Superconductive quantum interference device having two cavities isolated by a superconductive weak link

Claim 2

A superconductive quantum interference device comprising: a body of superconductive material having a plurality of holes extending therethrough, said body further having wall structure defining an aperture extending through said body, said aperture connecting at least a pair of said holes, and a superconductive weak link connected across said aperture.

Claim 3

A superconductive device comprising: a rod of superconductive material having a hole extending longitudinally therethrough, the cross section of said hole having a dumbbell shape in the plane perpendicular to the longitudinal axis of said rod and being disposed symmetrically about said longitudinal axis; and first means for establishing a superconductive weak link across said hole.

Claim 4

A superconductive device comprising: a rod of superconductive material having a hole extending longitudinally therethrough, the cross section of said hole having a dumbbell shape in the plane perpendicular to the longitudinal axis of said rod and being disposed symmetrically about said longitudinal axis; and first means for establishing a superconductive weak link across said hole.

Claim 5

The device of claim 2 wherein said first means is composed of the same superconductive material as said rod.

Claim 6

The device of claim 3 wherein said first means comprises means for establishing a point contact across the center of said hole.

Claim 7

The device of claim 3 wherein said first means comprises first and second screws threaded transversely through the center of said rod and meeting in the narrow portion of said dumbbell shaped hole.

Claim 8

The device of claim 5 wherein the end of said first screw which extends into said rod is pointed, and the end of said second screw which extends into said rod is flat.

Claim 9

The device of claim 6 wherein said first means additionally comprises second and third means for locking said first and second screws respectively into position.

Claim 10

The device of claim 7 wherein said second and third means are nuts.

Claim 11

The device of claim 8 wherein said superconductive material is selected from the group consisting of niobium, vanadium, and tantalum.

Claim 12

The device of claim 2 additionally comprising magnetiC material in one of the circular portions of said dumbbell shaped hole; said magnetic material having a susceptance which varies with temperature.

Claim 13

The device of claim 10 additionally comprising a coil wound around the outer cylindrical surface of said rod.

Claim 14

The device of claim 10 wherein said magnetic material is cerous magnesium nitrate powder.

Claim 15

The other hole of the SQUID was magnetically coupled to external electronic measuring circuitry 128 by coaxial cable 142. The SQUID-flux transformer combination was mounted in glass dewar 138 of about 2 liter capacity, which contained liquid helium 140. Dewar 138 was mounted in glass dewar 134 which contained liquid nitrogen 136. The entire cryogenic apparatus was mounted in cylindrical aluminum shield 132, to prevent interference from radio and radar apparatus. The shield must be placed symmetrically about the cryogenic apparatus to yield optimum results. The mounting apparatus (not shown) comprised non-metallic materials such as micarta sheets and nylon screws. Battery package 126 supplied power to electronic measuring circuitry 128. This circuitry was the same as that shown in FIG. 9, except for the addition of a gain-of-10-differential post detection amplifier (following detector and filter circuit 72 of FIG. 9). Chart recorder 130 was connected in parallel with the oscilloscope contained in circuitry 128. The field measurements indicate sensitivities of the order of 10 10 gauss/cm. Sensitivities of better than 10 11 gauss/cm. have been theoretically predicted for this device. OTHER APPLICATIONS OF THE SYMMETRIC SQUID The symmetric SQUID can be used for sensitive measurement of temperature by inserting into one hole of the device a magnetic material whose susceptance varies with temperature. The device is then cooled to its superconducting state in the presence of a magnetic field. Once in the superconducting state, the flux lines will be trapped in the device and the external field can be removed. Then, as changes in temperature cause changes of susceptance of the magnetic material, flux lines will shift across the point contact of the device. The changes in the interference pattern of the device as a function of temperature, which can be measured by the circuitry heretofore described, will yield a sensitive measure of temperature. In one embodiment of the invention, 6.6 milligrams of cerous magnesium nitrate (CMN) was inserted into one hole of the SQUID. A magnetic field of 300 gauss was applied to the SQUID by means of a coil wound around its outer cylindrical surface. The device was then cooled to 4.2*K. and then the applied field was removed. Interference pattern measurements, as a function of temperature, were made using the circuitry of FIG. 9. The results are shown in FIG. 16. The deviations from the striaght line oF FIG. 16 are accounted for by imprecision in measurements of phase shift and temperature. The inherent resolution of the symmetric SQUID as a temperature sensor has been computed to be of the order of 10 5 degrees. In a similar manner the device can be used to accurately measure susceptance of magnetic materials. It will be apparent to those skilled in the art that the symmetric SQUID can be used as an ultrasensitive magnetometer in other applications, such as the examination of airlines passengers for metal objects or the measurement of the magnetic field associated with the human heart. For example, the device was used in a probe shown in FIG. 17 as part of a magnetocardiograph. The probe was connected to external electronic circuitry (not shown), similar to that shown in FIG. 9, by stainless steel coaxial cable 148. A 10-turn coil of copper wire 154 was positioned in one hole of SQUID 152 and coupled to coaxial cable 148 and tank circuit capacitor 150. Flux transformer 156, which was constructed of 0.25 millimeter niobium wire, was coupled to the other hole of the SQUID. The entire lower end of the probe was surrounded by mylar sleeve 158. The probe was positioned in front of the subject''s chest. FIG. 18 shows the magnetocardiogram obtained from a human subject. It can be seen that the magnetocardiogram pattern compares favorably with the electrocardiogram, of the same subject, shown in FIG.

