JPH03212980A - Method for formation of magnetic shield - Google Patents

Abstract

PURPOSE:To execute a magnetic shielding operation at a high shielding rate by a method wherein a cylindrical superconducting loop where a superconducting film has been formed on the surface of a flexible film is formed, a space area surrounded by the loop becomes minimum at a normal conducting state and it becomes maximum after having been changed to a superconducting state. CONSTITUTION:In a normal conducting state the side face of a super-conducting loop 3 is first pressed and the area of a space surrounded by the loop is made as small as possible. Then, the superconducting loop 3 is immersed in a liquid cryogen and is changed to a superconducting state; after that, the area of the space surrounded by the superconducting loop 3 is made maximum and is expanded, e.g. to be nearly cylindrical. By this constitution, even when a slight magnetic flux is passed though the space inside the superconducting loop 3 in a pressed state, a flux density, inside the loop, which is generated by an external magnetic field can be reduced sharply and can be reduced easily to 1/100 when the space is expanded. Consequently, when the loop is used in combination with amu metal shield, a shielding rate against the external magnetic field can be improved to a prescribed value (1/tens of thousands).

Description

[Detailed Description of the Invention] [Summary] This invention relates to a method for forming a magnetic shield for shielding devices such as Josephson integrated circuits that are sensitive to external magnetic fields and may malfunction from external magnetic fields, using superconducting metals. For the purpose of shielding the above-mentioned devices etc. from external magnetic fields with a higher shielding rate than before when performing magnetic shielding, etc., it is configured to include a superconducting film formed on a flexible film, The film with the superconducting film is configured to have a structure that can be transformed into a cylindrical superconducting loop as a whole, and on the other hand, when the superconducting loop is in a normal conducting state, the superconducting loop is The area of the surrounding space is minimized, and then, after the superconducting loop is transitioned to a superconducting state, the area of the space is expanded to a maximum.

[Industrial application field]

The present invention applies to devices that are sensitive to external magnetic fields, such as Josephson integrated circuits,
The present invention relates to a magnetic shield for shielding a device that may malfunction from an external magnetic field, and a method for forming the same.

In recent years, progress has been made in the development of systems that perform ultra-high-speed arithmetic processing using Josephson elements. Furthermore, in fields such as medical electronics, attempts have been made to measure minute magnetic fields generated by objects to be measured, such as the brain or heart, using Josephson elements as highly sensitive magnetic sensors. In these cases, the entire device including the Josephson element etc. must be placed in a low magnetic field environment isolated from magnetic noise caused by external magnetic fields such as earth's magnetism. The present invention refers to one way to create this low-field environment.

[Conventional technology]

FIG. 5 is a front view showing a cross section of an example of a conventional magnetic shield. Here, a cylindrical structure is formed by doubling a magnetic material having high magnetic permeability, for example μ metal 7, and a device 8 including a Josephson element is placed in the space surrounded by the double μ metal 7. There is. In this way, external magnetic fields such as terrestrial magnetism can be shielded with a shielding rate of about one-tenth in the X direction (horizontal direction), and on the other hand, in the Y direction (vertical direction)
In this case, shielding can be achieved with a shielding rate of - several hundredths. However, in order to further reduce the malfunction rate of the Josephson element, it is necessary to use a It is required to further increase the shielding rate (several tenths to tens of thousands of times). For this reason, a hollow cylindrical superconducting metal 1 made of lead, niobium, etc. is placed inside the double μ metal 7 and around the device 8.
7 is placed. This superconducting metal 17 is heated at an extremely low temperature below the critical temperature (for example, liquid helium temperature 23'K).
It has the property of becoming a perfect conductor with an electrical resistance of 000, and becoming a perfect diamagnetic material that completely eliminates external magnetic fields in a low magnetic field below the critical magnetic field. In this way, it is considered possible to achieve a predetermined shielding rate by using both the μ metal shield and the superconducting shield.

