金属圆柱腔体中使用非均一背景增强微波断层成像

针对基于圆柱金属腔体的微波断层成像系统,提出了一种利用非均一背景增强系统获取目标信息能力的方法.该方法通过在腔体内放置已知物体构成非均一背景,这样不但能利用背景的先验信息,而且可以增加等效辐射源对目标进行探测.首先,利用矩量法计算圆柱金属腔体内非均一背景的格林函数和离散积分算子,并对离散积分算子进行奇异值谱和条件数分析,在理论上证明该方法的可行性;然后,利用基于有限元的对比源逆成像法对均一背景、有耗非均一背景和无耗非均一背景三种情况进行仿真研究;最后对仿真结果进行了误差分析和比较.仿真结果表明,该方法可以提高反演收敛速度和结果准确度,有耗非均一背景略优于无耗非均一背景.该方法可以在不改变硬件系统和算法的情况下得到更准确的反演结果,可应用于医学成像与工业无损探测.     

Persistence in random walk in composite media

We consider a class of inhomogeneous media known as composite media that is often encountered in experimental sciences, and investigate the persistence probability of a random walker in such a system. Analytical and numerical results for the crossover time scales are obtained for a composite system with two homogeneous components and three homogeneous components respectively.     

Radiation transfer by a finite-emitting inhomogeneous medium

A functional relation is obtained between radiative transfer in an inhomogeneous finite planar layer with an internal energy source and diffuse reflection. The intensity is derived for the emerging radiation of a polynomial energy source. We use Padé approximants to calculate the emitted intensity for a linear energy source when the single scattering albedo decreases exponentially with optical depth. Numerical results are given for both homogeneous and inhomogeneous media.     

非相干光时延四波混频的多能级理论——非均匀加宽情形

本文将我们建立的非相干光时延四波混频(TDFWM-IL)的多能级理论由均匀加宽情形发展到非均匀加宽情形,得到吸收带同时存在均匀和非均匀加宽时,TDFWM-IL多能级理论给出的信号强度随两束泵光相对延时的一般关系式,讨论非均匀加宽占绝对优势时,非相干光光子回波的多能级理论结果,及其与二能级理论结果的差异,论证等效二能级模型同样适用于近似分析存在非均匀加宽影响时,吸收带TDFWM-IL的实验结果,报道在红宝石零声子线和甲酚紫固体膜吸收边上的实验结果,并与上述理论做了比较。     

Laser diagnostics of nanoscale dielectric films on absorbing substrate by differential reflectivity and ellipsometry

The differential reflection characteristics for ultrathin inhomogeneous dielectric film on absorbing substrate are investigated in the long-wavelength approximation. The obtained first-order expressions for differential reflectivity and changes in the ellipsometric angles caused by ultrathin layer are of immediate interest to the solution of the inverse problem. The method to determine the averaged values (not the realistic profile) of refractive index for inhomogeneous nanometric films are shown. The novel possibilities for determining the dielectric constant and thickness of nanoscale homogeneous films by the differential ellipsometric and reflectivity measurements are developed, and a simple method to estimate whether the nanometric film is homogeneous or not is also discussed.     

Exact sum rules for inhomogeneous drums

We derive general expressions for the sum rules of the eigenvalues of drums of arbitrary shape and arbitrary density, obeying different boundary conditions. The formulas that we present are a generalization of the analogous formulas for one dimensional inhomogeneous systems that we have obtained in a previous paper. We also discuss the extension of these formulas to higher dimensions. We show that in the special case of a density depending only on one variable the sum rules of any integer order can be expressed in terms of a single series. As an application of our result we derive exact sum rules for the homogeneous circular annulus with different boundary conditions, for a homogeneous circular sector and for a radially inhomogeneous circular annulus with Dirichlet boundary conditions.     

Vector Diffraction Integrals for Solving Inverse Problems of Radio-Holographic Sensing of the Earth's Surface and Atmosphere

Vector relationships between the fields on a certain surface confining an inhomogeneous three-dimensional volume and the fields inside this volume are obtained by the Stratton–Chu method developed for the case of homogeneous media. The vector relationships allow us to solve the direct and inverse problems of determining the fields inside an inhomogeneous medium given the field on its boundary. The vector equations take into acount the polarization changes of direct and inverse waves propagated in an inhomogeneous medium. In the case of a two-dimensional homogeneous medium, the vector equations reduce to the previously obtained scalar equations used in the approximation of spherical symmetry to describe the process of backward wave propagation during the atmospheric and ionospheric radio-occultation monitoring. It is shown that the Green's function of the scalar wave equation in an inhomogeneous medium should be used as the reference signal for solving the inverse problem of radio-occultation monitoring. This validates the method of focused synthetic aperture previously used for high-accuracy retrieval of the vertical refractive-index profiles in the ionosphere and atmosphere. In this method, the reference-signal phase was determined from a model which describes with sufficient accuracy the radiophysical parameters of a refracting medium in the region of radio-occultation sensing. The obtained equations can be used for the high-accuracy solving of inverse problems of radio-holographic sensing of the Earth's atmosphere and surface by precision signals from radio-navigation satellites.     

