Passive magnetic cylindrical shielding at gauss-range static fields

A study has been performed in order to find the optimal solution for the magnetic shielding of the 10 in. photomultipliers which will be used in the Double Chooz neutrino experiment under a very low magnetic field (less than 2 G). The results obtained with analytical and numerical calculations are compared with measurements made using test prototypes of several magnetic materials, with different dimensions and from different manufacturers. An exhaustive analysis of the magnetic materials was needed to understand the observed disagreement between calculations and test results obtained at low field values.     

基于超导磁场环境真空测量电离规磁屏蔽结构仿真研究

聚变装置超导磁场环境下真空测量电离规因灯丝变形、带电粒子轨迹变化等物理机制导致真空测量精度不高,不能满足工程精确测量的需求.基于磁屏蔽的理论,构建了半封闭式和开放式的两种多层磁屏蔽结构模型,利用Maxwell有限元软件,仿真研究了结构材料层不同厚度和不同磁场方向等条件下磁屏蔽效能的变化规律.结果 表明,屏蔽效能随铁层材...     

Influence of an applied magnetic field on shielding current paths in a high TC superconductor

The influence of an applied magnetic field on shielding current paths in a bulk high T_c superconductor was investigated. This issue is very important for the quantitative analysis of levitation force, since the distortion and localization of the current paths have been found to be a major source of discrepancy between computed and experimental results. Furthermore, it has been speculated that the current paths vary, depending on the applied magnetic field. The magnetic field due to superconducting currents was measured by a Hall sensor when a permanent magnet was positioned at different gaps from a superconductor. The current paths were then reconstructed from the measured field data by an inverse analysis. A genetic algorithm, which is a robust probabilistic optimizing method, was used for the inverse analysis. It was found that the current paths merged into larger loops as the permanent magnet was retreating from the superconductor. It implies that a stronger applied magnetic field locally degrades the current density (J_c) and the shielding current paths are localized by these low-J_c areas which act as insulating boundaries.     

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.     

Numerical Simulation on the Shielding Efficiency of Magnetic Shielding Enclosures in the ITER Applications

Magnetic shielding needs to be employed to ensure proper operation of some electronic equipment which are sensitive to external magnetic interference, such as cryogenic valves located inside the ITER feeder cubicles. This paper is concerned with the shielding efficiency of the magnetic shielding enclosures. A 3-D theoretical model for Fe-Ni alloy magnetic shielding enclosures based on finite element method (FEM) is obtained with the nonlinear law of magnetization. The influence of shielding materials, enclosure configurations, single or multi- layer designs, and apertures on the shielding efficiency is investigated. It is shown that the proposed model can predict the shielding efficiency of shielding devices well with nonlinear magnetize relation, and some recommendations for an optimized magnetic shield design in the ITER feeder cubicles are given.     

The effect of a uniform magnetic field on the onset of steady Bénard-Marangoni convection in a layer of conducting fluid

In this paper we use a combination of analytical and numerical techniques to analyse the effect of a uniform vertical magnetic field on the onset of steady Bénard-Marangoni convection in a horizontal layer of quiescent, electrically conducting fluid subject to a uniform vertical temperature gradient. The critical values of the Rayleigh and Marangoni numbers for the onset of steady convection are calculated and the latter is found to be critically dependent on the non-dimensional Crispation and Bond numbers. The stability of the layer to long wavelength disturbances is analysed and the two different asymptotic limits of strong surface tension (small Crispation number) and strong magnetic field (large Chandrasekhar number) are investigated. In the latter case analytical results for the critical Rayleigh and Marangoni numbers are obtained and are found to be in excellent agreement with the results of numerical calculations. We conclude that the presence of the magnetic field always has a stabilising effect on the layer. Treating the Marangoni number as the critical parameter we show that if the free surface is non-deformable then any particular disturbance can be stabilised with a sufficiently strong magnetic field, but if the free surface is deformable and gravity waves are excluded then the layer is always unstable to infinitely long wavelength disturbances with or without a magnetic field. Including gravity has a stabilising effect on the long wavelength modes, but not all disturbances can be stabilised no matter now strong the magnetic field is.     