[Problem to be solved by the invention]

As described above, conventionally, the double μ metal 7 and the hollow cylindrical superconducting metal 17 are used in combination to shield the device 8 including the Josephson element etc. from external magnetic fields.

However, in this case, the hollow cylindrical superconducting metal 17, as shown in FIG. It has the property that a persistent current is induced so as to maintain the amount, that is, the magnetic flux, as it is. Therefore, even if the temperature falls below the critical temperature, the external magnetic field H in the Y direction of the device 8 is maintained in the same state as before the superconducting state. For this reason, a problem arises in that the shielding rate is not significantly improved with respect to the external magnetic field in the Y direction.

In order to deal with this problem, it is conceivable to provide the hollow cylindrical superconducting metal 17 with a bottom portion 27, as shown in FIG. In this way, the magnetic flux passing through the superconducting loop can be reduced and the shielding rate in the Y direction can be improved. However, since the superconducting metal 17 needs to be used in a liquid cryogen L (FIG. 5), holes 37 must be formed in the bottom 27 to drain the liquid cryogen. Since the external magnetic field H enters through this hole 37,
There is a limit to the improvement of the shielding rate in the Y direction, and the above problem still remains.

The present invention has been made in view of the above problems, and provides a method for forming a magnetic shield that can shield from external magnetic fields with a higher shielding rate than conventional methods when magnetic shielding is performed using superconducting metal. The purpose is to provide the following.

[Means to solve the problem]

The magnetic shield of the present invention includes a superconducting film formed on a flexible film, and the film with the superconducting film forms a cylindrical superconducting loop that encloses a space as a whole. It has a deformable structure.

On the other hand, in the method for forming a magnetic shield of the present invention, when a cylindrical superconducting loop formed by a flexible film with a superconducting film is in a normal conduction state, a space surrounded by the superconducting loop is The superconducting loop consists of a step of configuring the superconducting loop so that the area of the space is minimized, and a step of expanding the superconducting loop so that the area of the space is maximized after transitioning to a superconducting state.

[Function] In the present invention, a closed superconducting loop formed by a flexible film with a superconducting film is used instead of the conventional non-flexible superconducting metal 17 (FIG. 5). There is. When this superconducting loop is transitioned from a normal conducting state to a superconducting state, the magnetic flux that has passed through the superconducting loop is retained at the time the superconducting state is entered. If the superconducting loop is deformed in advance so that the area surrounded by the superconducting loop in the normal conduction state is as small as possible, then
The magnetic flux retained within this loop will also be small.

Furthermore, even if the superconducting loop is expanded so that the area surrounded by the superconducting loop becomes as large as possible after transitioning to a superconducting state, the total amount of magnetic flux within this loop does not change, so the magnetic flux per unit area In other words, the magnetic flux density can be significantly reduced. Furthermore, as long as the superconducting loop is in a superconducting state, no new magnetic field will enter the loop, so devices such as Josephson circuits placed in the space surrounded by the loop will be almost completely protected from external magnetic fields. can be shielded.

Thus, with the present invention, it is possible to shield a device such as a Josephson circuit from an external magnetic field with a higher shielding rate than before.

〔Example〕

1A and 1B are diagrams for explaining a method of forming a magnetic shield according to a first embodiment of the present invention, and FIG.
The figure is a perspective view thereof, and FIG. 1B is a sectional view taken along line BB in FIG. 1A. However, here, the magnetic shield (for example, the fifth
Only the portion of the superconducting loop 3 in Figure 1) is shown enlarged. Furthermore, (1) in FIGS. 1A and 1B shows the shape of the superconducting loop 3 in the normal conduction state, and on the other hand,
(n) in FIGS. 1A and 1B shows the shape of the superconducting loop 3 after transitioning to a superconducting state. In addition,
Also in the second to fourth embodiments (FIGS. 2A to 4) to be described later, (I) indicates the shape in the normal conduction state, and (n) indicates the shape in the superconductivity state. Further, components similar to those described above will be described using the same reference numerals.

