Giichiro Uchida

Dynamics of fine particles in magnetized plasmas

Here are presented experiments on fine particles levitating in low-pressure weakly ionized plasmas under a vertical magnetic field. The magnetic field is useful to provide a vertically long cylindrical column of fine-particle clouds, yielding even string-shaped vertically aligned fine particles, under the double-plasma configuration. Measurements show that fine-particle clouds rotate in the azimuthal direction on the horizontal plane even in such a weak magnetic field that positive ions are slightly magnetized. With an increase of the magnetic field, the rotation speed increases, being followed by subsequent saturation. The rotation speed and direction can be controlled by varying radial plasma potential and/or density profiles. The rotation is induced under the condition that the interparticle distance is small enough for the strong Coulomb coupling among fine particles. A mechanism of the rotation could be explained by effects of ion motions on fine particles, which are modified in the presence of the v...

Fine-particle clouds controlled in a DC discharge plasma

The spatial shape of fine-particle clouds in a strongly coupled state is controlled by varying a radial potential profile provided by radially segmented electrode for particle levitation and confinement under a completely DC configuration. Fine-particle clouds change their shapes from a usual two-dimensional disk to a three-dimensional cone, and finally concave cone with an increase in the radial potential slope. These profiles are closely related to an axial variation of the radial potential profile in the ion sheath above the segmented electrode. When we apply to the segmented electrode a potential profile with peaks between the radial center and edge, fine particles are rearranged to form azimuthal rings on the horizontal plane. The radial width of the rings is controlled by changing the peak width of the radial potential profile.

Generation of two-dimensional dust vortex flows in a direct current discharge plasma

The two-dimensional dust vortex flows are observed in a direct current discharge plasma near the edge of a metal plate which is situated in the dust-particle levitation region. Applying negative dc potential to the metal plate, dust particles are strongly accelerated toward the metal plate edge, and two symmetric dust vortex flows are generated on both sides of the metal plate. Numerical calculation including the effect of the ion drag force well demonstrates the dust vortex formation as in the experiment. A mechanism of the dust vortex generation could be explained by effect of an asymmetry of ion drag force near the metal plate.

Structure Controls of Fine-Particle Clouds in DC Discharge Plasmas

Publisher Summary As a series of the experiments on fine-particle clouds in plasmas, this chapter describes various kinds of experiments on structure controls of the fine-particle clouds in low-pressure dc discharge plasmas. The chapter presents a review of essential points of the results on vertical and radial profiles, phase transition, azimuthal votices and rotations, vertical spread of particle clouds, and vertical strings of periodic alignment of particles. The chapter also discusses active structure controls of the fine-particle clouds in plasmas, which are in liquid or in solid (Coulomb lattice) state, under several different dc situations.

Fine-particle Coulomb lattices formed and controlled in DC discharge plasmas

The Coulomb lattices of strongly-coupled fine particles, which have a two-dimensional hexagonal structure, are formed in dc discharge plasmas. Static and dynamic properties of the fine-particle clouds are controlled by changing plasma parameters and their spatial structures. A double-plasma method is employed to trigger the phase transition and to increase a vertical spread of the fine-particle clouds. Vortex flows of the particles are generated by two methods: insertion of a small metal plate biased electrically and application of a magnetic field in the vertical direction.

Liquid-crystal phase transition by electron shower in a direct current complex plasma

The Coulomb crystal of strongly coupled fine particles is formed by electron shower in a direct current discharge plasma, where the electron shower is injected from an auxiliary plasma which is situated below a main plasma. In case of electron shower injection, the phase transition of dust particles is controlled under the constant argon pressure. The characteristic properties of electron shower injection are discussed theoretically by using a simple collisional sheath model. The calculation analysis clearly shows that electron injection causes the change in charges on fine particles, resulting in a liquid-solid phase transition as in the experiment.

Potential-driven vortices of strongly-coupled fine particles in a plasma

Generation of dust vortexes is observed in a dc discharge plasma near the edge of a metal plate which is situated on a particle levitation electrode. The dust particles are accelerated away from the edge of the metal plate by the strong electric field concentrated near the tip of the metal plate. The dust particles behave like liquid molecule, forming vortex structures with a shear flow on both sides of the metal plate.

Vertical Spread of Fine-Particle Clouds in a Magnetized DC Plasma

Publisher Summary This chapter explains the vertical spread of fine-particle clouds in a weakly ionized argon dc plasma by applying axial magnetic field in vertical direction, using a double-plasma method. In order to investigate 3D behaviors of strongly-coupled dust particles, it is necessary to build up a long dusty plasma-column filled with the particles. Therefore, the formation of large-volume dusty plasma extending in vertical and radial directions is of crucial importance for the investigations of the 3D dust vortices, convection, instabilities, and particle transportation. The fine particles are trapped radially to form a large-volume fine-particle cloud along the magnetic field. The fine-particle cloud rotates around its vertical axis, as a whole, under the presence of the vertical magnetic field. A small-scale convection motion of fine particles is observed near the surface of the cloud. Changing the discharge conditions also excites unstable fluctuations propagating along the magnetic field.

Vertical String Structure of Fine Particles in a Magnetized DC Plasma

Publisher Summary This chapter explains vertical string structures of fine particles in a weakly ionized argon dc plasma by applying axial magnetic field in vertical direction. The particles are confined in an extremely narrow radial potential produced by a double plasma method, which enables a formation of a chain-like structure of particles in vertical direction. The fine particles constitute a very thin layer extended in horizontal direction and the properties of the crystal formation are actively investigated theoretically and experimentally The chapter reports that the crystal structures consisting of single chain-like string of fine particles, which is extended in either vertical or horizontal direction, are formed in an rf discharge plasma in the absence of the magnetic field. On the other hand, the collective motions, such as the potential driven vortex flow of fine particles, are observed in fine particle clouds in a dc plasma as well as a vortex formation under the weak magnetic field. Several vertical strings are coupled with each other to constitute three dimensionally characteristic and fundamental lattice structures. It is noted that these coupled vertical strings rotate, as a whole, around the axis that is directed along the vertical magnetic field.

Formation of Ring-Shaped Fine-Particle Clouds in a DC Plasma

Publisher Summary Fine-particles in the plasma, negatively charge up due to fast electrons, are known to form Coulomb crystal under strong repulsive force. Fine particles interact with the plasma, and various collective behaviors are observed. The collective behaviors of fine particles, especially in strongly coupled state, are very complicated, because the various forces, such as electrostatic force, electromagnetic force, ion drag force, and friction force with gas, act on these particles. This chapter discusses various structures of fine-particle clouds by controlling the potential profile in a DC ion sheath. Fine particles in a strongly-coupled state are found to form a ring-shaped and a cone-shaped cloud. Wave propagations in the cloud are also observed. By compressing the fine particle clouds in radial direction with increasing the external electric field, the Coulomb crystal changes into an unstable state, and the particles begin to oscillate in vertical direction.