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BLISS cryo-chain demonstrator
Cryogenics 70 (2015) 70–75
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Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
SPICA/BLISS cryo-chain demonstrator T. Prouvé c,a,⇑, L. Duband c, J. Hodis b, J.J. Bock a,b, C. Matt Bradford b,a, W. Holmes b a
Department of Physics and Astronomy, California Institute of Technology, Pasadena, CA 91106, United States Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, United States c Univ. Grenoble Alpes, CEA INAC-SBT, F-38000 Grenoble, France b
a r t i c l e
i n f o
Article history: Received 17 March 2015 Received in revised form 3 June 2015 Accepted 4 June 2015 Available online 11 June 2015 Keywords: Astrophysics Sub-Kelvin Space SPICA Sorption-cooler ADR
a b s t r a c t The Background Limited Infrared Submillimeter Spectrometer (BLISS) is an instrument proposed for SPICA, the Japanese–European space-borne telescope mission under study for a possible launch in the next decade. The BLISS concept is a suite of aluminum spectrometer modules totaling 10 kg cooled to 50 mK. Cooling this ambitious instrument with high-duty cycle within the stringent heat-rejection allocations envisioned for SPICA is a challenge. We have developed a solution consisting of two stages: (1) a continuous 300 mK intercept stage provided by two 3He sorption coolers operated sequentially, and (2) a 50 mK adiabatic demagnetization refrigerator (ADR) operated in single-shot mode. We have built a prototype cooler and demonstrated it in a dedicated SPICA-like thermal testbed with regulated stages enabling measurement of rejected heat at 1.7 K and 4.5 K. The approach offers lower mass than a dual-stage ADR, and lower rejected power to 1.7 K and 4.5 K than a comparable single-shot 300 mK system, while insuring a high duty cycle. As a demonstration of feasibility for SPICA and future cryogenic missions, we show long-term cooling with flight-like parasitics at 50 mK and 300 mK requiring only 3 mW and 8 mW rejected at 1.7 K and 4.5 K, respectively. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction SPICA is a proposed 3 m class space telescope optimized for mid- and far-IR (5–500 lm) observations of galaxies throughout the age of the Universe and star-formation sites in our own Galaxy [1]. It will be actively cooled to approximately 6 K in order to reduce the thermal emission from the telescope [2,3]. This low-background platform offers the potential for 2–4 orders of magnitude improvement in sensitivity in the important but relatively unexplored far-IR waveband. To capitalize on this, SPICA’s greatest science return will come from cryogenic instruments with arrays of low-background detectors. In the far-IR beyond 40 lm, high sensitivity requires bolometers with arrays of direct detectors such as transition-edge-sensed (TES) bolometers designed for the low-backgrounds, cooled to approximately 50 mK. SPICA must cool both the large telescope and provide heat lift to support this sub-Kelvin instrumentation. The SPICA spacecraft will use passive cooling available at L2, [7], and 4 K and 1.7 K closed-cycle Joule–Thompson coolers filled with 4He and 3He, respectively, working fluids. Heat lift at 4 K is used to both cool the telescope and provide a heat sink for the instrument suite. ⇑ Corresponding author at: Univ. Grenoble Alpes, CEA INAC-SBT, F-38000 Grenoble, France. http://dx.doi.org/10.1016/j.cryogenics.2015.06.001 0011-2275/Ó 2015 Elsevier Ltd. All rights reserved.
The 1.7 K lift is an additional support for the sub-Kelvin instrumentation. Cooling power is limited, so careful integration of the full system will be the key to SPICA’s success [6]. Stringent limits are placed on total instrument mass staged at 4 K, since 4 K mass drives total observatory parasitic load, and on the heat lift at 4 K and 1.7 K for the instruments. Two sub-K far-IR instruments have received substantial study for SPICA: a Fourier-transform spectrometer (FTS) named SAFARI is proposed by a European team led by SRON [4], and a sensitive grating spectrometer concept named BLISS has been developed by a team centered at JPL/Caltech [5]. Both studies have sought a 50 mK cooling scheme which is sufficiently low mass and high Carnot efficiency for lifting 50 mK power and rejecting it at 1.7 K and 4.5 K. 1.1. Approach Thus far, the least-massive solution to stage from 1.7 K to an ultimate 50 mK is a combination including a 3He sorption cooler and an ADR [9–11]. Both BLISS and SAFARI are using this hybrid approach. For BLISS, however, we have incorporated the additional advance of a continuous 300 mK intercept stage using two identical sorption refrigerators cycled sequentially. This approach, shown schematically in Fig. 1, offers three key advantages. First,
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Fig. 1. Schematic of the BLISS cryogenic chain design.
