Shigeharu Kabashima

Oscillatory‐to‐nonoscillatory transition due to external noise in a parametric oscillator

A degenerate parametric oscillator which is driven by a random current additionally applied to a sinusoidal pumping current is found to exhibit a new type of oscillatory‐to‐nonoscillatory transition. This noise‐induced phase transition has a different kind of critical behavior from that observed in usual phase transitions, that is, near the threshold the variance of the fluctuating output current remains constant, however, the critical slowing‐down does occur. A stochastic equation for the parametric oscillation which includes a random force multiplied by the amplitude itself is derived and the experimental results are fully accounted for in terms of that equation.

External Noise Effect on the Onset of Williams Domain in Nematic Liquid Crystals

In order to examine the effect of external noise on the electrohydrodynamic instability, we have measured the wave number spectrum of the pattern of Williams domain in nematic liquid crystals MBBA in the presence of external noise superposed to the a c voltage. The threshold voltage for the onset of Williams domain is found to shift to higher values with increasing noise power. Its noise dependence is interpreted in terms of a theory presented by Dubois-Violette et al.

Critical Fluctuation near Threshold of Gunn Instability

The low frequency noise and the oscillatory microwave power spectrum have been simultaneously measured on the Gunn diode in order to investigate the critical phenomenon near the threshold of the Gunn instability. In a rather wide bias range an anomalously large low frequency noise and a noisy power spectrum are observed. The appearance of a wide critical region is discussed with the use of an equation for the negative-resistance oscillator.

High Frequency Conductivity of NiO

The conductivities of 1.2, 7.9 and 12.6 ppm (weight ratio) Li-substituted NiO are measured at high frequencies up to 24 GHz in the temperature range 110° to 450°K. The conductivity at high frequencies can be expressed as the sum of a temperature-dependent conductivity σ 1 and a temperature-independent additive conductivity σ 2 . The former σ 1 is independent of frequency until microwave region but the latter σ 2 is strongly dependent on freqnency. The frequency dependence of σ 2 can be represented by the dispersion formula for the dielectric loss due to the hopping of charge carriers around imperfections. The staying time at a lattice site is determined: τ=2.2×10 10 sec. A lack of temperature dependence of σ 2 implies that the staying time τ does not depend on temperature and further that the holes are not selftrapped. The conduction mechanism in NiO is rather illustrated as something analogous to impurity conduction.

Numerical studies on the nonequilibrium inductively coupled plasma with metal vapor ionization

Nonequilibrium inductively coupled plasma (ICP) where cesium metal atoms contained in argon gas as a seed material are dominantly ionized is investigated with two-dimensional (2-D) numerical calculations which are based on a fully time-dependent (FTD) model and sinusoidal approximation (SA) model. Calculation results with the FTD model indicate that the amplitude of electron temperature oscillation over one radio frequency cycle is below about 120 K for the mean value of about 5600 K, and that of the electron number density is negligible due to its long relaxation time. Results with the SA model coincide with that with the FTD model, and it is valid to predict the plasma properties with the SA model. Under the suitable operating conditions, the region where electron number density is kept constant is formed and extended in the ICP, since the electron temperature ranging from 4000 to 6000 K is realized, and cesium atoms are fully ionized while the ionization of argon atoms is not significant. Since no current is induced at the center axis of ICP, the density profile around the axis is almost determined by the diffusion of electrons from the region of full seed ionization.

Plasma stabilization and improvement in the performance of a nonequilibrium disk MHD generator by a radio-frequency electromagnetic field

By applying a radio-frequency (RF) electromagnetic field, the feasibility of improvement in the performance of a nonequilibrium disk-shaped magnetohydrodynamic (MHD) generator suffering from water vapor contamination in the working gas is investigated with r-/spl theta/ two-dimensional numerical simulations. Attention is paid to the relation between the behavior of MHD plasma in the presence of the RF electric field and the generator performance. The water contamination causes the strongly nonuniform and unsteady plasma, and deteriorates its performance. The fluctuations of the electron temperature and of the ionization degree of seed atoms are found to be suppressed by applying the RF electric field. As a result, the enthalpy extraction ratio and the isentropic efficiency of the generator improve. The ratio of the required additional joule heating by the RF electric field to the thermal input to the generator for the stabilization of the plasma and the improvement in the performance is estimated to be about 0.9%.

