Characterisation of net type thermal insulators at 1.8 K low boundary temperature

The Large Hadron Collider's superconducting magnets are cooled by superfluid helium at 1.8 K and housed in cryostats that minimise the heat inleak to this temperature level by extracting heat at 70 K and 5 K. In the first generation of prototype cryostats, the radiative heat to the 1.8 K temperature level accounted for 70% of the total heat inleak. An alternative to enhance the cryostat thermal performance incorporates a thermalised radiation screen at 5 K. In order to avoid contact between the 5 K radiation screen and the cold mass, insulators are placed between both surfaces. Sets of commercial fibre glass nets (spacers) are insulator candidates to minimise the heat inleak caused by any accidental contact between the two temperature levels. A model to estimate their performance is presented. A set-up to thermally characterise them has been designed and is also described in the paper. Finally, results as a function of the number of nets forming the spacer, the boundary temperatures and the compressive force in the spacer are presented.