Small-signal power flow and energy density for streaming carriers in the presence of collisions

The Poynting theorem, or equation of continuity relating energy density and power flow, is extended to include the effects of diffusion and collisions. It is shown that the presence of the thermal power due to diffusion is accompanied by an increase in electrokinetic power. The effects of collisions on the electrokinetic power and energy density are examined in detail in the absence of diffusion. It is found that the kinetic power is zero in isolated streams when collisions are frequent (nu > 2omega_{p}). However, when such streams couple to "circuit-like" positive-energy waves (as in the acoustic amplifier) the stream's kinetic power becomes finite; in particular, it becomes negative if the stream drifts faster than the wave. Thus the usual picture, used in collision-free theory, in which the active wave must carry negative power, is preserved. It is also shown that, on the other hand, if the stream is lossless but interacts with a lossy and nonpropagating medium (as in a stationary collisional plasma), then the stream's finite-energy waves are coupled by the lossy medium. In contrast to the previous case where both systems propagate, it is now the negative-energy wave that grows. It is found that the electrokinetic energy density in a collision-dominant stream is negative for both modes due to collisional losses.     

Calcul du second terme de l’exposant linéique de propagation des ondes rampantes par une méthode de couche limite

Creeping waves propagate and decay on the boundary of convex obstacles. The propagation constant of these waves has an asymptotic expansion in fractional powers of the wavenumber k. The first term of this expansion is well-known, but the second term has only been determined for perfectly conducting objects, and specific shapes : bodies of revolution, canonical problems. In this paper, we compute this second term in the case of a general convex object satisfying an impedance boundary condition, by using a boundarylayer method. The result shows the effects of the geometrical parameters of the geodesic along which the creeping wave propagates, and of the variations of the surface impedance.     

Microwave-induced auditory effect in a dielectric sphere

The acoustic pressure wave generation inside an electromagnetically lossy dielectric sphere from an incident microwave pulse is analyzed rigorously. The pressure wave equation derived using the first-order approximation of a thorough formulation on microwave-induced thermoacoustic effect in dielectrics, is used. The inhomogeneous hyperbolic-type pressure wave differential equation is solved by a Green's function theory approach. The boundary conditions on the dielectric sphere-air interface are taken into account. The power is computed by applying the exact Mie theory solution for the dielectric sphere. Two types of acoustic waves are derived inside the sphere: (a) a transient burst type pressure wave, corresponding to the free-space contribution of Green's function; and (b) an infinite set of damped oscillations related to the normal acoustic modes of the spherical resonator. Numerical results are computed and presented for several cases     

Electromagnetic Modeling for Microwave Imaging of Cylindrical Buried Inhomogeneities

Many diagnostic techniques in geophysics and civil engineering are based on the interaction of electromagnetic waves with objects buried in homogeneous or stratified media. Most of the investigations are concerned with the detection of buried objects, but a few papers have dealt with the problem of identifying the objects. The proposed method is based on the integral representation for a plane wave incident on a lossy half-space containing a cylindrical object of arbitrary cross section and electrical properties. The induced current distribution in the object is obtained from the backscattered field measurement in amplitude and phase. In order to improve the spatial resolution of the image, the scattered field is measured for different plane wave incidence and frequencies. Results of numerical simulations concerning the shape and size of the object for different values of soil electromagnetic parameters are presented in this paper.     

Diffusion near an absorbing boundary

Using a microscopic model for collisions, an analysis is given of the diffusion of minority carriers in the field free base of a transistor, in the proximity of the base/collector junction where the carrier density gradient and the electric field vary strongly over a mean free path. This is done both on an idealised model which assumes that no carriers going into the junction return to the base, (perfect sink) and on a more realistic model which takes an approximate account of backscattering. In the case of the idealised model a diffusion equation is obtained which contains a new term due to the sink. The minority carrier density n₀ at the boundary is limited by the thermal energy of the carriers, and the density gradient is increased in the immediate vicinity of the sink. This increase has, however, a very small effect on the value of the carrier densities and on the base transit time. The value n₀ of the carrier density at the boundary becomes larger in the presence of backscattering from the junction. Calculations show that n₀ lies within 25–50 per cent of the value obtained from the standard transport equation in the base/collector junction, which includes carrier heating effects. A more detailed model of backscattering would however be desirable to confirm the latter results.     

