Cryogenic environment and performance for testing the Planck radiometers

0.75 MB·English

by  L. Terenzi

PreprinttypesetinJINSTstyle-HYPERVERSION
Cryogenic environment and performance for testing
the Planck radiometers
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n
a L. Terenzia∗, M.Lapollad,M. Laaninenb, P.Battagliad,F.Cavalierec, A.DeRosaa,N.
J
Hughese, P.Jukkalae,V.- H.Kilpiäe, G.Morgantea, M.Tomasic,J. Varisf,M.
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2 Bersanellic, R.C.Butlera,F.Ferrarid, C.Franceschetd, P.Leuteneggerd,N.

Mandolesia, A.Mennellac,R.Silvestrid, L.Stringhettia,J.Tuovinenf,L.Valenzianoa
]
M andF.Villaa
I aIstitutodiAstrofisicaSpazialeeFisicaCosmica,INAF
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viaP.Gobetti,101–I40129Bologna,Italy
p
- bYlinenElectronicsOy
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Kauniainen,Finland
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t cUniversitádegliStudidiMilano
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a ViaCeloria16,20133Milano,Italy
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dThalesAleniaSpaceItalia,SedediMilano

1 S.S.PadanaSuperiore,290–I20090Vimodtrone,Italy
v eDA-DesignOy
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4 Jokioinen,Finland
6 fMilliLab,VTTTechnicalResearchCentreofFinland
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Espoo,Finland
.
1 Email:[email protected]
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: ABSTRACT: The Planck LFI Radiometer Chain Assemblies (RCAs) were calibrated in two dedi-
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cated cryogenic facilities. Inthispaperthefacilities andtherelatedinstrumentation aredescribed.
i
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Themainsatellitethermalinterfacesforthesinglechainshadtobereproducedandstabilityrequire-
r
a ments had to besatisfied. Setup design, problems occurred and improving solutions implemented
arediscussed. Performanceofthecryogenic setuparereported.
KEYWORDS: experimental cosmology; spaceinstrumentation; radiometers; calibration methods;
cryogenics.
∗
Correspondingauthor.
Contents
1. Introduction 1
2. Cryogenicfacility setupfor30–44GHzradiometers 2
2.1 Theradiometer thermalscheme 2
2.2 Thecryogenic chamber 2
2.3 Thermalsetupandmaininterfaces 3
2.3.1 Thewarmstage 5
2.3.2 Thecoldstage 5
2.4 Thecoolers 6
2.5 Temperaturesensors, monitoring andcontrol 7
2.6 Thermalperformance 8
2.6.1 Temperaturedistribution 8
2.6.2 Temperaturestability 8
3. Facilitysetupforthe70GHzradiometers 9
3.1 Mainthermalinterfaces 10
3.2 Thecoolers 10
3.3 Temperaturesensors, monitoring andcontrol 11
3.4 Thermalperformance 11
4. Conclusions 12
1. Introduction
ThePlanck ([10])LowFrequency Instrument (LFI,[6,1])isanarrayof22pseudo-correlation ra-
diometers, whose main sensitive devices are cryogenic HEMT-based low noise amplifiers located
in the Front End Modules (FEMs). The signal coming from the sky is compared continously to
the emission of a reference load (4K RL, [14]) in order to reduce the impact of gain fluctuations.
The reference loads are small blackbodies connected to the outer shield of the High Frequency
Instrument (HFI,[4])andkeptatastabletemperature ofabout4K.
A flight-like low temperature environment is needed during ground calibrations [15, 7] of the in-
strument.
Animportant requirement forthetestingfacilities istoprovide thepossibility tochange, inacon-
trolled way, the temperatures of some of the devices in order to ensure tests to be run in a correct
way.
Radiometers’ linearity andcalibration constant [8]areverifiedbychanging thetemperature ofthe
reference loadsandofdedicated skysimulators, whilefocalplanetemperature variations effecton
–1–
theradiometers[11]isevaluatedbychangingthetemperatureofthefrontendmodules. Astability
controlat1%levelwithrespecttosingletemperaturesteps,typicallyrangingfrom2to5K,isthen
required forthesemainstagesduringcalibration tests.
