“Tolerance Current” of Bi-2223/Ag Coil

Different from low temperature superconductors, Bi-2223/Ag HTS tapes have lower ldquonrdquo value. This characteristic brings rather ldquofuzzyrdquo notification of the critical current. It depends on the cooling condition. This means that the critical current of Bi-2223/Ag tape can not be decided absolutely. Rather we should focus on the temperature rise characteristics of the coil against over-current exceeding the critical current. If the temperature of the coil keeps a finite value without thermal runaway, the coil can be used continuously. We call the uppermost current without thermal runaway as ldquotolerance currentrdquo. Considering the situation mentioned above, the authors have studied the temperature rise characteristics of HTS coil for both DC and AC currents. Two cooling conditions are adopted. One is a cooling by immersion in liquid nitrogen bath, and the other is a conduction cooling by GM refrigerator. From the experimental result, the ldquotolerance currentrdquo of the Bi-2223/Ag coil for various conditions are obtained.