Diffraction by a perfectly conducting semi-infinite screen in an anisotropic plasma

The scattering of a plane electromagnetic wave by a perfectly conducting semi-infinite screen embedded in a homogeneous plasma is investigated. A uniform magnetic field is assumed to be impressed externally in a direction parallel to the edge of the half plane. The plasma is idealized to be a dielectric characterized by a tensor dielectric constant. The direction of the incident wave is assumed to be in a plane perpendicular to that of the screen. This vector problem is separable into two equivalent scalar problems for which either the electric or the magnetic vector is parallel to the edge of the half plane. It is found that for the case of theEmode, the magnetic vector parallel to the edge of the half plane satisfies a simple wave equation and a new type of impedance boundary condition on the screen. This problem is formulated in terms of an integral equation which specifies the current induced on the screen. The integral equation is of the Wiener-Hopf type and is solved by the usual function-theoretic methods. For a given orientation of the external magnetic field, a surface wave is found to exist along the screen but on one side only. The characteristics of this surface wave are determined.     

Complex space-time rays and their application to pulse propagation in lossy dispersive media

Asymptotic transient field solutions of the form A(r,t) exp [iS (r, t)], where S is a rapidly and A a slowly varying function of space and time, may be analyzed in terms of wave packets with central frequency ω =-∂S/∂t and central wavenumber k = ∇S. When the (dispersive) medium is lossless, stationary, and homogeneous, wave packets with constant real ω and k move along straightline trajectories called space-time rays. In the presence of dissipation and (or) when the input signal has an exponential amplitude dependence, S is complex. The corresponding wave packets with constant complex ω and k move along complex space-time rays, i.e., along trajectories defined in a complex (r, t) coordinate space. The properties of complex space-time rays and of the fields propagating along them, and their relation to physical fields observed on real (r, t) coordinates, are illustrated for a plane pulse with Gaussian envelope and frequency swept carrier, launched into a lossy environment. Tracking of spatial and temporal maxima is performed by ray techniques, and a paraxial ray regime is defined that permits discussion of a signal velocity. Special attention is given to ray focusing and the associated phenomena of pulse compression. It is shown how a complex input frequency profile can be synthesized so as to achieve optimum compression at a real space-time observation point in a lossy medium. The general results are applied in detail to a cold dissipative plasma, and a representative set of numerical calculations is included.     

抛物线坐标系非傍轴矢量光束的解及聚焦特性

为了求解柱坐标系下非傍轴矢量波动方程,得到光束的电场解析表达式,基于轴对称情况下沿角向偏振的电场,将非傍轴近似情况下的矢量波动方程进行了抛物线坐标的转化,利用分离变量法进行了相应求解,并给出了相应的数值计算。结果表明,非傍轴近似情况下,矢量波动方程的解能描述一种光束的电场,该场的解析表达式与合流超几何函数以及梅杰函数的解有关;光束的光强分布与第1类零阶贝塞尔模式光束类似;光束在近光轴处的光强表现为无限大并且沿边缘方向急剧衰减;在焦平面上沿着径向方向光强急剧减小。所得结果对于探究非傍轴近似情况下矢量光束的传输特性有一定的意义。     

非平行分层等离子体鞘套电波传播的复射线方法

基于损耗媒质的复Snell定律提出采用复射线方法研究电磁波在非均匀且非平行分层的等离子体媒质中的传播特性.该方法考虑非均匀平面电磁波的复射线(包括等幅度面的传播射线和等相位面的传播射线), 追踪电磁波复射线在每层媒质中的传播路径以及它们透过媒质分界面时的折射射线, 同时根据复射线的传播方向计算电磁波在每层媒质中的传播衰减, 数值累计整个传播过程中的传播衰减即可获得电磁波穿过等离子体鞘套的总衰减.由钝锥体仿真流场数据简化出非平行分层几何模型并采用复射线方法进行计算分析.计算结果表明:飞行器头部至尾部的传播衰减相差巨大且呈现迅速减小趋势, 非平行分层非均匀等离子体媒质存在某特定入射角, 能使传播衰减达到最小值.     

