The fabrication of a high temperature superconducting magnet and critical current characterisation of the component Bi(_2)Sr(_2)Ca(_2)Cu(_3)O(_x) tapes and filaments in high magnetic fields

Abstract

The transport critical current density (J(_c)) of a 37 filament Bi-2223/Ag tape has been measured as a function of field and temperature from 4.2 K up to 90 K. Data have been obtained over a large current range from 10 mA up to 100 A and in fields up to 23 T with the tape in 3 orientations with respect to field. These comprehensive data have been used to test the predictions of the flux creep and weak link models used to explain J(_c) in Bi-2223 tapes.The J(_c)(B,T) dependence of optimised Bi-2223 tapes has been calculated using a curved film model. The model assumes perfect grain connectivity and that the local superconducting properties are equivalent to those in the best reported thin films. A comparison between the calculations and measured J(_c)(B,T) dependencies suggest that in high fields at 20 K, J(_c) in presently available industrially processed tapes is only a factor of 8 below the performance of ideal fully optimised tapes. Transport measurements have been made on Bi-2223 single filaments extracted from an alloy sheathed multifilamentary tape in liquid nitrogen at 77 K in fields up to 300 mT with the field aligned parallel and perpendicular to the a-b planes. Further Jc(B,T) data have been taken in a variable temperature insert at temperatures between 60 to 90 K in fields up to 15 T. In a study of the electric field-current density {E-J) characteristics of the c-axis orientated data at 77 K, negative curvature is observed in traces below 280 mT. However, the 280 mT trace exhibits both positive and negative curvature in different current regimes in contrast to the predictions of standard theory. A laboratory scale Bi-2223 superconducting magnet producing a maximum field of 1.29 T at 4.2 K has been designed and fabricated. The magnet comprises 6 resin impregnated double wound pancakes with a 40 mm bore fabricated via the react and wind route. Critical current density measurements have been made as a function of magnetic field, angle and strain at 4.2 K and 77 K on short samples of the constituent tape. The E-J characteristics of all component coils have been measured and a comparison with short sample data shows that minimal additional damage occurred beyond that produced by the bending strain on the tape and the long length variation in J(_c). Sufficient detail is provided for the non-specialist to assess the potential use of brittle superconducting tapes for magnet technology and construct a laboratory scale magnet. [brace not closed]