Abstract.
A magnetic filed relaxation at the center of a pulse-magnetized single-domain Y-Ba
Introduction.Textured high-temperature superconductors (HTS) of Re(Y)-Ba-Cu-O family appear to be very promising materials for manufacturing superconducting magnets with very large magnitudes of trapped magnetic fields [1,2]. For example, trapping a field as high as 17 T between two 26.5-mm diameter Y-Ba-Cu-O disks at 29K has been demonstrated recently [3]. A comprehensive review of synthesis, characteristics and applications of high-quality HTS suitable for applications in superconducting magnets can be found in [4]. Recently significant research efforts have been directed towards using a pulse magnetization of HTS instead of an isothermal one [5]. An obvious advantage of the pulse magnetization with a typical pulse duration in order of 1 to 100 msec is a smaller amount of energy needed to create a magnetic field of a desired magnitude. There is, however, a complication that needs to be considered when using this method: an electromagnetic heating of a superconductor during the pulse magnetization plays much bigger role than during a slow magnetization because there is not enough time for the temperature to reach an equilibrium distribution within the superconducting volume. For example, a thermal time constant of a HTS sample at 78K with characteristic dimensions in order of 10 mm would be in order of a few seconds, which is much large than the pulse duration. This circumstance allows one to analyze the pulse magnetization as an adiabatic process. A negative effect of the heating during a single-pulse magnetization can be reduced by using a multi-pulse method. Using a combination of the multi-pulse magnetization and a step-by-step temperature decrease has been shown to allow trapping magnetic fields as high as 4.5T at 30K [6].One of the key performance characteristics of any magnet is a long-term stability of its magnetic field. Here superconducting magnets have a fundamental limitation due to a phenomenon known as a thermally-activated magnetic flux creep. According to Kim-Andersen theory a thermally-activated magnetic flux creep in type II superconductors [7], supercurrents and magnetization decrease logarithmically in time as
Feiwibility of utilization of cold switches for qriericli protection of very large sc niitgnets (e.g., for SMES) is considered. The scheme of quench protection of large SMES is suggested. The necessary uumber of sections can be easily evaluated. Destructive sc switches seem to be the best solution. The switch has to be properely desigired to avoid arcing and to offer a possibility to c l i a i i g c the destructive elements in a reasonable time without warming up of large portions of the winding. A suggcstioii is also made to make use of the temperature depeiidence of the electrical resisttirice of the driinp resistor. A proper choice of its mass can result iii 25 percents decrease of a quench load.
Magnetic flux flow in layered NbTi/Nb and textured superconducting ceramics Bi2Sr2CaCu2O8+d with the nearly reversible magnetization curve are measured. The characteristic time of the magnetic moment relaxation of such superconductors is determined by the viscous flux flow. It is observed, that in a parallel to superconducting layers external magnetic field the characteristic time of the magnetic flux inflow into the sample and the time of the flux outflow from the sample differ in several times. This difference in the characteristic times is unequivocally due to the flux flow into the sample through the end plates of the thin superconducting layers (plates), where Bean-Livingston barrier is effectively suppressed, but the flux flow out from the superconducting layers (plates) through the wide sides of the plates freely. The experimental proof of the barrier absence for the outflow of the magnetic flux on the border of the superconductor is received. Thus is shown, that the border (surface) of the type-II superconductor is the "semiconductor" for the magnetic flow: it lets the flux to flow out from the superconductor freely, but it interferes with the flux to flow into the superconductor. Index Terms-Bean-Livingston barrier, layered superconductor, magnetic flux flow.
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