Metallic thermal connectors for use in nuclear refrigeration

Abstract

Measurements of the electrical resistance at 4.2 K are used to infer the thermal conductance at ultralow temperatures of (1) copper magnet wires with Formvar and Allex insulations, (2) inert gas welded and silver-soldered joints, and (3) various demountable metallic connectors. The purpose of this study is to determine a wire suitable for use as the refrigerant in a nuclear adiabatic demagnetization refrigerator and an appropriate method of joining the wire to a heat exchanger in a 3He cell. It was found that ordinary magnet wire insulated with Allex can easily be annealed to have a very low resistance at low temperatures. Further, annealed copper-to-copper welds are indistinguishable from bulk OFHC copper, while alloying makes the resistance of silver-soldered joints somewhat larger than would be calculated from the bulk resistivity of the silver solder. An improved version of the ’’squeeze’’ connector and a simple pressure contact using a stainless steel screw are described. These have too high a resistance for nuclear refrigeration but are good enough for most other low-temperature applications. Finally, a large copper tapered screw joint was found to make relatively poor thermal contact.