Instrument Science Report ACS 2001-05
ACS Thermal Control
with ASCS
Colin Cox and Christine Cottingham1
May 7, 2001
ABSTRACT
The components of the ACS need to be maintained within certain temperature limits for
health and safety considerations and for the optimal scientific use of the cameras. The
ACS and a NICMOS cryocooler are planned to be installed on the same mission. To provide the necessary additional cooling, there will be a new Aft Shroud Cooling System
(ASCS). Heat produced by STIS, COS and ACS will be dissipated by radiators on the
shaded side of HST. The NICMOS cryocooler will be attached to a separate NCS radiator.
The addition of COS will eventually mean that not all instruments can run simultaneously at their design operational temperatures. Current planning calls for the NICMOS
cryolcooler to be installed at the same time as the ACS but the ASCS to be placed on a
later mission. The ACS will operate for at least a year without the cooling benefit of the
ASCS..
Introduction
Currently, most HST instruments dissipate their heat through the Aft Shroud Radiative
Sink which is radiatively connected to the aft shroud itself, except for WFPC2 which has
its own external radiator. WFPC2 uses thermo-electric coolers (TECs) to reach the
required low operating temperature at the detector of -85°C. and in so doing generates
some heat. Even so, without intervention the instrument mechanics and electronics would
eventually become too cold to operate. Temperature stabilization is achieved by controlled
heaters at many positions within the telescope.
In the servicing mission SM3B, the NICMOS cryocooler and the ACS will be
installed. The extra heat generated by the cryocooler and the ACS TECs cooling the larger
area ACS CCDs changes the heat balance considerably, to the extent that, without extra
1.
Thermal Analysis Group. Lockheed Martin Corporation / Goddard Space Flight Center
Copyright© 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Instrument Science Report ACS 2001-05
cooling mechanisms, ACS, STIS, and COS would run above the desired operating temperature. A new aft shroud cooling system (ASCS), has been designed and is being built. Two
radiators will be added, connected to thermal interface plates by Capillary Pumped Loops
(CPLs). CPLs are similar to heat pipes but afford an extra method of controlling temperatures by moderating the rate of heat transfer. The ASCS is also designed to accommodate
another instrument, the Cosmic Origins Spectrograph (COS), to be installed in SM4.
Detector dark counts
Detector dark current is a very sensitive function of temperature. Figure 1 shows some
measurements of the CCD dark counts. The WFC measurements were made at Ball Aerospace in January 2000 on two chips of the type to be installed as the WFC. At
temperatures above -80°C the dark rate increases sharply. The requirement is that the
detector temperature be held at -83°C or lower with -90°C being the target temperature.
Although not much variation is shown below -80°C for the dark current, there will be a
stronger effect on isolated hot pixels. The HRC measurements were made on the flight
unit, taken during the thermal vacuum test of March 1999 at GSFC.
Figure 1: Temperature dependence of CCD dark currents
Counts per pixel per second
0.015
0.010
HRC
WFC (a)
WFC (b)
0.005
0.000
-95
-90
-85
-80
-75
-70
-65
-60
CCD Temperature (degrees C)
The Solar Blind Channel uses a completely different device, a Multi-Anode Multi-channel
Array (MAMA) which does not need to run as cold as the CCDs, but which also experiences higher dark current with increasing temperature. SBC data from the same thermal
vacuum test are shown in Figure 2.
The line through the points is the result of a linear fit of log(count rate) against temperature. The dark count rate increases by about 30% for each degree temperature rise. The
MAMA used is a flight spare for STIS and is already about ten times as noisy as the flight
STIS MAMA. The specifications call for a rate less than 6.25x10-5 counts pixel-1 second-1, which is achieved below 34°C, but is missed by about a factor of 4 at the expected
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Instrument Science Report ACS 2001-05
operational temperature of 39°C.
Figure 2: Temperature dependence of SBC dark current
Counts per pixel per second
0.00025
0.00020
0.00015
C = 2.2x10-5e0.29(T-30)
0.00010
0.00005
0.00000
25
30
35
40
Temperature (degrees C)
System Description
Figure 3 shows the heat flow between components of the instrument system that will be in
place following SM4. Most current components dissipate heat radiatively to the aft
shroud. Only WFPC2 is connected to its own external radiator. This passive cooling system was adequate until the installation of NICMOS, which has more stringent cooling
requirements for operation in the infra-red. NICMOS carried its own cooling source, a
volume of solid Nitrogen which provided cooling as it melted by extracting latent heat
from its surroundings. Because of a heat leak in this system through a thermal contact, the
refrigerant did not last for its 5-year design period and in December 1998, NICMOS was
turned off. An active refrigerator, a pumped cryogenic loop called the NICMOS Cryocooler, will be added during SM3B so that NICMOS may be operated again. While
extracting heat from one location a refrigerator transfers heat to another and overall
increases the heat dissipation requirement. So in conjunction with the NICMOS cryocooler (NCC), one radiator plate will be attached to the telescope in SM3B and another in
SM4, as shown in Figure 4. One will be connected solely to the NCC and the other takes
heat from STIS, ACS and COS. The ACS will be installed at the same time as the NCC.
