高功率电磁波脉冲在电离层中的传播

 用时域伪谱(PSTD)算法数值模拟了高功率电磁波脉冲在整个电离层中的传播。利用缩减的电离层模型并忽略非线性过程,脉冲在电离层中的传播可用脉冲在冷等离子体中的传播方程来描述。时域有限差分(FDTD)法解决这个问题将耗费大量的时间和内存,因此,使用PSTD方法来解决冷等离子体中的方程,并采用完全匹配层边界条件截断求解空间。所得脉冲在整个电离层传播的数据显示,在265 km附近有一个分界点,其以下是反射波,以上既有反射波又有前向波。这与用高阶(FD)2TD方法数值模拟结果一致,表明时域伪谱算法模拟电磁波在电离层中长距离传播的有效性。     

JEMS-FDTD软件在电磁脉冲区域传播数值模拟中的应用

电磁脉冲区域传播数值模拟是电磁环境效应分析的重要环节, 面临空间尺度巨大、多辐射源、复杂地貌等技术挑战。本文介绍了三维时域全波电磁模拟并行软件JEMS-FDTD的研制进展。在大规模并行计算FDTD方法的基础上结合自适应网格技术, 研制了一种适应于大区域电磁脉冲传播的高效时域全波计算方法。并基于该技术实现了千km2级城市电磁脉冲区域传播的数值模拟, 获取了全空间的时域电磁场信息, 验证了软件在电磁脉冲区域传播仿真应用中的可行性。     

微波谐振腔的瞬态响应研究

应用非正交网格时域有限差分算法，模拟了在高斯脉冲和高斯脉冲调制正弦波激励下有激励孔微波谐振腔的激励和谐振过程，分析了谐振频率。模拟结果表明；在微波谐振腔内，激发出了最接近脉冲激励源主频的若干个谐振态。     

基于高斯伪谱法的高超声速飞行器再入制导研究

针对高超声速飞行器再入标准轨迹制导方法中存在的制导准备周期长、弹上需存储标准轨迹参数、制导鲁棒性较差等缺点,本文提出了一种基于高斯伪谱法与滚动时域控制技术相结合的高超声速飞行器再入预测-校正制导方案。其中,在线高斯伪谱法采用纵/侧向、全程一体化的制导算法思路,从而实现了对高超声速飞行器再入弹道的全程预测-校正。同时结合滚动时域控制技术从工程上实现了高超声速飞行器再入制导中对开环制导信息的闭环应用,完成了飞行器预测-校正制导方案。通过对高超声速飞行器再入制导过程进行仿真分析,其结果表明应用本文设计的基于高斯伪谱法与滚动时域控制技术相结合的高超声速飞行器再入预测-校正制导方案,飞行器再入过程中具有良好的制导性能。     

水平极化电磁脉冲模拟器空间场的数值模拟

 采用时域有限差分方法，对水平极化电磁脉冲模拟器所产生的电磁场进行数值模拟，在时域中计算了电磁场的时间 空间分布。讨论了电磁脉冲模拟器产生的电磁脉冲波形和场的空间变化、均匀性等重要因素。结果表明，模拟器产生的电磁脉冲前沿为10ns，接近脉冲电压前沿，波形与脉冲电压波形（双指数波）相差较大；峰值场强在对称轴上并非与距离成反比，而是随距离增大而迅速减弱；该模拟器的双锥 笼形天线能在大范围内（大于等于50m,水平方向）产生均匀分布、高峰值场强、快前沿的电磁脉冲。     

并联介质加载偶极天线脉冲辐射特性的研究

应用FDTD方法和PML技术，对馈电区一侧放置介质块的电阻加载偶极天线的时域辐射特性进行了研究，发现这种天线具有良好的方向性.分析了加载介质块的尺寸和介电常数对脉冲辐射特性的影响，给出了能够产生良好方向性而且辐射效率又不至于很低的天线结构参数.并联介质加载脉冲天线的低轮廓特点使其容易与电路集成，非常适用于UWB这样的超宽带无线通信系统. 关键词： 脉冲天线 辐射特性 介质加载 时域有限差分方法     

