JPH09133744A - Squid fluxmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a SQUID fluxmeter wherein a device is miniaturized for its constitution and carried without limiting an article to be measured by providing a SQUID sensor, an input coil, a pickup, a superconducting wire and a heat conductive material. SOLUTION: Since a superconducting material such as a pickup coil 1 or the like is cooled by using a freezer 7, no liquid helium is needed. Thus, handling of a device is facilitated and the device is miniaturized, reduced in weight and carried. In addition, since the pickup coil 1 is cooled by using a long heat conductive material 12 thermally connected to the freezer 7, an article positioned in a place away from a SQUID sensor 6 is measured. Also, by covering a superconducting wire 11 with a superconductor 13 so as to magnetically shield the same, the superconducting wire 11 is prevented from picking up a noise signal during measuring. An inductance adjusting device 5 is provided in the middle way of the superconducting wire 11 and by changing inductance, a dynamic range is widened without changing the coil 1.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an SQUID magnetometer for measuring a minute magnetic field.

[0002]

2. Description of the Related Art The SQUID magnetometer is an ultrasensitive magnetometer utilizing the Josephson effect, which is one of the characteristic phenomena of superconductivity. FIG. 5 shows the configuration of a conventional SQUID magnetometer. In the conventional SQUID magnetometer, the SQUID sensor 6, the pickup coil 1, the input coil 3 and the superconducting wire 11 are immersed and cooled in liquid helium 22 in the cryostat 21 and kept operating in a superconducting state. Then, when measuring the magnetic field, the cryostat 21 and the subject 23 are brought close to each other, and the drive circuit 1
The magnetic flux was measured by 0.

[0003]

However, the above-mentioned conventional S
In the QUID magnetometer, the movement of the cryostat 21 containing the liquid helium 22 is difficult, so it is difficult to measure the cryostat 21 close to the subject 23,
The subject 23 was brought close to the cryostat 21 for measurement. Therefore, there is a problem that it is difficult to measure the magnetic flux in the fixed object, and the measurement target is limited.

Further, since liquid helium 22 is a valuable resource, a device for recovering helium which is vaporized and released into the atmosphere is used. However, if a device for recovering helium is provided, the device becomes bulky, and there is a problem that portability is lost.

Further, the SQUID magnetometer can measure a very small magnetic field, but when a magnetic flux exceeding a certain value is applied to the superconductor, the SQUID sensor functions because the superconducting state changes to the normal conducting state. Will not do. Therefore SQ
The UID magnetometer also has a problem that it can only measure a magnetic field in a limited range of about three digits. Therefore, when measuring a subject having a different measurement level of magnetic flux, a pickup coil having a different inductance was used for the measurement. However, replacing the pickup coil 1 in the cryostat 21 for each subject having a different measurement level requires extremely complicated work and takes a lot of time.

Further, since the sensitivity is high, there is a problem that the magnetic field signal (noise) passing through the loop formed by the pickup coil 1, the input coil 3 and the superconducting wire 11 is easily picked up by the superconducting wire 11. An object of the present invention is to reduce the size of the device configuration and to make it portable.
An object is to provide an SQUID magnetometer that does not limit the measurement target.

[0007]

In order to solve the above problems and achieve the object, the SQUID magnetometer of the present invention is constructed as follows. (1) SQUID sensor and input coil that are thermally connected to a refrigerator, a pickup coil that picks up magnetic flux in the vicinity of a subject, a superconducting wire that connects the input coil and the pickup coil, and the refrigerator And a long heat conducting material for cooling the pickup coil and the superconducting wire. (2) The heat conductive material is made of copper. (3) The superconducting wire is covered with a superconductor via the heat conducting material, and magnetically shielded by the superconductor. (4) A part of the superconducting wire constitutes an inductance adjusting device including a coil whose inductance can be changed.

According to the present invention, since the superconducting material such as the pickup coil is cooled by using the refrigerator, it is not necessary to use liquid helium. Therefore, the handling becomes easy, the size and weight can be reduced, and the portability can be provided, so that it is possible to measure a subject that cannot be moved.

In addition, since the pickup coil is cooled by using a long heat conductive material that is thermally connected to the refrigerator,
The pickup coil and the SQUID sensor can be separated. Therefore, it is possible to measure a subject at a position distant from the SQUID sensor.