and most important, the 300 mK cycle time is removed from the duty cycle; a substantial gain since cycling a single-shot sorption fridge cycle can require about 10 h when limited to the 10 mW at 4.5 K and 5 mW at 1.7 K on current allocations for far-IR instruments. Second, both the 50 mK cold stage (10 kg of aluminum spectrometers for BLISS) and the intercept stage (5 kg of structure and shields) are maintained under 300–400 mK. This eliminates the reduction of efficiency from warming and cooling the >15 kg of instrument mass from 1.7 K to 300 mK each cycle which would require 1 J of cooling energy. Third, the thermalisation time associated with cycling up to 1.7 K is eliminated. For an instrument similar to BLISS called Zspec, these thermal transients were 8 h [8]. This approach offers a high duty cycle and thermodynamic efficiency which would otherwise be impossible under the stringent requirements for instantaneous heat lift at 4.5 K and 1.7 K. The cycling phases, shown in Fig. 2 are as follows. One of the two 3He coolers is used to maintain 300 mK while the other is recycling, dumping heat at 1.7 K and 4.5 K. Since the recycle time can be long, essentially the hold time of one of the coolers in light of the parasitics, the instantaneous loads to 1.7 K and 4.5 K, essentially due to the cooling and condensation of the 3He gas, can be low. The 50 mK stage is still a single-shot ADR, but its duty cycle can be in excess of 80%. The cold stage is cooled to 50 mK for an integer number of 3He cooler cycling times. 2. Demonstrator design The basic design of the CEA–SBT sorption cooler has been presented in a previous paper [9]. Two 6-l 3He adsorption coolers provide the continuous 300 mK cooling. The constructed prototypes are nearly identical to the flight units made for Herschel [12] except that the titanium pump tubes have been replaced with stainless steel. The replacement of the stainless-steel tubes by TA6V tubes would improve the system at two levels:
could be recycle with lower ultimate resource at 4.5 K. This gain is about 30%. Of course the duty cycle would be impacted. But for a double stage system, the duty cycle is not really an issue as long as both stages are tuned properly. Low T phase: Going to titanium will of course lower the load on the sorption stages (by 30% if we do not include ADR cycling). Meaning that a redesign toward a smaller system could be made. Smaller system means less energy to be dissipated at 1.7 and 4.5 K and thus we could either lower the dissipation and keep the same duty cycle, or keep the same dissipation and gain on the duty cycle (if needed). The general architecture is shown schematically in Fig. 1. Cooler 1 (C1) and cooler 2 (C2) are operated inside a 1.7 K aluminum box. A copper bus reaches out of the box to interface with the 4.5 K heat sink. The charcoal pumps and evaporators are shielded. In this way the two coolers do not radiatively couple to one another when one is cycling. Heat Switch one (HS1) and HS4 are charged with 4He, while the other heat switches are charged with 3He. HS2 and HS5 connect the evaporators to 1.7 K when the coolers are recycling. HS3 and HS6 connect the evaporators to the 300 mK stage when the coolers are cooling at low temperature. HS7 connects the ADR salt pill during the magnetization phase to extract the heat of magnetization. Initial operation of the fridge indicated that HS3 and HS6 were difficult to turn OFF when a cooler needed to be recycled [9]. This led to large temperature oscillations of the 300 mK stage and to an
Recycling phase: The gain on the duty cycle is not significant since it only impacts the heating phase of the pump. However when the pump is at 30 K the power dissipated thru the various links between the pump and the 4.5 K interface is lower with Titanium; meaning the cooler
Fig. 2. Schematic showing the BLISS refrigerator operation approach. (A) Cycling of one sorption cooler at a time, (B) cycling of the ADR.