Peeling stress analysis for an inhomogeneous high-Tc superconductor with a discontinuous interface at the substrate

This paper presents an analysis of a superconductor–substrate system to calculate the peeling stress of a high temperature superconductor (HTS) when the temperature decreases from ambient to operating conditions (cryogenic temperatures). Firstly, the values for the material properties of the inhomogeneous high temperature superconductor (HTS) were obtained by fitting a second order polynomial to the experimental data. It is assumed that the material properties of the inhomogeneous HTS vary with varying height coordinate and temperature. Then, through the proposed graded finite element method, the coupled thermo-mechanical equations were solved numerically. The numerical results show that the thermal stress generated in the inhomogeneous HTS is larger on a SiTiO₃ substrate than on a MgO substrate. The maximum thermal stresses, i.e., the peeling stresses, occur near the bottom corner of the inhomogeneous HTS and may induce fracture behavior at the bi-material interface. The inhomogeneous HTS cools at a slower pace than the homogeneous HTS from the room temperature to the operating temperature. It is also shown that the magnitude of the peeling stress for a homogeneous HTS is larger than that for an inhomogeneous HTS. It is intended that the model presented here be useful to researchers who are interested in the mechanical properties of an inhomogeneous HTS.     

Secondary bistable solitons in non-linear gradient index media

Secondary bistable solitons have been calculated for a non-linear optical inhomogeneous medium with parabolic gradient index and quadratic intensity-dependent refractive index change. The remarkable effect of inhomogeneities on the existence and stability of bistable solitons has been studied. The results obtained are compared with those from homogeneous solitons.     

Sound radiation from a thin infinite plate in contact with a layered inhomogeneous fluid

In this paper the sound radiation from a thin infinite plate in contact with a layered inhomogeneous fluid subjected to single point excitation is studied. Sources contributing to the inhomogeneity are discussed, and the fluid is analytically expressed as a layered inhomogeneous field. Using Hankel transform, the equations governing the fluid-structure interaction are solved. The asymptotic form of the radiation pressure at far field is obtained using the method of stationary phase. Numerical examples show that the sound radiation patterns from a plate in contact with an inhomogeneous fluid is much more complicated than that in a homogeneous fluid. Three different radiation patterns (bell, disc, and their combination) were observed in the numerical examples, and the radiation patterns are sensitive to the field parameters. Depending on frequency, the sound pressure at far field is either magnified or reduced compared with that in a homogeneous fluid. And beyond some frequency the sound pressures are almost zero.     

Some comments on modeling the linearized Boltzmann equation

Some exact solutions of the homogeneous and the inhomogeneous linearized Boltzmann equation (LBE) for rigid-sphere collisions are used to define two model equations in the general area of rarefied-gas dynamics. These equations are obtained from a systematic development of two synthetic scattering kernels that yield model equations that have as exact solutions certain known exact solutions of the homogeneous and of the inhomogeneous LBE. The first model established is defined in terms of the collisional invariants and the Chapman-Enskog integral equations for viscosity and for heat conduction. An extended model is defined also in terms of the collisional invariants and the Chapman-Enskog functions for viscosity and heat conduction, but the first and second Burnett functions are also included in the model. The variable collision frequency or generalized BGK model is also obtained as a special case. In addition, the exact mean-free paths defined, for rigid-sphere collisions and the LBE, in terms of viscosity or heat conduction are employed to define approximations of these quantities that are consistent with the use of the variable collision frequency model.     

FDTD Simulation of Electromagnetic Reflection of Conductive Plane Covered with Imhomogeneous Time-Varying Plasma

A finite-difference time-domain (FDTD) algorithm is applied to study the electromagnetic reflection of conduction plane covered with inhomogeneous, collision, warm, time-varying plasma. The collision frequency of plasma is a function of electron density and plasma temperature. Under the one-dimensional case, transient electromagnetic propagation through various plasmas have been obtained, and the reflection coefficient of EM wave through inhomogeneous time-varying plasma (ITVP), homogeneous time-varying plasma (HTVP) and inhomogeneous plasma (IP) are calculated under different conditions. The results illustrate that a plasma cloaking system can successfully absorb the incident EM wave.     

