Power transmission efficiency and low-inductance magnet applications could be significantly improved by bundling high-temperature superconductor cables. Unfortunately, cabling techniques used so far have led either to fragile cables that are flexible, or robust cables that are stiff. US researchers have now shown that it is possible to create mechanically robust and flexible superconducting cables by making the cables from a conductor coated with a high-temperature superconductor.

The cables, developed by researchers at the University of Colorado and the National Institute of Standards and Technology, both in Boulder, Colorado, can be made much thinner and more flexible than demonstration cables used in the electric power grid [van der Laan et al., Supercond Sci Technol (2011) 24, 042001; doi:10.1088/0953-2048/24/4/042001].

In work supported by the US Department of Energy, the Boulder team has wound multiple HTS-coated conductors around a multi-strand copper core, with superconducting layers of GdBa2Cu3O7-d forming spirals in alternating directions. Although the prototype cables are wound by hand, several manufacturers say mass production is feasible.

The single most innovative step in producing these compact cables is in their tolerance for compressive strain, which allowed the team to use an unusually slender copper core. One prototype superconducting cable is just 6.5 mm in outer diameter but can carry 1200 A, while a second cable 1 mm thicker can sustain a current of more than twice that, at 2800 A at a temperature of 76 K. Conventional electrical transmission cables operate at below 1000 A, but these newly wound cables are a tenth of the diameter of the demonstration superconducting cables tested so far. For example, a three-phase cable installed in Columbus, Ohio, uses bismuth-strontium copper oxide tapes wound around a large core to form a 70 mm cable that can carry an average current of 3000 A at 73 K. NIST researchers are now developing prototype compact HTS cables for the military, which requires small size and light weight, as well as flexibility to pull transmission lines through conduits with tight bends. Tests on the new cables showed them to potentially have a bend radius of just 125 mm.

Besides power transmission, the flexible cabling concept could be used for superconducting transformers, generators, and magnetic energy storage devices that require high-current windings. The compact cables might be used in high-field magnets for medical applications such as next-generation magnetic resonance imaging and proton cancer treatment systems, and perhaps even in fusion reactors.

David Bradley