A cooling device for a superconducting magnetic sensor and a control method thereof.

Abstract

To mount a temperature sensor to a heat conductor to which a SQUID is mounted, or to an expansion part supporting the heat conductor.SOLUTION: The present invention relates to a superconductive magnetic sensor cooling device comprising: an outer container including a cap part including a vacuum window and a main body part; an inner container incorporating a coolant; and a rod-like heat conductor which includes a first end protruding from the inner container and opposing to the vacuum window of the cap part, and in the first end of which a substrate of a superconductive magnetic sensor can be disposed. The inner container further includes an expansion part, and a second end of the heat conductor is embedded in the expansion part. The superconductive magnetic sensor cooling device further comprises a pressure regulator of the inner container, a temperature sensor and a control unit. The temperature sensor is mounted to the expansion part or the heat conductor, and the control unit controls an inner pressure of the inner container by controlling the pressure regulator on the basis of a temperature measured by the temperature sensor.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an improvement in a cooling device for a superconducting magnetic sensor and a control method thereof. [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 is protruding from the inner container containing the refrigerant as a thermal conductor, 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 placed between the sapphire rod and the inner container (Patent Documents 1 and 2). Please refer to Non-Patent Document 1 as a document disclosing a technique related to the present invention. [Prior art documents] [Patent documents]

[0003] Patent Document 1: JP 2000-258520 A Patent Document 2 Patent No. 5145552 [Non-patent literature]

[0004] Non-Patent Document 1: 79th Autumn Meeting of the Japan Society of Applied Physics, Abstracts 20a-212B-2 (SUSTERA) DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0005] In a microscopic type cooling device for superconducting magnetic sensors, the thermal conductor is affected by the external temperature, so the temperature of the SQUID placed at the tip does not necessarily match the temperature of the coolant in the inner container. According to the inventor's investigation, when liquid nitrogen is used as the coolant, the temperature of the SQUID becomes high by about 2 K. Furthermore, the temperature of the SQUID may change depending on the external temperature, and may not be stable. In other words, there is a risk that sensing performed using a microscopic type cooling device for superconducting magnetic sensors may be performed while the actual temperature of the SQUID placed at the tip of the thermal conductor remains indefinite. It is therefore desirable to place a temperature sensor near the SQUID so that the actual temperature of the SQUID can be measured.

[0006] However, if a temperature sensor is placed near the SQUID, there is a risk that the temperature of the SQUID will be affected not only by changes in the external temperature, but also by the temperature sensor itself. Therefore, there was some hesitation in attaching a temperature sensor to the thermal conductor to which the SQUID was attached or to the bulge that supports the thermal conductor. [Means for solving the problem]

[0007] As a result of extensive research into solving the above problems, the inventors came to the conclusion that it might be possible to cancel out or greatly reduce the influence of the temperature sensor on the temperature of the SQUID by controlling the temperature of the refrigerant. As a premise, it has been confirmed that the critical current of the SQUID increases when the refrigerant temperature is lowered in a microscopic type cooling device for a superconducting magnetic sensor. The relationship between the coolant temperature and the critical current is shown in Figure 2. In Figure 2, the critical current is the value for one parallel Josephson junction.

[0008] It is known that the critical current of a SQUID changes depending on its temperature, so the results in Figure 2 suggest that even in the microscopic type, the temperature of the SQUID changes when the temperature of the refrigerant changes. In other words, by controlling the temperature of the coolant, the temperature measured by the temperature sensor attached to the thermal conductor or the bulge can be kept at a constant temperature, and the temperature of the SQUID attached to the tip of the thermal conductor can also be kept constant. In this way, when the temperature of the SQUID stabilizes, the temperature of the SQUID becomes substantially equal to the temperature measured by the temperature sensor placed nearby.

[0009] Because the temperature required for the operation of a SQUID is extremely low, even in the case of a so-called high-temperature superconducting SQUID, it is not realistic to control the temperature by cooling the refrigerant itself with another refrigerant. Therefore, it was decided to adjust the temperature of the refrigerant by controlling the pressure of the inner container in which the refrigerant is stored, because the refrigerant can be brought into a supercooled state by reducing the pressure of the environment of the refrigerant.