Frequency Characteristics of the Satureted Gain in Nonequilibrium Gas Flow and Doppler-Broadened Chemical Laser Amplifiers

We present "convection-nonequilibrium cross relaxation" model for a gas flow and chemical lasers, Teh experessions of the saturated gain spectrum are obtained for the two-type flow models with layer or turbulent flow mixture respectively. The numerical results show that the gain profile of a saturated amplifier with Doppler-broadening is a mixture of inhomogeneous and homogeneous profiles     

Current layers and filaments in a semiconductor model with an impact ionization induced instability

Dissipative structures associated with an instability in a semiconductor far from equilibrium are studied. A generation-recombination mechanism, which effects anS-shaped current-voltage characteristics, is coupled to diffusion and drift of the electrons. The spectrum of linear recombination-diffusion modes is computed for the homogeneous steady state with negative differential conductivity. The obtained soft mode instability gives rise to the bifurcation of a family of transversally modulated inhomogeneous steady states and longitudinal travelling waves. The inhomogeneous steady states are calculated from the full nonlinear transport equations for plane and cylindrical geometries. They correspond to oscillatory and solitary concentration profiles, including depletion and accumulation layers and cylindrical filaments. Conditions for the formation of kink-shaped coexistence profiles are established in terms of equal area rules. The current-voltage characteristics are extended to include inhomogeneous current states. Nonequilibrium phase transitions between various branches of these characteristics are associated with switching through filamentation.     

Casimir stress in an inhomogeneous medium

The Casimir effect in an inhomogeneous dielectric is investigated using Lifshitz’s theory of electromagnetic vacuum energy. A permittivity function that depends continuously on one Cartesian coordinate is chosen, bounded on each side by homogeneous dielectrics. The result for the Casimir stress is infinite everywhere inside the inhomogeneous region, a divergence that does not occur for piece-wise homogeneous dielectrics with planar boundaries. A Casimir force per unit volume can be extracted from the infinite stress but it diverges on the boundaries between the inhomogeneous medium and the homogeneous dielectrics. An alternative regularization of the vacuum stress is considered that removes the contribution of the inhomogeneity over small distances, where macroscopic electromagnetism is invalid. The alternative regularization yields a finite Casimir stress inside the inhomogeneous region, but the stress and force per unit volume diverge on the boundaries with the homogeneous dielectrics. The case of inhomogeneous dielectrics with planar boundaries thus falls outside the current understanding of the Casimir effect.     

Acoustic field of a vertical cylindrical antenna in an inhomogeneous waveguide with an impedance lower boundary

An expansion of the field of a vertical antenna located in an inhomogeneous waveguide in terms of the normal waves of a homogeneous reference waveguide is obtained. The frequency dependence of the radiation resistance is analyzed numerically for various antenna depths and sound velocity profiles. Variations in the radiation resistance are correlated with the variations in the sound velocity.     

Effects of inhomogeneous distribution of Si–Nc and Er ions on optical amplification in Si–Nc Er doped fiber

In this article, the effects of Si–Nc and Er³⁺ ions distribution parameters including inhomogeneous and homogeneous distribution profile are studied on the optical parameters such as gain, population inversion and Si–Nc induced losses. We have shown that by increasing of the concentration of Si–Nc particles the net gain and induced Si–Nc losses increased in homogeneous and inhomogeneous distributions. In practice, the homogeneous distribution of Er ions and Si–Nc is hard to be realized. Therefore, the inhomogeneous distributions of ions cased to perturb state in mode shape of optical signal then the investigations of those effects are important for high speed optical communications. In this article, a method for evaluation of the effects of inhomogeneous distribution of impurities on performance of optical amplifier is developed and the managing of the gain with use of suitable distribution functions is proposed.     

ABNORMAL NEUTRON STARS WITH ABNORMAL PROTONS(II) —INHOMOGENEOUS STELLAR MODELS

The homogeneous models of the abnormal neutron stars with abnormal protons were discussed in paper [4]. As its refinement, the inhomogeneous models of the abnormal neutron stars with abnormal protons are discussed in this paper. The results obtained are given in Table 2 and shown in Figs. 1-4.     

Theory of an intracavity Raman laser

The quantum mechanical Langevin equations of a cavity where laser action takes place both by ordinary population inversion and stimulated Raman scattering is discussed for both single mode and multimode oscillation of the pump and Raman laser. Expressions for amplitude correlation and linewidth due to phase diffusion are obtained both for homogeneous and inhomogeneous broadening.     

One-Parameter Inhomogeneous Differential Realizations and Boson–Fermion Realizations of the gl(2¦1) Superalgebra

One-parameter homogeneous and inhomogeneous differential realizations of the gl(2¦1) superalgebra on the spaces of homogeneous and inhomogeneous polynomials and the corresponding boson–fermion realizations are studied. The parameter has relation to the Hubbard interaction parameter U in the Hubbard model for correlated electrons.