Relative transmission loss of TE- and TM-like modes in metal-coated coupler halves

The relative transmission losses of TE- and TM-like waves through a metal-coated coupler half is measured as a function of the overlay index. For very thin metal films, the TE-like wave was found to be more lossy than the TM-like wave for certain overlay indices. The present study should find applications in the design of inline fibre-optic polarisers and sensors.<>     

Riemann-Green function solution of transient electromagnetic plane waves in lossy media

B. Riemann's (1953) solution of the Cauchy problem for the linear wave equation is used to find a closed-form solution for the problem of transient nonsinusoidal waves is lossy media. A method for finding the required Riemann-Green function is discussed. The evolution of a wavefront propagating in a semi-infinite lossy medium is studied, and a series solution for the corresponding electric field is obtained.<>     

Π-pulse propagation in the presence of host dispersion

A steady-state solution describingPi-pulse propagation in a homogeneously broadened laser amplifier embedded in a dispersive lossy host is found. For a given model of dispersion, the solution satisfies exactly the second-order wave equation together with the equations of motion for the density matrix of the two-level resonant system. The salient feature of the solution is that a small amount of dispersion in the host group velocity causes a large monotonic frequency sweep during the pulse. The effects of this chirp on the pulse properties are described in detail, and the relation of the theory to experiments is discussed.     

Analysis of energy coupling into spheroidal ferritic implants forhyperthermia applications

The coupling of energy of RF power into lossy tissues using a needle-shaped ferromagnetic implant excited by an externally applied current loop is analyzed theoretically. The ferritic implant is assumed to be of prolate spheroidal shape, while the tissue medium is modeled as a lossy sphere. The electromagnetic fields inside the ferritic implant, the surrounding lossy tissue, and the free space are appropriately expressed in terms of spherical and spheroidal wave functions. Application of the boundary conditions results in an infinite system of equations for the unknown field expansion coefficients. This system is truncated and solved and the electromagnetic field is computed numerically. Absorbed power density inside the implant and the surrounding medium is computed, and the efficiency of the method in producing in-depth energy deposition is examined     

Numerical modeling of an enhanced very early time electromagnetic(VETEM) prototype system

In this paper, two numerical models are presented to simulate an enhanced very early time electromagnetic (VETEM) prototype system, which is used for buried-object detection and environmental problems. Usually, the VETEM system contains a transmitting loop antenna and a receiving loop antenna, which run on lossy ground to detect buried objects. In the first numerical model, the loop antennas are accurately analyzed using the method of moments (MoM) for wire antennas above or buried in lossy ground. Then, the conjugate gradient (CG) methods, with the use of the fast Fourier transform (FFT) or MoM, are applied to investigate the scattering from buried objects. Reflected and scattered magnetic fields are evaluated at the receiving loop to calculate the output electric current. However, the working frequency for the VETEM system is usually low and, hence, two magnetic dipoles are used to replace the transmitter and receiver in the second numerical model. Comparing these two models, the second one is simple, but only valid for low frequency or small loops, while the first modeling is more general. In this paper, all computations are performed in the frequency domain, and the FFT is used to obtain the time-domain responses. Numerical examples show that simulation results from these two models fit very well when the frequency ranges from 10 kHz to 10 MHz, and both results are close to the measured data     

Calculated and measured absorption cross sections of lossy objects in reverberation chamber

Reverberation chambers can be used to measure radiation efficiency of small antennas when these are located close to lossy objects. The lossy objects represent a heavy loading of the chamber. This loading is characterized by the mean absorption cross section of the lossy objects. This paper describes how this mean absorption cross section can be calculated from the scattered far field of an object by using the forward scattering theorem, or from a more laborious near-field evaluation. Results for lossy spheres and cylinders are calculated by using three different codes, based on spherical mode expansion, finite difference time domain techniques, and moment methods, respectively. The results for the cylinder are compared with measured levels in a reverberation chamber.     

Infrared surface waves in circular hollow waveguides with smallcore diameters

An asymptotic form of the characteristic equation that describes wave propagation at near-infrared wavelengths in small core hollow circular waveguides is developed. Analytic solutions for the transverse and axial propagation constants are obtained. These demonstrate the transition of the TE₁₁ and TM₀₁ modes to surface waves as the guide radius is increased to values much greater than at cutoff. Relative power density distributions illustrating these mode transitions are shown     

A moment method solution for the shielding properties of three-dimensional objects above a lossy half space

In this paper, a moment method (MM) solution for analyzing the electromagnetic (EM) shielding properties of three-dimensional (3-D) lossy dielectric and magnetic objects over a lossy half space is presented. An MM, based on a volume formulation and a special Green's function in the spectral domain, is developed. Plane waves with TMz and TEz polarizations incident upon 3-D lossy material structures are demonstrated for the shielding effects of those bodies in the presence of a lossy ground. Some of the results are compared with those evaluated by applying the finite difference time domain method, and good agreements are obtained. The MM solution can be used to study the shielding problems for 3-D objects located above a lossy ground.     