Inthispaper, thecryogenic facilities usedfortheground tests oftheLFIsingle Radiometer Chain
Assemblies(RCAs)aredescribed.
Twodifferentchamberssharing thesamedesignphilosophy wereused:
(1) the 30 and 44 GHz channel radiometers ([2]) were tested in the Thales Alenia Space - Italy
(Milano)laboratories, while
(2)the70GHzchannelradiometerchains([3])weretestedintheDA-Design(YlinenElectronics)
laboratories, inFinland.
InSection2thethermalsetupoftheradiometerchainassembliesisdescribedandanoverview
of the chamber used inMilano isgiven, specifically the mechanical setup, the main thermal inter-
facesdesignandthecoolersusedtoobtainthedesiredtemperatures, togetherwiththetemperature
monitoring andcontrolsetupandadescription ofthesensortypesandlocations. Theperformance
reached duringthecalibration testsintermsoftemperature stability, togetherwithsomeoptimiza-
tionstudy,isprovidedaswell.
Themaindifferences inthe70GHzchannelchamberaredetailedinSection3.
Finally, inSection4,conclusions areaddressed.
2. Cryogenicfacilitysetup for30–44GHzradiometers
2.1 Theradiometerthermalscheme
A single LFI radiometer chain spans a large range of temperatures from the Back End Module
(BEM), at about 300 K room temperature, to the Front End Module at 20 K. They are connected
by means of four long waveguides, about 1.7 meters long. The materials used for the waveguides
allow a good inner signal transmission while having a good thermal insulation; in particular, the
warmestpartisbuiltinstainlesssteelinordertocreatealargethermalgradient betweenthewarm
300Kstageandthecoldest stages.
Atsatellitelevel,threelargeradiativeshields(theV-grooves)areinterfacedtothewaveguidesand,
inflight,willhelpthemtodissipate heatloadstothesurrounding 3Kenvironment.
A set of 4K reference loads is part of the LFI instrument, although mechanically and thermally
linked totheHFIouter shield. Theloads, together withadedicated skysimulator load, have tobe
taken into account in the design of a ground test cryogenic facility as well. The overall thermal
schemeofaRadiometerChainAssembly,integrated inthefacility, isshowninFig. 1a.
2.2 Thecryogenicchamber
The size of the facility is about 2 × 1.2 × 1 m3. Such size is compatible with the volume of the
radiometer chains, drivenbythewaveguideslengthandfine-shaping.
Acoppershieldatatemperature ofabout50Kisusedtoradiatively insulatethecoldestparts
of the system, namely the FEM and horn at 20K, the 4K reference load and a passive calibration
tool(theskyload)simulating theskymicrowavebackground emission,tobekeptatabout4K.
–2–
(a)
(b)
Figure 1. (a) A schematic view of the 30 and 44 GHz RCA thermal setup integrated in the cryogenic
chamber. Main thermal interfaces are shown. (b) Setup of a 30 GHz RCA, integrated in the chamber;
insulatingsupportoftheBEMandthirdV-grooveinterfacearevisible.
The chamber is provided with three cryocoolers and a vacuum bench allowing to reach pressures
lowerthan10−5 mBar.
2.3 Thermalsetupandmaininterfaces
Asmentionedabove,thePlanckLFIcoversalargerangeoftemperaturestages,startingfromabout
300Katthebackendleveldowntothe4Kreferenceload.
A set of thermal interfaces in the cryogenic chamber provides an optimized and stable working
–3–
environment fortheradiometers.
The seven main interfaces are listed here together with their working temperatures (see also Fig.
1):
• thebackendmoduleiscontrolled atatemperature ofabout310K;
• the third and coldest V-groove interface with the waveguides is controlled at a temperature
ofabout60K;
• thecopper-stainless steelinterface ofthewaveguides istemperature controlled at22K;
• theshieldsurrounding thelowertemperaturezoneiskeptatabout50K;
• thefrontendmoduleistemperaturecontrolled at20K;
• the4Kreference loadiscontrolled inarange8-20K;
• theskyloadistemperaurecontrolled intherange8-20K.
