An accurate method of calculating multilayer, multimaterial cylindrical magnetic shields

The basic concepts of shielding theory have existed since the last century [1,2]. There have been many publications on the subject of magnetic shielding, treating the case of shielding apparatus from static fields by means of multiple concentric shields and deriving several principles of fundamental importance. Unfortunately, however, theory has been applied to only the most ideal shield configurations, for the case of constant permeability [3-5]. This paper covers the analysis of shielding effectiveness of variable Permeability cylindrical shielded enclosures for the DC magnetic field case. When the permeability is a function of magnetic induction, the simple boundary solution for spherical or cylindrical shields can no longer be applied since the induction, through the permeability, is caused to vary as much as two orders of magnitude, causing nonuniformity in the field in the cavity and inside the shield. Thus, the permeability of the shielding 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 a multiple-shell geometry are treated in the equations of this analysis. This method gives fast, accurate results and can be run on a small computer for shielding optimization. Results of these magnetic field calculations allowed the selection of optimization criteria and showed how system requirements could be met by choosing a suitable shell structure arrangement. Experimental measurements on real materials for various shell structures confirmed the accuracy of this method.     

Magnetic field shielding concepts for power transmission lines

High current carrying cables used in power transmission lines create strong magnetic fields in their vicinity. For ac lines at 50-60 Hz, the magnetic field is quasi-static and hence uncoupled from the electric field. Shielding of such low frequency magnetic fields is a challenge. In this paper, finite element modeling is used to study the effect of various shield geometries and shield materials around a current carrying wire. Two-dimensional analysis is sufficient for this problem because the cables are very long compared to the wire diameters. Shielding effectiveness, defined as the difference in the magnetic field intensity with and without the shield, is presented. It is concluded that a partial shield is the optimal design for shielding the region below the cable. To achieve this, the shield gap must be oriented above the high current wire     

Microstructure and magnetic properties of plasma-sprayed Fe73.5Cu1Nb3Si13.5B9 coatings

Amorphous and nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉ coatings were formed by plasma-spraying micron-sized powders onto H62 brass substrates and aluminum pipes. The coatings are about 0.2-0.3 mm in thickness with fully dense and low porosity. The microstructure of the coatings is classified into two regions, namely, a full amorphous phase region and homogeneous dispersion of α-Fe nanoscale particles with a scale of 30-70 nm. The hardness of the amorphous and nanocrystalline coatings is about 960 HV_100g. Coercivity (Hc), saturation induction (B₈₀₀), and initial relative permeability (μ_i) of the coatings are 144 A/m, 0.27 T, 249, respectively, under 800 A/m direct current (DC) magnetic field. The magnetic shielding performance is good under DC magnetic field and its magnetic shielding effectiveness (SE) is 10-12 dB at coating thickness of 0.45 mm under static magnetic field of 2-40 Oe. The SE increases by increasing the coating thickness when the magnetic field frequencies are 50, 100 and 200 Hz with an intensity of 0.85 Oe. The results indicate that the amorphous and nanocrystalline alloy coatings can be good for some magnetic shielding applications.     

Multipole shaking field for magnetic shielding

The authors propose a novel method of magnetic shaking for enhancing the performance of ferromagnetic shielding. The method employs a new type of planar coil which generates higher-order multipole fields. Ferromagnetic shielding walls placed close to the planar coil can be suitably shaken by its field, whereas unwanted leakage of the shaking field into the shielded space can be avoided due to the highly localized nature of the multipole field. The configuration of the novel planar coil is similar to a square mesh in which alternating cells are clockwise or counterclockwise current loops. The shaking effect on the shielding performance and the leakage of the shaking field are evaluated using cylindrical shields and compared with results obtained with toroidal shaking coils     

