Effect of multilayer insulation on radiation heat transfer at cryogenic temperatures

Measurement results are reported for the heat transfer rate, as a function of the number of layers of different types of multilayer insulation, for temperature differences of 284 K to 77 K and 77 K to 4.2 K. The insulation was found to reduce heat transfer between the higher temperatures but generally had a detrimental effect at the lower temperatures. An exception was noted in double-aluminium-coated NRC2, which also gave a slight reduction in heat transfer for up to ten layers at the lower temperatures. Attempts to explain the results using the proximity effect were not successful. The authors, however, suppose the results may be explained by outgassing effects: the outgassing materials are cryopumped into the spaces between the layers and frozen, causing thermal shorts, so that in effect the many layers become one laminated layer. Also they are cryopumped onto the outer aluminium surface, so that the effective emissivity worsens. Eventually the thickness of the cryopumped material becomes thick enough for it to have the bulk material emissivity and no further deterioration occurs.