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内容記述 |
In order to avoid errors in the operation of devices subject to strong magnetic <br />fields, shielding systems are applied to such devices that would be affected by magnetic <br />fields. In big science projects such as the magnetically confined fusion-plasma science <br />and the high-energy accelerator science, highly efficient magnetic shields are required <br />for various devices sensitive to magnetic fields in order to reduce the strong fields <br />generated by large magnets, including superconducting magnets, to fields less than the <br />geomagnetic field within a limited space to have no influence on their operation. <br /> In physical science experiments including the above big science projects, the <br />ferromagnetic shield employing enclosures made from pure iron, Permalloys or mild <br />steel is usually applied for the required shielding. However, there have stilI been <br />difficulties with designing the shielding system for devices operating under the <br />large-scale equipment generating the strong field, probably because subjects on the <br />prediction of the shielding effectiveness and the material technology for the shielding <br />shell have not been definitely resolved. <br /> For the subject on the prediction of the shielding effectiveness, it is proposed in <br />this thesis that the effect of the hysteresis of the shell material on the shielding efficacy <br />should be considered. The magnetic hysteresis is well known as a phenomenon <br />characterizing ferromagnetic materials. The ferromagnetic shielding effectiveness must <br />be affected by the hysteresis of the shell material because its magnetized condition is <br />strongly dependent on the previous state. <br /> For the subject on the material technology, there has been less knowledge of <br />preparing shell materials simultaneously satisfying both high flux density and soft <br />magnetism. For example, the magnetic flux density is sufficiently high on pure iron <br />with moderate permeability while it is insufficient on Permalloy in contrast with its <br />high permeability. It should be also considered that the magnetic properties of soft <br />magnetic materials are strongly affected by the strain, especially for the residual strain, <br />due to their structural sensitivities. <br /> Although these subjects still remain to be solved, there have been few reports <br />discussing effects of the hysteresis on the magnetic shielding effectiveness theoretically, <br />and some only treat them from a phenomenological point of view. Furthermore, there <br />have also been few reports quantitatively investigating effects of the strain of the shell <br />materials on the magnetic shielding effectiveness. <br /><br /> This thesis aims at two subjects. One is to optimize the designing techniques of <br />the ferromagnetic shielding from the strong magnetic fields for devices operating under <br />large-scale equipments, based on the research on shell materials from a viewpoint of <br />the material science. The other is to clarify the influences of the magnetic properties on <br />the shielding effectiveness from a viewpoint of the electromagnetism, leading to further <br />understanding of the static ferromagnetic shield. <br /> First, research on the shell materials is focused on the improvement of the soft <br />magnetism without reduction of the magnetic flux density. Additionally, the <br />deterioration of the soft magnetism due to a tiny amount of strain in the shell material <br />is quantitatively investigated because the shell material is subject to be strained by <br />stress and deformation during the assembly and installation of the shielding system. <br /><br /> Next, the effect of the hysteresis of the shell material on the shielding <br />effectiveness is discussed against static magnetic fields. It has been qualitatively <br />understood that the shielding effectiveness can be seriously affected by strain of the <br />shell material. Therefore, an influence of a tiny amount of strain of the shell material <br />on the magnetic shielding effectiveness is analyzed. <br /> Finally, a guiding principle for evaluating the maximum leakage field as a <br />shielding efficacy of the shell material, which is a reasonable and obtainable field in <br />the worst-case scenario for the magnetized condition of the shielding shell, is discussed. <br />It is determined by considering the hysteresis of the shell material, and it is revealed <br />that the concept of the maximum leakage field is effective whether the shell material is <br />strained or not. <br /><br /> The thesis consists of five chapters, which are summarized below. <br /> Chapter 1 states the purpose and background of this study, providing an overview <br />of the prior researches, and describes the significance of this study. The first section of <br />this chapter explains the necessity of the magnetic shielding from strong static fields <br />for devices operating under large-scale equipments, by introducing specific situations <br />inherent in big science projects, such as the magnetically confined fusion-plasma <br />science and the high-energy accelerator science. In the second section, the novelties <br />and goals of this study are summarized. The structure of this thesis is outlined in the <br />third section. <br /><br /> Chapter 2, consisting of five sections, describes the research and development of <br />the soft magnetic materials for the use of the magnetic shielding system. In the first <br />section, the purposes of the research in this chapter are addressed. <br /> The second and third sections of this chapter focus on the strategies to improve the <br />soft magnetic properties of pure iron and to develop soft magnetic materials with both <br />higher permeability and lower coercivity than those of pure iron without deteriorating <br />its high-magnetic flux density by the grain-coarsening technique. The developed <br />Fe-1%Al alloy exhibits extremely large ferrite grains in its microstructure, and