Theoretical explanation of the non-equipotential quench behaviour in Y–Ba–Cu–O coated conductors

YBa2Cu3O7−x (YBCO) coated conductors are realized by the deposition of a YBCO film atop thin (10−6 m) ceramic buffer layers, previously deposited upon a high resistance nickel alloy. On the other side of the tape a copper layer is often added to create an alternative current path during quenches. Several investigations have shown a different qualitative behaviour of the voltages measured on the nickel and copper sides of the tape. In this work we give a theoretical explanation for this phenomenon and point out the technical consequences it might have for the design of devices constructed using YBCO coated conductors. The understanding and assessment of quenching phenomena is an important step in the design of superconducting magnets. This field is fairly well understood for magnets using metallic low temperature superconductors. For magnets to be built with high temperature superconducting (HTS) tapes, however, there is little understanding of critical behaviours. This lack of results is especially marked for YBa2Cu3O7−x (YBCO) coated conductors (CC) [1]. Different experimental procedures have been adopted using lengths of YBCO CC tape to initiate quenches under controlled environments that simulate those occurring during magnet operation. Several researchers have induced quenches using a current pulse greater than the tape’s critical value [2]. With this technique, the quench is initiated in a ‘weak’ region where the critical current is lower than in the rest of the tape. A different approach to quench initiation has been followed in [3] and [4]. The quench is in this case initiated by a heat pulse through a heater mounted on the tape. In both cases experimental investigations have shown that the voltage traces measured on the nickel substrate and on the silver (or copper) layer have different qualitative behaviours, indicating a contact relationship between nickel and silver (or copper) that cannot be treated as a simple parallel contact [4, 5]. In both cases the voltage traces recorded on the nickel side rise simultaneously along the tape length as soon as the current redistributes from the YBCO layer to the nickel layer at initiation of the quench. On the other hand, the voltage traces along the silver or copper side do not appear simultaneously. The first voltage traces appear across the ‘weak’ regions in overcurrent experiments and across the heated region in pulsed heater experiments. The voltage signals recorded at distances from the heater arise after a delay and reach much lower values than in the nickel side at the same positions or are too low to be measured. A dedicated experiment was realized to analyse this phenomenon [5]. A difference in the voltage traces was found even if continuous direct contact was realized between the silver and nickel layers along the whole sample. The driving mechanism for this discrepancy remained unclear. To explain this phenomenon we have developed an electrical model that takes into account that the tape cross section is not equipotential. The conductor is divided into Nsec sectors along the length (in this case Nsec is set to 140 in order to reach numerical convergence). Each layer sector is represented by means of its longitudinal resistance and selfinductance, as shown in figure 1. The different layers interact through electrical contact resistances between adjacent layers and mutual inductances. The YBCO layer is described through the power law model, EYBCO = Ec(J/Jc) , where Jc is 0953-2048/07/040009+03$30.00 © 2007 IOP Publishing Ltd Printed in the UK L9