Yoshifumi Saitou

Observation of ion-acoustic waves in two-ion-species plasmas

Coexistence of two modes of ion-acoustic waves in two-ion-species plasmas is investigated both theoretically and experimentally. Two ions are combinations of H2, He, Ne, Ar and Xe. The boundary of existence of two modes in the plane of an electron–ion temperature ratio and heavy–light ion mass ratio is obtained from the two-species ion-acoustic dispersion relation with a real frequency and a complex wavenumber. The boundary is given approximately by Te/Ti = 2Mh/Ml, where Te and Ti are temperatures of electrons and ions, respectively, and Mh and Ml are masses of heavy and light ions, respectively. The experiment is perfomed with a double-plasma device. Ion-acoustic waves are transmitted by applying a sinusoidal pulse to the anode of the source plasma. A single mode is observed for Ne–Ar, He–Ne, Ne–Xe and Ar–Xe plasmas and two modes are detected for H2–Ar, H2–Ne and He–Ar plasmas.

Observation of ion-acoustic shock waves undergoing Landau damping

Numerical results of a modified Korteweg–de Vries equation with an integral term which describes Landau damping have been compared with the experimental findings. In the linear regime, the wave damping rate is proportional to wave number, which conforms to a kinetic dispersion relation. Transition of an oscillatory ion-acoustic shocklike structure into a monotonic shock has been observed when Landau damping is introduced by mixing H2 ions with an Ar plasma. Quenching of the oscillatory stucture behind the wave front is identified as a dissipation charateristic governed by the strength of Landau damping.

Reduction effect of neutral density on the excitation of turbulent drift waves in a linear magnetized plasma with flow

The importance of reducing the neutral density to reach strong drift wave turbulence is clarified from the results of the extended magnetohydrodynamics and Monte Carlo simulations in a linear magnetized plasma. An upper bound of the neutral density relating to the ion-neutral collision frequency for the excitation of drift wave instability is shown, and the necessary flow velocity to excite this instability is also estimated from the neutral distributions. Measurements of the Mach number and the electron density distributions using Mach probe in the large mirror device (LMD) of Kyushu University [S. Shinohara et al., Plasma Phys. Control. Fusion 37, 1015 (1995)] are reported as well. The obtained results show a controllability of the neutral density and provide the basis for neutral density reduction and a possibility to excite strong drift wave turbulence in the LMD.

Ion-acoustic shock waves undergoing Landau damping

The Korteweg–de Vries equation with an additional term of Landau damping is numerically and analytically investigated. It is shown that the equation has a shock-like solution for an initial ramp signal. The temporal evolution of waveforms with various magnitudes of the Landau damping is studied for several values of the initial amplitude. Dependences of widths and velocities of the leading part on initial conditions are shown. It is found that a steepening is suppressed due to the Landau damping even when its coefficient is two orders less than those of nonlinear and dispersive terms. There is a critical relation for such a steepening to take place for a fixed height of the initial ramp. An analytical estimate of the magnitude of temporal Landau damping is given for a linear sinusoidal wave.

Observation of ion-acoustic shock wave transition due to enhanced Landau damping

Ion-acoustic shock waves are observed experimentally introducing strong Landau damping by increasing the ion temperature in a double plasma device. An oscillatory ion-acoustic shock wave undergoes transition with enhanced Landau damping and forms a monotonic shock wave. Numerical results of the Korteweg–de Vries equation with an additional integral term to account for the strength of Landau damping are compared with the experimental findings. Enhancement of Landau damping is found to increase the dissipation of the wave, which manifests in quenching of the oscillatory structure behind the shock front.

Observation of spontaneously excited chaos‐like ion plasma oscillations

An oscillation which behaves quite similarly to chaos under some conditions is observed to exist among ion plasma oscillations spontaneously excited in an ion beam–plasma system. There are two different states of the system, the ‘‘silent’’ and ‘‘chaotic’’ states, sensitively depending on the value of a direct current (DC) voltage VB, which determines the beam energy and is adopted as a control parameter here. In the chaotic state, a spontaneously excited ion plasma oscillation is observed to become chaotic. Here, the correlation dimension for the oscillation in the chaotic state is calculated to be 1.64±0.22. The result shows that an attractor for the oscillation has a low degree of freedom and a noninteger dimension.

