低频脉冲磁场中磁屏蔽体磁饱和效应的试验研究

针对磁屏蔽体在低频脉冲磁场环境中可能存在的磁饱和问题,利用试验方法开展了磁饱和效应研究,证实了常规工程屏蔽体可在低频脉冲磁场环境中达到磁饱和状态,并通过观测屏蔽效能的变化获得了磁饱和规律,同时分析了磁饱和效应对屏蔽效能的影响及其与屏蔽体的材料磁导率、壳体厚度、外形尺寸等参数的关系.研究表明：磁屏蔽体屏蔽效能在磁饱和效应影响下,呈现出明显的动态变化特点,具有与屏蔽壳体磁导率类似的变化趋势;壳体厚度2 mm以内、长宽高为2 m×2 m×2 m左右的屏蔽体在上升时间为300μs、持续时间为1.2 ms的磁场环境中,达到磁饱和状态的磁化场强度约为10 mT,其磁饱和难易程度与磁导率及外形尺寸负相关,与壳体厚度正相关.试验研究结果与理论分析结论一致,可为磁屏蔽体的科学合理设计提供参考,具有较高的工程应用价值.     

Magnetic Shielded Enclosure Design in the DC and VLF Region

A review of magnetic, shielding concepts and equations as applied to ideal shield configurations has provided a basis for the analysis of the shielding effectiveness of practical shielded enclosures to dc and VLF magnetic fields. The permeability of the shield material is considered as a function of the induction, and a significantly improved method of estimating the induction and permeability of the shield is presented. The effects of multiple shell geometry are given by the equations of this analysis, which are indeterminable with a transmission line analysis. The degrading effects of other departures from ideal shield materials and configurations are analyzed qualitatively; equations for estimating the magnitude of those effects are developed where possible.     

Optimization of Anisotropic Magnetic Shielding

The concept of anisotropy is introduced in a laminar structure of magnetic sheet and the condition for minimum volume and maximum effectiveness of the magnetic shield of a spherical shell is investigated. The advantage of this structure over the conventional layered structure is shown. Figures and tables of this condition for typical shielding factor and permeability values are given.     

Shielding Circuits from EMP

It is generally impractical to filter low-frequency electromagnetic pulse (EMP) signals from victim circuits. Twisting signal pair conductors is helpful but often results in insufficient isolation. The remainder must be provided by shielding. Highly permeable ferritic materials have generally been found to provide maximum shielding from low-frequency magnetic fields. It is shown that this may not be the case when the signal source is relatively distant from the shield. With large separation, there appears to be a greatly increased mismatch between the wave impedance at the shield and the intrinsic impedance of the metal. This results in much greater reflection of the impinging wave than occurs for the same signal strength with small source to shield separation. The mismatch is greatest with a highly conductive shield material. All common highly permeable materials have low relative conductivity. High permeability does not improve the shielding effectiveness at low audio frequencies because no significant attenuation occurs as the wave passes through the shield. It is concluded that materials such as copper or aluminum are logical choices for shielding circuits from distant, high-intensity, low-frequency EMP.     

电磁屏蔽中的难题—磁场屏蔽

磁场屏蔽是电磁屏蔽中的难点.本文分析了磁场干扰的产生机理,进而分别对高频磁场屏蔽和低频磁场屏蔽进行了分析并提出了一些抑制磁场干扰的措施.     

Moment method analysis of the magnetic shielding factor of aconducting TM shield at ELF

This paper presents an integral equation and method of moments (MoM) solution to the problem of TM transmission by a metallic conducting shield at extremely low frequencies (ELF). In order to accurately compute the total fields interior to the shield, equivalent problems are formulated which avoid the numerically difficult problem of computing the total fields as the sum of the incident plus scattered fields. In particular, the total electric field on the interior surface of the shield is obtained by a volume current equivalent problem, and then the total magnetic field interior to the shield is formulated in terms of equivalent magnetic surface currents flowing on the interior surface of the shield replaced by a perfect conductor     

Admittance calculation of a slot in the shield of a multiconductortransmission line

The admittance calculation for a narrow slot in the conducting shield of a multiconductor transmission line (MTL) is presented. An approximate calculation is given for the admittance seen looking into a slot in the shield of a multiconductor transmission line when a uniform magnetic current is placed over the slot. The calculation presented is approximate in that only the transmission line mode is used in the calculation. All higher-order waveguide modes are neglected. This allows the magnetic current to be replaced by a point voltage source at the slot location. The admittance calculation then corresponds to calculating the driving point admittance of a point voltage source located in the shield of an MTL     

