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Description
Adequate heat transfer is essential for components and systems that operate in cryogenic environments. In conduction-cooling applications, bolted joints are commonly employed to facilitate the heat transfer between the devices and cryocoolers. The performance of these joints is dependent on a multitude of factors such as material properties, surface topography, pressure distribution, and the use of interfacial materials. In support of the development of conduction-cooled superconducting radio-frequency (SRF) niobium cavities for use in continuous-wave linear accelerators, an experimental study of thermal contact conductance was performed on bolted joints using high-purity niobium, high-purity aluminum, and OFHC copper. The study investigates the relative performance of various combinations of these materials as well as two cryogenic thermal interface materials, Apiezon N grease and indium foil, by measuring the thermal resistance across each joint interface as well as the thermal conductivity of each material in the temperature range 3 - 8 K. The geometry of the joints investigated aims at replicating the design parameters of a 915 MHz conduction-cooled SRF Nb cavity currently under development at Jefferson Lab. This consisted of a single 3-bolt joint utilizing a Nb to Al plate contact as well as two 4-bolt joints, one comprised of a Cu to Al plate contact and one Cu to Nb. Prior to cold-testing, the interfacial contact pressure of each joint was determined using Fujifilm Prescale pressure-measuring film, providing characterization of the pressure distribution within the joint. The specimens were mounted to the second stage of a Sumitomo RDE-418D4 4 K cryocooler and tested in a high-vacuum dewar using the "two-heater" method. Additionally, the thermal conductivity of Al and Nb bar samples was measured using a simple setup holding one end in contact with a liquid helium bath through a "cold-finger" and with a heater mounted at the opposite end. The thermal conductivity of the Al was also measured with a strained sample. A finite-element analysis of the experimental joint setup was carried out using ANSYS to further investigate the results.