COS follows on a later mission, SM4. The ACS uses thermo-electric coolers which, while
providing local cooling, again add to the overall heat burden since they are only about
10% efficient. That is, it takes about 10 watts of electrical energy to extract one watt of
heat from the detectors and so now 10 watts of heat have to be expelled. The ACS TEC
heat transfer to the ACS radiator is via a Capillary Pumped Loop (CPL) which is a type of
heat pipe whose heat transfer properties can be controlled.
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Instrument Science Report ACS 2001-05
Figure 3: . Aft Shroud Cooling System Schematic
NICMOS
AFT SHROUD
EXTERIOR
AFT SHROUD RADIATIVE SINK
NCC
ESM
Thermal
Interface
Plates
+V2 NCS Radiator
to be installed
on SM3B
ACS
-V2 ASCS Radiator
to be installed
on SM4
STIS
COS
WFPC2
WFPC2
Radiator
Heat Pipe
CPL
Radiation
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Instrument Science Report ACS 2001-05
Figure 4: Aft Shroud Radiators
Heat Pipes and Capillary Pumped Loops
A heat pipe consists of a closed tube containing a fluid, in our case ammonia, in a mixed
liquid and vapor state such that at the hot end evaporation occurs and at the cold end condensation takes place. Latent heat of evaporation is extracted at one end and deposited at
the other. Heat is transferred very efficiently by flow of gas in one direction and liquid in
the other. The two flows are separated by dividing the pipe into concentric sections with a
wick in one section to assist the liquid flow by capillary action, while the vapor flows back
through the other compartment. For cooling the ACS, these heat pipes are required to
transfer 30 watts of heat over a 1.1 metre length for a temperature difference of one
degree. An illustration of the effectiveness of heat pipes is that to provide such a conductivity with a solid copper connection would take a cylinder of diameter more than 30
cm.An extension of the heat pipe concept is the capillary pumped loop illustrated in
Figure 5. The liquid and vapor flows are now separated in two sections of a loop. There is
an additional chamber reservoir with its own heater. The liquid to be used for the ACSC
loops is ammonia which remains liquid down to -78 degrees Celsius
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Instrument Science Report ACS 2001-05
Figure 5: .Capillary Pumped Loop
Evaporator
Condenser
Vapor Flow
Thermal
Interface
Plate
Radiator
Reservoir
Wick
Wick
Heater
Liquid Flow
Supplying heat to the reservoir is the key to controlling the heat transfer rate. While
the temperature (and therefore the pressure) is lower at the reservoir than at the evaporator,
boiling will occur at the evaporator and heat will be transferred along the vapor line. Heating the reservoir slows the transfer rate and causes the evaporator attached to the thermal
interface plate to increase its temperature to match that at the reservoir. If the total heat
input to the CPL is less than 15 W, the flow stops and vapor bubbles can form in the wick
areas. Each CPL is required to transfer 11 watts along an eighteen foot pipe for a one
degree temperature difference. Again, the equivalent conducting copper cylinder would
have to be about 45 cm in diameter.
ACS Temperature Control
The ACS optical bench is contained in a compartment thermally insulated from the electronics region. Whereas the electronics compartment is subject to moderate temperature
variations according to the behavior of the aft shroud, the optical bench requires a stable
temperature. Temperature variations can cause flexing of the bench, leading to instability
of the image position or focus. Temperatures are maintained by use of proportionally controlled heaters and by changing the temperature of the interface plate. Figure 6 illustrates
the ACS heat flow mechanisms. The WFC and HRC detectors are in contact with thermoelectric coolers which can bring the detector temperatures down to about -83°C and -85°C
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Instrument Science Report ACS 2001-05
respectively. Heat is transferred by flexible heat pipes to the interface plate which is in turn
connected to the aft shroud radiator by two CPLs, one being redundant and normally in
standby mode. This is achieved by setting its reservoir heater at a temperature higher than
the interface plate. To activate the second CPL it is only necessary to turn down its reservoir temperature below that of the interface plate temperature. This will become necessary
for normal ACS operation when COS is installed, or if a temperature lower than -85°C is
required. The interface plate will normally be at around -2°C or a few degrees colder for a
heat load of 50 watts from the ACS. Varying operational conditions can change this heat
load but the interface temperature can be controlled by adjustment of the active CPL’s
reservoir.
