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Physicists set 'speed limit' for future
superconducting magnet
Evanston, IL, 11 February 2007: A research
team led by a Northwestern
University physicist has identified a high-temperature superconductor --
Bi-2212, a compound containing bismuth -- as a material that might be suitable
for the new wires needed to one day build the most powerful superconducting
magnet in the world, a 30 Tesla magnet.
The material currently used in magnetic resonance
(MR) imaging machines in both hospitals and research laboratories -- a
low-temperature superconducting alloy of the metallic element niobium -- has
been pushed almost as far as it can go, to around 21 Tesla. (Tesla is used to
define the intensity of the magnetic field.) There are no superconducting magnet
wires currently available that can generate 30 Tesla.
"A new materials technology -- such as a
technology based on high-temperature superconductivity -- is required to make
the huge leap from 21 Tesla to 30 Tesla," said William P. Halperin, John Evans
Professor of Physics and Astronomy in the Weinberg College of Arts and Sciences
at Northwestern, who led the team. "We have shown that Bi-2212 could be operated
at the same temperature as is presently the case for magnets made with niobium
-- 4 degrees Kelvin -- and also achieve the stable state necessary for a 30
Tesla magnet."
The findings will be published online Feb. 11 by
the journal Nature Physics.
"We are exploring nature's limitations, and our
discovery has basic implications for the study of superconductors and for
applications to magnetic resonance imaging," said Halperin. "The dream would be
to have powerful magnets that don't require helium for cooling. Some day new
materials might be discovered where this restriction is lifted, but it isn't
possible at the present time."
A superconductor, when cooled to its appropriate
temperature, conducts electricity without any resistance. Superconductivity
first appears in Bi-2212 at a high temperature of 90 degrees Kelvin, but
Halperin and his colleagues found that the stable state required in
high-magnetic fields can be established only when the temperature falls below 12
degrees Kelvin. The team is the first to establish this limit for Bi-2212.
"Sometimes what seems to be bad can be good,"
said Bo Chen, lead author of the paper and a graduate student of Halperin's.
"Our findings set a speed limit. If you go beyond this speed you may have
trouble. Knowing the upper temperature limit is a kind of security."
"To create a 30 Tesla magnet, we need a
superconducting material that can carry the required amount of electricity
without blowing up," said Halperin. "We have found that the operating
temperature for Bi-2212 must be below 12 degrees Kelvin. The good news is that
this temperature can be reached by cooling the magnet with liquid helium. If we
had found the upper limit to be 2 degrees Kelvin then the cryogenic requirements
would be intractable."
MR imaging is widely used by hospitals for
medical diagnosis, and scientists at universities, national laboratories and
pharmaceutical companies use even more powerful MR technology to study DNA,
proteins and other complex molecules. About a dozen labs around the country take
advantage of the highest magnetic field now in use -- 21.1 Tesla, which produces
a magnetic field 10 times larger than your average hospital machine. Increasing
the field of the magnet even a small amount, from 21.1 to 22.2 Tesla, would
increase the cost of the machine by two million dollars.
"A holy grail of the scientific community, as set
out recently by the National Research Council, is to build a superconducting
magnet of 30 Tesla," said Halperin. "In MR imaging, the higher the magnetic
field, the higher the resolution, which provides scientists with more detail for
analysis. A 30 Tesla magnet could drive significant advances in chemistry,
biology and medicine."
Using MR techniques at the National High Magnetic
Field Laboratory in Tallahassee, Fla., Halperin and his team studied Bi-2212,
one of the "darlings" of superconductivity. To measure its properties, they put
the rare isotope oxygen-17 into a crystal of Bi-2212, with the isotope acting as
a probe, much like a fluorescent dye. They then determined the phase diagram of
the material where superconductivity is stable, which showed high temperature
and high magnetic field could not be achieved together.
"Now that we have this information about Bi-2212,
the next question is, 'Can such a magnet actually be made?'" said Halperin. "I
really don't know -- it depends on engineering and processing the materials to
make them into wires. My fellow scientists and engineers will have to solve the
materials problems, and they don't like to accept no as an answer."
In addition to Halperin and Chen, the research
team includes Vesna F. Mitrovic, of Brown University, and Arneil P. Reyes and
Philip L. Kuhns, of the National High Magnetic Field Laboratory, as well as
crystal growers Prasenjit Guptasarma, of the University of Wisconsin-Milwaukee,
and David G. Hinks, of Argonne National Laboratory.
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