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Design of the PIXIE cryogenic system
Cryogenics 52 (2012) 134–139
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Design of the PIXIE cryogenic system M. DiPirro a,⇑, D. Fixsen b, A. Kogut a, X. Li a, J. Marquardt c, P. Shirron a a
NASA/Goddard Space Flight Center, Greenbelt, MD, United States University of Maryland, College Park, MD, United States c Ball Aerospace Technology Center, Boulder, CO, United States b
a r t i c l e
i n f o
Article history: Available online 2 February 2012 Keywords: Cryogenic space instrument ADR Cryocooler
a b s t r a c t The Primordial Inflation Explorer (PIXIE) is a proposed mission to study the polarization of the remnant cosmic microwave background with the goal of finding and understanding primordial gravity waves. The instrument has been designed to capture this information across the entire sky by rejecting foreground signals and suppressing systematic error by multiple differencing methods. The instrument operates at a temperature very close to the cosmic microwave background of 2.7 K, while the detectors operate at 0.1 K. The PIXIE cryogenic system provides this in low Earth orbit by making use of three subsystems. Lightweight, simply deployed shields provide protection against the Earth and Sun while passively cooling wiring and instrument supports at 150 K. A mechanical cryocooler precools wires and supports at 68, 17, and 4.5 K while its compressors operate at room temperature. And finally two adiabatic demagnetization refrigerators cool the instrument from 4.5 to 2.7 K and cool the detectors to 0.1 K. Staged cooling in this manner allows a thermodynamically efficient use of relatively mature technologies that can be fully demonstrated before flight. Published by Elsevier Ltd.
1. Introduction The Primordial Inflation Explorer (PIXIE) (see Fig. 1) is a proposed mission to measure the cosmic microwave background (CMB) polarization over a wide range of frequencies to set limits on several cosmological theories. To accomplish this, a wide field spectrometer operating near the CMB temperature of 2.725 K will be flown on in a sun synchronous, dawn-dusk, 660 km altitude orbit and completely cover the sky in 6 months see [1]. The nominal lifetime is 2 years. To achieve the sensitivity level required, large polarization-sensitive bolometers will be operated at 0.1 K continuously throughout the mission. To achieve these goals a cryogenic system that is robust and relatively low cost will be developed based on several technologies that have been previously proven. The PIXIE cryogenic system is a robust, staged thermal design. It utilizes three independent and complementary cooling methods to reach 2.6 K at the instrument and 0.1 K at the detectors. The three subsystems are: (1) a thermal shield composed of a set of 4 nested truncated cones, which provide passive cooling at 150 K and thermal protection against the Sun, Earth, and warm spacecraft; (2) a mechanical cryocooler spanning room temperature to 4.5 K with intermediate cooling stages at 17 and 68 K; and (3) two adiabatic demagnetization refrigerators (ADR) systems, an instrument ADR (iADR) which cools the instrument to 2.6 K from the 4.5 K heat sink ⇑ Corresponding author. E-mail address: [email protected] (M. DiPirro). 0011-2275/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.cryogenics.2012.01.017
provided by the cryocooler, and a detector ADR (dADR) cooling the detectors to 0.1 K. The heat flow map (Fig. 2) shows staging the cooling provides the most efficient means of removing heat – heat is removed at the highest possible temperature. Interestingly, the moon is one of the largest sources of parasitic heat to the cold instrument. The subsystem technologies were selected based on their high technology readiness level and heritage, their thermodynamic efficiency, and their ability to be independently verified before final system-level test. As an example, a mechanical cryocooler was selected over a stored cryogen dewar. A masterful use of stored cryogens was realized for the Spitzer cooling system with a liquid helium dewar that lasted over 5 years. To achieve this, though, additional cold mass of 141 kg (helium tank plus liquid) was necessary. In contrast, the PIXIE cryocooler cold components and iADR weigh 6 kg. On Spitzer long lifetime was achieved by limiting the cooling required by the telescope to just 6 mW at 5.5 K, and just 17.5 mW of vapor cooling could be used at 24 K to intercept heat around the telescope. A 34 K radiator to deep space, which is not possible in low Earth orbit, was also required to keep the heat load down. In contrast, the PIXIE cryocooler provides substantial cooling as shown in Table 1 which allows for more freedom in design and test, and a system that is more immune to late developing thermal problems. Also note that Spitzer, with a heavier and larger cold instrument than PIXIE, has a smaller parasitic heat (6 vs. 9.2 mW), which gives us a sanity check on our predicted parasitics at 4.5 K. The cooling capabilities vs. heat loads for each of PIXIE’s
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Earth peeks into the aperture, and that the lifetime clock begins ticking after launch with anomaly work-arounds on orbit eating into scientific data collection. These considerations add cost and risk to a stored cryogen system that is avoided by using a cryocooler. 2. Thermal shields
Fig. 1. The PIXIE instrument (center) and four thermal shields. There are two 550 mm diameter barrels that feed the two primary mirrors. The cryocooler radiator is visible at the bottom between the outer shield (Shield #1) and Shield #2. The ADRs are attached to the 2.7 K instrument. Shield #1 is approximately 3.5 m across when deployed.