The HRC has a 30W heater on the baffle near its window because the CCD housing
becomes cold enough to cause contaminants to settle on the window. If contaminants do
settle on the window, the heater can be used to evaporate them. About once a month the
ACS CCD temperatures will be brought up to about 10°C to anneal hot pixels. The TECs
are turned off, the HRC window heater and interface heater turned on while the CPL reservoir is set to a higher temperature in the region of +5°C depending in the prevailing aft
shroud temperature. It will take up to eight hours for thermal equilibrium to be reached.
The annealing takes about 12 hours including this transition and will be, followed by
about three hours to return to operating temperature.
The MAMA is connected by rigid axial grooved heat pipes to the aft bulkhead which
has an aluminum radiator to reject heat. There is no other cooling mechanism for the
MAMA, and it will operate at around 39°C.
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Instrument Science Report ACS 2001-05
Figure 6: ACS Connections
ACS Concerns
Maintaining operating temperatures
The Aft Shroud Cooling System was designed principally to provide cooling for ACS.
Cooling for STIS and COS was included in the design, but we have to consider whether
the ACS operates in its optimal thermal environment and whether there is adequate cooling for all instruments. Preliminary thermal modelling indicates that, until COS is
installed, all instruments can be operated simultaneously at their required temperatures.
The extra load of COS will require that at any time, one of STIS, ACS or COS will have to
turn off its active cooling. These three instruments all connect to the -V2 radiator and are
largely independent of NICMOS which sends its heat to the +V2 radiator.
Thermal Instability
The TECs are relatively inefficient devices, generating 10 watts of heat for each watt
extracted from the cold junction. In an attempt to reach the coldest temperatures, below
-90°C, the system might be unable to transport the generated heat fast enough. The result
would be that the temperature rises and, because the target temperature has not been
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Instrument Science Report ACS 2001-05
reached, the TEC increases its current, leading to an unstable thermal situation with the
TEC current and CCD temperature both increasing. The temperature drift is fairly slow,
amounting to one or two degrees in a 24 hour period. The situation can easily be monitored and controlled, but places a lower limit on the attainable stable temperatures. Early
in the mission, we will establish stable temperature set points that do not lead to this
situation
Annealing
Approximately once a month the CCDs will be allowed to warm up to anneal hot pixels
and cosmic ray induced traps. The initially planned method was simply to turn off the
TECs without changing the interface plate temperature, and the original requirement to
reach a temperature of -5°C for this operation would be met. During November and
December 1999 when HST was unable to conduct observations because of the gyro failure, STIS spent a long time at about -10°C without any noticeable reduction of hot pixels
or other defects. This suggests that we may not be able to anneal out defects at -5°C. Consequently we requested thermal modelling be carried out to see if higher CCD
temperatures could be achieved. Repeatedly depriming and restarting the CPLs is considered inadvisable, but a higher set point for the interface plate can be allowed. A value of
5°C for the interface plate allows the HRC to reach 19°C and the WFC to reach 10°C
which should provide more effective annealing. For the HRC, use is made of an included
window heater. A CPL set point of 11C would allow the WFC to reach about 13°C with a
larger risk of depriming the CPL. The HRC temperature does not change. It does take the
WFC eight hours to reach this temperature, but annealing will take place as this value is
approached. At the time of this writing the CPL set point has not been settled, but it is
clear that we can warm the WFC to about 10°C for the annealing process. A control law
which sets the CPL reservoir temperature according to the prevailing aft shroud temperature is under consideration.
CPL Malfunction
The CPLs function properly only for a certain range of heat transfer rates. If the temperature at the evaporator end is too low to cause any vapor to be present the CPL is said to
flood. This could be counteracted by increasing the interface plate temperature using heaters and causing a greater heat flow along the CPLs. No part of the CPL may be below
-78°C since this would cause the ammonia to freeze, permanently damaging the CPL. This
is a concern for the region closest to the radiators. Conversely, CPLs can self start and provide more cooling than is intended if sufficient heat is applied to the hot end, but this can
be intercepted by increasing the reservoir heater current. There are survival heaters on the
reservoir that ensure that if the CPL self starts. Temperatures on ACS will not violate limits. In the unlikely event of a leak in a CPL, ACS has the option of switching to the second
unit.
