Quench induced degradation in Bi2Sr2CaCu2O8+x tape conductors at?4.2?K

With the growing interest in commercial Ag-alloy sheathed Bi2Sr2CaCu2O8+x powder-in-tube conductors (Bi2212) for use in high-field magnets, it is important to understand the quench behavior and limiting criteria of a quench, including conditions that will result in a decrease in critical current. Even though the quench characteristics of low-temperature superconductors NbTi and Nb3Sn are well understood, there is still a lack of data and understanding of what conditions during quenches in high-temperature superconducting (HTS) materials cause degradation of the conductor. In this investigation, quenches are induced in short samples of Bi2212 tape conductors with local heat disturbances using a resistive heater. The voltage and temperature evolution during quenching were recorded and analyzed to determine the normal zone propagation velocity. Furthermore, the quench conditions where varied to identify the threshold quench conditions which result in conductor damage. These conditions are quantified in terms of three parameters: the maximum temperature (Tmax), the maximum rate of temperature increase (dT/dt|max) and the maximum temperature gradient (dT/dx|max). It is found that the normal zone propagation velocity for Bi2212 tape conductor is 20–30 mm s−1. The samples showed high sensitivity during quench, exhibiting large losses in critical current under certain conditions. In all cases the sections of the tape closest to the quench initiation exhibited the highest peak temperatures and loss in critical current. It was found that conductor damage is avoided under the following conditions: dT/dt|max<250 K s−1, dT/dx|max<100 K cm−1 and Tmax<250 K.