脉冲激励混响室测试区域电磁场均匀性

研究了电磁脉冲激励混响室内电磁波传播规律,通过仿真建模和实验两方面对脉冲激励混响室测试区域内电磁场均匀性进行了研究。提出了混响室时域均匀性的概念及其计算方法,并利用此方法计算了仿真和实验结果。结果表明:脉冲激励混响室内电磁环境无论在时域上还是频域上,均满足国际标准规定的均匀性要求;在一定空间内,混响室内也可以形成统计平均的脉冲场电磁环境;在混响室内进行电磁脉冲场的电磁敏感度测试是可行的。     

电磁场2.5维时域间断有限元方法

介质沿空间固定方向均匀分布的结构在电磁导波器件中有十分广泛的应用,对这类器件的分析通常被称为2.5D电磁问题。利用器件在固定方向介质分布均匀的特点,将电磁场量沿该方向进行空间傅里叶变换,可以把对三维问题的分析转化为两维问题求解,从而极大地减小计算开销。针对传统基于差分的2.5D电磁场算法在弯曲形状逼近上有阶梯误差的缺陷,本文提出了基于三角形网格的2.5D时域间断有限元方法（DGTD）,并用它模拟了电偶极子与光纤的耦合效率和光子晶体光纤的色散特性。与基于规则网格的2.5D差分方法进行对比。结果表明,文中建立的2.5D DGTD方法对弯曲形状的模拟更加逼真,计算内存占用最大减少10.4%,计算精度最大相差0.011%,计算时间缩短74.9%,计算效率提高。     

两种电磁场谱间断元方法的比较

时域间断元方法是近年来电磁场计算领域的重要进展之一。将基函数的插值点和数值积分点重合的质量集中技术是降低该间断元方法质量矩阵存储开销和提高计算效率的重要手段。通过谐振腔、带通滤波器以及光子晶体内的电磁场等数值算例,在四边形网格上比较了传统的质量集中算法和近来提出的Weight-Adjust算法之间的差异。计算结果表明,尽管两种方法的存储量一样,但Weight-Adjust算法具有更高的精度。     

基于辛Runge-Kutta-Nystrom方法的雷达散射截面计算

从基本的差分概念和Maxwell方程出发, 引入电磁场方程的Hamilton函数.提出一种基于Runge-Kutta-Nystrom辛算法的高阶时域有限差分方法，该方法保持了系统的相空间体积不变和总能量不变，并导出了迭代公式.在此基础上计算了一种金属圆柱的雷达散射截面.计算结果表明该方法的正确性及快速、精确的特性. 关键词： 雷达散射截面 高阶算法 辛Runge-Kutta-Nystrom方法 时域有限差分     

Single-element ultrasonic transducer modeling using a hybrid FD–PSTD method

In a recent publication [E. Filoux, S. Callé, D. Certon, M. Lethiecq, F. Levassort, Modeling of piezoelectric transducers with combined pseudospectral and finite-difference methods, J. Acoust. Soc. Am. 123 (6) (2008) 4165–4173], a new finite-difference/pseudospectral time-domain (FD–PSTD) algorithm was presented and used to model the generation of acoustic waves by a piezoelectric resonator and their propagation in the structure and the surrounding water. In this paper, the model has been extended to simulate the two-dimensional behaviour of a complete single-element transducer, composed of the resonator, a backing and a front matching layer. This further version of the model takes into account the mechanical loss in materials, and enables the calculation of electrical impedance, which is a characteristic of high interest to optimize the performance of ultrasonic transducers. The impedance curves of a PZT [URL: http://www.ferroperm-piezo.com (last viewed 04/2008); B. Jaffe, R.S. Roth, S. Marzullo, Piezoelectric properties of lead zirconate-lead titanate solid-solution ceramics, J. Appl. Phys. 25 (1954) 809–810] plate-based high-frequency transducer, with a 50 MHz thickness resonant frequency, were compared to those of a KLM model [R. Krimholtz, D.A. Leedom, G.L. Matthei, New equivalent circuit for elementary piezoelectric transducers, Electron. Lett. 6 (1970) 398–399] in the one-dimensional case. The acoustical properties were also found to be in good agreement with those obtained using the finite element (FE) method of ATILA^® software in two-dimensional configuration.     

