Non-destructive evaluation of longitudinal uniformity for twisted Bi2223 tapes using scanning Hall-probe microscopy

In general, shorter filament twisting should be required for substantial AC loss reduction of Bi2223 tapes under an AC external field. However, the longitudinal uniformity of both transport property and wire structure of a tightly twisted tape could be easily deteriorated. To qualify the uniformity of twisted tape, simple and non-destructive evaluation techniques should be urgently required. In this study, we non-destructively measured the remanent magnetic field distributions for twisted Bi2223 tapes using scanning Hall-probe microscopy (SHM) with an active area of 50 μm x 50 μm. Twist pitch lengths of the tapes used for the measurements were 10 mm and 6 mm. After the tape was fixed on the sample holder at 77 K and zero fields, the magnetic field in perpendicular to the broader face of the tape was applied by a rectangular permanent magnet moving along a tape length. After removing the field, the distributions of remanent field Brz in perpendicular to tape surface were measured by SHM at a fixed distance of 0.5 mm away from a tape surface. For tightly twisted tape with twist pitch length of 6 mm, the longitudinal uniformity of Brz and transport critical current Ic were degraded remarkably and the local positions at which Brz greatly drops were well corresponding to low Ic region. It was also confirmed that the defects in filaments caused by tight twisting strongly affect on the intensity and shape of Brz profiles. The results suggest that SHM has the potential for simple and non-destructive characterization to qualify the longitudinal uniformity of twisted tapes.