JPH06174811A - Squid magnetic flux meter - Google Patents

Abstract

PURPOSE:To simplify an electronic circuit and reduce magnetic flux noise due to the change of a bias current without complicating a superconductive circuit by setting bias current infection positions in asymmetry for a SQUID loop and comparing output voltage with reference voltage. CONSTITUTION:Inductances 3, 4 and Josephson junctions 5, 6 are provided to form a SQUID loop 1, part of which is earthed, so that a bias current Ib can be injected to the inductances 3, 4 at asymmetrical positions. In this way, when the current Ib is changed, magnetic flux-voltage property is shifted to the axis of magnetic flux, the change of voltage with the change of the current Ib is poor and the change of the current Ib is cancelled. If a method of comparing output voltage with reference voltage is used, an electronic circuit can be simplified and magnetic flux noise due to the change of the current Ib can be reduced without complicating a superconductive circuit.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is suitable for biomagnetism measurement such as magnetocardiogram wave, electroencephalogram wave, and eye muscle magnetic field, geomagnetism measurement, or magnetic susceptibility measurement of a substance, and further an interface for magnetic signal transmission. SQUID (Superconducting Qu
antum Interference Device: superconducting quantum interference device)

[0002]

2. Description of the Related Art Conventionally, an SQUI used for an SQUID magnetometer
As an ID, 2 Josephson junctions in the superconducting loop
There are dcSQUIDs including one and rfSQUID including one Josephson junction, but the former is widely studied because it has better noise characteristics than the latter. Below, d
The cSQUID (simply referred to as SQUID) will be described. FIG. 5 is a schematic diagram of a conventional SQUID, which has a configuration in which a DC voltage terminal is provided in a superconducting loop 21 including two Josephson junctions 25 and 26. In FIG. 5, Ic indicates the critical current of the Josephson junction, and Ls indicates the inductance of the superconducting loop. R is the normal conduction resistance of the Josephson junction, and is the shunt resistance added to eliminate the hysteresis in the case of the tunnel junction. The characteristics and operation of the above SQUID will be described with reference to FIG. When a magnetic flux is applied to the superconducting loop 21 from the outside, the SQUID is a circulating current induced in the loop, and a minute change in the applied external magnetic flux is caused by a quantum interference effect in a Josephson junction in the loop. Is used to measure a minute change in magnetic flux by utilizing the fact that appears as a large change in the circulating current. In this case, when the external magnetic flux Φext entering the superconducting loop 21 is an integral multiple of the magnetic flux quantum Φo, the maximum current (hereinafter referred to as the maximum current) that can flow without generating a voltage in the SQUID is 2Ic. , SQU
The current-voltage characteristic (IV characteristic) of ID is a curve indicated by A in FIG. When the external magnetic flux Φext is (integer +1/2) times the magnetic flux quantum Φo, the maximum current is (2I
c-ΔI), and the IV characteristic of SQUID is shown in FIG.
It becomes a curve shown by B in (A). When Φ is continuously increased, the IV characteristic reciprocates between the above two curves A and B, and its cycle becomes Φ o. Therefore, when biasing the current so that I = 2Ic and observing the voltage, the magnetic flux-voltage curve (Φ− of the period Φo as shown in FIG. 6B is obtained.
V curve) is obtained. The value of ΔI / 2Ic is called the SQUID modulation depth, and the shielding parameter β (= 2Ls
It is calculated in detail as a function of Ic / Φo). In general, ΔI / 2Ic = 1 / (1 + β), and the larger β, the shallower the modulation. Next, FIG.
The configuration of the conventional SQUID magnetometer 20 is shown in FIG. SQU
Since the ID loop 21 is not normally large, it is inefficient to directly insert the magnetic field to be measured into the loop. There, the magnetic field to be measured is detected by the pickup coil 23 and coupled to the SQUID loop 21 via the input coil 24. The coil 28 adjacent to the input coil 23 is a modulation coil.
The modulated magnetic flux driven at 9 is applied to the SQUID loop 21. In this case, the SQUID loop 21, the input coil 24, and the modulation coil 28 are stored inside the ultra-low temperature storage container or the like and maintained in the ultra-low temperature state (about 4K). The output voltage of the SQUID is passed through an impedance matching circuit 32 and a preamplifier 33, and a PSD (Phase Sensit)
The ive detector (phase discriminator) 35 performs lock-in detection, and the first derivative of the Φ-V curve is obtained. When this output is added to the modulation coil 28 and negatively fed back, it stabilizes at the point (peak or valley) where the first derivative of the Φ-V curve becomes zero, and the magnetic field to be measured outputs the above feedback amount. It can be obtained by monitoring at Vout. This state is referred to as "locked". The above method is called a FLL (Flux Locked Loop) method, is a kind of so-called “zero-point method”, and is characterized in that the input / output relationship is linear. Figure 7
In the above, the impedance matching circuit 32 is for impedance matching between the SQUID 21 in the ultra-low temperature state (about 4K) and the preamplifier 33 in the room temperature state, and in addition to the illustrated LC circuit, a superconducting transformer is used. In many cases. As described above, the magnetic flux voltage characteristic (Φ
In the method of applying a modulation magnetic flux of a certain frequency to the periodic function of −V characteristic) and detecting the output voltage at the same frequency, as shown in FIG.
2 was located symmetrically with respect to the inductance of the SQUID loop 21.

