电子束二极管脉冲磁场的涡流分析及抑制

针对强流脉冲电子束二极管金属表面涡流引起的器件内部磁场分布不均、电子束辐照材料表面处理效果不佳等现象,提出了多板钢联接式内壁与玻璃外壁相结合的双层结构设计。基于有限元仿真软件对电子束系统的磁场与涡流进行了数值计算,分析了涡流的影响因素和涡流对于电子束源磁场的影响。通过数值计算表明,设计的新型介质壁结构能有效降低涡流对于电子束的影响。该分析为消除感应涡流对真空二极管内部磁场的影响提供了理论依据,具有良好的参考价值。     

三相行波磁场中涂层超导线圈的电-磁-力特性研究

随着超导线圈在电力领域中的应用越来越广泛,该文利用有限元仿真软件对处于交变磁场中载流高温超导带材的电-磁-力特性进行了研究,通过超导体的非线性E-J本构关系和基于棱边元法的磁场强度法来求解超导线圈内的电流分布、线圈周围的磁场分布及其受到的电磁力大小等电磁特性,重点研究了在以涂层超导磁体为励磁线圈的直线牵引系统中超导线圈和三相绕组之间的电磁力大小.基于仿真结果,分析了超导线圈内电流分布随气隙的变化关系;线圈周围的磁场分布情况及其对超导线圈临界电流的影响;线圈受到的电磁力随时间和气隙的变化关系等,所得结果为高温超导直线电机的设计和应用提供参考依据.     

基于FPGA的CMP电涡流终点检测装置设计

为实现对晶圆表面金属层的化学机械抛光（CMP）过程中的终点检测和对抛光速率进行监控的要求,设计了一种基于电涡流测量原理的测量装置。该装置以FPGA器件作为控制核心,由其控制高速D/A转换器生成正弦交流信号,并驱动测量电桥。由于测量线圈产生的交变磁场在晶片金属薄膜上产生电涡流,引起测量线圈的阻抗发生变化。通过测量相应的阻抗变化产生的信号,可以计算出相应的晶片表面金属薄膜的厚度。实验表明该装置可以满足对晶圆表面100～1000nm厚度金属层的测量要求。     

涡流及其应用

 大块金属在交变磁场中或相对磁场运动时,在金属内会出现流线为闭合涡旋状的感应电流,该电流叫涡旋电流,简称涡流。一、涡流产生的原因由法拉第电磁感应定律知,当通过闭合回路的磁通量发生变化时,将产生感生电动势,形成感生电流。由于金属内部处处可以构成回路,当大块金属处在变化着的磁场中或相对磁场运动时,穿过金属任意回路的磁通量都可能发生变化,在磁通量变化过程中,金属块内将产生感应电流,这种电流的流线在金属块内自行闭合,形成涡流。     

MFCG 2维磁流体力学程序中磁场计算的改进

 定子线圈中的电流在空间中产生的磁场是金属导体（套筒）膨胀压缩的主要对象，该磁场直接影响爆磁压缩发生器输出电流脉冲的大小。给出了面电流分布下2维磁场的计算公式，较点电流分布下的磁场公式更能细致准确地反映出磁场的变化。数值结果表明:面电流分布下的磁场公式不仅能准确地描述磁场的变化，而且计算效率大大提高，节约了计算时间。     

用于超导脉冲变压器的双饼线圈交流损耗优化

基于二维轴对称H方程法对用于高温超导脉冲变压器的YBCO双饼线圈进行了有限元建模与仿真,研究了分磁环参数和开槽数对YBCO双饼线圈传输原边电流时的磁场特性和交流损耗的影响。仿真结果表明,分磁环能够显著减弱双饼线圈的径向磁场,降低其交流损耗。对分磁环开槽后,径向磁场主要分布在开槽处。当分磁环相对磁导率为100,与线圈间距为0.25 mm,厚度为1 mm,开槽数为3时,双饼线圈交流损耗降低了64.9%。     

霍尔推力器磁场位形及其优化的数值研究

基于麦克斯韦方程,在轴对称假设下建立了霍尔推力器磁场的数学模型.用有限差分方法对模型进行了离散.给出了数值求解模型的迭代法.通过对模型的数值求解,得到了相应的数值结果.通过对所得数值结果的分析,研究了磁场线圈电流变化对霍尔推力器磁场位形的影响.通过调整磁场线圈电流的大小找到了理想磁场位形.研究表明,对于理想磁场位形,内通道的磁镜比在3—3.5之间,外通道的磁镜比在0.4—0.9之间;增加磁场线圈的电流,出口的磁场强度随着增加,但不能增加磁镜比.通道内部的磁场强度几乎不随着磁场线圈电流的变化而变化. 关键词： 霍尔推力器 磁场位形 磁场线圈电流 磁镜比     