Improvement of YBCO Superconductor Magnetic Shielding by Using Multiple Bulks

HTS bulks present a high critical current density which can be used as magnetic shields. Previous works showed that BSCCO bulks can screen magnetic fields up to 0.1 T. For large scale applications like electrical machines, stronger magnetic field is usually needed. In so doing, (RE)BCO materials are more suitable since they can shield much higher magnetic fields. Another key issue concerns the size of the bulks. Nowadays, it is possible to manufacture 150-mm diameter class cylindrical YBCO bulk. In order to get larger magnetic shielding areas, multiple bulk superconductors should be arrayed and stacked in layers. This paper presents experimental results on screening performances of layered YBCO pellets. These results are compared with 2D simulations. The experiments are carried out at 77 K under external magnetic fields of 150 mT. Different topologies are considered: single-layer configuration with 9 square pellets and double-layer configuration with respectively 9 and 4 square pellets. Experimental and simulation results show that a checkerboard of one layer configuration does not fully shield the external magnetic field. Improved shielding properties are obtained when the double-layer configuration is used.     

Low frequency magnetic shielding of YBa2Cu3O7-δ: Agx high temperature superconductors

Two measurement systems were used to investigate the low frequency magnetic shielding behavior of pure YBa₂Cu₃O_7−δ, silver doped YBa₂Cu₃O_7−δ, a packed powder, a laser ablated thin film and copper. It was found that factors which influence low frequency magnetic shielding include material composition, microstructure and properties, external a.c. and d.c. fields and sample dimensions. Among the experimental materials, only bulk ceramics showed good shielding factors which were constant with frequency in the range 10–3000 Hz. A macroscopic shielding current was necessary for a good shield. The thickness of the ceramic played a crucial role in shielding. Flux breakthrough occurred when the external field exceeded a certain value. The breakthrough field increased with sample thickness. The influence of d.c. fields on the shielding behaviour depended on the history and magnitudes of the fields. Magnetic shielding hysteresis was observed and its potential application is discussed.     

Magnetic Shielding Properties of MgB2Fe Superimposed Systems

We studied the magnetic shielding properties of a MgB₂/Fe hybrid structure consisting of two coaxial cups subjected to a magnetic field applied parallel to their axis. This study was performed to analyze the magnetic shield properties of a system contemporary exploiting the opposite magnetic properties of superconducting/ferromagnetic materials. The MgB₂ cup was grown by a microwave-assisted Mg-infiltration technique that allows obtaining samples with different shapes and easily scalable sizes, meeting the requirements of different shielding applications. Measurements of mitigation of the magnetic induction were performed in applied magnetic field up to 1.5 T and in different positions along the cup axis at temperature T=20 K. A decrease of the shielding factor (SF) of the hybrid system with respect to the MgB₂ cup alone turns out at low magnetic field. On the contrary, at higher magnetic field the superposition of the two cups increases the SF of the hybrid system up to 3 times over that one of the single MgB₂ cup.     

Fe-Ga合金发音振子换能器的多场耦合模型

建立了Fe-Ga合金弓张结构发音振子换能器电场、磁场和固体力学场的多场耦合模型,研究了发音振子换能器磁场强度、磁感应强度和应变分布情况,并进行了换能器在不同频率下的磁场强度分析和模态分析。分析发现,随着频率的增高,Fe-Ga合金发音振子换能器的磁场强度逐渐减小,计算得到换能器的共振频率为984 Hz。测试了Fe-Ga合金换能器中的核心元件Fe-Ga合金的磁场强度与应变的关系,当磁场强度饱和值为40 k A·m-1时,应变为70×10-6。搭建了磁致伸缩材料磁特性测试系统,测试了Fe-Ga合金磁场频率为5、20、50 Hz的磁滞曲线,并测试了Fe-Ga合金的应变随频率的变化曲线,实验结果与换能器中Fe-Ga合金的应变仿真结果一致。     

Evaluation of a high-performance magnetically shielded room forbiomagnetic measurement

A spherical magnetically shielded room called COSMOS and a method for measuring the shielding factor of well-shielded rooms have been developed. A dc-superconducting quantum interference device is used to detect faint magnetic fields, and the detected signals are averaged to increase the signal-to-noise ratio. The measured magnetic shielding factor for COSMOS is 420000 at 1 Hz     

