A cooling apparatus for a superconducting magnetic sensor.

Abstract

To solve the problem that a non-metal such as resin or ceramic is used for a peripheral member of a SQUID magnetic sensor in order to avoid Johnson noise (thermal noise), but since an electromagnetic wave is transmitted, the magnetic sensor is exposed to the electromagnetic wave, thereby causing an increase in high frequency noise and destabilization of operation.SOLUTION: A rod-shaped thermal conductor comprising an outer container including a cap portion having a vacuum window and made of a non-magnetic non-metal material and a main body portion made of a non-magnetic metal material, an inner container containing a refrigerant, and a first end portion protruding from the inner container and facing the vacuum window of the cap portion, A thermal conductor capable of disposing a substrate of the superconducting magnetic sensor at the first end portion, wherein the inner container further includes a bulging portion, and a second end portion of the thermal conductor is embedded in the bulging portion, wherein the cap portion having the vacuum window is coated with a conductive paint having a desired layer thickness or plated or deposited with a conductive film.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an improvement in a cooling device for a superconducting magnetic sensor. [Background technology]

[0002] In order to increase the sensitivity of a superconducting magnetic sensor (hereinafter sometimes referred to as "SQUID"), it is preferable to make the distance between the sensor and the object to be measured as short as possible. Therefore, a sapphire rod as a thermal conductor is protruded from the inner container containing the refrigerant, and a sensor is placed at the tip of the rod. In order to ensure the cooling efficiency and mounting stability of the sapphire rod, a bulge made of a material with high thermal conductivity is interposed between the sapphire rod and the inner container (see Patent Documents 1, 2, and 3). Please refer to Non-Patent Document 1 as a document disclosing technology related to the present invention. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-258520 [Patent Document 2] Patent No. 5145552 [Patent Document 3] Japanese Patent Publication No. 2022-071735 [Non-patent literature]

[0004] [Non-Patent Document 1] Saburo Tanaka, Yoshihiro Kitamura, Yoshimi Hatsukade, Takeyoshi Ohtani, Shuichi Suzuki, “Metallic contaminant detection system using multi-channel high Tc SQUIDs,” Journal of Magnetism and Magnetic Materials 324 (21) Oct. 2012, p.3487-3490. Summary of the Invention [Problem to be solved by the invention]

[0005] Conventionally, superconducting magnetic sensors have generally been cooled by immersing them in liquid nitrogen, but in nondestructive testing, a method known as the microscope type, in which the sensor is cooled via a thermal conductor, is used, as shown in Patent Documents 1, 2, and 3. Generally, in this type of device, if metal is present near the SQUID magnetic sensor, noise called Johnson noise or Nyquist noise is generated due to the thermal vibration of free electrons in the metal, so nonmetallic materials such as resin and ceramic are used for the surrounding components. However, because electromagnetic waves can pass through these nonmetallic materials, exposure of the magnetic sensor to electromagnetic waves can lead to increased high-frequency noise and unstable operation. Therefore, there is a demand for magnetic sensors that are not exposed to electromagnetic waves, have low noise, and operate stably.

[0006] One possible solution is to cover the vacuum window and nearby components with a conductive mesh metal, conductive cloth, or metal foil, but this would increase the distance between the vacuum window and the object in the atmosphere, reducing the signal strength obtained from the SQUID magnetic sensor. [Means for solving the problem]

[0007] As a result of extensive research into solving the above problems, the inventors of the present invention came to the conclusion that electromagnetic waves could be shielded by applying a conductive paint to the vacuum window or its vicinity, or by plating or vapor-depositing a conductive film. In this case, the conductive paint is simply applied, or the conductive film is plated or vapor-deposited, so the thickness of the paint layer is thin, and the distance increases by only a few tens to a hundred microns, with almost no reduction in signal strength.

[0008] The conductive coating, plating, or vapor deposition film should preferably be in the range of 20 to 180 microns, and more preferably 50 to 100 microns. If it is too thick, it will peel off easily, and if it is too thin, sufficient conductivity will not be obtained.

[0009] For the conductive coating, plating or vapor deposition, a non-magnetic, highly conductive material such as gold, silver, copper, tin, or silver-copper is selected.