对亥姆霍兹线圈磁场分布的分析

利用DH4501D实验仪探究了亥姆霍兹线圈轴线上和轴线外的磁场分布问题。首先通过对单个线圈不同电流下的磁场分布的实验值与理论值对比发现电流大的磁场更接近理论值,更适合用来研究磁场分布问题。然后利用较大电流分别研究了单个线圈以及亥姆霍兹线圈轴线上和轴线外的磁场分布的规律及产生原因。最后利用磁场分布拟合了真空磁导率并与理论值进行了对比。通过对这些问题的研究不仅加深了对毕奥-萨伐尔定律的理解,更加明确了亥姆霍兹线圈的磁场分布规律。     

脉冲磁场中波导管涡流影响的数值模拟

 提出了脉冲磁场中波导管涡流影响的数值模拟方案，编制了EDDYC程序，研究了波导管电导率、管径、壁厚和激励电流角频率等对波导管涡流的影响，对将在二极管-速调管实验中使用的不锈钢（1Cr18Ni9Ti）波导管的涡流影响进行了模拟。     

多脉冲磁场下含热开关高温超导环的励磁研究

针对堆叠式高温超导磁体的持续电流模式运行,提出一种由多脉冲磁场和热开关组合的励磁方法。在77 K温度下对高温超导环进行了系统的励磁实验研究,并采用有限元方法建立模型对高温超导环中的电流及产生的磁场进行了仿真。励磁结束后高温超导环产生的磁场稳定,磁场大小与多脉冲磁场幅值呈正相关。结果表明,该励磁方法可行,有望应用于堆叠式高温超导磁体的持续电流模式运行。     

Improved convection compensating pulsed field gradient spin-echo and stimulated-echo methods

The need for convection compensating methods in NMR has been manifested through an increasing number of publications related to the subject over the past few years (J. Magn. Reson. 125, 372 (1997); 132, 13 (1998); 131, 126 (1998); 118, 50 (1996); 133, 379 (1998)). When performing measurements at elevated temperature, small convection currents may give rise to erroneous values of the diffusion coefficient. In work with high resolution NMR spectroscopy, the application of magnetic field gradients also introduces an eddy-current magnetic field which may result in errors in phase and baseline in the FFT-spectra. The eddy current field has been greatly suppressed by the application of bipolar magnetic field gradients. However, when introducing bipolar magnetic field gradients, the pulse sequence is lengthened significantly. This has recently been pointed out as a major drawback because of the loss of coherence and of NMR-signal due to transverse relaxation processes. Here we present modified convection compensating pulsed field gradient double spin echo and double stimulated echo sequences which suppress the eddy-current magnetic field without increasing the duration of the pulse sequences.     

轴线起爆式螺线管型爆磁压缩发生器理论模型

 对轴线起爆式螺线管型爆磁压缩发生器进行了理论模型研究，建立了爆炸管的一维爆轰驱动模型、螺线管内空间磁场强度分布模型、爆炸管外表面磁压力模型和发生器系统的等效电路模型等，对此类发生器的物理过程进行系统描述。在此基础上，编制了相应的零维数值模拟程序CEMG 1.0，利用该程序分别对四种不同模型参数的发生器进行了理论计算和参数优化，并对其中一模型发生器爆炸管外表面的磁压力及其引起的剩余电感进行了计算，给出了剩余电感与初始输入条件及负载电感的关系，从而得到该模型的输出性能极限。对理论模型的正确性进行了实例验算证明。     

New Method for 3D Transient Eddy Current Field Calculation and Its Application in Magneto-Acoustic Tomography

A new method of 3 D transient eddy current field calculation is proposed. The Maxwell equations with time component elimination(METCE) are derived under the assumption of magnetic quasi static approximation, especially for the sample of low conductivity. Based on METCE, we deduce a more efficient reconstruction algorithm of a3 D transient eddy current field. The computational burden is greatly reduced through the new algorithm, and the computational efficiency is improved. This new algorithm decompounds the space-time variables into two individual variables. The idea is to solve the spatial vector component firstly, and then multiply it by the corresponded time component. The iterative methods based on METCE are introduced to recover the distribution of conductivity in magneto-acoustic tomography. The reconstructed images of conductivity are consistent with the original distribution, which validate the new method.     

