1. Field of the Invention
The present invention relates to apparatus for the measurement of magnetic fields, and more specifically, to a gradiometer based on a superconducting magnetometer which may be used to measure the magnitude of a weak magnetic field from a nearby source in the presence of a stronger background field.
2. Description of Related Art
Magnetometers are devices which are used to measure the change in magnitude of a magnetic field. Sensitive magnetometers have been constructed from superconducting quantum interference devices (SQUIDs). A SQUID is a magnetic flux detector which transduces the flux into a voltage as an output signal. A SQUID can be used for a range of applications in which physical phenomena are converted to magnetic flux, e.g., current, voltage, magnetic field, and magnetic field gradient can all be measured or inferred via magnetic flux measurements.
A dc SQUID may be formed from two "weak links", that is a material in which a local breakdown of the superconducting property is not accompanied by a global loss of the superconducting properties. In such a configuration, the SQUID is constructed by connecting two weak link devices in parallel in a closed loop of superconducting material and biasing them with a static current. A commonly used weak link device from which a SQUID may be formed is a Josephson tunnel junction. A SQUID sensor (sometimes referred to as a SQUID) may be formed from one or more Josephson junctions arranged in a superconducting loop. A signal can be inductively coupled through an input coil or directly injected into the SQUID.
A SQUID based magnetometer is formed from a SQUID sensor and some form of pickup device for coupling a signal to the SQUID sensor. An applied magnetic field creates a current in the pickup coil which is inductively coupled to the SQUID via an input coil. A magnetometer may also be formed by directly connecting a pickup coil to a SQUID sensor. This is termed a "directly coupled magnetometer". A SQUID based magnetometer has several advantages over other commonly used magnetic sensors, including extremely high sensitivity, broad bandwidth, and small size. A gradiometer is a device which measures the gradient of a magnetic field, i.e., the variation in magnetic field strength (.delta.B) across a known distance. It may also be used to make measurements of a source magnetic field in the presence of a background field in situations in which the source field changes sufficiently over the dimensions of the gradiometer.
An important characteristic of a superconducting material is its critical or transition temperature, T.sub.c. This is the temperature below which the material exhibits its superconducting behavior. Thus, if the material is kept at a temperature above the critical temperature, the superconducting properties will disappear. Superconducting materials may broadly be classified as "low T.sub.c " and "high T.sub.c " materials. Low T.sub.c materials are those which must be cooled below the boiling point of liquid nitrogen, typically by liquid helium (4.2 degrees Kelvin), to exhibit superconductivity; high T.sub.c materials can be cooled by liquid nitrogen (77 degrees Kelvin) and still exhibit superconductivity. This is a significant difference because liquid nitrogen is less expensive, easier to handle and store, and evaporates much more slowly than liquid helium.
A SQUID may be used with a flux transformer in order to increase a SQUID based magnetometer's sensitivity. A flux transformer is a closed loop of superconductor consisting of a large area pick-up loop and an input coil. The input coil is inductively coupled to the SQUID via a mutual inductance. This forms a magnetometer of the type described above, in which the pickup loop and SQUID sensor input coil are directly electrically connected to form a flux transformer. Placement of the pickup loop in a magnetic field generates a current of magnitude .PHI./L (where .PHI. is the magnetic flux through the pickup loop and L is the total inductance of the flux transformer) in the input coil. This current couples a magnetic flux into the SQUID, with the magnitude of the flux being equal to the product of the magnitude of the current generated in the flux transformer and the mutual inductance between the input coil and SQUID. In order to achieve a high coupling efficiency between the flux transformer and the SQUID, the input coil of the flux transformer (i.e., the SQUID sensor input coil) may be fabricated as a planar spiral in a film arranged directly over the SQUID in what is termed a "square washer" configuration.
Excess noise has been a serious limitation to the sensitivity of SQUID-based devices. This noise typically arises from two sources. First, there is ambient noise arising from a variety of background sources, including fluctuations in the Earth's magnetic field, power lines, electrical appliances, etc. Second, there is a noise source arising from magnetic flux vortices which penetrate a superconducting film as it is cooled in a static ambient magnetic field (in particular, the Earth's magnetic field). The present invention is primarily concerned with the measurement problems arising when attempting to measure a relatively weak magnetic field signal from a source in the presence of stronger, fluctuating background magnetic fields. An invention directed to reducing the measurement noise associated with the penetration of a superconducting film by flux vortices is described and claimed in U.S. patent application Ser. No. 09/032,171, now U.S. Pat. No. 6,023,161, entitled "Low-Noise SQUID", filed Feb. 27, 1998, assigned to the assignee of the present invention, and the contents of which are incorporated herein by reference.
In many applications of SQUIDS, particularly biomagnetic measurements such as magnetoencephalography and magnetocardiology, it is necessary to measure relatively weak magnetic signals from a nearby source in the presence of a much higher background field. One approach to solving this problem is to use a magnetically-shielded enclosure with a magnetometer, a spatial gradiometer (assuming the source field falls off rapidly enough over the dimensions of the gradiometer), or a combination of both.
