The high-energy cosmic-rays come from a wide variety of high energyastrophysical sources; their generation mechanisms are a fundamental problem inastrophysics and have been attracting significant interest for over six decades.

Stochastic acceleration and shock acceleration are well recognized as key mechanismsfor cosmic ray generation since first proposed by Fermi. So far, these twomechanisms have been investigated widely by analytical models and numericalsimulations, but often modeled separately. In a recent paper published in SciChina-Phys Mech Astron 58(10), 105201 (2015) by Cui et al. [1], it is foundthat the two mechanisms can occur naturally in sequential two stages when alepton flow propagates in a background interstellar plasma. This relativelysimple interaction configuration suggests that experimental tests of Fermi-typeacceleration are possible in laboratory.

In their work, Cui et al. found that an electron-positron jet can drive atype of beam-plasma instabilities called the Weibel instability when ittransports through a background plasma composed of electrons and ions. Strongelectromagnetic turbulences develop as a result of the Weibel instability,which can accelerate background ions effectively. The accelerated ions form aperfect inverse-power energy spectrum as expected for Fermi II typeacceleration. After certain interaction period, the electron-positron jetfurther drives a collisionless shock wave in the second stage. Some ions can betrapped and accelerated further. This is the first time that the twoacceleration mechanisms for energetic ions are illustrated clearly in a simpleinteraction configuration.

Even though, many theoretical models have been proposed for stochasticacceleration and shock acceleration to develop, practically it is verydifficult to verify these models based upon current astronomical observations. In the last decade, there is increasinginterest to test some theoretical models in laboratory, which has brought abouta new field called laboratory astrophysics. In particular, the development ofhigh power laser technologies enables one to create a variety of uniqueconditions to mimic some astrophysical processes in much reduced temporal andspatial scales. For example, recent studies suggest that a lepton flow composedof dense electron and positron beams may be generated from relativisticlaser-plasma interaction.  Electron-positronjets are found widely in astrophysical environments such as quasars and blackholes.

According to Prof. ZhengMing Sheng, the corresponding author of thisarticle from Shanghai Jiao Tong University and University of Strathclyde, theacceleration scenario found from their numerical simulations could be tested inthe near future given the fact that high current electron-positron beams havebeen demonstrated recently in some laser-plasma experiments with high powerlasers. Three years ago, Prof. Sheng and his collaborator Prof. G. R. Kumarfrom Tata Institute for Fundamental Research in India first reported themeasurement of the turbulent magnetic field structures generated via the Weibelinstability when a dense electron beam transports through a metal [2].Detection of ion acceleration from Fermi acceleration in laboratory would be extremelyinteresting and is highly valuable to our understanding of the origin of cosmic-rays,according to Prof. Kumar. When commenting on this work, Prof. J. Zhang fromShanghai Jiao Tong University, who is one of the pioneers in promotinglaboratory astrophysics in China, pointed out that this work not only revealedvery important new physics related to beam transport and Fermi acceleration,but also suggested the great potential of laboratory astrophysics forscientific discoveries.  

Figure1: Snapshot of plasma turbulence and ion acceleration at the first stage.

[1] Y.Q. Cui, Z.M. Sheng, Q.M. Lu, et al. Two-stageacceleration of interstellar ions driven by high-energy lepton plasma flows. SciChina-Phys Mech Astron 58, 105201 (2015)

[2] S. Mondal, V. Narayanan, W. J. Ding, et al. Direct observation ofturbulent magnetic fields in hot, dense laser produced plasmas. PNAS, 109, 8011–8015 (2012)

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