Planck Early Results. II. The thermal performance of Planck

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by  Planck Collaboration

Astronomy&Astrophysicsmanuscriptno.Planck2011-1.3 (cid:13)c ESO2012
January4,2012
Planck Collaboration: Planck Early Results. II. The thermal
performance of Planck
PlanckCollaboration:P.A.R.Ade76,N.Aghanim49,M.Arnaud62,M.Ashdown60,4,J.Aumont49,C.Baccigalupi74,M.Baker34,A.Balbi29,
A.J.Banday81,8,67,R.B.Barreiro56,E.Battaner82,K.Benabed50,A.Benoˆıt48,J.-P.Bernard81,8,M.Bersanelli26,41,P.Bhandari58,R.Bhatia5,
J.J.Bock58,9,A.Bonaldi37,J.R.Bond6,J.Borders58,J.Borrill66,77,F.R.Bouchet50,B.Bowman58,T.Bradshaw73,E.Bre´elle3,M.Bucher3,
C.Burigana40,R.C.Butler40,P.Cabella29,P.Camus48,C.M.Cantalupo66,B.Cappellini41,J.-F.Cardoso63,3,50,A.Catalano3,61,L.Cayo´n19,
A.Challinor53,60,11,A.Chamballu46,J.P.Chambelland58,J.Charra49,M.Charra49,L.-YChiang52,C.Chiang18,P.R.Christensen70,30,
D.L.Clements46,B.Collaudin79,S.Colombi50,F.Couchot65,A.Coulais61,B.P.Crill58,71,M.Crook73,F.Cuttaia40,C.Damasio35,L.Danese74,
R.D.Davies59,R.J.Davis59,P.deBernardis25,G.deGasperis29,A.deRosa40,J.Delabrouille3,J.-M.Delouis50,F.-X.De´sert44,K.Dolag67,
S.Donzelli41,54,O.Dore´58,9,U.Do¨rl67,M.Douspis49,X.Dupac33,G.Efstathiou53,T.A.Enßlin67,H.K.Eriksen54,C.Filliard65,F.Finelli40,
S.Foley34,O.Forni81,8,P.Fosalba51,J.-J.Fourmond49,M.Frailis39,E.Franceschi40,S.Galeotta39,K.Ganga3,47,E.Gavila79,M.Giard81,8,
G.Giardino35,Y.Giraud-He´raud3,J.Gonza´lez-Nuevo74,K.M.Go´rski58,84,S.Gratton60,53,A.Gregorio27,A.Gruppuso40,G.Guyot43,
2
1 D.Harrison53,60,G.Helou9,S.Henrot-Versille´65,C.Herna´ndez-Monteagudo67,D.Herranz56,S.R.Hildebrandt9,64,55,E.Hivon50,M.Hobson4,
0 W.A.Holmes58,A.Hornstrup13,W.Hovest67,R.J.Hoyland55,K.M.Huffenberger83,U.Israelsson58,A.H.Jaffe46,W.C.Jones18,M.Juvela17,
2 E.Keiha¨nen17,R.Keskitalo58,17,T.S.Kisner66,R.Kneissl32,5,L.Knox21,H.Kurki-Suonio17,36,G.Lagache49,J.-M.Lamarre61,P.Lami49,
A.Lasenby4,60,R.J.Laureijs35,A.Lavabre65,C.R.Lawrence58(cid:63),S.Leach74,R.Lee58,R.Leonardi33,35,22,C.Leroy49,81,8,P.B.Lilje54,10,
n
M.Lo´pez-Caniego56,P.M.Lubin22,J.F.Mac´ıas-Pe´rez64,T.Maciaszek7,C.J.MacTavish60,B.Maffei59,D.Maino26,41,N.Mandolesi40,
a
J R.Mann75,M.Maris39,E.Mart´ınez-Gonza´lez56,S.Masi25,S.Matarrese24,F.Matthai67,P.Mazzotta29,P.McGehee47,P.R.Meinhold22,
A.Melchiorri25,F.Melot64,L.Mendes33,A.Mennella26,39,M.-A.Miville-Descheˆnes49,6,A.Moneti50,L.Montier81,8,J.Mora58,G.Morgante40,
2
N.Morisset45,D.Mortlock46,D.Munshi76,53,A.Murphy69,P.Naselsky70,30,A.Nash58,P.Natoli28,2,40,C.B.Netterfield15,D.Novikov46,
] I.Novikov70,I.J.O’Dwyer58,S.Osborne78,F.Pajot49,F.Pasian39,G.Patanchon3,D.Pearson58,O.Perdereau65,L.Perotto64,F.Perrotta74,
M F.Piacentini25,M.Piat3,S.Plaszczynski65,P.Platania57,E.Pointecouteau81,8,G.Polenta2,38,N.Ponthieu49,T.Poutanen36,17,1,G.Pre´zeau9,58,
M.Prina58,S.Prunet50,J.-L.Puget49,J.P.Rachen67,R.Rebolo55,31,M.Reinecke67,C.Renault64,S.Ricciardi40,T.Riller67,I.Ristorcelli81,8,
I
. G.Rocha58,9,C.Rosset3,J.A.Rubin˜o-Mart´ın55,31,B.Rusholme47,M.Sandri40,D.Santos64,G.Savini72,B.M.Schaefer80,D.Scott16,
h
M.D.Seiffert58,9,P.Shellard11,G.F.Smoot20,66,3,J.-L.Starck62,12,P.Stassi64,F.Stivoli42,V.Stolyarov4,R.Stompor3,R.Sudiwala76,
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- J.-F.Sygnet50,J.A.Tauber35,L.Terenzi40,L.Toffolatti14,M.Tomasi26,41,J.-P.Torre49,M.Tristram65,J.Tuovinen68,L.Valenziano40,L.Vibert49,
o P.Vielva56,F.Villa40,N.Vittorio29,L.A.Wade58,B.D.Wandelt50,23,C.Watson34,S.D.M.White67,A.Wilkinson59,P.Wilson58,D.Yvon12,
r A.Zacchei39,B.Zhang58,andA.Zonca22
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Preprintonlineversion:January4,2012

2
v
3 ABSTRACT
2
0 TheperformanceofthePlanck’sinstrumentsinspaceisenabledbytheirlowoperatingtemperatures,20KforLFIand0.1KforHFI,achieved
2 through a combination of passive radiative cooling and three active mechanical coolers. The scientific requirement for very broad frequency
. coverageledtotwodetectortechnologieswithwidelydifferenttemperatureandcoolingneeds.Activecoolerscouldsatisfytheseneeds;ahelium
1
cryostat,asusedbypreviouscryogenicspacemissions(IRAS,COBE,ISO,Spitzer,Akari),couldnot.RadiativecoolingisprovidedbythreeV-
0
grooveradiatorsandalargetelescopebaffle.Theactivecoolersareahydrogensorptioncooler(<20K),a4HeJoule-Thomsoncooler(4.7K),anda
1
3He-4Hedilutioncooler(1.4Kand0.1K).Theflightsystemwasatambienttemperatureatlaunchandcooledinspacetooperatingconditions.The
1
HFIbolometerplatereached93mKon3July2009,50daysafterlaunch.ThesolarpanelalwaysfacestheSun,shadowingtherestofPlanck,and
:
v operatesatameantemperatureof384K.Attheotherendofthespacecraft,thetelescopebaffleoperatesat42.3Kandthetelescopeprimarymirror
i operatesat35.9K.Thetemperaturesofkeypartsoftheinstrumentsarestabilizedbybothactiveandpassivemethods.Temperaturefluctuations
X
aredrivenbychangesinthedistancefromtheSun,sorptioncoolercyclingandfluctuationsingas-liquidflow,andfluctuationsincosmicrayflux
r onthedilutionandbolometerplates.Thesefluctuationsdonotcompromisethesciencedata.