In the case of low-critical temperature (T.sub.c) SQUIDs, an axial gradiometer formed from a SQUID sensor and two pickup loops has been developed. The two loops are wound in opposite directions from niobium wire and connected by a common line to the SQUID sensor input coil which inductively couples a flux into the SQUID. A uniform magnetic field across the two pickup loops provides the same flux through each of the loops, thereby generating oppositely directed currents of the same magnitude in the two loops. Thus, in the presence of a time varying, uniform ambient field, the loops generate no net current in the input coil, and no net magnetic flux signal is inductively coupled into the SQUID. The result is that the effect of a uniform background noise source is effectively removed from the measurement. On the other hand, if the magnetic fields through the two pick up loops differ, the magnetic field gradient produces a signal proportional to the magnitude of the field gradient. This permits measurement of the first order derivatives, e.g., .differential.B.sub.z /.differential.Z, of the magnetic field (where B.sub.z is the component of the magnetic field in the z direction). The addition of more loops enables the measurement of higher order derivatives of the magnetic field, e.g., .differential.B.sub.z /.differential.z.sup.2. The distance between the pickup loops (i.e., the baseline for such a device) is typically 50-100 mm. The disadvantages of such devices include the relatively poor "balance" (the ability of the device to cancel out a uniform background field), and the fact that at present time, such designs are limited to operation with low T.sub.c SQUIDs. This approach is currently impracticable for high-T.sub.c devices because of the difficulties of winding coils from high-T.sub.c wire and making superconducting connections between such wires.
Planar, monolithic thin film gradiometers measuring an off-diagonal gradient, such as .differential.B.sub.z /.differential.x, have also been developed. Such devices involve the fabrication of a SQUID and two pickup loops on the same substrate. The pickup loops are of the same dimensions to provide for a substantially equal flux through the two loops (and hence the generation of substantially equal, but opposing currents) and are connected by a common line at their center to the SQUID. In such a design, a uniform magnetic field applied over both pickup loops produces currents of the same magnitude but opposite sense in the two pickup loops. This results in no net current being coupled to the SQUID. Although this design generally has better balance than the niobium wire-wound design, the balance is difficult to adjust after fabrication of the device. In addition, the baseline of the device is limited by presently available fabrication technology, which prevents fabrication of the SQUIDs on a larger substrate.
An alternative approach to using a single magnetometer as the basis for a gradiometer is to subtract the signals electronically from two or more magnetometers to eliminate a uniform, time varying background field and form diagonal or off-diagonal derivatives of the magnetic field. This design permits the use of an arbitrary baseline since the baseline is not limited by the need to fabricate the entire gradiometer on a single substrate. However, the disadvantages of such a design include the relatively higher cost (owing to the use of multiple magnetometers and hence a greater number of SQUIDs), and the higher dynamic range, linearity and slew-rate required.
Even though several types of low T.sub.c SQUID based gradiometers have been constructed, a high T.sub.c device having a longer baseline is desirable. As noted, this is in part because a high critical temperature device requires a relatively inexpensive and more convenient cooling method compared to a device based on a low critical temperature superconductor.
In terms of high-T.sub.c SQUID instruments, gradiometers are presently made in two general ways. In the first method, previously referred to as a planar monolithic gradiometer, a thin film flux transformer with two or more pickup loops is coupled to a SQUID, or, alternatively, the SQUID loop is fabricated in the form of a gradiometer. As noted, in the case of a two pickup loop flux transformer electrically coupled to a SQUID, a uniform ambient magnetic field produces opposite direction currents in the pickup loops. The currents cancel each other out so that no net current is present at the SQUID. However, while this overcomes the critical temperature related disadvantages of low T.sub.c devices, the high T.sub.c devices have generally been limited to baselines of 10 mm or less, a value too small to be useful in most medical applications. As noted, this is because the SQUID and pickup loops are fabricated on a common substrate.
The second technique for fabricating high-T.sub.c SQUID based gradiometers involves the formation of an electronic gradiometer of the type previously described for low T.sub.c devices. In such a device, the outputs of two or more separated SQUID magnetometers are subtracted to form a spatial magnetic field gradient. However, as noted with respect to the low T.sub.c devices, this technique is expensive due to the number of SQUIDs and readout electronics required. The technique is also constrained by the linearity and common-mode rejection ratio of the subtraction electronics and requires high slew rate and dynamic range.
A significant disadvantage of the smaller baseline devices (e.g., planar, monolithic gradiometers) is that the relatively small baseline causes the device to detect the differentiated source signal unless the source signal decreases sufficiently rapidly over the distance of the baseline. This makes such designs inappropriate for some intended applications. In addition, in the case of the planar, monolithic devices, the magnetometer sensitivity is reduced because the applied flux is divided between two pickup loops. This reduces the flux which is sensed by the SQUID in the form of the induced currents. A further, general problem with the described devices is that either the balance is poor or is difficult to adjust after fabrication.
Thus, planar gradiometers, albeit with shorter baselines than desired for biomagnetic measurements, have been fabricated, as have gradiometers which operate by electronic subtraction. However, there has not yet been a demonstration of a high-T.sub.c gradiometer incorporating a flux transformer and having a baseline that is sufficiently long for biomagnetic measurements.
What is desired is a gradiometer having a sufficiently long baseline for use in making biomagnetic measurements and which is based on a high critical temperature superconducting SQUID.