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

J.-M. Lamarre et al.: Planck pre-launch status: the HFI instrumentFig. 21. Simulation of the gradients induced on the bolometer plate byparticles when the PID is off.Fig. 20. Thermal architecture of the 100 mK stage.requirement of 28 µK Hz −0.5 (in the 0.016 Hz to 100 Hz frequencyrange). For both the 4 K and 1.6 K stages, the requiredstabilities were achieved during system tests as discussed in detailby Pajot et al. (2010).5.4.2. 100 mK stageThe temperature stability of the bolometer plate is achieved withboth an active control and a passive thermal filter as shown inFig. 20 (Piat 2000; Piat et al. 2000).Two stages of PID regulation are included. The first one(PID1) is on the dilution itself and provides stability on long timescales. When no thermal perturbation is applied to the bolometerplate, PID1 alone provides the required stability. The secondregulation system (PID2) is on the bolometer plate. The passivethermal filter is mounted between the dilution cold tip andthe bolometer optical plate. The mechanicallinkbetweenthesetwo stages is built from a material made of an holmium-yttrium(HoY) alloy with high heat capacity at 100 mK. It gives a thermaltime constant of several hours between these stages (Madet2002).The behaviour of the 100 mK part of HFI was extensivelymodelled. The heat input expected on the bolometer plate are theinput from the bias current of the bolometers (less than 1 nW),the microwave radiation reaching the bolometers (0.12 nW),cosmic rays that penetrate the FPU box and deposit energy inthe bolometer plate (0.2 nW), the heat dissipated by microvibrationsin the bolometer plate, and heating from the PID2(about 1 mK temperature increase for 30 nW).Micro-vibrations from the 4 K compressors can add extraheating power on the 100 mK bolometer plate (as was seen duringthe instrument and system tests). The average amount of heatdissipated in the bolometer plate was around 10 nW in the instrumenttests and 40 nW during some periods of the system testsat CSL.Figure 21 shows an example of the predicted gradients onthe bolometer plate when the PID is off. ThethreeHoYlinksto the dilution cooler are seen as the blue (cold) spots throughwhich the heat from the cosmic rays and bolometer bias currentdissipation is transferred towards the dilution stage. The yellowareas are due to recent cosmic rays hits. If the PID2 is turnedon with a typical heat input of 100 nW, the gradients will be300 times steeper. When the heat input varies, the top of theHoY feet will readjust very slowly because of their long timeconstants. The gradient will change accordingly. It is thus believedthat using the PID2 will not improve the temperature stabilityof the bolometer plate for rapid changes of heat inputs. Theslow drifts are damped by PID1 on the dilution plate. Thus PID2is considered as a back-up of PID1 and will not be used in flight.The main temporary perturbations on the 100 mK stageshould come from solar flares. Only a few events at most areexpected during the mission leading to the loss of a few days ofoperation of the dilution cooler.6. Model philosophy and calibrations6.1. Model philosophyThe Planck spacecraft (S/C) development philosophy was basedon two models, the qualification model and the Planck Flightmodel (equipped with the flight models of the instruments).The HFI followed the same approach. The testing programmeof the S/C imposedacryogenicverificationmodelinadditionto the usual space qualification. This was called the CryogenicQualification Model (CQM). The S/C CQMwasrequiredtobeidentical to the PFM for the cold part (PPLM) and similar forthe SVM. The goal was to verify as far as necessary the conceptof the entire cryogenic chain andassociatedancillaryunits.TheHFI model philosophy, numbering and naming were the same,with a CQM HFI partly composed of flight units, or candidatesto be swapped with flight ones later on or to be used as spareunits for the PFM. The qualification of HFI followed a standardtesting sequence for the warm units in compliance with theenvironmental conditions imposed by the mission through theS/C interfaces.AmajorstepfortheHFIwastheperformanceverification of the FPU including the cold parts of the 1.6 K and100 mK of the dilution cooler, equipped with a limited number ofoptical channels. This test allowed the validation and the characterizationof the whole detection chain in a thermal configurationidentical to the flight one, i.e. bolometers at 100 mK, cold filtersat 100 mK, 1.6 K, and 4 K, JFET preamplifier at 110 K, andPage 17 of 20