Threshold Field for Runaway Instability of Bilayer Hard Type-II Superconductor

Abstract

We have solved the problem of the critical state stability of a bilayer type-II superconductor, whose layers have different physical properties (critical current density, specific heat, and critical temperature). We have found the optimal thickness for the outer layer, which results in a maximal increase in the stability threshold—the magnetic field at which the first thermomagnetic avalanche occurs (the jump of the magnetic flux and other physical properties). The calculations were performed for a Nb\(_3\)Sn superconductor coated with a layer of NbTi. It is shown that by choosing the optimum coating thickness, the stability threshold of the magnetic field increases by more than 60 %.