JPH05196709A - Squid fluxmeter - Google Patents

Abstract

PURPOSE:To measure magnetic flux accurately even if the input magnetic flux is largely changed in a short time. CONSTITUTION:The magnetic coupling part of a pickup coil 10, a feedback loop 24 and a magnetic coupling line 26 are provided in the vicinity of a SQUID element 12. An AC bias current IB1 and an offset current IC having the same frequency are supplied into the SQUID element 12 and the magnetic coupling line 26, respectively. Then, the positive pulse voltage and the negative pulse voltage having the widths corresponding to magnetic flux phi, which is supplied into the SQUID element 12 from the pickup coil 10 and the feedback loop 24, are taken out of the SQUID element 12. The width of the positive pulse and the width of the negative pulse are measured with currents IP and IN having the higher frequencies than the AC bias current IB1. The difference between both pulses is approximately proportional to the supplied magnetic flux phi.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a SQUID magnetometer.

[0002]

2. Description of the Related Art The SQUID magnetometer has the highest sensitivity among all magnetometers, and is particularly used for measuring the magnetic field emitted from the living body. From the viewpoint of clinical application, the SQUID magnetometer is desired.

FIG. 10 is a circuit diagram of a conventional SQUID magnetometer.

The primary differential type pickup coil 10 for canceling the earth magnetism is magnetically coupled to the SQUID element 12. The SQUID element 12 is constructed by interposing a Josephson junction J1 and a Josephson junction J2 in a superconducting loop. An AC bias current as shown in FIG. 11A is supplied to the SQUID element 12 from the AC bias current source 14.

On the other hand, the write gate 16 has a SQUID
A magnetic coupling wire 20 is provided near the element 18. One end of the magnetic coupling wire 20 is connected to the SQUID element 18,
The other end is connected to the output end of the AC bias current source 14. The SQUID element 18 is connected in a ring with a superconducting storage loop 22 and a feedback loop 24.
The feedback loop 24 is magnetically coupled to the SQUID element 12.

In the above structure, when the SQUID element 12 receives the input magnetic flux through the pickup coil 10,
A write pulse as shown in FIG. 11B is supplied to the magnetic coupling line 20, and a magnetic flux Ψ as shown in FIG. 11C is stored in the superconducting storage loop 22 via the SQUID element 18. A magnetic flux proportional to the stored magnetic flux Ψ is fed back to the SQUID element 12 via the feedback loop 24, canceling the input magnetic flux from the pickup coil 10 and maintaining the operating point so that the total magnetic flux in the SQUID element 12 becomes zero. ..

In the write pulse shown in FIG. 11B, the positive pulse is SQU from the pickup coil 10.
This occurs when the magnetic flux supplied to the ID element 12 and the magnetic flux supplied from the feedback loop 24 to the SQUID element 12 are superposed on each other, and the supplied magnetic flux φ increases by a magnetic flux quantum. It occurs when the amount of quantum decreases (the magnetic flux in the opposite direction increases).

Such an SQUID magnetometer has an advantage that it can be integrated into one chip and a large number of magnetometers can be arranged in parallel in one chip to measure the magnetic field distribution.

[0009]

However, only one flux quantum can be written into the superconducting storage loop 22 for each positive and negative cycle of the alternating bias current.
On the other hand, the frequency of the AC bias current source 14 is SQUID
Although it is necessary to match the resonance frequency of the element 12, this resonance frequency cannot be made too high because the coupling capacitance between the pickup coil 10 and the SQUID element 12 is usually relatively small. Therefore, when the input magnetic flux changes greatly in a short time, the magnetic flux cannot be measured accurately.

In addition, one-chip multi-channel SQUI
SQU to configure the D magnetometer and increase the response speed
When the resonance frequency of the ID element 12 is increased, a relatively large high frequency bias current must be supplied to the SQUID element for detecting the input magnetic flux, which increases crosstalk and causes the circuit to malfunction.

In view of such problems, an object of the present invention is to provide an SQUID magnetometer capable of accurately measuring the magnetic flux even if the input magnetic flux changes largely in a short time.

