Planck is the third Medium-Sized Mission (M3) of ESA Horizon 2000 Scientific Programme. It is designed to image the anisotropies of the Cosmic Background Radiation Field over the whole sky, with unprecedented sensitivity and angular resolution.
Planck carries two main experiments named HFI (High Frequency Instrument) and LFI (Low Frequency Instrument). The first is based on bolometers, the latter is an array of tuned radio receivers, based on High Electron Mobility Transistors (HEMTs) amplifier technology, and covering the frequency range from 30 to 70 GHz. The Front-End Electronics Modules (FEM’s) are cooled at 20K by a H2 sorption cooler. The high frequency signals (up to 70 GHz) are amplified, phase lagged and transported by means of waveguides to the warm back-end electronics at temperatures of the order of 300K.
The 20 K cooling is achieved exploiting a two-stage cooling concept. The satellite is passively cooled to temperatures of the order of 60K using special designed radiators called V-grooves. An H2 sorption cooler constitutes the second active cooling stage, which allows focal plane temperatures of 20K, i.e. compatible with the tight noise requirements of the Low Noise Amplifiers (LNA’s).
Each FEM needs 22 bias lines characterised by a high immunity to external noise and disturbances. The power required for each FEM ranges from 16 to 34mW, depending on the radiometer frequency. Due to the limited cooling power of the sorption cooler (about 2W), the heat transport through the harness and therefore the parasitics on the focal plane, shall be minimised. A total of 290 wires have to be routed from the warm electronics (300K) to the cold focal plane (20K), along a path of about 2200mm, transporting currents ranging from a few uA up to 240mA.
The present paper analyses the thermal and electrical problems connected with the design of a suitable cryo-harness for the bias of the radiometers cryogenic front-end modules of LFI. Two possible approaches are proposed, and a solution presented.