[0003]

However, in the above-mentioned conventional SQUID magnetometer, an oscillator for applying modulation, a phase discriminator for detecting, etc. are required, and the electronic circuit becomes complicated. was there. On the other hand, D. Drung et al. Proposed a method of directly comparing an output voltage with a reference voltage without applying modulation. Also, in addition, SQ
We proposed a method that can reduce the inductance by connecting the UID loops in parallel and directly couple the magnetic field to the SQUID loop. This eliminates the need for the conventional input coil in their system. The outline of their SQUID magnetometer will be described below. That is, as shown in FIG.
In the SQUID magnetometer 40 of Ung et al., the output voltage of the SQUID 41 is compared with the reference voltage in the integrating amplifier 54 and amplified. Then, the feedback to the feedback coil 48 locks the magnetic flux where the output voltage of the SQUID 41 matches the reference voltage. In the above FLL of the normal method, the lock is applied when the first derivative of the Φ-V curve becomes zero, but in the method of Drung et al., The lock is applied where the value becomes zero in the Φ-V curve before differentiation. It is about to call. In this Drung et al. Scheme,
It is necessary to oscillate the output voltage of SQUID positively and negatively,
A bias voltage is applied to the Φ-V curve. In other words, the output voltage is being compared with the reference voltage. Since the method of Drung et al. Is not lock-in detection,
Although the resolution deteriorates, as can be seen by comparing FIGS. 7 and 9, an oscillator, a phase discriminator, etc. are not required, and the electronic circuit is prevented from becoming complicated. However, in their method, when the bias current fluctuates, the output voltage fluctuates, which becomes the magnetic flux noise of the magnetometer. To prevent this, Drung et al.
CF (Bias Current Feedback) was proposed. This is to cancel the variation of the bias current by feeding back the variation of the bias current to the SQUID itself through the inductance (D. Drung and H. Koch, "An Electr.
onicSecond-Order Gradiometer for Biomagnetic A
pplications in ClinicalShielded Rooms ", ASC'92,
Chicago, August, 1992 (The Institute of Electrical and Electronics Engineers (IEEE)
E) Published "Transaction on Superconductivity" 1993
reference)). However, the BCF method shown in FIG. 10 has a problem that the superconducting circuit portion including the SQUID loop becomes complicated, as can be seen from comparison with FIG. Even in this BCF method, as shown in FIG. 10, the injection position 42 of the bias current is SQUID.
The position is symmetrical with respect to the inductance of the loop 41. The present invention has been made in order to solve these problems, and simplifies an electronic circuit by using a method of comparing an output voltage with a reference voltage, and a bias current of a bias current without complicating a superconducting circuit. An object is to provide an SQUID magnetometer capable of reducing magnetic flux noise due to fluctuations.

[0004]

In order to solve the above-mentioned problems, the SQUID magnetometer according to the present invention is provided with a bias current injection position of the SQUID loop at an asymmetrical position, and the output voltage is compared with a reference voltage. It is configured to form a lock loop and reduce magnetic flux noise due to fluctuations in bias current.

