Double-Spiral Current Leads of Bi-2223 Tapes for a SMES Magnet

A pair of double-spiral current leads of Bi-2223 tapes are developed for a 0.5 MJ SMES magnet. Two stacks of live Bi-2223 tapes (4.8 x 0.32 mm²; Ic > 115 A at 77 K, zero field, 1 uV/cm) with a length of 1500 mm are wound separately along the double-spiral slots in an epoxy-resin bobbin that has a height of only 90 mm and a diameter of 105 mm. The critical current of the leads measured in liquid nitrogen at self-field is about 410 A. The degradation of the critical current is mainly due to the magnetic field generated by the tape stacks themselves. After being assembled in the SMES magnet the critical current of the Bi-2223 tape leads will be further degraded by the magnetic field produced by the SMES coil. However, if the temperature of the warm end of the leads is kept below 64 K, the critical current will not be smaller than the value at 77 K and zero field. The experimental results of the magnet indicate that the double-spiral current leads of Bi-2223 tapes work very well.