Current–voltage curve of the high temperature superconductor with local reduction of critical current

Abstract

Particular feature of the 2nd generation of conductors, based on the high-temperature superconducting compounds and produced nowadays in industrial process, is the fluctuation of properties along the conductor length. As a consequence, the voltage appearing at approaching the maximum transportable DC current could come from just one single weak spot. In such a place, characterized by the critical current reduced with respect to the rest of the conductor, the excess dissipation results in a locally elevated temperature. We developed an analytical theory predicting the experimentally observed current–voltage characteristics of such conductor, based on the weak spot properties and the cooling conditions. It extends our previous work, that was focused on forecasting the value of current at which would happen the thermal runaway, accompanied by an abrupt growth of the measured voltage. Here we derive the analytical expression that provides prediction for the current–voltage curve in wider range of currents, without necessity to reach thermal runaway. Then, a more reliable determination of the superconductor properties, in particular the flux creep exponent, can be achieved. Use of the model in the analysis of experimental data is illustrated, and its limitations resulting from the simplifications, introduced in order to reach the analytical predictions, are discussed.