Planck Pre-Launch Status Papers - APC - Université Paris Diderot ...

J.-M. Lamarre et al.: Planck pre-launch status: the HFI instrumentFig. 18. Position of thermometers (left)andofthereferenceloadsfortheLFI(centre and right).compressors in series. A description of this system is givenin Bradshaw & Orlowska (1997, p.465).Itwasdeveloppedat RAL. The compressors for the HFI 4 K cooler were suppliedby EADS Astrium in Stevenage, UK. The drive electronicswere designed and built by a consortium of RAL and SystemsEngineering and Assessment (SEA) in Bristol. The pre-chargeregulator was built by CRISA in Madrid with supervision fromthe University of Granada.The two compressors are mounted in symmetrical positionsas shown in Fig. 17 first and foremost to cancel momentumtransfer to the spacecraft. Furthermore, force transducers betweenthe two compressors provide an error signal that is processedby the drive electronics servo system, which controls theprofile of the piston motions to minimise the first seven harmonicsof the periodic vibration injected into the spacecraft.The 4 K cold stage is a small liquid helium reservoir, wherethe helium is contained in a sinter material. This is located on the4KstageaftertheexpansionofthegasthroughtheJTorifice(JT). This is an important point, as the JT orifice is thermallyisolated from the stage. It is attached to the bottom of the 4 Kbox of the HFI focal plane unit (FPU) as can be seen in Fig. 19.It provides cooling for the 4 K shield and also pre-cooling forthe gas in the dilution cooler pipes described in the next section.The cooling power and the thermal properties of this coolerwere measured by the RAL team and are summarised in theequation below, which gives its linear dependence on the mainparameters in the vicinity of the flight operating point. Theseparameters are the pre-cooling temperature T pc ,theV-groove3temperatureT vg3 ,thestrokeamplitudeofthecompressorsS aand the helium filling pressure P fill .Heat lift = 18.3mW+ 3.4mW/mm × (S a − 7.5mm)− 1.1mW/K × (T pc − 17 K)+ 0.6mW/bar × (P fill − 4.5bars)Heat load = 10.6mW+ 0.5mW/K × (T pc − 17 K)+ 0.065 mW/K × (T vg3 − 45 K) + Heater powerT = 4.47 K − 0.12 K/mm × (S a − 7.5mm)+ 0.007 × (T pc − 17 K)− 0.032 K/mW × (Heat lift − Heat load).The stroke amplitude and to some degree the temperature of thesorption cooler are adjustable in flight by telecommand. The4Kcoolingpowermarginstronglydependsonthestrokeamplitude,as the heat load increases and the heat lift decreaseswith the sorption cooler pre-cooling temperature. The temperatureof the sorption cooler is thus the most critical interface ofthe HFI cryogenic chain. It is mostly driven by the warm radiatoron the satellite, which will be at 272 K (±10 K, maximumrange allowed), leading to sorption cooler temperatures between16.5 K to 17.5 K (Bersanelli et al. 2010). The warm radiatortemperature is also a critical interface.Athermalbalancesystemtestshowedthatthepre-coolingtemperature will be close to 17.5 K. The performance givenabove indicates that the 4 K cooling power margin is about5.2 mW for a stroke amplitude of 7 mm, which is well belowthe maximum value of 8.8 mm. In these conditions the temperatureis about 4.4 K, well below the maximum of about 4.7 Krequired for the operation of the dilution cooler with reasonablemargins.The two mechanical compressors produce micro-vibrationsand also induce electromagnetic interference affecting the sciencesignals of bolometers. The risks associated with these effectswere taken into account early in the design of the HFI byphase-locking the sample frequency of the data to a harmonic ofthe compressors’ frequency. No microphonic noise was seen insystem tests when the vibration control option was activated inthe drive electronics of the compressors. However, electromagneticinterferences were seen in the qualification and the flightmodel system tests at several beat frequencies of the compressorfrequency and sampling frequency. But they are extremely narrowand can be removed completely from the signal as a resultof the harmonic ratio between them.Vibration from the compressors could affect the HFI datain a different way. During the instrument and system tests, the100 mK bolometer plate was heated by micro-vibrations. Theaverage amount of heat dissipated in the bolometer plate wasaround 10 nW in the instrument tests and 40 nW during some periodsof the system tests at CSL. The heat inputs on the bolometerplate are discussed in the dilution cooler section below.The basic characteristics of the 4 K cooler are summarizedin Table 6.ThesorptioncoolercoldheadLVHX1isonthe18Kplate of the HFI FPU, where the helium of the 4 K cooler ispre-cooled. The LFI reference loads (Bersanelli et al. 2010) onthe 4 K box of the FPU are shown in Fig. 18. Thepositionsofthe thermometers and heaters on the FPU 4 K box are shown inFig. 19. Theheatingbeltofthe4KPIDisbetweenthe70GHzreference loads and those for the 30 GHz and 44 GHz channels.The temperature stability is not as good for the latter asthat obtained for the HFI horns and 70 GHz loads. The twoPage 15 of 20