Radiation Modes of an Accelerated Cavity

The paper presents a theory of radiation by an accelerated linear laser cavity at imaginary eigenfrequencies. Expressions for the phase, frequency, and allowed directions of radiation are given. It is found that, in a rigid cavity moving with a constant acceleration together with a photodetector, two groups of frequencies with four different complex arguments are radiated. A relation between the earlier introduced radiation vector and the wave vector is established. It is shown that the radiation vector, which determines the eigenvalues of the spatial problem, is a multiple of the wave vector if boundary conditions are imposed along only one coordinate, when the side walls of the cavity with a plane front are located at infinity.     

Scattered and internal intensity of a sphere illuminated with aGaussian beam

A method of calculating the internal and scattered electric fields of a spherical dielectric object illuminated with a Gaussian beam is presented. The vector nature of the beam is considered. The fields satisfy Maxwell's equations, and the beam can be located arbitrarily with respect to the object. A polarized Gaussian beam is first represented as an angular spectrum of plane waves. These waves are then expanded in vector spherical harmonics. Although the details of the expansion are presented for a lowest-order Gaussian beam, the method can be applied to any wave which can be expressed as a sum of homogeneous plane waves. The interaction of an arbitrarily located Gaussian beam with a spherical object is analyzed using the T-matrix method. Calculated results for beams having waists much smaller than the radius of the sphere help in visualizing how a narrow beam reflects and refracts at the spherical dielectric interfaces. The combination of the plane-wave spectrum technique and the T-matrix method can be applied to the problem of an arbitrary beam interacting with an axisymmetric, nonspherical, homogeneous or layered object     

Beam scattering of the Luneberg lens by a complex source point

The complex source point method and vector mode function expansion are used to calculate the electromagnetic beam scattering by the Luneberg lens. A six-layered medium with different radii is considered as a model of the Luneberg lens. A comparison between the well-known numerical result in the Cartesian coordinate system and the new numerical one in the vector mode function is made for the incident Gaussian beam propagation in free space. The propagation properties are in good agreement with each other. The total electric field distributions along the transversal and longitudinal axes are examined for scattering by the Luneberg lens, whose dimension is assumed to be comparable with the beam width. The focusing effect of the Gaussian beam is much lower than that of the plane wave. The spot size focused by the Luneberg lens almost equals the wavelength for the various beam parameters including the plane wave.     

Scattering by a two-dimensional periodic array of conducting plates

The boundary value problem of an infinite array of thin plates arranged in a doubly periodic grid along any two coordinates is formulated in a general form for an arbitrarily polarized plane wave incident from any oblique angle. The induced current on the plate, the near-field distribution, and the distant reflected waves can be obtained to a very close accuracy. Both magnitudes and phases of the reflection coefficients for some specific examples are determined explicitly. For the case of a wave incident normally on a rectangular lattice array of narrow rectangular plates, the calculated values are in excellent agreement with the measurements in a previously published paper.     

Rigorous formulation of scattering at a randomly varying impedanceplane: general case of oblique incidence

The problem of scattering of electromagnetic plane waves at one-dimensional surfaces (random gratings) is solved in the general case in which the incident wave vector does not lie on the main section of the cylindrical surface (oblique incidence). The scatterer is simulated by a plane boundary characterized by a coordinate-dependent impedance that varies along one of the two coordinates on the surface. This representation could be regarded as a canonical model of one-dimensional surfaces with height corrugations. A rigorous electromagnetic formalism for calculating the fields scattered at the impedance plane is presented. The fields above the scatterer are represented by spectral domain expansions. It is shown that the wave vectors of the scattered waves lie on the surface of a cone containing the direction of specular reflection and whose axis coincides with the direction of the grooves of the random grating. The theory is exemplified by calculating the angular distribution of the mean intensity scattered from an ensemble of surfaces with similar statistical parameters     

Waveguides Containing Moving Dispersive Media

Perturbation formulas are derived for the changes in the dispersion curves and phase velocity for the modes in an arbitrary composite waveguide structure containing dispersive media in relative motion. The formulas are also valid when the media are fluids with arbitrary velocity distributions. It is shown that the relativistic transformation laws for the frequency and wave vector of uniform plane waves are also valid for waveguide modes provided that all moving media that make up the guide move with the same velocity. There are also difficulties when the moving media are dispersive. In general, one most therefore obtain the dispersion relation directly from the field equations or from the perturbation formulas. An example involving a simple surface wave along the interface of a moving plasma and a dielectric is worked out by both methods. As an interesting side result, it is found that plane waves in an unbounded isotropic plasma have phase velocities independent of the motion of the plasma.     