has <br />almost the same high-permeability and low-coercivity as those of Permalloy B. <br />Moreover, it indicates high saturation magnetization, which is lower only by 2% than <br />that of pure iron. <br /> The fourth section of this chapter discusses an influence of a tiny amount of plastic <br />strain on the magnetic properties. It is revealed that the deterioration of coercivity due <br />to the plastic strain is alleviated in the coarsened grain microstructure. Therefore, it is <br />understood that the improvement of the soft magnetism by the grain-coarsening <br />technique is effective for the prevention of the deterioration of coercivity by strain <br />caused through damage. The results of the research in this chapter are summarized in <br />the fifth section. <br /><br /> Chapter 3, containing six sections, explains a prediction method for the magnetic <br />shielding effectiveness and a design concept for the double-layer shielding techniques. <br />The first section of this chapter addresses the purpose of the research in this chapter. <br /> In the second and third sections of this chapter, a new approach to the estimation <br />of the static ferromagnetic shielding effectiveness is proposed, in which the magnetic <br />hysteresis of the shielding materials is considered. The hysteresis effects of the shell <br />materials are discussed on the ferromagnetic shielding, in which a relatively strong <br />external field is reduced below the geomagnetic field in a shielded space. The measured <br />leakage field in the shielding enclosure corresponds to the results of the finite element <br />method (FEM) analysis when permeability considering the effect of coercivity is used <br />for the calculation as a parameter representing the hysteresis of the shielding material. <br /> The effects of both permeability and coercivity on the leakage fields are discussed <br />in the fourth section of this chapter, with regard to the magnetic properties of the <br />shielding materials by using the FEM analysis in combination with the results <br />obtained in chapter 2. The maximum leakage field, which is regarded as a figure of <br />merit of the shielding efficacy, is determined by the coercivity of the material used for <br />the shield, and it is clarified that the coercivity should be considered for the estimation <br />of the leakage field in an actual design of the shielding system. It is also confirmed that <br />the deterioration of the soft magnetic properties, not only permeability but also <br />coercivity, due to the residual strain causes the reduction of the magnetic shielding <br />efficacy. Finally, it is concluded that the maximum leakage field is dominated by the <br />coercivity of the shell material regardless of the shell material condition being strained <br />or not. <br /> In the fifth section, the effect of the double-layer structure on the shielding design <br />is investigated by using the analysis with considering the hysteresis of the shell <br />material. The double-shell structure is definitely effective for abating the degradation <br />of the shielding efficacy in the strained shielding material in that the influence of the <br />deterioration of permeability of the inner shell material due to the strain is lowered <br />because the external field applied to the inner shell is reduced by the shielding effect of <br />the outer shell. Accordingly, the enhancement of leakage field by the strain can be <br />suppressed below a field corresponding to the coercivity of the strained inner shell <br />material in the double-shell structure. It is concluded that the required property for the <br />shielding material is low coercivity that is subject to little change due to the residual <br />strain, adding to high saturation induction and high permeability which are <br />conventionally required. The above results are summarized in the sixth section. <br /><br /> Chapter 4, containing three sections, focuses on the application techniques, and <br />describes the actual performance of the ferromagnetic shielding for devices operating <br />under the large-scale equipment in big science projects. The first section of this chapter <br />provides some practical uses of the soft magnetic alloy developed in chapter 2. The <br />practical applications of Fe-1%Al alloy to the magnetic shielding are presented for <br />photomultiplier tubes (PMT), neutral beam injectors (NBI), and so on, which are used <br />in the magnetically confined fusion-plasma science and the high energy accelerator <br />science. In the second section, the shielding design for a neutralizing cell of the NBI, <br />which is used for the heating of the magnetically confined fusion-plasmas, are <br />discussed as an example for the design optimization with the double-layer shielding <br />system. The role of the outer shell under a severe condition, where a large volume is <br />shielded from strong external magnetic field, is addressed, and it is clarified that the <br />inner shell design including the structure and the shell materials can be optimized by a <br />proper selection of the outer shell materials. It is also confirmed that the double-layer <br />shielding is effective not only for reducing the leakage field in the shielded space but <br />also for abating the deterioration of the shielding efficacy in the strained shell material. <br />The summary of this chapter is provided in the third section. <br /><br /> Chapter 5 is the conclusion of this thesis. The results obtained in this study are <br />summarized, and the future prospects of the ferromagnetic shielding techniques are <br />presented. Additional challenges for the further improvement are proposed from a <br />viewpoint of both the material science and the electromagnetism. The results obtained <br />here on the designing techniques and the material techniques for the magnetic <br />shielding should contribute to future development in not only big science projects but <br />also general physics researches as invaluable techniques. <br /><br /> |
要旨・審査要旨 (459.9 kB)