cooling stages comply with the factor of two margin recommended by the AIAA [2] for this point in a program. Finally, all cryocoolers flown to date have met their minimum lifetime requirements and many have far exceeded those (Ross, Ron, Jr., private communication). Unfortunately, there have been several stored cryogen missions that have not met their on-orbit requirements: Hubble/ NICMOS (which was resuscitated by a cryocooler!), WIRE, and Astro-E2/XRS. There are also many pre- and post-launch operational restrictions that apply to stored cryogens and not cryocoolers such as frequent ground servicing, requirements to service at the launch pad hours before lift-off, more severe consequences if the Sun or
The PIXIE thermal shields are simply constructed from aluminized Kapton, honeycomb aluminum, and M55J carbon composite. The shields are symmetric because PIXIE rotates about its vertical axis. The shields must keep the sun off the instrument and inner shields at all times. Due to the relative low altitude of the orbit during two months of the year some Earthshine will peek over the rim of the shield, but the residual Earthshine over the top of the shields is slightly less than moonshine, which can come from directly overhead. The four shields when stowed can fit in the fairing of a Taurus rocket (Fig. 3). The four shields are deployed at the same time using a single deployment lanyard and redundant non-explosive actuators. There are no critical angles or deployment spacing in the design and the M55J batons that form the structure of the cones are rigid, except for hinges at the base. This design was developed and demonstrated under a NASA/ Goddard Space Flight Center IR&D program and a Space Technology-9 Phase A study. This study focused on the technology required to cool large space telescopes. The shields tested had a somewhat different configuration, but the materials and test GSE are the same as that to be used for PIXIE. The results of the test showed that the shields: have thermal performance insensitive to wrinkles, do not need to be highly specular, are testable in a flight-like condition, and have test results and thermal modeling that not only agree, but need only minor changes to model the test vs. the flight conditions [3]. Thermal modeling of the PIXIE
Fig. 2. Heat Map of the PIXIE cryogenic system. Red indicates heat inputs and blue shows heat outputs either radiated or absorbed by coolers. Dotted lines indicate radiative heat and solid lines indicate conducted heat flow. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Temperatures and heat loads. Cooler stage
T and heat sources
mW Nom. loads (CBE)
mW Derated capability
Margin (%) Nominal operation
Floating Shield #2
150 K Radiation from/to shields Conduction
4560 300
4560
N/A
Stirling (upper stage) Shield #3
68 K (TBD) Radiation Harnesses Support structure ADR leads Moon Cal drive shaft Other parasitics
2363 326 71 1156
4613
95
17 K (TBD) Support structure Radiation HTS leads Harnesses Moon Cal drive shaft
131.9 120.7
278
111
4.5 K Support structure ADR Radiation Harnesses HTS leads
19.86 7.00
40
101
2.7 K Moon Support structure ADR Harnesses Cal drive shaft HTS leads Instrument temp control
6.002 1.000 0.816
12.0
100
0.1 K
1.4E-03
Stirling (lower stage) Shield #4
4.5 K JT
iADR
dADR
4260
791 1 18
2.4 1.4 6.4 1.0 3.0
12.00 0.00 0.68 0.18
The cryocooler 68 K stage is attached to the base of shield 3 enabling it to maintain that temperature even up to the outer edges of the shield. Shield 4 is attached to the cooling lines providing 17 K to the inlet of the Joule–Thomson (JT) expansion stage. Once again, due to the layered concept, by cooling just the main conducted sources of heat – the structure and wiring – shield 4 is able to operate at 17 K, even at the top of the shield. All of these temperatures predicted by the model are in line with our ST-9 measured results. The shields are 6 sided cones with angles to vertical stepping down from 45° for shield 1° to 11° for shield 4. The step-down in angle, shown schematically in Fig. 2, allows each shield’s length to be slightly lower than the next outer one and still prevent thermal energy ‘‘jumping’’ over one of the shields. The outermost shield (shield 1) angle is a compromise between length and tolerance to slight deviations in deployed length and angle. The innermost shield 4 is below 20 K and does not present any stray light issue for the instrument. The hexapod support structure is a commonly used configuration, especially in cryogenic payloads. Spitzer, for instance, used two hexapods sets – one to support the telescope, instruments, and dewar from the spacecraft, and one to support the helium tank within the dewar. PIXIE will use S2 glass composite, a commonly available and standard material for cryogenic structure. A somewhat better material, gamma-alumina composite, which was used on Spitzer, is no longer being manufactured. If this were to become available in time for PIXIE, this could be easily substituted. The structure was analyzed and meets all requirements for mechanical and structural integrity through a typical launch and cool down environment. As it is, with the S2 glass support structure, PIXIE meets all of its cooling requirements with a factor of two or greater margin.
