Measured Performance of Different Solders in Bi2223/Ag Current Leads

In superconducting magnet systems the current leads are usually divided into two parts. Normal metal like brass or copper is often used in the first part from the room temperature to the temperature of the radiation shield. The second part down to the magnet is made of high temperature superconductors (HTS). HTS leads can reduce the conductive heat load because they have poor thermal conductivity. Since HTS wires are lossless with direct current and have fair tolerance to the magnetic field only Ohmic losses are generated in the contact resistances at the current terminals. Thus, efficient current leads require that appropriate solders are used to reduce the contact resistances. In this paper, the thermal and contact resistances as well as thermal conduction losses of Bi-2223/Ag current leads are experimentally investigated using indium- and tin-based solders at operation temperatures between 20 and 77 K. Finally, the measured data is utilized to design an efficient HTS current lead to the current range of 0-1000 A.