[0012]

FIG. 1 shows the principle configuration of the SQUID magnetometer according to the present invention.

This SQUID magnetometer is a superconducting ring.
Two or more Josephson junctions J1 and J2 are interposed and
Current bias current I_BSQUID element 1 supplied with
Offset current I having the same frequency as the AC bias current_C
The magnetic coupling line 2a supplied with is installed close to the SQUID element 1.
The SQUID element 1 state change (superconducting state / normal
Supply to the SQUID element 1 based on the change of conduction state)
Magnetic flux that outputs positive pulse and negative pulse with width according to magnetic flux
/ Pulse width conversion circuit 2 and AC bias current I _Bthan
High frequency signal I_RMeasure the pulse width based on
The pulse width measuring circuit 3 is provided.

In the present invention, the magnetic flux supplied to the SQUID element 1 is converted into a pulse width, and the pulse width is measured based on the signal I _R having a frequency higher than the AC bias current I _B supplied to the SQUID element 1. Therefore, it is possible to accurately measure the magnetic flux even if the input magnetic flux largely changes in a short time without increasing the frequency of the AC bias current I _B.

In the first aspect of the present invention, in addition to the above structure, a superconducting storage loop 4 and a feedback loop 5 for giving a negative feedback magnetic flux to the SQUID element 1.
A feedback circuit in which a write gate 6 that stores magnetic flux in a superconducting storage loop 4 in response to a pulse from the pulse width measurement circuit 3 is annularly connected is provided, and the pulse width measurement circuit 3 has a number corresponding to the pulse width. The pulse of is output.

As the feedback circuit, various known circuits can be combined, but in this configuration, the SQUID magnetometer including the feedback circuit can be integrated into one chip, and one chip can be used for multichannel SQUI.
A D magnetometer can be constructed. Also, by providing this feedback loop, the magnetic flux measurement range can be widened.

In the second aspect of the present invention, for example, as shown in FIG. 2, write gate 16W includes positive write gates 18P and 20P that cause positive flux quanta to accumulate in superconducting storage loop 22 in response to a positive pulse. Negative write gates 18N and 20N that store negative magnetic flux quanta in the superconducting storage loop 22 in response to the negative pulse are connected in series, and the pulse width measurement circuits 28P, 30P, 28N, and 30N are the same as those of the positive pulse. The number of positive pulses corresponding to the width is supplied to the positive write gates 18P and 20P, and the number of negative pulses corresponding to the width of the negative pulse is supplied to the negative write gates 18N and 20N.

In the case of this configuration, the difference between the positive pulse width and the negative pulse width is determined by the write gate 16W and the superconducting storage loop 22.
Since the calculation can be performed by and, the circuit becomes relatively simple.

In the third aspect of the present invention, for example, as shown in FIG. 6, pulse width measuring circuits 28P, 30P, 28N and 30 are provided.
N and 34 supply the write gate 16 with positive or negative pulses corresponding to the sign of the difference between the width of the positive pulse and the width of the negative pulse.

With this structure, it is possible to reduce the fluctuation of the magnetic flux φ supplied to the SQUID element 12 for each cycle of the bias current I _B1 .

In the fourth aspect of the present invention, for example, as shown in FIG. 8, the write gate has a plurality of write gates 161 which respond to one pulse and accumulate a plurality of mutually different quantum fluxes in the storage superconducting loop. 16n are connected in series, and pulse width measuring circuits 28P, 30P, 28
A selection circuit 36 is provided which selects the write gate 16i according to the pulse width measured at N and 30N and supplies a pulse to the selected write gate 16i.

In the case of this configuration, by supplying one pulse to the selected write gate 16i, it is possible to accumulate a magnetic flux of, for example, i times the magnetic flux quantum in the superconducting loop for storage, so that it can be further increased in a short time. It is possible to measure the changing magnetic flux.

[0023]

Embodiments of the present invention will be described below with reference to the drawings.

