The Consequence of Self-field and Non-uniform Current Distribution on Short Sample Tests of Superconducting Cables

Electrical measurements on samples of superconducting cables are usually performed in order to determine the critical current $I_c$ and the n-value, assuming that the voltage U at the transition from the superconducting to the normal state follows the power law, U\sim($I/I_c$)$^n$. An accurate measurement of $I_c$ and n demands, first of all, good control of temperature and field, and precise measurement of current and voltage. The critical current and n-value of a cable are influenced by the self-field of the cable, an effect that has to be known in order to compare the electrical characteristics of the cable with those of the strands from which it is made. The effect of the self-field is dealt with taking into account the orientation and magnitude of the applied field and the n-value of the strands. An important source of inaccuracy is related to the distribution of the currents among the strands. Non-uniform distributions, mainly caused by non-equal resistances of the connections between the strands of the cable and the current leads, can easily result in a misinterpretation of the measured critical current and n-value by 5% and 50% respectively. In this paper this effect is explained in detail, taking also into account the influence of the current ramp-rate (during a voltage-current measurement), the sample length, the contact resistance between the strands and the placement of the voltage taps.