JPH04235369A - Squid magnetic fluxmeter - Google Patents

Abstract

PURPOSE:To make it possible to make inductance of a pickup coil small without making the area of the pickup coil small, regarding a SQUID fluxmeter which is used for measurement of a very minute magnetic flux (or a magnetic field) generated from an organism or the like. CONSTITUTION:A SQUID fluxmeter is so constructed as to comprise a pickup coil 10 detecting a magnetic flux to be measured, an input coil 20 connected to the pickup coil 10 and forming a superconducting closed loop together with the pickup coil 10, and SQUID 30 coupled magnetically with the input coil 20 and formed of a superconducting loop including a Josephson device. The pickup coil 10 is composed of a plurality of coils located on the same plane substantially and also it is so constructed that parts of coils adjacent to each other, out of those coils in a plurality, are in proximity to each other and that the aforesaid coils in a plurality are connected in parallel so that the direction of the flow of an induced current outputted from each coil by a uniform input magnetic field from outside be the same.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SQUID magnetometer used to measure extremely small magnetic fluxes (or magnetic fields) generated from living bodies and the like. More specifically, the present invention relates to improvement of a pickup coil that is a part of this SQUID magnetometer.

0002

2. Description of the Related Art A typical configuration example of a conventional SQUID magnetometer is shown in FIG. In the same figure, when the magnetic flux to be measured φ1 is input to the pickup coil 1, the pickup coil 1
A current I1 expressed by equation (1) flows through the superconducting loop formed by the input coil 2 and the input coil 2 connected thereto.

0003

[Math 1]

In equation (1), L1 and L2 are the inductances of the pickup coil 1 and the input coil 2, respectively. Now, if the magnetic coupling coefficient between the input coil 2 and the superconducting loop of the SQUID (superconducting quantum interferometer) 3 is k, then the magnetic flux φs input to the SQUID 3 is:

[
0005

[Math 2]

[0006] However, in equation (2), LS is the inductance of the superconducting loop of SQUID 3. Here, to simplify the explanation, assuming k=1, the above formula (2) becomes

[0007]

[Math 3]

[0008] Furthermore, in conventional devices, in order to make φS as large as possible, the input coil 2 is wound many times (n times) as shown in FIG. At this time, the inductance L2 of the input coil 2 seen from the pickup coil 1 is L2 = n2 LS, and φS is

[0009]

[Math 4]

It is expressed as: Here, the maximum value of φS is

[0011]

[Math 5]

Find n that satisfies [0012]

[0013]

[Math 6]

[0014] φS at this time is φSm
When expressed as ax, substituting equation (6) into equation (4),

00
15]

[Math 7]

[0016] Now, the ratio of L1 and LS is 104
:1, the value of n is 100. Conventionally, the input coil 2 in which the coil is wound many times in this manner has been used.

[0017]

Problem to be Solved by the Invention: As mentioned above, if the number of turns of the input coil 2 is large, the input coil 2
The magnetic field coupling coefficient k between the input coil 2 and the SQUID 3 decreases, resulting in a decrease in sensitivity. Also, by winding the input coil 2 many times, the magnetic flux that crosses the superconducting conductor forming the input coil 2 increases. There was a drawback that the probability that magnetic flux would be trapped within this superconducting conductor increased, making the operation of the SQUID 3 unstable.

Note that by reducing the area of the pickup coil 1, the inductance L of the pickup coil 1 can be reduced.
1 can be made smaller, thereby increasing φS without increasing the number of turns of input coil 2. However, this reduces the magnetic flux crossing in pickup coil 1, making it difficult to use as a magnetometer. This results in the disadvantage of reduced sensitivity.

An object of the present invention is to make it possible to reduce the inductance of a pickup coil without reducing the area of the pickup coil.

[0020]

[Means for Solving the Problems] FIG. 1 is a diagram showing the basic configuration of the present invention. As shown in the figure, the SQUID magnetometer of the present invention includes a pickup coil 10 that detects a magnetic flux to be measured, an input coil 20 that is connected to the pickup coil 10 and forms a superconducting closed loop with the pickup coil 10, and an input coil 20 that forms a superconducting closed loop with the pickup coil 10. 20 and a SQUID 30 made of a superconducting loop including a Josephson element.

Although not shown in detail in FIG. 1, a feature of the present invention is the pickup coil 10. That is, the pickup coil 10 is made up of a plurality of coils located on substantially the same plane. The plurality of coils are arranged in such a way that parts of the coils adjacent to each other among the plurality of coils are close to each other, and the flow direction of the induced current output from each coil is the same due to a uniform input magnetic field from the outside. are connected in parallel with each other.

