The 4 K cooler on the Planck spacecraft
Illustration of the active cooling system on board the Planck spacecraft, with the components of the 4 K cooler highlighted. Also the three V-grooves of the passive cooling system are coloured, representing their progressively lower temperature from bottom to top. To see where on the spacecraft these components are located, see the related video and image in the right-hand menu for context.
The 4 K cooler is the second cooler in the active cryogenic cooling chain and cools the focal plane unit (FPU) of the High Frequency Instrument (HFI) down to 4 K. It is a Joule-Thomson cooler in a closed-cycle system, with 4He as the working medium.
Two frictionless mechanical compressors (highlighted in orange, at lower left) are situated in the service module and pump 4He gas into the cooler pipe work, up towards the cold end located in the HFI FPU (highlighted in blue). On its way there the gas is pre-cooled through a heat exchanger on the spacecraft's coldest V-groove and one on the 18 K stage of the 20 K cooler.
At the cold end the 4He gas undergoes a Joule-Thomson expansion leading to small amounts of liquid helium that provides the cooling power and cools the HFI FPU.