(1) First Embodiment FIG. 2 is a circuit diagram of the SQUID magnetometer of the first embodiment. The same components as those in FIG. 10 are designated by the same reference numerals and the description thereof will be omitted.

A magnetic coupling line 26 is provided near the SQUID element 12 in addition to the magnetic coupling portion of the pickup coil 10 and the feedback loop 24. One end of the magnetic coupling line 26 is grounded, and the other end is connected to the output end of the AC bias current source 14 via the resistors R3 and R1. The connection point between the resistor R1 and the resistor R3 is SQU via the resistor R2.
Connected to the ID element 12, the resistor R2 and the SQUID element 1
The connection point of 2 is the buffer gate 28P via the resistor R4.
And 28N input terminals.

The buffer gates 28P and 28N are superconducting circuits, and the output terminals of the current sources 30P and 30N are connected to the respective power source input terminals. Current source 30P and 30N outputs a positive pulse current I _P and the negative pulse current I _N, respectively. Positive pulse current I _P and negative pulse current I
_N has the same frequency and the same phase, and is higher than the frequency of the AC bias current source 14. Buffer gate 28
The output terminals of P and 28N are connected to the write gate 16W.

In the write gate 16W, an SQUID element 18P and an SQUID element 18N are connected in series,
A magnetic coupling line 20P is provided near the UID element 18P, and SQ
A magnetic coupling wire 20N is provided near the UID element 18N. The SQUID elements 18P and 18N are respectively SQ
The SQUID element 18 has the same configuration as the UID element 12.
P comprises Josephson junctions J3P and J4P, SQU
The ID element 18N includes Josephson junctions J3N and J4N. The magnetic coupling line 20P has one end connected to the output end of the buffer gate 28P and the other end connected to the resistor R.
It is grounded via P, and the positive pulse current I _CP is supplied from the buffer gate 28P. Similarly, the magnetic coupling line 2
One end of 0N is connected to the output end of the buffer gate 28N and the other end is grounded via the resistor RN, and the negative pulse current I _CN is supplied from the buffer gate 28N.

The write gate 16W is annularly connected to the superconducting storage loop 22 and the feedback loop 24. At the connection point between the SQUID element 18P and the superconducting storage loop 22, the output terminal of the AC bias current source 14 is a resistor R.
It is connected via 5.

The output terminals of the buffer gates 28P and 28N are connected to the clock input terminals U and D of the up / down counter 32, respectively.
The output terminals of the current sources 30P and 30N are connected to the power source input terminal of No. 2. The up-down counter 32 is a superconducting circuit and increments the count value each time one pulse is supplied to the clock input terminal U, and increments the count value each time one pulse is supplied to the clock input terminal D. Decrement and output the count value to the outside.

The resistors R1 to R3 and R5 are for determining the shunt ratio of the current supplied from the AC bias current source 14, and the resistor R4 is for the voltage V _O input to the buffer gates 28P and 28N. It is for adjusting the level. The resistors RP and RN are for adjusting the currents flowing through the magnetic coupling lines 20P and 20N, respectively.

The circuit shown in FIG. 2 has an AC bias current source 1
4. Except for the current sources 30P and 30N, they can be integrated into one chip.

Next, the operation of this embodiment constructed as described above will be explained.

When the SQUID element 12 is in the superconducting state,
The magnetic flux supplied from the pickup coil 10, the feedback loop 24, and the magnetic coupling line 26 to the SQUID element 12 is superposed, and is canceled by the magnetic flux generated in the SQUID element 12. In this case, no current flows through the resistor R4 and the voltage V _O becomes 0. Pickup coil 10, feedback loop 24 and magnetic coupling wire 26
If the combined magnetic flux supplied from the SQUID element 12 to the SQUID element 12 has a non-zero finite value, the SQUID element 1
Since the threshold value of 2 moves as shown in FIG. 3 (A), the SQUID element 12 always switches with a bias on the positive or negative side. At this time, J1 and J2 are switched in sequence and SQUID
The element 12 becomes excessively normal and the voltage V _O rises.

