PORTLAND, Ore. Researchers at Florida State University's National High Magnetic Field Laboratory in Tallahassee achieved the holy grail of magnetism recently when their high-temperature superconductor attained the coveted 25-Tesla field strength record.
"When we founded the National High Magnetic Field Laboratory, a part of our charter was to someday produce a 25-Tesla magnetic field, which sounded futurist in 1994 at the dedication. Now we have finally achieved it, and with an oversized bore of 38 mm," said project leader Justin Schwartz. The record-setting superconducting magnet was jointly developed with Oxford Superconducting Technology (Carteret, N.J.).
The bore of a high-temperature superconductor (HTS) insert coil is the spot where researchers place their experiments, and thus the bigger the better. The nominal size is 33 mm, which makes the 25-Tesla HTS insert coil larger than most, but bores as large as 105 mm are under development at the lab for nuclear magnetic resonance (NMR) applications.
NMR was also what drew Oxford to the project. "For us, it was the drive toward ever-higher field strengths for commercial NMR spectroscopy that motivated Oxford's participation," said Ken Marken, project leader at Oxford. "We plan to use these technological advances in our high-field business."
All high-field-strength magnets make use of a descending hierarchy of coils. Unfortunately, no material is strong enough to make a single-coil electromagnet that can withstand such a high field strength. (The generated field tries to uncoil its own wire in direct proportion to the strength of the magnetic field it is generating.) To compensate, multiple coils in high-field-strength magnets generate fields inside fields inside fields, since one coil does not affect the unraveling tendency of contained coils.
For the experiment, a conventionally generated 20-Tesla field served as the baseline, with the final 5 Tesla generated by the coil with a 38-mm bore using Bi-2212 superconducting wire from Oxford. "We've been working on this milestone for a long time, and have had two previous versions with Oxford that generated less than 3 Tesla," said Schwartz.
From those previous prototypes, the researchers knew that Oxford's Bi-2212 superconducting wire, though near-atomically perfect, still could not withstand the uncoiling force of a single 5-Tesla coil. So their final superconducting magnet was composed of three coils, two pancake-wound "warmup" coils plus the final high-field-strength superconducting coil with the 38-mm bore.
For the future, the lab plans to propose to the National Science Foundation that it create a "desktop" version of the 25-Tesla HTS coil for labs around the world. The NSF sponsors the laboratory jointly with the state of Florida. Florida State, the University of Florida and Los Alamos National Laboratories operate the lab, which offers state-of-the-art experimental facilities at all three sites. "Now labs are paying as much as $5 million for 20-Tesla fields, so we think our 25-Tesla fields could be sold for as much as $10 million," said Jeff Crow, the lab's director.
The National High Magnetic Field Laboratory, which operates similar high-field-strength magnets at the University of Florida (Gainesville) and Los Alamos National Laboratories (Albuquerque, N.M.), is also working with the Navy to create a sea-worthy superconducting motor. Only the most central core coils in the motor use superconducting wire, but the resulting efficiency means the resulting motor can be vastly smaller, which is important for space-constrained Naval ships.
The laboratory also aims to create a superconducting transformer for in-house use.