Effect of Bubbling on the Current Carrying Capacity of Small BSCCO Coils

Abstract

One of the problems facing the applications of BSCCO/Ag tapes in HTC magnets is their deformation during heat treatment and/or after testing in liquid cryogen, known as bubbling. In this paper, we studied the effect of bubbles which occur during heat treatment, on the current carrying capacity of two single layer coils. The first coil was made from 2212/Ag Powder In Tube (PIT) monofilament tape which was fabricated by Oxford Instrument, Inc. The second coil was made from another 2212/Ag (PIT) monofilament tape which was fabricated at the University of Wisconsin-Madison. After heat treatment, the first coil had several bubbles, while the second had only one. The current-voltage characteristic (I-V) curves were measured for both coils at 4.2 K and a zero external magnetic field. I-V curves for bubbled and unbubbled sections of the first coil were measured in external magnetic fields up to 0.5 Tesla (T). The critical current (I_c) of the first coil was 20 A (1 μV/cm criterion), which was only 20 % of the I_c of the good section of the coil. The I_c of the second coil was 147 A (1 μV/cm criterion). The critical current density (J_c) in the BSCCO core were 1.4×10⁴ A/cm² and 1.7×10⁵ A/cm² for the first and second coil respectively. We also observed that the I-V curve of the second coil had an unusual shape, consisting of a linear voltage section after the zero voltage section, followed by the usual non-linear I-V dependence.

In spite of a large reduction of local critical current (I_c) values due to bubbling, both coils can carry currents close to the critical currents of the best coil sections.

A microstructure study was also conducted on the bubbled and unbubbled sections of the coils and the results are discussed.