[0022]

[Operation] In the present invention, the pickup coil 10 has a configuration in which a plurality of coils are connected in parallel on almost the same plane,
By placing some of the adjacent coils close to each other among the multiple coils connected in parallel, the sensitivity is almost equal to that of a single conventional coil, which has an area equal to the sum of the areas of the coils connected in parallel. Moreover, by having a configuration in which individual coils with small inductance are connected in parallel, it is possible to reduce the inductance of the pickup coil 10 to approximately 1/the number of coils connected in parallel. Therefore, even if the number of turns of the input coil 20 remains small, φS can be increased, and there is no fear that the sensitivity of the magnetometer will deteriorate.

[0023]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 is a block diagram of the first embodiment of the present invention.

In this embodiment, the pickup coil 10 is
It is constructed by connecting two semicircular coils 11 and 12 in parallel. That is, the two semicircular coils 11 and 12 are connected in parallel to the input coil 20 on substantially the same plane, and the linear portions (circular diameter portions) of each coil 11 and 12 are close to each other, They are arranged so that the entire outer circumference is approximately equivalent to one circle. Moreover,
The connection relationship between the two coils 11 and 12 is designed so that the induced currents I11 and I12 output from each coil 11 and 12 flow in the same direction due to a uniform input magnetic field φ1 from the outside.

Here, the effect of the pickup coil 10 of this embodiment having the above-mentioned configuration and the effect of the conventional circular pickup coil 1 whose size just surrounds the coils 11 and 12 as shown in FIG. compare.

First, when a uniform magnetic field φ1 is similarly input to the pickup coils 1 and 10, and if the magnetic flux passing through the pickup coil 1 is φ1, the induced current I1 flowing through the pickup coil 1 is φ1/L1 (here, L1 is the inductance of pickup coil 1)
On the other hand, the magnetic fluxes passing through the two coils 11 and 12 of the pickup coil 10 are both φ1/2. Also,
The induced currents I11 and I12 flowing through each of the coils 11 and 12 have the same magnitude, but the directions of flow are opposite to each other in the straight portion of the coil. Therefore, depending on the current flowing in this portion, the magnetic flux passing through the coils 11 and 12 may vary. cancel each other out and become almost zero. It is only the currents on the arcs of the coils 11 and 12 that contribute to the generation of cross magnetic flux. Since the total magnetic flux crossing the two semicircular coils 11 and 12 is φ1, the magnitude of the induced current generated in the coils 11 and 12 to cancel this magnetic flux is that generated in one circular coil 1. It is equal to the induced current I1. That is,

[0027]

[Math. 8]

[0028] In this way, as the current I1 flows through each of the coils 11 and 12, φ1/
Therefore, the inductances L11 and L12 of the coils 11 and 12 are equal as shown in equation (9).

[0029]

[Math. 9]

On the other hand, regarding the circular coil 1,

00
31]

[Math. 10]

Since there is the following relationship, equation (10) is replaced by equation (9
), we get

[0033]

[Math. 11]

The values of the inductances L11 and L12 of the semicircular coils 11 and 12 are half of the inductance L1 of the circular coil 1. Moreover, coil 11,
12 are connected in parallel with each other, the inductance of the pickup coil 10 is as small as L1/4. Therefore, when the input coil 20 and the SQUID 30 are connected as shown in FIG. 2, the magnetic flux φS input to the SQUID 30 is

[0035]

[Math. 12]

In this case, the value of n that maximizes φS is:

[0037]

[Math. 13]

[0038] Then, φ which is the maximum value of φS
Smax is calculated by substituting equation (13) into equation (12),

0
039]

[Math. 14]

[0040] This value is the same as the value for the conventional circular coil 1 shown in equation (7). As described above, in the pickup coil 10 (coils 11, 12) of this embodiment, the inductance can be reduced to 1/4 (=1/22) without reducing the sensitivity compared to the conventional one. For this reason, input coil 2
The number of turns n of 0 can be reduced to 1/2 compared to the conventional input coil 2 (FIG. 6). For example, if the conventional input coil 2 requires 100 turns, the input coil 20 in this embodiment only requires 50 turns. Therefore, the input coil 20
Even if the number of turns is reduced to 1/2 of the conventional value, the same large φ
S can be obtained, and there is no fear that the sensitivity of the magnetometer will decrease.