[0010] From the above, the first aspect of the present invention is defined as follows: an outer container including a cap portion made of a non-magnetic, non-metallic material and having a vacuum window, and a main body portion made of a non-magnetic metallic material; an inner container containing a refrigerant; a rod-shaped thermal conductor having a first end portion protruding from the inner container and facing the vacuum window of the cap portion, the first end portion being capable of accommodating a substrate of a superconducting magnetic sensor; The inner container further includes a bulging portion, and a second end of the heat conductor is embedded in the bulging portion. The cooling device for a superconducting magnetic sensor has a cap portion having the vacuum window, the surface of which is coated with a conductive paint or provided with a conductive film by plating or vapor deposition. [Effects of the Invention]

[0011] According to the cooling device for a superconducting magnetic sensor defined in the first aspect as defined above, electromagnetic waves are reflected, so that the electromagnetic waves do not affect the magnetic sensor, enabling stable operation of the SQUID magnetic sensor and enabling the magnetic sensor to exhibit its original performance.

[0012] In the above, it is desirable that the material of the conductive film, which is coated with conductive paint or plated or vapor-deposited, is non-magnetic, and it is preferable to use materials such as gold, silver, copper, tin, or silver-copper (second aspect).This is because using a non-magnetic material can eliminate undesirable magnetic effects from the conductive film on the SQUID. The thickness of the conductive coating, plated or vapor-deposited conductive film is preferably in the range of about 20 to 180 microns (third aspect), and more preferably about 50 to 100 microns. If it is too thick, it will be prone to peeling and will undesirably increase Johnson noise, while if it is too thin, sufficient conductivity will not be obtained. It is preferable that the electrical resistance of the conductive coating or plated or vapor-deposited conductive film be 0.8 Ω or less (fourth aspect), and even more preferable that it be 0.3 Ω or less. If the resistance is too high, it will not function effectively as an electromagnetic shield. A conductive film formed by coating, plating, or vapor deposition of conductive paint is also formed in the counterbore for the fixing bolt in the cap part having the vacuum window, and by tightening the metal fixing bolt, conductivity between the conductive film and the main body part can be ensured through the metal bolt (fifth aspect). [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic diagram showing the configuration of a cooling device for a superconducting magnetic sensor according to an embodiment of the present invention. [Figure 2] The relationship between time and SQUID output voltage before and after applying the conductive paint is shown. DETAILED DESCRIPTION OF THE INVENTION

[0014] FIG. 1 is a schematic diagram showing the structure of a cooling device 1 for a superconducting magnetic sensor according to an embodiment of the present invention. The cooling device 10 comprises an outer container 11, an inner container 30, and a conductive film 60 that is coated with conductive paint or plated or vapor-deposited. The outer container 11 has a main body 12 made of a non-magnetic metal and a cap 20 made of a non-magnetic non-metal, and a vacuum window 21 made of transparent sapphire is attached to the top of the cap 20. The main body 12 is provided with a vacuum passage 13 with a valve for drawing a vacuum inside the main body 12. The cap portion 20 is fastened to the main body portion 12 by a fixing bolt 22 so as to maintain airtightness by an O-ring seal (not shown).

[0015] The inner container 30 is housed within the outer container 11 and is filled with liquid nitrogen as a refrigerant. Reference numeral 32 denotes a liquid nitrogen filling port, which is closed during use. Reference numeral 31 denotes an exhaust port for discharging nitrogen gas. A bulge 33 made of a material with good heat conductivity is connected to the bottom wall of the inner container 30. In the figure, reference numeral 50 denotes a support leg, and reference numeral 53 denotes a heat insulating sheet.

[0016] A thermal conductor made up of a plurality of sapphire rods 5 is inserted into the bulging portion 33 from the bottom side in the figure. A SQUID is attached to the bottom end of each sapphire rod 5. The cap portion 20 is provided with a counterbore 23 so that the head of the fixing bolt does not protrude from the surface of the cap portion 20.

[0017] A conductive film 60, which is applied with conductive paint or plated or vapor-deposited, is provided so as to cover the entire outer surface of the vacuum window 21 made of transparent sapphire and the cap portion 20 made of a non-magnetic, non-metallic material. The material of the conductive film 60, which is coated with conductive paint or plated or vapor-deposited, can be selected from a variety of materials, but it is desirable that it be a non-magnetic material, and it is preferable to use a material with high conductivity such as gold, silver, copper, tin, or silver-copper. The thickness of the conductive coating 60, whether coated, plated, or vapor-deposited, is preferably in the range of approximately 20 to 180 microns. Approximately 50 to 100 microns is even more preferable. If it is too thick, it will peel off easily, and if it is too thin, sufficient conductivity will not be obtained. Here, the conductive coating (Polycarm Conductive Coating Spray Silver Copper PCS-107AgCu) was sprayed in several layers to achieve a layer thickness of 50 microns.