[0005]

According to the present invention having the above configuration, when the bias current is changed by setting the injection position of the bias current to be an asymmetric position, the magnetic flux-voltage characteristic (Φ-V characteristic) is shown in the graph on the left side of FIG. As shown in, the magnetic field shifts in the direction of the magnetic flux axis (Φ axis). When this is viewed on the bias current-voltage axis, as shown in the graph on the right side of FIG. 3, the fluctuation of the voltage becomes dull with respect to the fluctuation of the bias current, and the fluctuation of the bias current is canceled.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an equivalent circuit diagram showing the configuration of the SQUID loop 1 in the SQUID magnetometer which is an embodiment of the present invention.

The SQUID loop 1 is an SQUID loop having inductances 3 and 4 and Josephson junctions 5 and 6, and a part of the SQUID loop is grounded at a position 7. The SQUID loop 1 is biased at a position 2 which is asymmetric with respect to the inductance. The current Ib is configured to be injected.

FIG. 2 is a diagram showing a part of the configuration of a double-washer type dcSQUID magnetometer which directly compares an output voltage with a reference voltage using the above SQUID loop. Is. As shown in the figure, in the SQUID loop 11 having a predetermined inductance,
Tunnel junctions 13 and 13, which are Josephson junctions, are provided so that the bias current Ib is injected into the position 12 asymmetric with respect to the inductance. Further, 14 is an input coil, and this input coil 14 is connected to a pickup coil (not shown). The SQUID magnetometer of the present embodiment is the same as the above SQUID.
The configuration other than the loop 1 or the SQUID loop 11 is the same as that shown in FIG.

FIG. 3 shows the relationship between the external magnetic flux Φext, which is a magnetic flux applied from the outside of the dcSQUID, and the output voltage V (Φ-V characteristic), and the relationship between the current and the voltage when the bias current is changed. It is a conceptual characteristic graph explaining (IV characteristic). When the injection position of the bias voltage is made asymmetric with respect to the inductance of the SQUID loop as shown in FIGS. 1 and 2, the magnetic flux-voltage characteristic (Φ-V characteristic) shifts in the direction of the magnetic flux axis (Φ axis) ( Figure 3
Left graph).

When this is viewed from the relationship between the bias current and the voltage (IV characteristic), the stationary point a is shown on the characteristic curve.
Occurs, the change in the output voltage becomes dull even if the bias current changes. Therefore, if we use the characteristics of this stopping point,
It is possible to suppress the magnetic flux noise due to the fluctuation of the bias current to a predetermined value or less.

FIG. 4 shows the dcSQUID of FIG.
Bias current 0 μA (microampere), 66, 8
External magnetic flux (1 scale = 0.43Φo, Φo: magnetic flux quantum) when changing to 8, 110, 132, 154 μA, and SQUID output voltage in that case (1 scale = 2)
4 is a Φ-V characteristic graph showing 0 μV, μV: microvolts, and corresponds to the graph on the left side of FIG. 3 (magnetic flux-voltage characteristic).

When dIb / dV, which is the fluctuation of the bias current value with respect to the fluctuation of the output voltage value, is read from the point b in FIG. 4, an output voltage curve with a bias current of 88 μA and an output voltage curve with a bias current of 110 μA are read. The difference in bias current between the two is dIb = 22 μA, and the difference between the voltage values of the two output voltage curves at the position b in FIG. 4 (in other words, the separation between the two curves) is dV = 2 μV. Since dIb / dV = 22 μA / 2 μV = 11 A / V, it can be seen that the point b is the stationary point in the case of the characteristics shown in FIG.

Therefore, the magnetic flux noise

[Equation 1] To keep it within

[Equation 2] When,

[Equation 3] The bias current fluctuation is

[Equation 4] It can be seen that it is sufficient to keep it within the range. here,
nA is nanoamps.

Normally, when a commercially available amplifier is used as the preamplifier 53 shown in FIG. 9, the fluctuation value of the bias current is

[Equation 5] However, if a commercially available amplifier is used as the preamplifier 53 shown in FIG. 9, an electronic circuit that can be suppressed within the above range of the value of In can be sufficiently realized by the present technology. Where pA is picoampere. Therefore, by using the method of comparing the output voltage with the reference voltage, it is possible to simplify the electronic circuit and reduce the magnetic flux noise due to the fluctuation of the bias current without complicating the superconducting circuit.

