JPS61195390A - Superconducting shielding body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a superconducting shield body used in a 5QtJID (superconducting quantum interference device) magnetometer and the like.

(Prior art and its problems) When measuring minute magnetic flux with a SQUID magnetometer, for example when trying to detect a monoball (magnetic monopole), it is necessary to avoid the influence of induced electromagnetic waves from peripheral equipment and the earth's magnetism. Since it is necessary to block the magnetic field and perform precise measurements, some kind of magnetic shielding is required. Magnetic shields used for this purpose include those using high magnetic permeability alloys such as permalloy,
There are superconducting shield bodies that use superconductors.

Of these, the former requires a large magnetic shield made of a high magnetic permeability alloy to be provided around the cryogenic container filled with liquid helium, resulting in considerably high material and processing costs. Furthermore, even if a large amount of money is spent in this way, there is a drawback that the magnetic shielding effect is not very large.

On the other hand, in the latter method, a superconducting shield made of a superconductor such as lead (Pb) or niobium (Nb) is immersed in liquid helium, and the pickup coil is surrounded by it. Because it is mechanically weak, it is difficult to make the superconductor layer thin while maintaining its magnetic properties uniformly, and it is generally formed as a fairly thick superconductor layer.
In cases where a large-area magnetic shield is required, the heat capacity becomes considerably large, resulting in a decrease in the cooling efficiency of liquid helium. In addition, even if you try to make it larger, the shape is not uniform due to the above-mentioned mechanical weakness, and it is difficult to handle, which causes noise, so the magnetic shielding effect is not necessarily sufficient. There is also.

On the other hand, when Nb or the like is used, the depth of magnetic flux penetration (P eneration Depth)
Since λ is relatively small, the superconducting shield can be made thin, and there are fewer problems such as heat capacity. However, when the superconductor layer is made thinner, the effects of lattice defects and impurities become relatively large, causing magnetic flux to enter the inside of the superconducting shield, and this penetrating magnetic flux fluctuates due to changes in the external magnetic field, etc. However, there is a problem in that noise is generated.

(Object of the Invention) The present invention was made in order to overcome the above-mentioned problems. The purpose is to provide a superconducting shield body that is inexpensive and has a high magnetic shielding effect.

(Means for Achieving the Object) In order to achieve the above object, a superconducting shield according to the present invention is formed to have a laminated structure of an electrically good conductor layer and a superconductor layer.

(Example) FIG. 1 is a diagram showing the basic configuration of a SQU ID1i force meter as an example of a device that can utilize the superconducting shield of the present invention. In the figure, the system under test (
(not shown) interlinks with the pickup coil 1 formed of Nb or Nb Ti wire, etc.
This minute magnetic flux is converted into magnetic flux of the input coil 2 connected to the pickup coil 1. The inductance of the 5QUID element 4 having the Josephson junction 3 changes periodically depending on the magnitude of the magnetic flux from the input coil 2.

On the other hand, the RF lightning current applied from the RF (radio frequency) electric field FiLWA5 to the tank circuit 6, this 5QtJI
It is modulated by the inductance change of the D element 4, and the modulated wave is amplified by the RF amplifier 7 and then detected by the detector 8, thereby measuring the magnetic flux of the system under test. Among these, input coil 2.5Q
Magnetic coupling system 9 including tJID element 4 and tank circuit 6
The pickup coil 1 is immersed in liquid helium LHe. Further, the superconducting shield body 10 has a container shape that surrounds the pickup coil 1, and is formed as a superconducting shield body as shown below.

FIG. 2(a) is a partial cross-sectional view showing the structure of a superconducting shield according to a first embodiment of the present invention. In this figure, this superconducting shield body 20 is formed as a laminated structure in which a superconductor layer 23 is provided on the surface of an electrically conductive layer 22 supported by a support base material layer 21. Among these, the support base material layer 21 is, for example, FRP (fiber reinforced plastic) such as a glass-based epoxy board, and may also be a phenol board or a polyester board.

Further, the electrically conductive layer 22 may be a Cu (copper) layer or an AI layer.
(aluminum) layer or the like is used, and for example, high-purity Cu is provided by adhesion or electroless plating. The combination of the support base material layer 21 and the electrically conductive layer 22 is a printed wiring board (typically, a support base material llI
21 with a thickness of 1.6 m+) that is commonly used can also be used.