应用于铯喷泉钟的磁屏蔽系统的设计

高质量磁屏蔽系统是铯喷泉钟的重要部件之一。首先给出评定磁屏蔽效果的3个指标,即磁场均匀区长度、磁场均匀区起点位置和磁屏蔽效率。在此基础上,讨论了利用有限元计算3层磁屏蔽系统的层间径向间距、轴向间距、端盖孔套管长度和端盖连接方式对磁屏蔽的影响,给出了相应的优化参数,并对比了按优化参数设计的3层磁屏蔽与原有4层磁屏蔽系统用有限元计算的磁屏蔽效果。有限元计算结果为中国计量科学研究院新铯喷泉钟磁屏蔽系统的技术设计提供了理论指导。     

Arrangement of high-energy neutron irradiation field and shielding experiment using 4 m concrete at KENS

An irradiation field of high-energy neutrons produced in the forward direction from a thick tungsten target bombarded by 500 MeV protons was arranged at the KENS spallation neutron source facility. In this facility, shielding experiment was performed with an ordinary concrete shield of 4 m thickness assembled in the irradiation room, 2.5 m downstream from the target centre. Activation detectors of bismuth, aluminium, indium and gold were inserted into eight slots inside the shield and attenuations of neutron reaction rates were obtained by measurements of gamma-rays from the activation detectors. A MARS14 Monte Carlo simulation was also performed down to thermal energy, and comparisons between the calculations and measurements show agreements within a factor of 3. This neutron field is useful for studies of shielding, activation and radiation damage of materials for high-energy neutrons, and experimental data are useful to check the accuracies of the transmission and activation calculation codes.     

Magnetic field measurement near a superconducting film using a 2-dimensional field-dependence of Josephson current through a Nb tunnel junctions sensor

Two-dimensional (2-D) magnetic field dependences of Niobium/niobium tunnel junction current Ic were first used in order to measure the magnetic field near the superconducting film. The 2-D magnetic field dependences of superconducting Josephson current Ic through the niobium/niobium tunnel junction can be changed by the external magnetic field. So, we measured the magnetic field using this superconducting tunnel junction fabricated by DC-magnetron apparatus as a magnetic sensor. The 2-D magnetic field dependences of Ic through the junction without and with the Nb thin-film sample were compared. With the Nb thin-film sample, extension of the characteristics in the perpendicular direction Hz to the sample was observed because of the Meissner effect of the superconducting thin-film sample. Moreover, the magnetic field Hz perpendicular to the Nb thin-film sample has been added in the triangle shape as a following sequence: (0)-(1000)-(0)-(2000)-(0)-(3000)-(0)-(4000)-(0)-(5000 A/m)-(0). Significant changes in the measured Ic-H (Hy,Hz) characteristics were observed above the certain value of Hz > 3000 A/m. We consider the shift of the measured Ic-H dependence in the Hz minus direction occurred because of the flux quanta were trapped in the sample superconducting niobium film after the Hz value had been added greater than 3000 A/m.     

Thermal analysis of magnetic shields for induction heating

For the design of magnetic shields for induction heating, it is useful to analyse not only the magnetic field reduction but also the temperature behaviour of the shield. The latter is heated by its electromagnetic losses and by thermal radiation from the workpiece. A coupled thermal-electromagnetic axisymmetric finite element model is used to study the temperature of a shield for an axisymmetric induction heater, highlighting the effect of the radius, length, thickness and material of the shield on its temperature and magnetic shielding factor. Also the effect of frequency and workpiece dimensions is investigated. The model is validated by measuring magnetic induction, induced currents in the shield and temperature of the shield on the experimental setup. The temperature is unacceptably high for shields close to the excitation coil, especially if the shield length is lower than the workpiece length. Although the study is carried out for one specific induction heater geometry, the paper indicates the effect of parameters such as geometry, material and frequency on shield temperature so that the results are also useful for other induction heating configurations.     

Microstructure and Magnetic Properties of Soft Magnetic Composites with Silicate Glass Insulation Layers

In this study, a new soft magnetic composite (SMC) material design scheme was put forward. According to this design scheme, SMC with silicate glass insulation layers was prepared by powder metallurgy with nanometer glass powder and micron iron powder. It can be annealed at high temperature for reducing residual stress. The magnetic properties were good. The maximum permeability, saturation magnetic induction, residual magnetic flux density and coercivity were 467, 1.47 T, 1.25 T and 394 A/m, respectively. The core loss versus alternating magnetic field frequency showed linear growth. The core loss was 4.4 W/kg at an induction level of 1 T, 50 Hz.     