The possibilty to control and induce temperature changes is also needed in order to perform
calibrations [8]andcheckthetemperature fluctuationeffectsontheinstrument [11,13].
Multilayerinsulation(MLI)isextensivelyusedinordertoinsulatefromradiativeheatloadmainly
the sky load large surface and the long waveguides path. Due to the different routing of the RCA
waveguides, different assemblingandsupporting structures weredesigned andbuiltinordertoget
the optimal test setup. This led to some differences in the heat load and temperature control of
some stages. In Fig. 2, the thermal network and main heat routes, detailed in the following, are
represented.
Figure2.Thermalnetworkofthechamber.Mainthermalinterfacesandheatloadsareshown,togetherwith
coolingpowersofthedifferentcoolersstages.
–4–
2.3.1 Thewarmstage
Themechanicalsupportinthestageswarmerthanthecoldshieldisprovidedbytwomainstructures
(visibleinFig. 1b),onesupportingtheBEMtheotheratthelevelofthethirdV-grooveinterfaceof
the waveguides, andafurther support provided byakevlarwireholding thecopper-steel interface
ofthewaveguides. MLIisusedtoradiatively shieldallthewaveguides route.
Thelayout andmaterialsusedforthetwomainsupports arethesameforallthechains under test;
fourcolumns ofPEEKpolymer(halfinchsection fortheBEM,quarter inchfortheVG3support)
are alternated to small insulating bases of hostaform and teflon. The length of the supports is
instead depending on the orientation of the waveguides and horns in the chamber. This kind of
insulation is enough for allowing a good control of the BEM temperature, since the latter is very
closetoroomtemperature.
ThemaximumpassiveloadestimatedontheVG3interface, keptat60K,isabout0.2W.
The thin kevlar wire linked to a stainless steel 304L support have a negligible impact (less than 1
mW)intermsofheatloadontheinterface,whilewarmerstagesofwaveguidescontributelessthan
50mW.
2.3.2 Thecoldstage
Thelarge shieldcovering thecoldstagehasasizeof0.72×0.53×0.60m3. Itismadeofcopper
and all its surfaces are covered with an MLIinsulating layer. Some mechanical details have been
takenintoaccount forthecorrectintegration inthechamber.
Thelargedimensionoftheshieldsuggestedtoavoidtoomanymechanicalboundariesinfixingthe
shield tothechamberbase. Onlyoneofthesupports, consisting inhostaform columns, isscrewed
tothebase. Theothershaveanhollowbasewhereastainless steelballsits.
This design reduces the stress due to thermal contraction during cooldowns while minimizing the
contact surface to the chamber at room temperature. Another set of thin hostaform supports at its
sideskeepsthecoldshieldstable.
Thesesolutionsallowtokeeptheconductiveloadontheshieldtolessthan200mWandtheglobal
loadtoabout450mW,whentakingintoaccount alsotheradiative loadonthelargereflecting sur-
face.
The 50 K shield contains, also radiatively insulating them from the room temperature walls, the
coldest unitsoftheLFIradiometers.
The front end module is connected by means of a stainless steel thin frame, alternated with teflon
supporting bars,totheshieldbase. Thesupports arealsoholdingthefeedhornflangebymeansof
adedicated insert.
Thepurposeoftheseinsulating supports istokeeptheheatloadontheFEMwithin100mW.
The reference load is mechanically supported by the FEM itself, by means of a similar dedicated
stainless steel frame, in order to thermally insulate itfrom the 20 K stage. Theconductive load is
measured tobelessthan100mW.
Thelargeststructureinsidethecoldpartofthechamberistheskysimulatorcylinder,whosediam-
eterandlengthareabout 25cmandweightabout 8Kg. Itisbuiltinabsorbing material (Eccosorb
CR110) and its external surface is covered with MLI. It is supported by a stainless steel AISI304
structurewhosebaseislinkedtothe40Kshieldbasebymeansofsmallhostaformbars. Theglobal
–5–
loadonitismeasuredtobeabout200mW.