On the other hand, the superconductor layer 23 in this embodiment includes a second type superconductor layer 24 formed of a second type superconductor such as Nb, and a first type superconductor layer 24 formed of a first type superconductor such as Pb. It contains 1!25 of the first type superconductor. This is done by using, for example, a Nb plate plated with PB as the electrically good conductor 1!
It can be formed by adhering to the surface of 22.

Next, the effect of the configuration shown in FIG. 2(a) will be explained. First, the reason for the existence of the electrically good conductor layer 22 is to prevent minute magnetic flux from entering from the outside due to lattice defects or impurities in the superconductor layer 23, and to prevent the flux from changing due to fluctuations in the external magnetic field. It's about doing. In other words,
When the penetrating magnetic flux attempts to fluctuate, eddy currents are generated within the electrically conductive layer 22 to prevent or suppress the fluctuation. Therefore, noise generation due to intruding magnetic flux is reduced, and a high shielding effect can be obtained. In addition, since fluctuations in the penetrating magnetic flux can be prevented in this way, the superconductor layer 23
There is no need to increase the thickness; it is sufficient that the thickness is larger than the order of the magnetic flux penetration depth λ (~10'as+). In practice, this means that it can be quite cheap, so the heat capacity is quite small, and it is possible to prevent the cooling efficiency from decreasing due to liquid helium.

On the other hand, in this embodiment, a support base material layer 21 is provided to support the electrically conductive layer 22 and the superconductor layer 23, but this is not suitable for mechanical The purpose is to ensure strength. Superconductor layer 2 which is difficult to handle in order to be mechanically robust
3. The reason why such a support base layer 21 can be used without increasing the thickness of the superconductor layer 23 is that the superconductor layer 23 can be made thin while maintaining the shielding effect as described above.

Incidentally, in this embodiment, the superconductor layer 23 has a two-layer structure consisting of the second type superconductor layer 24 and the first type superconductor layer 25, and the reason for this will be described below. Figure 3 shows the phase diagram of well-known superconductors, of which Figure 3(a) is the phase diagram of a type 1 superconductor, which is superconducting
(Meissner) region SP and the normal conduction region NM are in contact with each other with the critical curve CR as a boundary. In addition, the figure (b) shows the second
Phase diagram of a seed superconductor, with two critical curves CR1°C
There is a superconducting region SP, a mixed state region MX and a normal conducting region NM bounded by R2.

In these figures, H is the magnetic field, T is the temperature, and T.

is the critical temperature, H is the critical magnetic field, Hcl and HC2 are the lower and upper critical magnetic fields (temperature variables in parentheses, O
and To mean absolute zero and liquid helium temperature, respectively. ) are shown respectively.

The superconductor layer 23 shown in FIG. 2(a) is formed using type 1 and type 2 superconductors having such phase regions, and the temperature T is set to the liquid helium temperature T. Let us consider the case where the external magnetic field φ gradually increases while keeping it at . Then, when the external magnetic field φ is small, the type 2 superconductor layer 24 in FIG. 2(a)
At the same time, the first type superconductor layer 25 is in a completely diamagnetic (Meissner) state 1, and almost completely shields magnetic flux from the outside. Next, the external magnetic field φ increases as shown in Figure 3 (
a) critical magnetic field H(To) (usually 10A/mα125
(approximately G), the first type superconductor layer 25 undergoes a first-order phase transition to the normal conduction region NM and loses its shielding effect. However, in general, Ho (To)
Because of the relationship C2(To), at this magnetic field magnitude, the second type superconductor layer 24 is in the mixed state region MX, and although there is an intrusion of vortices (vortex system), it has a considerable shielding effect. present. Even if the magnetic flux penetrating in the form of vortices attempts to move due to fluctuations in the external magnetic field φ, the eddy current generated in the electrically conductive layer 22 will cause This movement is prevented or suppressed, thereby preventing the generation of noise. Of course, the external magnetic field φ
The size of is the temperature T. The upper critical magnetic field H62 (T.

