JPH02206101A - Reduction in magnetization by superconductor - Google Patents

Abstract

PURPOSE:To reduce magnetization as viewed from the outside of an object, to prevent fluctuation in the magnetic field in a circumferential space of the object and to prevent inconvenience caused by the fluctuation by a method wherein the surface of the object composed of a material whose permeability is larger than 1 is covered with a superconductor in an area ratio according to a shape and permeability of the object. CONSTITUTION:A part corresponding to an area ratio according to permeability and the shape of an object 10 on the surface of the object 10 constituted of a substance whose permeability is larger than 1, e.g. iron, Permalloy or the like, is covered by using a method by which e.g. a required number of panel- shaped superconductors 22 are glued on or the like; a permeability as viewed from the outside of the object 10 is approached to a permeability in a circumferential space. Thereby, magnetization as viewed from the outside of the object 10 is reduced; when this object is executed as a circumferential substance of an apparatus requiring a highly uniform magnetic field, it is possible to use the apparatus in a stable state where disturbance of the magnetic field has been prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magnetization reduction method for reducing the magnetization of an object as seen from the outside using a superconductor.

[Prior Art] Objects whose magnetic permeability is larger or smaller than the surrounding space are magnetized in the presence of a magnetic field. This magnetization changes the magnetic field distribution in the space around the object.

[Problems to be Solved by the Invention] Therefore, in fields where it is inconvenient if the magnetic field distribution in the surrounding space changes, it is desirable to prevent this.

An example of where it is desirable to prevent this change in magnetic field distribution is M
RI (Magnetic Re5onance I)
Examples include the use of devices that require extremely homogeneous magnetic fields, such as image processing (magnetic resonance imaging) devices.

If this type of device is used in the presence of objects that are easily magnetized (e.g. reinforcing bars of structures, iron casings of peripheral devices, etc.), the uniform magnetic field will be disturbed by the magnetization of the nearby objects, making it difficult to use the device. is hindered. Therefore, when using the device, it is desirable to perform some kind of magnetization reduction treatment on objects around the device.

Further, another example in which it is desirable to prevent the above-mentioned change in till field distribution is prevention of detection of ships such as submarines.

A ship whose frame is made of a material such as iron with high magnetic permeability changes the magnetic field distribution in the surrounding space by magnetizing the frame. Therefore, ships are detected by measuring changes in this magnetic field distribution.

On the other hand, from the perspective of a ship being detected, it is desirable to reduce or prevent the magnetization of the frame as a measure to prevent detection.

Conventionally, one method for this purpose is to form the frame with a non-magnetic metal such as titanium instead of iron, which has high magnetic permeability. The resulting ship will be referred to as a ``conventional non-magnetic metal ship'').

However, in this method, the cost is much higher than in the case where the frame is made of iron.

Furthermore, magnetic fields generated from magnetic field sources inside the ship's hull (such as electric propulsion motors and transformers) leak to the outside through the non-magnetic metal frame, necessitating a magnetic shield. Generally, in this type of magnetic shield, a ferromagnetic material is placed around a magnetic field generation source to form a magnetic circuit.
By passing magnetic flux through this, leakage of the magnetic field to the outside is prevented. That is, a ferromagnetic material that causes fluctuations in the magnetic field in the surrounding space is used as a shield.

Therefore, if the frame is made of non-magnetic metal to prevent detection, a paradoxical situation arises in that the ferromagnetic material of the magnetic shield ends up being the cause of detection.

The present invention has been made in view of these problems of the prior art, and its purpose is to provide a magnetization reduction method for reducing the magnetization of an object as seen from the outside.

[Means for Solving the Problems] In order to achieve the above object, the magnetization reduction method according to the present invention reduces the surface of an object made of a material with a magnetic permeability greater than 1 by reducing the permeability and shape of the object. By covering the object with a superconductor at an area ratio corresponding to the area ratio, the magnetization seen from the outside of the object is reduced.

[Function] The magnetization reduction method according to the present invention reduces the area ratio of the surface of an object made of a material with magnetic permeability greater than 1 (for example, iron, permalloy, etc.) according to the magnetic permeability and shape of the object. The area corresponding to the object is covered by a method such as attaching a required number of panel-shaped superconductors, so that the magnetic permeability seen from the outside of the object approaches the magnetic permeability of the surrounding space.

The principle will be explained with reference to FIG.

In the figure, the object generally designated 10 has a thickness of 1
A cube (object) with a side of 2 m formed by a cm iron plate
On the surface of the cube 110, 10 cm square superconductor unit panels 22 are pasted at approximately regular intervals, making up 40% of the surface area of the cube 11.
It is covered.

Each superconductor unit panel 22 is a 1 mm thick oxide superconductor (not shown) placed in a heat insulating case and cooled in advance with liquid nitrogen to maintain superconductivity.

Let us now consider the magnetic state of the surface of the cube 11.

As is well known, the strength of magnetization (magnetic efficiency per unit volume)
Between ■ and magnetic flux density B, B=μ. There is a relationship of (1+H). μ here. is the vacuum permeability and H is the strength of the magnetic field.

First, the surface of the cube 11 on which iron is exposed is magnetized in the direction of the magnetic field, with the magnetization strength I being proportional to the magnetic field strength H, and I=χH. Here, χ is the magnetic susceptibility.

On the other hand, on the surface of the cube 11 covered with the superconductor unit panel 22, since the superconductor has the property of B=O, the magnetization strength is I=-H. That is, it is magnetized in the opposite direction to the magnetic field.

