JPH06310897A - Ferromagnetic magnetic shield body - Google Patents

Abstract

PURPOSE:To prevent generation of trap magnetic field without hindering mea surement, by folding ferromagnetic magnetic shield body over a vessel aperture end portion and inside the aperture end portion, so as to cover the aperture end surface and the cylindrical side surface of a superconducting magnetic shield vessel and to wrap the aperture end top part of the magnetic shield vessel. CONSTITUTION:A ferromagnetic magnetic shield body 1 has an external form somewhat larger than a superconducting magnetic shield vessel 2 in order to cover the closed aperture end surface and the cylindrical side surface of the vessel 2, and is folded over the aperture end top part and the aperture end inside upper part. The shield body is constituted as one body member. The length L and the inner diameter D of a folded part 3 are larger than or equal to one-half of the inner diameter W of the vessel 2. Thereby the trap of earth magnetism at the time of cooling the superconducting magnetic shield vessel can be completely prevented by a comparatively small-sized ferromagnetic body.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferromagnetic magnetic shield body of a superconducting magnetic shield container utilizing a superconducting phenomenon (excluding magnetic flux), and particularly when cooling the superconducting magnetic shield container to a temperature below its critical temperature Tc. The present invention relates to a ferromagnetic magnetic shield that can prevent an environmental magnetic field (especially geomagnetism) that causes magnetic noise from being trapped in a superconductor.

[0002]

2. Description of the Related Art Recently, much research has been conducted on applying an oxide superconductor to a magnetic shield container by utilizing the superconducting phenomenon of eliminating the magnetic flux. In this case, when the magnetic shield container made of oxide superconductor is cooled to be in a superconducting state, the superconductor traps magnetic flux so as to preserve the environmental magnetic field (earth magnetism in normal environment) in which it is placed. Resulting in. In such a superconducting magnetic shield container, SQUID (superconducting quantum interference device)
If the trapped magnetic flux fluctuates or relative vibration occurs between the SQUID magnetometer and the shield container when operating the magnetometer to detect an extremely weak magnetic field, the SQUID magnetometer detects this as a magnetic fluctuation. , This becomes the source of magnetic noise.

Therefore, in cooling the superconductor, it is disclosed in JP-A-3-242134 that the outside of the superconductor is covered with a ferromagnetic material in order to reduce the environmental magnetic field in the ferromagnetic body as much as possible. Although this ferromagnetic material makes it easy to carry the DUT into the shield and allows quick transition to measurement, it has the drawback of incomplete reduction of the environmental magnetic field near the opening end due to the leakage magnetic field from the opening end. In addition, the trap magnetic field becomes strong at the opening end and becomes nonuniform. This can lead to migration of the trap flux to a deeper part of the container away from the open end, which can be problematic. Therefore, it is necessary to make the ferromagnetic material sufficiently longer than the superconductor at the opening end of the superconductor, which causes a problem that the device itself becomes large. Further, JP-A-3-218695 discloses that an opening end of a superconductor is covered with a ferromagnetic lid.
However, although the reduction of the environmental magnetic field is uniform and sufficient, it takes a lot of time to remove the lid when shifting to the measurement after cooling, and there is a problem that it becomes very complicated especially with the increase in size of the shield container. It was a thing.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a ferromagnetic magnetic shield which can sufficiently prevent the occurrence of a geomagnetic trap when the oxide superconductor magnetic shield container is cooled without hindering the measurement. It is a thing.

[0005]

The ferromagnetic magnetic shield of the present invention is used for preventing a trap magnetic field when cooling a magnetic shield container made of a cylindrical or rectangular tube type superconductor with one end closed / one end open. A ferromagnetic magnetic shield, which is folded into the container opening end and the inside of the opening end so as to cover the closed end surface and the tubular side surface of the superconducting magnetic shield container and wrap around the opening end top of the magnetic shield container. Is.

FIG. 1 shows a ferromagnetic magnetic shield body according to the present invention. The ferromagnetic magnetic shield body 1 covers the closed end surface and the cylindrical side surface of the superconducting magnetic shield container 2 more or less than the container 2. It has a large outer shape and is folded so as to wrap around the top of the open end of the container 2 and the upper inside of the open end, and these are integrally formed. The length L and the inner diameter D of the folded portion 3 are at least 1 / the inner diameter W of the container 2.
It is preferable that the number is 2 or more from the viewpoint of the attenuation factor of the external magnetic field and insertion of the object to be measured from the opening end.

