Hall magnetometer for AC characterization and test results of Bi-2223 tape specimens

Abstract

The economically viable use of HTS superconductors poses strict demands on the manufacturing and operational losses of superconducting tapes. Both these losses are minimized with an appropriate choice of tape cross-section and manufacturing process. They can be optimized with systematic testing of different combinations, but such testing is expensive, consumes plenty of time and provides only few answers as to the sources of the internal losses in the tested tapes.

In this thesis, Hall sensor modification for AC characterization has been made to enable novel AC measurements and to gain new information about the current distributions causing AC maps. The work involved analysis of the reproducibility and accuracy of the measurement and calculation methods. In addition, DC measurements and their analysis were included to help visualize the differences between DC and AC maps. New tools based on the discrete Fourier transformation were developed to determine the optimal current distribution among several assumed current penetration models.

The modified AC Hall magnetometer proved to be inexpensive, robust, simple to use, and well suited for tape manufacturers to test systematically their specimens. Test tools for the estimation of the inversion errors proved to give tight error limits. Comparison between Hall sensor and magnetic knife measurements showed that these methods should be used complementary to examine possible current variations in the cross-section of the tape. DFT analysis was used to test penetration models and find estimates on tape Ic and penetration model parameter G, even thicknesswise evolution of current density could be distinguished. The third harmonic content was shown to be crucial in optimization.