利用偏置磁极周期会切磁铁产生边聚焦场

 利用理论分析和数值计算的方法,研究了偏置磁极周期会切磁铁产生的、可用于带状电子束宽边聚焦的边聚焦场。结果表明:增加偏置长度,减小电子束通道宽度,增大磁极轴向长度可以在基本不改变边聚焦场在x方向上分布特性的前提下提高幅值;增大电子束通道的高度,增大磁极厚度均可以减小边聚焦场在x方向上分布曲线的曲率,但同时其幅值也会降低,该降低可利用前述方法予以补偿。在对带状电子束宽边聚焦进行束匹配时,可以先进行曲率匹配,再进行幅值匹配。在进行参数选择时,应合理选择束通道高度和磁极厚度的取值,以避免束通道内的边聚焦场在x方向上的分布出现曲率反向。     

激光空间相干合成效果评价方法的研究

根据目标上实际最大光强与理想合成效果下最大光强的比值定义了合成效果因子,用以评价相干合成效果.在此基础上研究了影响相干合成效果的主要因素,并对合成效果因子的传输特性进行了研究.结果表明:阵列紧密程度对合成效果影响不大,在较小的空间占空比下有可能得到较好的合成效果.影响合成效果的主要因素是随机相位差,当随机相位差增大到一定程度时,光束合成其实已经由相干合成过渡到了非相干合成,得不到预期的相干合成效果;随着传输距离的增大,空间占空比对合成效果因子的影响将进一步减小,影响合成效果的主要因素之一是随机相位差.     

双曲余弦高斯列阵光束在湍流大气中的光束传输因子

本文推导出了双曲余弦高斯(ChG)列阵光束在湍流大气中的光束传输因子( M ^{2因子)的解析公式,并采用相对 M 2因子研究了湍流对 M 2因子的影响.研究表明,在湍流大气中 M 2因子不再是一个传输不变量,湍流使得 M 2因子增大.非相干合成情况下, M 2因子随着传输距离、光束参数、相对子光束间距和子光束数目的增大而增大.相干合成情况下, M 2因子随光束参数和相对子光束间距的增大呈现振荡上升.相干合成情况下的 M 2因子比 关键词： M2因子)')" href="#">光束传输因子(M2因子) 光束质量 双曲余弦高斯列阵光束 大气湍流}     

基于M2因子测量的大气湍流参数的确定方法

基于广义惠更斯-菲涅尔原理以及大气湍流理论,推导出部分相干光束在大气湍流中传输的光束传输M ^{2因子的解析表达式.定量分析了表征大气湍流参数的折射率结构常数 C2_n}和涡旋内尺度 l _{0对 M ^{2因子的影响,并由此提出了一种通过实验测量大气湍流中光束的 M 2因子,进而确定出大气湍流参数的新方法.研究结果表明,由于大气湍流对相干性好的光束影响更为明显,在测量中可采用具有高相干性的基模高斯光束作为测量光源,而测量装 关键词： 2因子')" href="#">光束传输 M 2因子 大气湍流参数 湍流折射率结构常数 湍流涡旋内尺度}}     

三层单负材料结构中电磁波模分析

本文分析了由负介电常数材料和负磁导率材料交替排列组成的一维三层结构中的电磁波模式,包括传播模及波导模.传播模对应于入射的行波模,着重研究了发生共振隧穿时场分布和透射谱随入射角度的变化关系,发现改变结构参数时共振隧穿频率会劈裂成两个,且这两个隧穿模的频率间距随着中间层厚度的减小而逐渐加大.波导模是指在结构两边的半无限真空中以倏逝场形式存在的电磁模式,分析了它的存在条件即色散关系,发现这种模在材料的交界面处会出现较强的局域场. 关键词： 单负材料 共振隧穿 传播模 波导模     

