LBL Scientists have developed a new generation of superconducting electronic devices able to measure extremely faint magnetic fields. Practical and affordable, these "magnetometers" make possible a broad range of new medical instruments and industrial tools.
Superconducting magnetometers have existed for decades but until the recent advent of high transition temperature (high Tc) superconductors that can operate in liquid nitrogen, they relied on low Tc helium-cooled superconductors. These sensors have a rich variety of applications but their actual use has been restricted because of the cumbersome and costly nature of helium cooling.
Explained LBL Materials Sciences Division physicist John Clarke, "The central problem in creating usable high Tc superconducting magnetometers has been that these materials produce copious quantities of noise. Through multiple breakthroughs in material science, we have managed to beat the noise power down to one part in ten-million of the level five years ago."
The magnetometers were developed by a team led by Clarke that includes researchers at LBL, the University of California at Berkeley, and Conductus Inc., a private firm based in Sunnyvale, California. The collaboration includes Berkeley postdocs Dieter Koelle and Frank Ludwig, Berkeley graduate students Gene Dantsker, Andy Miklich, and David Nemeth, and Conductus scientists Kookrin Char and Ward Ruby.
Clarke, a professor in the UC Berkeley Department of Physics, said fetal cardiology is likely to be the first commercial usage. This was presaged in 1991 when the collaborators developed the first magnetocardiogram with a high Tc thin film magnetometer. The device detects faint magnetic signals produced by the beating of the human heart, creating a waveform which a doctor can examine much like an electrocardiogram.
Said Clarke, "Measuring the cardiogram on an unborn child's heart can provide invaluable imformation, but after about 22 weeks, it is generally impossible with existing electrical techniques. With the improvements in magnetic field sensitivity we've made, we can now do this with a high Tc magnetometer. This was long possible with low Tc instruments but not practical. But you can now imagine having these instruments in a doctor's office."
People with heart arrhythmias also should benefit. In Germany and Nova Scotia, for example, studies are underway using arrays of helium-cooled magnetometers to image spurious electrical pathways in the heart magnetically. These arrays paint a picture showing the location within the heart where the electrical short-circuiting responsible for the arrhythmia is occurring. This procedure helps doctors zero in and selectively treat the defective area using catheter ablation.
While extremeley promising, this approach is handicapped by the use of helium-cooled materials. The high performance, high Tc magnetometers should help transform this experimental medical program into a more affordable and widely-available procedure.
Geophysics and the search for underground natural resources also should be a major beneficiary. Collaborating with Conductus, Clarke's team recently field-tested a sensor that provides a three-dimensional measurement of time-varying magnetic fields. The device makes possible a family of instruments useful in oil exploration, geological surveying, and environmental investigative work.
Said Clarke, "One interesting geophysical application involves instruments that are lowered down boreholes. These holes can go down 10 kilometers. For instance, companies do electromagnetic sounding looking for oil deposits using coils that may be a foot or more in length, too large to be turned sideways inside the borehole. Consequently, what they can see is limited to a one-dimensional image. Using pickup loops one-half an inch across (which is the size with our magnetometers) would provide a three-dimensional picture. This would be the first cryogenic borehole instrument."
Magnetometers actually consist of two coupled devices, a superconducting quantum interference device (SQUID) and a flux transformer. The flux transformer serves as an amplifier, picking up weak magnetic signals over a large area and inducing a proportional magnetic flux in the tiny hole of the SQUID. The SQUID, which consists of a loop of superconducting film interrupted by two weak links or "Josephson junctions," responds to the magnetic field by producing a corresponding voltage signal.
Clarke said the key to increasing the sensitivity of the devices was reducing the noise that had been inherent in the superconducting materials.
"The noise was particularly bad at low frequencies," observed Clarke, "say one hertz, or once a second, which is about the frequency of a human heart beat. We have had to learn how to deposit yttrium-barium-copper oxide superconducting films so that these have minimal defects and are precisely oriented."
The teams at both LBL and Conductus use a pulsed excimer laser deposition process. The film from which the flux transformer was made was deposited at Conductus using a technique in which the substrate is rotated during the process.
Another factor involved in the increased sensitivity is the use of single as versus multiple layer films. Single layer devices do not suffer from the degree of noise that still limits magnetometers with multilayer flux transformers.
In work reported in the October 18 and December 27, 1993 editions of Applied Physics Letters, the team describes a magnetometer with a single-layer flux transformer with the lowest magnetic field noise yet achieved by a high Tc instrument. The work was funded by the U.S. Department of Energy and the California Competitive Technology Program, and Conductus.
While sufficient sensitivity has been achieved for most applications, further improvements are necessary before medical diagnostic work can be done on the brain. Sensitivity must be increased and size reduced by three times. Because of the need for miniaturization, multilayer devices will be required, motivating further work on noise reduction.
Said Clarke, "The U.S. has dominated the science of thin film superconductors. Japan, on the other hand, has a government funded institute -- at about $50 million over six years -- for developing medical applications for the market, and they are moving quickly. So far, there is no U.S. federal investment of this magnitude. The U.S. has the scientific lead but it remains to be seen who is going to cash in on the medical and industrial applications."