The present invention is not limited to the above embodiment. The above-mentioned embodiment is an exemplification, has substantially the same configuration as the technical idea described in the scope of the claims of the present invention, and has any similar effect to the present invention. It is included in the technical scope of the invention.

In the above embodiment, the case where the dcSQUID is a double-washer type having two rings has been described as an example. However, this is a dcSQUID of another format.
D, eg single-washer type with one ring, 3
Triple-washer type with n rings, or generally n
It may be of the form having a number of washers,
As long as the bias current is injected so that the inductance is unbalanced (unbalanced, that is, asymmetrical) in these rings, the effect of the present invention can be obtained regardless of the dcSQUID.

In the above embodiments, the case where the Josephson junction is a tunnel junction has been described as an example, but this may be another Josephson junction (micro bridge, point contact, etc.). .

[0018]

As described above, according to the present invention having the above structure, when the bias current is changed by setting the injection position of the bias current to be an asymmetric position, the magnetic flux-voltage characteristic is changed in the direction of the magnetic flux axis. Shift. When this is viewed on the bias current-voltage axis, the fluctuation of the voltage becomes dull with respect to the fluctuation of the bias current, and the fluctuation of the bias current is canceled. Therefore, by using the method of comparing the output voltage with the reference voltage, the electronic circuit can be simplified, and the magnetic flux noise due to the fluctuation of the bias current can be reduced without complicating the superconducting circuit. is doing.

[Brief description of drawings]

FIG. 1 is an equivalent circuit diagram of an SQUID loop in an SQUID magnetometer that is an embodiment of the present invention.

FIG. 2 is a diagram showing a partial configuration of an SQUID magnetometer which is an embodiment of the present invention.

FIG. 3 is a diagram showing a Φ-V characteristic and an IV characteristic of the SQUID loop shown in FIG.

FIG. 4 shows a bias current of 0, 66, 88, 110, 132, 154 μA in the SQUID loop shown in FIG.
External magnetic flux when changing to S, and S in that case
It is a figure which shows the (phi) -V characteristic which showed the output voltage of QUID.

FIG. 5 is a schematic diagram of SQUID in a conventional SQUID magnetometer.

6 is a diagram showing characteristics of the SQUID shown in FIG.
FIG. 6A shows current-voltage characteristics, and FIG. 6B shows magnetic flux-voltage characteristics.

FIG. 7 is a circuit diagram showing an overall configuration of a conventional SQUID magnetometer.

FIG. 8 is an equivalent circuit diagram of a SQUID loop in a conventional SQUID magnetometer.

FIG. 9 is a circuit diagram showing the configuration of the SQUID magnetometer proposed by Drung et al.

FIG. 10 is an equivalent circuit diagram of the SQUID loop proposed by Drung et al.

[Explanation of symbols]

 1 SQUID Loop 2 Bias Current Injection Position 3,4 Inductance 5,6 Josephson Junction 7 Grounding Position 11 SQUID Loop 12 Bias Current Injection Position 14 Input Coil 15, 16 Tunnel Junction 20 SQUID Fluxmeter 21 SQUID Loop 22 Bias Current Injection Position 23 Pickup coil 24 Input coil 25,26 Josephson junction 27 Grounding position 28 Modulation coil 29 Oscillator 30 Phase shifter 31 Bias current source 32 Impedance matching circuit 33 Preamplifier 34 Integral amplifier 35 PSD 40 SQUID magnetometer 41 SQUID loop 42 Bias current injection Position 43 Pickup coil 44 Input coil 45,46 Josephson junction 47 Grounding position 48 Feedback coil 53 Preamplifier 54 Integral amplifier 60 Over Temperature storage container

Claims (1)

[Claims] 1. A magnetic flux lock loop is formed by providing a bias current injection position of the SQUID loop at an asymmetrical position, and comparing an output voltage with a reference voltage to reduce magnetic flux noise due to fluctuations in the bias current. SQU
ID magnetometer.