Cooling System

The sensitivity of the HFI critically depends on the temperature of the detectors. The cooling scheme that allows to cool at 0.1K the 48 bolometers and their filters is based on technical solutions that have been successfully tested in flight or have been demonstrated on ground applications and are being qualified for space. Each cooling system takes advantage of the previous one in an optimal way, following the arrangement of Figure 1.

 

Figure 1: A sketch of the cooling system and its links to the HFI ``cold box".

The precooling of the instrument to 50/60K is insured by the Planck Payload Module thanks to passive radiation to free space, which can be very efficient in the environment of the Earth-Sun Lagrangian L2 orbit of Planck. A series of radiators (called V-grooves) thermally decouple the payload from the warm Service Module, and provide intermediate temperature stages for the payload structures, piping, harness, and waveguides.

A closed cycle cooler using Joule-Thomson (J-T) expansion of hydrogen and sorption compressors insures cooling of both the LFI and the HFI to about 20K (the sorption cooler, see Figure 2). The J-T valve delivers a mixture of liquid and gas at about 17.5K. The 20 K sorption cooler is being developed for HFI and LFI by NASA/JPL.

A first high efficiency heat exchanger cools to 18K the helium flows of the 4K and the 0.1K stages. This heat exchanger is thermally decoupled from the one used to cool the LFI 20K plate, for which a larger temperature drop in the exchanger is acceptable. The shielded cable from the 4K box to the JFET box is thermally attached to the 20K stage in order to reduce heat loads on the 4K stage. Joule-Thomson expansion of helium compressed by mechanical compressors (Figure 3) is used to cool the 4K stage of the HFI. That stage supports the back-to-back horns that insure the optical coupling of the detectors with the telescope.

 

Figure 2: Schematic of the 20 K sorption cooler which serves both LFI and HFI.

 

Figure 3: A sketch of the major components of the 4K mechanical cooling system.

Lower temperatures (to 0.1 K) are obtained by dilution of ³He in ⁴He (Figure 4). A 1.6K stage is generated by J-T expansion of mixed helium. This stage supports filters and intercepts heat from the 4K stage. The 0.1K stage supports the bolometers, thermometers, heaters, and filters. Its temperature is controlled thanks to a closed loop active system. The additional cooling power available from the mixture of ³He and ⁴He under 1.6K is used to intercept heat inputs along the mechanical support of the 0.1K stage. The tubes from and to each stage are attached to form heat exchangers for all circulating fluids in order to minimise thermal losses.

 

Figure 4: A sketch of the 4K to 0.1 K cooling system.

The 0.1 K Open Cycle Dilution/Joule-Thomson Refrigerator has been developed at the Centre de Recherches des Très Basses Temperatures (CRTBT) in Grenoble (France) (Benoit et al.994). It uses a new dilution principle based on friction that does not need gravity to operate. Its cooling power depends on the gas flow, which is very low and thus allows sufficient gas storage to achieve long mission life. For a total (³He and ⁴He) flow rate of 12 $\mu$mole/s, a cooling power of 100 nW at 0.1 K has been demonstrated. In the same process, the mixture is expanded in a Joule-Thomson valve, producing a cooling power of several hundreds of $\mu$W at 1.6 K. This is enough to insure a proper insulation of the 0.1 K stage from the radiative and conductive thermal loads coming from the 4 K stage. The principle of this cryogenic architecture and its +/-1 g operation have been successfully proven with a demonstration model, and qualification of this system for space operation is in progress.

Bolometric detectors of the type to be used in the HFI are known to be sensitive to microphonic noise induced by mechanical vibrations. Since the instrument will operate in conjunction with mechanical compressors, it is important to verify that the cooler vibration does not cause degradation of the performance of the detectors. As initial measures, the compressors will be mounted back-to-back (in the case of the 50-80 K cooler resulting in the addition of one completely redundant cooler), and driven with electronic circuits especially designed to minimize vibrations. Secondly, the mechanical design of the platforms which support the coolers can be optimized to an extent where the transfer of vibrations from the platforms to the focal plane is suppressed to acceptable levels. It is worth noting here that the mechanical excitation by the compressors -if at all present- occurs at frequencies higher than 35 Hz, which would only contaminate the measurement of spherical harmonics higher than l ~ 2000.