In other words, when the magnetic flux supplied from the pickup coil 10 to the SQUID element 12 and the magnetic flux supplied from the feedback loop 24 to the SQUID element 12 are superposed, the supplied magnetic flux φ is 0, the resistance R3 is Depending on the offset current I _C flowing and the bias current I _B1 flowing through the resistor R2, as shown in FIG. 3A, the superconducting state is set within the threshold curve TC0 and the normal conducting state is set outside the threshold curve TC0. When the supply magnetic flux φ is not 0, the threshold curve TC0 moves in parallel in the axial direction of the offset current I _C according to the magnitude of the supply magnetic flux φ, and becomes the threshold curve TC1. In this case, when the bias current I _B1 changes as shown in FIG. 3 (B), the SQUID element 12 is moved to the point P of FIG. 3 (A).
The state transitions at points M and M, and the voltage V _O becomes as shown in FIG. That is, in one cycle of the bias current I _B1 , a positive pulse of width t _{P and} a negative pulse of width t _N are obtained. t _P and t _N vary depending on the supplied magnetic flux φ, and φ =
When 0, t _P = t _N. t _P −t _N is almost proportional to the supplied magnetic flux φ. Therefore, t _P , t _N or t _P −t _N
The supply magnetic flux φ can be measured by measuring

For the positive pulse current I _P and the negative pulse current I _N shown in FIGS. 4C and 4D, the voltage V _O shown in FIG. 4B is applied to the buffer gates 28P and 28N.
Is supplied to the buffer gates 28P and 28N, the positive pulse current I _CP and the negative pulse current I _CN as shown in FIGS. 4E and 4F are output, respectively. On the other hand, the bias current I _B2 flowing through the resistor R5 is as shown in FIG.

Positive pulse current I with respect to bias current I _B2
Changes in _CP and negative pulse current I _CN , and SQUID element 18
The relationship between the P and 18N state changes is shown in FIG. The line CP in the figure is the SQUI for one positive pulse current I _CP .
The state change of the D element 18P is shown, the Josephson junction J3P switches at the point A to generate a positive magnetic flux quantum in the SQUID element 18P, and the Josephson junction J4P switches at the point B to superconduct this magnetic flux quantum. It is written in the storage loop 22. Likewise, CN in the figure indicates a change in the state of the SQUID 18N for one of the negative pulse current I _CN, a negative flux quantum to the SQUID element 18N to switch the Josephson junction J3N at point A occurs , The Josephson junction J4N switches at point B, and this flux quantum is written in the superconducting storage loop 22.

In this way, the pickup coil 10
A magnetic flux proportional to the input magnetic flux to the superconducting storage loop 22 is accumulated, and a magnetic flux proportional to the accumulated magnetic flux Ψ is fed back to the SQUID element 12 via the feedback loop 24 so that the supply magnetic flux φ becomes zero. Controlled. Further, the count value of the up / down counter 32 becomes equal to the value of the accumulated magnetic flux Ψ in units of magnetic flux quantum.

In the first embodiment, the SQUID element 12 is used.
Of the supplied magnetic flux φ of the SQUID element 12
AC bias current I supplied to_B1Higher frequency electricity
Flow I _PAnd I_NMeasure the width of this pulse based on
Therefore, the AC bias current I_B1The frequency of
Even if the input magnetic flux φ changes greatly in a short time, the magnetic flux
It is possible to measure accurately.

(2) Second Embodiment FIG. 6 is a circuit diagram of the SQUID magnetometer of the second embodiment.
The same components as those in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted.

In this SQUID magnetometer, the write control circuit 34 calculates the difference i between the number of positive pulses from the buffer gate 28P and the number of negative pulses from the buffer gate 28N for each cycle of the bias current I _B2. , When the difference i is positive, i positive pulses are supplied to the clock input terminal U of the write gate 16 and the up / down counter 32, and when i is negative, −i negative pulses are supplied to the write gate 16 and the up / down counter 32. Is supplied to the clock input terminal D. The other points are the same as in the first embodiment.

