Quasi elasto-electromagnetic surface waves on a piezo-plasma-like layer

Considering a piezo-plasma-like layer with finite thickness and hexagonal symmetry whose main symmetry axis is parallel to the z axis and approximating it by an isotropic medium, we study the coupling of the elastic wave with plasma properties of the medium with and without spatial dispersion and collisions. In this case we investigate the coupled surface quasi elasto-electromagnetic wave propagating on the interface of piezoelectric layer with vacuum. Furthermore, the coupling of elasticity and ion-acoustic waves is investigated.     

Quasi elasto-electromagnetic surface waves on a piezo-plasma-like layer

Considering a piezo-plasma-like layer with finite thickness and hexagonal symmetry whose main symmetry axis is parallel to the z axis and approximating it by an isotropic medium, we study the coupling of the elastic wave with plasma properties of the medium with and without spatial dispersion and collisions. In this case we investigate the coupled surface quasi elasto-electromagnetic wave propagating on the interface of piezoelectric layer with vacuum. Furthermore, the coupling of elasticity and ion-acoustic waves is investigated.     

A geometric theory for scroll wave filaments in anisotropic excitable media

Scroll waves are an important example of self-organisation in excitable media. In cardiac tissue, scroll waves of electrical activity underlie lethal ventricular arrhythmias and fibrillation. They rotate around a topological line defect which has been termed the filament. Numerical investigation has shown that anisotropy can substantially affect the dynamics of scroll waves. It has recently been hypothesised that stationary scroll wave filaments in cardiac tissue describe geodesics in a space whose metric is the inverse diffusion tensor. Several computational studies have validated this hypothesis, but until now no quantitative theory has been provided to study the effects of anisotropy on scroll wave filaments. Here, we review in detail the recently developed covariant formalism for scroll wave dynamics in general anisotropy and derive the equations of motion of filaments. These equations are fully covariant under general spatial coordinate transformations and describe the motion of filaments in a curved space whose metric tensor is the inverse diffusion tensor. Our dynamic equations are valid for thin filaments and for general anisotropy and we show that stationary filaments obey the geodesic equation. We extend previous work by allowing spatial variations in the determinant of the diffusion tensor and the reaction parameters, leading to drift of the filament.     

Mapping misoriented fibers using X-ray dark field tomography

X-ray grating interferometers produce three distinct signals; an absorption signal, a differential phase signal and a dark field signal. Until now a method for successfully creating dark field tomograms of nonisotropic samples has not been demonstrated. In this paper we test a method for creating such tomograms on a highly nonisotropic sample, i.e. a five layer “sandwich” of oriented carbon fibers. The fibers are parallel within the individual sandwich layers, but perpendicular to the fibers in the adjacent layers. We show that by choosing a rotation axis parallel to the grating stepping direction (i.e. a horizontal rotation axis in most setup configurations) it is possible to produce a darkfield tomogram where fibers parallel to the probed scattering direction appear to have no dark field signal. The method produces a tomogram in the form of a scalar field of dark field scattering values.     

Detection of the decay of an acoustic soliton arising during pulsed breakdown of a glassy semiconductor film

The results of an experiment demonstrating the appearance of a soliton under certain conditions of pulsed breakdown of a glassy semiconductor film in a magnetic field are reported. The wave is excited by a current filament moving at a velocity close to the speed of sound between two parallel electrodes in an external magnetic field. To distinguish the direction of motion of the acoustic wave and that of the current filament along the substrate, electrodes with a bend that changes the direction of motion of the filament are deposited. Two “frozen” structural-excitation fronts, diverging at an angle to one another and attesting to the decay of the soliton at the moment the current filament vanishes, are observed at the location of the bend in the electrodes. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 70–72 (25 July 1999)     

Covariant stringlike dynamics of scroll wave filaments in anisotropic cardiac tissue

It has been hypothesized that stationary scroll wave filaments in cardiac tissue describe a geodesic in a curved space whose metric is the inverse diffusion tensor. Several numerical studies support this hypothesis, but no analytical proof has been provided yet for general anisotropy. In this Letter, we derive dynamic equations for the filament in the case of general anisotropy. These equations are covariant under general spatial coordinate transformations and describe the motion of a stringlike object in a curved space whose metric tensor is the inverse diffusion tensor. Therefore the behavior of scroll wave filaments in excitable media with anisotropy is similar to the one of cosmic strings in a curved universe. Our dynamic equations are valid for thin filaments and for general anisotropy. We show that stationary filaments obey the geodesic equation.     