Further, by covering the superconducting wire with a superconductor and magnetically shielding it, it is possible to prevent the superconducting wire from picking up a noise signal during measurement. Also, by providing an inductance adjusting device in the middle of the superconducting wire and changing the inductance, it is possible to widen the dynamic range without changing the pickup coil.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) FIG. 1 is a schematic diagram of an SQUID magnetometer according to a first embodiment of the present invention. In FIG. 1A, reference numeral 1 is a pickup coil that converts a magnetic field of a subject into an electric signal, and the coil 1 is stored in a cryostat 2. The pickup coil 1 and the input coil 3 are connected via a superconducting wire contained in the flexible tube 4 (the structure inside the flexible tube 4 will be described later). Flexible tube 4
An inductance adjusting device 5 described later is provided in the middle of. The input coil 3 is for transmitting the signal from the pickup coil 1 to the SQUID sensor 6. The SQUID sensor 6 has a Josephson junction (marked with X in the figure) in a loop made of a superconducting material. The input coil 3 and the SQUID sensor 6 are installed on a stage 8 that is thermally connected to the refrigerator 7, and are stored in a cryostat 9. And the SQUID
The magnetic flux is measured by connecting the drive circuit 10 to the sensor 6.
The drive circuit 10 measures the magnetic flux by applying a high frequency bias current to the coil 10a and observing the output voltage. The drive circuit 10 is composed of, for example, a flux locked loop (FLL) circuit or the like.

The SQUID shown in this embodiment is an rf-SQUID having one Josephson junction, but it may be a DC-SQUID having two Josephson junctions.

FIG. 1B is a view showing the structure inside the flexible tube 4. In FIG. 1B, the periphery of two superconducting wires 11 that connect the pickup coil 1 and the input coil 3 is covered with copper 12 that is thermally connected to the refrigerator 7. The copper 12 is for efficiently cooling the superconducting wire 11 by the refrigerator 7, and may not be copper as long as it has a high thermal conductivity. The periphery of the copper 12 is covered with a superconductor 13 that shields an external magnetic field. The structure described above is put in the flexible tube 4 and the inside is evacuated to fill the multilayer heat insulating material 14.

Two superconducting wires 11 and copper 12 inside the copper 12
The external superconductor 13 is cooled by the copper 12 that is thermally connected to the refrigerator 7, and is in a superconducting state. Therefore, the superconductor 13 outside the copper 12 shields the external magnetic field by the Meissner effect, and the two superconducting wires 1 inside the copper 12 are shielded.
1 does not pick up an external magnetic field (noise).
Since the copper 12 has a large resistance when viewed from the two superconducting wires 11 and the superconductor 13, no current flows from the two superconducting wires 11 or the superconductor 13 to the copper 12, and the The conductive wire 11 and the superconductor 13 are insulated from each other.

FIG. 2 is a schematic diagram showing the structure of the inductance adjusting device 4. Two coils 15, 16 are inserted in series in the middle of the superconducting wire 11, and these coils 15, 1
6 is magnetically arranged with its polarity reversed. The inductances of the coils 15 and 16 are L1 and L2.
The inductance of the pickup coil 1 is Lp and the inductance of the input coil 3 is Li. Then, first, the inductances Lp and Li of the pickup coil 1 and the input coil 3 are made uniform in size.
Next, the inductance L of the inductance adjusting device is L =
It becomes L1 + L2 -2M. Here, M is the mutual inductance of L1 and L2. By changing the mutual inductance M, the value of the current flowing through the pickup coil 1, the superconducting wire 11 and the input coil 3 is changed. Especially, when M is maximum, L is minimum and the current value is maximum. If the mutual inductance M is reduced, the inductance L is increased and the current flowing through the input coil 3 is reduced. Actually, the mutual inductance M is changed by changing the distance between the two coils 15 and 16.
Is changed to change the inductance L.

By changing the inductance by the inductance adjusting device 4, the current flowing through the input coil 3 can be adjusted. for that reason,
The value of the magnetic flux generated by the input coil 3 is large and SQU
When the superconducting state of the ID sensor 6 is broken, the current value is reduced by the inductance adjusting device, and the magnetic flux generated by the input coil 3 can be reduced to a value at which the SQUID sensor 6 operates.

The SQUID magnetometer having the above structure operates as follows. The input coil 3 and the SQUID sensor 6 on the stage 8 which is thermally connected to the refrigerator 7 are cooled and are in a superconducting state. Further, the pickup coil 1, the superconducting wire 11, and the superconductor 13 connected to the copper 12 which is thermally connected to the refrigerator 7 are also cooled and are in a superconducting state.

When the pickup coil 1 picks up a magnetic signal, a shield current is induced according to the law of conservation of magnetic flux in the superconducting closed loop. This shield current passes through the superconducting wire 11 and flows into the input coil 3 without being attenuated. The input coil 3 generates a magnetic flux, and the generated magnetic flux is S
The magnetic flux value detected by the QUID sensor 6 is measured by the drive circuit 10.

When the value of the magnetic flux to be measured is large and the superconducting state of the SQUID sensor 6 is destroyed, the value of the current flowing through the input coil 3 is reduced by the inductance adjusting device 4 so that the magnetic flux value at which the SQUID sensor 6 operates. By measuring, a large magnetic flux value is measured.

The SQUID magnetometer constructed as described above is easy to handle because it does not use liquid helium, and since the SQUID sensor 6 and the pickup coil 1 are separated, the operability of the pickup coil 1 is high. Further, since it is shielded by the superconductor 13, an external magnetic field is not picked up, and the dynamic range can be widened by the inductance adjusting device, so that it can be used in a wider range of applications than the conventional SQUID magnetometer.

