Application of a multiregion model to the EM scattering from a rough surface with or without a target above it

An efficient multiregion model is introduced to calculate the electromagnetic scattering from a perfectly electrical conducting(PEC) rough surface with or without a PEC target above it.In the multiregion model,the rough surface is split into multiple regions depending on their position along the rough surface.Two intermediate regions are chosen as the dominant region.If a target is located above the rough surface,the target will also be included in the dominant region.The method of moments(MOM) is only adopted on the dominant region to ensure validity.Hence,the new model can greatly reduce the number of unknowns associated with full MOM analysis.The induced electric currents on the other regions are obtained by approximately considering the mutual coupling between different regions along the rough surface.Compared with the published hybrid method,this new model is not only suitable for EM scattering from a target above a rough surface but also applicable for just rough surfaces.Several numerical simulations are presented to show the validity and efficiency of the multiregion model.     

Electromagnetic scattering from two-layered rough interfaces with a PEC object: vertical polarization

Electromagnetic(EM) scattering from a stack of two rough interfaces separating a homogeneous medium with a perfectly electric conducting(PEC) object has been calculated through the method of moments for vertical polarization.Theoretical formulations of EM scattering from multi-layered rough interfaces with a PEC object have been derived in detail and the total fields and their normal derivatives on the rough interfaces are solved.The two-layered model is a special case.In this work,a Gaussian rough surface was applied to simulate the rough interface.A cylinder was located above,between or below the two-layered rough interfaces.Through numerical simulations,the validity of this work is demonstrated by comparing it with existing scattering models,which are special cases that include a PEC object located above/below a single-layered rough interface and two-layered rough interfaces without an object.Subsequently,the influences of characteristic parameters,such as the relative permittivity of the medium,as well as the average height between the two rough surfaces,on the bistatic scattering coefficient are discussed.     

On the Frenkel problem of equivalent steady currents in a sphere

The Frenkel problem of substituting the 3D system of steady currents given in one of two concentric spherical regions by an equivalent system of currents (i.e., by that inducing the same external magnetic field) that is distributed over the surface of the other region is considered. A method of multipole moments providing the direct solution (without calculating the fields) of the problem is described. The case of currents with the density components represented by cubic polynomials of the Cartesian coordinates is considered as an example.     

宽范围偏置电流下的GaAlAs红外发光二极管1/f噪声特性

在宽范围偏置条件下,测量了GaAlAs红外发光二极管(IRLED)的低频噪声,发现1/f噪声幅值与偏置电流的γ次方成正比,在小电流区,γ≈1,在大电流区γ≈2.基于载流子数涨落和迁移率涨落机制建立了一个GaAlAs IRLED 1/f噪声模型,该模型的分析表明,低电流区GaAlAs IRLED的1/f噪声源于体陷阱对载流子俘获和发射导致的扩散电流涨落,高电流区的1/f噪声源于结空间电荷区附近氧化层陷阱对该处表面势的调制而引起载流子表面复合速率的涨落.该研究结果为1/f噪声表征GaAlAs IRLED的可靠性提供了实验基础与理论依据.     

Electromagnetic backscattering from one-dimensional drifting fractal sea surface Ⅱ:Electromagnetic backscattering model

Sea surface current has a significant influence on electromagnetic(EM) backscattering signals and may constitute a dominant synthetic aperture radar(SAR) imaging mechanism. An effective EM backscattering model for a one-dimensional drifting fractal sea surface is presented in this paper. This model is used to simulate EM backscattering signals from the drifting sea surface. Numerical results show that ocean currents have a significant influence on EM backscattering signals from the sea surface. The normalized radar cross section(NRCS) discrepancies between the model for a coupled wavecurrent fractal sea surface and the model for an uncoupled fractal sea surface increase with the increase of incidence angle,as well as with increasing ocean currents. Ocean currents that are parallel to the direction of the wave can weaken the EM backscattering signal intensity, while the EM backscattering signal is intensified by ocean currents propagating oppositely to the wave direction. The model presented in this paper can be used to study the SAR imaging mechanism for a drifting sea surface.     

On the role of the surface currents in conducting targets in the formation of the diffraction and transition radiation of relativistic electrons

The surface current method and the pseudophoton method are widely used in the study of the interaction between relativistic electrons and matter. A simple analysis reveals the contradictions between these methods as to the excitation of the currents on the surface of the conducting target. To solve this contradiction, the surface currents on the downstream and upstream surfaces of the conducting target were measured in the geometry of the diffraction radiation. The surface currents were experimentally recorded on the upstream target surface, from which the backward diffraction radiation is generated. At the same time, the surface currents are absent on the downstream target surface, which is conventionally considered as a source of diffraction radiation in the direction of the motion of electrons. Analogous results were obtained in the same geometry in a beam of real photons. On the whole, these results confirm the applicability of the pseudophoton method for the analysis of the effects of the interaction between the field of relativistic photons and the thick (thicker than the skin layer) conducting targets and inapplicability of the surface current method for the radiation in the direction of motion of electrons.     