The write control circuit 34 uses the positive pulse current I
_{It is composed} of an up-down counter that counts up by _{CP and} counts down by a negative pulse current I _CN , and a circuit that supplies a pulse to the counter and the write gate 16 so that the count value of the counter becomes 0. Can be configured.

According to the SQUID magnetometer of the second embodiment, the fluctuation of the magnetic flux φ supplied to the SQUID element 12 for each cycle of the bias current I _B1 can be made smaller than that of the first embodiment. it can.

(3) Third Embodiment In the above first and second embodiments, the relationship between the supply magnetic flux φ and the pulse width difference t _P -t _N is as shown in FIG. If the input magnetic flux changes further with time, it saturates and t _P -t _N is no longer proportional to the supply magnetic flux φ.

Therefore, in order to solve this, the SQUID magnetometer of the third embodiment is constructed as shown in FIG.

The write gate of this SQUID magnetometer has a structure in which write gates 161 to 16n are connected in series. Write gate 16i (i = 1 to n)
Writes in the superconducting storage loop 22 a positive magnetic flux that is i times the quantum magnetic flux for one positive pulse in the superconducting storage loop 22 for one negative pulse. Write.

Further, for each cycle of the bias current I _B2 , the difference i between the pulse number of the positive pulse current I _{CP and} the pulse number of the negative pulse current I _CN is set by the prediction selection circuit 36 as in the second embodiment. Is measured and stored in the pulse width memory 38. Then, when trying to exceed the proportional range shown in FIG. 7, the supply magnetic flux φ is predicted from the change of the difference i according to the same logic as that of a known prediction filter, and an adjustment proportional to the magnitude of this supply magnetic flux φ is made. When k is positive with respect to the numerical value k, one positive pulse is supplied to the write gate 16k, and when k is negative, one negative pulse is supplied to the write gate 16 | k |. For the up / down counter 32, k pulses are supplied to the clock input terminal U when k is positive, and -k pulses are supplied to the clock input terminal D when k is negative. These prediction selection circuit 36 and pulse width memory 38
Can be composed of a superconducting circuit.

The other points are the same as those of the first embodiment.

The write gate 16i is constructed, for example, as shown in FIG. 9A, 9B or 9C.

(A) shows SQUID elements 181 to 18i.
Are connected in series, the magnetic coupling lines 201 to 20i are respectively provided near the SQUID elements 181 to 18i, and the magnetic coupling lines 201 to 20i are connected in series and grounded via a resistor R6 for current adjustment. It is a composition. Therefore, when one current pulse is supplied to the magnetic coupling line 201, the SQUID elements 181 to 18i perform the writing operation of one magnetic flux quantum.

(B) is a structure in which magnetic coupling lines 201 to 20i connected in series are provided close to one SQUID element 18.

In (C), the inductance of the SQUID element 18A is i times, and one magnetic coupling line 20 is brought close to the SQUID element 18A.

According to the third embodiment, it is possible to measure the magnetic flux that changes more greatly in a shorter time than in the first and second embodiments.

[0053]

As described above, the SQU according to the present invention
In the ID magnetometer, the magnetic flux supplied to the SQUID element 1 is converted into a pulse width, and the pulse width is measured based on a signal having a frequency higher than the AC bias current supplied to the SQUID element 1. There is an excellent effect that the magnetic flux can be accurately measured even if the input magnetic flux changes greatly in a short time without particularly increasing the frequency of, and it greatly contributes to the expansion of applications of the SQUID magnetometer.

Although various known feedback circuits can be combined, according to the first aspect of the present invention, the SQUID magnetometer can be integrated into one chip including the feedback circuit, and one feedback chip can be used in a large number. A channel SQUID magnetometer can be configured, and
By providing this feedback loop, it is possible to widen the magnetic flux measurement range.

According to the second aspect of the present invention, since the difference between the positive pulse width and the negative pulse width can be calculated by the write gate and the superconducting storage loop, the circuit configuration becomes relatively simple. Produce an effect.

According to the third aspect of the present invention, it is possible to reduce the fluctuation of the magnetic flux supplied to the SQUID element for each cycle of the bias current.

