Yoshimitsu Amagishi

Experiment on the Toda Lattice Using Nonlinear Transmission Lines

A nonlinear transmission line equivalent to the exponential lattice of Toda was obtained with the proper understanding of nonlinear properties of a capacitor used in a shunt branch. The character of a soliton is observed to follow the prediction given by Toda with respect to the shape and velocity of the soliton. The details of interaction of two solitons moving in the opposite or same direction are also found to be in good agreement with the theory. The dissipation effect on the soliton due to the small losses in the passive elements of the network is discussed in the continuum limit.

Observation of spatial Alfven resonance

Abstract By exciting axisymmetric Alfen waves with Stix coils, the spatial properties of shear Alfven resonance are investigated. The maximum azimuthal magnetic field of the excited waves is located at this resonance layer, which is a function of both frequency and wavelength. The polarization also changes continuously through the resonance layer.

Non-Axisymmetric Alfvén Wave Excited by a Helical Coupler in an Inhomogeneous Plasma

Non-axisymmetric Alfven waves are excited with a helical coupler in a finite beta and a cylindrical inhomogeneous plasma surrounded by a conducting wall. The helical coupler, which consists of two symmetrical helical windings, exhibits a strong propagation directionality; slow waves of the m=-1 mode and fast waves of the m=+1 mode can be launched simultaneously from the coupler, but propagate in opposite directions along the static magnetic field. Dispersion relations including the attenuation length for both modes are compared with a magnetohydrodynamic theory given by Woods on the assumption of appropriate boundary conditions. As a nonlinear phenomenon, subharmonic slow Alfven waves of the m=-1 mode have been observed when the pump frequency is close to or above the ion cyclotron frequency.

Excitation of MHD surface waves propagating with shear Alfven waves in an inhomogeneous cylindrical plasma

Experimental results are presented which show that the MHD surface waves or fast waves of m=+or-1 have been excited in the range of frequency less than ion cyclotron frequency using a helical antenna in a finite beta and cylindrical plasma. Their dispersion relation and spatial structure were successfully confirmed after the wave signals were separated from those of the local shear Alfven waves that propagated together with the surface waves. In order to obtain the dispersion relations from the signals, the authors adopted three kinds of methods: (1) conventional cross-power spectrum density using fast Fourier transform, (2) maximum entropy method, and (3) direct estimation of the phase velocity of wave packets. The observed properties of both the surface waves and the SAWs are in general agreement with theory.

Formation of spatial Alfven resonance and cutoff fields in an inhomogeneous plasma

By measuring the time evolution of the perturbed magnetic field vectors, the formation of the spatial Alfven resonance layer and cutoff fields has been first observed in an inhomogeneous plasma. A simple MHD theory including the effect of the ion-neutral collision is described as a background to understand the experimental results.

Excitation of Electron Plasma Oscillation by Ion-Acoustic Waves

Electron plasma oscillations were generated by application of one characteristic low frequency fields to a mesh anode. Conditions for the excitation of electron plasma oscillations are that the positive column is bounded by the ion-sheath and the voltage of the applied signal is above a certain value. These excited oscillations, which are of the Looney-Brown type, are possible only on a certain phase of the low frequency fields. The low frequencies associated with the excitation coincide with those of radial ion-acoustic wave modes. Therefore only the external frequency which satisfies the boundary condition determined by the argument of the Bessel function can participate in the excitation of electron plasma oscillation.

MHD Instabilities Developing to Alfven Waves Observed during Plasma Production by MPD Arcjet

We have observed two kinds of magnetohydrodynamic (MHD) instabilities excited in a current-carrying cylindrical plasma that was produced by a magneto-plasma-dynamic (MPD) arcjet. The “net current” in the plasma excites the instability with the azimuthal mode number m =1, which is deduced to be a kink mode from MHD theory. On the other hand, the m =0 instability is excited by the “total current” flowing to the cathode, and this mode was found to be responsible for the modulation of electron temperature. They are converted respectively to m =1 compressional Alfven wave and m =0 global Alfven eigenmode, propagating along the axial magnetic field in the current-free region.

Ring-Shaped Model of the Pacemaker in Oscillatory Reaction-Diffusion System.

This paper presents a new model of a pacemaker in the oscillatory reaction-diffusion system called “the ring-shaped model”. The present model has a frequency higher than that of the bulk oscillations in a ring-shaped region. Theoretical and experimental studies have revealed that the “ring-shaped model” explains the experiments with a piece of disk-shaped filter paper as a pacemaker, indicating that the wavelength is a function of the radius of the pacemaker and has a minimum value.

A numerical study of the decomposition of chemical waves in a closed system

Abstract A photosensitive Oregonator that has three steady states (one is stable and the others unstable) is investigated in a one-dimensional reaction-diffusion system to study the decomposition of chemical waves in a closed excitable medium. A time-dependent concentration of reactant, a , plays the role of an internal bifurcation parameter. A part of the system is set to be oscillatory to produce a wave train. The chemical waves in the excitable medium become irregular with time, and before the system reaches a steady state a new oscillation mode characterized by a small amplitude and a high frequency emerges. This phenomenon implies that a basin of the unstable focus in the excitable Oregonator is perceptible in the pattern decomposition process in the reaction-diffusion system.

Wave Field Structures in a Toroidal Plasma at the Frequency Range of Alfvén Resonance

Wave field structures at the frequency range of Alfven resonance are studied in a toroidal plasma, laying stress on discrete Alfven waves and local Alfven resonances.