(Second Embodiment) FIG. 3 shows an example of performing a capillary inspection using the present invention. The pickup coil 1 is housed in the probe 17. By pushing out the flexible tube 4 with the pusher 18, the probe 17 scans the inside of the thin tube 19. Tube 4 here
The inside has the same structure as the first embodiment, and the pickup coil 1 is cooled by the copper in the tube 4. Further, although not shown, similarly to the first embodiment,
It has a cryostat 9 and the like in which the inductance adjusting device 4 and the SQUID sensor 6 are stored.

The main component of the probe 17 is only the pickup coil 1, and since liquid helium is not used in the probe 17, the probe 17 can be miniaturized. By using this probe 17, the deterioration inspection of the thin tube 19 can be performed. Therefore, the thin tube 19 which cannot be inspected because the probe 17 could not be inserted conventionally can be inspected by this embodiment.

A magnetic field is applied to the thin tube 19 in advance by using, for example, an exciting coil, and then the probe 17 is placed in the thin tube 19 to detect the residual magnetic field as a change in magnetic permeability due to deterioration of the material of the thin tube 19. The deterioration is inspected. At this time, by changing the dynamic range with the inductance adjusting device 4 according to the deterioration level,
It is possible to detect the degree of deterioration in a wide range.

(Third Embodiment) FIG. 4 is a schematic view of a measuring unit of a brain magnetic field measuring apparatus according to the third embodiment of the present invention. In FIG. 4, the flexible tube 4 has the same structure as in the first embodiment, and the pickup coil 1 is cooled by the copper in the tube 4. Although not shown in the figure, it also has a cryostat 9 and the like storing the SQUID sensor 6 and the like similar to the first embodiment.

By measuring the cerebral magnetic field, the activity state of each part of the brain can be known, and there are expectations for the treatment of epilepsy and the like. However, since the cerebral magnetic field is an extremely weak magnetic field signal, it is difficult to measure with a measuring device other than the SQUID magnetometer. Generally, a multichannel SQUID sensor in which a large number of SQUID sensors and pickup coils are installed in a helmet-shaped cryostat is used for measuring brain magnetic fields. However, since the pickup coil was fixed in the cryostat, the pickup coil could not be brought into close contact with the head according to the difference in the shape and size of the head. Therefore, since the pickup coil is separated from the head, it is more difficult to measure the brain magnetic field, which is a minute magnetic field.

On the other hand, in the apparatus of this embodiment, since the pickup coil 1 is cooled via the heat conducting material in the flexible tube 4, the position of the pickup coil 1 can be easily changed. . Therefore, it is possible to adjust the position of the sensor according to the shape of the subject's head and bring the pickup coil 1 into close contact with the head 20. Therefore, it becomes possible to measure the brain magnetic field with high sensitivity.

[0027]

According to the present invention, the SQU as described below is provided.
An ID magnetometer can be provided. It is a heat-conducting material that is thermally connected to a freezer. It is easy to handle because it cools superconducting materials such as SQUID, and it has portability, so it does not matter what is measured. Further, since the pickup coil is cooled by the long heat conductive material that is thermally connected to the refrigerator, the pickup coil can be operated at a position away from the SQUID sensor.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the configuration of an SQUID magnetometer according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing the configuration of an inductance adjusting device according to the first embodiment of the present invention.

FIG. 3 is a schematic diagram showing a configuration of a capillary inspection device according to a second embodiment of the present invention.

FIG. 4 is a schematic diagram showing the configuration of a brain magnetic field measurement apparatus according to a third embodiment of the present invention.

FIG. 5 is a schematic diagram showing a configuration of a conventional SQUID magnetometer.

[Explanation of symbols]

 1 pickup coil 2 cryostat 3 input coil 4 flexible tube 5 inductance adjusting device 6 SQUID sensor 7 refrigerator 8 stage 9 cryostat 10 driving circuit 10a coil 11 superconducting wire 12 copper (heat conducting material) 13 superconductor 14 multi-layer heat insulating material 15 , 16 coils 17 probes 18 pushers 19 capillaries 20 heads

Claims (4)

[Claims] 1. A SQUID sensor and an input coil that are thermally connected to a refrigerator, a pickup coil that picks up a magnetic flux in the vicinity of a subject, and a superconducting wire that connects the input coil and the pickup coil. An SQUID magnetometer, comprising a long heat conductive material that is thermally connected to the refrigerator and cools the pickup coil and the superconducting wire. 2. The SQUID magnetometer according to claim 1, wherein the heat conducting material is made of copper. 3. The SQUID magnetometer according to claim 1, wherein the superconducting wire is covered with a superconductor via the heat conducting material and magnetically shielded by the superconductor. 4. The SQUID magnetometer according to claim 1, wherein a part of the superconducting wire constitutes an inductance adjusting device including a coil whose inductance can be changed.