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

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.