The effects of boundary layer phenomena on the performance of disk CCMHD generator

Two dimensional calculations were carried out to clarify the behavior of boundary layer and its effects on performance of closed cycle MHD (CCMHD) generator and to investigate the relation between enthalpy extraction ratio and adiabatic efficiency. Calculation results suggest that the large Lorentz force causes propagation and separation of boundary layer where reverse current flows, because of small electromotive force. For large load resistance boundary layer becomes very thick and the eddy current arises in broad region. The push work of working gas against Lorentz force is effectively converted into electric energy under the condition at which the Lorentz force decelerates the working gas to Mach number in the range between 1.0 and 1.5 in this case of the generator. Stagnation pressure loss increases with load resistance until enthalpy extraction ratio takes maximum value. The entropy production due to Joule heating and viscosity increases with load resistance. The difference between the load resistances for which the enthalpy extraction ratio and the adiabatic efficiency take maximum value can be explained with the entropy production of Joule heating and viscosity. >

Critical Slowing-Down near Threshold of Electrical Oscillation

The probability distribution and the autocorrelation function of the fluctuating output voltage of a Wien-Bridge oscillator have been measured as a function of the feedback factor β. It has been found that both the variance and the correlation time of the fluctuation which are determined from the probability distribution and the correlation function, respectively, become infinite in a manner like (β c -β) -1 as β approaches the threshold β c of the oscillation. The aspect is analogous to the Curie-Weiss law in the second-order phase transition. The results are discussed within the linear response scheme starting from a simplified kinetic equation for the fluctuating output voltage of the Wien-Bridge oscillator.

Structure of inductively coupled plasma with metal vapor ionization

Structures of nonequilibrium inductively coupled plasmas with cesium metal vapor ionization in argon gas ar revealed experimentally and are compared with ones from two-dimensional numerical simulations. The main object of the present paper is to clarify the behavior of the plasma in which cesium atoms as seed atoms are completely ionized, and the ionization of the argon gas as a mother gas is negligible, that is the fully ionized seed (FIS) plasma, produced by the inductive radio-frequency electric fields. The plasmas are generated in a cylindrical quartz discharge tube around which a four-turn induction coil is wound, under the conditions of currents of <10 A, excitation frequencies ∼10 MHz, argon gas pressures 30-50 torr, and cesium mole fractions on the order of 10 -5 . By cesiun seeding, the quite uniform plasma in the azimuthal direction of the discharge tube is realized even at small coil currents. Under suitable operating conditions, the electron temperatures are in the range of 5000-7000 K near the tube wall, whereas the temperature around the axis is relatively low (∼3000 K). Then, the plasma consists of three layers, that is, the weakly ionized argon plasma, the FIS plasma in which the electron density is maintained almost uniform, and the partially ionized seed plasma. The thickness of the FIS plasma is determined by the distribution of the inductive electric field. The experimenta results can be explained well by the numerical simulation based on two-dimensional vector potential and two-temperature plasma models.

Heat transport in He II channel with phase transition

Abstract Experimental and numerical investigations have been carried out on the heat transport characteristics in a channel where atmospherically pressurized He II coexists with He I. The channel has a rectangular cross-section and both ends are open to the bath. One of the side walls of the channel is heated uniformly. The channel is placed vertically or horizontally in the bath. The experimental results are analysed by two-dimensional time-dependent numerical calculations. An explicit finite element scheme is applied to the two-fluid equations which include the Gorter-Mellink mutual friction term. It is found that the applied heat is transported by one-dimensional internal convection through the channel filled with single phase He II. Once He I is generated in the channel, however, two-dimensional flow occurs due to natural convection and this influences the transport of heat.