Observation of Weibel magnetic fields in laser-produced interpenetrating flows

Weibel instability is a promising candidate mechanism for collisionless shock formation in astrophysical systems.Capturing the underlying physics of Weibel instability will help us to understand the astrophysical shock formation,magnetic field generation and amplification,particle acceleration,and so on.Laboratory astrophysics,provides a new way to study these microphysics in controlled conditions.At Shenguang-Ⅱlaser facility,the interpenetrating plasma flows are generated by eight laser beams irradiating a pair of opposing foils to mimic the supernova explosion and the ejecta sweeping up the surrounding medium.Evolution of collisionless interpenetrating plasma flows is observed using optical diagnostics.Filamentary structures appear in the interaction region and the associated magnetic strength is measured about 40 T.Theoretical analysis and simulations indicate that these characteristics are induced by nonlinear Weibel instability.

Collisional Efect on Weibel Instability with Semi-Relativistic Maxwellian Distribution Function

In this paper, the Coulomb collisional effect of electron-ion on the growth rate of Weibel instability is investigated based on the semi-relativistic Maxwellian distribution function in dense and unmagnetized plasma. An analytical expression was derived for the dispersion relation of Weibel instability for two limit cases [ξ = ω＇/k‖T‖ 〉〉 1 and ｜ξ｜ 〈〈 1. In limit ｜ξ｜ 〉〉 1 the dispersion relation only includes a real part and in limit ｜ξ｜ 〈〈 1 the imaginary part of the frequency of waves＇ instability plays a role in the dispersion relation. In limit ｜ξ｜ 〈〈 1, the two quantities μ and η, that are due to the relativistic and collisional effects, will appear in the growth rate of Weibel instability. The growth rate of Weible istability will be increased through decreasing the Coulomb collisional frequency and also increasing the temperature anisotropic parameter in strong relativistic limit.

Weibel Instability Growth Rate in Magnetized Plasmas with Quasi-Relativistic Distribution Function

The mechanism of the Weibel instability is investigated for dense magnetized plasmas. As we know, due to the electron velocity distribution, the Coulomb collision effect of electron-ion and the relativistic properties play an important role in such study. In this study an analytical expression for the growth rate and the condition of restricting the Weibel instability are derived for low-frequency limit. These calculations are done for the oscillation frequency dependence on the electron cyclotron frequency. It is shown that, the relativistic properties of the particle lead to increasing the growth rate of the instability. On the other hand the collision effects and background magnetic field try to decrease the growth rate by decreasing the temperature anisotropy and restricting the particles movement.

Filamentation instability in two counter-streaming laser plasmas

The filamentation instability was observed in the interaction of two counter-streaming laser ablated plasma flows, which were supersonic, collisionless, and also closely relevant to astrophysical conditions. The plasma flows were created by irradiating a pair of oppositely standing plastic （CH） foils with Ins-pulsed laser beams of total energy of 1.7 kJ in two laser spots. With characteristics diagnosed in experiments, the calculated features of Weibel-type filaments are in good agreement with measurements.

Proton acceleration in plasma turbulence driven by high-energy lepton jets

The interaction of high energy lepton jets composed of electrons and positrons with background electron–proton plasma is investigated numerically based upon particle-in-cell simulation,focusing on the acceleration processes of background protons due to the development of electromagnetic turbulence.Such interaction may be found in the universe when energetic lepton jets propagate in the interstellar media.When such a jet is injected into the background plasma,theWeibel instability is excited quickly,which leads to the development of plasma turbulence into the nonlinear stage.The turbulent electric and magnetic fields accelerate plasma particles via the Fermi II type acceleration,where the maximum energy of both electrons and protons can be accelerated to much higher than that of the incident jet particles.Because of background plasma acceleration,a collisionless electrostatic shock wave is formed,where some pre-accelerated protons are further accelerated when passing through the shock wave front.Dependence of proton acceleration on the beam-plasma density ratio and beam energy is investigated.For a given background plasma density,the maximum proton energy generally increases both with the density and kinetic energy of the injected jet.Moreover,for a homogeneous background plasma,the proton acceleration via both turbulent fields and collisionless shocks is found to be significant.In the case of an inhomogeneous plasma,the proton acceleration in the plasma turbulence is dominant.Our studies illustrate a scenario where protons from background plasma can be accelerated successively by the turbulent fields and collisionless shocks.