Electromagnetic properties of a lattice of anisotropically conducting strips with a high filling factor

A planar small-period lattice of anisotropically conducting strips separated by extremely narrow gaps is considered. An explicit analytic expression for the coefficient of reflection of a plane linearly polarized wave from this lattice is derived. Computation results illustrating the resonance properties of the structure are presented. It is shown that surface waves can propagate along the lattice. The corresponding dispersion equation is derived and analyzed.     

Electromagnetic plane wave scattering by a system of two uniformlylossy dielectric prolate spheroids in arbitrary orientation

By means of modal series expansions of electromagnetic fields in terms of prolate spheroidal vector wave functions, an exact solution is obtained for the scattering by two uniformly lossy dielectric prolate spheroids in arbitrary orientation embedded in free space, the excitation being a monochromatic plane electromagnetic wave of arbitrary polarization and angle of incidence. Rotational-translational addition theorems for spheroidal vector wave functions are employed to transform the outgoing wave from one spheroid into the incoming wave at the other spheroid. The field solution gives the column vector of the unknown coefficients of the series expansions of the scattered and transmitted fields expressed in terms of the column vector of the known coefficients of the series expansions of the incident field and the system matrix which is independent of the direction and polarization of the incident wave. Numerical results in the form of curves for normalized bistatic and monostatic radar cross sections are given for a variety of two-body system of uniformly lossy dielectric prolate spheroids in arbitrary orientation having resonant or near resonant lengths and different distances of separation     

FDTD simulation of TE and TM plane waves at nonzero incidence in arbitrary Layered media

Plane wave scattering is an important class of electromagnetic problems that is surprisingly difficult to model with the two-dimensional finite-difference time-domain (FDTD) method if the direction of propagation is not parallel to one of the grid axes. In particular, infinite plane wave interaction with dispersive half-spaces or layers must include careful modeling of the incident field. By using the plane wave solutions of Maxwell's equations to eliminate the transverse field dependence, a modified set of curl equations is derived which can model a "slice" of an oblique plane wave along grid axes. The resulting equations may be used as edge conditions on an FDTD grid. These edge conditions represent the only known way to accurately propagate plane wave pulses into a frequency dependent medium. An examination of grid dispersion between the plane wave and the modeled slice reveals good agreement. Application to arbitrary dispersive media is straightforward for the transverse magnetic (TM) case, but requires the use of an auxiliary equation for the transverse electric case, which increases complexity. In the latter case, a simplified approach, based on formulating the dual of the TM equations, is shown to be quite effective. The strength of the developed approach is illustrated with a comparison with the conventional simulation based on an analytic incident wave specification with half-space, single frequency reflection and transmission for the edges. Finally, an example of a possible biomedical application is given and the implementation of the method in the perfectly matched layer region is discussed.     

Computation of the RCS of coated objects by a generalized PTD approach

We consider the diffraction of an incident plane wave by 3D objects, such as missiles. These objects can be either perfectly conducting or coated with dielectric or magnetic coatings. Their shape, usually described in cad format, can be quite complex, with reentrant cavities, wings, ..., such that multiply reflected, reflected diffracted, and multiple diffracted rays exists. Morover, as these objects are designed to have a low rcs, these rays can be, for some angles of incidence, the main contributions to thercs. ptd is an efficient technique to compute thercs of complex objects, that has the desirable property of providing bounded results for any angle of incidence and observation. However, this theory has been mainly applied to perfectly conducting objects, the fringe wave is computed for wedge-like discontinuities, it does not include multiple phenomena. We have generalizedptd in three directions. First, we have extended the fringe wave concept to the case of an edge satisfying an impedance boundary condition. Second, we have computed diffraction by rounded edges or by tips by using an hybrid asymptotic-MoM method. Third, we have included multiply reflected or diffracted rays. We present some examples ofrcs and comparison with other methods.     