a
Keywords. Cosmology–Cosmicmicrowavebackground–Spaceinstrumentation–Instrumentdesignandcalibration
1. IntroductionPlanck
Planck1 (Tauberetal.2010;PlanckCollaboration2011a)isthe
thirdgenerationspacemissiontomeasuretheanisotropyofthe
1 Planck (http://www.esa.int/Planck) is a project of the
EuropeanSpaceAgency(ESA)withinstrumentsprovidedbytwosci-
(cid:63) Corresponding author: C. R. Lawrence <charles.lawrence@ entificconsortiafundedbyESAmemberstates(inparticularthelead
jpl.nasa.gov> countriesFranceandItaly),withcontributionsfromNASA(USA)and
1
PlanckCollaboration:ThethermalperformanceofPlanck
cosmic microwave background (CMB). It observes the sky in ements,threeV-grooveradiators,andatelescopebafflewithlow
ninefrequencybandscovering30–857GHzwithhighsensitiv- emissivityinsideandhighemissivityoutside.
ity and angular resolution from 31(cid:48) to 5(cid:48). The Low Frequency
Instrument (LFI; Mandolesi et al. 2010; Bersanelli et al. 2010;
2.1.1. Missiondesign,scientificrequirements,andthermal
Mennella et al. 2011) covers the 30, 44, and 70GHz bands
architecture
withamplifierscooledto20K.TheHighFrequencyInstrument
(HFI;Lamarreetal.2010;PlanckHFICoreTeam2011a)covers
Planck is designed to extract all information in the tempera-
the 100, 143, 217, 353, 545, and 857GHz bands with bolome- tureanisotropiesoftheCMBdowntoangularscalesof5(cid:48),and
ters cooled to 0.1K. Polarisation is measured in all but the
to provide a major advance in the measurement of polarisation
two highest bands (Leahy et al. 2010; Rosset et al. 2010). A
anisotropies. This requires both extremely low noise and broad
combination of radiative cooling and three mechanical cool-
frequencycoveragefromtenstohundredsofgigahertztosepa-
ers produces the temperatures needed for the detectors and op-
rateforegroundsourcesofradiationfromtheCMB.Theneces-
tics(PlanckCollaboration2011b).Twodataprocessingcentres
sary noise level can be reached only with cryogenically-cooled
(DPCs) check and calibrate the data and make maps of the sky
detectors.Thelowestnoiseisachievedwithamplifierscooledto
(Planck HFI Core Team 2011b; Zacchei et al. 2011). Planck’s
≤ 20Kandbolometerscooledto∼ 0.1K.Temperaturefluctua-
sensitivity,angularresolution,andfrequencycoveragemakeita
tionsmustnotcompromisethesensitivity.Additionalconstraints
powerfulinstrumentforGalacticandextragalacticastrophysics on Planck that affect the thermal design include: 1) no deploy-
as well as cosmology. Early astrophysics results are given in
ables (e.g., a shield that could block the Sun over a large solid
PlanckCollaboration(2011d–u).
angle);2)noopticalelementssuchaswindowswithwarmedges
The unprecedented performance of the Planck instruments
between the feed horns and telescope; 3) 1.5yr minimum total
in space is enabled by their low operating temperatures, 20K lifetime;4)aspinningspacecraft;5)anoff-axistelescopebelow
for LFI and 0.1K for HFI, achieved through a combination of
60K;6)feedhornsforthebolometersbelow5Kandabolome-
passive radiative cooling and three active coolers. This archi-
ter environment below 2K; 7) reference targets (loads) for the
tecture is unlike that of the previous CMB space missions, the
pseudo-correlationamplifierradiometersbelow5Ktominimize
CosmicBackgroundExplorer(COBE;Boggessetal.1992)and
1/f noise;and8)0.5Wheatliftforthe20Kamplifiers.
the Wilkinson Microwave Anisotropy Probe (WMAP; Bennett
These requirements (stated precisely in Tables 1, 2, and 5,
etal.2003).COBEusedaliquidheliumcryostattoenablecool-
andinSect.2.1.2)ledtoadesignthatincludesthefollowing:
ing of the bolometers on its Far-Infrared Absolute spectropho-
tometer(FIRAS)instrument(Matheretal.1990)to1.5K.This
approachwasnotadoptedforPlanckasitwouldrestricttheon- – A“warmlaunch”scenario,inwhichtheentireflightsystem
orbitlifetimefortheHFI,requireadditionalcoolerstoreachsub- isatambienttemperatureforlaunch.Thisallowsaveryclean
Kelvin temperatures, and be entirely infeasible for cooling the environmentfromthestraylightpointofview.
activeheatloadfromtheLFI.WMAPreliedonpassiveradiative – The overall thermal architecture shown in Fig. 1, with the
coolingalonewhich,whilesimpler,resultedinahigheroperat- solarpanelactingasaSunshield,andtemperaturedecreas-
ingtemperatureforitsamplifiersandahighernoisetemperature. ingalongthespinaxistowardthecoldendandthepassively
Additionally,purelypassivecoolingisunabletoreachthesub- cooledtelescope.
kelvinoperatingtemperaturesrequiredbyHFI’shigh-sensitivity – Extensive use of passive (radiative) cooling, especially the
bolometers. V-grooveradiatorsandthetelescopeandtelescopebaffle.
In this paper we describe the design and in-flight perfor- – Detectorsbasedonamplifiersat30,44,and70GHz,andon
manceofthemission-enablingPlanckthermalsystem. bolometersat100,143,217,353,545,and857GHz.
