Yoshihiro Kazimura

Generation of a Small-Scale Quasi-Static Magnetic Field and Fast Particles during the Collision of Electron-Positron Plasma Clouds

We present the results of analytical studies and 2D3V PIC simulations of electron-positron plasma cloud collisions. We concentrate on the problem of quasi-static magnetic field generation. It is shown from linear theory, using relativistic two-fluid equations for electron-positron plasmas, that the generation of a quasi-static magnetic field can be associated with the counterstreaming instability. A two-dimensional relativistic particle simulation provides good agreement with the above linear theory and shows that, in the nonlinear stage of the instability, about 5.3% of the initial plasma flow energy can be converted into magnetic field energy. It is also shown from the simulation that the quasi-static magnetic field undergoes a collisionless change of structure, leading to large-scale, long-living structures and the production of high-energy particles. These processes may be important for understanding the production of high-energy particles in the region where two pulsar winds collide.

Magnetic Field Generation during the Collision of Electron-Ion Plasma Clouds

We present the results of analytical studies and computer simulations of electron-ion plasma cloud collisions using 2D3V particle-in-cell and 2D two-fluid collisionless relativistic codes. We address the problem concerning the generation of a quasistatic magnetic field. Using relativistic two-fluid equations for the two counterstreaming electron populations, we show with the help of linear theory that the generation of a magnetic field can be associated with the “electromagnetic counterstreaming instability”. Two-dimensional (2D) particle-in-cell provide good agreement with the results of linear theory. We show that the quasistatic magnetic field undergoes a collisionless change of structure, leading to large-scale, long-lived structures. These processes may be important for the understanding of magnetic field generation in laser plasmas and in space plasmas in the regions where two stellar winds collide.

Magnetic Field Generation and Subsequent Field Dissipation with Plasma Heating in Relativistic Streaming Pair Plasmas

Relativistic plasma streaming in background plasmas becomes unstable against the Weibel-type electromagnetic instability, which is one of effective mechanisms of magnetic field generation in cosmic plasmas. It is shown by using 3-D fully relativistic particle-in-cell (PIC) simulation code that magnetic fields perpendicular to the stream in an electron-positron (pair) plasma are generated in the early stage of the instability with small-scale sizes of the electron skin-depth. In the subsequent nonlinear stage there occurs magnetic reconnection which causes to make large-scale structure of magnetic fields in pair plasmas. The magnetic field dissipation through the magnetic reconnection leads to heating of the background plasma. These sequential physical processes are important for understanding of magnetic field generation in the relativistic shock of gamma-ray burst (GRB) sources in astrophysical plasmas.

Relaxation of an electron beam during the onset of the electromagnetic filamentation instability

Results are presented from three-dimensional particle-in-cell simulations of relaxation of an electron beam in a plasma. When penetrating into the plasma, the electron beam generates the return current carried by the plasma electrons. In a collisionless plasma, the relaxation mechanism is related to the onset of an electromagnetic filamentation instability. The instability leads to the generation of a quasistatic magnetic field, which decays due to the magnetic field reconnection in the final stage of the system evolution.

Electromagnetic fluctuations near the electron plasma frequency from an electron/electron instability

Enhanced electromagnetic fluctuations near the electron plasma frequency ωe are observed in the auroral and polar regions of the terrestrial magnetosphere; until recently, their origin has not been understood. Linear Vlasov theory and a particle-in-cell simulation with a two-dimensional fully relativistic electromagnetic code have been carried out for an electron/electron instability in a homogeneous, collisionless, magnetized plasma model under the condition ωe/|Ωe| < 1 where Ωe is the electron cyclotron frequency. At propagation parallel to the background magnetic field the instability is strictly electrostatic, but at oblique propagation significant electromagnetic fluctuations are also excited. The simulation reproduces the following essential features of the observed plasma frequency fluctuations: (1) narrowband frequencies near ωe, (2) relatively large electric fields (δE/δB ≫ 1), and (3) episodes of both right-hand and left-hand polarization. This polarization result is new and has not been published before. Furthermore, the present theory and simulation show that the right-hand polarized fluctuations are on a short-wavelength beam mode and are excited by linear instability, growth; the left-hand fluctuations are at relatively long wavelengths and are apparently driven by nonlinear processes.

Numerical simulation of electromagnetic filamentation and hollowing instabilities during collisions of counter-streaming plasmas

We present the results of two dimensional PIC (Particle-in-Cell) simulations of plasma cloud collisions. We investigate two different types of the plasma configuration: in the first one the initial configuration is assumed to have the slab form and the second configuration has a form of the cylindrical. In both cases we found that the counterstreaming motion of the plasmas is unstable with respect to the electromagnetic mode excitation with the developing of the hollow structures in the electron density distribution, the formation of the electric current filaments and the merging of the filaments and quasistatic magnetic field generation.

Magnetic Field Generation and Its Nonlinear Evolution of the Weibel Instability

The Weibel instability caused by anisotropic velocity distribution may play an important role for the generation of magnetic field in the laser produced plasmas. A three dimensional electromagnetic and relativistic particle-in-cell (PIC) code is used to show the generation of magnetic field and its nonlinear evolution. As a result, we confirm that strong magnetic fields are generated with coherent structures. Loop-like magnetic fields are generated in “football-shaped” anisotropy and sheet-like magnetic fields are generated in “pancakeshaped” anisotropy.

Wave Excitations from High Dense and Hot Plasmas

In cosmic plasmas magnetized plasmas with high density and high temperature are often observed. In such plasmas, we investigate what kinds of interaction and wave excitation are developed, by using two dimensional in real space and three dimensional in velocity space fully relativistic electromagnetic PIC code. As the remarkable results, strong electric fields are produced by the electron thermal expansion from the high density and high temperature region. And particles expanded by the thermal energy interact with background cold plasma, resulting in several waves generated both perpendicular and parallel direction to the ambient magnetic field. We propose the new mechanism of 2ωe mode excitation from the hot plasma.

Magnetic Field Generation During the Collision of Narrow Plasma Clouds

We investigate the dynamics of the collision of narrow plasma clouds, whose transverse dimension is on the order of the electron skin depth. A 2D3V (two dimensions in space and three dimensions in velocity space) particle-in-cell (PIC) collisionless relativistic code is used to show the generation of a quasi-static magnetic field during the collision of narrow plasma clouds both in electron-ion and electron-positron (pair) plasmas. The localized strong magnetic fluxes result in the generation of the charge separation with complicated structures, which may be sources of electromagnetic as well as Langmuir waves. We also present one application of this process, which occurs during coalescence of magnetic islands in a current sheet of pair plasmas.