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

High temperature superconducting current leads are composed of a norma l metal part, conducting the current from room temperature to an inter mediate 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 ele ctrical power consumption of an equivalent all metal current lead. The temperature profiles and the 4.2 K heat load of the superconducting p art are evaluated by a computer calculation. The calculation uses mate rial parameters from sintered Y-123 and melt-cast processed Bi-2212 tu bes, materials which already exist in usable dimensions for current le ads. The 1 muV cm-1 criterion turns out to be much too conservative an d 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-fi eld 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 curre nts. The use of very short high T(c) conductor parts (50 mm) and the p ossibility of fabricating a short metallic lead for 77 K operation all ows us to design very short (<0.3 m) and high performance current lead s for 4.2 K applications. This current lead can be designed to pass ri ght through the insulating vacuum of a cryostat, presenting new opport unities for a more compact cryostat system. Stability considerations i ndicate that the current lead is not sensitivie to flux jumps. As curr ents I > I(TR) cannot be accepted, a safety margin must be considered in the design of the current lead.