The effect of sample holder and current ramp rate on a conduction-cooled V -I measurement of MgB2

In a voltage-current measurement of a superconductor the sample must be kept at a constant temperature. Otherwise, the measured critical currents and n values are distorted. However, when conduction cooling is applied and currents are high keeping a constant temperature is difficult, or even impossible, to achieve. We measured voltage-current characteristics of a conduction-cooled multifilamentary MgB2 tape in the self-field above 23 K. Simultaneously, the sample temperature was monitored. Due to the losses at the contact resistances, the sample holder and the sample, considerable sample warming was detected already below the critical current. Therefore, at low ramp rates, the critical current and the conductor n value cannot be determined from the measured voltage-current characteristic. However, when the ramp rate was increased, the critical currents and the n values levelled out. The required increase depended on the sample holder. We studied two sample holders and scrutinized the effect of thermal and electrical conductivities on the distorted critical currents and n values