Portland, Ore. An enormous magnet, boasting a rock-steady field and a bore twice the size found elsewhere, is expected to draw the global scientific community to its Florida home.
Opened last month in Tallahassee, the magnetic system offers a 105-millimeter bore and a record birth "weight," or magnetic field. Standing 16 feet tall and weighing more than 15 tons, the National High Magnetic Field Laboratory magnet is expected to live at least 10 years, and could survive for decades.
On its inception date, July 21, the magnet produced a uniform field of 21.1 Tesla, which is expected to hold steady for years, a product of the resistanceless state of the superconducting electrons orbiting its bore. The magnet is now being readied for an international user community and, thanks to its superconducting coils, it will stay running for years to clean up a backlog of experiments waiting for such a device.
"Other systems have only a 52-mm bore, which makes it impossible to perform some experiments. Our 105-mm bore enables unique new scientific experiments to be performed, especially in the fields of chemical and biomedical discoveries," said Tom Painter, one of several engineers at the National High Magnetic Field Laboratory, with locations in Tallahassee and Gainesville, Fla. The 13-year effort to develop, design, manufacture, and test the magnet also required the contributions of engineers Iain Dixon, Jim Ferner and team leader Denis Markiewicz.
Last year the Lab announced an experiment designed exclusively to generate a 25-Tesla field (see www.eet.com/showArticle.jhtml?articleID=18309382), but it used a conventionally generated 20-Tesla field as the baseline, with the final 5 Tesla generated by a concentric coil with a 38-mm bore.
That "holy grail" system's bore was too small for many experiments and its field was not uniform enough for the delicate procedures in many proposed studies. But the new low-temperature system's bore has a volume 64 times larger than that of typical nuclear magnetic resonance (NMR) systems. The entire field is generated by the superconducting coils around the bore and the engineers have fine-tuned its specifications to make sure it meets the stringent requirements of a wider variety of experiments. Field-strength homogeneity, for instance, has been held to one part per billion and drift to one part per billion per hour. These specs already have researchers worldwide lining up to use the lab.
"Our low-temperature superconductors are operated at just 1.7 Kelvin, the temperature of liquid helium, whereas our previous 'holy grail' demonstration of a 25-Tesla field was just to prove the concept that a high-temperature superconductor, operating at 77 Kelvin, the temperature of readily available liquid nitrogen, could produce such high field strengths," said Painter.
Some day, high-temperature superconductors will enable laboratories to set up their own temporary high-magnetic-field experiments, then put the superconductors away in a closet. Today such high-temperature superconductors are still experimental and low-temperature superconductors are so expensive to set up and maintain round-the-clock for years, that only specially funded labs can afford to set up a low-temperature facility.
24/7 for decades
Once a low-temperature superconductor is turned on, the time and expense of setting it up pays off by allowing experiments to run day-and-night for decades with no sag in field strength (since the electrons orbiting in the circular cores of the superconducting magnets have zero resistance as long as the temperature stays below 4 Kelvin.
"Right now we have the world's highest frequency for NMR at 900 MHz that corresponds to 21.1 Tesla. It's a linear scale. For instance, zero Tesla is zero megahertz, but next we are going for higher fields, shooting for 25 Tesla in a nuclear magnetic resonance system within five to 10 years," said Painter.
In magnetic resonance imaging (MRI), an NMR spectrometer produces electronic images of specific atoms and molecular structures in solids, especially human cells, tissues and organs. Many experiments to discover new chemicals and biomolecular mechanisms depend on high field strength. The Florida lab's magnet achieves its high field strength with a concentric assembly of 10 superconducting coils connected in series and operated at 1.7 K ( - 456.6° F).
Each coil is wound with a monolithic superconductor, composed of niobium-tin (Nb3Sn) or niobium-titanium (NbTi) filaments suspended in a copper matrix. Magnetic loading was supported by coils configured with stainless steel overbanding and vacuum impregnated with cryogenically tough epoxy for structural support.
"There are many new areas of chemical and biomedical science that will make use of our uniquely high operating frequency of 900 MHz for nuclear magnetic resonance spectroscopy and magnetic resonance imaging," said Tim Cross, the lab's NMR director. "With this instrument, scientists from around the world will expand the horizons of materials research, determining the macromolecular structure of biological samples, and the noninvasive magnetic resonance imaging of laboratory animals."
The project was conducted with funds from the National Science Foundation and the State of Florida. The National High Magnetic Field Laboratory operates two high-field-strength magnets at the University of Florida (Gainesville) and Los Alamos National Laboratories (Albuquerque, N.M.). It also works with the U.S. Navy to create seaworthy superconducting motors that could save space in Naval ships.