– An active cooling chain with three mechanical coolers. No
large helium cryostat had ever been flown on a spinning
2. ThermalDesign spacecraft. A focal plane with detectors at 0.1K and 20K
and reference loads at less than 5K would have been diffi-
2.1. Overview,philosophy,requirements,andredundancy culttoaccomodateinacryostat.Aheatliftof0.5Wat20K
would have required a huge cryostat of many thousands of
The thermal design of Planck is driven by the scientific re-
liters.
quirement of very broad frequency coverage, implying two in-
strumentswithdetectortechnologiesrequiringdifferentcooling 1. The “sorption cooler” (Fig. 2), a closed-cycle sorption
temperatures (20K and 0.1K) and heatlifts (0.5W and 1µW). coolerusinghydrogenastheworkingfluidwithaJoule-
This, combined with a scanning strategy based on a spinner Thomson (JT) expansion, which produces temperatures
satellite,ledtothechoiceofanactivecoolingsystem.Theover- below 20K and a heat lift close to 1W. The sorption
all architecture can be understood from Fig. 1. The solar panel coolercoolstheLFIfocalplaneto< 20Kandprovides
always faces the Sun and the Earth, the only two significant precooling to lower temperature stages. Although this
sources of heat in the sky, and operates at 385K. The service cooler was a new development, it was the only one that
vehiclemodule(SVM)operatesatroomtemperature.Thetele- could provide such a large heatlift at the required tem-
scope, at the opposite end of the flight system, operates below perature.
40K. The detectors at the focus of the telescope are actively 2. The“4He-JTcooler”(Fig.3),aclosed-cyclecoolerusing
cooled to 20K (amplifiers) or 0.1K (bolometers). Between the aStirlingcyclecompressorand4Heastheworkingfluid
SVMandthe“coldend,”aggressivemeasuresaretakentomin- withaJTexpansion,whichproducestemperaturesbelow
imize heat conduction and to maximize the radiation of heat to 5K. The 4He-JT cooler cools the structure hosting the
coldspace.Thesemeasuresincludelow-conductivitysupportel- HFIfocalplaneandtheLFIreferenceloadsto<5Kand
provides precooling to the dilution cooler. The chosen
telescopereflectorsprovidedbyacollaborationbetweenESAandasci- technologyhadalongflightheritageforthecompressors
entificconsortiumledandfundedbyDenmark. andaspecificdevelopmenttominimizemicrovibrations.
2
PlanckCollaboration:ThethermalperformanceofPlanck
36 K Primary mirror
42 K Telescope baffle
FPU
Secondary mirror
46 K V-groove 3
90 K V-groove 2
140 K V-groove 1
270 K SVM
385 K Solar panel
Fig.1.CutawayviewofPlanck,withthetemperaturesofkeycomponentsinflight.Thesolarpanelatthebottomalwaysfacesthe
SunandtheEarth,andistheonlypartoftheflightsystemilluminatedbytheSun,theEarth,andtheMoon.Temperaturedecreases
steadily towards the telescope end, due to low-conductivity mechanical connections and aggressive use of radiative cooling. The
focalplanedetectorsareactivelycooledto20Kand0.1K.
3. The“dilutioncooler”(Fig.4),a3He-4Hedilutioncooler made the cryogenic chain the most challenging element in the
that vents combined 3He and 4He to space, and which Planckmission.
producestemperaturesof1.4KthroughJTexpansionof
the3Heand4He,and∼0.1Kforthebolometersthrough
the dilution of 3He into 4He. Although this was a new 2.1.2. Requirementsonthecoolers
technology,itwaschosenforitssimplearchitecture,con-
The three coolers were required to deliver temperatures of <
tinuous operation, and temperature stability. An ADR
20K, < 5K, and 0.1K, continuously. Although use of proven
suitable for HFI would have to operate from a 4K heat
technology is preferred in space missions, the special require-
sinkratherthanthe1.5KheatsinkavailableonAstro-E,
mentsofPlanck ledtothechoiceoftwonew-technologycool-
and would have to be scaled up in mass by a factor of
ers.
10tolift8µWat0.1K(Triqueneauxetal.2006).Thisis
The first is a hydrogen sorption cooler developed by the
becausethedilutionliftsheatnotonlyat100mKwhere
JetPropulsionLaboratoryinCalifornia,whichprovidesalarge
the3Heand4Helinescombine,butalsoallalongthere-
heat lift with no mechanical compressors (avoiding vibration).
turn line (which is attached to the struts and wiring) as
The sorption cooler requires precooling of the hydrogen to ≤
themixturewarms,reducingtherequiredliftat100mK
60K.Theprecoolingtemperaturerequiredbythe4He-JTcooler,
fromabout8µWtoonly1µW.IfanADRsimilartothat
which is supplied by the sorption cooler, is ≤ 20K. This in-
ofastro-ewereusedforPlanck,itwouldhavetoliftthe
terface temperature is critical in the cooling chain: the 4He-JT
entire8µWat100mK.Furthermore,ADRsrequirehigh
coolerheatloadincreaseswiththeinterfacetemperaturewhen,
magnetic fields and cycling, not easily compatible with
atthesametime,itsheatliftdecreases.Thusthegoalforthepre-
continuousmeasurementsandtheveryhighstabilityre-
cooltemperaturesuppliedbythesorptioncoolerwas18K,with
quirementsofPlanck.
astrictrequirementof≤ 19.5Ktoleaveadequatemargininthe
coolingchain.
The above design results in a complicated architecture and The second new cooler is a 3He-4He dilution cooler. The
testphilosophy.Thetwoinstruments,passiveradiators,andac- microgravity dilution cooler principle was invented and tested
tivecoolerscannotbeseparatedeasilyfromthespacecraft,either by A. Benoˆıt (Benoˆıt et al. 1997) and his team at Institut Ne´el,
mechanicallyorthermally.Thecomponentsoftheflightsystem Grenoble,anddevelopedintoaspacequalifiedsystembyDTA
are highly interdependent, and difficult to integrate. All of this Air Liquide (Triqueneaux et al. 2006) under a contract led by
3
PlanckCollaboration:ThethermalperformanceofPlanck
Fig.2. Sorption cooler system. The system is fully redundant. One of the compressor assemblies, mounted on one of the warm
radiator panels, which faces cold space, is highlighted in orange. Heat pipes run horizontally connecting the radiators on three
sidesoftheservicevehicleoctagon.Thesecondcompressorassemblyistheblackboxontheright.Atube-in-tubeheatexchanger
carrieshighpressurehydrogengasfromthecompressorassemblytothefocalplaneassemblyandlowpressurehydrogenbackto
thecompressor,withheat-exchangingattachmentstoeachofthethreeV-grooves.Coloursindicatetemperature,fromwarm(red,
orange,purple)tocold(blue).
