Development status of the inner thermal shielding for the ET-LF cryogenic payloads

Cryogenic operation of ET-LF is imperative for exploiting the full scientific potential of ET,
with mirrors operated at temperatures of 10 K to 20 K in order to limit thermal noise. Thermal
shielding is essential to support the cool-down process and to reduce both the particle adsorption
and the heat load on the optics. Additionally in steady-state operation, mechanical vibrations
must be kept to an absolute minimum to limit the noise input by scattered light.
We present the development progress of a thermal shield surrounding the cryogenic payloads
of ET-LF, which considers rapid cool-down and low vibration in steady-state operation. During
cool-down, cooling tubes enable the flow of supercritical helium, driving the shield temperature
decrease by forced convection. For steady-state operation, the shield cooling mechanism is
converted to heat conduction at 2 K via static He-II in the same tubes. The conceptual design
status is explained by means of analytical and numerical modeling results.