[0010] In view of the above, the first aspect of the present invention is defined as follows: an outer container including a cap portion having a vacuum window and a body portion; 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 receiving a substrate of a superconducting magnetic sensor; The inner container further includes a bulging portion, and a second end of the thermal conductor is embedded in the bulging portion, The pressure regulator, the temperature sensor and the control unit of the inner container are further provided, The temperature sensor is attached to the bulge or the heat conductor, A cooling device for a superconducting magnetic sensor, wherein the control unit controls the pressure regulator based on the temperature measured by the temperature sensor to control the internal pressure of the inner container.

[0011] According to the cooling device for a superconducting timing sensor defined in the first aspect thus defined, the boiling point of the refrigerant is changed by controlling the internal pressure of the inner container, thereby making it possible to control the temperature of the refrigerant. The temperature of the refrigerant is subjected to so-called feedback control (e.g., PID control) so that the temperature measured by the temperature sensor is stabilized at a predetermined temperature. The temperature of the refrigerant is adjusted by controlling the pressure regulator with the control unit, thereby controlling the internal pressure of the inner container. When the temperature of the temperature sensor is stable, it is strongly assumed that the temperature of the SQUID located near the temperature sensor is also stable, and that the temperature is considered to be substantially equal to the temperature measured by the temperature sensor. In other words, even in the SQUID microscope type, the temperature of the SQUID attached to the tip of the thermal conductor can be specified and stabilized.

[0012] In the above, it is preferable to use a thermocouple or a resistance temperature detector as the temperature sensor (second aspect), because this makes it possible to reduce the thermal effect of the temperature sensor on the SQUID. To get closer to the SQUID, a thermocouple is preferred (third aspect), which can be attached to a thermal conductor, since it does not produce electrical current. The coolant that provides the operating temperature of the SQUID is generally liquid nitrogen, and the internal pressure of the inner container filled with this liquid nitrogen can be controlled by a pressure reducing pump (fourth aspect). [Brief description of the drawings]

[0013] [Figure 1] FIG. 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. [Diagram 2] 1 shows the relationship between the refrigerant temperature and the critical current in a microscope-type cooling device for a superconducting magnetic sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 includes an outer container 11, an inner container 30, a temperature sensor 60, a control unit 70, and a pressure reducing pump 80. The outer container 11 has a body portion 12 and a cap portion 20, and a vacuum window 23 made of transparent sapphire is attached to a top portion 21 of the cap portion 20. The main body 12 is provided with a vacuum path 13 connected to a vacuum pump (not shown) for evacuating the inside of the main body 12 . The cap portion 20 is attached to the main body portion 12 so as to be movable in the vertical direction in the figure. This allows the distance between the SQUID 1 and the vacuum window 23 to be adjusted.

[0015] The inner container 30 is housed in 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 a pressure reduction port connected to a pressure reduction pump 80. A bulge 33 made of a material with good thermal conductivity is connected to the bottom wall of the inner container 30. In this example, a hole is drilled in the bottom wall and the liquid nitrogen in the inner container comes into contact with the bulge 33. The upper edge of the bulge 33 can be inserted into the inner container 30 to increase the contact area between the bulge 33 and the liquid nitrogen. A flow path for liquid nitrogen can be formed within this bulge 33 . In the figure, reference numeral 50 denotes a support leg, and reference numeral 53 denotes a heat insulating sheet.

[0016] A thermal conductor made of a pair of sapphire rods 5 is inserted into the bulging portion 33 from the bottom side in the figure. A SQUID 1 is attached to the bottom end of each sapphire rod 5. A temperature sensor 60 is attached to the sapphire rod 5 or the bulge 33 . The temperature sensor 60 is preferably attached to the tip side of the sapphire rod 5 where the SQUID 1 is attached, and to the bulging portion 33, it is preferably attached near the sapphire rod 5 on the bottom surface shown in the figure. The temperature sensor 60 is preferably one that can measure the temperature of liquid nitrogen in the atmosphere and generates as little temperature as possible by itself. For example, a thermocouple or a resistance thermometer can be used. Since a weak current flows in a resistance thermometer type temperature sensor, it is preferable to attach it exclusively to the bulge 33 in order to minimize the influence on the SQUID circuit. A thermocouple type temperature sensor that barely flows current can be attached to the sapphire rod 5.

[0017] The measurement result by the temperature sensor 60 is sent to the control unit 70 via a signal line 72. The control unit 70 sends a control signal to the decompression pump 80 via a signal line 74 based on the temperature measured by the temperature sensor 60. In accordance with this control signal, the decompression pump 80 reduces the pressure inside the inner container 30. This makes it possible to lower the temperature of the liquid nitrogen. Generally, it is easier to obtain high output and high performance from a SQUID by lowering the environmental temperature. Therefore, it is preferable to lower the temperature of the liquid nitrogen as much as possible and to keep the temperature of the SQUID 1 attached to the tip of the sapphire rod 5 low.