3. Cryocooler
1.000 0.183 0.110 0.043 2.850 3.00E-02
2043
configuration further shows that the thermal performance is insensitive to deployment errors of a few degrees and insensitive to external changes, such as Earth albedo. The shield material is conductive, and ground straps will be used to eliminate space charging effects. The multilayer design also makes the shield subsystem less sensitive to micrometeorite damage or external contamination due to spacecraft outgassing. The outermost of the four shields views the Earth and Sun and uses an SiO coating which makes it tolerant to atomic oxygen. The cryocooler radiators are housed between the outermost shield 1 and shield 2. This prevents Sun- and Earthshine from hitting them and allows them to run at 270 K using very conservative properties. The radiative loading of the outside of shield 2 by the cryocooler radiators has been included in the thermal model. Shields 2, 3 and 4 are all supported by a hexapod, which also supports the instrument. The lightweight (see schematic Fig. 2) shields have a negligible effect on the structure’s strength or stiffness. The inside of shield 2 is currently black Kapton, which gives it more radiative cooling power while not increasing the net heat load on the inner shields. This radiator is used to intercept parasitic heat conducted by the support structure and instrument wiring.
The PIXIE Stirling/JT cryocooler derives from Ball Aerospace work with Stirling coolers on High Resolution Dynamic Limb Sounder (HIRDLS), and the Thermal Infrared Sensor (TIRS) cooler delivered for flight on the LandSAT program. The JT stage development occurred during the Advanced Cryocooler Technology Development Program (ACTDP) as a possible cooler for the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) and on the 10 K Cooler Program see Fig. 4. The coolers have long life due to non-contacting moving parts and reed valves with extensive life tests behind them. The hybrid Stirling/JT cryocooler combination is one of the most efficient methods to get from room temperature to 4.5 K. Referring to Fig. 5, the Stirling portion goes from room temperature to 17 K, which is the Stirling’s most efficient range. The JT completes the cooling to 4.5 K. The JT cooling loop is located remotely within the instrument while the compressors, which contain all the moving parts, are located in the lower, room temperature compartment. This simplifies integration into the instrument and isolates any exported disturbances from the cooler into the instrument. Ball has previously demonstrated 3.4 K cooling using 3He for the JWST MIRI cooler proposal. The proposed JT cooler has flight heritage to the Cryogenic On-Orbit Long Life Active Refrigerator (COOLLAR) JT cooler flown on STS-85. BATC Stirling coolers have logged over 200,000 h in life testing including more than 59,000 h on-orbit for HIRDLS. PIXIE uses a loop heat pipe to move the heat from the cryocooler to the radiator with <10 K temperature drop. The loop heat pipe is oversized and will not be the limiting factor in heat transport. This ensures a generous operating margin for the cryocooler compressors which are specified as operating at 300 K.
M. DiPirro et al. / Cryogenics 52 (2012) 134–139
Fig. 3. PIXIE stowed and deployed. PIXIE can fit into a relatively small rocket fairing and is simply and reliably deployed.
Fig. 4. Heritage parts of the cryocooler.
Fig. 5. Schematic of the PIXIE Stirling/JT cooler.
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4. Adiabatic demagnetization refrigerator The PIXIE ADRs derive from previous work beginning in the 1980s. Flight heritage comes from Astro-E/XRS (launch 2000 but had rocket failure) and Astro-E2/XRS2 (launch 2005, and operated completely successfully until helium dewar failed – to date the lowest temperature achieved in space, 0.052 K) and 4 sounding rocket flights. Ongoing work on Astro-H/SXS has resulted in a flight-qualified design of newer, lighter, and more robust components. Development funded by NASA’s Cross Enterprise Technology Development Program (CETDP) resulted in a prototype which provided continuous cooling from 5 K to as low as 0.035 K. ADRs have a well-understood principal of operation. The thermodynamic cycle produced by alternately ramping a magnetic field up and down on a magnetocaloric material is close to Carnot efficiency in practice and our group has developed software that very accurately simulates ADR performance under a variety of conditions. ADRs have no moving parts, using magnetic coils and heaters.
Referring to Fig. 6, the iADR uses a pair of cooling stages and 4 heat switches to achieve a 6 mW heat lift and maintain a constant thermal bus temperature of 2.6 K, above which the various components of the instrument are independently regulated. Even if one of these stages were to fail, the instrument could be operated in a pulsed mode where it is alternately cooled and recharged on an approximately orbital time scale. The dADR consist of three stages which provide a 1.4 lW heat lift and maintain the detector mount at 100 mK. 21 PIXIE-type magnets have been wound and tested through several thermal cycles from 4 K to room temperature and several times more magnetization–demagnetization cycles without a single failure. The PIXIE ADR magnets are custom wound using techniques developed under the CETDP and proven in the Astro-H/SXS program where the three-stage engineering model ADR is about to begin integration and test. All magnets use 2 amps or less to charge, allowing efficiencies in the drive electronics and harnessing as well. High temperature superconducting leads developed for Astro-H will be used to eliminate joule heating at temperatures
Detectors
4a 1
2
3
100 mK
0.09-0.5 K
0.45-2.8 K
2.6-5 K
Cryocooler Cold Tip 4b
2.6-5 K
Continuous ~2.6K
Fig. 6. Schematic of the iADR (two stages on the right) and dADR (three stages on the left). The dADR maintains the bulk of the instrument continuously at 2.6 K, while the iADR continuously maintains the detector mount at 100 mK.