Fig.3. 4He-JT cooler system. The back-to-back compressors are highlighted in gold on the left-hand side of the service vehicle,
adjacenttocontrolelectronicsboxes.Thehighandlowpressure4HegastubesconnectingthecompressorswiththeJTvalveinthe
focalplanearecolouredfrompurpletoblue,indicatingtemperatureasinFig.2.
4
PlanckCollaboration:ThethermalperformanceofPlanck
Fig.4.Dilutioncoolersystem.Fourhighpressuretanksof3Heand4Hearehighlightedinsilver.Thedilutioncoolercontrolunit
(DCCU),towhichpipingfromthetanksandtothefocalplaneunitisattached,ishighlightedontheleft.
GuyGuyotandthesystemgroupatIAS.Itprovidestwostages mance significantly. The 4He-JT cooler provides the reference
of cooling at 1.4K and 100mK. The total heat lift requirement loadsfortheLFI,andisthusaquasi-singlepointfailure;how-
at100mKwas0.6µW,andcouldbeachievedwithanopencir- ever, it was not made redundant because of its good flight her-
cuit system carrying enough 3He and 4He for the mission. The itage and in view of the extra resources it would have required
precoolingtemperaturerequiredwaslessthan4.5K. fromthespacecraft.
Thisprecoolisprovidedbythethirdcooler,aclosed-circuit
4He JT expansion cooler driven by two mechanical compres-
2.1.4. Thecriticalimportanceofpassivecooling
sors in series (Bradshaw & Orlowska 1997, p.465) developed
byRALandEADSAstriuminStevenage,UK(formerlyBritish Nearly14kWofsolarpowerilluminatesthesolarpanel.Ofthis,
Aerospace).Thedriveelectronicsweredesignedandbuiltbya less than 1W reaches the focal plane, a result of careful ther-
consortium of RAL and Systems Engineering and Assessment mal isolation of components, extremely effective passive cool-
(SEA)inBristol.Thepre-chargeregulatorwasbuiltbyCRISA ingfromtheV-groovesandthetelescopebaffle,andtheoverall
in Madrid with oversight from ESA and the University of geometry.Thelowtemperaturesachievedpassivelyhaveadra-
Granada.Thenetheatliftrequirementforthiscoolerwas15mW maticeffectonthedesignoftheactivecoolers.Inparticular,the
ataprecoolingtemperatureof20K.Sincebolometersaresensi- efficiency and heat lift of the sorption cooler increase rapidly
tivetomicrovibrations,thiscoolerincludesanewvibrationcon- astheprecoolingtemperatureprovidedbytheV-grooves,espe-
trol system. The major components of the system are shown in cially V-groove 3, decreases. The 4He-JT cooler heat lift also
Fig.3,andthebasiccharacteristicsaresummarizedinTable2. increases as its precool temperature — the ∼18K provided by
the sorption cooler — decreases. The low temperatures of the
primaryandsecondarymirrorsmeanthattheirthermalemission
2.1.3. Redundancyphilosophy
contributesnegligiblytotheoverallnoiseoftheinstruments(see
A redundant sorption cooler system was required because both Sect. 4.1.), and the radiative heat load from the baffle and mir-
instruments depend on it. Furthermore the hydride used in the rorsonthefocalplaneunit(FPU)isextremelylow.Noneofthis
sorption beds ages. The other two coolers are needed only by couldworkasitdoeswithoutthepassivecooling.
theHFI,andwerenotrequiredtoberedundant.Thecriticalel-
ements of the dilution cooler are passive. Although the flow of
3Heand4Heisadjustable,aminimumflowisalwaysavailable. 2.2. Passivecomponents
Theonlysinglepointfailureistheopeningofthevalvesonthe
2.2.1. V-grooves
high pressure tanks of 3He and 4He at the start of the mission.
Making those redundant would increase the risk of leaks with- TheV-grooveradiatorsareconesbuiltfromflatwedgesofcar-
outimprovingreliabilitysignificantly.Thecompressorsusedin bonfibrehoneycombpanelcoveredwithaluminiumfacesheets.
the4He-JTcoolerhavegoodflightheritage.Thevibrationcon- All surfaces are low emissivity except the exposed top of V-
trolsystemwasnotconsideredasinglepointfailureforthehigh groove 3, which is painted black for good radiative coupling
frequencyinstrument,althoughitsfailurewoulddegradeperfor- to cold space. The vertex angle of successive cones decreases
5
PlanckCollaboration:ThethermalperformanceofPlanck
byabout 7◦, thus facing cones are not parallel, and photons be- Table1.Requirementsonthesorptioncoolersystem.
tween them are redirected to cold space in a few reflections.
V-grooves are extremely effective at both thermal isolation and Item Requirement
radiativecooling,withmanyadvantagesovermultilayerinsula-
Coldendtemperature .... 17.5K<LVHX1<19.02K
tion (MLI), including negligible outgassing after launch. Three
17.5K<LVHX2<22.50K
V-grooves are required to achieve the ≤ 60K requirement on
precool temperature for the sorption cooler, with margin. More Coolingpower......... atLVHX1>190mW
wereunnecessary,butwouldhaveaddedcostandmass. atLVHX2>646mW
Inputpower........... <426W,beginningoflife
2.2.2. Telescopebaffle Coldendtemperature .... ∆T atLVHX1<450mK
fluctuations ∆T atTSA<100mK
The telescope baffle provides both radiative shielding and pas-
sivecooling.Itsinterioriscoveredwithpolishedaluminiumfor
lowemissivity,whileitsoutsideiscoveredwithopenhex-cells
paintedblack,forhighemissivity. valves that allow gas flow in a single direction only. The high
pressure is stabilized by a 4litre ballast tank, the high-pressure
stabilization tank (HPST). On the low pressure side, the low
2.3. Activecomponents pressurestoragebed(LPSB),filledwithhydrideandmaintained
at a temperature near that of the warm radiator, stores a large
2.3.1. Sorptioncoolerandwarmradiator
fraction of the H required to operate the cooler during flight
2
Eachofthetwohydrogensorptioncoolerscomprisesacompres- and ground testing, while minimizing the pressure in the non-
sorassembly,warmradiator,pipingassembly(includingheatex- operationalcoolerduringlaunchandtransportation.