[0018] Even if the temperature of the liquid nitrogen is kept constant, the temperature of the object to be measured by the temperature sensor 60 fluctuates due to the influence of the external temperature. In the cooling device 10, the pressure reducing pump 80 is operated to control the internal pressure of the inner container 30 so that the temperature of the object to be measured by the temperature sensor 60 is kept constant, thereby controlling the temperature of the liquid nitrogen. Reference numeral 82 denotes a pressure reducing line. Considering that the temperature of the liquid nitrogen can be lowered by lowering the internal pressure of the inner container 30, when using the pressure reducing pump 80, it is preferable to keep the internal pressure of the inner container 30 in a default state reduced below atmospheric pressure. When a pressure adjusting device capable of arbitrarily adjusting the internal pressure of the inner container 30 is used, the internal pressure of the inner container 30 in the default state can be arbitrarily selected.

[0019] In the cooling device 10 configured in this manner, the temperature of the object to be measured by the temperature sensor 60 can be stabilized at a desired temperature. Stabilization of the temperature of the object to be measured by the temperature sensor 60 means that the temperature of the tip of the sapphire rod 5, i.e., the SQUID 1, is also stable, and this temperature can be said to be equal to the temperature measured by the temperature sensor 60. That is, according to the cooling device 10, in a microscope type cooling device for a superconducting magnetic sensor, the actual temperature of the SQUID attached to the tip of the thermal conductor can be confirmed and measured.

[0020] The present invention is not limited to the above-mentioned embodiment and examples. Various modifications within the scope of the claims and within the scope that can be easily conceived by a person skilled in the art are also included in the present invention. [Explanation of symbols]

[0021] 1. Cooling device for superconducting magnetic sensors 10 Cooling device 11 Outer container 12 Main body 20 Cap section 23 Vacuum window 30 Inner container 33 Bulge 60 Temperature Sensor 70 Control section 80 Pressure reducing pump

Claims

1. an outer container including a cap portion having a vacuum window and a body portion; 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 receiving a substrate of a superconducting magnetic sensor; The inner container further includes a bulging portion, and a second end of the thermal conductor is embedded in the bulging portion, The pressure regulator, the temperature sensor and the control unit of the inner container are further provided, The temperature sensor is attached to the bulge or the heat conductor, A cooling device for a superconducting magnetic sensor, wherein the control unit controls the pressure regulator based on the temperature measured by the temperature sensor to control the internal pressure of the inner container.

2. The cooling device according to claim 1 , wherein the temperature sensor is a thermocouple or a resistance temperature detector.

3. The cooling device according to claim 1 , wherein the temperature sensor is a thermocouple attached to the heat conductor.

4. 4. The cooling device according to claim 1, wherein the refrigerant is liquid nitrogen, and the pressure regulator is a pressure reducing pump connected to the inner container.

5. 5. The cooling device according to claim 1, wherein the control unit performs PID control of the pressure regulator based on the temperature detected by the temperature sensor.

6. an outer container including a cap portion having a vacuum window and a body portion; 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 receiving a substrate of a superconducting magnetic sensor; The inner container further includes a bulging portion, and a second end of the thermal conductor is embedded in the bulging portion, A control method for a cooling device further comprising a pressure regulator for the inner container, a temperature sensor attached to the bulging portion or the heat conductor, and a control unit, a temperature measuring step of causing the temperature sensor to measure a temperature of the bulge portion or the thermal conductor; The control method for a cooling device for a superconducting magnetic sensor, further comprising the step of: causing the control unit to control the pressure regulator based on the measured temperature, thereby maintaining the internal pressure of the inner container at a desired pressure.

7. The control method according to claim 6 , wherein the temperature sensor is a thermocouple or a resistance temperature detector.

8. The control method according to claim 6 , wherein a thermocouple is attached to the heat conductor as the temperature sensor.

9. 9. The control method according to claim 6, wherein the refrigerant is liquid nitrogen, and the pressure regulator is a pressure reducing pump connected to the inner container.

10. The control method according to claim 9 , wherein the internal pressure of the inner container is reduced in a default state.

11. The control method according to any one of claims 6 to 10, further comprising causing the control unit to perform PID control of the pressure regulator based on the temperature detected by the temperature sensor.

Images