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The equilibrium and stability of a sheared force-free magnetic field in a collisionless plasma are investigated, and the main features of charged particle motion in such a field are analyzed. A steady solution is derived to the Vlasov-Maxwell equations for the charged particle distribution function that describes different equilibrium configurations. The tearing instability of the magnetic field configurations is studied both analytically and by particle-in-cell simulations.

By using a two-dimensional (2D) relativistic fully electromagnetic particle-in-cell code, the interaction process of counterstreaming pair (electron–positron) plasmas is investigated. The counterstreaming plasmas become unstable against the collisionless electromagnetic counterstreaming instability, similar to the Weibel instability. In the linear phase, magnetic and electric fields with the scale of skin depth size are generated through the electromagnetic counterstreaming instability. The behavior of plasma in the nonlinear phase is also made clear. The small-scale magnetic fields coalesce with each other, merging through the inverse cascade process which occurs characteristically in 2D dynamics, and change into larger unit. The large-scale magnetic fields propagate more slowly than the initial plasma flow as a low-frequency wave. Behind the magnetic fields, plasmas are isotropically heated by the mixing of counterstreaming plasmas. On the other hand, the electric fields propagate the same as the initial plasma flow. Ahead of the generated magnetic fields, the electric field component along the stream is amplified through the electrostatic counterstreaming instability and the electrostatic shock waves are formed. High-energy particles are also produced from the electrostatic shocks. These simulation results may be applied to the synchrotron gamma-ray burst model.

It is shown by using a 2-dimensional fully relativistic electromagnetic particle-in-cell (PIC) code that a force-free collisionless pair plasma which is stable against collisionless tearing instability becomes unstable when driven by relativistic plasma flows and the magnetic field energy can be strongly dissipated and converted to heat plasmas as well as high-energy particle production. The force-free current sheet becomes unstable against the firehose instability and eventually breaks up into current filaments which are associated with the magnetic reconnection. The energy spectrum in the high-energy region shows an exponential-type law. The interaction process between the force-free collisionless plasmas and the relativistic plasma flows is an important mechanism for understanding the effective magnetic energy conversion, filament structures and high-energy particle production in astrophysical plasmas.

It is shown by using a two-dimensional fully relativistic electromagnetic particle-in-cell code that the tearing instability in a current sheet of pair plasmas is caused by Landau resonances of both electrons and positrons. Strong magnetic flux can be generated during coalescence of magnetic islands in the nonlinear phase of the tearing instability. The magnetic flux produced in an O-type magnetic island is caused by the counterstreaming instability found by Kazimura et al. [Astrophys. J. Lett. 498, L183 (1998); J. Phys. Soc. Jpn. 67, 1079 (1998)]. It is also shown that charge separation with a quadrupole-like structure is generated from the localized strong magnetic flux. During the decay of the quadrupole-like charge structure as well as the magnetic flux, there appear wave emissions with high-frequency electromagnetic waves and Alfvén waves as well as Langmuir waves.

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.

A dense plasma focus device can produce dense and high energy plasma in a short time. Recently, it has been proposed that the device could be applied to fusion for clean energy production (focus fusion). In order to understand the behavior of the plasma current filaments in the device, two-dimensional, relativistic, fully electromagnetic, particle-in-cell simulations were performed. Sixteen plasma current filaments were initially located on the edge of a circle in our model. They begin to interact with each other while pinching, and then coalesce in the vicinity of the center of system. In the pinch phase (during the coalescence), there appears dense plasma, whose maximum number density is 10 times larger than the initial value. The ions are accelerated, but the rate of the number of them is somewhat small. After that, the current becomes unstable and jumps out from the center. These results are useful for understanding the coalescence process of current filaments.

It is shown by using a two-dimensional, fully relativistic, electromagnetic particle-in-cell simulation code that a force-free magnetic configuration in collisionless, electron-positron (pair) plasmas becomes unstable against current-driven Buneman instability and subsequently there occurs a strong magnetic field energy dissipation associated with collisionless magnetic reconnection, which can be driven by particle trapping due to two-dimensional electric potentials remaining in the nonlinear stage of the initial current-driven Buneman instability. About 43% of the initial magnetic field energy dissipates and is transformed to plasma heating as well as high-energy particle production. The energy spectrum in the high-energy region is given by an exponential type with two temperatures. We also show the simulation results for the electron-ion plasma to compare the effect of the mass ratio of the electron and proton.

High-energy particles of a few hundred keV for electrons and up to MeV for ions were observed in a plasma focus device. Haruki et al. [Phys. Plasmas 13, 082106–1 (2006)] studied the mechanism of high-energy particle production in pinched plasma discharges by use of a 3D relativistic and fully electromagnetic particle-in-cell code. It was found that the pinched current is unstable against a sausage instability, and then becomes unstable against a kink instability. As a result high-energy electrons were observed, but protons with MeV energies were not observed. In this paper the same pinch dynamics as Haruki and co-workers is investigated, focusing on the shock formation and the shock acceleration during the pinched current. It is found that a fast magnetosonic shock wave is produced during the pinching phase which, after the maximum pinch occurs, is strongly enhanced and propagates outwards. Some protons trapped in the electrostatic potential produced near the shock front can be accelerated to a few MeV by...

In an experimental plasma, high-energy particles were observed by using a plasma focus device, to obtain energies of a few hundred keV for electrons, up to MeV for ions. In order to study the mechanism of high-energy particle production in pinched plasma discharges, a numerical simulation was introduced. By use of a three-dimensional relativistic and fully electromagnetic particle-in-cell code, the dynamics of a Z-pinch plasma, thought to be unstable against sausage and kink instabilities, are investigated. In this work, the development of sausage and kink instabilities and subsequent high-energy particle production are shown. In the model used here, cylindrically distributed electrons and ions are driven by an external electric field. The driven particles spontaneously produce a current, which begins to pinch by the Lorentz force. Initially the pinched current is unstable against a sausage instability, and then becomes unstable against a kink instability. As a result high-energy particles are observed.