changers onthree V-groove radiators),a JTexpander, and con- Asinglecompressorelementcomprisesacylindersupported
trol electronics. The major components of the system are high- at its ends by low thermal conductivity tubes connected to a
lightedinFig.2.Theonlymovingpartsarepassivecheckvalves. largersemi-cylinderwithaflatside.Theinnercylindercontains
Six “compressor elements” containing a La Ni Sn alloy the La Ni Sn ; the outer semi-cylinder creates a volume
1.0 4.78 0.22 1.0 4.78 0.22
absorb hydrogen at 270K and 1/3atmosphere, and desorb it at aroundtheinnercylinder,anditsflatsideisboltedtothe“warm
460Kand30atmospheres.Byvaryingthetemperatureofthesix radiator.”Thevolumebetweenthetwoisevacuatedorfilledwith
beds sequentially with resistance heaters and thermal connec- lowpressurehydrogenbyagas-gapheatswitchusingasecond
tions to the warm radiator, a continuous flow of high-pressure metal hydride, ZrNi. When filled with low pressure hydrogen,
hydrogenisproduced. there is a good thermal connection from the inner hydride bed
Sorption coolers provide vibration-free cooling with no ac- tothewarmradiator.Whenhydrogenisevacuated,theinnerhy-
tive moving parts, along with great flexibility in integration of dridebedisthermallyisolated,andcanbeheatedupefficiently.
the cooler to the cold payload (instrument, detectors, and tele- Thecompressorelementsaretakensequentiallythroughfour
scope mirrors) and the warm spacecraft. No heat is rejected in steps:heatuptopressurize;desorb;cooldowntodepressurize;
ornearthefocalplane.TherefrigerantfluidinthePlancksorp- absorb.Atagiveninstant,oneofthesixcompressorelementsis
tioncoolersishydrogen,selectedforoperationatatemperature heatingup,oneisdesorbing,oneiscoolingdown,andthreeare
of ∼17K. The Planck sorption coolers are the first continuous absorbing. Heating is achieved by electrical resistance heaters.
cyclesorptioncoolerstobeusedinspace. Coolingisachievedbythermallyconnectingthecompressorel-
Table 1 gives the requirements on the sorption cooler sys- ementtotheso-calledwarmradiator,whosetemperatureiscon-
tem. The temperature stability requirement listed is an inade- trolled(Sect.2.4.2)byelectricalheatersatatemperatureinthe
quatesimplificationofacomplicatedreality.Fluctuationsinthe range272±10K.
temperaturesofthesorptioncoolerinterfacestotheLFIandHFI The warm radiator covers three of the eight panels of the
havenointrinsicsignificance.Whatmattersistheeffectoftem- SVM. The two compressor assemblies are mounted on the end
perature fluctuations on the science results. Fluctuations at the panelsofthethree,eachofwhichcontains16straightheatpipes
cooler interfaces with HFI and LFI (LVHX1 and LVHX2, re- running parallel to the spacecraft spin axis and perpendicular
spectively)propagatetothedetectorsthemselvesthroughcom- tothecompressorelements.Theseheatpipesmaintainanearly
plicated conductive and radiative paths (quite different for the isothermal condition across the panel, in particular distributing
twoinstruments).Temperaturecontrols,passivecomponentsof the heat of the compressor element that is in the cooldown cy-
varyingemissivitiesintheopticalpaths,thestructureofthede- cle.Eightlong,bent,heatpipesrunperpendiculartotheothers,
tectorsandtheeffectofthermalfluctuationsontheiroutput,and connectingallthreepanelsoftheradiatortogether.Theexternal
theeffectsofthespinningscanstrategyanddataprocessingall surfacesofallthreepanelsarepaintedblack.
mustbetakenintoaccount.Fluctuationsatfrequencieswellbe- Upon expansion through the JT valve, hydrogen forms liq-
lowthespinfrequency(16.67mHz)cannotbeduetothesky,and uiddropletswhoseevaporationprovidesthecoolingpower.The
areeasilyremovedbythespinanddataprocessing.Fluctuations liquid/vapourmixturethenflowsthroughtwoliquid/vapourheat
atfrequencieswellabovethespinfrequencyareheavilydamped exchangers (LVHX), the first thermally and mechanically cou-
bythefront-endstructureoftheinstruments.Theimpactofthese pled to the HFI interface, where it provides precooling for the
factors could not be calculated accurately at the time a cooler 4He-JT cooler (Sect. 2.3.2) and the 3He-4He dilution cooler
fluctuation requirement had to be devised, and therefore it was (Sect.2.3.3).ThesecondiscoupledtotheLFIinterface,whereit
not possible to derive a power spectral density limit curve — coolstheLFIfocalplaneassemblyto∼20K.Anyremainingliq-
theonlykindofspecificationthatcouldcapturethetruerequire- uid/vapourmixtureflowsthroughathirdLVHX,whichismain-
ments—withhighfidelity.WewillreturntothispointinSect.7. tained above the hydrogen saturated vapour temperature. This
Eachcompressorelementisconnectedtoboththehighpres- thirdLVHXservestoevaporateanyexcessliquidthatreachesit,
sureandlowpressuresidesofthepipingsystemthroughcheck preventing flash boiling and thereby maintaining a nearly con-
6
PlanckCollaboration:ThethermalperformanceofPlanck
4K optical plate Table2.Requirementsandcharacteristicsofthe4He-JTcooler.
1.4 K optical plate
Workingfluid .......................... 4Helium
bolometer plate Heatliftat17.5Kpre-cooltemperature
100 mK dilution plate Maximum ......................... 19.2mW
dilution Required .......................... 13.3mW
Pre-coolrequirements
ThirdV-groove...................... ≤54K
20 K 20 K
mechanical mechanical LFI SorptioncoolerLVHX1................ 17.5–19K
interface dilution interface Nominaloperatingtemperature .............. 4.5K
exchanger
Mass
1.4K box Compressors,pipes,coldstage........... 27.7kg
1.4K JT Electronicsandcurrentregulator ......... 8.6kg
heat switch LVHX2 Powerintocurrentregulator ................ ≤120W
4K box
4K cold
stage heat switch
18K stage
tached to the bottom of the 4K box of the HFI FPU, as can be
LVHX1 seeninFig.5.Itprovidescoolingforthe4Kshieldandalsopre-
coolingforthegasinthedilutioncoolerpipesdescribedinthe
V-groove
interfaces nextsection.
The cooling power and thermal properties of the 4He-JT
4HeJT sorption warm cooler,measuredbytheRALteamatsubsystemlevelandthen
compressors DCCU
and panel cooler radiator in the system thermal vacuum tests, are summarised in the fol-
lowing relationships, which depend linearly on the adjustable
Fig.5. Schematic of the HFI FPU. The designations “20K”,
parametersinthevicinityoftheflightoperatingpoint:
“18K”, “4K”, “1.4K”, and “100mK” are nominal. Actual op-
eratingtemperaturesaregiveninSect.4.