Fig. 7. Model of the thermal shields and cold instrument in the center. On this scale each of the shields is nearly uniform in temperature. The inner two shields’ temperatures are set by the cryocooler stage at the base of each.
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below 77 K. The relatively large heat capacity of the ADR stages and the large inductance of the magnets that control them make a very thermally stable system that is easy to temperature control. In fact the complete ADR operation is one of temperature control by simple proportional–integral–derivative parameters of each of the ADR stages. The ADR heat switches are either controlled passively (as the stage warms or cools it automatically turns a heat switch on or off), or actively (by using a small amount of heat – typically 0.25 mW – on an external charcoal getter). The lowest temperature stage in the dADR uses a superconducting heat switch, which is activated by a small Helmholtz coil. A more complete description of the PIXIE ADR is found in [4]. 5. Instrument thermal control Twenty separate items within the instrument will be individually adjusted in temperature about 5 mK above and below the nominal CMB temperature of 2.725 K. To accomplish this with a minimum of excess heating, several of the temperature-controlled zones will use magnetocaloric material surrounded by coils, similar to an ADR stage, but without the thermal insulation, heat switches or large coils. These thermal controllers have the benefit of operating without much loss, can cool as well as heat, and have a very high effective cooling/heating power with very quick response times. As currently designed, the largest heat capacities in the instrument – the external calibrator and the two optical barrel baffles – will be temperature-controlled in this way. These items will also have heaters for redundancy, or in case a larger temperature excursion is desired which may be the case during ground tests. All parts of the instrument will be thermally connected together by a backbone of high conductance copper. The heating and cooling of these parts will be done asynchronously to capture individual components’ thermal signatures. The time averaged estimate for all of these instrument heaters is conservatively calculated to be 2.85 mW, and is the major heat load at 2.6 K. 6. Thermal modeling Thermal desktopÒ (TD) (a commercial product by Cullimore and Ring) was used to perform thermal analysis for this system involving radiation and conduction (see Fig. 7). The thermal model of the PIXIE shields includes an outer shield (Shield #1), a shield allowed to float in temperature (Shield #2), a 68 K shield (Shield #3), a 17 K shield (Shield #4), the cold instrument at 2.7 K, the support struts, the cryocooler, and the radiator. Joule heating for
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non-superconducting portion of the ADR wires also is included in the thermal model. Shield #1 is conservatively set to the spacecraft temperature of 300 K. There are more than 10,000 nodes, including 7506 TD/RC nodes and 2584 boundary nodes. Several parametric studies were performed. First, the angles of shields were varied. The outer layer angle was varied from 45° to 30°. The Inner layers were varied proportionally. It was found that the floating shield angle has a significant effect on the 68 K stage heat load. The higher the angle is, the more surface area is facing space, and the lower the load is. Therefore, 45° was chosen for the angle of the outer shields. Shields 2, 3 and 4 were spaced evenly in angle using 33.75°, 22.5°, and 11.25°, respectively. Second, changing the surface coating material from low emissivity to high emissivity, such as black Kapton, on the inner-facing side of Shield #3 reduces the load on the 68 K cryocooler stage by almost one half watt. Thermal analysis also found that the location of the connecting points between the cryocooler stages and the instrument support struts are very important. Accurately positioning the thermal connection between the two can effectively intercept the heat and improve efficiency. Table 1 lists the heat loads on all the cryocooler stages. The temperature of the base of Shield #2 is 150 K, while the average temperature of the conical portion is only 83.5 K. Shield #2 emits 4.56 W to space. The margins listed in Table 1 are calculated as cooling capability minus load divided by load. Thus 100% margin means that the cooling capability is twice the expected heat load. Shield #2 is essentially a 150 K radiator which provides additional cooling to the system. Margin is not calculated for this component. 7. Summary The PIXIE cryogenic system uses stages of passive, mechanical, and magnetic cooling to produce a long life, low cost solution for a cosmology payload in low Earth orbit. The compact size and geometry of the cryogenic system allows it to be fully tested on the ground using shrouds similar to that used in Ref. [3]. References [1] Kogut A et al. J Cosmol Astropart Phys, in press, arXiv:1105.2044. [2] Donabedian M, editor. Spacecraft thermal control handbook, vol. 2 – cryogenics. American Institute of Aeronautics and Astronautics/Aerospace Press; 2003. [3] DiPirro M et al. High fidelity cryothermal test of a subscale large space telescope. Proc SPIE 6692. vol. 1–9; 2007. p. 669202. [4] Shirron P et al. Adiabatic demagnetization refrigerators for the PIXIE instrument, Cryogenics; this proceedings [companion paper].
