The role of nickel substrates in the quench dynamics of silver coated YBCO tapes

A pair of silver coated YBCO tapes with varying degrees of electrical contact between the silver and the YBCO nickel substrate were studied to examine the impact of nickel on normal zone formation and stability. The YBCO tapes were fabricated using the rolling assisted bi-axially textured (RABiTS) method and 15-cm-long samples with 2 mum of silver were prepared. The samples were place in a conduction cooling environment at 45 K to study quench or recovery when a series of dc transient over-current pulses were applied. We used a series of voltage taps on both the silver and nickel to characterize the nature of the contact between the silver and the nickel through the measured voltages. In as-manufactured samples and those sample with continuous contact between the nickel and silver, we found that the silver and nickel can not be treated as conductors in parallel when a normal zone is present because of current present in the nickel even while sections of the sample remains superconducting. This nonparallel contact makes stability characterization difficult although the samples with intentional contact were able to withstand larger current pulses. In addition, the samples with intentional electrical contact between the silver and nickel exhibited normal zone propagation in both the silver and the nickel with speeds between 4 mm/s and 8 mm/s.