HL = 15.9mW+6.8(∆S −3.45mm)
max
−1.1(T −17.3K)+0.6(P −4.5bar); (1)
pc fill
stantpressureinthelow-pressurepiping.Low-pressuregaseous
Heatload = 10.6mW+0.5(T −17.3K)
hydrogenisre-circulatedbacktothecoolsorbentbedsforcom- pc
pression. +0.065(Tvg3−45K)+Heaters; (2)
Regulationofthesystemisdonebysimpleheatingandcool-
T = 4.4K−0.035(HL −Heatload). (3)
ing;noactivecontrolofvalvesisnecessary.Theheatersforthe JT4K max
compressorsarecontrolledbyatimedon-offheatersystem.
The flight sorption cooler electronics and software were HereHLmaxisthemaximumheatlift,Tpcisthepre-coolingtem-
developed by the Laboratoire de Physique Subatomique et de perature,Tvg3 isthetemperatureofV-groove3,∆S isthestroke
Cosmologie (LPSC) in Grenoble. These electronics and their halfamplitudeofthecompressors,andPfill istheheliumfilling
controllingsoftwareprovideforthebasicsequentialoperationof pressure.
the compressor beds, temperature stabilization of the cold end, The heat load on the 4K box was predicted using the ther-
and monitoring of cooler performance parameters. In addition, malmodelandverifiedontheflightmodelduringtheCSLther-
they automatically detect failures and adapt operations accord- malbalance/thermalvacuumtest.Performanceinflightwasun-
ingly. Operational parameters can be adjusted in flight to max- changed(Sect.4.3).
imisethelifetimeandperformanceofthesorptioncoolers. Thestrokeamplitude,andtosomedegreethesorptioncooler
The total input power to the sorption cooler at end of life precooltemperature,areadjustableinflight.Theinterfacewith
(maximumaveragepower)is470W.Another110Wisavailable thesorptioncooler,includingthewarmradiatortemperature,is
tooperatethesorptioncoolerelectronics. themostcriticalinterfaceoftheHFIcryogenicchain.The4He-
JT cooler heat load increases and its heat lift decreases as the
sorption cooler precool temperature increases (see Fig. 6). The
2.3.2. 4He-JTcooler
4He-JTcoolingpowermargindependsstronglyonthistemper-
Figure 5 shows schematically the thermal interfaces of the ature,itselfdrivenmostlybythetemperatureofthewarmradi-
HFIcoolingsystem.Thecompressorsofthe 4He-JTcoolerare ator. Warm radiator temperatures of 272K (±10K, Sect. 2.3.1)
mounted in opposition (Lamarre et al. 2010) to cancel to first leadtosorptioncoolertemperaturesbetween16.5Kand17.5K
order momentum transfer to the spacecraft. Force transducers (Bersanellietal.2010).ItcanbeseenfromFig.6thatata20K
betweenthetwocompressorsprovideanerrorsignalprocessed precool temperature even the largest possible stroke amplitude
bythedriveelectronicsservosystemthatcontrolstheprofileof leavesnomargin.
thepistonmotionstominimisethefirstsevenharmonicsofthe The two mechanical compressors produce microvibrations
periodicvibrationinjectedintothespacecraft. andalsoinduceelectromagneticinterference,potentiallyaffect-
The4Kcoldheadisasmallreservoirofliquidheliumina ingthesciencesignalsofbolometers.Therisksassociatedwith
sinteredmaterial,locatedaftertheexpansionofthegasthrough these effects were taken into account early in the design of the
theJTorifice.Thisprovidesanimportantbufferwithhighheat HFIbyphase-lockingthesamplefrequencyofthedatatoahar-
capacitybetweentheJTorificeandtherestoftheHFI.Itisat- monicofthecompressorfrequency.
7
PlanckCollaboration:ThethermalperformanceofPlanck
Fig.6. Heat lift of the 4He-JT cooler as a function of precool Fig.7. Heat lift margin of the dilution cooler as a function of
temperature, stroke half amplitude ∆S (“DS” in the figure), dilution temperature and helium flow, with the dilution cooler
and proportional, integral, differential (PID) control power. For controlunitat19◦C.Foroperationinflightat101mK,115nW
∆S = 3.45mm,aprecooltemperatureof19.5Kgivesthemin- is available to accommodate the heating from cosmic rays and
for temperature regulation even at the lowest flow (Fmin2 =
imumrequiredheatlift(Table2)of13.3mW.Thein-flightpre-
19.8µmol−1)usedinflight(seeTable3andSect.4.4).
cooltemperatureof∼17.0K(verticaldottedline,andTable10)
allowstheuseofalowstrokeamplitude,minimisingstresseson
thecooler,andprovidesalargemargininheatlift.
Thismarginisgivenby:
Table3.Heliumflowoptionsforthedilutioncooler. (cid:18)T (cid:19)−1.5
HL [nW]=3.210−3He [µmols−1]T2 DCCU
margin flow dilu 273
4He 4He+3He 3He −250(T1.4K−1.28)−20(Tbolo−Tdilu)
FlowLevel [µmols−1] [µmols−1] [µmols−1] −490, (4)
Fmin2 ........ 14.5 19.8 5.4
wherethelasttwolinesare,respectively,theheatloadsfromthe
Fmin ......... 16.6 22.9 6.3
1.4Kstage,thebolometerplate,andfixedconductionparasitics,
FNOM1 ....... 20.3 27.8 7.5
allinnanowatts.
FNOM2 ....... 22.6 30.8 8.2
As shown in Fig. 7, even at the highest temperature of the
dilutionpanelinthespacecraft(19◦C,thusminimumflowfora
givenrestrictionduetohigherviscosityofthehelium),101mK
2.3.3. Dilutioncooler can be achieved in flight with the lowest flow (Fmin2), with
115nWofpoweravailableforregulation,andwiththeextraheat
The dilution cooler operates on an open circuit using a large
inputinflightfromcosmicraysandvibration.
quantityof4Heand3Hestoredinfourhighpressuretanks.The
majorcomponentsofthesystemareshowninFig.4,including
a JT expansion valve producing cooling power for the “1.4K 2.4. Temperaturecontrol
stage” of the FPU and pre-cooling for the dilution cooler. The
2.4.1. ServiceVehicleModule
gasfromthetanks(300baratthestartofthemission)isreduced
to19barthroughtwopressureregulators,andtheflowthrough The SVM thermal control system maintains all SVM compo-
thedilutioncircuitsisregulatedbyasetofdiscreterestrictions nentsattheirpropertemperature,minimizestheheatfluxtothe
chosenbytelecommand.Theflowratesfordifferentconfigura- payload module, and guarantees a stable thermal environment
tionsoftherestrictionsaregiveninTable3forthehotspacecraft tothepayloadmodule.Nospecificeffortismadetocontrolthe
case. The flow depends on the restriction temperature through temperatureofthethrustersortheirheatingeffect.
changesoftheheliumviscosity. The solar panels are nearly normal to the Sun and, as men-
TheheatliftmarginHLmargin (availablefortemperaturereg- tionedinSect.2.1,operateat385K.Tominimizetheirheatflux
ulation)isdeterminedby: totheSVMtheyarecoveredby20layersofmultilayerinsula-
tion(MLI)andmountedwithlowthermalconductivitytitanium
– He ,theflowrateoftheheliumisotopesinµmols−1given brackets.ThelaunchvehicleadaptorisalwaysSun-exposed;to
flow
bythechosenrestrictionconfiguration—foreachrestriction minimize its flux to the SVM, it is covered with MLI to the
the flow is expressed when the temperature of the dilution maximumextentand,wherenotpossible,withaproper“cold”
coolercontrolunit(DCCU)isat273K; thermo-opticalcoating.