Electromagnetic wave scattering by a system of two spheroids ofarbitrary orientation

An exact solution to the problem of the scattering of a plane electromagnetic wave by two perfectly conducting arbitrarily oriented prolate spheroids is obtained by expanding the incident and scattered electric fields in terms of an appropriate set of vector spheroidal eigenfunctions. The incident wave is considered to be a monochromatic, uniform plane electromagnetic wave of arbitrary polarization and angle of incidence. To impose the boundary conditions, the field scattered by one spheroid is expressed in terms of its spheroidal coordinates, using rotational-translational addition theorems for vector spheroidal wave functions. The column matrix of the scattered field expansion coefficients is equal to the product of a square matrix which is independent of the direction and polarization of the incident wave, and the column matrix of the known incident-field expansion coefficients. The unknown scattered-field expansion coefficients are obtained by solving the associated set of simultaneous linear equations. Numerical results for the bistatic and backscattering cross sections for prolate spheroids with various axial ratios and orientations are presented     

Analysis of Electromagnetic Scattering from an Eccentric Multilayered Sphere

An exact analytic solution of a plane electromagnetic (EM) wave scattered by an eccentric multilayered sphere (EMS) is obtained. It is assumed that the layers are perfect dielectrics and that the innermost core is a perfectly conducting sphere. Each center of a layer is translated along the incident axis. All fields are expanded in terms of the spherical vector wave functions with unknown expansion coefficients. The addition theorem for spherical wave functions is used prior to applying the boundary conditions. The unknown coefficients are determined by solving a system of linear equations derived from the boundary conditions. Numerical results of the scattering cross sections are presented on the plane of φ=0 degrees and φ=90 degrees. The convergence of modal solutions and the characteristics of patterns are examined with various geometries and permittivity distributions     

符号相干系数加权的超声平面波复合成像算法

平面波超声成像算法运用广泛,具有较高的成像帧率,但成像质量较差.相干平面波复合成像（CPWC）算法采用对多个方向平面波成像输出直接叠加的方式进行成像,有效的提高了成像质量,但忽略了平面波之间的相干性.本文提出符号相干系数加权的平面波复合成像算法（SCF-CPWC）.该算法将不同角度的平面波对同一个成像点的成像结果做为向量,计算出该向量的符号相干系数（SCF）,对该成像点的相干平面波复合成像的输出结果进行加权成像,得到SCF-CPWC算法的最终结果.仿真和实验结果表明SCF-CPWC算法具有更好成像质量.相对于传统的CPWC成像算法,可以明显改善成像的横向分辨率和暗斑对比度.     

The theory of twisted transverse-wave couplers

The coupled-mode theory of a class of transverse-wave couplers in which the plane of polarization of the electric field rotates along the length of the coupler is described. The synchronism conditions between the circuit wave and the beam that lead to strong interaction with one or more of the transverse beam waves are given and the case of equal interaction with the two cyclotron waves is discussed in detail. It is observed that any two transverse waves of opposite polarization can be excited equally in a twisted coupler, or that coupling to one of two waves with the same phase velocity but opposite polarizations can be achieved.     

光轴任意取向单轴晶体非正入射工作状态

在单轴双折射晶体中，考虑到折射Ｏ光和ｅ光的两者的主截面实际上是不相同的，选择一个主轴坐标系，使其某一坐标平面为ｅ光线的主截面，并在这个坐标系中分解出ｅ光的Ｄ振动矢量。这样就得出了对ｅ光振动矢量的较为严格的描述。     

平面波谱法分析电磁波在管道中的传播

通过分析电磁波从管道开口耦合进入管道的绕射机理及场在腔体内的射线特征，将进入管道的绕射场展开为平面波谱，利用平面波在自由空间的传播特性分析了电磁波沿管道的传输和反射特征。通过算例验证了该算法的有效性。