An analytical derivation of the optimum source patterns for the pseudospectral time-domain method

In the computational electromagnetics and acoustics, spatially smoothed sources are often utilized to alleviate the aliasing errors in the pseudospectral time-domain (PSTD) algorithms. In our work, an analytical derivation of the optimum source patterns is presented according to the accurately derived expressions of the dominant source-introduced aliasing errors according to the circular discrete convolution and Tailor series expansion method. We quantitatively demonstrate, for the first time in literature, that the aliasing errors can be optimally suppressed and rapidly reduced to the negligible levels by these optimum patterns and with the increment of source cells. We also provide the different implementation schemes of the optimal patterns both for the soft and hard source cases. The numerical calculation and 1D PSTD transient simulations are conducted to verify the excellent performance of these optimum sources.     

A spectral collocation algorithm for two-point boundary value problem in fiber Raman amplifier equations

A novel algorithm implementing Chebyshev spectral collocation (pseudospectral) method in combination with Newton’s method is proposed for the nonlinear two-point boundary value problem (BVP) arising in solving propagation equations in fiber Raman amplifier. Moreover, an algorithm to train the known linear solution for use as a starting solution for the Newton iteration is proposed and successfully implemented. The exponential accuracy obtained by the proposed Chebyshev pseudospectral method is demonstrated on a case of the Raman propagation equations with strong nonlinearities. This is in contrast to algebraic accuracy obtained by typical solvers used in the literature. The resolving power and the efficiency of the underlying Chebyshev grid are demonstrated in comparison to a known BVP solver.     

Perfect plane-wave source for a high-order symplectic finite-difference time-domain scheme

The method of splitting a plane-wave finite-difference time-domain (SP-FDTD) algorithm is presented for the initiation of plane-wave source in the total-field / scattered-field (TF/SF) formulation of high-order symplectic finite-difference time-domain (SFDTD) scheme for the first time. By splitting the fields on one-dimensional grid and using the nature of numerical plane-wave in finite-difference time-domain (FDTD), the identical dispersion relation can be obtained and proved between the one-dimensional and three-dimensional grids. An efficient plane-wave source is simulated on one-dimensional grid and a perfect match can be achieved for a plane-wave propagating at any angle forming an integer grid cell ratio. Numerical simulations show that the method is valid for SFDTD and the residual field in SF region is shrinked down to -300 dB.     

时域磁场积分方程时间步进算法稳定性研究

从时域磁场积分方程时间步进算法出发,结合范数概念,从理论上推导得到了感应电流稳定的充分条件. 满足该条件,可以保证任何形式的入射波入射时,计算结果都后时稳定. 同时通过推导得到了表征感应电流递推关系的因子,该因子的大小可以表征感应电流收敛性的相对好坏. 最后通过数值算例验证了感应电流稳定的充分条件, 以及利用本文推导的感应电流递推因子表征感应电流后时稳定性的准确性. 关键词： 时域磁场积分方程 时间步进算法 后时不稳定性     

Time-Domain Numerical Solutions of Maxwell Interface Problems with Discontinuous Electromagnetic Waves

