Electricity: measuring and testing – Magnetic – Magnetometers
Patent
1995-03-17
1997-02-04
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
Magnetometers
505846, G01R 33035
Patent
active
056002432
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an improved coupling structure for a superconducting quantum interference device (SQUID) incorporating a flux pick-up loop. More particularly, it relates to a nearly planar arrangement of coupling structure, SQUID, and ground plane which is self-shielding.
BACKGROUND
Superconducting quantum interference devices (SQUIDs) are extremely sensitive detectors of magnetic flux. When paired with appropriate feedback and readout electronics, SQUIDs can detect magnetic fields corresponding to fractions of a flux quantum (.PHI..sub.0).
Because of the nature of magnetic fields, SQUIDs can be used in applications where light does not penetrate and sound is distorted. SQUIDs can be used to detect underwater objects such as mines and submarines, or to determine probable locations of oil and mineral deposits. They can detect magnetic signals produced by the body as well, detecting the firing of neurons in the is brain in magnetoencephalography (MEG), or disease in soft tissues in magnetic resonance imaging (MRI).
In certain cases, however, this extreme sensitivity to magnetic fields can be detrimental to a SQUID's performance in an application. For example, a large background field can mask a smaller field that is of interest. In this case the SQUID must be very sensitive to see the small target signal. However, if the large background field penetrates even a small portion of the SQUID's field of view it can swamp the signal.
Typically in such uses of SQUIDs as magnetometers, both the SQUID and the object under study are located within a magnetic shield. This can be a "shielded room" which is available commercially and which is simply a room built to reject any external magnetic or s electromagnetic signals. Another option is to completely enclose the object and sensor in a superconducting enclosure. Since superconductors are perfect diamagnets, no magnetic field can penetrate a superconducting plate or box. (Under certain conditions, magnetic flux can penetrate a superconductor. However, it is easy to predict the magnetic field strength which will be shielded by any superconducting shield, and to design the shield to accomplish this task.) Unfortunately, this shielding is not possible in all situations.
One example of such a situation is the use of SQUIDs for magnetic microscopy and non-destructive testing (NDT), or evaluation (NDE). In these applications, spatial resolution is very important. The change in a magnetic signature over regions a few micrometers in diameter can be important for pathologists looking at a biopsy sample or for aircraft maintenance engineers looking for an incipient crack in a corroded weld. The small field of view and the typically small changes in magnetic field that must be detected require the use of extremely sensitive SQUIDs. At the same time, the small field of view or the fineness of the array precludes the use of external shields to block background magnetic fields from equipment like computers or from the earth itself. The combination of very sensitive detectors and an unshielded environment places stringent requirements on the magnetic sensing system.
DISCUSSION OF THE ART
In order to operate SQUIDs as flux sensors, it is usually necessary to have a means for applying magnetic flux feedback, possibly in addition to a small alternating modulation flux, in order to implement the flux-locked loop. This requires a mutual inductance between a coil (the feedback coil) and the SQUID loop. The mutual inductance is defined as the flux introduced into the loop per unit current in the coil. When the SQUID encounters a magnetic field, the resulting change in flux in the SQUID loop causes the electrical output of the SQUID to change. The feedback electronics counteracts this change by introducing into the feedback coil a current which produces in the SQUID loop a flux equal and opposite to that produced by the applied magnetic field. By monitoring the current required to stabilize the flux in the SQUID, the readout electronics measures the ma
REFERENCES:
patent: 5012190 (1991-04-01), Dossel
patent: 5185527 (1993-02-01), Bluzer
patent: 5287057 (1994-02-01), Gotoh
patent: 5319307 (1994-06-01), Simmonds
Conductus, Inc.
DeFranco, Esq. Judith A.
Strecker Gerard R.
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