The design of an apparatus and methodology for testing the thermal conductivity of various materials at subkelvin temperatures

We present the design of an apparatus and methodology for testing the thermal conductivity of various materials at subkelvin temperatures. These tests are performed using a dilution refrigerator which is commonly used to cool superconducting devices such as TES bolometers. These devices need to be cooled to very low temperatures below (0.1K) to operate properly. For operating large arrays of these devices it is necessary to reduce systematic effects in the instrumentation. One of the areas of concern is conductive heat loads between different temperature stages of the cryostat, which arise from the complex mechanical supports and cryogenic wiring supporting the focal plane and its sensors. Having precise thermal conductivity versus temperature values is vital for an accurate cryogenic model, including predicting heat loads and temperatures for each cryogenic stage. To test the thermal conductivity, heat is applied to a sample, and the temperature increase is measured. To ensure an accurate model all parasitic heat loads, such as radiative loading, have to be accounted for. This project will create a library of thermal conductance of materials at subkelvin temperatures.