This paper is devoted to time domain numerical solutions of two-dimensional (2D) material interface problems governed by the transverse magnetic (TM) and transverse electric (TE) Maxwell's equations with discontinuous electromagnetic solutions. Due to the discontinuity in wave solutions across the interface, the usual numerical methods will converge slowly or even fail to converge. This calls for the development of advanced interface treatments for popular Maxwell solvers. We will investigate such interface treatments by considering two typical Maxwell solvers – one based on collocation formulation and the other based on Galerkin formulation. To restore the accuracy reduction of the collocation finite-difference time-domain (FDTD) algorithm near an interface, the physical jump conditions relating discontinuous wave solutions on both sides of the interface must be rigorously enforced. For this purpose, a novel matched interface and boundary (MIB) scheme is proposed in this work, in which new jump conditions are derived so that the discontinuous and staggered features of electric and magnetic field components can be accommodated. The resulting MIB time-domain (MIBTD) scheme satisfies the jump conditions locally and suppresses the staircase approximation errors completely over the Yee lattices. In the discontinuous Galerkin time-domain (DGTD) algorithm – a popular Galerkin Maxwell solver, a proper numerical flux can be designed to accurately capture the jumps in the electromagnetic waves across the interface and automatically preserves the discontinuity in the explicit time integration. The DGTD solution to Maxwell interface problems is explored in this work, by considering a nodal based high order discontinuous Galerkin method. In benchmark TM and TE tests with analytical solutions, both MIBTD and DGTD schemes achieve the second order of accuracy in solving circular interfaces. In comparison, the numerical convergence of the MIBTD method is slightly more uniform, while the DGTD method is more flexible and robust.     

乳腺癌检测的三维微波热声成像技术

提出了三维微波热声成像技术检测早期乳腺肿瘤,成像同时具有高对比度和高分辨率的特点.该技术利用自适应的鲁棒Capon波束成形(RCB)算法进行信号处理和图像重建.基于电磁学和声学时域有限差分(FDTD)法,进行微波热声成像仿真,算例结果证实了该技术用于诊断早期乳腺肿瘤的可行性. 关键词： 乳腺癌检测 微波热声成像 鲁棒的Capon波束成形 时域有限差分     

Beaming of light through depth-tuned plasmonic nanostructures

A nanostructure comprising dielectric/Ag interface with central slit surrounded by grooves is presented for beam focusing and collimating. Desired phase retardations of the beam emerging through slits are manipulated by tuning the depth of grooves. Numerical simulations are performed through a finite-difference time-domain (FDTD) algorithm. Results reveal the suitability of parabolic depth-tuned structure for enhanced transmission and beam collimation. Enhanced transmission and beam collimation are the result of transportation of electromagnetic energy in the form of surface plasmon polariton (SPP) waves.     

SAR EVALUATION IN THE 0.25 MM HUMAN EYE MODEL EXPOSED TO 30 GHz MILLIMETER WAVE

We have evaluated the specific absorption rate (SAR) in the eye induced by the perpendicular incidence of 30 GHz uniform plane millimeter wave (MMW). An anatomically based 0.25 mm voxel model of the eye with 10 types of tissue is constructed. Debye’s dispersion equation is used to derive the dielectric properties of the eye tissues and biological materials in the neighboring region. The three-dimensional (3-D) finite-difference time-domain (FDTD) algorithm and the perfectly matched layer (PML) absorbing boundary conditions are used in the numerical simulation. The FDTD code is validated against the Mie theory. The SAR deposition in the eye is calculated and displayed, with the analysis on the SAR distribution characteristics. The average SAR values in the cornea, the lens cortex, the lens nucleus, the humor, and the whole eye are statistically derived and examined against present irradiation safety standard.     

Terahertz Time-Domain Spectroscopy of Solids: A Review

Recent development of the terahertz time domain spectroscopy (THz-TDS) and its application to solids have been reviewed. This spectroscopy is unique in that the time-domain wave forms are measured at first and the complex optical constants are deduced directly by the Fourier transformation of them without resort to the Kramers-Kronig analysis. Various types of the THz-TDS systems are briefly described. Applications of the THz-TDS to various solids, i.e., semiconductors, superconductors, polymers, photonic crystals, and so on are also presented to demonstrate how widely this spectroscopy is applicable to characterization of solids.