Contents lists available at SciVerse ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Design of the PIXIE cryogenic system M. DiPirro a,⇑, D. Fixsen b, A. Kogut a, X. Li a, J. Marquardt c, P. Shirron a a
NASA/Goddard Space Flight Center, Greenbelt, MD, United States University of Maryland, College Park, MD, United States c Ball Aerospace Technology Center, Boulder, CO, United States b
a r t i c l e
i n f o
Article history: Available online 2 February 2012 Keywords: Cryogenic space instrument ADR Cryocooler
a b s t r a c t The Primordial Inflation Explorer (PIXIE) is a proposed mission to study the polarization of the remnant cosmic microwave background with the goal of finding and understanding primordial gravity waves. The instrument has been designed to capture this information across the entire sky by rejecting foreground signals and suppressing systematic error by multiple differencing methods. The instrument operates at a temperature very close to the cosmic microwave background of 2.7 K, while the detectors operate at 0.1 K. The PIXIE cryogenic system provides this in low Earth orbit by making use of three subsystems. Lightweight, simply deployed shields provide protection against the Earth and Sun while passively cooling wiring and instrument supports at 150 K. A mechanical cryocooler precools wires and supports at 68, 17, and 4.5 K while its compressors operate at room temperature. And finally two adiabatic demagnetization refrigerators cool the instrument from 4.5 to 2.7 K and cool the detectors to 0.1 K. Staged cooling in this manner allows a thermodynamically efficient use of relatively mature technologies that can be fully demonstrated before flight. Published by Elsevier Ltd.
1. Introduction The Primordial Inflation Explorer (PIXIE) (see Fig. 1) is a proposed mission to measure the cosmic microwave background (CMB) polarization over a wide range of frequencies to set limits on several cosmological theories. To accomplish this, a wide field spectrometer operating near the CMB temperature of 2.725 K will be flown on in a sun synchronous, dawn-dusk, 660 km altitude orbit and completely cover the sky in 6 months see [1]. The nominal lifetime is 2 years. To achieve the sensitivity level required, large polarization-sensitive bolometers will be operated at 0.1 K continuously throughout the mission. To achieve these goals a cryogenic system that is robust and relatively low cost will be developed based on several technologies that have been previously proven. The PIXIE cryogenic system is a robust, staged thermal design. It utilizes three independent and complementary cooling methods to reach 2.6 K at the instrument and 0.1 K at the detectors. The three subsystems are: (1) a thermal shield composed of a set of 4 nested truncated cones, which provide passive cooling at 150 K and thermal protection against the Sun, Earth, and warm spacecraft; (2) a mechanical cryocooler spanning room temperature to 4.5 K with intermediate cooling stages at 17 and 68 K; and (3) two adiabatic demagnetization refrigerators (ADR) systems, an instrument ADR (iADR) which cools the instrument to 2.6 K from the 4.5 K heat sink ⇑ Corresponding author. E-mail address: [email protected] (M. DiPirro). 0011-2275/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.cryogenics.2012.01.017
provided by the cryocooler, and a detector ADR (dADR) cooling the detectors to 0.1 K. The heat flow map (Fig. 2) shows staging the cooling provides the most efficient means of removing heat – heat is removed at the highest possible temperature. Interestingly, the moon is one of the largest sources of parasitic heat to the cold instrument. The subsystem technologies were selected based on their high technology readiness level and heritage, their thermodynamic efficiency, and their ability to be independently verified before final system-level test. As an example, a mechanical cryocooler was selected over a stored cryogen dewar. A masterful use of stored cryogens was realized for the Spitzer cooling system with a liquid helium dewar that lasted over 5 years. To achieve this, though, additional cold mass of 141 kg (helium tank plus liquid) was necessary. In contrast, the PIXIE cryocooler cold components and iADR weigh 6 kg. On Spitzer long lifetime was achieved by limiting the cooling required by the telescope to just 6 mW at 5.5 K, and just 17.5 mW of vapor cooling could be used at 24 K to intercept heat around the telescope. A 34 K radiator to deep space, which is not possible in low Earth orbit, was also required to keep the heat load down. In contrast, the PIXIE cryocooler provides substantial cooling as shown in Table 1 which allows for more freedom in design and test, and a system that is more immune to late developing thermal problems. Also note that Spitzer, with a heavier and larger cold instrument than PIXIE, has a smaller parasitic heat (6 vs. 9.2 mW), which gives us a sanity check on our predicted parasitics at 4.5 K. The cooling capabilities vs. heat loads for each of PIXIE’s
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Earth peeks into the aperture, and that the lifetime clock begins ticking after launch with anomaly work-arounds on orbit eating into scientific data collection. These considerations add cost and risk to a stored cryogen system that is avoided by using a cryocooler. 2. Thermal shields
Fig. 1. The PIXIE instrument (center) and four thermal shields. There are two 550 mm diameter barrels that feed the two primary mirrors. The cryocooler radiator is visible at the bottom between the outer shield (Shield #1) and Shield #2. The ADRs are attached to the 2.7 K instrument. Shield #1 is approximately 3.5 m across when deployed.