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Instrument Science Report ACS 2001-05
Activation of Aft Shroud Cooling System
Since this document was started, SM3B has been reconsidered and the ASCS will not
be installed due to the number of other tasks that are scheduled for that mission. The NICMOS cryocooler will be installed as planned. The ASCS will be delayed until SM4 which
will not take place before 2003. For approximately two years we will have considerably
less cooling capacity and the ACS will therefore have to operate at the higher
temperatures.
The conditions will be similar to the current situation but with the added burden of the
ACS radiating onto the Aft Shroud Radiative Sink. The thermal effect of the NICMOS
cooling system is negligible because it is in a separate quadrant and has its own radiator
Under these circumstances, thermal modelling indicates we could reach temperatures of
-75°C at the WFC detector and -81°C at the HRC with consequently much higher dark
count rates. The thermal modelling results have been provided by Jorge Piquero of GSFC.
Another scenario to consider is that the ASCS might be installed but the cryo-cooler
not operated. In the Report of the Independent Science Review: NICMOS Cryocooler of
March, 1999, the project’s “Do-No-Harm” policy was endorsed and stated that the NCS
will not be powered on if it substantially degrades the performance of HST or its complement of other instruments. In this situation we would have the extra cooling capacity
provided by the new radiators without the NICMOS load. This would not greatly change
the parameters for ACS since it is connected to a separate radiator. There would be less
load on the existing aft shroud radiator sink and this might allow ACS to run a little cooler.
Thermal Vacuum Test Results.
In October 2000, a test was performed in a Thermal Vacuum Facility at Goddard
Space Flight Center. The setup included the ASCS with its CPLs, the ACS, the NICMOS
Cryocooler and thermal emulators for NICMOS and COS. Tests relating to the ACS were
the initial turn-on and detector temperature stabilization, the annealing procedure and a
test to see how cold the ACS WFC could be driven. Much of the following information is
presented by Greg Johnson in the Ball SER THM-033.
Initial Turn-on
The procedure for turning on the ACS and cooling the detectors to operating temperature and that for performing a monthly anneal; has been the subject of several meetings. A
version of the procedures arrived at following the thermal vacuum test is illustrated in
detail in Figure 7. We start with ACS in HOLD, and the CPLs are brought to a state known
as pressure primed, which involves using a CPL boost heater. The CPL connected to the
ACS radiator is activated first by turning off this boost heater and turning on a 50 watt
CPL startup reservoir heater. Next, to provide adequate initial heat flow through the CPL
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Instrument Science Report ACS 2001-05
requires turning on a 100W ACS interface heater. Once the ACS interface plate is warmer
than the reservoir the CPL will start running and this will push a cold ‘slug’ of ammonia
into the reservoir. The reservoir will initially cool down because of this ‘slug’, then will
need to be warmed back up and stabilized. This can take a much as six hours. The CPL
reservoir temperature is commanded to a desired set point between -12°C to -18°C
designed to keep the ACS interface plate at a fixed temperature which will be chosen
between -2°C and -7°C. This temperature will be reached by controlling the reservoir
heater and the action of the CPL itself. The ACS interface plate temperature will be controlled to -2°C or possibly a lower temperature. A lower temperature has the effect of
sending more heat through the CPL and a smaller fraction to the ACS aft shroud radiative
sink. This should solve a problem seen during the thermal vacuum test in which the CPL
turned off because insufficient heat was being transported through it. As the reservoir temperature reaches 0°C, after half an hour, the 100W heater is turned off but the CPL
reservoir heater remains on for another hour, again to ensure that the CPL has sufficient
heat flow. The Thermoelectric coolers (TECs) attached to the detectors are now turned on
as is the ACS main electronics box. After 30 minutes the HRC will be at its operating temperature while the WFC takes about one hour. At this point the evaporator heater is turned
off and the ACS is ready to operate. The heat generated by the TECs flows through the
CPLs which should operate stably in this configuration.
During the thermal vacuum test this stable situation was not reached. There was insufficient heat passing through the CPL to keep the ammonia circulating. To obtain a steady
state, the interface plate temperature had to be set at -7°C. Then the WFC and HRC stabilized at -82°C and -84°C respectively. The HRC reached its operating temperature in 20
minutes and the WFC took 1 hour 40 minutes. The need for the required lower interface
temperature may be explained by the discovery that the ACS radiator has a kind of thermal
short which causes only one third of it to be effective. Much of the ACS heat went to the
aft shroud radiative sink. (see figure 3). The short comes about from the attachment of survival heaters which are designed to prevent the ammonia in vapor lines on the radiator
from freezing in extremely cold situations. These heaters were thermally sunk to the vapor
lines which were in turn accidentally attached to the radiator. Attaching these in a way
which insulates them more from the radiator is expected to solve this problem. At the new
interface temperature the thermal control worked very well; the interface plate temperature always stayed within +/- 0.5°C of the set point and normally was within +/- 0.25°C.
