Fabrication and properties of pancake coils using Bi-2223/Ag tapes

Small double pancake coils with a 15 mm bore were fabricated from Ag sheathed mulifilamentary Bi-2223 tapes by the 'wind and react' technique. By optimizing the parameters of the final heat treatment, the critical current density of the tapes could be achieved also in the winding of the coils. A typical value of the critical current density at 77 K is 18 000 A/cm(2) for a 90 m long tape. In a double pancake coil with a total tape length of 48 m, a magnetic field of 1 T could be generated at 20 K. A filling factor of 15 % was obtained for the superconductor in the winding of the coil, compared with a value of 25 % for the Bi-2223/Ag tapes. The critical current in different sections of the coils was controlled by electric field-current measurements. Another problem is the electric connection between both pancake coils. A contact resistivity of less than 2.10(-7) ZZ was achieved at 20 K. Investigations of the current limiting mechanisms were performed on short tapes in the temperature range between 4.2 K and 77 K.