Planck-HFI thermal architecture: from requirements to solutions

The Planck-High Frequency Instrument (HFI) will use 48 bolometers cooled to 100mK by a dilution cooler to map the Cosmic Microwave Background (CMB) with a sensitivity of DeltaT/Tsimilar to2.10(-6) and an angular resolution of 5 minutes of arc. This instrument will therefore be about 1000 times more sensitive than the COBE-DMR experiment. This contribution will focus mainly on the thermal architecture of this instrument and its consequences on the fundamental and instrumental fluctuations of the photon flux produced on the detectors by the instrument itself. In a first step, we will demonstrate that the thermal and optical design of the HFII allow to reach the ultimate sensitivity set by photon noise of the CMB at millimeter wavelength. Nevertheless, to reach such high sensitivity, the thermal behavior of each cryogenic stages should also be controlled in order to damp thermal fluctuations that can be taken as astrophysical signal. The requirement in thermal fluctuation on each stage has been defined in the frequency domain to degrade the overall sensitivity by less than 5%. This leads to unprecedented stability specifications that should be achieved down to 16mHz. We will present the design of the FIR thermal architecture, based on active and passive damping, and show how its performances were improved thanks to thermal simulations.