Improved field localization in transcranial magnetic stimulation of the brain with the utilization of a conductive shield plate in the stimulator

In this paper, a carefully designed conductive shield plate is presented, which helps to improve localization of the electric field distribution induced by transcranial magnetic stimulation for neuron stimulation. The shield plate is introduced between a figure-of-eight coil and the head. In order to accurately predict the field distribution inside the brain and to examine the effects of the shield plate, a realistic head model is constructed from magnetic resonance image data with the help of image processing tools and the finite-element method in three dimensions is employed. Finally, to show the improvements obtained, the results are compared with two conventional coil designs. It is found that an incorporation of the shield plate into the coil, effectively improves the induced field localization by more than 50%, and prevents other parts of the brain from exposure to high pulsed magnetic fields.     

On the effect of a spherical shield on the radiation Q and efficiency of ideal dipole antennas

A method is developed for calculating the radiation efficiency and quality factor, Q, for azimuthally symmetric electric (Tm) and magnetic (Te) multipole fields surrounded by a semi-transparent spherical shield of variable thickness, dielectric and magnetic constants involving losses. A matrix method is used to connect the transverse field components at adjacent interfaces. The Q and efficiency are determined by computing the energy stored in the near field of the multipole as well as the power radiated and dissipated in the shield. A numerical comparison of the performance of electric and magnetic dipole radiators is given as a function of frequency. An expression is derived for the permittivity of the shield which optimisés the Q.     

低频脉冲磁场对屏蔽体耦合特性的试验研究

低频脉冲磁场对屏蔽体的耦合效应与低频磁场自身参数、屏蔽体尺寸、材料特性及屏蔽体结构有关,作用过程比较复杂,实际耦合强度和规律应以试验研究结果确定。目前关于时变磁场的研究成果主要集中在单频磁场、快沿磁场等方面,而且效应试验研究较少。针对此种情况,文章选取钢板、纯铁板和铜板等几种常用的金属材料,制作了相应的屏蔽体缩比模型,利用低频脉冲磁场模拟试验系统研究了两者的耦合规律,分析了电磁参数等因素对耦合特性的影响,在一定程度上弥补了目前研究成果的不足。     

Noncontact conveyance of conductive plate using omni-directional magnet wheel

A magnet wheel is chosen as the driving method to transfer a conductive plate without mechanical contact in space, due to its high force density. When permanent magnets (hereafter PMs) constituting the magnet wheel rotate below the conductive plate, not only a repulsive type of normal force but also a traction torque is generated on the plate. To utilize the torque as a thrust force, the magnetic fields from PMs are covered partially using a magnetic shield plate. So, in-plane positions of the conductive plate are controlled by turning the shield plate opened partially. In this paper, the operating principle of the noncontact conveyance system using a magnet wheel is discussed, including experimental verification. Specially, the resulting force from the suggested magnet wheel is AC force with oscillation, but it can be minimized through varying entry or exit shape of the open area of the magnetic shield plate.     

A sheet impedance approximation for electrically thick multilayered shields

This paper describes an extension of the sheet impedance concept to treat inhomogeneous or multilayered shields that may be thick in terms of material shield wavelengths. For shields with magnetic materials, a simple relation between the equivalent electric and magnetic currents representing the shield is obtained. This allows the magnetic current to be treated as a dependent unknown and the electric current to be found as the solution of a single surface integral equation shown to be a perturbation of that for a perfect electric conducting (PEC) surface. By using the proper interior equivalent problem, it is shown that the method produces accurate and stable results for shielding by a rectangular box.     

300mm硅单晶的生长技术

讨论了 30 0 mm硅单晶的工艺控制 ,分析了拉晶工艺、热屏及磁场对晶体质量的影响。合理的工艺参数是拉制无位错单晶的前提 ,热屏和磁场的应用有效地控制了晶体的氧含量和微缺陷     

300mm硅单晶的生长技术

讨论了300mm硅单晶的工艺控制,分析了拉晶工艺、热屏及磁场对晶体质量的影响。合理的工艺参数是拉制无位错单晶的前提,热屏和磁场的应用有效地控制了晶体的氧含量和微缺陷。     

