On the interaction of magnetic fields and magnetic shields

Some general mathematical expressions have been derived for the induced self and mutual inductances of the wires inside a cylindrical ferromagnetic shell. An application of these results is given. It is shown that the effect of the ferromagnetic shell on the inductances of the multiconducting wires is strongly geometry dependent; the closer the wires to the magnetic shell, the stronger the effect. For wires in the center of the cylinder, no significant result will be caused by the presence of the magnetic shell.     

Superconducting shields for alternating magnetic fields

This paper describes a shield to guard a dc superconducting field from an ac field.     

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     

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.     

Analysis of perforated magnetic shields for electric power applications

The shielding performance of perforated magnetic shields for electric power applications is described. The shielding of an axisymmetric induction heating device is studied as a function of frequency, number of perforations and dimensions of the perforations. From the numerical point of view, the perforations cause the numerical model to be 3D. A numerical optimisation is carried out to find the optimal geometry with respect to the shielding factor and the volume of the shield. For the optimisation, two approaches are presented. The first approach is fast and easy-to-implement, but has limited accuracy. It uses a classical 2D axisymmetric model where the perforations are approximated by 'axisymmetric air gaps' resulting in a segmented shield. It is shown how to modify the 2D model to obtain results that are similar to the ones of a 3D model. The second approach is more accurate although quite fast, but more difficult to implement. It combines a 3D thinshell finite element model with the unmodified 2D model in a space mapping optimisation algorithm. The validation of both models is based on experimental work for an unperforated shield and for the optimised perforated shield.     

Superconducting spherical magnetic shields on the basis of niobium and its compounds

The method of fabrication of superconducting spherical magnetic shields on the basis of super-conducting electrolytic coatings made of niobium, Nb₃Sn, and NbC is developed. These shields can be used for weakening the external magnetic fields with strength up to 800 kA/m and obtaining a magnetic vacuum lower than 0.8 × 10⁻⁵ A/m.     

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.     

Contribution to the use of superconducting shields for magnetic field shaping

The low-field diamagnetism of superconductors has often been used to design quasi-ideal magnetic circuits, especially for traveling wave maser (TWM) magnets. An analogical approach to the calculation of the efficiency of such nonideal cylindrical superconducting shields is presented. In most cases, the problem reduces to a 2-dimensional Laplace equation with given boundary conditions. The logarithmic complex potentiallog (Bx - iBy)has been used to obtain the field homogeneity and disorientation. Partially penetrated shields may be replaced by perfect equivalent ones. The analog measurements on conducting paper permit quick iterations of the solution. Various field component measurements have been done on TWM magnets using bismuth thin film Hall probes and found to be in good agreement with the analog measurements.     

One-component cesium magnetometer for measuring the residual magnetic induction in ferromagnetic shields

A simple design for a miniature cesium magnetometer intended for measuring the residual magnetic induction (B ₀≤1000 nT) in ferromagnetic shields of cylindrical shape with an internal diameter ⊘≥15 mm is described. Pis’ma Zh. Tekh. Fiz. 25, 42–46 (May 12, 1999)     

Shielding against space debris. A comparison between different shields: The effect of materials on their performances

The space environment requires the Space Station to be shielded against orbital debris. A technological programme undertaken by the European Space Agency has led to a preliminary definition of the shield configuration for the European Attached Pressurized Module. The envisaged shield is a modified Whipple shield. A second bumper is located midway between the first bumper and the backwall.

The work described has been initiated to quantify experimentally the merits of different shields compatible with the APM system requirements. For this technological investigation, two requirements had to be satisfied. The spacing between the front bumper and the backwall had to be limited to 120 mm. The backwall thickness could not be reduced to technological limits as it has structural functions as well. In addition, the long life requirements of the Space Station precludes the use of unproved materials for the external parts of the shield.

Different materials have been tried as second bumper. The effect of the first bumper thickness on the projectile fragmentation has been explored as well. Shields based on Aluminium, Kevlar and Glare have been investigated. Kevlar 29 fabrics impregnated with epoxy resin were used for this work. Glare is a material developed to improve the fatigue strength of metal structures. It is primarily intended for aircraft skin applications. Glare consists of a 60 percent fibre volume adhesive prepreg with high-strength unidirectional or cross-ply R-glass fibres. A variety of lay-up sequences is available ranging from 2/1 (two layers of aluminium alloy sheet bonded by one layer of prepeg) to any number of layers. The 2/1 layers version of the Glare material has been used for this work.

The tests results indicate the performances of materials can change significantly with the impact conditions. Glare shows the best performances in the low velocity regime while Kevlar is very promising in the high velocity regime. It is concluded the use of Kevlar can improve substantially the performances of the APM shield.     

The effect of elastic stresses on super-high-frequency magnetic impedance of amorphous magnetic microwires

It has been demonstrated experimentally that, in magnetic microwires with a negative sign of the magnetostrictive constant, imposing an axial stress results in an increase of the effective field of circular anisotropy. Torsion stresses form a helical magnetic structure that yields hysteresis and irreversible jumps on giant magnetic impedance curves. The field at which the stability of a magnetic system deteriorates depends on the values of axial and torsion stresses.     