太赫兹波在一维反铁磁/电介质准周期光子晶体的传输性质

本文利用传递矩阵方法研究了电磁波倾斜入射时,一维反铁磁/电介质准周期光子晶体的透射性质.反铁磁层和电介质层按Fibonacci数列排列构成准周期光子晶体.通过数值模拟发现光子禁带中有透射峰出现, 而且随着准周期光子晶体级数的增加透射峰的数目也在增加.此外,透射峰受电介质层的介电常数、入射角度和电磁波偏振影响明显. 关键词： 反铁磁层 准周期光子晶体 太赫兹波传输性质     

Long distance laser ultrasonic propagation imaging system for damage visualization

Wind turbine blade failure is the most prominent and common type of damage occurring in operating wind turbine systems. Conventional nondestructive testing systems are not available for in situ wind turbine blades. We propose a portable long distance ultrasonic propagation imaging (LUPI) system that uses a laser beam targeting and scanning system to excite, from a long distance, acoustic emission sensors installed in the blade. An examination of the beam collimation effect using geometric parameters of a commercial 2 MW wind turbine provided Lamb wave amplitude increases of 41.5 and 23.1 dB at a distance of 40 m for symmetrical and asymmetrical modes, respectively, in a 2 mm-thick stainless steel plate. With this improvement in signal-to-noise ratio, a feasibility study of damage detection was conducted with a 5 mm-thick composite leading edge specimen. To develop a reliable damage evaluation system, the excitation/sensing technology and the associated damage visualization algorithm are equally important. Hence, our results provide a new platform based on anomalous wave propagation imaging (AWPI) methods with adjacent wave subtraction, reference wave subtraction, reference image subtraction, and the variable time window amplitude mapping method. The advantages and disadvantages of AWPI algorithms are reported in terms of reference data requirements, signal-to-noise ratios, and damage evaluation accuracy. The compactness and portability of the proposed UPI system are also important for in-field applications at wind farms.     

An improved algorithm for detecting point of impact in anisotropic inhomogeneous plates

Conventional triangulation techniques fail to correctly predict the acoustic source location in anisotropic plates due to the direction dependent nature of the elastic wave speeds. To overcome this problem, Kundu et al. [1] proposed an alternative method for acoustic source prediction based on optimizing an objective function. They defined an objective function that uses the time of flight information of the acoustic waves to the passive transducers attached to the plate and the wave propagation direction (θ) from the source point to the receiving sensors. Some weaknesses of the original algorithm proposed in Ref. [1] were later overcome by developing a modified objective function [2]. A new objective function is introduced here to further simplify the optimization procedure and improve the computational efficiency. A new algorithm for source location is also introduced here to increase the source location accuracy. The performance of the objective function and source location algorithm were experimentally verified on a homogeneous anisotropic plate and a non-homogeneous anisotropic plate with a doubler patch. Results from these experiments indicate that the new objective function and source location algorithm have improved performance when compared with those discussed in Refs. [1] and [2].     

可控空心光束在单轴晶体中的传输特性

基于光束在各向异性单轴介质中的傍轴矢量传输理论,对可控空心光束(CDHB)在单轴晶体中的传输特性作了研究.得出该光束在单轴晶体中传输的横向慢变振幅的表达式,并利用此表达式作数值计算,研究了CDHB在单轴晶体中的传输特性.研究表明,可控空心光束在单轴晶体中传输时,控制参数ξ和光束阶数N的取值对其光强分布有较大的影响.随着...     

Akhmediev breather evolution in optical fiber for realistic initial conditions

We study numerically Akhmediev breather dynamics in optical fibers under initial conditions that do not correspond to an ideal infinitesimal modulation on a plane wave. We examine two modifications that are commonly encountered in realistic experiments: (i) a finite-amplitude (not necessarily weak) initial modulation; (ii) the use of pulsed excitation. In the first case, we derive an expression for the propagation distance required to generate a train of compressed breather pulses. In the second case, we show that breather dynamics with pulsed excitation can be interpreted in terms of local breather states at different points on the pulse envelope.