Measurement of In-Field E-J Characteristics in Multi-Filamentary Bi-2223 Tapes at Ultra-Low Electric-Field Down to Around 10−13 V/m

We have developed a method to investigate in-field electric-field v.s. current-density (E-J) characteristics at ultra-low electric field criterion around 10⁻¹³ V/m in a multi-filamentary Bi-2223 tape at 5 K based on scanning Hall probe microscopy (SHPM). Bi-2223 is widely used in high field applications, e.g., a 30.5 T NMR magnet system is currently being developed in Japan. Ultra-low electric field is induced in such magnets operating in persistent current mode with extremely steady field. Namely, E-J characteristics need to be clarified around that criterion. E-J characteristics can be estimated from decay of magnetization in the whole sample, assuming the uniformity of J. But for multi-filamentary Bi-2223 tape, magnetization current path should be clarified. In this study, we measured local magnetic field profile as a function of time for Bi-2223 tapes using SHPM. Additionally, we carried out “flux annealing” measurements to realize ultra-low electric field induced in the tape. As a result, two main messages we obtained. Experimental field profile or sheet current density obtained from the inverted Biot-Savart law indicated that filaments remain coupled up to high field, and even at ultra-low E criterion. Moreover, local E-J characteristics around 10⁻¹³ V/m are obtained at 5 K and 4.5 T of perpendicular magnetic field, which is comparable perpendicular field component in a Bi-2223 insert coil in the high field NMR magnet system. I_c properties at 10⁻¹² V/m decrease to nearly 50%, in comparison to the transport I_c properties at 10⁻⁴ V/m.