– T ,thetemperatureoftheDCCUinflight; The SVM is octagonal. Internal SVM components are dis-
DCCU
– the heat loads from the bolometer plate, determined by the tributed on the eight panels, each of which is provided with its
temperaturedifferencebetweenT andT ,andfromthe own external radiator. Three of the panels — power, downlink
bolo dilu
1.4KstageatatemperatureofT ;and transponder,andstartracker/computer(STR/DPU)—havetem-
1.4K
– T ,thetemperatureofthedilutioncoldend. peraturecontrolofsomesort.Thepowerpanelisstableindis-
dilu
8
PlanckCollaboration:ThethermalperformanceofPlanck
Table 4. Power levels and temperature ranges of the seven in-
dependent heater lines of the warm radiator temperature con-
trolsystem.Heatersarealwayson,alwaysoff,orcontrolledon-
off(“bang-bang”control).Thesampleconfigurationwastypical
duringthefirstmonthsofoperation.
Power Sample T Range
LoopNumber [W] Configuration [◦C]
13............. 78 On −8to−9
14............. 78 On −9to−10
8............. 91 On-Off −10to−11
12............. 91 Off −11to−12
32............. 91 Off −12to−13
28............. 91 Off −13to−14
27............. 91 Off −14to−15
Fig.8. Schematic of the temperature stabilization assembly
(TSA).TheheateriscontrolledbyahybridPID+predictivecon-
troller. Stainless steel strips provide thermal resistance with a
sipation and consequently in temperature. The other two pan-
highheatcapacity.
elsexperiencetemperaturefluctuationsfordifferentreasons(see
Sect.5.1).
Another three panels are dedicated to the sorption coolers
2.4.3. 20Kstage
(Sect.2.3.1).Thesorptioncoolercompressorelementshavein-
termittenthighdissipation.Tominimizetheimpactontherestof LVHX1 provides a temperature below 18K, with fluctuations
SVM,thesorption coolercavityisinternallywrappedby MLI. drivenbythecooler(bed-to-bedvariations,cycling,instabilities
To maintain the sorption coolers above their minimum temper- inthehydrogenliquid-gasflowaftertheJT,etc.).Stabilizationof
aturelimits(253Knon-operatingand260Koperating),several the temperature of this interface with the HFI is not necessary,
heaters have been installed on the eight horizontal heat pipes as temperature control of the subsequent colder stages is more
andgroupedinsevenheaterlinesworkingatdifferenttempera- efficientandveryeffective.
tures. Temperature control of the warm radiator is described in LVHX2 provides a temperature of about 18K. To reduce
Sect.2.4.2below. coldendfluctuationstransmittedtotheradiometers,aninterme-
The warm compressors of the 4He-JT cooler are installed diatestage,thetemperaturestabilizationassembly(TSA,Fig.8),
on another panel equipped with heaters to maintain a mini- is inserted between LVHX2 and the LFI FPU. The TSA com-
mumtemperature.TheLFIradiometerelectronicbackendunits prises a temperature sensor and heater controlled by a hybrid
(REBAs) and the dilution cooler control unit (DCCU) are in- PIDandpredictivecontroller,plusahigh-heat-capacitythermal
stalledonthelastpanel.HeaterswithPIDcontrolmaintainthe resistance.Theset-pointtemperatureoftheTSAisanadjustable
REBAsandtheDCCUatastabletemperatureat2.75 ◦C. parameter of the sorption cooler system, chosen to provide dy-
namic range for control, but not to require more than 150mW
ThetopsurfaceoftheSVMiscoveredwith20layersofMLI
ofpowerfromtheheater.Asthehydrideinthesorptioncooler
tominimizeradiationontothepayload.
ages,thereturngaspressureandthusthetemperatureofLVHX2
rise slowly. The temperature of the warm radiator (Sect. 2.4.2)
2.4.2. Warmradiator also affects the temperature of LVHX2. Small adjustments of
theset-pointtemperaturearerequirednowandthen.Thereisno
Thewarmradiatoristhemeansbywhichtheheatgeneratedin- othertemperatureregulationintheLFIfocalplane.
side the compressor elements during heat-up and desorption is The heater/thermometer is redundant. The thermal resis-
rejected to cold space. (Most of the heat lifted from the focal tance between LVHX2 and the TSA causes a temperature dif-
plane is rejected to space by the V-groove radiators.) The tem- ference proportional to the heat flux through it; thus the stage
peratureofthewarmradiatordeterminesthetemperatureofthe is stabilized to (or above) the highest temperature expected for
hydridebedsduringtheabsorptionpartofthecoolercycle.The LVHX2.Thermalvariationofthecontrolledstageisdetermined
lowerthetemperature,thelowerthepressureofhydrogenonthe byseveralfactors,includingthermometerresolution,thermome-
low pressure side of the JT expansion, and therefore the lower ter sampling/feedback cycle time, heater power resolution, and
the temperature of the thermal interfaces with the HFI and LFI heater power slew rate. Attached on the other side of the con-
(LVHX1 and LVHX2, respectively). The strict requirement on trolledstageisanotherthermalresistance,throughwhichflows
thetemperatureofLVHX1giveninTable1translatesintoare- alloftheheatliftedfromLFI.Thetransferfunctionofthermal
quirementonthetemperatureofthewarmradiator. noisefromthecontrolledstagetotheLFIFPUisdeterminedby
thisresistanceandbytheeffectiveheatcapacityofLFI.
