四面体磁梯度张量系统的载体磁干扰补偿方法

针对搭载于水下无人航行器(UUV)的四面体磁梯度张量系统易受载体磁场干扰的问题,提出了一种载体磁干扰补偿方法.该方法在载体磁干扰产生机理的基础上,利用磁梯度张量差分测量算法融合四面体磁梯度张量系统中四个矢量磁力仪的载体磁干扰,建立了磁梯度张量系统载体磁干扰数学模型;然后在此数学模型的基础上提出了磁干扰补偿方法,并根据磁梯度张量9分量的数学关系提出了补偿参数辨识方法;最后通过仿真实验对方法进行了验证,结果表明该补偿方法可以有效补偿磁梯度张量系统95.9%的载体磁干扰.该方法利用补偿参数对磁梯度张量系统的输出值直接进行磁干扰补偿,从理论上解决了磁梯度张量系统中各个矢量磁力仪载体磁干扰的统一补偿问题.     

超导磁梯度张量探头结构优化方法研究

针对传统超导磁梯度张量探头中多个磁梯度计间的噪声互扰会降低系统灵敏度的问题,本文提出了一种基于二维傅里叶变换的磁梯度张量探头结构优化方法。通过二维傅里叶变换及逆变换将磁场矢量Z分量沿着Z轴方向的二阶空间导数B_zzz转换为磁场矢量的3个分量以及一阶磁梯度张量的5个独立元素,把通常至少需要6个SQUID一阶平面磁梯度计的探头结构简化为只需要1个二阶SQUID磁梯度计的探头结构,消除了磁梯度计之间的噪声干扰,系统灵敏度提高了1个数量级。实验结果表明该探头具有良好的探测能力,证明了这种优化方法的可行性。     

基于SQUID梯度计的单磁源定位及磁矩反演误差分析

磁源定位及磁矩反演技术在空间探测、无损检测以及目标追踪等领域有着广泛的应用前景。利用磁场梯度张量信息进行磁源定位及磁矩反演的思想是在1975年首次提出的,2006年提出的磁性目标线性方程定位方法以其快速准确求解的特点而备受关注。但是,对于该梯度张量矩阵求逆法进行磁源定位的全方位的误差分析仍有待完善。在简要回顾梯度张量矩阵求逆法进行磁源定位的方法的基础上,提出了一种基于该方法的系统误差和随机误差分析的理论;研究结果表明定位误差是由于将差分近似为微分而引起的系统误差和传感器测量的随机误差构成。以目前具有超高精度、超高灵敏度的超导量子干涉仪作为典型磁场测量传感器,分析了梯度张量矩阵求逆法进行磁源定位及磁矩反演误差与传感器分辨率和相对误差水平之间的关系。     

基于速度梯度张量的四元分解对若干涡判据的评价

本文基于速度梯度张量分析,对其中四种ω判据、Q判据、w判据、λci判据的物理意义和局限性进行分析,揭示各判据常用等值面展示的涡形态或强度的实际物理意义.首次采用基于速度梯度张量正规性的四元分解,将流体微元的运动分解为胀缩、沿正规标架的轴向变形、做平面运动和简单剪切,使得各涡判据的运动学意义更加清晰.涡量ω反映的流体微元的平均转动中总是包含简单剪切运动;Q判据可揭示流体微元在复特征向量平面上净转动相对于轴向变形的强弱,是净转动存在的充分但非必要条件;fi判据能准确辨别净转动是否存在,却无法表示出净转动的强度;在净转动存在的前提下,λci可反映其绝对强度大小,净转动是复特征向量平面内正规转动和简单剪切的总和效果,正规转动是最基本的转动.新引入的四元分解方法有利于深入了解流体的涡及其运动.     