According to the fourth aspect of the present invention, by supplying one pulse to the selected write gate, it is possible to accumulate a plurality of magnetic fluxes of the magnetic flux quantum in the storage superconducting loop. It is possible to measure a magnetic flux that changes further with time.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram of the SQUID magnetometer of the first embodiment of the present invention.

FIG. 3 is a diagram showing a magnetic flux / pulse width conversion operation.

FIG. 4 is a waveform diagram showing a pulse width measurement operation.

FIG. 5 is a diagram showing a magnetic flux quantum write operation.

FIG. 6 is a circuit diagram of a SQUID magnetometer according to a second embodiment of the present invention.

FIG. 7 is a diagram showing a relationship between a magnetic flux passing through the SQUID element and an output pulse width difference t _P −t _N.

FIG. 8 is a circuit diagram of an SQUID magnetometer according to a third embodiment of the present invention.

9 is a configuration diagram of a write gate 16i of FIG.

FIG. 10 is a circuit diagram of a conventional SQUID magnetometer.

11 is a waveform chart showing an operation of the circuit of FIG.

[Explanation of symbols]

10 Pickup coil 12, 18, 18P, 18N SQUID element 14 AC bias current source 16, 16W, 161-16n Write gate 20, 20P, 20N, 26 Magnetic coupling line 22 Superconducting storage loop 24 Feedback loop J1-J4, J3P, J4P, J3N, J4N Josephson junction 28P, 28N Buffer gate 30P, 30N Current source 32 Up-down counter 34 Write control circuit 36 Prediction selection circuit 38 Pulse width memory

Claims (5)

[Claims] 1. A superconducting ring having two or more Josephson junctions (J1, J2) interposed, and an alternating bias current (I).
_B ) is supplied to the SQUID element (1) and an offset current (I) having the same frequency as the AC bias current.
_C ) is supplied to the magnetic coupling line (2a) close to the SQUID element, and outputs a positive pulse and a negative pulse having a width corresponding to the magnetic flux supplied to the SQUID element based on the state change of the SQUID element. the flux / pulse width conversion circuit (2), based on the high frequency signals than said AC bias current (I _R), a pulse width measuring circuit for measuring the width of the pulse (3), that it has a Characteristic SQUID magnetometer. 2. A superconducting storage loop (4) and said SQU.
A feedback loop (5) for giving a negative feedback magnetic flux to the ID element (1), and a write gate (6) for accumulating the magnetic flux in the superconducting accumulation loop in response to a pulse from the pulse width measuring circuit (3). 2. The SQUI according to claim 1, further comprising a feedback circuit connected in a loop, wherein the pulse width measuring circuit outputs a number of pulses corresponding to the pulse width.
D magnetometer. 3. The write gate (6, 16W) is
A positive write gate (18) for storing a positive flux quantum in the superconducting storage loop (4, 22) in response to a positive pulse.
P, 20P) and a negative write gate (18) that stores negative flux quanta in the superconducting storage loop in response to a negative pulse.
N, 20N) are connected in series, and the pulse width measuring circuit (3, 28P, 30P, 28N,
30N) supplies the positive write gate with a number of positive pulses corresponding to the width of the positive pulse, and supplies the negative write gate with a number of negative pulses corresponding to the width of the negative pulse. The SQUID magnetometer according to claim 2. 4. The pulse width measuring circuit (3, 28P, 3
0P, 28N, 30N, 34) outputs a positive or negative pulse depending on the sign of the difference between the width of the positive pulse and the width of the negative pulse to the write gate (6, 1
6. The SQU according to claim 2, wherein the SQU is supplied to
ID magnetometer. 5. The write gate (6, 161-116)
n) has a plurality of write gates connected in series for accumulating a plurality of quantum fluxes different from each other in the storage superconducting loop in response to one pulse, and the pulse width measuring circuit (3, 28P, 30P, 28N,
The SQUID magnetometer according to claim 2, further comprising a selection circuit (36) for selecting the write gate according to the pulse width measured in 30 N) and supplying a pulse to the selected write gate.