Characterization of acoustic disturbances in linearly sheared flows

The equation describing the plane wave propagation, the stability, or the rectangular duct mode characteristics in a compressible inviscid linearly sheared parallel, but otherwise homogeneous, flow, is shown to be reducible to Whittaker's equation. The resulting solutions, which are real, viewed as functions of two variables, depend on a parameter and an argument the values of which have precise physical meanings depending on the problem. The exact solutions in terms of Whittaker functions are used to obtain a number of known results of plane wave propagation and stability in linearly sheared flows as limiting cases in which the speed of sound goes to infinity (incompressible limit) or the shear layer thickness, or wave number, goes to zero (vortex sheet limit). The usefulness of the exact solutions is then discussed in connection with the problems of plane wave propagation and stability of a finite thickness shear layer with a linear velocity profile. With respect to the plane wave propagation it is shown that, unlike the compressible vortex sheet, the shear layer possesses no resonances and no Brewster angles, whereas with respect to the stability problem it is shown that, again unlike the compressible vortex sheet, the thin layer is unstable to long wavelength disturbances for all Mach numbers. These results imply that the reflection and stability characteristics of a non-zero thickness but thin shear layer (i.e., the long wavelength characteristics) do not go over smoothly into the results of the compressible vortex sheet as the wave number approaches zero, except for a limited range of generally subsonic relative flow of the two parallel streams bounding the shear layer.     

Effects of reduced discrete coupling on filament tension in excitable media

Wave propagation in the heart has a discrete nature, because it is mediated by discrete intercellular connections via gap junctions. Although effects of discreteness on wave propagation have been studied for planar traveling waves and vortexes (spiral waves) in two dimensions, its possible effects on vortexes (scroll waves) in three dimensions are not yet explored. In this article, we study the effect of discrete cell coupling on the filament dynamics in a generic model of an excitable medium. We find that reduced cell coupling decreases the line tension of scroll wave filaments and may induce negative filament tension instability in three-dimensional excitable lattices.     

Wave propagation through a dielectric layer containing densely packed fibers

This paper presents the theoretical formulation for the propagation of electromagnetic wave through a dielectric layer containing a random dense distribution of fibers. The diameter of the fibers is comparable to the inter-fiber spacing and wavelength of the incident radiation, but is much smaller than the thickness of the layer. Discontinuity of refractive index across the boundaries of the dielectric layer resulted in multiple internal reflection of both the primary source wave and the scattered waves. As a result the incident waves on the fibers consist of the multiply-reflected primary waves, scattered waves from other fibers, and scattered-reflected waves from the boundaries. The effective propagation constant of the dielectric fiber layer was developed by utilizing the Effective field-Quasicrystalline approximation. The influence of the refractive index of the dielectric medium on the radiative properties of a dense fiber layer was examined by means of numerical analyses.     

Synchrotron x-ray study of the smectic layer directional instability

We have investigated the phenomenon of field-induced smectic layer instability, as monitored by synchrotron x-ray scattering. This instability means that, upon application of time-asymmetric electric fields to chiral smectics, the layer direction seems to "rotate" locally around an axis given by the direction of the applied field. For moderate values of field amplitude and asymmetry, domains with a favored layer inclination grow at the expense of unfavored ones, while larger fields and asymmetries generally lead to a chaotic flow behavior. At moderate amplitudes, we have followed the process of the horizontal layer folding (or horizontal chevron domain formation) and the smectic C* layer reorientation of ferroelectric liquid crystals by applying symmetric and asymmetric wave forms, respectively, and performing time resolved x-ray measurements. The studies unambiguously show the formation of a horizontal (in-plane, i.e., in a plane parallel to the cell substrates) chevron domain structure from a nonoriented sample by application of a symmetric electric field of sufficient amplitude. It is then demonstrated that a transition from the horizontal chevron domain structure to an in-plane uniform smectic layer direction takes place on application of asymmetric electric wave forms. Reversal of the field asymmetry reverses the inclination direction and selects the other layer normal direction as the uniform end state. The in-plane smectic layer reorientation process is followed here as it evolves, and analyzed directly by means of x-ray scattering.     

Acoustic-Gravity Waves in the Atmosphere with a Piecewise-Linear Temperature Profile

We consider the problem of propagation of acoustic-gravity waves in the atmosphere with a constant temperature gradient in the near-surface layer. The assumption of linear temperature dependence on height allowed us to reduce the wave equation to the hypergeometric form, regardless of the compressibility of the medium. The solution of this equation is represented in terms of degenerate hypergeometric functions. To analyze the obtained solution, we consider a two-layer model of a half-bounded atmosphere with a height-independent background temperature in the upper layer. The results are studied in detail under the approximation of an incompressible medium. For the model specified above, we find analytical expressions for the perturbation fields and obtain a characteristic equation whose solution allows us to calculate wave dispersion characteristics at frequencies close to the Brunt-Väisälä frequency for large horizontal scales as compared to the layer thickness.     

Numerical Simulation of Shock Wave Generation for Ignition of Precompressed Laser Fusion Target

In this work, we investigate the formation of a converging shock wave in a homogeneous spherical target, whose outer layer was heated by a flux of monoenergetic fast electrons of a given particle energy. Ablation pressure generating the wave forms at spherical expansion of a layer of a heated substance, whose areal density remains constant throughout the entire heating process and equal to the product of the initial heating depth and density of the target. The studies are carried out based on numerical calculations using a one-dimensional hydrodynamic code as applied to ignition of a precompressed target by a shock wave (shock ignition), one of the most promising techniques of laser fusion ignition.     

