【作者】 徐利军；

【导师】 袁乃昌；

【作者基本信息】 国防科学技术大学 ， 电子科学与技术， 2006， 博士

【摘要】 等离子体技术作为一种全新的隐身手段,由于它不涉及飞行器本身的空气动力特性、可隐身性以及实际应用方面的廉价性,尤其是对于现役武器系统的易隐身改造等一系列优点,使得它得到世界很多国家的高度重视。同时,等离子体隐身技术是一项非常有希望但又十分复杂的系统工程,它涉及到等离子体产生技术、电磁理论与工程、雷达原理、机械工程等众多学科,但随着科学技术的飞速发展,隐身兵器必定会开创一片新天地,并在军事领域展示出广阔的前景。本文主要从非磁化等离子体电磁散射特性、各向异性磁化等离子体电磁散射特性以及复杂军用目标散射建模三个方面开展了相关研究工作。用交替方向隐式时域有限差分(ADI-FDTD)方法分析非磁化等离子体的电磁散射特性,研究了非磁化等离子体涂敷二维、三维导体目标后其RCS的缩减情况。用电流密度卷积-时域有限差分(JEC-FDTD)方法分析各向异性磁化等离子体的电磁散射特性,研究了各向异性磁化等离子体涂敷二维、三维典型目标及军用复杂目标后其RCS的缩减情况,这些工作在国内都是首创。本文的创造性工作及成果主要体现在以下几个方面:针对ADI-FDTD方法色散较大的缺点,提出了高阶ADI-FDTD算法,并对其增长因子相位的分析,巧妙得到数值色散关系。同时基于辅助差分方程给出一种适合于ADI-FDTD方法的完全匹配层—ADE-PML,这种匹配层,不需要在时域分裂场量。采用Z变换方法将ADI-FDTD推广应用于色散媒质,得到了二维情况下色散媒质中的迭代公式。同时给出了一种采用无限脉冲响应(IIR)滤波器结构来减少存储量的方法,并与色散媒质中的FDTD结果进行比较,在ADI-FDTD计算时间大量减少的情况下,两者结果符合很好。基于分段线性电流密度递归卷积(PLJERC)方法将ADI-FDTD推广应用于非磁化等离子体中,得到了二维情况下非磁化等离子体的迭代差分公式,为了验证该方法的有效性和可靠性,计算了等离子体涂敷导体圆柱的RCS和非均匀等离子体平板的反射系数,数据仿真结果表明,本文给出的算法与传统的FDTD相比,在计算结果吻合的情况下,存储量相当,计算效率更高,时间步长仅仅由计算精度来决定。并将此方法进一步推广到三维情况,研究了三维非磁化等离子体的电磁散射特性。提出了一种求解各向异性不均匀等离子体圆柱电磁散射的解析方法,不均匀等离子体圆柱用多层同心圆柱壳来近似模拟,每层圆柱壳的电子密度为常数,此方法简单并便于编程计算。采用此解析方法研究了各向异性分层均匀等离子体圆柱的RCS与等离子体层数、外磁场强度、等离子体碰撞频率以及圆柱轴心电子密度的关系。采用Z变换方法和JEC方法把FDTD推广应用到二维各向异性磁化等离子体中,该算法同时解决了电磁波在各向异性和频率色散介质中传播的难题,着重研究了等离子体参数对其RCS的影响,计算结果表明磁化等离子体不仅吸收频带宽,而且吸收性能更好。进一步把JEC-FDTD推广到三维各向异性磁化等离子体,这种算法既可以计算三维磁化等离子体的角域散射特性,又可以计算其宽频散射特性。计算了各向异性磁化等离子体涂敷二面角反射器和球锥体等典型三维目标后其RCS的变化情况,分析了电子回旋频率对其RCS的影响。计算结果表明磁化等离子体吸收性能更好。分别给出了基于实体模型和面元模型的FDTD自动网格剖分技术,两种方法机理完全相异,分别针对不同的目标模型,剖分平台也不同。其中,基于面元模型的剖分方法更具独立性,利于建模、剖分、计算程序的一体化集成。我们在此基础上研究磁化等离子体涂敷三维复杂军事目标的电磁散射特性,分析磁化等离子体的隐身性能。我们主要给出两个算例:等离子体涂敷导弹模型和等离子体涂敷军用飞机模型。更多还原