磁共振扩散张量成像中扩散敏感梯度磁场方向分布方案的研究进展

磁共振扩散张量成像可以定量无创研究人体内水分子在三维空间中的各向异性扩散规律,进而获取重要的病理及生理信息.为了得到水分子各向异性扩散信息,需要按照一定的方案依次施加不同方向的扩散敏感梯度磁场,测量水分子在这些方向上的扩散系数用以估算扩散张量.扩散张量成像测量结果的准确程度受梯度磁场方向分布方案的影响,本文对扩散敏感梯度磁场方向分布方案进行综述,包括完全随机方案、启发式方案、规则多面体式方案和数值优化方案等,分析这些方案的优势与局限性,并提出需进一步研究的问题.     

核磁共振水分子扩散张量成像中基于广义Fibonacci数列的扩散敏感梯度磁场方向分布方案

为了准确得到人体内水分子各向异性扩散信息,在核磁共振扩散张量成像及高角分辨率扩散成像实验中,需要在众多空间均匀分布的方向上依次施加扩散敏感梯度磁场,测量水分子在不同方向上的扩散系数.目前方向分布方案的缺点有方向数目不连续、均匀性有待提高及部分方向数据的损坏会影响整个数据集等.本文以广义Fibonacci数列为基础,提出新的可以产生连续方向数目的扩散敏感梯度磁场方向分布方案,整个方案的方向均匀性较好,数据集内的部分数据仍然具有很好的空间均匀性,而且本方案中相邻两个扩散敏感梯度磁场方向接近相反,可以减小快速变化的高强度梯度磁场产生的涡流对结果的影响.     

从不变量看电磁场

本文从电磁场不变量的角度 ,考察电磁场 ,并与时 -空间隔不变量类比 ,达到和谐统一的认识高度     

CTF磁压缩器的误差研究

磁压缩器广泛应用在自由电子激光装置中. 电子束团的归一化发射度增长由二极磁铁的高阶分量误差及准直误差引起. 本文描述了CTF (CXFEL Test Facility)的第一个磁压缩器的误差研究.     

基于结构张量的变分多源图像融合

提出了可以保持源图像特征和细节信息的基于结构张量的变分多源图像融合算法. 首先叙述基于结构张量的融合梯度场, 然后测量每幅源图像的特征图, 根据特征图为源图像的每个梯度构造一个权值, 将携带明显特征的梯度在融合的梯度场中凸显出来, 从而使源图像的特征和细节得到保持, 最后应用变分偏微分方程理论从目标梯度场重建出融合的图像. 实验结果表明, 本文算法融合图像的灰度平均梯度和信息熵均高于小波变换算法、塔分解法和直接梯度融合算法, 视觉效果上,融合图像很好的保留了源图像的特征和细节, 为图像目标检测和识别提供了高质量的图像信息. 关键词： 图像融合 梯度场 结构张量 变分法     

Translation invariant tensor product states in a finite lattice system

We show that the matrix (or more generally tensor) product states in a finite translation invariant system can be accurately constructed from a same set of local matrices (or tensors) that are determined from an infinite lattice system in one or higher dimensions. This provides an efficient approach for studying translation invariant tensor product states in finite lattice systems. Two methods are introduced to determine the size-independent local tensors.     

Effect of a weak static magnetic field on nitrogen-14 quadrupole resonance in the case of an axially symmetric electric field gradient tensor

The application of a weak static B₀ magnetic field (less than 1 mT) may produce a well-defined splitting of the ¹⁴N Quadrupole Resonance line when the electric field gradient tensor at the nitrogen nucleus level is of axial symmetry. It is theoretically shown and experimentally confirmed that the actual splitting (when it exists) as well as the line-shape and the signal intensity depends on three factors: (i) the amplitude of B₀, (ii) the amplitude and pulse duration of the radio-frequency field, B₁, used for detecting the NQR signal, and (iii) the relative orientation of B₀ and B₁. For instance, when B₀ is parallel to B₁ and regardless of the B₀ value, the signal intensity is three times larger than when B₀ is perpendicular to B₁. This point is of some importance in practice since NQR measurements are almost always performed in the earth field. Moreover, in the course of this study, it has been recognized that important pieces of information regarding line-shape are contained in data points at the beginning of the free induction decay (fid) which, in practice, are eliminated for avoiding spurious signals due to probe ringing. It has been found that these data points can generally be retrieved by linear prediction (LP) procedures. As a further LP benefit, the signal intensity loss (by about a factor of three) is regained.     