2.4 Thecoolers
Thecontrolofmanytemperaturestagesisrequired,inparticularforthecoldesttemperatureswhich
have to be varied during many of the calibration tests. Three two-stage coolers are then used to
provide therightreference temperature tothedifferentpartsoftheinstrument.
• Both stages of a 20 K CTI Cryogenics cooler are used to keep the temperature of the 60 K
V-grooveandthe22Kstainlesssteel-copperwaveguidesinterface attheirnominalvalues.
• Firstandsecondstagesofafurther20Kcooler(LeyboldCoolpower5/100)areusedtokeep
the temperature of the radiative shield surrounding the cold part of the instrument at about
50Kandthetemperature oftheFEMat20K,respectively.
• First and second stages of a 4 K cooler (Advanced Research System Inc. DE210) are used
to keep the temperature of the 50 K radiative shield, as above, and the temperature of the
reference andskyloadsatabout8and10K,respectively.
Thecoolers’ model and cooling powerare reported in Table1. Asexpected, comparing their
cooling powers to the heat loads on the various stages, the coolers have a good margin and then
easilykeepstagesabove20Kattheirnominaltemperatures.
CoolerModel ARS Leybold CTICryogenics
DE-210 CoolPower5/100 Model1020C
1stStageNominalT(K) 41.0 80.0 65
1stStageCoolingPower@Nom. T(W) 35 100 30
2ndStageNominalT(K) 4.2 20.0 20
2ndStageCoolingPower@Nom. T(W) 0.8 5 12
Table1. Maincharacteristicsofthecoolersusedinthecryogenictestchamber.
Skyandreferenceloadsareinsteadoperated attemperatures higherthanthenominalones.
Intheearlyruns, the4Kcooler, actually, showedahightemperature instability (1Kpeaktopeak
amplitude) andacooling powerlowerthannominal(Fig. 3).
After some dedicated dry tests, a different configuration was decided for the cooler. It was
mountedonthesideofthechamber,withthecoldfingerpointingattheground. AnU-likethermal-
vacuumpathwasbuiltbymeansofacopperrod,whichhavebecomethenewcoldreferenceinside
thecryogenic chamber.
Asaresult, thetemperature during thetestsisabout6K,comparable totheformercoldheadone,
butwithaconsiderably improvedstability.
As a consequence less power is needed for the stabilization of the loads (Fig. 4), keeping their
temperature aslowaspossible.
Thisallowsteststobeperformed inalmostnominalconditions.
–6–
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SKY CTRL
REF
9 4K ARS
8
7
K)
T (
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65:00 66:00 67:00 68:00 69:00 70:00 71:00 72:00
Time (hh:mm)
(a) (b)
Figure 3. (a) Coolingpower at differentcold end temperatures: in the early setup (red curve), the cooler
is far fromthe nominal(bluecurve)behaviour,in particularat the lower temperatures. (b) Thisresults in
an unstable temperaturesink for the referenceand sky loads, which have to be controlledat temperatures
higherthan8Kand10K,respectively.

8
7 SKY CTRL
REF CTRL
Cooler
T (K) 6 Finger
5
4
3
103:40 105:36 107:31
Time (hh:mm)
Figure4.Inthenewsetupthecoolercoldheadreachesnominaltemperaturesandthenewcoldfingerinside
thechamberismuchmorestable,allowingacontrolofbothloadsatlessthan8K.
2.5 Temperaturesensors,monitoringandcontrol
The large temperature range spanned by the instrument for the calibration tests needs an accurate
monitoring andcontrol. Asetof18temperature sensorsisthenusedinthetestcampaign.
Acouple ofLakeshoreTM PT-100platinum resistance thermometers areused fortheBEMcontrol
and monitoring close to the room temperature. Silicon diodes (DT470 and DT670) and Cernox
resistors with a sensitivity better than 1 mK are used for the low temperatures to control stages
whosestability isrelevantforasuccessful testcampaign.