聚变装置中真空室上涡流的分析

本文研究了聚变装置真空室上的涡流问题,将真空室上的涡流看作面电流密度并定义一矢势的法向分量来描述它。详细地叙述了求解真空室上感应涡流的计算方法,对HTU托卡马克实验装置真空室上涡流的大小、分布形式及衰减情况进行了分析。     

Passivation techniques for InAs/GaSb strained layer superlattice detectors

InAs/(In,Ga)Sb Strained Layer Superlattices (SLSs) have made significant progress since they were first proposed as an infrared (IR) sensing material more than three decades ago. The basic material properties of SLS provide a prospective benefit in the realization of IR imagers with suppressed interband tunneling and Auger recombination processes, as well as high quantum efficiency and responsivity. With scaling of single pixel dimensions, the performance of focal plane arrays is strongly dependent on surface effects due to the large pixels’ surface/volume ratio. This article discusses the cause of surface leakage currents and various approaches of their reduction including dielectric passivation, passivation with organic materials (polyimide or various photoresists), passivation by overgrowth of wider bandgap material, and chalcogenide passivation. Performance of SLS detectors passivated by different techniques and operating in various regions of infrared spectrum has been compared.     

Broadband IF matching for quasioptical mixers

Everyone recognizes the need to drive symmetric quasioptical antennas in a symmetric way to maintain clean antenna patterns; in this note we report on the advantages of bringing out the IF in a symmetric (balanced) way as well. The main difference in IF circuits between waveguide and open structure mixers is that the quasioptical mixers are usually also open at IF wavelengths, so IF currents can flow on the outside of the mixer mounting structures. We measured these surface currents and their associated resonances on a scale model of our mixer block for a 690 GHz SIS mixer. Bringing the IF off the mixer with a balanced circuit solves the surface current problems, yielding a broad bandwidth with predictable impedances. We successfully tested an octave bandwidth IF matching circuit for open structure mixers that incorporates a commercial 180° hybrid at cryogenic temperatures. We also found that surface currents are not significant for corner cube mixers because they generate their own balancing currents.     

Effects of spin-orbit interaction on superconductor-ferromagnet heterostructures: Spontaneous electric and spin surface currents

We find proximity-induced spontaneous spin and electric surface currents at all temperatures below the super-conducting T _c in an isotropic s-wave superconductor deposited with a thin ferromagnetic metal layer with spin-orbit interaction. The currents are carried by Andreev surface states and generated as a joint effect of the spin-orbit interaction and the exchange field. The background spin current arises in the thin layer due to different local spin polarizations of electrons and holes, which have almost opposite velocities in each of the surface states. The spontaneous surface electric current in the superconductor originates in the asymmetry of Andreev states with respect to sign reversal of the momentum component parallel to the surface. The conditions for electric and spin currents to show up in the system significantly differ from each other.     

Classical solutions of a model of quark confinement

We find the classical solutions of a model of quark confinement defined by the vanishing of colour currents. Both plane-wave type of solutions extending all over space as well as string-type of solutions confined to restricted regions of space are found.     

Vertical displacements of the current sheet and Hall currents

Vertical displacements of the peripheral regions of the current sheet with respect to its mean plane have been detected in the case of the sheet formation in the three-dimensional configuration with the X line and longitudinal component of the magnetic field. It has been shown that this effect is due to the generation of Hall currents and the appearance of vertically directed forces. Change in the sign of vertical displacements at the late stage of the sheet evolution has been detected, indicating the generation of opposite Hall currents when the direction of the main current near the side edges of the sheet changes. It has been shown that Hall currents significantly affect the structure of current sheets, and vertical oscillations of the peripheral regions of the current sheet, which appear owing to oppositely directed Hall currents, are involved in the dynamic processes in current sheets.     

High temperature and wavelength dependence of avalanche gain of AlAsSb avalanche photodiodes

The evolution of the dark currents and breakdown at elevated temperatures of up to 450 K are studied using thin AlAsSb avalanche regions. While the dark currents increase rapidly as the temperature is increased, the avalanche gain is shown to only have a weak temperature dependence. Temperature coefficients of breakdown voltage of 0.93 and 1.93 mV/K were obtained from the diodes of 80 and 230 nm avalanche regions (i-regions), respectively. These values are significantly lower than for other available avalanche materials at these temperatures. The wavelength dependence of multiplication characteristics of AlAsSb p-i-n diodes has also been investigated, and it was found that the ionization coefficients for electrons and holes are comparable within the electric field and wavelength ranges measured.     