Since the present invention is configured as described above, the closed end surface and the cylindrical side surface of the superconducting magnetic shield container are all covered with the ferromagnetic magnetic shield body, and the opening end portion of the superconducting magnetic shield container is at the top of the container. Since the inside is covered with the folded portion of the ferromagnetic material, the trap of the geomagnetism at the time of cooling the superconducting magnetic shield container is completely prevented by the relatively small ferromagnetic material.

[0008]

According to the present invention as described above, the following effects can be obtained. (1) The magnetic field shield near the open end can be reduced without a lid by using a one-piece open / one-closed ferromagnetic magnetic shield with a folded part in the opening to reduce the environmental magnetic field of the entire superconducting magnetic shield container. The trap magnetic field can be prevented. (2) Since it is not necessary to attach a lid to the opening end during cooling,
It is not necessary to remove the lid after cooling, and the measurement can be started immediately. (3) Since the superconductor and the ferromagnetic body are integrated, even when the superconductor becomes superconducting after cooling and effectively shields the magnetic field as a magnetic shield,
The external magnetic field is first shielded by the surrounding ferromagnetic shield, and the magnetic field is attenuated by the superconductor, so that an extremely weak magnetic field space can be created.

[0009]

Example 1 A thickness of 1 mm according to the present invention as shown in FIG.
The magnetic field distribution in the ferromagnet when a uniform external magnetic field of about 400 milligauss is applied by the Helmholtz coil in the cylindrical μ-metal cylinder axial direction of the fluxgate meter 4
At the z-axis shown in FIG.
= Defined by the ratio of external magnetic field / internal magnetic field), the result is as shown in FIG. From this result, the magnetic field in the folding part 3 near the opening end is attenuated to 1/10 to 1/100 of the environmental magnetic field, reaching a level equivalent to a position deeper than this part, and the effect of the present invention can be confirmed. It was

[0010]

COMPARATIVE EXAMPLE 1 An external magnetic field was applied in the axial direction to a ferromagnetic cylinder having one opening / one closing as shown in FIG. The magnetic field in the ferromagnetic cylinder at that time was measured on the z axis in the same manner as in Example 1, and the result of obtaining the shield effect is shown in FIG. From this result, it is understood that the vicinity of the opening end is affected by the leakage magnetic field, and the attenuation of the environmental magnetic field is significantly worse than that of the first embodiment.

[0011]

[Embodiment 2] As shown in FIG. 6, a strong magnetic substance was applied to a ferromagnetic material similar to that of Embodiment 1 in the same manner as in Embodiment 1 in the case of applying an external magnetic field of about 400 milligauss in the y direction perpendicular to the cylindrical axis. The magnetic field in the magnetic cylinder was measured on the z-axis shown in the figure by the same method as in Example 1 to obtain the shield effect. The result is shown in FIG. 7. From this result, the magnetic field in the folded portion near the opening end is attenuated to about 1/40 of the environmental magnetic field, and becomes the same level as the in-cylinder magnetic field at a position deeper than this portion, as in the first embodiment. The effect of the invention was confirmed.

[0012]

[Comparative Example 2] An external magnetic field is applied to the same ferromagnetic body as that shown in FIG. 4 in the y direction perpendicular to the cylinder axis as shown in FIG. FIG. 9 shows the result of measuring the shield effect in the same manner as in Example 2. From this result, it can be seen that the vicinity of the opening end is affected by the leakage magnetic field as in the case of Comparative Example 1, and the attenuation of the environmental magnetic field is significantly worse than that of Example 2.

[0013]

[Embodiment 3] As shown in FIG. 10, a superconducting magnetic shield container with one end open / one end closed (inner diameter 100 mm, length 25)
0 mm) in the ferromagnetic body shown in FIG. 2 of the present invention,
This was cooled in the earth's magnetic field environment. In this case, the magnetic shield effectively shielded the earth's magnetic field, and the trap field was significantly reduced. The magnetic field was measured in the axial direction and the radial direction at a position 150 mm inside from the opening end on the central axis of the superconductor magnetic shield container. Table 1 shows the measured values of the trap magnetic field.