The temperature readout consists of two LakeshoreTM LSC331 temperature controllers (two read
–7–
outandcontrolchannels)andtwoLSC340temperaturecontrollers(onewithtworeadoutandcon-
trolchannelsand8readout-onlychannels,theotherwithtworeadoutandcontrolchannelsandtwo
readout only channels). These allows to control six of the seven stages listed in the Section 2.3.
Everytemperaturestage,withtheexceptionoftheradiativeshield,isactivelycontrolledbyheaters
usingPIDalgorithms.
Temperaturecontrollersandmonitorsareinterfaced,viaGPIBboards,toadedicatedPCanddriven
by adedicated software developed inthe LabWindows environment, which also handles thecom-
munication with a breadboard model of the Data Acquisition Electronics (DAE) supplying the
powertotheradiometer devices andacquiring datafromthedetectors. Finallyaquicklookmoni-
toringisperformedbyanothersoftware,RACHEL[5],whichalsoconvertsdatainFITSformatto
prepare theoff-lineanalysis[12].
2.6 Thermalperformance
The main performance requirements for the cryogenic chamber are the cooling of the entire LFI
radiometerchainsdowntothenominallowtemperatures,allowingboththecorrectthermalstability
andthecorrecttemperature steps, whenneededbytheprocedures.
2.6.1 Temperaturedistribution
During the whole flight model test campaign, the chamber setup and the temperature control has
allowedagoodrepeatibility inthetemperaturedistribution forthestagesintherange20-310K,as
reported intheTable2.
Temperaturestage T(K)
FEMcontrol 20.0
CU/SSInterface 22.0
BEMcontrol 310.0
BEMbody ∼310.0
ThirdV-Groove ∼60.0
ColdShroud ∼50.0
Table2. TypicalsetoftemperaturesobtainedforthewarmeststagesoftheRCAs.
Due to the different orientation and routing of the feed horns and waveguides, different sky
loadsupportstructures areusedleadingtodifferentthermalpathsbetweenthe50Kshieldandthe
loadbase. Thisleadstoaheatloadvariation onthe4Kcooler, depending ontheRCAundertest.
InTable 3, as representative for theminimum controlled temperature reached forthe sky and
reference loadbackplates, valuesfromnoiseproperties dataanalysis arereported foreachRCA.
2.6.2 Temperaturestability
Temperature instabilities are the most important thermal systematic effects for the LFIinstrument
alsoduringcalibrations.
Thelargenumberoftemperaturestagescontrolledduringthetestsisrequiredtokeepundercontrol
this dangerous source of systematics. The design of the test and dedicated corrections adopted
throughout thecampaignensurethehighlevelofstabilityneeded formanyofthetests.
–8–
RCAID# SkyLoadT(K) RefLoadT(K)
24 8.5 8.5
25 10.5 8.0
26 13.3 8.0
27 12.8 9.5
28 8.6 8.5
Table 3. Minimumstabletemperaturesreachedforthenoisepropertiestestbytheskyandreferenceload
controlbackplates
Typically, the short term peak-to-peak temperature variation is kept under a level of 5 mK for the
lowesttemperature stages.
3. Facilitysetupforthe 70GHzradiometers
Thecryogenicfacilityhasavolumeallocationofabout1.6×1×0.3m3. Thedesignofthefacility
isverysimilartothe30-44GHzone,eventhoughthe70GHzfacilityisdesignedinordertohouse
two radiometer chains at once (Fig. 5). This is possible in terms of thermal budget, due to the
smallersizeoftheradiometer units.
The heat transfer through the waveguides is reduced mainly by the effective section being about
0.45timesthesectionofthe30GHzguides. Intermsofglobalenvelopeinsidethecoldshield,the
sizesofthefeedhornsandfrontendmodulesarereducedandthechoicetousetwosmalldedicated
sky loads directly infront ofthe horns allowstouse thesame volume than forthe30 and44 GHz
one.
Figure5. AglobalviewoftwoRCAsintegratedinthe70GHzchannelcryogenicchamber.
–9–