by R. Mayrhofer and J. Stipsitz, RUAG Space GmbH, cryo.at.space@ruag.com
The 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. This coating is an electrical conductor and in applications with variable magnetic fields eddy currents are induced in the coating.
These eddy currents can have two effects. In high energy applications, for example in superconducting fault current limiters, heat is dissipated in the superinsulation, degrading its performance and potentially even damaging the superinsulation. For high precision magnetic measurements using SQUIDs, switching ambient fields induce eddy currents in the superinsulation which results in a transient response super-posing the signal to be measured. This, in addition to a higher thermal noise background, reduces the quality of the magnetic measurements.
A former material type with etched gridlines is no longer available. Our study investigated various processes to structure the metallic coatings to develop a better product for our customers. The aim was to retain the high reflectivity provided by the aluminum and to reduce the eddy currents to a level acceptable for different applications. This is a compromise between thermal and electrical properties. Several calorimetric and magnetic experiments were conducted. Based on these, the most effective structured coating was chosen and a respective product was developed.
Coolcat 2 NI (Non Inductive) is a spacered superinsulation composed of 10 layers of 12µm polyester foil, single-side aluminized in squares of 10 mm x 10 mm, interleaved with 10 layers of non-woven polyester spacer material (see Figure 1). It is available with two different surface resistances, 0.8 ohms per square and 1.6 ohms per square. A lower surface resistance means the aluminum coating is thicker. The distance between the squares is approximately 100 µm.
Magnetic thermal noise measurements were performed with 20 layers of structured foil interleaved with 20 layers of polyester spacer material, on a sample size of 100 mm × 100 mm.
As can be seen in Table 1, the surface coating thickness is not a very important factor for the spectral noise density. The measurements were conducted in the test chamber (see Figure 2) of the Physikalische Technische Bundesanstalt (PTB) in Berlin. The PTB has one of the best magnetically shielded rooms worldwide [1].
RUAG can now offer blankets of Coolcat 2 NI preshaped by laser cutting, which assembles the layers along the edges, similar to our standard Coolcat 2 NW product line. Coolcat 2 NI is available in sheets 3 m long and 0.75 m wide; other sizes on request.
The emissivities of the different foils were measured and compared to a former material type, which is no longer available. In Figure 3 one can compare the emissivities of a continuously coated VDA foil of the old patterned material and with the new patterned material utilized in Coolcat 2 NI. The emissivity for the new material is significantly better than the values of the older material. The measurements were performed using a measurement calorimeter device described in [2].
Figure 4 shows Coolcat 2 NI MLI blankets at THISTA [3] at Karlsruhe Institute of Technology (CSA CSM). For calorimetric measurements on the MLI blankets, see the performance values in Table 2.
For the sizing of superinsulation for real applications it is recommended to multiply these heat flux values by a factor of 1.3–1.5. This is assuming good design, installation and vacuum conditions.
Coolcat 2 NI has a nominal compressed thickness of 1.4 mm per 10 layers. For good thermal performance, compression of the superinsulation should be avoided. It is recommended to allow a minimum insulation gap thickness of 3 mm per 10 layers and to install the insulation loosely.
For low outgassing Coolcat 2 NI contains 100% polyester spacer without binder. The maximum baking temperature should not exceed 423 K.
Conclusion
The work provided a new material for application in SQUIDs and high energy superconducting applications to reduce eddy currents in superinsulation. The thermal performance was improved compared to state-of-the-art materials, and important properties of the materials were tested with novel future applications in mind.
The work was carried out cooperatively by the Vienna University of Technology and the Thermal Systems department of RUAG Space GmbH, the largest supplier of space products and related ground support equipment in Austria, focusing on electronics, mechanisms and thermal insulation. The state-of-the-art equipment comprises leading-edge design and analysis tools, clean rooms and automated cutting machines. The “Coolcat” line of high quality cryogenic multilayer insulation is efficiently produced on a large industrial scale.
The project was funded by RUAG Space GmbH and the Austrian Federal Ministry of Transport, Innovation and Technology (Austrian Re-search Promotion Agency). www.ruag.com/thermal
References:
[1] Bork J., Hahlbohm HD., Klein R., Schnabel A., “The 8-layered magnetically shielded room of the PTB: Design and construction,” Proceedings of the 12th Int. Conf. on Biomagnetism, Biomag2000, Helsinki (2000), pp. 970-973.
[2] Kralik T., et al., “Device for measurement of thermal emissivity at cryogenic temperatures,” Proceedings of the 8th IIR Int. Conf. Cryogenics, Prague (2004), 23-29.
[3] Neumann H.: “Thermal performance of different glass microspheres in comparison to perlite between 77K and 300K”; CEC-ICMC; 28.06. – 02.07.2009; Tucson, Arizona, USA.