[0014]

Comparative Example 3 In FIG. 11, the superconductor alone was cooled in an earth magnetic field environment and the strength of the trap magnetic field was examined. In this case, it can be seen that the superconductor traps geomagnetism (300 to 400 milligauss) almost completely in both the axial and radial directions (Table 1).

[0015]

[Comparative Example 4] A superconducting magnetic shield container was installed in a ferromagnetic body having one end opening / one end closing shown in FIG. 12 and cooled in the same manner. At this time, since the whole body is covered with the ferromagnetic material except for the opening end, the number of traps of geomagnetism is relatively small, but as shown in Table 1, the strength of the trapped magnetic field is larger than that of the third embodiment. Is ten times stronger.

[0016]

Comparative Example 5 As shown in FIG. 13, a ferromagnetic ring having a diameter of 162 mm and a length of 90 mm (Japanese Patent Laid-Open No. 3-242134).
The same method) was placed at the open end of the superconductor, cooled as described above, and the trap field was measured.

[0017]

Comparative Example 6 In addition to the ring of Comparative Example 5, the diameter is 90 m.
A ferromagnetic ring having a length of m and a length of 90 mm was arranged so that a part thereof was inside the superconductor as shown in FIG. 14, and the trap magnetic field was measured after cooling. In Comparative Examples 5 and 6, it can be seen that the geomagnetism is completely trapped in the superconductor as shown in Table 1.

[0018]

[Table 1]

As described above, when the ferromagnetic magnetic shield of the present invention is used, the trap magnetic field can be reduced 10 to 100 times as much as that of the comparative example, and an extremely weak magnetic field space can be easily created. .

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing a ferromagnetic magnetic shield of the present invention with a superconducting magnetic shield container housed therein.

FIG. 2 is a schematic explanatory diagram of a method of measuring the cylindrical axial shield effect of the first embodiment.

FIG. 3 is a result diagram showing a shield effect in the cylinder axis direction of Example 1.

FIG. 4 is a schematic explanatory diagram of a method of measuring a cylindrical axial shield effect of Comparative Example 1.

FIG. 5 is a result diagram showing a shield effect in a cylinder axis direction of Comparative Example 1.

FIG. 6 is a schematic explanatory diagram of a method for measuring the cylindrical radial shield effect of the second embodiment.

FIG. 7 is a result diagram showing the shielding effect in the radial direction of the cylinder in Example 2;

FIG. 8 is a schematic explanatory diagram of a method of measuring the cylindrical radial shield effect of Comparative Example 2.

9 is a result diagram showing a shielding effect in a radial direction of a cylinder of Comparative Example 2. FIG.

FIG. 10 is an explanatory view showing an oxide high temperature superconducting container installed in a ferromagnetic body having folds of Example 3.

11 is an explanatory view showing an oxide high temperature superconducting container of Comparative Example 3. FIG.

FIG. 12 is an explanatory diagram showing an oxide high temperature superconducting container installed in a one-end open / one-end closed ferromagnetic body of Comparative Example 4.

13 is an explanatory diagram showing an oxide high temperature superconducting container in which a ferromagnetic cylinder is arranged outside the open end of Comparative Example 5. FIG.

FIG. 14 is an explanatory view showing an oxide high temperature superconducting container of Comparative Example 6 in which ferromagnetic cylinders are arranged inside and outside an open end.

[Explanation of symbols]

 1 Ferromagnetic Magnetic Shield 2 Superconducting Magnetic Shield Container 3 Folding Part 4 Fluxgate Meter

Claims (2)

[Claims] 1. A ferromagnetic magnetic shield body for preventing a trap magnetic field when cooling a magnetic shield container made of a cylindrical or prismatic type superconductor with one end closed / one end opening, the superconducting magnetic shield container comprising: A ferromagnetic magnetic shield body which is folded inside the opening end and inside the opening end of the magnetic shield container so as to cover the closing end surface and the cylindrical side surface and wrap around the opening end top of the magnetic shield container. 2. The ferromagnetic magnetic shield according to claim 1, wherein the inner fold length and the diameter of the ferromagnetic magnetic shield at the open end are at least ½ or more of the inner diameter of the superconducting magnetic shield container.