{"created":"2023-06-20T13:20:30.713583+00:00","id":538,"links":{},"metadata":{"_buckets":{"deposit":"37fd2454-54ad-4b27-ae6c-199d0520700a"},"_deposit":{"created_by":1,"id":"538","owners":[1],"pid":{"revision_id":0,"type":"depid","value":"538"},"status":"published"},"_oai":{"id":"oai:ir.soken.ac.jp:00000538","sets":["2:427:12"]},"author_link":["0","0","0"],"item_1_creator_2":{"attribute_name":"著者名","attribute_type":"creator","attribute_value_mlt":[{"creatorNames":[{"creatorName":"大森, 俊道"}],"nameIdentifiers":[{"nameIdentifier":"0","nameIdentifierScheme":"WEKO"}]}]},"item_1_creator_3":{"attribute_name":"フリガナ","attribute_type":"creator","attribute_value_mlt":[{"creatorNames":[{"creatorName":"オオモリ, トシミチ"}],"nameIdentifiers":[{"nameIdentifier":"0","nameIdentifierScheme":"WEKO"}]}]},"item_1_date_granted_11":{"attribute_name":"学位授与年月日","attribute_value_mlt":[{"subitem_dategranted":"2008-03-19"}]},"item_1_degree_grantor_5":{"attribute_name":"学位授与機関","attribute_value_mlt":[{"subitem_degreegrantor":[{"subitem_degreegrantor_name":"総合研究大学院大学"}]}]},"item_1_degree_name_6":{"attribute_name":"学位名","attribute_value_mlt":[{"subitem_degreename":"博士（工学）"}]},"item_1_description_12":{"attribute_name":"要旨","attribute_value_mlt":[{"subitem_description":" In order to avoid errors in the operation of devices subject to strong magnetic
fields, shielding systems are applied to such devices that would be affected by magnetic
fields. In big science projects such as the magnetically confined fusion-plasma science
and the high-energy accelerator science, highly efficient magnetic shields are required
for various devices sensitive to magnetic fields in order to reduce the strong fields
generated by large magnets, including superconducting magnets, to fields less than the
geomagnetic field within a limited space to have no influence on their operation.
In physical science experiments including the above big science projects, the
ferromagnetic shield employing enclosures made from pure iron, Permalloys or mild
steel is usually applied for the required shielding. However, there have stilI been
difficulties with designing the shielding system for devices operating under the
large-scale equipment generating the strong field, probably because subjects on the
prediction of the shielding effectiveness and the material technology for the shielding
shell have not been definitely resolved.
For the subject on the prediction of the shielding effectiveness, it is proposed in
this thesis that the effect of the hysteresis of the shell material on the shielding efficacy
should be considered. The magnetic hysteresis is well known as a phenomenon
characterizing ferromagnetic materials. The ferromagnetic shielding effectiveness must
be affected by the hysteresis of the shell material because its magnetized condition is
strongly dependent on the previous state.
For the subject on the material technology, there has been less knowledge of
preparing shell materials simultaneously satisfying both high flux density and soft
magnetism. For example, the magnetic flux density is sufficiently high on pure iron
with moderate permeability while it is insufficient on Permalloy in contrast with its
high permeability. It should be also considered that the magnetic properties of soft
magnetic materials are strongly affected by the strain, especially for the residual strain,
due to their structural sensitivities.
Although these subjects still remain to be solved, there have been few reports
discussing effects of the hysteresis on the magnetic shielding effectiveness theoretically,
and some only treat them from a phenomenological point of view. Furthermore, there
have also been few reports quantitatively investigating effects of the strain of the shell
materials on the magnetic shielding effectiveness.
This thesis aims at two subjects. One is to optimize the designing techniques of
the ferromagnetic shielding from the strong magnetic fields for devices operating under
large-scale equipments, based on the research on shell materials from a viewpoint of
the material science. The other is to clarify the influences of the magnetic properties on
the shielding effectiveness from a viewpoint of the electromagnetism, leading to further
understanding of the static ferromagnetic shield.
First, research on the shell materials is focused on the improvement of the soft
magnetism without reduction of the magnetic flux density. Additionally, the
deterioration of the soft magnetism due to a tiny amount of strain in the shell material
is quantitatively investigated because the shell material is subject to be strained by
stress and deformation during the assembly and installation of the shielding system.
Next, the effect of the hysteresis of the shell material on the shielding
effectiveness is discussed against static magnetic fields. It has been qualitatively
understood that the shielding effectiveness can be seriously affected by strain of the
shell material. Therefore, an influence of a tiny amount of strain of the shell material
on the magnetic shielding effectiveness is analyzed.
Finally, a guiding principle for evaluating the maximum leakage field as a
shielding efficacy of the shell material, which is a reasonable and obtainable field in
the worst-case scenario for the magnetized condition of the shielding shell, is discussed.
It is determined by considering the hysteresis of the shell material, and it is revealed
that the concept of the maximum leakage field is effective whether the shell material is
strained or not.
The thesis consists of five chapters, which are summarized below.