Observation of the ion–ion instability and its suppression mechanism in a dusty double plasma device

In a plasma system, ion–ion instability is excited due to the counter streaming of ion beams. An experiment has been carried out to observe the ion–ion instability in a dusty plasma device and in the presence of Ar as a background gas. The experiment is performed in a double plasma device which is 90 cm in diameter and 120 cm in length, separated by a mesh grid of 81% optical transparency. Glass beads of 4 µm average diameters are used as dust grains for the whole set of experiments and are allowed to fall within a particular region of the plasma column inside the system. The growth and decay rate of the ion–ion instability is observed by shifting a probe spatially away from the grid in the target section with changing dust density in the system. Experiments have been carried out under different dust density conditions. According to our experimental findings, the suppression mechanism can be discussed in terms of the dust density inside the system. The distributions of ions, which are taking part in the formation of the instability, are also observed at the respective dust composition. The instability is found to be suppressed completely at a certain critical value of the dust density (Ndcr).

Sheath-Plasma Criterion of Fluid Theory with Finite Ion Temperature

with a polytropic index . The value of is important in relation to probe measurements, and one may set 1⁄4 3 for an adiabatic case (one-dimension), or 1⁄4 1 for an isothermal case. In fact, varies from three to unity owing to limited heat flow, since there is no heat flow in the adiabatic case and unhindered heat flow in the isothermal case. The value of for the sheath-plasma criterion has remained a problem. To derive the sheath-plasma criterion, equations applicable in a sheath region with space charge are used in general. However, there is another method of deriving the sheath-plasma criterion, in which equations applicable in a plasma or presheath region are used. Since quasi-neutrality is assumed in the equations in the second method, the equations become invalid in the sheath region owing to space charge, and a singularity gives a sheath-plasma boundary, where the ion drift speed is given by eq. (1) for Ti 1⁄4 0. The extension of the second method for Ti 61⁄4 0 is achieved by considering ion pressure, and shows the sheathplasma criterion indicating that the ion drift speed at the sheath-plasma boundary is given by eq. (2). As for the first method for Ti 61⁄4 0, the generalized sheath-plasma criterion of the ion velocity distribution function is obtained, although the details of the ion velocity distribution function are not determined. The purpose of this report is to find a solution of the equations in the second method for a finite ion temperature, which may vary in accordance with the polytropic approximation, because only the solution under the condition of Ti 1⁄4 0 or Ti 1⁄4 const: is known. Basic equations of the fluid theory treating a steady-state one-dimensional plasma with a uniform ion generation G (1⁄4 const:) are written as

Changes of plasma potential induced by ion‐beam injection

It is experimentally clarified that injection of a stationary ion beam always changes the background plasma potential in a beam–plasma system. In this experiment, a technique of ion energy analysis is adopted to determine the plasma potential in the system. Results show that the plasma potential changes sensitively depending on the beam‐to‐plasma density ratio, even if other parameters are not allowed to vary. Finally, all the results obtained here are tentatively explained with the help of a Boltzmann relation.

Motions of dust particles in a complex plasma with an axisymmetric nonuniform magnetic field

We investigate the motions of dust particles in a complex plasma by applying an axisymmetric nonuniform magnetic field, B, introduced with a permanent magnet. The magnetic field changes its direction from upward to downward within the experimental area. The distribution of dust particles is conical in the meridional plane, and its central area is a void. The dust particles are generally stagnant in the vertical direction and distributed in multiple layers. The horizontal plane is separated into two regions where the vertical component of B can and cannot be regarded as zero. The distribution of the dust particles in the horizontal plane is concentric. The dust particles along the inner and outer edges rotate in opposite directions due to the direction of the vertical component of B and generate shear flow at a certain height. The rotation velocities of the particles at the edges are compared with the theory of Kaw et al. [Phys. Plasmas 9, 387 (2002)]. The vortex-like structure is not easy to observe even ...