Claim 16

The other hole of the SQUID was magnetically coupled to external electronic measuring circuitry 128 by coaxial cable 142. The SQUID-flux transformer combination was mounted in glass dewar 138 of about 2 liter capacity, which contained liquid helium 140. Dewar 138 was mounted in glass dewar 134 which contained liquid nitrogen 136. The entire cryogenic apparatus was mounted in cylindrical aluminum shield 132, to prevent interference from radio and radar apparatus. The shield must be placed symmetrically about the cryogenic apparatus to yield optimum results. The mounting apparatus (not shown) comprised non-metallic materials such as micarta sheets and nylon screws. Battery package 126 supplied power to electronic measuring circuitry 128. This circuitry was the same as that shown in FIG. 9, except for the addition of a gain-of-10-differential post detection amplifier (following detector and filter circuit 72 of FIG. 9). Chart recorder 130 was connected in parallel with the oscilloscope contained in circuitry 128. The field measurements indicate sensitivities of the order of 10 10 gauss/cm. Sensitivities of better than 10 11 gauss/cm. have been theoretically predicted for this device. OTHER APPLICATIONS OF THE SYMMETRIC SQUID The symmetric SQUID can be used for sensitive measurement of temperature by inserting into one hole of the device a magnetic material whose susceptance varies with temperature. The device is then cooled to its superconducting state in the presence of a magnetic field. Once in the superconducting state, the flux lines will be trapped in the device and the external field can be removed. Then, as changes in temperature cause changes of susceptance of the magnetic material, flux lines will shift across the point contact of the device. The changes in the interference pattern of the device as a function of temperature, which can be measured by the circuitry heretofore described, will yield a sensitive measure of temperature. In one embodiment of the invention, 6.6 milligrams of cerous magnesium nitrate (CMN) was inserted into one hole of the SQUID. A magnetic field of 300 gauss was applied to the SQUID by means of a coil wound around its outer cylindrical surface. The device was then cooled to 4.2*K. and then the applied field was removed. Interference pattern measurements, as a function of temperature, were made using the circuitry of FIG. 9. The results are shown in FIG. 16. The deviations from the striaght line oF FIG. 16 are accounted for by imprecision in measurements of phase shift and temperature. The inherent resolution of the symmetric SQUID as a temperature sensor has been computed to be of the order of 10 5 degrees. In a similar manner the device can be used to accurately measure susceptance of magnetic materials. It will be apparent to those skilled in the art that the symmetric SQUID can be used as an ultrasensitive magnetometer in other applications, such as the examination of airlines passengers for metal objects or the measurement of the magnetic field associated with the human heart. For example, the device was used in a probe shown in FIG. 17 as part of a magnetocardiograph. The probe was connected to external electronic circuitry (not shown), similar to that shown in FIG. 9, by stainless steel coaxial cable 148. A 10-turn coil of copper wire 154 was positioned in one hole of SQUID 152 and coupled to coaxial cable 148 and tank circuit capacitor 150. Flux transformer 156, which was constructed of 0.25 millimeter niobium wire, was coupled to the other hole of the SQUID. The entire lower end of the probe was surrounded by mylar sleeve 158. The probe was positioned in front of the subject''s chest. FIG. 18 shows the magnetocardiogram obtained from a human subject. It can be seen that the magnetocardiogram pattern compares favorably with the electrocardiogram, of the same subject, shown in FIG.

Claim 17

The symmetric SQUID 144 was magnetically coupled from one hole to flux transformer 146, which is of the type shown in FIG.

Claim 18

The symmetric SQUID 144 was magnetically coupled from one hole to flux transformer 146, which is of the type shown in FIG.