cooling stages comply with the factor of two margin recommended by the AIAA [2] for this point in a program. Finally, all cryocoolers flown to date have met their minimum lifetime requirements and many have far exceeded those (Ross, Ron, Jr., private communication). Unfortunately, there have been several stored cryogen missions that have not met their on-orbit requirements: Hubble/ NICMOS (which was resuscitated by a cryocooler!), WIRE, and Astro-E2/XRS. There are also many pre- and post-launch operational restrictions that apply to stored cryogens and not cryocoolers such as frequent ground servicing, requirements to service at the launch pad hours before lift-off, more severe consequences if the Sun or
The PIXIE thermal shields are simply constructed from aluminized Kapton, honeycomb aluminum, and M55J carbon composite. The shields are symmetric because PIXIE rotates about its vertical axis. The shields must keep the sun off the instrument and inner shields at all times. Due to the relative low altitude of the orbit during two months of the year some Earthshine will peek over the rim of the shield, but the residual Earthshine over the top of the shields is slightly less than moonshine, which can come from directly overhead. The four shields when stowed can fit in the fairing of a Taurus rocket (Fig. 3). The four shields are deployed at the same time using a single deployment lanyard and redundant non-explosive actuators. There are no critical angles or deployment spacing in the design and the M55J batons that form the structure of the cones are rigid, except for hinges at the base. This design was developed and demonstrated under a NASA/ Goddard Space Flight Center IR&D program and a Space Technology-9 Phase A study. This study focused on the technology required to cool large space telescopes. The shields tested had a somewhat different configuration, but the materials and test GSE are the same as that to be used for PIXIE. The results of the test showed that the shields: have thermal performance insensitive to wrinkles, do not need to be highly specular, are testable in a flight-like condition, and have test results and thermal modeling that not only agree, but need only minor changes to model the test vs. the flight conditions [3]. Thermal modeling of the PIXIE
Fig. 2. Heat Map of the PIXIE cryogenic system. Red indicates heat inputs and blue shows heat outputs either radiated or absorbed by coolers. Dotted lines indicate radiative heat and solid lines indicate conducted heat flow. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Temperatures and heat loads. Cooler stage
T and heat sources
mW Nom. loads (CBE)
mW Derated capability
Margin (%) Nominal operation
Floating Shield #2
150 K Radiation from/to shields Conduction
4560 300
4560
N/A
Stirling (upper stage) Shield #3
68 K (TBD) Radiation Harnesses Support structure ADR leads Moon Cal drive shaft Other parasitics
2363 326 71 1156
4613
95
17 K (TBD) Support structure Radiation HTS leads Harnesses Moon Cal drive shaft
131.9 120.7
278
111
4.5 K Support structure ADR Radiation Harnesses HTS leads
19.86 7.00
40
101
2.7 K Moon Support structure ADR Harnesses Cal drive shaft HTS leads Instrument temp control
6.002 1.000 0.816
12.0
100
0.1 K
1.4E-03
Stirling (lower stage) Shield #4
4.5 K JT
iADR
dADR
4260
791 1 18
2.4 1.4 6.4 1.0 3.0
12.00 0.00 0.68 0.18
The cryocooler 68 K stage is attached to the base of shield 3 enabling it to maintain that temperature even up to the outer edges of the shield. Shield 4 is attached to the cooling lines providing 17 K to the inlet of the Joule–Thomson (JT) expansion stage. Once again, due to the layered concept, by cooling just the main conducted sources of heat – the structure and wiring – shield 4 is able to operate at 17 K, even at the top of the shield. All of these temperatures predicted by the model are in line with our ST-9 measured results. The shields are 6 sided cones with angles to vertical stepping down from 45° for shield 1° to 11° for shield 4. The step-down in angle, shown schematically in Fig. 2, allows each shield’s length to be slightly lower than the next outer one and still prevent thermal energy ‘‘jumping’’ over one of the shields. The outermost shield (shield 1) angle is a compromise between length and tolerance to slight deviations in deployed length and angle. The innermost shield 4 is below 20 K and does not present any stray light issue for the instrument. The hexapod support structure is a commonly used configuration, especially in cryogenic payloads. Spitzer, for instance, used two hexapods sets – one to support the telescope, instruments, and dewar from the spacecraft, and one to support the helium tank within the dewar. PIXIE will use S2 glass composite, a commonly available and standard material for cryogenic structure. A somewhat better material, gamma-alumina composite, which was used on Spitzer, is no longer being manufactured. If this were to become available in time for PIXIE, this could be easily substituted. The structure was analyzed and meets all requirements for mechanical and structural integrity through a typical launch and cool down environment. As it is, with the S2 glass support structure, PIXIE meets all of its cooling requirements with a factor of two or greater margin.
