Random instabilities of current-voltage curves of BSCCO-2223/Ag multifilamentary tapes in LN2 at 77 K

The measurement of the current-voltage (I-V) characteristics of BSCCO-2223/Ag multifilamentary tapes in a silver matrix has been performed on short samples (of several centimetres) as well as on long tape (I m), wound in the form of a helical one-layer coil. Measurements at 77 K and in zero external magnetic field have revealed good reproducibility of the I-V hysteresis in most runs. Nevertheless, strange irregularities have sometimes been observed in the I-V curve behaviour during current ramping up and down. Quasi-reproducible drops from the ascending hysteretic branch in the direction of the descending one have been measured at higher voltage levels (similar to1 mV cm(-1)) on the curve measured on the helical coil. These have recently been explained by a sudden change in the heat transfer coefficient [I]. Rarely and non-reproducibly we have also observed these drops on short samples at E similar to 1 x 10(-2) V m(-1), (and even under 1, x 10(-3) V m(-1)). The accidental drops have also been sporadically measured by other experimenters. We also interpret these effects as a result of changes in heat transfer dynamics to LN2 coolant, on a local scale, in the vicinity of a small locally degraded portion of the superconducting multifilament tape. The implication of the known differences in the heat transfer dynamics between ramp-up and ramp-down phases to the thermal interpretation of the I-V curve hysteresis, observed on short samples of multifilament tapes, however, fails to explain why already a rather small external magnetic field (similar to15 mT) suppresses this hysteretic effect.