Claim 19

One advantage of the magnetocardiograph is that terminals need not be connected to the subject. The symmetric SQUID can also be used as a detector of R.F. energy. In this application the device can be used to terminate a section of waveguide. When R.F. is applied to the waveguide its presence is detected as a change in impedance of the tank circuit used with the device. Alternatively, the R.F. signal to be detected can be applied to one end of the device while a local oscillator signal of slightly different frequency is applied to the other end of the device. The tank circuit can then be tuned to the difference between these two frequencies, to provide an I.F. pick off. I claim:

Claim 20

The loose coupling provides a high impedance connection to the tank circuit which assures that the bias current is supplied from a constant current source (i.e. so that the bias level will not be a function of tank circuit impedance). Audio oscillator 74 is connected by switch 76 to either oscillator 62 (position a) or choke 64 (position b). When the V-I characteristic (FIG. 4) is to be measured, switch 76 is thrown to position a, and the audio signal is used to amplitude modulate the output of oscillator 62. This, in effect, varies the bias current supplied to the SQUID. Audio oscillator 74 is also connected across the X (or horizontal) axis of XY scope 78. Thus when switch 76 is connected in position a, the variation in the amplitude of the SQUID bias current is displayed on the X-axis on the face of scope 78 and the variation in voltage across the tank circuit is displayed across the Y-axis (as shown in FIG. 4). When the field characteristic (FIG. 5) is to be measured, switch 76 is thrown to position b, and the audio signal, which is applied via choke 64 and coaxial cable 86 to coil 82, is used to vary the field strength in the SQUID. In this manner, the variation in field strength is displayed on the X-axis on the face of scope 78 and the variation in voltage across the tank circuit is displayed across the Y-axis (as shown in FIG. 5). USE OF THE SYMMETRIC SQUID AS A MAGNETOMETER AND GRADIOMETER By replacing the X-Y scope, in the circuit of FIG. 9, by a phase-locked detector and feeding the D.C. output of the phase-locked detector back to the SQUID, the system can be made to function as a lock-on magnetometer. The magnetometer circuit is shown in FIG. 10. Components similar to those of the FIG. 9 circuit are similarly numbered. A suitable phase locked detector 88 is a Princeton Applied Research Model HR-8, having the functional characteristics shown in Table 2. TABLE 2 SENS. 50 volts FREQ. 95 Hz PHASE about 0 SIG Q about 10 MODE INT. TIME CONST. 3 SEC REF ATTN. about 2 METER MONITOR - OUT ZERO OFFSET - 0 The phase locked detector output is coupled back to the tank circuit coil 82 of the SQUID by coupling resistors 90 and 92, choke 64, and coaxial cable 86. This output is also applied to chart recorder 98 by the low pass filtering network comprising resistor 94 and capacitor 96. In operation, changes in external field which are sensed by the flux-transformer will cause the phase-locked detector to produce a D.C. output signal which is fed back to coil 82 to produce a field in the SQUID which tends to oppose the external field until the output of the phase detector is nulled. The change in field strength needed to oppose the external field is recorded on chart recorder 98 as the magnetometer output. The external field is applied to the symmetric SQUID 80a by inserting in the other hole of the SQUID a coil 100 which is coupled to flux transformer 102. The flux transformer-SQUID interconnection is represented schematically in FIG. 11. The flux transformer is a closed loop comprising a superconductive loop, represented schematically by coil 104, connected to coil 106. Coil 106 is inserted into one of the holes of symmetrical squid 108 thereby magnetically coupling the flux transformer in the SQUID by mutual inductance M. A gradient-sensitive flux transformer is shown schematically in FIG. 12. The external flux sensing portion of the gradient sensitive flux transformer is represented by two coils 110 and 112, of equal inductance, connected in series opposition. Thus, there is no response to a uniform applied field, but a current proportional to the gradient of the field will be induced. The gradient-sensitive flux transformer can be repreSented spatially as shown in FIG. 13, where the two external superconductive loops, 114 and 116, are aligned coaxially. The flux transformer responds to delta Bz/ delta Z, where z represents distance along the line through the center of the two loops, and Bz represents the flux density of the external field as a function of z. The sensitivity of this device varies inversely with the distance between the sensing loops, l. The gradient-sensitive flux transformer used in the preferred embodiment of the invention is shown in FIGS. 14 and 14A. The flux sensing superconductive loops, 120 and 124, are each formed by a single turn of thin niobium sheet wrapped on quartz tube

