The Influence of Local Critical Current Degradation on the Quench Characteristics of GdBCO Tapes and Coils

We experimentally investigated the influence of local critical current degradation on the thermal and electromagnetic behavior around the local degradation region and the time-dependent characteristics of the voltage across it. The local critical current degradation was produced by cutting a part of the width of a piece of GdBCO tape. The current that caused the tape to burn out and the voltages across the degradation region were measured in short tape samples and pancake coils made using GdBCO tape. In a sample without copper stabilizer covered with Styrofoam, the voltage rapidly increased over a short time period and the sample burned out soon after the operating current exceeded the local critical current. On the other hand, the tape with the copper stabilizer cooled by an LN 2 was able to stably maintain the voltage even when the operating current largely exceeded the local critical current. The current sharing effect of the copper stabilizer greatly contributed to an improvement in stability because the current sharing effect was much larger than the LN 2 cooling effect. The cooling effect from the top and bottom surfaces of the coil was also important and should be improved upon as much as possible to suppress the temperature rise during coil excitation. The heat transfer effect in the radial direction between coil layers was smaller than the LN 2 cooling effect.