Chapter 1 states the purpose and background of this study, providing an overview
of the prior researches, and describes the significance of this study. The first section of
this chapter explains the necessity of the magnetic shielding from strong static fields
for devices operating under large-scale equipments, by introducing specific situations
inherent in big science projects, such as the magnetically confined fusion-plasma
science and the high-energy accelerator science. In the second section, the novelties
and goals of this study are summarized. The structure of this thesis is outlined in the
third section.
Chapter 2, consisting of five sections, describes the research and development of
the soft magnetic materials for the use of the magnetic shielding system. In the first
section, the purposes of the research in this chapter are addressed.
The second and third sections of this chapter focus on the strategies to improve the
soft magnetic properties of pure iron and to develop soft magnetic materials with both
higher permeability and lower coercivity than those of pure iron without deteriorating
its high-magnetic flux density by the grain-coarsening technique. The developed
Fe-1%Al alloy exhibits extremely large ferrite grains in its microstructure, and has
almost the same high-permeability and low-coercivity as those of Permalloy B.
Moreover, it indicates high saturation magnetization, which is lower only by 2% than
that of pure iron.
The fourth section of this chapter discusses an influence of a tiny amount of plastic
strain on the magnetic properties. It is revealed that the deterioration of coercivity due
to the plastic strain is alleviated in the coarsened grain microstructure. Therefore, it is
understood that the improvement of the soft magnetism by the grain-coarsening
technique is effective for the prevention of the deterioration of coercivity by strain
caused through damage. The results of the research in this chapter are summarized in
the fifth section.
Chapter 3, containing six sections, explains a prediction method for the magnetic
shielding effectiveness and a design concept for the double-layer shielding techniques.
The first section of this chapter addresses the purpose of the research in this chapter.
In the second and third sections of this chapter, a new approach to the estimation
of the static ferromagnetic shielding effectiveness is proposed, in which the magnetic
hysteresis of the shielding materials is considered. The hysteresis effects of the shell
materials are discussed on the ferromagnetic shielding, in which a relatively strong
external field is reduced below the geomagnetic field in a shielded space. The measured
leakage field in the shielding enclosure corresponds to the results of the finite element
method (FEM) analysis when permeability considering the effect of coercivity is used
for the calculation as a parameter representing the hysteresis of the shielding material.
The effects of both permeability and coercivity on the leakage fields are discussed
in the fourth section of this chapter, with regard to the magnetic properties of the
shielding materials by using the FEM analysis in combination with the results
obtained in chapter 2. The maximum leakage field, which is regarded as a figure of
merit of the shielding efficacy, is determined by the coercivity of the material used for
the shield, and it is clarified that the coercivity should be considered for the estimation
of the leakage field in an actual design of the shielding system. It is also confirmed that
the deterioration of the soft magnetic properties, not only permeability but also
coercivity, due to the residual strain causes the reduction of the magnetic shielding
efficacy. Finally, it is concluded that the maximum leakage field is dominated by the
coercivity of the shell material regardless of the shell material condition being strained
or not.
In the fifth section, the effect of the double-layer structure on the shielding design
is investigated by using the analysis with considering the hysteresis of the shell
material. The double-shell structure is definitely effective for abating the degradation
of the shielding efficacy in the strained shielding material in that the influence of the
deterioration of permeability of the inner shell material due to the strain is lowered
because the external field applied to the inner shell is reduced by the shielding effect of
the outer shell. Accordingly, the enhancement of leakage field by the strain can be
suppressed below a field corresponding to the coercivity of the strained inner shell
material in the double-shell structure. It is concluded that the required property for the
shielding material is low coercivity that is subject to little change due to the residual
strain, adding to high saturation induction and high permeability which are
conventionally required. The above results are summarized in the sixth section.
Chapter 4, containing three sections, focuses on the application techniques, and
describes the actual performance of the ferromagnetic shielding for devices operating
under the large-scale equipment in big science projects. The first section of this chapter
provides some practical uses of the soft magnetic alloy developed in chapter 2. The
practical applications of Fe-1%Al alloy to the magnetic shielding are presented for
photomultiplier tubes (PMT), neutral beam injectors (NBI), and so on, which are used
in the magnetically confined fusion-plasma science and the high energy accelerator
science. In the second section, the shielding design for a neutralizing cell of the NBI,
which is used for the heating of the magnetically confined fusion-plasmas, are
discussed as an example for the design optimization with the double-layer shielding
system. The role of the outer shell under a severe condition, where a large volume is
shielded from strong external magnetic field, is addressed, and it is clarified that the
inner shell design including the structure and the shell materials can be optimized by a
proper selection of the outer shell materials. It is also confirmed that the double-layer
shielding is effective not only for reducing the leakage field in the shielded space but
also for abating the deterioration of the shielding efficacy in the strained shell material.
The summary of this chapter is provided in the third section.
Chapter 5 is the conclusion of this thesis. The results obtained in this study are
summarized, and the future prospects of the ferromagnetic shielding techniques are
presented. Additional challenges for the further improvement are proposed from a
viewpoint of both the material science and the electromagnetism. The results obtained
here on the designing techniques and the material techniques for the magnetic
shielding should contribute to future development in not only big science projects but
also general physics researches as invaluable techniques.
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