Claim 21

The loose coupling provides a high impedance connection to the tank circuit which assures that the bias current is supplied from a constant current source (i.e. so that the bias level will not be a function of tank circuit impedance). Audio oscillator 74 is connected by switch 76 to either oscillator 62 (position a) or choke 64 (position b). When the V-I characteristic (FIG. 4) is to be measured, switch 76 is thrown to position a, and the audio signal is used to amplitude modulate the output of oscillator 62. This, in effect, varies the bias current supplied to the SQUID. Audio oscillator 74 is also connected across the X (or horizontal) axis of XY scope 78. Thus when switch 76 is connected in position a, the variation in the amplitude of the SQUID bias current is displayed on the X-axis on the face of scope 78 and the variation in voltage across the tank circuit is displayed across the Y-axis (as shown in FIG. 4). When the field characteristic (FIG. 5) is to be measured, switch 76 is thrown to position b, and the audio signal, which is applied via choke 64 and coaxial cable 86 to coil 82, is used to vary the field strength in the SQUID. In this manner, the variation in field strength is displayed on the X-axis on the face of scope 78 and the variation in voltage across the tank circuit is displayed across the Y-axis (as shown in FIG. 5). USE OF THE SYMMETRIC SQUID AS A MAGNETOMETER AND GRADIOMETER By replacing the X-Y scope, in the circuit of FIG. 9, by a phase-locked detector and feeding the D.C. output of the phase-locked detector back to the SQUID, the system can be made to function as a lock-on magnetometer. The magnetometer circuit is shown in FIG. 10. Components similar to those of the FIG. 9 circuit are similarly numbered. A suitable phase locked detector 88 is a Princeton Applied Research Model HR-8, having the functional characteristics shown in Table 2. TABLE 2 SENS. 50 volts FREQ. 95 Hz PHASE about 0 SIG Q about 10 MODE INT. TIME CONST. 3 SEC REF ATTN. about 2 METER MONITOR - OUT ZERO OFFSET - 0 The phase locked detector output is coupled back to the tank circuit coil 82 of the SQUID by coupling resistors 90 and 92, choke 64, and coaxial cable 86. This output is also applied to chart recorder 98 by the low pass filtering network comprising resistor 94 and capacitor 96. In operation, changes in external field which are sensed by the flux-transformer will cause the phase-locked detector to produce a D.C. output signal which is fed back to coil 82 to produce a field in the SQUID which tends to oppose the external field until the output of the phase detector is nulled. The change in field strength needed to oppose the external field is recorded on chart recorder 98 as the magnetometer output. The external field is applied to the symmetric SQUID 80a by inserting in the other hole of the SQUID a coil 100 which is coupled to flux transformer 102. The flux transformer-SQUID interconnection is represented schematically in FIG. 11. The flux transformer is a closed loop comprising a superconductive loop, represented schematically by coil 104, connected to coil 106. Coil 106 is inserted into one of the holes of symmetrical squid 108 thereby magnetically coupling the flux transformer in the SQUID by mutual inductance M. A gradient-sensitive flux transformer is shown schematically in FIG. 12. The external flux sensing portion of the gradient sensitive flux transformer is represented by two coils 110 and 112, of equal inductance, connected in series opposition. Thus, there is no response to a uniform applied field, but a current proportional to the gradient of the field will be induced. The gradient-sensitive flux transformer can be repreSented spatially as shown in FIG. 13, where the two external superconductive loops, 114 and 116, are aligned coaxially. The flux transformer responds to delta Bz/ delta Z, where z represents distance along the line through the center of the two loops, and Bz represents the flux density of the external field as a function of z. The sensitivity of this device varies inversely with the distance between the sensing loops, l. The gradient-sensitive flux transformer used in the preferred embodiment of the invention is shown in FIGS. 14 and 14A. The flux sensing superconductive loops, 120 and 124, are each formed by a single turn of thin niobium sheet wrapped on quartz tube

Claim 22

Connection to the SQUID is made by twisted niobium leads

Claim 23

Connection to the SQUID is made by twisted niobium leads

Claim 24

These leads are connected to a flux sensing coil (not shown) which is inserted into one hole of the symmetric SQUID. The flux sensing coil comprises five turns of No. 36 (0.25 mm) niobium wire. The dimensions of the flux transformer of FIG. 14 are shown in Table 3. TABLE 3 DEVICE DIMENSION SIZE millimeters f 50 g 50 h 50 i 50 Field measurements were made with the apparatus shown in FIG.

Claim 25

These leads are connected to a flux sensing coil (not shown) which is inserted into one hole of the symmetric SQUID. The flux sensing coil comprises five turns of No. 36 (0.25 mm) niobium wire. The dimensions of the flux transformer of FIG. 14 are shown in Table 3. TABLE 3 DEVICE DIMENSION SIZE millimeters f 50 g 50 h 50 i 50 Field measurements were made with the apparatus shown in FIG.