3. Cryocooler
1.000 0.183 0.110 0.043 2.850 3.00E-02
2043
configuration further shows that the thermal performance is insensitive to deployment errors of a few degrees and insensitive to external changes, such as Earth albedo. The shield material is conductive, and ground straps will be used to eliminate space charging effects. The multilayer design also makes the shield subsystem less sensitive to micrometeorite damage or external contamination due to spacecraft outgassing. The outermost of the four shields views the Earth and Sun and uses an SiO coating which makes it tolerant to atomic oxygen. The cryocooler radiators are housed between the outermost shield 1 and shield 2. This prevents Sun- and Earthshine from hitting them and allows them to run at 270 K using very conservative properties. The radiative loading of the outside of shield 2 by the cryocooler radiators has been included in the thermal model. Shields 2, 3 and 4 are all supported by a hexapod, which also supports the instrument. The lightweight (see schematic Fig. 2) shields have a negligible effect on the structure’s strength or stiffness. The inside of shield 2 is currently black Kapton, which gives it more radiative cooling power while not increasing the net heat load on the inner shields. This radiator is used to intercept parasitic heat conducted by the support structure and instrument wiring.
The PIXIE Stirling/JT cryocooler derives from Ball Aerospace work with Stirling coolers on High Resolution Dynamic Limb Sounder (HIRDLS), and the Thermal Infrared Sensor (TIRS) cooler delivered for flight on the LandSAT program. The JT stage development occurred during the Advanced Cryocooler Technology Development Program (ACTDP) as a possible cooler for the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) and on the 10 K Cooler Program see Fig. 4. The coolers have long life due to non-contacting moving parts and reed valves with extensive life tests behind them. The hybrid Stirling/JT cryocooler combination is one of the most efficient methods to get from room temperature to 4.5 K. Referring to Fig. 5, the Stirling portion goes from room temperature to 17 K, which is the Stirling’s most efficient range. The JT completes the cooling to 4.5 K. The JT cooling loop is located remotely within the instrument while the compressors, which contain all the moving parts, are located in the lower, room temperature compartment. This simplifies integration into the instrument and isolates any exported disturbances from the cooler into the instrument. Ball has previously demonstrated 3.4 K cooling using 3He for the JWST MIRI cooler proposal. The proposed JT cooler has flight heritage to the Cryogenic On-Orbit Long Life Active Refrigerator (COOLLAR) JT cooler flown on STS-85. BATC Stirling coolers have logged over 200,000 h in life testing including more than 59,000 h on-orbit for HIRDLS. PIXIE uses a loop heat pipe to move the heat from the cryocooler to the radiator with <10 K temperature drop. The loop heat pipe is oversized and will not be the limiting factor in heat transport. This ensures a generous operating margin for the cryocooler compressors which are specified as operating at 300 K.
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Fig. 3. PIXIE stowed and deployed. PIXIE can fit into a relatively small rocket fairing and is simply and reliably deployed.
Fig. 4. Heritage parts of the cryocooler.
Fig. 5. Schematic of the PIXIE Stirling/JT cooler.
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4. Adiabatic demagnetization refrigerator The PIXIE ADRs derive from previous work beginning in the 1980s. Flight heritage comes from Astro-E/XRS (launch 2000 but had rocket failure) and Astro-E2/XRS2 (launch 2005, and operated completely successfully until helium dewar failed – to date the lowest temperature achieved in space, 0.052 K) and 4 sounding rocket flights. Ongoing work on Astro-H/SXS has resulted in a flight-qualified design of newer, lighter, and more robust components. Development funded by NASA’s Cross Enterprise Technology Development Program (CETDP) resulted in a prototype which provided continuous cooling from 5 K to as low as 0.035 K. ADRs have a well-understood principal of operation. The thermodynamic cycle produced by alternately ramping a magnetic field up and down on a magnetocaloric material is close to Carnot efficiency in practice and our group has developed software that very accurately simulates ADR performance under a variety of conditions. ADRs have no moving parts, using magnetic coils and heaters.
Referring to Fig. 6, the iADR uses a pair of cooling stages and 4 heat switches to achieve a 6 mW heat lift and maintain a constant thermal bus temperature of 2.6 K, above which the various components of the instrument are independently regulated. Even if one of these stages were to fail, the instrument could be operated in a pulsed mode where it is alternately cooled and recharged on an approximately orbital time scale. The dADR consist of three stages which provide a 1.4 lW heat lift and maintain the detector mount at 100 mK. 21 PIXIE-type magnets have been wound and tested through several thermal cycles from 4 K to room temperature and several times more magnetization–demagnetization cycles without a single failure. The PIXIE ADR magnets are custom wound using techniques developed under the CETDP and proven in the Astro-H/SXS program where the three-stage engineering model ADR is about to begin integration and test. All magnets use 2 amps or less to charge, allowing efficiencies in the drive electronics and harnessing as well. High temperature superconducting leads developed for Astro-H will be used to eliminate joule heating at temperatures
Detectors
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3
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Fig. 6. Schematic of the iADR (two stages on the right) and dADR (three stages on the left). The dADR maintains the bulk of the instrument continuously at 2.6 K, while the iADR continuously maintains the detector mount at 100 mK.
