DESIGN OF SUPERCONDUCTING CURRENT LEADS

The thermal behaviour of superconducting current leads operating between 4.2 and 293 K has been studied. For operating current densities above 10 000 A cm(-2), textured Bi-2223/Ag tapes are an alternative to Bi-2212 bulk material. Results for these two cases will be compared. The room temperature refrigerator power required to cool a superconducting current lead has been calculated for different cooling concepts, taking into consideration the normal conducting copper part. Use of high T-c superconductors has the potential to reduce the required room temperature refrigerator power to one-fifth of that consumed by a conventional current lead. The effects of complete coolant loss have been simulated numerically. To avoid burn-out the current has to be switched off, i.e. the supplied superconducting magnet has to be discharged. The maximum possible operating current densities in the superconducting part of the current lead have been calculated assuming an exponential decay of the current with a time constant tau = 10 s and allowing a maximum peak temperature of 400 K in the current lead. The operating current density should not exceed 500 A cm(-2) for the Bi-2212 bulk material, whereas values above 10 000 A cm(-2) are possible for Bi-2223/Ag tapes. In both cases it has been assumed that the current starts to decay when a voltage of 0.1 V has been developed across the superconducting part of the current lead.