Quench propagation in high Tc superconductors

Abstract

The quenching process in composite high T c and low T c superconductors has been studied in detail. Specifically, thermal stability and quench protection issues were addressed to gain a better understanding of how high T c superconductors may be best used in superconducting magnet applications. A series of experiments has been performed to measure quench propagation velocities - a parameter that quantifies the quenching process - along short samples (≈12 cm) of both low T c (niobium tin) and high T c [BiPbSrCaCuO(2223)] composite superconductors as a function of operating temperature, transport current and background magnetic induction. A computer simulation has also been developed to predict the behaviour of these conductors during a quench. The simulation includes the temperature and magnetic field dependence of material properties and generates temperature profiles that are marched out in time. Simulated voltage traces are also generated for comparison with the experimentally recorded voltage traces. The assumption that quench propagation velocities are constant, generally used in quench analysis for low T c conductors, is also reasonable for high T c conductors and thus, velocities may still be experimentally extracted from high T c conductors. In general, quench propagation velocities are two or three orders of magnitude slower in high T c conductors than in low T c conductors. This indicates that magnets wound with these conductors are unlikely to be self-protecting. For the same reasons that they are difficult to protect, they are likely to be much more stable in response to thermal disturbances than their low T c counterparts.