This invention relates generally to superconductors, and more particularly the invention relates to improving the operating stability of superconductors by reducing decay of magnetic shielding current.
The so-called high temperature superconductors are materials such as copper oxides (e.g. 1-2-3 compounds) which exhibit superconducting characteristics when cooled with liquid nitrogen (boiling point of 77.degree. k). While high temperature superconductors offer much promise for commercial applications since the need for expensive and difficult to use helium is obviated, a serious problem has been recognized regarding a magnetic flux lattice weakness which results in a decay of magnetic shielding current. See Pool"Superconductivity: Is the Party Over?," Science, May 1989. As noted therein, the unexpected weakness of magnetic flux lattice means that high temperature superconductors may not be able to hold a high enough magnetic field in the "persistent current" mode to perform many of the tasks once envisioned for them. In many applications the superconductors are used in the so-called persistent current mode. A constant DC current flow is maintained inside the superconductor without the need of an external power supply. The lack of electrical loss in the superconductor makes such an operation possible. An important criterion of a superconductor is the critical current density J.sub.c, or maximum amount of current the DC superconductor can carry before it becomes lossy electrically. Persistent currents can be established in a variety of ways; but in all previously used methods the electrical critical current density, J.sub.c, is reached in at least some portions of the circuit. It has been found that in all the high T materials operation near the critical current density results in electrical loss so that the persistent current decays significantly over useful periods of time. Typical decay can 10 be 10-20% over the first 1000 seconds which limits practical utilization. It is recognized that when the high T.sub.c superconductors are operated near their critical current density the persistent currents decay significantly over time due to the undesirable loss in the superconducting material. Typical relaxation causes a large amount of decay in the persistent current (on the order of 10-20% for the first 1000 seconds) which limits the practical utilization of the material.
High T.sub.c superconductors are the so called type II superconductors. Inside such superconductors, magnetic fields do not spread uniformly but form in lines of quantized flux, called fluxoids. Electric currents passing through the superconducting material push against the lines of flux, and if they move energy is dissipated. The dissipation of energy appears as electrical resistance which defeats the purpose of the superconductor. When the force of the electrical current pushing against the fluxoids becomes large enough, the fluxoids aided by thermal energy start creeping over defects. This creep also represents loss that appears as electrical resistance.