In this case, as the film 1, a flexible polymer film made of polyimide or the like and having a thickness of approximately 100 to 300 mm is used. Further, as the superconducting film 2, a thin film made of lead, niobium, etc. and having a film thickness of about one layer is formed on the film 1 by sputtering or the like. Note that the superconducting film 2
It is preferable to process the film 1 into a cylindrical shape in advance so that the electrical connection can be made by itself.

Next, a method of forming a magnetic shield using the superconducting loop 3 formed from the cylindrical film 1 and the superconducting film 2 will be explained.

First, in the normal conduction state (Fig. 1A and 1
(I) in Figure B, the side surfaces of the superconducting loop 3 are pressed so that the area of the space surrounded by the loop is made as small as possible. Next, the superconducting loop 3 is immersed in a liquid cryogen L (Fig. 5) to transition to a superconducting state (
(■) in FIGS. 1A and 1B, the superconducting loop 3 is expanded to have a substantially cylindrical shape, for example, so that the area of the space it surrounds is maximized. In this way, (I
Even if a small amount of magnetic flux passes through the space inside the superconducting loop 3 in state (II), the space is greatly expanded in state (II), so the magnetic flux density inside the loop caused by the external magnetic field is significantly reduced Ru. The rate of reduction is determined by the area ratio before and after deformation, and can easily be 1710 to 1/100.

Therefore, when used in combination with a μ metal shield, the shielding rate against external magnetic fields can be set to a predetermined value (several tenths to tens of thousands of parts).
can be improved to.

In the first embodiment (FIGS. 1A and 1B), the cylindrical superconducting loop does not need to have a bottom or a hole for draining liquid, so the structure of the superconducting loop is improved compared to the conventional one. It doesn't get complicated.

2A and 2B are diagrams for explaining a method for forming a magnetic shield according to a second embodiment of the present invention, and FIG.
The figure is a perspective view thereof, and FIG. 2B is a sectional view taken along line BB in FIG. 2A.

Here, the first film 1 and the superconducting film 2 are
The same members as in the embodiment (Fig. 1Δ and Fig. 1B) are used. Furthermore, both ends of the superconducting film 2 formed on the two films 1 by sputtering or the like are connected to each other by a connecting member 4 such as solder made of a superconducting material to form a superimposed superconducting loop 13. There is. When this superconducting superconducting loop 13 is in a normal conduction state ((■) in FIGS. 2A and 2B), the two films 1 are superimposed as close as possible to each other and are contained between each other. Keep the space in which it is exposed as small as possible. Next, after being immersed in liquid cryogen L (Figure 5) to transition to a superconducting state (see Figure 2A),
(■) in the figure and FIG. 2B, both ends of the two films 1 are pressed to enlarge the space as much as possible. In this way, the space surrounded by the superconducting loop 13 is greatly expanded after the superconducting transition, so that the magnetic flux density within the loop is significantly reduced. Therefore, in the second embodiment (FIGS. 2A and 2B) as well, the shielding rate against external magnetic fields can be significantly improved as in the first embodiment (FIGS. 1A and 1B).

In the second embodiment, the superconducting films on the two films must be connected to each other by soldering or the like, but since the two films can be brought into close contact with each other in the normal conduction state, the Compared to the first embodiment, this embodiment has the advantage that the shape of the loop in the normal conduction state can be easily set.

3A and 3B are diagrams for explaining a method for forming a magnetic shield according to a third embodiment of the present invention, and FIG.
Fig. A is a perspective view thereof, and Fig. 3B is a sectional view taken along the line BB in Fig. 3A.

Here again, the first film 1 and the superconducting film 2 are
Example (FIGS. 1A and 1B) and second example (FIG. 1A and FIG. 1B)
The same members as in FIGS. 2A and 2B) are used. Furthermore, one film 1 on which the superconducting film 2 is formed is spirally wound. Furthermore, one end of the superconducting film located near the center on the inside and the other end of the superconducting film located on the outer periphery are connected by a connecting wire 5 made of a superconducting material such as lead or niobium to form a spiral shape. A superconducting loop 23 is formed.