Effect of surface preparation technique on the radiation detector performance of CdZnTe

Synthetic CdZnTe (CZT) semiconducting crystals are highly suitable for the room temperature-based detection of gamma radiation. The surface preparation of Au contacts on surfaces of CZT detectors is typically conducted after (1) polishing to remove artifacts from crystal sectioning and (2) chemical etching, which removes residual mechanical surface damage however etching results in a Te rich surface layer that is prone to oxidize. Our studies show that CZT surfaces that are only polished (as opposed to polished and etched) can be contacted with Au and will yield lower surface currents. Due to their decreased dark currents, these as-polished surfaces can be used in the fabrication of gamma detectors exhibiting a higher performance than polished and etched surfaces with relatively less peak tailing and greater energy resolution.     

Magnetic lines of force in a Tornado trap

Numerical methods are used to analyze the structure of the magnetic field of an ideal Tornado trap; i.e., a trap consisting of linear currents whose magnitude and location in space are such as to produce a spherical separatrix with a radius η _c between the helices of the trap and partition the magnetic field into two regions so that the lines of force from the working volume η<η _c do not pass beyond its boundary. It is found that the magnetic lines of force of the working volume form a series of regions, each with its own properties, and do not penetrate from one region to another. In particular, one of the regions is a layer of magnetic toroidal surfaces adjacent to the inner surface of the separatrix. Zh. Tekh. Fiz. 67, 30–34 (January 1997)     

Gaseous discharge plasmas produced by high-energyelectron-irradiated insulators for spacecraft

High-energy electron irradiation of insulators in vacuum causes both internal regions and surfaces of insulators to achieve high (negative) static voltage relative to nearby “ground”. Occasional spontaneous discharges inject pulses of partially ionized gas composed of the insulating material and surface gas-adatoms into the adjacent vacuum. The gas is capable of partially discharging the high surface potentials by carrying current across the vacuum to “ground”. The current-time waveforms were measured as a function of the spatial arrangement of the sample, test chamber electrodes, and static electric fields in order to investigate effects inside spacecraft boxes and cavities. It was found that plane-parallel electric fields are less able to sustain large discharge currents than are divergent electric fields. Also, physical confinement of the pulse of gas within the region of the electric field greatly increases the conductance of the gaseous discharge. Thus, the spatial arrangement of the sample, test chamber electrodes, and static electric field has a strong impact on the level of ESD-pulse threat to sensitive electronics posed by spontaneous discharges of irradiated insulators in spacecraft     

Analysis of multiple scattering from two-dimensional dielectric sea surface with iterative Kirchhoff approximation

<正>An iterative method in the Kirchhoff approximation is proposed for high frequency multiple electromagnetic scattering from two-dimensional dielectric sea surface.The multiple interaction of the scattering field is characterized with the corrected electromagnetic currents of the wind-driven sea surface.The actual surface currents are approximated with the iterative solution of the corrected currents.A newly developed sea spectrum,Elfouhaily spectrum,is utilized to build the sea surface model.The shadowing correction is improved by the Depth-Buffer algorithm.The validity of the iterative Kirchhoff approximation is verified by the agreement of backscattering coefficients with the measured data.     

Spinor Boltzmann Equation with Two Momenta at the Fermi Level

Based on the formalism of Keldysh's nonequilibrium Green function, we establish a two momenta spinor Boltzmann equation for longitudinal scalar distribution function and transverse vector distribution function. The longitudinal charge currents, transverse spin currents and the continuity equations satisfied by them are then studied, it indicates that both the charge currents and spin currents decay oscillately along with position, which is due to the momenta integral over the Fermi surface. We also compare our charge currents and spin currents with the corresponding results of one momentum spinor Boltzmann equation, the differences are obvious.     

Spinor Boltzmann Equation with Two Momenta at the Fermi Level

Based on the formalism of Keldysh's nonequilibrium Green function, we establish a two momenta spinor Boltzmann equation for longitudinal scalar distribution function and transverse vector distribution function. The longitudinal charge currents, transverse spin currents and the continuity equations satisfied by them are then studied, it indicates that both the charge currents and spin currents decay oscillately along with position, which is due to the momenta integral over the Fermi surface. We also compare our charge currents and spin currents with the corresponding results of one momentum spinor Boltzmann equation, the differences are obvious.