[0018] To block electromagnetic waves, it is important to ensure electrical continuity between the conductive paint coated, plated, or vapor-deposited conductive film 60 and the main body 12, and to electrically integrate them; the inner wall surface and bottom of the countersunk hole 23 in the cap 20 are also coated. This ensures electrical continuity between the conductive paint coated, plated, or vapor-deposited conductive film 60 and the main body 12 through the metal bolt when the metal bolt is fastened. Furthermore, to ensure electrical continuity, it is also effective to wrap conductive tape around the seam between the conductive paint coated, plated, or vapor-deposited conductive film 60 and the main body 12.

[0019] In the cooling device 10 configured in this manner, a conductive film 60, which is applied, plated, or vapor-deposited with conductive paint, is formed on the cap portion 20 made of a non-magnetic, non-metallic material and the vacuum window 21 made of transparent sapphire, thereby reflecting electromagnetic waves, allowing the SQUID magnetic sensor to operate stably and demonstrating its inherent performance.

[0020] Figure 2 shows the results of an experimental example of the SQUID output, showing time waveforms. Figure 2(a) shows the results without a conductive film, and Figure 2(b) shows the results with a conductive film. The conductive film used in the experimental example was formed by spraying the aforementioned conductive paint (Polycarm Conductive Paint Spray Silver Copper PCS-107AgCu) in several layers to a thickness of 50 microns. The magnetic sensor in this experimental example had a counterbore in the cap of the cooling device. A similar conductive paint film was also applied to the inner wall and bottom of the counterbore. The cap was fastened to the main body with a metal bolt. As shown in the figure, the experimental results showed that pulse-like noise was present before the application of the conductive paint (without the conductive film), but disappeared after the application (with the conductive film).

[0021] The present invention is not limited to the above-described embodiments and experimental examples. Various modifications within the scope of the claims and within the scope of those skilled in the art are also included in the present invention. As a modification, the configuration in which conductive tape is wrapped around the joint between the conductive film 60 and the main body 12 is not only used to supplement the electrical continuity provided by the fastening of the metal bolt, but also to ensure electrical continuity when no conductive film is formed in the counterbore. Furthermore, the conductive film can be formed by applying conductive paint to the surface of an already manufactured cap, or by laminating a conductive film by plating or vapor deposition during the manufacturing process of the cap. [Explanation of symbols]

[0022] 1. Cooling device for superconducting magnetic sensors 10 Cooling device 11 Outer container 12 Main body 20 Cap part 21 Vacuum window 30 Inner container 33 Bulge 60 Conductive paint coating or plated or vapor-deposited conductive film

Claims

1. an outer container including a cap portion made of a non-magnetic, non-metallic material and having a vacuum window, and a main body portion made of a non-magnetic metallic material; an inner container containing a refrigerant; a rod-shaped thermal conductor having a first end portion protruding from the inner container and facing the vacuum window of the cap portion, the first end portion being capable of accommodating a substrate of a superconducting magnetic sensor; The inner container further includes a bulging portion, and a second end of the heat conductor is embedded in the bulging portion. The cooling device for a superconducting magnetic sensor has a cap portion having the vacuum window, the surface of which is coated with a conductive paint or provided with a conductive film by plating or vapor deposition.

2. 2. The cooling device according to claim 1, wherein the conductive coating, plating or vapor deposition applied to the cap portion having the vacuum window is non-magnetic.

3. 2. The cooling device according to claim 1, wherein the thickness of the conductive coating applied to the cap portion having the vacuum window or the conductive film applied by plating or vapor deposition is 20 to 180 microns.

4. 2. The cooling device according to claim 1, wherein the electrical resistance of the conductive coating, plating, or vapor-deposited conductive film applied to the cap portion having the vacuum window is 0.8 Ω or less.

5. 2. The cooling device according to claim 1, wherein the cap portion having the vacuum window is provided with a countersunk hole for fastening to the main body portion, and the inner wall surface and bottom of the countersunk hole are provided with a conductive film that is coated with conductive paint or plated or vapor-deposited, so that when the cap portion and the main body portion are fastened with a metal bolt, there is electrical continuity between the conductive film and the main body portion through the metal bolt.

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