When the helical superconducting loop 23 is in a normal conducting state ((I) in FIGS. 3A and 3B), only a small amount of magnetic flux passes through a narrow space such as in the central core.

Next, the helical superconducting loop 23 is connected to the liquid cryogen L (fifth
If the superconducting loop 23 is unraveled after being immersed in the superconducting loop 23 ((■) in FIGS. 3A and 3B) to transition to a superconducting state, the space inside the loop will be greatly expanded.
The magnetic flux density within the loop caused by the external magnetic field is significantly reduced, and the external magnetic field can be almost completely shielded.

FIG. 4 is a plan view for explaining a method for forming a magnetic shield according to a fourth embodiment of the present invention. Note that here, only the above-mentioned plan view is shown in cross section.

In this case, two superconducting metal plates 6 made of lead, niobium, etc.
During this time, a film 2 on which a superconducting film 2 similar to that described above is formed is folded into a bellows shape. Furthermore, the space between the superconducting metal plate 6 and the superconducting film 2 and the folded portion of the superconducting film 2 are soldered with solder (not shown) made of a superconducting material.
etc., to form a bellows-shaped superconducting loop 33. Here, the film thickness of the film 1 on which the superconducting film 2 is formed is shown in an enlarged manner. The area covered is much larger than it actually is. The bellows-shaped superconducting loop 33 is connected to a liquid cryogen L (Fig. 5).
After transitioning to a superconducting state by immersing it in water (■ in Figure 4),
When the bellows-shaped superconducting loop is expanded, the space inside the loop can be greatly expanded.
As in Figures A to 3B), external magnetic fields can be almost completely shielded. Note that the superconducting film 2 formed on the film 1 can also be used instead of the superconducting metal plate 60.

In the examples so far, as various superconducting materials,
Although metal superconductors such as lead and niobium are used, a similar shielding effect can be produced by using oxide superconductors instead.

〔Effect of the invention〕

As explained above, according to the present invention, when magnetic shielding is performed using a cylindrical superconducting metal, the magnetic flux density inside the cylinder caused by an external magnetic field is significantly reduced to create an ultra-low magnetic field environment. Therefore, a magnetic shield having a higher shielding rate than the conventional one can be realized. Furthermore, since there is no need to provide a bottom or a hole for liquid drainage in the cylindrical superconducting metal, the structure of the magnetic shield does not become complicated.

[Brief explanation of drawings]

FIG. 1A is a perspective view for explaining the magnetic shield forming method according to the first embodiment of the present invention, and FIG. 1B is a B of FIG. 1A.
-B sectional view, Figure 2A is a perspective view for explaining the magnetic shield forming method according to the second embodiment of the present invention, and Figure 2B is B of Figure 2A.
-B sectional view, Figure 3A is a perspective view for explaining the magnetic shield forming method according to the third embodiment of the present invention, and Figure 3B is B of Figure 3A.
-B sectional view, FIG. 4 is a plan view for explaining the magnetic shield forming method according to the fourth embodiment of the present invention, FIG. 5 is a front view showing an example of a conventional magnetic shield in cross section, and FIG. 6 is a front view of a superconducting metal to explain the problem in Figure 5, and Figure 7 is a front view showing an improvement plan for Figure 6. In the figure, 1... film, 2... superconducting film, 3...
...Superconducting loop, 4...Connecting member, 5...Connecting wire, 6...Superconducting metal plate.

Claims (1)

[Claims] 1. When a cylindrical superconducting loop (3) formed by a flexible film (1) having a superconducting film (2) formed on its surface is in a normal conducting state, the superconducting loop (3) is surrounded by configuring the superconducting loop (3) so that the area of the space is minimized; and expanding the superconducting loop (3) so that the area of the space is maximized after transitioning to a superconducting state. A method for forming a magnetic shield, comprising the steps of: