Metallic thermal connectors for use in nuclear refrigeration

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.