Quench induced degradation inBi2Sr2CaCu2O8+x tape conductors at 4.2 K

Abstract

With the growing interest in commercial Ag-alloy sheathedBi2Sr2CaCu2O8+x powder-in-tube conductors (Bi2212) for use in high-field magnets, it is importantto understand the quench behavior and limiting criteria of a quench, includingconditions that will result in a decrease in critical current. Even thoughthe quench characteristics of low-temperature superconductors NbTi andNb3Sn are well understood, there is still a lack of data and understanding of what conditionsduring quenches in high-temperature superconducting (HTS) materials cause degradationof the conductor. In this investigation, quenches are induced in short samples of Bi2212tape conductors with local heat disturbances using a resistive heater. The voltage andtemperature evolution during quenching were recorded and analyzed to determine thenormal zone propagation velocity. Furthermore, the quench conditions where varied toidentify the threshold quench conditions which result in conductor damage. Theseconditions are quantified in terms of three parameters: the maximum temperature(Tmax), the maximum rate of temperature increase(dT/dt|max) and the maximumtemperature gradient (dT/dx|max).It is found that the normal zone propagation velocity for Bi2212 tape conductor is20–30 mm s−1. The samples showed high sensitivity during quench, exhibiting large losses in criticalcurrent under certain conditions. In all cases the sections of the tape closest to the quenchinitiation 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.