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Influence of Self-Heating on Measured n -Value

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4 Author(s)
Hiltunen, I. ; Inst. of Electromagn., Tampere Univ. of Technol., Tampere, Finland ; Lehtonen, J. ; Stenvall, A. ; Mikkonen, R.

Knowledge of the voltage current characteristics, and especially n-values, is very important for the development of superconducting applications such as NMR-magnets and fault current limiters. n-values are usually determined by fitting the power law into a measured voltage-current-characteristics (V(I)) 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 n-value of a poorly cooled short sample. In this paper, we test this method with two samples of different materials in different cooling conditions. V(I) -curves of a MgB2 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 n-value significantly. However, when correction method was applied acceptable results were obtained from all samples.

Published in:

Applied Superconductivity, IEEE Transactions on  (Volume:20 ,  Issue: 3 )