Influence of Self-Heating on Measured $n$-Value

Abstract

Knowledge of the voltage current characteristics, and especially -values, is very important for the development of superconducting applications such as NMR-magnets and fault current limiters. -values are usually determined by fitting the power law into a measured voltage-current -characteristics ( ( )) of a sample. However, the sample warms due to resistive losses even at subcritical currents, and thereby, measuring accurate characteristics at overcritical currents has proven to be difficult. Previously, we have developed a mathematical method to determine the critical current and -value of a poorly cooled short sample. In this paper, we test this method with two samples of different materials in different cooling conditions. ( )-curves of a MgB 2 tape were measured at 30 K in vacuum environment and of a Bi-2223/Ag tape at 73 K in gas cooled environment to verify the effect of warming during the sample characterization. With high currents and slow current ramps the sample warmed up the most distorting the -value significantly. However, when correction method was applied acceptable results were obtained from all samples.

Influence of the voltage taps position on the self-field DC and AC transport characterization of HTS superconducting tapes

“…Higher temperature is a consequence of the additional heat produced in the current leads and reduced cooling since the superconductor is not cooled directly by the nitrogen bath. An increase of the n-value due to heat produced in the samples was reported also in [16].…”