Strain Effects in Superconducting Compounds

This paper [1] was presented by invitation at the 1983 International Cryogenic Materials Conference at a crucial point in the scale-up of superconductor magnet applications. It synthesizes a wide range of electromechanical strain data into what has become known as the Strain Scaling Law, which has had a strong impact on the design of large-scale, high-field superconductor magnet coils since that time. The Strain Scaling Law was the result of a unique high-field electromechanical measurement capability, developed by Jack Ekin at NBS, which led to the discovery and systematization of the intrinsic strain effect in practical superconductors. In the years leading up to this work, it was known that superconductors have a “critical surface” that is a function of temperature, current density, and applied magnetic field. It had not been known that the critical surface also depends on an additional fundamental parameter, mechanical strain. Fig. 1 shows the scaling of the superconductor critical surface with this variable. This discovery was especially important to the development of large-scale superconductor applications because early work had focused only on small coils of superconducting wire in which the magnetic forces were small. However, the need to handle large forces emerged with the development of large-scale magnets for particle accelerator machines, magnetic plasma confinement in fusion energy, superconducting magnetic energy storage (SMES), and medical magnetic resonance imaging (MRI) systems. Ekin had focused primarily on the electrical properties of metals and superconductors in his thesis work at Cornell University and postdoctoral work at Rutgers University before joining NBS in 1975. The focus of his group at NBS was predominantly on mechanical properties, which led him to probe the unexplored niche between the electrical and mechanical research worlds. This coincidence of two fields of expertise ultimately resulted in the discovery of the superconductor strain effect and the Strain Scaling Law.