Weibel Mechanism of Magnetic-Field Generation in the Process of Expansion of a Collisionless-Plasma Bunch with Hot Elect
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Radiophysics and Quantum Electronics, Vol. 62, No. 12, May, 2020 (Russian Original Vol. 62, No. 12, December, 2019)
WEIBEL MECHANISM OF MAGNETIC-FIELD GENERATION IN THE PROCESS OF EXPANSION OF A COLLISIONLESS-PLASMA BUNCH WITH HOT ELECTRONS A. A. Nechaev,1∗ M. A. Garasev,1 V. V. Kocharovsky,1 and Vl. V. Kocharovsky1,2
UDC 533.9. . . 16+533.951+537.86
We perform qualitative physical analysis and particle-in-cell two-dimensional numerical simulation of the Weibel mechanism of magnetic-field generation due to the self-consistent formation of filaments of the hot-electron current with anisotropic velocity distribution, which occurs in the process of decay of a strong discontinuity in a collisionless plasma with a great difference in the number density and temperature of electrons. Anisotropy of the electron distribution is mainly stipulated by their quasiadiabatic expansion and a significant decrease in the thermal velocities in the normal direction to the front of the formed electrostatic shock wave at a weak decrease in the thermal velocities in the transverse directions. A strong magnetic field is formed in a layer with the maximum anisotropy under the front, covers the entire region of the expanding plasma, and, at the nonlinear evolution stage, acquires quasiperiodic modulation along the normal to the front, which correlates with the modulation of the electron anisotropy degree. The considered scenario of development of the Weibel instability is of interest for analysis of experiments with femtosecond laser plasma and interpretation of such phenomena in the case of a decay of strong discontinuities in a nonequilibrium space plasma.
1.
INTRODUCTION
A characteristic feature of the nonequilibrium collisionless plasma with an anisotropic particle velocity distribution is the presence of quasimagnetostatic turbulence, which largely determines both kinetics of individual particles and interparticle interaction, as well as dynamics of macroscopic quasineutral inhomogeneities and the structure of self-consistent currents in them [1–3]. One of the main mechanisms of turbulence occurrence is the Weibel instability [4–6], which accompanies various transient or explosive phenomena in laboratory and space plasmas. Such an instability is typical of the processes of nonequilibriumplasma expansion to the background plasma, including shock waves and jets, or explosive phenomena in active regions of the solar corona and chromospheres of the Sun and other stars [1, 7–10]. Recent development of superpower lasers made it possible to observe the Weibel instability in a laser plasma [11–16], which opens up new prospects for studies of quasimagnetostatic turbulence and modeling of the related phenomena in weakly collisional gas-discharge, ionospheric, and astrophysical plasmas. In the experimental works referenced above, an expanding plasmoid was produced by ablating various targets with laser pulses having subnanocecond-to-subpicosecond durations and energies from thousands to several joules, respectively. Preliminary experiments [17,
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