Therefore, on the surface of the cube 11, the direction of magnetization is opposite between the exposed iron part and the part covered with the superconductor unit panel 22. Therefore, both magnetizations cancel each other out,
Overall, the magnetization can be brought close to τ.

The area ratio of the surface that should be covered with the superconductor in order to bring the magnetization close to 7 depends on the shape of the object and the magnetic permeability of the material.

- By way of example, for objects made of iron or permalloy, it ranges from approximately 10% to 15%.

In the manner described above, the magnetization of the object viewed from the outside can be reduced by the superconductor.

[Embodiments] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 shows an embodiment of the magnetization reduction method according to the present invention, and its structure is as explained in the section of the above-mentioned operation. Objects whose magnetization is undesirable (e.g. M
The surface of the material surrounding equipment that requires a highly uniform magnetic field such as RI equipment, or a frame made of a material that is easily magnetized such as iron, is adjusted to the shape and magnetic permeability of the object as shown in Figure 1. By covering the object with the superconductor panel 22 at a corresponding area ratio, the magnetization seen from the outside of the object can be reduced.

Further, FIG. 2 shows a magnetization reduction type magnetic shielding device as an application example of the magnetization reduction method according to the present invention.

In FIG. 2, the magnetic shielding device, designated as a whole by the reference numeral 20, has a two-layer permalloy plate 21A having a thickness of 0.5 cm.
, 21B with a cubic shape of 2 m on a side.
40% of the surface area of the outer permalloy plate 21A is covered with a superconductor unit panel 22 similar to that shown in FIG. 1, and a magnetic field generation source 23 is installed inside the shield 21. There is. The magnetic field generated from the magnetic field generation source 23 is generated by a magnetic circuit mainly composed of permalloy in the inner layer.
Magnetic flux leakage to the outside is prevented.

In the magnetic shielding device 20 configured in this way,
The action and effect of reducing magnetization are the same as in the embodiment shown in FIG. That is, the exposed surface of the outer Permalloy layer is magnetized in the direction of the magnetic field existing outside the shield, and the surface covered with the superconductor unit panel 22 is magnetized in the opposite direction to the magnetic field, and the magnetizations of both cancel each other out. Therefore, the magnetization as a whole can be brought close to zero. Therefore, if this magnetic shielding device 20 is used for magnetic shielding of a magnetic field generation source mounted on a conventional non-magnetic metal ship, in addition to preventing leakage of the magnetic field generated from the magnetic field generation source to the outside of the ship's body,
It is also possible to prevent magnetic field fluctuations in the space surrounding the frame due to the shield.

Note that the ratio of the surface area that should be covered with the superconductor in order to bring the magnetization close to zero is as described above.

For reference, the measurement results of changes in the earth's magnetic field by the object shown in FIG. 1 and the magnetic shield device shown in FIG. 2 will be explained.

By moving the object 10 (or magnetic shielding device 20), the resulting change in the earth's magnetic field is measured as SQU I D (Super
conductive Quantum Interf
When measured with a magnetometer (Erence Qavice superconducting quantum interference device), the change was less than the noise fluctuation of the surrounding magnetic field until the distance between the magnetometer and the object 10 (or the magnetic shielding device 20) reached 5 m.

On the other hand, the cube 1 not covered with the superconductor unit panel 22
1 (or the shield 21 not covered with the superconductor unit panel 22), a change in the earth's magnetic field was detected at a distance of 60 m.

This measurement result shows that the superconductor has a great effect in preventing magnetization.

Incidentally, in each of the above embodiments, a plate-shaped unit panel 22 in which an oxide superconductor is housed in a thermally insulating case is shown as an example of a superconductor, but the structure and shape of the superconductor are not limited to this. It's not something you can do. Furthermore, the materials to be prevented from magnetization are not limited to iron and permalloy;
It is widely applied to things that are relatively easily magnetized, that is, have a magnetic permeability of 1 or more.

[Effects of the Invention] As explained above, according to the magnetization reduction method according to the present invention, the surface of an object made of a material with magnetic permeability greater than 1 is superimposed at an area ratio depending on the shape and magnetic permeability of the object. By covering with a conductor, the magnetization seen from the outside of the object can be reduced. Therefore, it is possible to prevent fluctuations in the magnetic field in the space surrounding the object and the associated problems.

If the above magnetization reduction method is applied to materials surrounding a device that requires a highly uniform magnetic field, such as an MHI device, disturbance of the magnetic field is prevented and the device can be used in a stable state.

Furthermore, if the above magnetization reduction method is applied to a ship body such as a submarine made of a material that is easily magnetized or a magnetic shielding device mounted thereon, the ship can escape detection based on magnetic field fluctuations.

・Magnetic shielding device ・Shielding body ・Superconductor unit panel ・Magnetic field source (superconductor) In addition, in each figure, The same code indicates the same or equivalent part. show.

Representative Patent Attorney Sa Fujimasa

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an embodiment of the magnetization reduction method according to the present invention, and FIG. 2 is a cross-sectional view showing a magnetic shielding device as an application example of the magnetization reduction method according to the invention. [Explanation of symbols of main parts] 10...Object 11...Cube (object) - Object/cube/superconductor unit panel, 20/21 A. 22 ・ 23 ・ ・Magnetic shield device・side, shield 21B・・permalloy plate・travel conductor vehicle position panel・magnetic field source

Claims (1)

[Claims] By covering the surface of an object made of a material with a magnetic permeability greater than 1 with a superconductor at an area ratio depending on the magnetic permeability and shape of the object, the magnetization seen from the outside of the object can be reduced. Characteristic magnetization reduction method.