Method and apparatus for contactless longitudinal and vertical homogeneity measurements of the critical current density jc in superconducting tape

摘要

A method and apparatus employs Hall probes for contactless, longitudinal and transversal homogeneity examination of a critical current density jc in a superconductor tape that is cooled to below a transition temperature Tc. The cooled superconductor tape is pulled through an external, locally limited magnetic field that has a constant gradient and a fixed magnetic field axis. The magnetic field generated by a screening current induced in a region of the superconductor tape penetrated by the external magnetic field is scanned using a first Hall probe at a distance of a few tenths of millimeters to the superconductor tape. The external magnetic field outside of a field range of the induced persistent currents is measured with a second Hall probe. A compensated difference between the first and second Hall probe signals is used as a qualitative measure for a critical current density jc(x) and its longitudinal homogeneity which varies locally with respect to a longitudinal axis of the superconductor tape. The compensated difference is formed by adjusting a test difference between the two Hall probe signals to zero as a result of measuring the external magnetic field by both Hall probes in a dummy measurement conducted without the superconductor tape. A transverse homogeneity with a linear Hall probe array that is positioned crosswise to the longitudinal axis of the superconductor tape and extends broadside over the superconductor tape is measured after all Hall probes in the array have first been adjusted to zero with a signal from the second probe. Quantitative locally critical current densities are specified from the Hall probe signals by a resistive calibration obtained from a current-voltage measurement.