一种磁场优化结构电磁超声传感器设计*

为了进一步增强电磁超声检测技术在管道厚度测量领域的检测能力,该文对电磁超声传感器(EMAT)的结构进行了优化。提出了多磁铁对称分布型EMAT,能实现更小的磁铁体积,产生更强的表面剩磁强度。采用在硅钢表面开槽的方式限制涡流形成的区域,解决了涡流对测量的影响。建立厚度测量实验系统,对比出单磁铁型与多磁铁对称分布型EMAT在不同提离距离上检测信号的变化规律。结果表明,多磁铁对称分布型结构可通过增强EMAT的偏置磁场达到信噪比更优的效果。采用耐高温探头外壳和钐钴磁铁,提高了EMAT探头在高温环境下的检测性能。     

Electric field distribution around the chain of composite nanoparticles in ferrofluids

Composite nanoparticles (NPs) have the ability of combining materials with different properties together, thus receiving extensive attention in many fields. Here we theoretically investigate the electric field distribution around core/shell NPs (a type of composite NPs) in ferrofluids under the influence of an external magnetic field. The NPs are made of cobalt (ferromagnetic) coated with gold (metallic). Under the influence of the external magnetic field, these NPs will align along the direction of this field, thus forming a chain of NPs. According to Laplace's equations, we obtain electric fields inside and outside the NPs as a function of the incident wavelength by taking into account the mutual interaction between the polarized NPs. Our calculation results show that the electric field distribution is closely related to the resonant incident wavelength, the metallic shell thickness, and the inter-particle distance. These analytical calculations agree well with our numerical simulation results. This kind of field-induced anisotropic soft-matter systems offers the possibility of obtaining an enhanced Raman scattering substrate due to enhanced electric fields.     

Eddy current simulation in thick cylinders of finite length induced by coils of arbitrary geometry

Eddy currents are inevitably induced when time-varying magnetic field gradients interact with the metallic structures of a magnetic resonance imaging (MRI) scanner. The secondary magnetic field produced by this induced current degrades the spatial and temporal performance of the primary field generated by the gradient coils. Although this undesired effect can be minimized by using actively and/or passively shielded gradient coils and current pre-emphasis techniques, a residual eddy current still remains in the MRI scanner structure. Accurate simulation of these eddy currents is important in the successful design of gradient coils and magnet cryostat vessels. Efficient methods for simulating eddy currents are currently restricted to cylindrical-symmetry. The approach presented in this paper divides thick conducting cylinders into thin layers (thinner than the skin depth) and expresses the current density on each as a Fourier series. The coupling between each mode of the Fourier series with every other is modeled with an inductive network method. In this way, the eddy currents induced in realistic cryostat surfaces by coils of arbitrary geometry can be simulated. The new method was validated by simulating a canonical problem and comparing the results against a commercially available software package. An accurate skin depth of 2.76 mm was calculated in 6 min with the new method. The currents induced by an actively shielded x-gradient coil were simulated assuming a finite length cylindrical cryostat consisting of three different conducting materials. Details of the temporal-spatial induced current diffusion process were simulated through all cryostat layers, which could not be efficiently simulated with any other method. With this data, all quantities that depend on the current density, such as the secondary magnetic field, are simply evaluated.     

Spatial distribution of magnetic field and persistent currents in Condon domain phase

The distributions of magnetic field above the surface of the sample due to existence of the diamagnetic domain structure are found. It is shown, that the constant magnetic induction splitting inside of a sample is caused by the magnetization current density, localised in the boundaries between adjacent domains, close to the sample surface. The properties of this current are studied. The influence of the domain wall thickness on the spatial distribution of magnetic field and magnetization current density is present. A possibility of detection of the changes in the magnetic field distribution in vacuum, close to the surface of the sample, by means of Hall probes, is discussed. The measurement of the spatial distribution of magnetic field can give lacking information about characteristic sizes of magnetic domain formation at the conditions of the strong de Haas van Alphen effect.     

Energy losses in a thin-wall tube multifilament composite superconductor carrying a transport current in a time-varying transverse magnetic field

An approach and formulae are given to calculate the hysteresis, coupling, eddy-current and Ohmic losses in a thin-wall tube multifilament composite superconductor carrying a transport current under low excitation by a homogeneous transverse time-varying magnetic field, taking into account possible filament saturation and current sharing effects. The technique can be applied to a single layer superconducting cable twisted around a thin-wall normal conducting tube.