CRYOGENIC LOAD CALCULATION OF HIGH-T(C) CURRENT LEAD

High temperature superconducting current leads are composed of a normal metal part, conducting the current from room temperature to an intermediate temperature (often 77 K), and a high T(c) part, conducting the current down to liquid helium temperature. The cryogenic load and the corresponding power consumption of both parts are compared to the electrical power consumption of an equivalent all metal current lead. The temperature profiles and the 4.2 K heat load of the superconducting part are evaluated by a computer calculation. The calculation uses material parameters from sintered Y-123 and melt-cast processed Bi-2212 tubes, materials which already exist in usable dimensions for current leads. The 1 muV cm-1 criterion turns out to be much too conservative and some dissipation due to flux flow in the upper part of the lead does not affect the overall performance of the current lead if the thermal runaway current I(TR) is not exceeded. The maximum stable current is evaluated as a function of conductor length for operation in a self-field and applied fields up to 0.2 T. Long leads result in very low 4.2 K heat loads but shorter leads result in higher stable transport currents. The use of very short high T(c) conductor parts (50 mm) and the possibility of fabricating a short metallic lead for 77 K operation allows us to design very short (<0.3 m) and high performance current leads for 4.2 K applications. This current lead can be designed to pass right through the insulating vacuum of a cryostat, presenting new opportunities for a more compact cryostat system. Stability considerations indicate that the current lead is not sensitivie to flux jumps. As currents I > I(TR) cannot be accepted, a safety margin must be considered in the design of the current lead.