Chaotic signal generation in low-voltage vircator with electron source shielded from external magnetic field

The effect of shielding an electron source from a homogeneous external magnetic field of the drift chamber on the nonlinear dynamics of the electron beam with a virtual cathode (VC) and on the characteristics of output microwave radiation in a low-voltage vircator have been numerically simulated within the framework of a two-dimensional model. It is established that the increased degree of shielding of the electron source from the external magnetic field leads to the complication of the VC dynamics in the system and the corresponding chaotization of the output microwave radiation. Physical processes that account for the observed effect of shielding are analyzed.     

Analysis of a novel laminated coil using eddy currents for AC highmagnetic field

The analysis and performance of an eddy current type laminated coil for a high AC magnetic fields are described. It is usually difficult to obtain a high AC magnetic field using an air-gap coil because of eddy currents. The present coil circumvents this limitation by making use of the magnetic shielding effect of eddy currents. Two different realizations of the coil are proposed to provide design flexibility. The field distributions are analyzed by the two-dimensional finite-element method. The coils can also be applied to an induction electromagnetic pump     

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.     

A magnetic field source system for magnetic refrigeration and its interaction with magnetocaloric material

The magnetic field source system constitutes an important component of magnetic refrigeration. A specific methodology for its' dimensioning is proposed in this paper. It is based on analytical calculation models and takes into account the geometry of the system, the magnetic properties of the magnetocaloric material and the magnetothermal cycle (direct or active magnetic regenerative refrigeration). The analytical calculation of the field is first developed and applied to usual permanent magnet-based field sources with and without soft magnetic materials. Then the forces generated by the interaction between the field and the magnetocaloric effect material are analytically evaluated considering the real field distribution. All calculations are validated thanks to two- or three-dimensional finite element method simulations.     

Analytical Modeling of a Permanent-Magnet Synchronous Machine in a Flywheel

We develop an analytical model for a radial-flux external-rotor permanent-magnet synchronous machine (PMSM) without slots in the stator iron and with a shielding cylinder. The machine is part of an energy storage flywheel, to be used as the peak-power unit in a hybrid electric passenger bus. To reduce the induced no-load losses due to the high rotational speed of the flywheel, the slots in the stator are made not of iron but of a nonmagnetic plastic material. This results in an air gap winding with a stator yoke consisting of stacked circular laminations. The analytical model includes the effect of the winding distribution on the field, the fact that it is in the air gap, and the effect of the eddy-current reaction field of the shielding cylinder. The two-dimensional magnetic field is solved in six defined machine layers and useful machine quantities are derived directly from it, leading to the machine voltage equation. We built a prototype flywheel machine. The locked-rotor machine resistance and inductance predicted by the analytical model was compared with the experimentally determined values. The values showed good agreement, thereby validating the analytical model of the machine.      

Shielding Effectiveness Measurements for Ferromagnetic Shields

This paper proposes a new way to measure the shielding effectiveness of ferromagnetic shields. The procedure combines an experimental characterization of the shielding material and numerical simulations. In a first case, magnetic hysteresis is reduced to a series of equivalent $B$ –$H$ linear relations through an optimization procedure that is applied to a measured set of symmetric minor loops, and an equivalent multilayer linear medium is defined through a subsequent iterative procedure. In a second case, magnetic hysteresis is reduced to a simple $B$–$H$ nonlinear relation, obtaining an equivalent nonlinear shield. In both cases, the obtained results are compared with the direct measurements of the shielding effectiveness at different operating frequencies, showing good agreement and the validity of the proposed method. A comparison and a discussion on the harmonic content of the attenuated field for the investigated shield geometry are reported. The same analysis is carried out for a basic diffusion problem, comparing the results with those obtained by the use of a Preisach model.      

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     

Smoothing of Magnetic Fields for Superconducting Bearings by Multilayers

Losses in magnetic bearings, whether conventional or superconducting, are caused by any magnetic inhomogeneities in the direction of motion. These can be smoothed by inserting a permeable sheet between the magnet and superconductor. However, this will also have the effect of reducing the field of the main magnet, which supplies the levitating force. Drawing on the parallel with shielding, where shields with air-gaps are more effective than solid shields of the same thickness (Rucker in Philos. Mag. 37:95, 1894), the same principle can be applied to smoothing layers. Analytic solutions are easy to obtain by using an anisotropic permeability in the smoothing layer. An air gap makes the normal permeability about unity while maintaining the parallel smoothing permeability which shorts out small wavelength inhomogeneities. For inhomogeneities which are mainly in the direction of motion we can striate the layer so that the in plane permeability is also inhomogeneous. The results are applied to a magnet track made of permanent magnets joined together with an effective gap of 0.1 mm. A suitable ferromagnetic layer can reduce the dip in field by a factor of 53 while only reducing the main field by 3 %.     

