Current-voltage measurements in a 2G YBCO coil

The Oak Ridge National Laboratory in collaboration with American Superconductor Corporation and Cryomagnetics Inc. has designed, fabricated, and tested an HTS coil wound with second-generation (2G) YBCO coated conductor tape. The purpose of the HTS coil project was to study the quench characteristics in 2G YBCO coils at 77 K and lower temperatures (∼30-45 K). These quench characteristics were investigated in both a pool boiling LN2 environment and in a conduction cooled configuration at ∼30 K and 45 K. Transport critical current (I_c) measurements taken on the very first thermal cycle of the YBCO coil in pool boiling LN2 showed an I_c ∼ 31 A corresponding to a central magnetic field of 0.32 T. The measured I_c value was consistent with the calculated value using the calculated maximum perpendicular B-field component and the measured short sample I_c at 77 K. Subsequent I_c measurements taken in the conduction cooling configuration at 34 K and 45 K, showed a steady-state I_c ∼ 45-49 A and 38-44 A, respectively. These I_c values were significantly lower than the calculated value assuming a literature derived temperature dependent I_c of the 2G YBCO tape. A steady degradation was observed in the I_c of the coil with each successive thermal cycle. In addition, the coil was also pulse tested up to 1-T in non-steady state transient conditions and for ramp rates varying between 0.01 and 5 A/s. The issues and limitations encountered during testing of this new type of 2G coil are briefly discussed.