T. Haruki

Simulation of high-energy proton production by fast magnetosonic shock waves in pinched plasma discharges

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...

Simulation of high-energy particle production through sausage and kink instabilities in pinched plasma discharges

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.

Generation of magnetic field and electrostatic shock wave driven by counterstreaming pair plasmas

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 initia...

Tearing instability of a force-free magnetic configuration in a collisionless plasma

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.

Rapid dissipation of magnetic field energy driven by plasma flows in force-free collisionless pair plasmas

It is shown by using a two-dimensional fully electromagnetic and relativistic particle-in-cell code that magnetic field energy can be strongly dissipated when external plasma flow interacts with the force-free magnetic field configuration in pair plasmas. During the early stage of the interaction, the streaming instability occurs, which induces the electromagnetic perturbations associated with the generation of a quasistatic magnetic field. In the nonlinear stage, the force-free magnetic field becomes unstable against the firehose instability, and then magnetic islands are formed through magnetic reconnection. The dissipated magnetic field energy is converted to plasma heating, as well as high-energy particle production. The energy spectrum in the high-energy region shows a law of the exponential type. When the plasma flow velocity becomes relativistic (0.9c), the effective energy conversion from the initial magnetic field energy is observed, with a conversion rate of about 90%. The interaction process be...

Simulations of plasma heating caused by the coalescence of multiple current loops in a proton-boron fusion plasma

Two dimensional particle-in-cell simulations of a dense plasma focus were performed to investigate a plasma heating process caused by the coalescence of multiple current loops in a proton-boron-electron plasma. Recently, it was reported that the electric field produced during the coalescence of two current loops in a proton-boron-electron plasma heats up all plasma species; proton-boron nuclear fusion may therefore be achievable using a dense plasma focus device. Based on this work, the coalescence process for four and eight current loops was investigated. It was found that the return current plays an important role in both the current pinch and the plasma heating. The coalescence of four current loops led to the breakup of the return current from the pinched plasma, resulting in plasma heating. For the coalescence of eight current loops, the plasma was confined by the pinch but the plasma heating was smaller than the two and four loop cases. Therefore the heating associated with current loop coalescence depends on the number of initial current loops. These results are useful for understanding the coalescence of multiple current loops in a proton-boron-electron plasma.

Electromagnetic wave emission during collision between a current sheet and a fast magnetosonic shock associated with coronal mass ejections

Aims. We investigate how the emission of electromagnetic waves can be enhanced when a fast magnetosonic shock wave associated with a coronal mass ejection (CME) collides perpendicularly to a coronal streamer with a stable current sheet. Methods. A two-dimensional relativistic and fully electromagnetic Particle-In-Cell (PIC) code is used. Results. It is shown that the ions in front of the shock can be accelerated by the surfatron acceleration mechanism. This shock compresses the current sheet, resulting in a local electron temperature anisotropy. The electron Bernstein waves are generated by the local electron temperature anisotropy and they are converted into electromagnetic waves (X-mode) through the linear mode conversion due to density inhomogeneity. As a result, the electromagnetic waves are observed in both forward and backward regions of the shock. The simulation results may be applied to the enhancement of electromagnetic wave emissions when a shock wave associated with CMEs collides with a coronal streamer.

Magnetic field energy dissipation driven by relativistic flows in force-free collisionless pair plasmas

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.

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.