Optimizing active and passive magnetic shields in induction heating by a genetic algorithm

This paper presents a method for designing optimal passive and active shields for axisymmetric induction heaters. Such shields are needed to protect human operators and external electronic equipment from stray magnetic fields. The method uses a genetic algorithm (GA) to minimize an objective function. This function reduces the magnetic field in the target area, the power dissipation in the active and passive shields, and the influence of the shields on the heating process. The GA returns the position and height of the passive shield, the optimal current for the active shield, and the number of turns of all coils. The paper describes two optimization modes: 1) optimization of only the active shield with fixed passive shield and 2) global optimization of both active and passive shields. Several passive shields are studied: electrically conductive shields and both electrically and magnetically conductive shields. The field reduction depends on the optimization mode and the passive shield properties, but always exceeds 25 dB for combined active and passive shields. Finally, the paper compares the results of the simulations to experimental measurements.     

The effect of radiation shields around the air condenser and compressor of a refrigerator on the temperature distribution inside it

In almost all domestic refrigerators–freezers all components are assembled in the same relative position since several years ago. It is also known that the condenser releases heat at high temperatures (first law of thermodynamics) as well as the compressor. This heat is rejected to the environment in almost all practical situations partially by natural air convection. However, part of it is due to thermal radiation that causes an overheating of the refrigerator–freezer surfaces adjacent to those equipments. As a consequence there are more heat gains to the refrigerator–freezer through these surfaces and hence higher air temperatures inside. This paper describes how a simple technique can be very useful in order to minimize that part of heat transfer by radiation. The improvement is achieved by placing a radiation shield – a sheet of aluminium foil – over the surfaces close to the condenser and the compressor. For validating this technique a refrigerator–freezer was monitored with thermocouples for the measurements of the inside air temperatures in two situations: with and without the radiation shield. Results show that with this practice the average inside air temperatures in the refrigerator–freezer could decrease to about 2 K. An available commercial code was used in order to simulate the air temperature distribution and air velocities inside the refrigerator cabinet in both situations. Results from the experimental apparatus and from simulations show that there is a good agreement between them which validates the experiments carried out.Also an available commercial code, the Fluent, was used to simulate the internal air temperature in both situations.     

The effect of magnetic interaction between medium and head on perpendicular magnetic recording characteristics

The high density recording characteristics of perpendicular magnetic recording using a single-pole head are affected by the magnetic interaction between the medium and the head. By decreasing the relative thickness of the Co-Cr layer in the double-layer medium to that of the main-pole of the head, and increasing the saturation magnetization of the Co-Cr layer, the high density recording characteristics are enhanced. When requisite conditions are realized, the reproduced voltage vs. bit density characteristics are improved considerably for a thinner main-pole of the single-pole head.     

The effect of weak pulsed magnetic fields on modified wood

The edge hardness of birch wood modified under the action of a weak (not exceeding 0.5 T) pulsed magnetic field is increased as compared to the initial level. The magnetic field effect has a threshold (0.2 T) and is explained by a decrease in the mobility of wood fibers as a result of the field-induced cross linking between side groups of cellulose macromolecules.     

The effect of substrate defects on epitaxial magnetic garnet films

The stress patterns associated with various defects in Czochralski-grown gadolinium gallium garnet (GGG) crystals have been observed using a polarizing microscope. The effect of these defects on both the surface topography and the magnetic behaviour of epitaxial magnetic garnet films grown on GGG substrates is reported. In particular, iridium inclusions and defects of a filamentary nature affect the surface topography. Other types of defect influence the magnetic behaviour of the films in a manner attributable to a change in the lattice parameter.     

The effect of magnetic layer thickness on magnetic properties of Fe/Cu multilayer nanowires

Fe/Cu multilayer nanowires were ac-pulse electrodeposited into the anodic aluminum oxide templates prepared by a two-step mild anodization technique. Transmission electron microscopy images showed the distinct layers with a relatively high contrast. A highly pure layer (∼99%) was achieved by tuning the proper ions ratio and optimizing the off-time between pulses of each layer in the single electrodeposition bath. Fe/Cu multilayer nanowires with 38 nm diameter were obtained. The effect of reducing the Fe layer thickness on the magnetic properties of Fe/Cu multilayer nanowires was investigated. It was seen that reducing the Fe layer thickness, thereby variation of rode- to disc-like multilayer nanowires, caused to rotate the magnetic easy axis from parallel to perpendicular to the wires axis.     

The effect of initial growth layer on magnetic properties of Co-Crfilms

The effect of a Ti underlayer, which increases the perpendicular coercive force of Co-Cr films, was investigated. To clarify the cause of this phenomenon, the film-thickness dependence of the magnetic properties was examined. It was found that the coercive force of a Co-Cr film deposited directly on a polymide substrate decreases drastically when it becomes thinner than 50 nm, whereas in the case of Co-Cr film on a Ti underlayer, a high coercive force is maintained even when the film becomes as thin as 20 nm. The film with the underlayer has a distinct uniform columnar structure, whereas the film without it has a 50-nm-thick initial growth layer with no clear structure. Measurements of the temperature dependence of magnetic properties and observations of segregated microstructures indicate that the improvement of magnetic properties by the insertion of the Ti underlayer is mainly due to the improvement of shape anisotropy resulting from the formation of a distinctly segregated microstructure