High temperature superconducting current leads for fusion magnet systems

Superconducting magnets for fusion applications typically have very high operating currents. These currents are transmitted from the room temperature power supplies to the low temperature superconducting coils by way of helium-vapor-cooled current leads. Because of the high current magnitude and the resistive characteristics associated with the normal metallic lead conductors, a substantial amount of power is dissipated in the lead. To maintain a stable operation, a high rate of helium vapor flow, generated by the boil-off of liquid helium, is required to cool the lead conductors. This helium boil-off substantially increases both the installation capacity and the operating cost of the helium refrigerator/liquefier. It has been demonstrated that the boil-off liquid helium can be significantly reduced by using ceramic high-temperature superconductors, such as Y-Ba-Cu-O, in the low temperature part of the lead conductor structure. This design concept has been conclusively demonstrated by a 2-kA current lead test model using Y-Ba-Cu-O(123) material. The potential cost saving derived from using such leads in fusion magnets is discussed, and a design concept for a 10-kA lead is presented