Collisional effects on the current-filamentation instability in a dense plasma

The collisional current-filamentation instability (CFI) is studied for a nonrelativistic electron beam penetrating an infinite uniform plasma. It is analytically shown that the CFI is driven by the drift-anisotropy rather than the classical anisotropy of the beam and the background plasma. Therefore, collisional effects can either attenuate or enhance the CFI depending on the drift-anisotropy of the beam-plasma system. Numerical results are given for some typical parameters, which show that collisional effects cannot stabilize but enhance the CFI in a dense plasma. Thus, the CFI may play a dominant role in the fast electron transport and deposition relevant to the fast ignition scenario (FIS).

Recent progress of laboratory astrophysics with intense lasers

Thanks to a rapid progress of high-power lasers since the birth of laser by T.H.Maiman in 1960,intense lasers have been developed mainly for studying the scientific feasibility of laser fusion.Inertial confinement fusion with an intense laser has attracted attention as a new future energy source after two oil crises in the 1970s and 1980s.From the beginning,the most challenging physics is known to be the hydrodynamic instability to realize the spherical implosion to achieve more than 1000 times the solid density.Many studies have been performed theoretically and experimentally on the hydrodynamic instability and resultant turbulent mixing of compressible fluids.During such activities in the laboratory,the explosion of supernova SN1987A was observed in the sky on 23 February 1987.The X-ray satellites have revealed that the hydrodynamic instability is a key issue to understand the physics of supernova explosion.After collaboration between laser plasma researchers and astrophysicists,the laboratory astrophysics with intense lasers was proposed and promoted around the end of the 1990s.The original subject was mainly related to hydrodynamic instabilities.However,after two decades of laboratory astrophysics research,we can now find a diversity of research topics.It has been demonstrated theoretically and experimentally that a variety of nonlinear physics of collisionless plasmas can be studied in laser ablation plasmas in the last decade.In the present paper,we shed light on the recent 10 topics studied intensively in laboratory experiments.A brief review is given by citing recent papers.Then,modeling cosmic-ray acceleration with lasers is reviewed in a following session as a special topic to be the future main topic in laboratory astrophysics research.

Mapping non-laminar proton acceleration in laser-driven target normal sheath field

We report on experimental observation of non-laminar proton acceleration modulated by a strong magnetic field in laser irradiating micrometer aluminum targets.The results illustrate the coexistence of ring-like and filamentation structures.We implement the knife edge method into the radiochromic film detector to map the accelerated beams,measuring a source size of 30-110μm for protons of more than 5 MeV.The diagnosis reveals that the ring-like profile originates from low-energy protons far off the axis whereas the filamentation is from the near-axis high-energy protons,exhibiting non-laminar features.Particle-in-cell simulations reproduced the experimental results,showing that the short-term magnetic turbulence via Weibel instability and the long-term quasi-static annular magnetic field by the streaming electric current account for the measured beam profile.Our work provides direct mapping of laser-driven proton sources in the space-energy domain and reveals the non-laminar beam evolution at featured time scales.

Filaments in high-speed counter-streaming plasma interactions driven by high-power laser pulses

Interactions of two counter-streaming plasmas driven by high power laser pulses are studied on Shenguang II laser facility.Filamentary structures were observed in the interaction region after the electrostatic shockwave decay.Theoretical analysis and observations indicate that the filaments are because of collisionless mechanisms,which are caused by the electromagnetic instability,such as the beam-Weibel instability.Collision experiments were also carried out for comparison and no filaments were generated.