Highly Sensitive Measurement of AC Loss Characteristics of Short and Straight HTS Tapes Under Transverse Magnetic Field by Pickup Coil Method

In order to develop high temperature superconducting (HTS) tapes with excellent low-loss characteristics, it is indispensable to have a measurement system capable of measuring the AC loss characteristics of HTS tapes with high sensitivity at 77 K. We have developed a measurement system using the pickup coil method, which can measure the AC loss characteristics of short straight HTS tapes under AC transverse magnetic fields with high sensitivity. In the pickup coil method, pickup coils consist of a main pickup coil for measuring the specimen signal voltage and a compensation coil used to extract only the magnetization signal voltage of the specimen contained in the main pickup coil voltage. In order to increase the sensitivity of AC loss measurement systems using the pickup coil method, background losses, which are apparent losses that occur even when there is no sample in the main pickup coil, must be reduced as much as possible. To reduce background losses, first, a magnet was designed and manufactured to apply external transverse magnetic fields so that a main pickup coil and a compensation coil could be arranged symmetrically. This makes it possible to reduce the phase difference between the main pickup coil voltage and the compensation coil voltage, which is one of the main causes of background losses. Next, background losses were measured using several kinds of pickup coils with different parameters. Furthermore, based on the measurement results, the optimum parameters of the pickup coils were investigated. Finally, background losses could be successfully reduced, and the sensitivity of the AC loss measurement system increased.