On the Causes of Degradation in Bi2Sr2CaCu2O8+x Round Wires and Coils by Quenching at 4.2 K

One of the remaining challenges for the implementation of commercial Ag-alloy-sheathed Bi2Sr2CaCu2O8+x (Bi2212) wires in high-field superconducting magnets is quench protection. To develop an effective quench protection system, it is important to understand the conditions that must be avoided during a quench so that the conductor is not degraded. While these conditions are understood for NbTi and Nb3Sn, they are conductor specific and there remains a lack of data and understanding of the limiting conditions for Bi2212 wires. Here, quenches are induced in short strands and small coils of Bi2212 round wires at 4.2 K. The quench conditions are varied to identify the threshold conditions resulting in wire degradation. These conditions are quantified in terms of the maximum temperature, the maximum time rate of change of the temperature, and the maximum temperature spatial gradient along the length of the wire. It is found that the time rate of change of the temperature (thermal shock) is not a primary driver for degradation but that both the maximum temperature and its spatial gradient play a key role. It is not clear, however, whether the temperature gradient along the length of the wire, or radially from the center of the wire to the surface, dominates. It is also found that threshold values for these parameters vary between different Bi2212 wires and, thus, must be identified for the specific wire to be used in a magnet system. Implications of these results on quench protection are discussed.