高精度光纤陀螺零位误差的磁温特性研究

介绍了高精度光纤陀螺零位误差的形成原理,并就温度和磁场对高精度光纤陀螺零位误差的影响进行了详细的理论分析。仿真结果表明:由于温度引起的零偏峰值漂移为0.06°/h。最后,对交变温度场和径向静磁场交联作用下的情况建立了理论模型并进行了实验研究,实验结果表明:在径向磁场和不同温度作用下,光纤陀螺由于交联耦合效应产生的径向磁场灵敏度变化,即交联耦合度<1%,这和理论模型具有较高的一致性。     

磁场梯度对Hall推力器放电特性影响的实验研究

为进一步探索Hall推力器通道内磁场优化设计理论,通过实验分析了强场区磁场梯度对推进剂的电离与加速等放电过程的影响. 研究发现,在本实验设计的磁场梯度范围内,磁场梯度大小对推进剂的电离过程影响较小,但是对离子流的加速特性会产生较为明显的影响.随着磁场梯度的增加,离子束的能量分布会趋于集中,推力器效率提高. 最后,对磁场轴向梯度进一步变大可能会引起的一系列物理问题如有限Larmor半径效应、电子传导机理转变规律和梯度漂移效应等进行了分析和思考.     

大口径方形离轴非球面的非球面度计算与应用研究

采用不同的数值计算方法求解了大口径(340mm×340mm)方形离轴非球面的最接近球面曲率半径和非球面度,得出了各种不同的磨削量及其对应的分布结构。对不同的最接近球面及非球面度所适用的加工工艺和检测方法进行了分析和评价。这对离轴非球面的加工和检测具有一定的指导意义。     

Study of electron temperature gradient instability in toroidal plasmas with negative magnetic shear

Electron temperature gradient driven instability in toroidal plasmas with negative magnetic shear is studied. Full electron kinetics is considered, and the behaviours of the modes and corresponding turbulent transport in the parameter regimes close to the instability threshold are emphasized. Growing and damped modes are both investigated. The singular points of the integrand are disposed of, and the fitting formulae for the critical gradient are given. The theoretical results are shown to be close to the experimental observations.     

Experiment on relationship between the magnetic gradient of low-carbon steel and its stress

In geomagnetic field, a series of tensile experiments on the low-carbon steel sticks were carried out. A special homemade detector was used to measure the magnetic gradient on the material surface. The results showed that the relationship between the magnetic gradient and the stress varied with different conditions of measurement. There was no obvious correlation between the magnetic gradient and the tensile stress if the sample remained on the material test machine. If the sample was taken off from the machine, the measured magnetic gradient was linear with the prior maximum stress. In Nanjing, PR China, a place of 32°N latitude, the slope of the linear relationship was about 67 (uT/m)/MPa. This offered a new method of non-destructive stress testing by measuring the magnetic gradient on the ferromagnetic component surface. The prior maximum applied stress of the sample could be tested by measuring the present surface magnetic gradient. Actually this phenomenon was the metal magnetic memory (MMM). The magnetic gradient near the stress concentration zone of the sample, the necking point, was much larger than other area. Thus, the hidden damage in the ferromagnetic component could be detected early by measuring the magnetic gradient distribution on its surface. In addition, the magnetic memory signal gradually weakened as the sample was taken off and laid aside. Therefore, it was effective for a given period of time to detect the stress or stress concentration based on the MMM testing.