), the second type superconductor layer 24 also undergoes a second-order phase transition and shifts to the normal conduction region NM, but as is well known, the upper critical magnetic field H62 (To) is about 10 to 100 times that of HC (To). However, it is quite large and poses no problem in practical terms.

FIG. 2(b) shows a second embodiment of the present invention which can be applied to a case where the external magnetic field φ is relatively small, for example, about the magnitude of earth's magnetic field. This superconducting shield body 3
0 differs from the case shown in FIG. 3A in that the type 2 superconductor layer is omitted and only the type 1 superconductor layer 25 is formed as a superconductor layer. If the magnitude of the external magnetic field φ is expected to be less than the critical magnetic field H (To), the shielding effect of the first type superconductor layer 25 will not be completely destroyed, so such a configuration is sufficient.

FIG. 2(C) shows a third embodiment of the invention. In the superconducting shield body 40 of this embodiment, electrically conductive layers 22a and 22b are provided on both sides of the support base material 21, and the first
Seed superconductor layers 25a and 25b are provided, respectively, to enhance the shielding effect against magnetic fields φ and φ' in both directions.

FIG. 4 shows an example of a cylindrical shield made using the superconducting shield of the present invention. This cylindrical shield body 5
0 is a relatively small device that can be used to house the magnetic coupling system 9 shown in FIG.
1, a concentric cylindrical electrically conductive layer 52 is provided, and a superconductor layer 53 is provided inside the electrically conductive layer 52. As mentioned above, this superconductor layer 53 is a type 1 superconductor layer (
For example, it is possible to have a single-layer structure with only Pb plating), or the first type superconductor @54 and the second type superconductor layer 55.
It may also have a two-layer structure. In addition, since this cylindrical shield body 50 is relatively small as mentioned above,
If the electrically conductive layer 52 is made thick to a certain extent, it becomes mechanically quite robust, and there is no need to provide a separate supporting base material layer.

By the way, the superconducting shield body of the present invention is not limited to the structure of the above-mentioned embodiment, and for example, the following modifications are possible.

(1) The stacking order of the electrically good conductor layer and the superconductor layer, and when the superconductor layer has a two-layer structure of a type 1 superconductor layer and a type 2 superconductor layer, The stacking order is not particularly limited.

(2) A higher shielding effect can be obtained by providing a plurality of each of the layers forming the superconducting shield and stacking them alternately.

(3) The superconductor layer is not limited to pb or Nb, but can also be made of W.

Any material selected depending on the application, such as Nb3Sn or NbTi, may be used. When using a support substrate, various materials such as various plastic materials can be considered in addition to those exemplified in the above embodiments.

(4) The superconducting shield body of this invention is 5QL11
It can be used not only for zero magnetometers but also for various devices that require a high degree of magnetic shielding.

(Effects of the Invention) As explained above, according to the present invention, it is possible to obtain a sufficient magnetic shielding effect even with a thin superconductor layer, so it is possible to reduce the heat capacity, and it is also mechanically stable. It is possible to obtain a superconducting shield body that can be constructed robustly, inexpensively, and has a high magnetic shielding effect.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the basic configuration of a 5QUID magnetic system that can utilize the superconducting shield of the present invention, Figure 2 is a partial cross-sectional view of an embodiment of the present invention, and Figure 3 is a diagram showing the structure of a superconductor. The phase diagram, FIG. 4, is a diagram showing a cylindrical shield body formed using a superconducting shield body according to an embodiment of the present invention. 1...Pickup coil, 4...5QUID element, 21...Support base material layer, 22.22a-b, 52...Good electrical conductor layer, 23.
53... Superconductor layer, 24.55... Second type superconductor layer, 25.25a-b, 54... First type superconductor layer. Patent applicant Shimadzu Corporation +2ffi (α) (4) (Figure 3 (a-) (↓)

Claims (3)

[Claims] (1) A superconducting shield body having a laminated structure of a good electrical conductor layer and a superconductor layer. (2) The superconductor layer according to claim 1, wherein the superconductor layer has a first layer containing a first type superconductor and a second layer containing a second type superconductor. conductive shield body. (3) The electrically good conductor layer and the superconductor layer are provided on a support base material layer and supported by the support base material layer, and the superconductor layer according to claim 1 or 2 is conductive shield body.