Temperature control of the warm radiator is achieved with
sevenindependentheaterlines.Thetemperatureofthewarmra-
diator depends on the total heat input from the sorption cooler
2.4.4. 4K,1.4K,and0.1Kstages
plustheheaters.Alistingoftheheatersalongwiththeircontrol
bands is given in Table 4. The average of three warm radiator The noise produced by thermal fluctuations of sources of stray
thermistors, calculated once per minute for each loop, is used radiationshouldbesmallrelativetothephotonnoiseifnocor-
forcontrolofallsevenheaters. rection is applied to the signal. This leads to a conservative re-
9
PlanckCollaboration:ThethermalperformanceofPlanck
Table 5. Temperature stability requirements on HFI compo-
nents,overthefrequencyrange16mHz–100Hz.
Component Requirement
4Khornsandfilters(30%emissivity) ........ ≤10µKHz−1/2
1.4Kfilters(20%emissivity) .............. ≤28µKHz−1/2
0.1Kbolometerplate .................... ≤20nKHz−1/2
quirement (Lamarre et al. 2003) that the temperatures of the
cryogenicstagesthatsupportopticalelementsmustmeetthesta-
bilityrequirementsgiveninTable5.
Activethermalcontrolofthe4K,1.4K,and100mKstages
(Piat et al. 2003, 2000) is needed to meet these requirements.
Fig.10.Heatliftofthesorptioncoolerasafunctionofprecool
Temperature is measured with sensitive thermometers made of
(V-groove3)temperature.
optimisedNTDGe(Piatetal.2001,2002)andreadoutbythe
same electronics as the bolometers. Details of the temperature
stabilitytestsaregivenbyPajotetal.(2010).
Regulationofthestagesisachievedbyactivecontroloflow aroundwhichthedilutiontubesarewound.Thefirstregulation
frequencyfluctuations(f <∼0.1Hz)andpassivefilteringofhigh system(PID1)isdirectlyattachedtothiscylinderandprovides
frequencyfluctuations(f >∼ 0.1Hz).Theactivesystemusesthe stabilityonlongtimescales.ItisahollowNb-Tialloycylinder
NTDGethermometersmentionedaboveandaPIDcontrolim- containinganI-shapedpieceofcopperwithredundantPIDs(i.e.,
plementedintheon-boardsoftware.Eachheaterisbiasedbya twoheatersandtwothermometers)inthethinpartoftheI(Piat
24bitADC(madeoftwo12bitADCs).Apassiveelectricalcir- etal.2003).Itsfunctionistoactivelydampfluctuationsinduced
cuit connects the ADC to the heater to fix the maximum heat bythedilutioncooler.ThedilutionplatesupportsthePID1box,
depositionandthefrequencyrange. wiring,andconnectors.Yttrium-holmium(YHo)strutssupport
4K — The main sources of thermal fluctuations on the 4K the bolometer plate. They provide passive filtering with a ther-
platearethe4He-JTcoolerandthemechanicalsupportstoLFI maltimeconstantofseveralhoursthankstoaverylargeincrease
of heat capacity in YHo at low temperature (Madet 2002; Piat
(Fig. 9), which conduct thermal fluctuations introduced by the
2000). The bolometer plate is made of stainless steel covered
sorptioncoolerontheLFIchassis.Theactivelycontrolledheater
with a 250µm film of copper, itself covered with a thin gold
isaringlocatedatthetopofthecylindricalpartofthe4Kbox
plating. This architecture was defined after thermal simulation
(Fig.9).Passivefilteringisprovidedbythethermalpathandthe
of high energy particle interactions. A second regulation stage
heatcapacityofthe4Kplateandbox.
(PID2)isplaceddirectlyonthebolometerplate.Itensurescon-
1.4K—The1.4Kstageisacylindricalboxwithaconicaltop
troloftheabsolutetemperatureoftheplateandcompensatesfor
(Fig.9).Temperaturefluctuationsarise:(i)atthebottomofthe
fluctuationsinducedbyexternalsourcessuchascosmicraysor
boxwheretheJTexpansionislocated;and(ii)onthesideofthe
backgroundradiationfluctuations.Groundtestsshowedthatthe
cylinder,atabout2/3ofitslength,wheremechanicalsupportsat-
regulation systems would meet requirements as long as the in-
tachatthreepointssymmetricallylocatedonthecircumference.
flightfluctuationsoftheheatloadsdidnotexceedpredictions.
Tousethenaturalsymmetryofthisstage,theheaterisaribbon
placed after the mechanical supports. A sensitive thermometer
on the 1.4K filter plate is used as the sensor in the regulation 2.5. Dependencies
loop. Passive filtering is provided by the long thermal path be-
ThePlanckcoolingchainiscomplicated,withcriticalinterfaces.
tweenthesourcesoffluctuationsandthefilters,aswellasbythe
Themostcriticalare:
heatcapacityoftheopticalfilterplate.
0.1K—Theprincipalsourcesoftemperaturefluctuationsonthe – thetemperatureofthepassivecooling/sorptioncoolerinter-
100mKbolometerplatearethedilutioncooleritself,theback- face at V-groove 3. The heat lift of the sorption cooler de-
groundradiation,andfluctuationsincosmicrays.Thisstagehas pendssteeplyonthisprecooltemperature(Fig.10).
been carefully optimised, since it is one of the main sources – the 4He-JT cooler helium precooling interface with the
of noise on the bolometers themselves. The principles of the LVHX1liquidreservoirofthesorptioncooler.Theheatlift
100mKarchitectureare(Piat2000):(i)controlallthermalpaths of the 4He-JT cooler depends steeply on the cold-end tem-
betweensourcesoftemperaturefluctuationsandpartsthatmust perature(Fig.11).Theprecooltemperatureisalsothedom-
bestable;(ii)activelycontrolthestructurearoundthebolometer inantparameterfortheheatloadofthe4He-JTcooler.
plateandthebolometerplateitselftoensurelongperiodstabil-
ity; and (iii) low-pass filter the structure to remove artifacts of Temperaturefluctuationsofthesorptioncooleraredrivenby
theactivesystemandtoallowshort-termstability. thevariationofpressurewithhydrogenconcentrationinthehy-
Figure9showsaschematicofthe100mKarchitecture.The dride beds during the desorption cycle, as well as by inhomo-
mechanical support of the heat exchanger consists of struts of geneitiesinthehydridebeds.Thisleadstofluctuationsofrather
Nb-Ti alloy and plates used to thermalise the heat exchanger large amplitude at both the bed-to-bed cycle frequency and the
tubesandthewiring.Acounterflowheatexchangeristhermally overall 6-bed cycle frequency. These are controlled at LVHX2
connectedtoallplatesexceptthecoldestone,thedilutionplate. (Sect. 2.4.3), but not at LVHX1, the 4He-JT cooler interface.
It goes directly from the next-to-last plate (at about 105mK) Temperature fluctuations in precooling the helium of the 4He-
to the dilution exchanger. The dilution exchanger is a cylinder JTcooleraretransferredtothecoldheadat7–8mKK−1.
10