Self field AC losses in Ag-sheathed PbBi2223 tapes

Hughes, Timothy John Stopford (1997) Self field AC losses in Ag-sheathed PbBi2223 tapes. University of Southampton, Doctoral Thesis.

Abstract

In many engineering applications of high temperature superconductors the AC losses of the superconductor is an important design parameter. In general there are two cases of AC losses, those produced when the superconductor carries an AC transport current (self-field losses) and those produced when it is placed in an external AC field (external field losses). The work presented here is focused on the characterization of the self-field losses in Ag sheathed PbBi2223 tapes. The self-field losses of the tape can be devided into different regimes. At currents below the critical current of the tape the losses are dominated by the hysteresis losses of the tape. The cross-section of the superconducting core in a tape is best represented as either a "thin" rectangle or ellipse and the hysteresis losses can be calculated by solving the critical state for these two types of geometry. A standard method of measuring the self-field losses is to attach voltage contacts to the sample and measure the loss component of the surface electric field. However for the extreme geometry of the tape this loss electric field will vary according to the configuration of the voltage loop with a difference of up to 300% possible according to whether the loop is closed on top of or on the side of the tape. Detailed modelling and experiments have shown that an accurate measurement of the losses (within 5%) can be made provided that a suitable measurement protocol is followed. This protocol requires that the voltage loop used for measurement of the loss electric field is closed in the same plane as the tape at a distance for three times the tape half-width. In order to provide validation of the electric measurement a novel thermometric measurement system was constructed. With this method the losses within the tape are determined by the measurement of the temperature profile along an adiabatic tape with both ends at a fixed temperature. Comparative measurements between the electrical and thermometric systems showed a good correspondence for a large range of losses. Studies using the two systems also revealed that some of the tapes measured have a considerable inhomogeneity in the critical current along the length of the tape. Long lengths of the tape capable of carrying a high critical current are separated by shorter regions only capable of carrying a much reduced current. It is likely that the long regions of high current represent the intrinsic critical current of the tape with the reduced current in the shorter regions due to mechanical defects. Although the DC characteristics are dominated by the critical current in the short bad regions the AC losses are dominated by the critical current of the longer good regions making the AC measurements more indicative of the intrinsic critical current of the tape.

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