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.