Hysteresis losses in a superconducting plate

On the basis of theoretical analysis expressions have been derived suitable for calculating the hysteresis losses in the presence of variable and constant components of both external magnetic field and transport current in a superconducting layer. The calculation scheme can be applied to any arbitrary shape of the field variation cycle, provided the variable components of the field and current are proportional. It has been assumed that the critical current density is dependent only on the external field. For the more common forms of this relationship the results have been expressed in an analytical form. Application of the results is illustrated by calculating the losses in a superconducting solenoid under variable current in the winding and the external field.     

Onset of Benard-Marangoni Ferroconvection with Internal Heat Generation

The effect of internal heat generation on the onset of Benard-Marangoni convection in a horizontal ferrofluid layer heated from below in the presence of a uniform vertical magnetic field is studied. The lower boundary is rigid and the upper free boundary is assumed to be flat and undeformable. Both the boundaries are considered to be perfectly insulated to temperature perturbations and the resulting eigenvalue problem is solved numerically using the Galerkin technique as well as analytically by regular perturbation technique with wave number as a perturbation parameter. It is observed that the analytical results agree well with those obtained numerically. It is noted that the combined effect of magnetic Rayleigh number and dimensionless internal heat source strength is to reinforce together and to hasten the onset of Benard-Marangoni ferroconvection compared to their presence in isolation. In addition, the nonlinearity of fluid magnetization is found to have no influence on the criterion for the onset of ferroconvection. Some existing results are reproduced as particular cases from the present study.     

Shielding design for the PANDA spectrometer at the Munich high-flux reactor FRM-II

The start-up of the Munich high-flux reactor FRM-II is in progress on. At the beam tube SR-2 the spectrometer PANDA has been installed. It is at three-axis neutron spectrometer looking onto a slightly under-moderated cold neutron source. For polarisation analysis, PANDA is equipped with a vertical cryomagnet producing fields up to 14.5 T for the sample. To get an appropriate shielding of the high-intensity instrument, one has to take into account the large cross section of the primary beam, several restrictions using magnetic materials, limitations in loading the site and finally, has to keep the lateral extent of the shielding small to allow for high-scattering angles. The shielding has been designed on the basis of the results, which were achieved by the combined use of both the Monte Carlo code MCNP-4B2 and an analytical method based on one-dimensional dose transmission functions.     

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。     

Unsteady MHD-boundary-layer of a source and vortex flow adjacent to a stationary surface

Summary The steady laminar incompressible flow of an electrically conducting fluid over an infinite permeable disk in the presence of an axial magnetic field has been investigated, and a self-similar solution of the boundary-layer equations is obtained numerically. For large values of the suction parameter, a closed form solution is obtained. Also, an asymptotic solution is found for large values of the independent variable. The surface-shear stresses in the radial and tangential directions and the surface heat transfer strongly depend on the suction parameter, the ratio of the source and vortex flow and the magnetic field except the surface heat transfer which weakly depends on the magnetic field. The similarity solution of the boundary-layer equations exists only when a certain minimum suction or magnetic field is applied. The results of the analytical solution are in good agreement with those of the numerical solution for the suction parameterf _w3.     

Wave Propagation in a Magneto-Micropolar Thermoelastic Medium with Two Temperatures for Three-Phase-Lag Model

The present paper is concerned with the wave propagation in a micropolar thermoelastic solid with distinct two temperatures under the effect of the magnetic field in the presence of the gravity field and an internal heat source. The formulation of the problem is applied in the context of the three-phase-lag model and Green-Naghdi theory without dissipation. The medium is a homogeneous isotropic thermoelastic in the half-space. The exact expressions of the considered variables are obtained by using normal mode analysis. Comparisons are made with the results in the two theories in the absence and presence of the magnetic field as well as the two-temperature parameter. A comparison is also made in the two theories for different values of an internal heat source.