the current system in the magnetic island. At the edge of the magnetic island, there exists a current anti-parallel to the in-plane magnetic field, while the current is parallel to the in-plane magnetic field inside the magnetic island. Such a dual-ring current system, which is attributed to the electron dynamics in the magnetic island, leads to the dip of the out-of-plane magnetic field in the center of the island. The relevance between our simulations and crater flux transfer events (C-FTEs) is also discussed.
Full-text · Article · Apr 2012 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: Using opened reentrant cone silicon targets, we have demonstrated the effect of micro focusing of fast electrons generated in intense laser-plasma interactions. When an intense femtosecond laser pulse is focused tightly onto one of the side walls of the cone, fast electron beam emitted along the side wall is observed. When a line focus spot, which is long enough to irradiate both of the side walls of the cone simultaneously, is used, two electron beams emitted along each side wall, respectively, are observed. The two beams should cross each other near the open tip of the cone, resulting in micro focusing. We use a two-dimensional Particle-In-Cell code to simulate the electron emission both in opened and closed cone targets. The simulation results of the opened cone targets are in agreement with the experimental observation while the results of the closed cone targets do not show the micro focusing effect.
No preview · Article · Jan 2012 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: Intense femtosecond laser-plasma interactions can produce high power terahertz radiations. In our experiment, the polished copper target was irradiated by a p-polarized laser with intensity of more than 1018 W/cm2 at an incident angle of 67.5° from the target normal. The THz energy from three different detection angles is measured. The maximum emission is found in the direction at an angle of 45° to the laser backward direction, which is more than one order of magnitude higher than in the other two directions. A simple theoretical model has been established to explain the measurements.
No preview · Article · Jan 2012 · Sciece China. Information Sciences
[Show abstract][Hide abstract] ABSTRACT: A survey on the mechanisms of powerful terahertz (THz) radiation from laser
plasmas is presented. Firstly, an analytical model is described, showing that a
transverse net current formed in a plasma can be converted into THz radiations at
the plasma oscillation frequency. This theory is applied to explain THz generation
in a gas driven by two-color laser pulses. It is also applied to THz generation in a
tenuous plasma driven by a chirped laser pulse, a few-cycle laser pulse, a DC/AC
bias electric field. These are well verified by particle-in-cell simulations,
demonstrating that THz radiations produced in these approaches are nearly
single-cycles and linear polarized. In the chirped laser scheme and the few-cycle
laser scheme, THz radiations with the peak field strength of tens of MV/cm and the
peak power of gigawatt can be achieved with the incident laser intensity less than
No preview · Article · Oct 2011 · Chinese Optics Letters
[Show abstract][Hide abstract] ABSTRACT: Magnetic reconnection is a process by which oppositely directed magnetic
field lines passing through a plasma undergo dramatic rearrangement,
converting magnetic potential into kinetic energy and heat. It is
believed to play an important role in many plasma phenomena including
solar flares, star formation and other astrophysical events,
laser-driven plasma jets, and fusion plasma instabilities. Because of
the large differences of scale between laboratory and astrophysical
plasmas, it is often difficult to extrapolate the reconnection phenomena
studied in one environment to those observed in the other. In some
cases, however, scaling laws do permit reliable connections to made,
such as the experimental simulation of interactions between the solar
wind and the Earth's magnetosphere. Here we report well-scaled
laboratory experiments that reproduce loop-top-like X-ray source
emission by reconnection outflows interacting with a solid target. Our
experiments exploit the mega-gauss-scale magnetic field generated by
interaction of a high-intensity laser with a plasma to reconstruct a
magnetic reconnection topology similar to that which occurs in solar
flares. We also identify the separatrix and diffusion regions associated
with reconnection in which ions become decoupled from electrons on a
scale of the ion inertial length.
[Show abstract][Hide abstract] ABSTRACT: Laboratory spectroscopy of non-thermal equilibrium plasmas photoionized by intense radiation is a key to understanding compact objects, such as black holes, based on astronomical observations. This paper describes an experiment to study photoionizing plasmas in laboratory under well-defined and genuine conditions. Photoionized plasma is here generated using a 0.5-keV Planckian x-ray source created by means of a laser-driven implosion. The measured x-ray spectrum from the photoionized silicon plasma resembles those observed from the binary stars Cygnus X-3 and Vela X-1 with the Chandra x-ray satellite. This demonstrates that an extreme radiation field was produced in the laboratory, however, the theoretical interpretation of the laboratory spectrum significantly contradicts the generally accepted explanations in x-ray astronomy. This model experiment offers a novel test bed for validation and verification of computational codes used in x-ray astronomy. Comment: 5 pages, 4 figures are included. This is the original submitted version of the manuscript to be published in Nature Physics
[Show abstract][Hide abstract] ABSTRACT: This review covers recent developments in multi-hundred-TW femtosecond laser systems and progress in intense laser-plasma interactions at the Institute of Physics, Chinese Academy of Sciences. The peak power of the institute's Xtreme Light III (XL-III) laser system has been upgraded from 350 to 725 TW, and the system used to observe the lateral transport of fast electrons due to magnetic and electrostatic fields at target surfaces. The surface fields cause fast-electron beams to be emitted along the front and rear target surfaces. Confinement and guiding of fast-electron propagation is demonstrated with wire- or wedge-shaped targets. Longitudinal transport is detected by optical and ion emission. Numerical simulations show that quasi-monoenergetic ion beams can be generated by collisionless electrostatic shock acceleration and phase-stable acceleration with a circularly polarized laser field. A new mechanism for high-power THz emission with plasmas as media is proposed; it can be used to obtain a single-cycle THz emission. The conversion efficiency from laser energy to Kalpha X-ray emission in a laser-copper target interaction is increased to 10-4 using high-contrast laser pulses.
Full-text · Article · Jan 2009 · Plasma and Fusion Research
[Show abstract][Hide abstract] ABSTRACT: We investigate polarization-dependent properties of the supercontinuum emission generated from filaments produced by intense femtosecond laser pulses propagating through air over a long distance. The conversion efficiency from the 800-nm fundamental to white light is observed to be higher for circular polarization than for linear polarization when the laser intensity exceeds the threshold of the breakdown of air.
[Show abstract][Hide abstract] ABSTRACT: The interaction of femtosecond laser pulses with solid targets was studied through experiments and particle-in-cell (PIC)
simulations. It is proved that the vacuum heating and the inverse bremsstralung process are the main mechanisms of the laser
pulse absorption under such conditions. The distribution of hot electrons and that of X-ray are found to have double-temperature
structure, which is confirmed by PIC simulations. While the lower temperature is attributed to the resonant absorption, the
higher one, however, is caused by the laser-induced electric field in the target normal direction. The time-integrated spectra
of the reflected laser pulse shows that the mechanism of electron acceleration is determined by the plasma density profile.
No preview · Article · Feb 2003 · Science in China Series G Physics Mechanics and Astronomy