Tightly wound superconductor gives magnet strength

Winding a thin tape of superconducting material tightly enough can form a powerful magnet the size of a wristwatch. This could enable the creation of more sensitive nuclear magnetic resonance devices for studying cells and materials (J. Magn. Reson. 2023, DOI: 10.1016/j.jmr.2023.107588).

The researchers used a commercially available high-temperature superconducting tape and wound it into a washer-shaped coil, which they then cooled to 4.2 K in liquid helium. The coil had an inner diameter of 8 mm and an outer diameter of 24 mm. When charged with 1,168 A, the coil generated a magnetic field with a strength of 7.3 Tesla (T), about 7,000 times the strength of a refrigerator magnet. Two similarly sized magnets connected in series produced a field of 12 T.

With the proper engineering, such coils could be made to generate much higher fields, says Alexander Barnes, a professor in the Institute of Molecular Physical Science at the Swiss Federal Institute of Technology (ETH), Zurich, who led the work. “We think we can hit 50 T next year. And I think that with this kind of technology, 100 T is within grasp in the next couple of years,” Barnes says. Higher fields could improve the ability of compact NMR devices to study drug reactions inside cells and the surface chemistry of catalysts.

NMR magnets that reach 28 T are already available, but those can weigh several tons and cost millions of dollars. Barnes says his device is much less expensive and can fit in someone’s pocket. The main cost lies in the tape, which runs about $100 per meter. The single coil required 9.2 m of tape wound 190 times around the spindle.

Barnes says there’s no particular trick to making the smaller coils. The main challenge lies in how much the tape can be bent without damaging the crystalline structure of the material, which must be free of defects for superconductivity to work. The researchers are trying to wrap the tape even more tightly—the more loops to the coil, the stronger the magnetic field it generates and the more quickly fringe fields beyond their edges drop off. The design uses no insulation, keeping the size and materials costs down, but it requires careful management of “quench,” which happens when the current heats up the material, reduces the superconductivity, and causes the helium coolant to boil away.

The superconducting material is a rare earth barium copper oxide (REBCO), with europium filling the rare earth role in this case. Barnes would like to explore other elements, such as yttrium, to see if they improve performance.

REBCO magnets have been explored since about 2010, including one concept for a 100 T device, says Mark Bird, associate director of the National High Magnetic Field Laboratory at Florida State University. Several research groups have pursued the coils for a variety of applications. “Many of those groups have come to understand that the quench dynamics of such coils are difficult to master and have moved on to other approaches,” Bird says.