Takuma Gohda

Levitation of positively charged fine particles in a cross-field sheath between magnetized double plasmas

Trapping of positively charged fine particles of micron size has been successfully achieved in a cross-field sheath between magnetized double plasmas with different potentials separated vertically by the horizontal magnetic field. In the sheath the charges on the particles become positive, because ion current flowing from a lower high-potential plasma surpasses electron current coming across the magnetic field from an upper low-potential plasma. Variation of particle levitation positions can be explained by the change of the electron current absorbed by the particles under an almost constant ion current.

Control of fine particles by time-averaged external forces in plasmas

A technique for the control of fine particle behavior is developed and demonstrated experimentally. In this method positive pulses are applied to two point-electrodes placed with some distance in plasmas containing fine particles. When the positive pulses are applied to these electrodes alternatively with a repetition period that is shorter than the particle response time, the particles feel only time-averaged force because of their large mass and are gradually transported toward the middle point between two point-electrodes wherever they are distributed initially. This method is quite effective for converging fine particles in the plasma.

Sustainment of Fine Particle Cloud by Means of Time‐Averaged Particle Driving Force in Plasmas

We have succeeded in sustaining fine particle cloud by using a time‐averaged particle driving (TAPD) method in the RF discharge plasma. The particles feel only time‐averaged force when the period of pulses applied to those point‐electrodes is shorter than the particle response time. The particles are transported to a middle point between two point‐electrodes.

Vortex Formation in a High Speed Dust Flow with Large Velocity Shear in RF Plasmas

We have investigated a rotation of a dust cloud disc with strong velocity shear in a radio frequency (RF) plasma. The flow pattern of the dusts was evaluated by the Navier Stokes Equation with shear viscosity due to the Coulomb interactions. We have clarified dynamic behaviors of the dusts and observed generation of micro‐vortices around rotational center, when the velocity shear is enhanced.

Sheath structure for positively-charged fine-particle levitation between magnetized double plasmas

We have investigated positively-charged fine particles in a cross-field sheath between magnetized double plasmas with different potentials separated vertically by the horizontal magnetic field. In the sheath the charges on the particles become positive, because ion current flowing from a lower high-potential plasma surpasses electron current coming across the magnetic field from an upper low-potential plasma. In this report, we have found a phenomenon that the particles can be sustained in the cross-sheath even when the levitation electrode is completely removed from the cross-sheath region. We also discuss the sheath structure between magnetized double plasmas.

Electrostatic Shocks Formed by Ion‐Beam Velocity Modulation in a Q‐Machine Plasma

Nonlinear spatial evolutions of velocity‐modulated ion beams along a magnetized plasma column are investigated by computer simulation for a Q machine where no electrostatic shock formation is observed for density‐modulated ion perturbations. In case of the velocity modulation, the perturbations grow spatially with subsequent saturation even if the system is stable. This is due to the ion bunching along the plasma column. With an increase in the velocity modulation, an electrostatic shock formation is observed to evolve at the propagating front along the plasma column. The observation is compared with the experimental results with the bunching process under the Landau damping. The phenomena are explained by taking into account of the nonlinear collective effect for the bunching of velocity‐modulated large‐amplitude ion perturbations.