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The Method to Diagnose Soundness of the High Temperature Superconducting Coil by Pick-Up Coil Pairs

We have proposed a new system using pick-up coil pairs to detect normal transitions in the high temperature superconducting (HTS) coils which are applied to several applications, high field magnets, SMES, transformer, and so on. In the previous study, we have demonstrated that our system can detect local normal transitions in the winding of the test transformer wound by Bi-2223 multifilamentary tapes even if the operation of the transformer is ongoing. However, it is not sufficient to detect normal transitions for operations of practical devices because sudden breaks are not allowable for practical operation. It is important that soundness of the coil, such as critical currents, are monitored and diagnosed for actual operation of practical superconducting devices. In this paper, the external magnetic fields and temperature dependences of signals measured by our system are investigated, in order to make possible to diagnose soundness of the windings from the signals. And then it is shown that critical current of the windings, as an indicator of soundness, can be estimated under ac operation. The diameter of the sample coil is 70 mm. The results indicate that the critical currents of the winding can be estimated from our measured signals.