O. Ishihara

Complex plasma: dusts in plasma

Dust particles in a plasma are charged negatively and are subject to various types of forces, including a drag force by plasma particles and a force due to the collective nature of a plasma. Dust particles are found in a sheath in laboratories balanced by the gravitational force and the electric force, while dust particles in space are ubiquitous, including planetary magnetospheres and interstellar space. Because of the novel nature of a complex system involving plasma particles and dust particles in a collective way, the dusty plasma is often called a complex plasma. The complex plasma is characterized by two distinctly different scales in time and in space. The plasma with electrons, ions and neutrals is characterized by the collective motion with a fast time scale and a short wavelength, while the dust particles move in a slow time scale and a long spatial scale. Some fundamental aspects of a complex plasma are reviewed and possible applications are discussed.

Wake potential of a dust grain in a plasma with ion flow

Debye screening potential and wake potential for a moving dust grain in a collisionless plasma with ion flow is studied. When a relative velocity of the dust grain exceeds the ion acoustic velocity, the oscillatory wake potential is formed in a circular cone behind the particle and produces potential minima in a periodic manner. The ion acoustic collective effects on dust particles contribute to the formation of the periodic structure. The characteristic spacing between the potential minima are several times of Debye wavelength in height and in radius. Such a periodic structure may be relevant to the formation of Coulomb quasilattices (plasma crystals) observed in the dusty plasma laboratory experiments.

On plasma crystal formation

It is shown that charged dust grains in a sheath region with plasma ion flow can attract each other in the wake potential cone of an upstream dust particulate. Because of the periodic nature of the potential, periodic structures of the dust grains can be formed in a base plane of the cone.

Nonlinear evolution of Buneman instability

The nonlinear evolution of one‐dimensional electron‐ion two‐stream instability in a field‐free plasma is studied both analytically and numerically (computer simulation). The instability is dominated by the fastest growing mode and its harmonics, provided that the initial fluctuation level is sufficiently small. A nonlinear dispersion relation is derived and solved numerically, taking into account; (a) the frequency and growth rate modulation, (b) the electric field up to ‖Ek‖4, and (c) the ’’renormalized’’ particle distribution functions. The model can successfully explain the results of a computer simulation, particularly the presence of an algebraic growth stage following the breakdown of the exponential linear growth, the appearance of harmonics, and the final saturation level. The minimum conductivity found scales as (M/m)0.61wpe, where M/m is the ion/electron mass ratio.

Plasma effects on electron beam focusing and microwave emission in a virtual cathode oscillator

The effect of anode and cathode plasmas on the electron beam dynamics in a virtual cathode oscillator is investigated. A cathode plasma is formed immediately after the rise of the electron beam current and is followed by an anode plasma. The anode plasma formation occurs well before beam focusing and microwave emission. Each plasma expands in the diode region with approximately the speed of 2.0 cm//spl mu/s. The electron beam current in the diode region is well characterized by the electron space-charge-limited current in bipolar flow with expanding plasmas in the anode-cathode gap. Particle-in-cell computer simulation reveals that in the presence of anode plasma the annular electron beam is focused down to small radius while oscillating between a real and a virtual cathode. These simulation results agree qualitatively with X-ray measurements of the electron beam current density profile across the anode. Such a focused beam is found to be responsible for the formation of a strong virtual cathode and microwave emission.

On the rotation of a dust particulate in an ion flow in a magnetic field

Spinning motion of a charged test dust particulate placed in a plasma sheath is studied. The shear of the ion flow velocity in the sheath is found to be responsible for a self-rotation of the particulate, resulting in a formation of a magnetic dipole moment. The presence of an external magnetic field will cause the spinning dust particulate to precess around the magnetic field direction, while the particulate will experience the cyclotron motion. The spinning frequency is on the order of 10 Hz, while the precession is much slower for the laboratory dust experimental conditions.

Molecular dynamics simulation of plasma flow around two stationary dust grains

Plasma kinetics in the presence of ions flowing around two stationary dust grains aligned perpendicularly to the direction of the flow is studied by a three-dimensional molecular dynamics simulation code. The dynamics of plasma electrons and ions as well as the dust particle charging are simulated self-consistently. Distributions of electron and ion number densities and the electrostatic plasma potential are obtained for various intergrain distances, including those much less, of the order of, and more than the plasma electron Debye length.

Reflection and absorption of ion‐acoustic waves in a density gradient

One problem with ion‐acoustic waves is that sometimes they are observed to be reflected from discharge tube walls, and sometimes to be absorbed. Theoretical computation reveals that a velocity gradient produced by a density gradient plays a significant role in the reflection. The velocity gradient produces a subsonic−supersonic transition and long wavelength waves are reflected before reaching the transition while short wavelength waves penetrate over the transition and are absorbed in the supersonic flow plasma.

Nonlinear evolution of Buneman instability. II. Ion dynamics

The nonlinear evolution of the one‐dimensional Buneman instability after saturation due to electron trapping is studied both by analysis and by computer simulation. A nonlinear dispersion relation proposed previously is shown to describe the nonlinear evolution even after the electron trapping sets in. After electron trapping is completed, amplitude oscillation at a frequency close to ωpi (ion plasma frequency) appears due to the exchange of energy between the ions and the electric field. The maximum energy acquired by the ions reaches more than 10% of the initial electron drift energy.

Mass Transport by a Vortex Ring

Mass transport by a vortex ring in water has been studied experimentally. A vortex ring with the Reynolds number in the vicinity of ∼1×10 5 is launched in water by means of an exploding wire. Fine particles injected into a vortex ring at the outlet of vortex driver are carried away with vortex motion. Some particles spin out of the vortex motion and are left behind a vortex ring. But many of them are trapped in a vortex ring and are carried up to the end of water tank which is located at 1.4 m from the vortex driver. Mass analyses of particles show that particles with specific gravity of larger than unity are scattered out of a vortex ring due to a centrifugal force, but those with specific gravity being less than unity are trapped in a vortex core and are carried for a long distance. The result indicates that an efficient mass transport by a vortex ring is realized when the specific gravity of particles is less than that of ambient fluid that supports a vortex ring.