High power lasers can create extreme conditions in the laboratory relevant to astrophysical systems[
High Power Laser Science and Engineering, Volume. 6, Issue 3, 03000e45(2018)
Laboratory study of astrophysical collisionless shock at SG-II laser facility
Astrophysical collisionless shocks are amazing phenomena in space and astrophysical plasmas, where supersonic flows generate electromagnetic fields through instabilities and particles can be accelerated to high energy cosmic rays. Until now, understanding these micro-processes is still a challenge despite rich astrophysical observation data have been obtained. Laboratory astrophysics, a new route to study the astrophysics, allows us to investigate them at similar extreme physical conditions in laboratory. Here we will review the recent progress of the collisionless shock experiments performed at SG-II laser facility in China. The evolution of the electrostatic shocks and Weibel-type/filamentation instabilities are observed. Inspired by the configurations of the counter-streaming plasma flows, we also carry out a novel plasma collider to generate energetic neutrons relevant to the astrophysical nuclear reactions.
1 Introduction
High power lasers can create extreme conditions in the laboratory relevant to astrophysical systems[
One of the hottest research fields is astrophysical shocks, which are ubiquitous and observed in a wide range of astrophysical environments, such as solar-terrestrial space, supernova explosions and gamma-ray bursts. Figure
Figure
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No matter which scheme is applied, it must achieve the collisionless conditions between CPFs, i.e., the MFP larger than the interaction scale, (target separation or shock transition width, in our experiment ). The simplified expression of MFP can be written in Gaussian units as[
Here we will review the achievements of the collisionless shock at SG-II laser facility[
2 Experimental results
2.1 The evolution of the symmetrical CPFs
The left panel in Figure
Relevant works[
2.2 Collisionless electrostatic shock formation and evolution in the CPFs
2.2.1 Unsymmetrical case
Figure
To clarify the generation mechanism of the shock, a quasi-one-dimensional particle-in-cell (PIC) simulation is performed under the same experimental conditions. From Figure 4 in Ref. [
2.2.2 Symmetrical case
Figure
2.3 Weibel/filamentation instability in the symmetrical case
Weibel instability is a promising candidate for creating astrophysical shocks. It is a typical electromagnetic phenomenon, driven by the plasma anisotropy. Under the current laser-plasma conditions, i.e., the electron thermal velocity is larger than the flow velocity and the ion thermal velocity is smaller than the flow velocity, the ions freely interpenetrate each other in the presence of a single thermalized electron background. Therefore, it is called ion–ion driven Weibel-type instability. The signature of Weibel instability is that the filamentary structures form and extend in the axis of flow direction. The self-generated magnetic field grows from linear phase to nonlinear phase until saturation. Although many groups[
According to Equation (
2.4 Other applications of the CPFs
The neutron yield in CPFs is an important tool to distinguish between collisionless and collisional effects. Neutrons generation in CPFs can originate from three sources: (i) the laser-induced fireballs from the target foils, (ii) the counter-propagating ions interaction with each other, and (iii) the scattering ions interaction with the ions from intra-flows. Here we carry out two neutron generation experiments for comparison[
3 Summary and outlook
The study of astrophysical shock formation is important for us to understand the particle acceleration and cosmic rays generation. The laboratory experiments provide us a new opportunity to investigate the physical mechanism behind these scenarios. Our experimental results show that both electrostatic instability and filamentation instability can grow up in CPFs, but compete with each other. The self-generated bipolar electrostatic field from the electrostatic instability can support the shock formation and evolution. However, the magnetic field is too weak to excite the shock generation.
Considering the characteristic of the cosmic ray spectrum with power law, Weibel-mediated shock is a promising candidate via diffusive shock acceleration. Such important issues of the acceleration mechanism are not fully understood and still a big challenge. A stronger magnetic field is necessary to excite the Weibel-type shock. This higher magnetic field could be achieved by optimizing filamentation instability[
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Dawei Yuan, Huigang Wei, Guiyun Liang, Feilu Wang, Yutong Li, Zhe Zhang, Baojun Zhu, Jiarui Zhao, Weiman Jiang, Bo Han, Xiaoxia Yuan, Jiayong Zhong, Xiaohui Yuan, Changbo Fu, Xiaopeng Zhang, Chen Wang, Guo Jia, Jun Xiong, Zhiheng Fang, Shaoen Jiang, Kai Du, Yongkun Ding, Neng Hua, Zhanfeng Qiao, Shenlei Zhou, Baoqiang Zhu, Jianqiang Zhu, Gang Zhao, Jie Zhang. Laboratory study of astrophysical collisionless shock at SG-II laser facility[J]. High Power Laser Science and Engineering, 2018, 6(3): 03000e45
Special Issue: LABORATORY ASTROPHYSICS
Received: Nov. 26, 2017
Accepted: Jun. 21, 2018
Published Online: Sep. 5, 2018
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