Electricity: measuring and testing – Magnetic – Magnetometers
Utility Patent
1998-08-13
2001-01-02
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C505S846000
Utility Patent
active
06169397
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to planar geometry, multiturn superconducting coils used with a ground plane, and more particularly to apparatus and methods for reducing resonances in such coils.
2. Description of the Prior Art
One of the basic circuit elements of superconducting electronic devices is the inductor. In order to obtain useful values of inductance, multiple windings, or turns, of the inductor coil are often required. Moreover, a basic method of fabricating superconducting electronic devices is thin film deposition and patterning, resulting in the widespread use of planar geometry spiral inductors in practical circuit design. Such nominally spiral coils may be of any symmetry (square, circular, octagonal, etc.). In those situations where planar spiral inductor coils are implemented in conjunction with a ground plane (particularly a superconducting ground plane), stray capacitance between the coil and the ground plane results in an inductive/capacitive resonant circuit with very low damping (“high Q”). Resonance induced changes in the impedance of either the coil or the ground plane at the resonance frequency or frequencies often unfavorably influence the operation of devices incorporating either of the two elements, and therefore damping of these resonances is desirable. The most common (though not exclusive) example of this situation is the input coil to a superconducting quantum interference device (SQUID).
Current technology for SQUID fabrication uses a planar fabrication process to create a washer geometry ground plane whose purpose is to focus magnetic flux from an input inductor or coil to the SQUID body; the ground plane often in fact forms the SQUID body. This geometry was developed by Jaycox and Ketchen (see, for example, “Planar coupling scheme for ultra low noise dc SQUIDS,” J. M. Jaycox and M. B. Ketchen, IEEE Trans. Magn., vol. MAG-17, pp. 400-403, January 1981). This geometry results in an inductive-capacitive resonant circuit as discussed above. The resulting resonances distort the output characteristics of the SQUID and introduce electronic noise. Both of these consequences degrade SQUID performance.
FIGS. 1 and 2
(prior art) show a conventional planar SQUID
100
, including a multiturn input coil
104
which couples external signals to the SQUID via SQUID washer
102
.
FIG. 1
is a simplified top view of the device, while
FIG. 2
is a schematic. A conventional dc SQUID
100
is formed with a loop of superconducting material (washer
102
) interrupted by two Josephson tunnel junctions
106
. Josephson junctions
106
are shunted with resistors
112
to remove hysteresis as necessary. In operation, SQUID
100
is biased with a constant current, I
b
130
. When a current, i
f
126
passes through input coil
104
, it causes a magnetic field which modifies the current flow in washer
102
, resulting in a change in the voltage across Josephson junctions
106
and the SQUID as a whole. Thus, the measured voltage (V)
124
across the SQUID is related in a predictable way to the current flowing in coil
104
, and can be used to determine the current flowing in coil
104
. This voltage can be measured by external circuitry.
However, high frequency currents which develop within Josephson junctions
106
cause resonances to develop in coil
104
, which cause voltage
124
to lock onto certain values, causing the relation between the value of current
126
introduced into coil
104
to become nonlinear. As a result of the nonlinearity, the SQUID is not as useful as it could be.
Techniques in the prior art which have been used to reduce the effects of resonances have met with limited success. Returning to
FIG. 1
, these include an external coil shunt
108
, a washer shunt
110
, overdamped junction shunts
112
, and/or coil/washer shunt
114
. In the cases of external coil shunt
108
, washer shunt
110
, and coil/washer shunt
114
, both resistive and resistive/capacitive networks have served as the shunting element.
All of the previous methods of damping resonances in planar geometry superconducting coils have attempted to damp the resonance of the coil as a whole. A need remains in the art for improved apparatus and methods for damping resonances in planar geometry superconducting coils.
SUMMARY
It is an object of the present invention to provide improved apparatus and methods for damping resonances in planar geometry superconducting coils. In order to meet this object, an internal damping resistor is applied across the windings of the coil. Thus resistive damping is added to each turn of the coil.
A damped superconductor coil according to the present invention comprises a planar geometry multiturn superconducting coil and an intracoil shunt connecting a plurality of turns of the coil with resistors.
An electrical ground plane is disposed parallel and proximate to the coil. Generally, the electrical ground plane consists of a superconductive material and forms at least one hole, which concentrates magnetic field lines from the coil to the hole. The ground plane may also form a gap extending from the hole to the edge of the ground plane to admit changing magnetic flux.
The coil may comprise a signal coil or a modulation coil of a superconducting quantum interference device (SQUID),an inductor in a filter, or a winding in a transformer.
The shunt may comprise a planar-film resistor which extends along a radius of the coil, or along more than one radius of the coil.
REFERENCES:
patent: 5319307 (1994-06-01), Simmonds
patent: 5656937 (1997-08-01), Cantor
Keene, Mark, Nicholas Exon, Julian Satchell, Richard Humphreys, Nigel Chew and Karan Lander, “HTS SQUID Magnetometers with Intermediate Flux Transformers,” IEEE Transactions on Applied Superconductivity, 1996.
Cantor, Robin, “DC SQUIDS: Design, Optimization and Practical Applications,”SQUID Sensors: Fundamentals, Fabrication and Applications,H. Weinstock (ed.), Netherlands: Kluwer Academic Publishers, 1996, pp. 179-233.
Ryhanen, Tapani, Heikki Seppa, Risto Ilmoniemi, and Jukka Knuutila, “Squid Magnetometers for Low-Frequency Applications,” Journal of Low Temperature Physics, vol.76, Nos. 5/6, 1989, 287-386.
Enpuku, K., R. Cantor and H. Koch, “Resonant Properties of a DC SQUID Coupled to a Multiturn Input Coil,” IEEE Transactions on Applied Superconductivity, vol. 3, No.1, 1993, 1858-1861.
Enpuku, K., K. Yoshida and S. Kohjiro, “Noise Characteristics of a DC SQUID with a Resistively Shunted Inductance,” J. Appl. Phys., vol. 60, No. 12, 1986, 4218-4223.
Sauvageau, J.E., C.J. Burroughs, P.A.A. Booi, M.W. Cromar, S.P. Benz and J.A. Koch, “Superconducting Integrated Circuit Fabrication with Low Temperature ECR-Based PECVD SiO2Dialectic Films,” IEEE Transactions on Applied Superconductivity, vol. 5, No. 2, 1995, 2303-2309.
Ketchen, M.B., “DC SQUIDS 1980: The State of the Art,” IEEE Transactions on Magnetics, vol. 17, No. 1, 1981, 387-394.
Jaycox, J.M., and M.B. Ketchen, “Planar Coupling Scheme for Ultra Low Noise DC SQUIDs,” IEEE Transactions on Magnetics, vol. 17, No. 1, 1981, 400-403.
Huber Martin Emile
Ono Ronald Hajime
Steinbach Andrew Harvey
Bales Jennifer L.
Macheledt Bales & Johnson LLP
Patidar Jay
University Technology Corp.
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