Development of high-frequency current probe utilizing high T_c Josephson junction

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In this research, we developed high T_c superconducting magnetic sensor for high-frequency current probe. Our results are summarized as follows.1. We developed the magnetic sensor utilizing bicrystal junctions. Using the change of the current-voltage characteristics of the junction with the magnetic field, we can convert the magnetic field to the voltage with the conversion efficiency of dV/dB. Compared to the SQUID, the present sensor can measure the absolute value of the magnetic field.2. The magnetic sensor is made using the 30-degree bicrystal junction. The I-V curve of the junction is studied both theoretically and experimentally in order to optimize the conversion efficiency dV/dB. Key factors which determine the conversion efficiency, such as the ration of the junction width to the Josephson penetration depth λ_j, flux focusing effect, and temperature dependence, are clarified.3. Performance of the series array of the junction is compared with that of the conventional induction coil. When the sensor area is 100 μm x 50 μm, the sensitivity of the junction array is more sensitive up to 7 GHz than the induction coil. Moreover, the sensitivity is independent of the frequency, unlike the case of the induction coil. Therefore, wide-band measurement is possible with the present sensor.4. Current probe is made with the magnetic sensor, and the magnetic field produced by the high frequency current is measured. The current with frequency of 130 MHz is successfully measured. With the improvement of the measurement system, we will demonstrate the measurement of GHz signal.