A near perfect Pendry lens: Projecting properties of a thin metal layer

We consider the field of an evanescent wave in a space with a thin metal layer (? < 0). The wavenumber of the evanescent wave appreciably exceeds the wavenumber k ₀ = 2π/λ₀ of a uniform plane wave in spatial regions adjacent to the metal. In accordance with the Pendry result (2000), the field behind the metal layer is amplified as compared to the field in the absence of the layer. Pendry predicted this effect for a metal whose dielectric permittivity is ? = ?1, whereas we show that the effect can also be observed for ? = ?15 and for arbitrarily thick adjacent regions. This extends the range of possible applications of the effect. We find that the losses in the metal only weakly affect the image quality behind the metal layer.     

电磁波与复合材料板上传输线的耦合分析

利用单层均匀模型等效非均匀碳纤维复合材料,获得等效电磁参数,进而采用基于全波分析方法的仿真软件研究了碳纤维的排列方向以及入射波参数对传输线终端负载感应电流的影响。结果表明,当导线与碳纤维方向平行时,其场线耦合规律与同电导率导电板上的场线耦合变化规律基本一致,且负载感应电流大于导线与碳纤维正交时的感应电流。当电磁波垂直于复合材料板入射时,负载感应电流大于同等条件下电磁波平行入射时的感应电流。     

Hollow-fiber delivery of high-power pulsed Nd:YAG laser light

We propose hollow fibers for delivery of high-peak-power pulsed Nd:YAG laser light. Hollow fibers with an internal polymer layer were fabricated by a liquid-phase coating technique. We reduced the losses of the fibers in the near-infrared region by producing a silver film that was very smooth owing to use of an ultrasonic wave for mixing of the silver and the reducer solutions in the silver-plating process. The straight losses of the 1-m-long polymer-coated fibers were 0.3 dB for the 700-mum bore size and 0.1 dB for the 1000-mum bore fiber.     

Simulation and experimental study of the influence of a front face layer on the response of ultrasonic transmitters

Using a theoretical model, we simulate the ultrasonic pressure generated by a piezoelectric transmitter whose active element has its front face perfectly connected to a layer with parallel faces. This simulation takes into consideration the characteristic parameters of the piezoelectric ceramic, the layer and the propagation medium.Verification of the validity of the model is carried out by assessing, qualitatively and quantitatively, theoretical and experimental responses obtained with different values of layer thickness. Having correctly verified the model solely by simulation, we examine the influence of the nature of the layer on the transmitter response and give an energetics interpretation of the results. We conclude that by using a simple theoretical model, we may forecast the experimental response of a transducer which has a front face layer.     

Doppler-shifted cyclotron absorption of electron Bernstein waves via N( parallel)-upshift in a Tokamak plasma

Extraordinary (X) waves are perpendicularly injected for electron Bernstein (B) wave heating into an Ohmically heated plasma from the inboard side in the WT-3 tokamak. Measurements show that absorption does not take place at the electron cyclotron resonance layer nor the upper hybrid resonance layer, but does happen midway between them. This is consistent with the ray tracing prediction, i.e., the poloidal field and poloidal inhomogeneity of toroidal field lead the B waves to have a large parallel refractive index N( parallel) (>1), and the B waves are damped away via the Doppler-shifted cyclotron resonance.     

Determination of isotropic layer parameters from spatiotemporal signals of an ultrasonic array

The paper discusses a method for measuring the velocities and attenuation of longitudinal and transverse ultrasonic waves and the density and thickness of the isotropic layer with an array placed in an immersion liquid parallel to the sample. The method is based on the recording of the total spatiotemporal signal of the array and its expansion into a spatial spectrum of pulse plane wave response. The ultrasonic velocity and sample thickness depend on the response delay of the plane wave in the layer from the transverse projection of the slowness vector. The density and attenuation are determined from the behavior of the amplitudes of spectral responses. To confirm this method in experiment, the parameters of a polystyrene plate have been measured using a linear 32-element array with a central frequency of 17 MHz.     

Shock waves in periodically inhomogeneous active fibers

The dynamics of the pulse-envelope shock wave is studied in amplifying fibers whose gain, groupvelocity dispersion, and nonlinearity are nonuniform with respect to the fiber length. It is demonstrated that, in inhomogeneous fibers, picosecond pulses can exhibit a substantial steepening of the leading edge at lengths of less than 10 cm.     

Fabrication and application of imaging holographic gratings (chirped gratings) for integrated optics

Holographic chirped gratings with parallel grating lines have been fabricated by recording the interference structure of a plane wave and a cylindrical wave in a photoresist layer on a monomode glass waveguide. These gratings will be useful elements for the imaging of guided waves in integrated optics. A theoretical approach for treating their imaging properties is presented. The experimental results are described theoretically.