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increase in the 3He consumption of the cold cooler. It was realized that because the HS3 and HS6 cold ends are maintained at 300 mK even when their respective evaporators are warmed, the cold ends were adsorbing 3He at rate comparable to the pumping speed of the mini pump, thus preventing complete evacuation of the heat switch. Small temperature variations on the continuous 300 mK stage lead to 3He gas bursts which turn HS on briefly, thus creating the oscillations. The solution [13] was to increase the pumping speed of the HS mini pump significantly above the rate of 3He adsorption rate. To do this, the long pumping tubes have been replaced by a 500 times more conductive pumping tube. The new low temperature gas gap HS design is presented in Fig. 3. For the ADR, we used an existing FAA salt pill [14], which was adapted to the BLISS cryo-chain demonstrator. The salt is supported from the 1.7 K superconducting magnet with a Kevlar suspension, which includes an intercept cooled by the 300 mK stage. While HS7 might be prone to the same problem of gas adsorption as HS 3 and HS 6, in practice it has no problem turning OFF. As the salt is demagnetized, the HS7 corresponding end is cooled accordingly. This naturally condenses the remaining 3He gas. At T < 200 mK the gas conduction in the heat switch was measured to be insignificant compared to other parasitics to the FAA salt pill. 2.1. 300 mK and ADR cycling control An important aspect of operating this integrated system is to tune the cycling parameters of the sorption refrigerators. Specifically, a higher desorption temperature for the pump yields a larger a longer autonomy and/or cooling power (more 3He condensed), but creates larger, possibly unacceptable, heat loads on the 4.5 K and 1.7 K stages. For our BLISS/SPICA prototype, the pump is heated to 30 K, this provides a desorbing fraction above 80% with a condensation point at 1.7 K. Higher temperatures would overload the 4.5 K stage with conductive load down the pump tube. The pumps heater is regulated with PID controllers which can be operated so as to not exceed desired thermal allocations at 4.5 K and 1.7 K. To cool down one of the evaporators, the pump is cooled by turning ON the pump heat switch (HS1 or HS4). As the latent heat of the pump is extracted, the heat switch is used as a clutch to insure that the load to 4.5 K does not exceed the allocation. The heat switch conductance is controlled by changing the heat flow to its miniature pump; it is servoed to maintain a constant heat flow to 4.5 K. The ADR magnetization occurs when both 3He coolers are at 300 mK. The current ramp rate is also regulated by a PID, in this case targeting either maximum 4.5 K power consumption or a salt pill temperature profile. The PID control allows optimization of the cycle time (thus duty cycle) and can provide robustness to degradations or failures of the satellite infrastructure. For example, the cycle could be slowed to accommodate a smaller maximum load to 4.5 K. 2.2. 300 mK and 50 mK thermal loads We have coupled the 300 mK and 50 mK coolers to aluminum dummy thermal ballast masses which mimic the masses, approximate physical aspect ratios, and parasitics of the shields and spectrometer designs for BLISS. The thermal demonstrator assembly is shown in Fig. 4. The 300 mK box is made with three trapezoidal shaped belts, which ensure the stiffness of the box, and 1 mm thick aluminum sheets cover the box. It is 450 mm high by 200 mm thick with 250 mm and 350 mm long trapezoid parallels. In total it consists of 5.4 kg of 6061 aluminum and 1.1 kg of copper including heat straps. It is supported by three carbon-fiber bipods [18], each
