High-temperature superconductors make major progress
This month's Nature Materials featured an important breakthrough for high-temperature superconductors. A new method has been found for processing Bi-2212 high-temperature superconducting round wire in order to drastically increase its critical current density. The result confirms that this conductor is a serious candidate for future very-high-field magnets.
This image shows the cross-section of two Bi-2212 wires. The bottom wire has less leakage and void porosity due to a heat treatment done at an overpressure of 100 bar - about 100 times the pressure used to produce the top wire (image from [Nature Materials, Vol. 13 (2014), 10.1038/nmat3887]).
The workhorse for building superconducting accelerator magnets has been, so far, the Niobium-Titanium (Nb-Ti) alloy superconductor. But with Nb-Ti having reached its full potential, other conductors must be used to operate in higher magnetic fields beyond those reached with the LHC magnets. Today, the intermetallic Niobium-Tin (Nb3Sn) is the most advanced superconducting material beyond Nb-Ti. Nb3Sn conductors are now ready for use at CERN in high-field accelerator magnets, like the 11 Tesla magnets, and large aperture quadrupole magnets, like those being developed for the LHC High-Luminosity upgrade. However, high-temperature superconductors are needed to reach the 20 Tesla domain.
“Recent spectacular performance improvements of Bi-2212 wire at the National High Magnetic Field Laboratory (NHMFL) have made this high-temperature superconductor a real candidate for future magnets that can produce magnetic fields beyond the reach of any Nb-based superconductor,” says Christian Scheuerlein, materials engineer in the Superconductors and Superconducting Devices section within the Magnet, Superconductors and Cryostats group of the Technology Department.
In the framework of the EuCARD-2 Future Magnets development programme, CERN is collaborating with NHMFL and industrial partners Nexans (who produce the state-of-the-art Bi-2212 precursor) and Oxford Superconducting Technology (the Bi-2212 wire manufacturer). Their goal is to develop Bi-2212 wire for use in accelerator magnets that can reach 20 Tesla.
A drawback of Bi-2212 is that the precursor material needs to be melted and re-solidified when the conductor is its final size and shape. This requires heating the entire magnet coil up to a maximum temperature of about 900 °C, controlling a homogeneously distributed temperature with high accuracy. In order to reach the full potential of the Bi-2212 conductor, the heat treatment must be done at overpressures, possibly as high as 100 bar. “In collaboration with researchers at the European Synchrotron Radiation Facility (ESRF), we are studying how to simplify this delicate process to make the use of Bi-2212 in accelerator magnets even more attractive,” says Scheuerlein, and he concludes: “Three decades after its discovery, Bi-2212 high-temperature superconductor is becoming a real candidate for application in very-high-field magnets!”
For more information about the Bi-2212 study, consult the full paper in Nature Materials: D.C. Larbalestier, J. Jiang, U.P. Trociewitz, F. Kametani, C. Scheuerlein, M. Dalban-Canassy, M. Matras, P. Chen, N.C. Craig, P.J. Lee, E.E. Hellstrom, “Isotropic round wire multifilament cuprate superconductor for generation of magnetic fields above 30 T”, Nature Materials, Vol. 13 (2014), 10.1038/nmat3887.
