Magnetically shielding structure

Details Magnetically shielding structure Magnetically shielding structure

1994-09-19

2002-11-26

Paladini, Albert W. (Department: 2827)

Electricity: conductors and insulators

Anti-inductive structures

Conductor transposition

C174S034000, C174S034000, C505S872000

Reexamination Certificate

active

06486393

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a magnetically shielding structure utilizing a superconductor cylinder and, in particular, it relates to a magnetically shielding structure in which magnetically shielding capability is made to improve by combining magnetically shielding materials of various different properties and in which a penetrating magnetic field from the opening portion of the superconductor cylinder is reduced to a very small value, the volume of a usable highly magnetically shielded space inside the cylinder bore is increased, or a very feeble magnetic field can be realized in the cylinder bore even in the case of a short superconductor cylinder. Thereby, a much feebler magnetic field than an external one can be effectively realized in the cylinder bore.
BACKGROUND ART
In a magnetically shielding structure constructed by a superconductor, the Meissner effect is utilized for the magnetic shield. That is, a material having the Meissner effect is, for example, formed into a cylinder shape to form a shielding body and is cooled below the critical temperature Tc for the transition to a superconductive state for making the shielding body a diamagnetic and, thereby, a magnetic flux is forced out to the exterior of the shielding body, and the internal space of the shielding body is magnetically shielded.
On the other hand, in the case of a shielding structure with a highly permeable material being commonly used without utilizing a superconductor, where a shielding body is formed into a cylinder with the highly permeable material, for example, and if the shielding body is held in a magnetic field, magnetic induction is generated in the wall of the shielding body and the magnetic field is short-circuited along the shielding body. The internal bore space of the cylinder is magnetically shielded thereby.
In such a magnetically shielding structure utilizing a superconductor, although the magnetically shielding capability of a cylindrical shielding body, for example, is high enough for a magnetic field parallel to the center axis of the cylinder (longitudinal magnetic field), the magnetically shielding capability for a magnetic field perpendicular to the center axis (lateral magnetic field) is not enough. Therefore, there is a problem in that the length of the cylinder has to be long in comparison with the inner diameter of the cylinder.
On the other hand, in the case of a cylindrical shielding body formed with a highly permeable material, for example, the shielding capability for longitudinal magnetic field is not enough in comparison with that for lateral magnetic field. Also, because of the limited value of permeability, the shielding efficiency of a single-layer cylinder can not be high enough, so that, for obtaining high shielding capability, a plurality of cylinder walls have to be laminated and a structure in which the inner layer is shorter than the outer layer has to be adopted. As a result, even in the case of a highly permeable material being used, there has been a problem in that the length of a cylinder outer layer becomes long. When a usable space is to be larger, the dimensions in the radial dirction and also in the axial direction have to be large, which causes a problem in that the shielding body becomes expensive.
DISCLOSURE OF INVENTION
The present invention is invented considering various aspects as described above, and the object of the invention is to provide a magnetically shielding structure in which the shielding capability is improved by combining magnetically shielding materials of various different properties. A volume of a highly usable magnetically shielded space is increased by suppressing a penetrating magnetic field through the opening portion of a superconductor cylinder to be at a low value. Alternatively, an extremely feeble magnetic field can be realized in a specified space in the cylinder even in the case of a short superconductor cylinder. Thereby an extremely feeble magnetic field can be efficiently realized in comparison with an external magnetic field.
According to the present invention, the above-mentioned object can be solved with a magnetically shielding structure having the features as described in each of the claims.
In a magnetically shielding structure according to the present invention, against a penetrating magnetic field, which shows an attenuating distribution toward the center on the longitudinal axis of a cylindrical shielding body composed of a material manifesting the Meissner effect, a wide variety of combinations of the cylindrical magnetic shielding body composed of a superconductive material, which manifests the Meissner effect, and a cylindrical collar means made of a highly permeable material having a through opening along the longitudinal direction of the cylindrical magnetic shielding body are disposed for absorbing and magnetically shorting the penetrating magnetic field by the magnetic induction generated in the highly permeable material.
FIG. 20
shows a schematic diagram of a penetrating magnetic vector inside a superconductor cylinder when a lateral magnetic field is applied to the superconductor cylinder. A penetrating magnetic field being thus distributed is attenuated to decrease the penetration quantity coming into the inside of the superconductor cylinder by utilizing a property of a highly permeable member, called magnetic induction, which magnetically shorts the penetrating magnetic field. Since a highly permeable member may have a residual magnetic field, it has to be disposed in a position where the residual magnetic field does not exert its magnetic influence on the objective space of a feeble magnetic field.
FIG. 1
is an illustrative representation showing the constitution of an ordinary embodiment of the present invention. In the figure, there is shown an end portion of a cylindrical shielding body (
1
) composed of an oxide superconductive material. The length of the shielding body (
1
) is generally required to be in a range of 2 times to 20 times the sectional diameter of the cylinder. The range, however, can be changed according to the size of a shielded space or to the intensity of a magnetic field inside the space. In the figure, a sectional view is shown in which a large diameter ferromagnetic cylindrical collar (
2
) and a small diameter ferromagnetic cylinderical collar(
3
) are disposed in the opening portion of the cylindrical magnetically shielding body (
1
) in a coaxial manner for the prevention of magnetic penetration. In the figure, there are shown a space G
1
and a distance d
1
between the magnetically shielding body (
1
) and the large diameter ferromagnetic cylindrical collar (
2
), a space G
2
, and a distance d
2
, between the large diameter ferromagnetic cylindrical collar (
2
) and the small diameter ferromagnetic cylindrical collar (
3
), and an internal space G
3
of the small diameter ferromagnetic cylindrical collar (
3
) and a distance of radius R
3
from the wall of the small diameter cylindrical collar (
3
) to the center axis. A part denoted with the numeral (
4
) is a magnetic field sensor placed in an objective feeble magnetic field space.
In the case where the superconductor cylinder (
1
) is applied alone, a penetrating magnetic field from the opening portion is expressed by following equations (I) and (II).
Ht = 10
−Kt′·Z/R1
(Kt′ = 0.79)
------(I)
Ha = 10
−Ka′·Z/R1
(Ka′ = 1.66)
------(II)
Equation (I) expresses the magnetic field intensity Ht when an external magnetic field is applied to the cylinder in the direction perpendicular to the cylinder axis (lateral magnetic field), and Equation (II) expresses the magnetic field intensity Ha when an external magnetic field is applied to the cylinder in the direction parallel to the cylinder axis (longitudinal magnetic field). R
1
is the radius of the superconductor cylinder and Z is the distance from the opening end of the cylinder. From the above equations, it is known that the quantity of a penetrating magnetic field is larger when the

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