Dynamic magnetic shield for the CLAS12 central TOF detector photomultiplier tubes

The Central Time-of-Flight detector for the Jefferson Laboratory 12-GeV upgrade is being designed with linear-focused photomultiplier tubes that require a robust magnetic shield against the CLAS12 main 5-T solenoid fringe fields of 100 mT (1 kG). Theoretical consideration of a ferromagnetic cylinder in an axial field has demonstrated that its shielding capability decreases with increasing length. This observation has been confirmed with finite element analysis using POISSON model software. Several shields composed of coaxial ferromagnetic cylinders have been studied. All difficulties caused by saturation effects were overcome with a novel dynamical shield, which utilizes a demagnetizing solenoid between the shielding cylinders. Basic dynamical shields for ordinary linear-focused 2-in. photomultiplier tubes were designed and tested both with models and experimental prototypes at different external field and demagnetizing current values. Our shield design reduces the 1 kG external axial field by a factor of 5000.     

Simple method to realize the zero-field transition temperature of a superconductive fixed point

Transition temperatures of superconductive fixed points are extremely sensitive to ambient magnetic fields. Careful magnetic shielding, either active or passive, is required to obtain the standard zero-field condition. Application of an open shield simplifies the task by reducing the requirement of field compensation to only one dimension and by removing the uncertainty about the field inside a completely closed shield.     

Application of flux trapping in hard superconductors to magnetic shielding and magnetic levitation

A thermally switched magnetic shield which can both shield against and trap magnetic fields is described. The shield, consisting of a Nb---Ti sheet, can trap stably fields of up to 0.3T, and shield against fields of up to 0.35 ± 0.05T, when the in superconducting state It provides large attenuation of alternating magnetic fields in the presence of a steady trapped field. It is shown to significantly reduce the eddy current damping of the mechanical vibration of a superconducting sample in a magnetic field, by shielding nearby normal metal surfaces. A specific application to magnetic levitation of gravitational radiation antennae is discussed.     

Inductance of magnetically shielded air-core coils of circular cross section

An expression for the inductance of shielded cylindrical air-core solenoids of arbitrary diameter to length ratio is developed. Three shielding configurations are investigated: 1) the use of a coaxial-concentric magnetic shield, 2) the coaxial magnetic shield with conducting end plates, 3) the coaxial magnetic shield with conducting end rings. Case 2) is solved in closed form by separation of variables, and from this result a method is given for predicting the length of the coaxial magnetic shield required to contain the field for the three cases.     

Magnetic Characterization of MgB2 Bulk Superconductor for Magnetic Field Mitigation Solutions

Magnetic shielding properties of MgB₂ bulk samples synthesized by the SPS (Spark?CPlasma?CSintering) technique were characterized in low applied magnetic fields at temperatures ranging from 20 to 37 K. The used growth technique allows one to produce this compound in different shapes and sizes required for shielding applications. In this framework, shielding magnetic-induction field profiles generated by MgB₂-based shield components, shaped as planar thick disks, were measured by means of a suitable Hall probe in-plane array. The magnetic field distribution at different vertical distances above the sample was also obtained by a micrometric motion of the probe ensemble. Magnetic field profiles were then analyzed in the framework of the critical state model and the critical current density, J _c , was evaluated. The J _c magnitude indicates that the material under test is a good candidate for passive magnetic shield manufacturing up to temperatures close to the transition one.     

3D nonlinear magnetostatic analysis of a double-walled cylindricalshield

The performance of a double-walled cylindrical shield for magnetostatic field has been optimized using a 3-D numerical code. The shielding factor of the double-walled screen has been evaluated by varying the geometrical parameters. The optimization of the screen and the design criteria are reported. The influence on the shielding factor of the presence of a hole perpendicular to the cylindrical shield has been evaluated by calculating the field penetration into the internal region of the screen. Finally, the FEM (finite-element method) results have been tested by means of measurements performed on a shield prototype. The comparison between the numerical and experimental data shows good agreement, especially in the region where the finite-element model has been refined with more elements