Variable Temperature Thermal Conductivity and Conductance Measurements Using a Gifford-McMahon Cryocooler

Abstract

The U.S. Navy is pursuing the design and fabrication of a superconducting magnet system cooled by closed cycle cryogenic refrigerators; no liquid helium will be used at all to cool the superconducting magnet. Well characterized superconducting magnet materials and thermally efficient interfaces between cryocoolers and the magnet, current leads, and radiation shields are critical because of the limited cooling capacity of cryocoolers being considered. In order to determine the thermal conductivity or conductance of several magnet composites and interfaces, respectively, a variable temperature apparatus has been developed which incorporates a Gifford-McMahon(GM) cryocooler. High heat capacity neodymium is used in the GM’s second stage regenerator and in a thermal filter used to smooth normally occurring temperature oscillations inherent to GM cryocoolers. In addition to good thermal performance, connections between the leads and coolers, and the magnet and coolers must be good electrical isolators to protect against damaging high voltages developed during magnet quenches. Measurements of thermal conductivity and conductance are made in the range of 6.0 K–35.0 K. This range is chosen to encompass the probable operating temperatures of conductively cooled NbTi, Nb₃Sn, and high T_c magnets. Design, fabrication, and general operation of the thermal conductivity measuring apparatus are presented and discussed.