ACS Annealing
To anneal out CCD charge traps that occur as a result of space radiation, the ACS detectors
will be warmed up once a month for about 24 hours. This process is more effective the
warmer the detectors get. The procedure is illustrated on the right side of Figure 7. First
the TECs are turned off and immediately the 100 watt interface heater and the HRC window heater are turned on. The CPL reservoir temperature is set to maintain a fixed
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Instrument Science Report ACS 2001-05
temperature difference between the interface plate and the evaporator. The details of this
control law have still to be worked out, but the aim is to provide adequate heating in the
cold environment case but not emit excessive heat in the hot case which might impact the
other instruments. We keep the CPL running even though we are not cooling so that we do
not have to go through the process of restarting the CPL. Under these conditions the HRC
will reach about 30°C and the WFC about 15°C.
The instructions to turn off the HRC window heater during initial turn-on and the CPL reservoir heater during the return from anneal are shown greyed out. These commands turn
off heaters that are already off but are placed there so that the same set of commands can
be used in each transition to ACS operate.
The thermal vacuum test did not quite reach these temperature targets. The WFC
reached 9°C and was still warming at about 3 degrees per hour after 6 hours. We estimate
that equilibrium would be attained after about 8 hours. The HRC reached 15°C.
ACS detector temperatures
A test was run with the interface plate temperature set to -10°C to see how cold the CCDs
could be driven. Both reached -92°C. This capability will become important after a few
years as the detectors are radiation damaged. The CTE effect can be lessened by running
at colder temperatures. For the first two years of operation the ACS will not have the benefit of the ASCS cooling. Therefore a test was run with the CPLs off, to see what the
normal conditions will be during the initial period. The coldest WFC temperature measured was -78°C, but this was with the environment still warming up after turning off the
CPLs. It would have taken another 24 hours to stabilize at a predicted temperature of
-74°C.
Further testing
At this time (December 2000) the ACS is back at Ball Aerospace having its flight detectors installed. ACS calibration will take place back at Goddard early in 2001. Although
another thermal vacuum test is scheduled for the ASCS around March 2001 after modifications a re made to the radiator. The ACS will probably not be available. Testing of the
rest of the system will take place using a thermal emulator for the ACS as was done for
STIS and COS in the recent test.
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ACS CPL Start-Up & Anneal Operations
CPL Start-Up
Anneal
Interface Control to Set Pt
(-2C)
ACS Interface Heater Off
ACS TECs to Set Pt (-83C)
-8.5C
(-2C at
Interface
Plate)
HRC Window Heater
ACS Interface Heater
CPL Reservoir Heater
90m
ACS Interface Heater
TECs
360m 390m 450m
3 hours
570m
TECs
T’=0 30m
ANNEAL DURATION ~24Hr
Figure 7. ACS Startup and Anneal
Startup
Heater Off
90m
210m
13
HRC Window
Heater Off
ACS to Operate
ACS start nominal ops
Reservoir Heater
Off
ACS start nominal ops
ACS HRC TEC Rollover
CPL Reservoir Temperature
Instrument Science Report ACS 2001-05
ACS to Hold
ACS Interface & Win Heater
On
Reservoir Control to maintain
fixed ∆T between interface
plate and evaporator
ACS to Operate
HRC Window
Heater Off
+15C
T=0
Interface Control to Set Point (-2C)
ACS Interface Heater Off
ACS TECs to Set Pt (-83C)
ACS HRC TEC Rollover
CPL Pressure ACS Interface Heater On
Prime (+15C) CPL Reservoir Heater On
Instrument Science Report ACS 2001-05
References
Johnson, G., Pre-Thermal Balance Design and Analysis Thermal Report, Ball System
Engineering Report, ACS THM-030, 1998
HST Aft Shroud Cooling System/ NICMOS Cooling System Operations Concept,
Lockheed report LMMS/P505610 - Version 1.14, December 1998
Rafal, M., Thermal Design for the Advanced Camera for Surveys, Space Telescopes
and Instruments V, Proceedings of SPIE, Vol 3356 p 301, 1998
Johnson, G,B., Thermal management for CCD performance on the Advanced Camera
for Surveys(ACS), Space Telescopes and Instruments V, Proceedings of SPIE, Vol
3356 p 284, 1998.
Johnson, G. B., Lessons Learned During ASCS Thermal Vacuum Testing, Ball System
Engineering Report, ACS THM-033, 2000
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