Mode-expansion method for calculating electromagnetic waves scattered by objects on rough ocean surfaces

A mode-expansion method that needs less than 6% the number of unknowns required by conventional method of moments is introduced in calculating two-dimensional electromagnetic wave scattering from perfectly conducting objects on rough ocean surfaces. Modes are selected for dominant propagation waves so that the number of unknowns in the matrix equation are minimized. In the numerical examples, ocean surfaces are modeled as perfectly conducting rough surfaces described by the Pierson-Moskowitz power spectrum. Bistatic radar cross-sections (RCS) of various objects, such as ship-like and low-observable targets, are calculated for a 1-GHz incident plane wave and are validated for accuracy against an iterative MoM solution.     

A blind mobility aid modeled after echolocation of bats

A new model of a mobility aid for the blind was designed using microprocessor and ultrasonic devices. This mobility aid was evaluated based on psychophysical experiments. In this model, a downswept FM ultrasound signal is emitted from a transmitting array with broad directional characteristics in order to detect obstacles. The ultrasound reflections from the obstacles are picked up by a two-channel receiver. The frequency of the emitted ultrasound is swept from 70 to 40 kHz within 1 ms, so it has almost the same characteristics as the ultrasound a bat produces for echolocation. The frequency of the reflected ultrasound wave is down converted by about 50:1 by using a microcomputer with A/D and D/A converters. These audible waves are then presented binaurally through earphones. In this method obstacles may be perceived as localized sound images corresponding to the direction and the size of the obstacles. From the results of psychophysical experiments, it was found that downswept FM ultrasound was superior for the recognition of small obstacles compared to other ultrasonic schemes. With it a blind person can recognize a 1-mm-diameter wire. It was also proved that the blind could discriminate between several obstacles at the same time without any virtual images. This mobility aid, modeled after the bat's echolocation system, is very effective at detecting small obstacles placed in front of the head.     

地下目标的瞬态电磁散射特性分析及成像

地下目标的瞬态电磁散射分析对于探测、检验及识别埋地目标，特别是冲击探地雷达的研究具有重要的理论和应用价值。本文用FDTD三维建模计算在收发天线作用下地下无限长管道的时域散射场，重点探讨适用于吸收土壤中凋落波的GPML吸收边界以及处理有耗色散媒质的(FD)^2TD算法，并利用所得数据进行成像分析。BP方法成像结果证明了整个模拟计算过程的正确性。     

Study of Electron Heat Pulse Propagation and Particle Diffusion During the Synergy of LHW and IBW Heating in the HT-7

A successful experiment on lower hybrid wave (LHW) and ion Bernstein wave (IBW) synergy has been carried out in the HT-7 superconducting tokamak. With 560KW of LHW heating power and 160 KW of IBW heating power, during the synergy of the LHW and IBW experiment we achieved an improved plasma: the electron density increased and peaked, the soft X intensity increased too. A five-channel far-infrared (FIR) hydrogen cyanide (HCN) laser interferometer was used to measure the electron density profile. The particle diffusion coefficient was studied for the experiment, and the result compared with the heat diffusion coefficient. With the same injected LHW and IBW power, high electron density n_e can deduce the electron pulse propagation and the particle diffusion.     

有损均匀传输线的PSpice仿真分析

有损均匀传输线的电压、电流既是时间的函数,也是距离的函数,需要用分布参数电路模型分析.有损均匀传输线方程是偏微分方程,正弦稳态解的手算求解比较复杂.应用PSpice对有损均匀传输线进行仿真分析,可得到传输线上任一点的电压、电流的频率特性和正弦稳态波形.本文介绍了有损均匀传输线的PSpice模型和参数,通过实例说明了其应用方法,通过对比可知仿真分析比手算求解更清晰地阐明了物理概念.     

Clutter return from vegetated areas

An analytical stochastic model to predict relevant statistical scattering features of electromagnetic waves propagating within vegetated environments is presented. The propagation phenomena are described by formulating the scattering associated with random permittivity fluctuations superimposed on a lossy deterministic background slab. The distorted wave Born approximation is employed to determine the backscattered power, and its temporal spectrum.     

Optically nonlinear waves in thin films

A new treatment of the behavior of TE nonlinear waves in an optically nonlinear film is given. The new mathematical results are expressed in terms of the physical parameters of the system and represent a straightforward way to introduce the necessary Jacobian elliptic functions. The optical nonlinearity is of the Kerr type and the numerical calculations are performed for a self-focusing medium. Dispersion curves labeled with optical power density at the lower film boundary, detailed plots of the variation of electric field amplitude as the wave number changes, and details of the power distribution across the guide are given. Since two values of a wave number can exist for the same power level and power thresholds exist, the system is of device interest in the area of optical switching.