【Abstract】 The stealth technology of plasma has attracted much attention from many countries around the world, due to the potential application of plasma as absorbers of electromagnetic waves. At the same time, the plasma stealth is also a promising and complex system engineering, it involves in many subjects, such as: the plasma generator technique, the theory and engineering of electromagnetic field, radar theory, mechanical and electrical engineering, and so on. But with the development of the science and the technology, the plasma stealth must exhibit expansive future.The research works of this dissertation are at the three parts: the scattering of unmagnetized plasma, the scattering of anisotropic magnetized plasma and the scattering modeling method of complex targets. The alternating direction implicit finite difference time domain (ADI-FDTD) method is applied to analyze the scattering of unmagnetized plasma. The RCS reduction for the two dimensional, three dimensional conducting target coated by unmagnetized plasma is studied. Furthermore, the finite difference time domain (FDTD) method is applied to analyze the scattering of anisotropic magnetized plasma. The RCS reduction for the two dimensional, three dimensional conducting target coated by anisotropic magnetized plasma is studied. The main innovations of this paper are as follows:The ADI-FDTD method has a larger dispersion error compared with the regular FDTD method. To improve the accuracy of the ADI-FDTD method, we introduce the higher order ADI-FDTD method. The dispersion relation of the higher order ADI-FDTD method can be derived by analyzing the phase. In addition, a new perfectly matched layer based on the auxiliary differential equation is presented for the ADI-FDTD method. The ADE-PML formulations only require two additional auxiliary variables per field component in the PML region without the need of splitting the field in the time domain. Two dimensional numerical examples are included to validate the proposed formulations.The ADI-FDTD method is extended to dispersive media based on the Z-transform method. Two-dimensional ADI-FDTD formulations for dispersive media are derived. And the same time, a method based infinite-impulse response digital filter (IIR) is presented to reduce memory requirements. The proposed method is applicable to arbitrary Mth-order dispersive media. Finally, some examples are calculated, the numerical results of ADI-FDTD for dispersive media are in good agreement with the results obtained by conventional FDTD method, but compared with conventional FDTD method, the proposed method is efficient without requiring additional memory.The ADI-FDTD method is extended to dispersive media—isotropic plasma based on the PLJERC(Piecewise Linear JE Recursive Convolution) method. Two-dimensional ADI-FDTD formulations for isotropic plasma are derived. Two examples are calculated, the numerical results of the ADI-FDTD method for isotropic plasma are compared to those obtained by the FDTD method to show the efficiency of the proposed method. And this method is applied to study the scatteing characteristics of two dimensional, three dimensional unmagnetized plasma.An analytical technique for solving the scattering of an anisotropic nonuniform plasma cylinder is presented. The plasma model chosen for the study is cold, nonuniform, collisional and magnetized. The nonuniform plasma cylinder is represented by a number of concentric cylindrical shells and each has a fixed electron density. The overall density profile follows any prescribed distribution function. The effect of the plasma parameters such as the number of layers, the collision frequency, and the central density on the backscattering cross section is investigated.The JE convolution fmite-difference-time-domain (JEC-FDTD) method is extended to the anisotropic magnetized plasma which incorporates both anisotropy and frequency dispersion at the same time, enabling the transient solution of the electromagnetic wave propagation in anisotropic magnetized plasmas. Two-dimensional JEC-FDTD formulations for magne-tized plasma are derived. The back scattering radar cross section (RCS) of a perfectly conducting cylinder coated by a layer of magnetized plasmas is calculated.The JEC-FDTD method is extended to three dimensional anisotropic dispersive media—magnetized plasma. The problem which incorporates both anisotropy and frequency dispersion at the same time is solved for the electromagnetic wave propagation. The three dimensional JEC-FDTD formulations for anisotropic magnetized plasma are derived. The method is applied to the electromagnetic scattering of dihedral corner reflector and sphere-cone coated with anisotropic magnetized plasma. By simulating the interaction of electromagnetic wave with magnetized plasma, some numerical results are obtained, which indicate that an appropriate plasma coating may efficiently reduce the RCS of a metallic target.As a supplement to the FDTD meshing technique based on solid model, which is not easy to combine with the FDTD program, a new meshing technique based on surface model was put forward. Using this technique, we can mesh most of the target models without caring about what file format they use or where they come from. The electromagnetic characteristics of the three dimensional large complex military objects coated by anisotropic mangtized plasma are investigated. Two models are calculated. They are the missile model and the airplane model coated by anisotropic magnetized plasma.更多还原