THERMODYNAMIC OPTIMIZATION OF CONDUCTION-COOLED HTS CURRENT LEADS

A theoretical optimization is performed for the conduction-cooling met hod of high Tc superconductor (HTS) current leads, which can be applie d to the superconducting systems cooled directly by cryogenic refriger ators without liquid helium. The current lead is a series combination of a normal metal conductor at the warmer part and a HTS at the colder part, and is cooled by a contact with distributed or staged refrigera tors instead of boil-off helium gas. An analytical method is developed to derive a mathematical expression for the required refrigerator pow er. By incorporating the critical characteristics of the HTS, it is de monstrated that there exist unique optimal values for the current dens ity of HTS and the joint temperature of the two parts to minimize the total refrigerator power per unit current, for a given length of the H TS. As results of the study, the absolute minimum in the refrigerator power per unit current is presented as a thermodynamic limit and the l eads cooled by a two-stage refrigerator are theoretically optimized. S ome aspects in practical design are also discussed with a new and usef ul graphical method. (C) 1998 Elsevier Science Ltd. All rights reserve d.