Next, FIG. 3 is a block diagram of a second embodiment of the present invention. In the first embodiment, the pickup coil 10 was composed of two semicircular coils, but in this embodiment,
Four coils 11 and 12 in the shape of a circular coil divided into four parts
, 13, and 14 are connected in parallel to form the pickup coil 10. That is, four quadrant coils 11, 121 are used for the input coil 20.
3 and 14 are connected in parallel on almost the same plane, and the straight portions (circular diameter portions) of each coil 11, 12, 13, and 14 are close to each other, and the entire outer circumference is approximately equivalent to one circle. It is arranged so that Moreover, four coils 11, 1
2, 13, and 14 are connected so that the direction of flow of induced currents I11, I12, I13, and I14 output from each coil 11, 12, 13, and 14 is the same due to a uniform input magnetic field φ1 from the outside. It is considered. In the figure, output lines a and b to which coils 11 and 12 are connected in parallel are output lines a' and b' to which coils 13 and 14 are connected in parallel, respectively.
It is connected to the.

In the above configuration, the four coils 11, 1
For the same reason as stated in the first embodiment, the inductance of the pickup coil 10 configured by connecting the coils 2, 13, and 14 in parallel is different from that of the conventional circular pickup coil 1 (FIG. 6). 1/16 compared to (
= 1/42), so in this case, the number of turns n of the input coil 20 can be reduced to 1/4 compared to the conventional one, and even 1/4 compared to the first embodiment.
/2 can be reduced.

Next, FIG. 4 is a block diagram of a third embodiment of the present invention. In the embodiments described so far, a coil in the shape of a divided circular coil was used as the pickup coil 10, but in this embodiment, two coils 11 and 12 in the shape of a rectangular coil divided into two were used. It is. The two coils 11 and 12 are connected in parallel to each other on almost the same plane, one side of each coil 11 and 12 is close to each other, and the entire outer circumference is approximately equivalent to one rectangular coil before division. It is arranged so that. Moreover, the connection relationship between the two coils 11 and 12 is the same as in each of the above embodiments.
Each coil 11, 1 is
The directions of the induced currents I11 and I12 outputted from 2 are made to be the same.

Even when using the coils 11 and 12 which are formed by dividing a rectangular coil into two in this way, as in the case where a semicircular coil is used as in the first embodiment, the original Compared to the case of one large rectangular coil, the inductance can be reduced to 1/4 (=1/22).

Note that in FIGS. 2 to 4, the coil 11,
The parts where the coils 12 (or coils 11, 12, 13, 14) are close to each other are drawn with a slight distance apart for ease of viewing, but in reality, this can be achieved by placing the conductor wires of the coils closely together or twisting them together. It is desirable to minimize the generation of magnetic fields from these parts.

[0046]Also, in the above embodiment, examples were shown in which the number of divisions was 2 and 4, but the number of divisions is not limited to these numbers, and in general, the number of divisions is N. A coil can be used as the pickup coil 10. When N-divided coils are connected in parallel, the inductance of the pickup coil 10 becomes 1/N2 of that of a single coil before division, so the number of turns of the input coil 20 is reduced to 1. /N.

Furthermore, in the above embodiment, the case where a circular or rectangular coil is divided is shown, but by dividing a coil of any shape into an arbitrary number and connecting them in parallel, the inductance of the pickup coil 10 can be reduced. It is possible to reduce the

In addition, it goes without saying that the pickup coil of the present invention can be used to construct a first-order differential pickup coil or a second-order differential pickup coil.

[0049]

According to the present invention, the inductance of the pickup coil can be reduced without reducing the sensitivity, and therefore the number of turns of the input coil can be reduced. Therefore, it is possible to improve the sensitivity by bringing the magnetic coupling coefficient between the input coil and the SQUID close to 1, and to reduce the probability that magnetic flux will be trapped within the superconducting conductor that is the material of the input coil. It can greatly contribute to stabilizing SQUID's operation.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the principle configuration of the present invention.

FIG. 2 is a configuration diagram of a first embodiment of the present invention.

FIG. 3 is a configuration diagram of a second embodiment of the present invention.

FIG. 4 is a configuration diagram of a third embodiment of the present invention.

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

FIG. 6 is a configuration diagram of a conventional SQUID magnetometer.

[Explanation of symbols]

10 Pickup coil 11, 12, 13, 14 Coil 20 Input coil 30 Squid

Claims (1)

[Claims] 1. A pickup coil (10) for detecting a magnetic flux to be measured; an input coil (20) connected to the pickup coil (10) and forming a superconducting closed loop together with the pickup coil (10); In the SQUID magnetometer, the pickup coil (10) includes a SQUID (30) magnetically coupled to a coil (20) and consisting of a superconducting loop including a Josephson element.
consists of a plurality of coils located on almost the same plane, parts of the coils that are adjacent to each other among the plurality of coils are close to each other, and the direction of the flow of induced current output from each coil due to a uniform input magnetic field from the outside. A SQUID magnetometer, characterized in that the plurality of coils are connected in parallel so that the values are the same.