Electrical Measurements on Superconductors by Transport

This article focuses on techniques for measuring the electrical behavior of technologically important superconducting wires, tapes, and other conductors. Depending on the conditions of measurement, electrical behavior can be interpreted to obtain many technologically important quantities. The interpretation for some of the most common of these quantities are discussed. When characterizing the electrical behavior of superconductors, it is primarily the voltage-current relationships that are measured. All other quantities are derived from these. Background conditions for the measurements, including temperature, magnetic field, and stress-strain state, vary with the specific measurement and type of information being sought. The most common parameter to be obtained from a transport current-voltage measurement is the transport critical current (), which is the maximum current that can be carried by the superconductor in the superconducting state. Often this is determined as a function of temperature and magnetic field. Critical current and the corresponding critical current density () are the most important parameters in determining the utility of a superconducting material for applications. The value of equals normalized by the cross-sectional area perpendicular to the direction of the current flow. This area can be defined by a variety of methods that are discussed. Electrical measurements of transport current are used to determine the critical temperature () of the superconductor. This is the temperature below which the material enters the superconducting state and above which it is in the normal state. By measuring as a function of magnetic field using such an electrical measurement, one can also learn about the magnetic behavior of a high-temperature superconductor. Electrical measurements are used to determine the properties of superconductors beyond the superconducting state. Measurements of the resistivity versus temperature (i.e., the current-voltage relationship as a function of temperature in the normal state) are important for the design of superconducting current leads and magnets. For cables and coils, electrical measurements provide insight into the stability and quench propagation (sudden reversion to the normal state from the superconducting state) behavior. This information is key for the design of a superconducting magnet. Electrical measurements can be used to estimate the AC losses (i.e., energy dissipation resulting from an alternating current) in a superconducting wire or cable. In this case, the hysteretic behavior is used to estimate the energy dissipation. This article concentrates on techniques for measuring the current-voltage () relationship in superconducting wires, tapes, and cables. Important differences between LTSs and HTSs are discussed. Emphasis is placed on the measurement and on the interpretation of the results in the context of the technical information sought from the particular experiment. Keywords: electrical measurements; current-voltage relationships; superconductors; sample shape; contacts; troubleshooting; instrumentation; cooling options; specimen modification; internet resources; instruments; current ramp rate