A sheet impedance approximation for electrically thick materialshields

This paper describes a simple extension of the sheet impedance concept to treat electromagnetic (EM) shields that may be thick in terms of material shield wavelengths. For magnetic shields, a simple relation between the equivalent electric and magnetic currents representing the shield is obtained, and the electric current is found as the solution of a single surface integral equation that is shown to be a simple perturbation of that for a perfect electric conducting (PEC) surface. Finally, it is shown that the computation of the small interior fields of good shields requires the use of the proper PEC interior equivalent problem     

Symmetrical Four-Port Edge-Guided Wave Circulators

Four-port microwave-integrated-circuit (MIC) edge-guided wave circulators (BGC) have been designed, fabricated, and tested. A mathematical characterization of the strip conductor's shape as well as a precise mechanical control of the bias inhomogeneity are provided. By means of these two techniques the reproducibility of the device is greatly improved with respect to that of the EGC obtained by the traditional cut-and-try methods. X-band performance data are presented and related to the spatial distribution of the effective magnetic permeability mu/sub eff/ in the ferrite substrate. Experimental evidence is reported that an efficient circulator action occurs when mu/sub eff/< 0 at some point under the central circular shield. The spatial distribution of the RF electric field at the circulator's surface is investigated by a mechanical probing technique. It is found that in the lower part of the operation band, RF fields of considerable amplitude extend in the air in the region between the guiding edge and the substrate's edge.     

A three dimensional analysis of slotted tube resonator for MRI

A three dimensional model of a slotted tube resonator (STR) used as a probe in the magnetic resonance imaging (MRI), which is loaded by a dielectric body and surrounded by a conducting shield, is analyzed by using the variational method and the dyadic Green's function of a circular waveguide having a dielectric core. Three surface current modes are properly assumed to expand the currents on the STR. The characteristics such as the input impedance, the resonance frequency, the Q value, and the magnetic field distribution are obtained to show the effects of the dielectric body and the conducting shield. Some theoretical results are compared with the measured data to confirm the validity of the present analysis.     

低温烧结平面六方铁氧体的非线性磁性研究

介绍了低温烧结Y型平面六方铁氧体的非线性磁性能。采用固相反应法制备样品,使用HP4284A,配合HP42841A直流源,测试外加磁场对软磁铁氧体高频磁导率的调制特性。研究表明,随外磁场强度增加,材料的磁导率呈非线性下降。定义了磁性可调率,用以表征外磁场调制下磁导率的改变量,此参数与材料本身的磁性能和显微结构密切相关。随Zn含量升高,材料的磁导率和可调率同时上升,可调率最高可达50%以上。     

Magnetic Nanocomposite for Potential Ultrahigh Frequency Microelectronic Application

Cobalt ferrite nanocomposites were investigated as example materials for microelectronic applications in the ultrahigh frequency range. Both static magnetic properties (magnetization versus applied field curve) and dynamic properties (complex permeability and permittivity at frequencies up to 1 GHz) were studied. When the ferritic density reached a certain level, the coercivity of the composite material decreased and the permeability increased noticeably. This phenomenon indicated the establishment of magnetic coupling between particles, which is necessary for magnetic nanocomposites to be used in electronic applications. Compared to magnetic composites made of micron-sized fillers, the magnetic loss of the nanocomposites was lower. However, the permeability of the nanocomposite was also lower. Cobalt zinc ferrite nanocomposites were prepared to explore the effect of composition, which revealed that the permeability of the magnetic nanocomposite could be improved by modifying the composition.     

Reduction of Lightning-Induced Magnetic Fields and Voltages Inside Struck Double-Layer Grid-Like Shields

This paper presents a numerical analysis of the reduction of lightning-induced magnetic fields and voltages inside double-layer grid-like spatial shields typically used in reinforced concrete buildings, e.g., nuclear power plants. The calculations are performed with the CONCEPT computer code, which solves Maxwell's equations using the method of moments in the frequency domain. The computer code is extended with the well-known transmission line model (TL-model) in order to simulate direct lightning strikes. The structure under study comprises a cubic cage of 2 m side length having single- or double-layer grid-like spatial shields with square meshes of 0.25 m width. Three lightning-channel attachment points are considered at the cage roof, namely, the center, the midedge, and the corner. The simulated lightning currents are the positive, the negative first, and the negative subsequent strokes at lightning protection level I (LPL I) according to the international standard series IEC 62305. The computed quantities comprise the currents through some selected wires of the grid-like spatial shields, the magnetic fields, the magnetic-field derivatives, and the induced voltage across a typical installation loop inside the shield. The results of the single-layer shield are compared to those of the double-layer one to evaluate the additional reduction of the latter shield.