Fig. 7. Model of the thermal shields and cold instrument in the center. On this scale each of the shields is nearly uniform in temperature. The inner two shields’ temperatures are set by the cryocooler stage at the base of each.
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below 77 K. The relatively large heat capacity of the ADR stages and the large inductance of the magnets that control them make a very thermally stable system that is easy to temperature control. In fact the complete ADR operation is one of temperature control by simple proportional–integral–derivative parameters of each of the ADR stages. The ADR heat switches are either controlled passively (as the stage warms or cools it automatically turns a heat switch on or off), or actively (by using a small amount of heat – typically 0.25 mW – on an external charcoal getter). The lowest temperature stage in the dADR uses a superconducting heat switch, which is activated by a small Helmholtz coil. A more complete description of the PIXIE ADR is found in [4]. 5. Instrument thermal control Twenty separate items within the instrument will be individually adjusted in temperature about 5 mK above and below the nominal CMB temperature of 2.725 K. To accomplish this with a minimum of excess heating, several of the temperature-controlled zones will use magnetocaloric material surrounded by coils, similar to an ADR stage, but without the thermal insulation, heat switches or large coils. These thermal controllers have the benefit of operating without much loss, can cool as well as heat, and have a very high effective cooling/heating power with very quick response times. As currently designed, the largest heat capacities in the instrument – the external calibrator and the two optical barrel baffles – will be temperature-controlled in this way. These items will also have heaters for redundancy, or in case a larger temperature excursion is desired which may be the case during ground tests. All parts of the instrument will be thermally connected together by a backbone of high conductance copper. The heating and cooling of these parts will be done asynchronously to capture individual components’ thermal signatures. The time averaged estimate for all of these instrument heaters is conservatively calculated to be 2.85 mW, and is the major heat load at 2.6 K. 6. Thermal modeling Thermal desktopÒ (TD) (a commercial product by Cullimore and Ring) was used to perform thermal analysis for this system involving radiation and conduction (see Fig. 7). The thermal model of the PIXIE shields includes an outer shield (Shield #1), a shield allowed to float in temperature (Shield #2), a 68 K shield (Shield #3), a 17 K shield (Shield #4), the cold instrument at 2.7 K, the support struts, the cryocooler, and the radiator. Joule heating for
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non-superconducting portion of the ADR wires also is included in the thermal model. Shield #1 is conservatively set to the spacecraft temperature of 300 K. There are more than 10,000 nodes, including 7506 TD/RC nodes and 2584 boundary nodes. Several parametric studies were performed. First, the angles of shields were varied. The outer layer angle was varied from 45° to 30°. The Inner layers were varied proportionally. It was found that the floating shield angle has a significant effect on the 68 K stage heat load. The higher the angle is, the more surface area is facing space, and the lower the load is. Therefore, 45° was chosen for the angle of the outer shields. Shields 2, 3 and 4 were spaced evenly in angle using 33.75°, 22.5°, and 11.25°, respectively. Second, changing the surface coating material from low emissivity to high emissivity, such as black Kapton, on the inner-facing side of Shield #3 reduces the load on the 68 K cryocooler stage by almost one half watt. Thermal analysis also found that the location of the connecting points between the cryocooler stages and the instrument support struts are very important. Accurately positioning the thermal connection between the two can effectively intercept the heat and improve efficiency. Table 1 lists the heat loads on all the cryocooler stages. The temperature of the base of Shield #2 is 150 K, while the average temperature of the conical portion is only 83.5 K. Shield #2 emits 4.56 W to space. The margins listed in Table 1 are calculated as cooling capability minus load divided by load. Thus 100% margin means that the cooling capability is twice the expected heat load. Shield #2 is essentially a 150 K radiator which provides additional cooling to the system. Margin is not calculated for this component. 7. Summary The PIXIE cryogenic system uses stages of passive, mechanical, and magnetic cooling to produce a long life, low cost solution for a cosmology payload in low Earth orbit. The compact size and geometry of the cryogenic system allows it to be fully tested on the ground using shrouds similar to that used in Ref. [3]. References [1] Kogut A et al. J Cosmol Astropart Phys, in press, arXiv:1105.2044. [2] Donabedian M, editor. Spacecraft thermal control handbook, vol. 2 – cryogenics. American Institute of Aeronautics and Astronautics/Aerospace Press; 2003. [3] DiPirro M et al. High fidelity cryothermal test of a subscale large space telescope. Proc SPIE 6692. vol. 1–9; 2007. p. 669202. [4] Shirron P et al. Adiabatic demagnetization refrigerators for the PIXIE instrument, Cryogenics; this proceedings [companion paper].