Improved Thermal Insulation Performance for Structured Metallic Coatings to Reduce Thermal Noise in Superinsulation

Abstract

Performance of multilayered superinsulation in cryogenic systems is highly dependent on the radiative properties of reflector foil surfaces. Aluminum coated polyester foils are utilized for their high reflectivity in the infrared spectrum. In an earlier work it was shown that by structuring these coatings the amount of eddy currents induced by switching ambient fields can be reduced considerably. This results in a much smaller transient response superposing the signal to be measured by high precision magnetic measurements using SQUIDs. In addition to a reduction of the thermal noise background, this considerably increases the quality of the magnetic measurements. In high energy applications with changing magnetic fields, such as superconducting fault current limiters, heat is dissipated in the superinsulation by eddy currents, degrading the performance of the superinsulation. In the past these materials showed rather high emissivity values and caused poor thermal performance when used in multilayered superinsulation. In fact the measured thermal performance was not correlating well with models. Using the measured emissivity some thermal coupling with next but one foil surfaces was indicated, by radiation through the plain grid lines of the partially transparent structured coatings. To improve these properties, the grid size was increased and a new structured coating on both sides of the foil was tested. Several calorimetric and magnetic experiments are presented and the results are discussed in this paper.