3 mm in diameter and 100 mm long. The high temperature ends of the bipods are decoupled from the 1 K plate, by graphite spacers, to be regulated to 1.7 K with 0.25 mW. The thermal budget for holding the 300 K box is estimated to 15 lW. The 50 mK stage consists of five massive 6061 aluminum pieces, 2 kg each. The central one features holes for the thermal link to the ADR, support and thermometers. The support is made by three carbon fiber bipods, each 1.5 mm in diameter by 30 mm long. The copper heat strap is going through the largest face of the 300 mK box inside a copper tube acting as a light baffle. The parasitic heat leak of the mechanical supports of the 50 mK box is estimated to 0.3 lW. 2.3. SPICA cryogenic testbed To perform the cryogenic tests, a dedicated cryostat has been developed at JPL. This cryostat is designed to mimic the SPICA 1.7 K and 4.5 K interfaces and to allow a demonstration of our refrigerator’s heat rejections at these two interfaces. It consists of a large 4 K cryostat built by High-Precision Devices [17] featuring a Cryomech PT 415. It has been equipped with a closed-cycle 1 K 4 He Joule–Thomson stage develop in house to provide cooling power at 1.7 K [15]. Both 4.5 K and 1.7 K heat sink stages are mechanically mounted to the 3.5 K and 1 K stages (respectively) with weak links, which are thermally regulated on both ends. The 4.5 K stage provides 45 mW of cooling power. The cold end of the 4.5 K stage is decoupled from the 3.5 K plate by a thin brass washer and regulated at 3.7 K using 4 mW. The 1.7 K stage is connected directly to the 1 K pot. A heat lift of 9 mW is available on this stage while the 1 K pot is regulated at 1.1 K with 5 mW. Fig. 4 gives an overview of the sub-Kelvin BLISS demonstrator. Precooling of this system from room temperature is a challenge due to the large heat capacity and necessarily low thermal conductance mechanical supports. The massive 10 kg 1 K stage, containing all the sub Kelvin cryogenics, is precooled with the 4 K pulse tube stage via two passive graphite heat switches [15] which have a thermal conductivity that decreases by a factor >105 from room temperature to 1 K. The measured cooled down time of the 1 K stage components to 1 K is 1 day. Once cold the net parasitic to the 1 K stage is <1 mW, because the graphite heat switches temperatures from 4 K to 1 K lead to very low thermal conductivity. More details are given in the paper [15]. The thermal connection of the 300 mK box and the 50 mK stage to the 1 K stage is required to be so small that passive heat switches are not feasible. Furthermore the 50 mK mass is only connected to 300 mK by HS7. To precool these stages from room temperature, we use two mechanical heat switches provided by HPD. These heat switches remove heat to the 3.5 K plate which is cooled by the pulse tube. The heat switches are actuated with room-temperature motors outside the cryostat. The 300 mK box has two copper links going through the 1 K plate, into the copper jaws of the mechanical heat switches. The sub Kelvin stages must be precooled to <10 K before the JT system can be activated. After which the 1 K stage and the sub Kelvin stages cool down to 1 K within 30 min. The cool down time of the sub Kelvin system is 2 days when no mass is cool down on the 50 mK stage and 4 days when 6 kg of aluminum are loaded. 3. Operating of the cryochain 3.1. Sub Kelvin heat switch operation In Fig. 5, the temperature of the 300 mK stage and the power consumption on the evaporators during 2 cycles of each cooler (with 8 mW at 4.5 K and 3 mW at 1.7 K) is presented. Peak heat load of 40 lW to the 300 mK stage occurs during the switching over from one 3He cooler to the other. This is due to the HS desorbing phase of the cycled cooler. The miniature pump efficiency
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Fig. 3. Low temperature gas gap HS design.
Fig. 4. The BLISS cryogenic demonstrator.
Fig. 5. Left: 300 mK HS4 and HS6 operating, right: turning OFF of HS7.
improvements are significant compare with previous results with the old heat switch design (1). The 300 mK temperature during the hand off increases from the nominal steady state 315 mK to a maximum of 340 mK for no more than a few minutes. The heat switch shut down energy on the 300 mK cooler is measured to be 80 mJ. The operating of HS7 is shown on the right plot of Fig. 5 by representing the 300 mK heat load variation during the ADR cooling. The HS7 shut down energy is lower than 5 mJ (which is 2% of the cooling energy of the ADR FAA salt pill). The total parasitics conductive heat load on the 50 mK stage is about 1 lW. This heat load
is dominated by the contribution from the stainless steel tube of the HS7 demonstrator model, estimated to 0.26 lW. The heat load would be reduced to 0.038 lW if the HS7 tube were made of titanium alloy TA6V or less than 8 nW with alloy 15-3-3-3 [16]. 3.2. ADR operating with a 6 kg dummy aluminum load The masse of the dummy 50 mK stage was loaded with 6 kg of aluminum at its maximum (and not 10 kg) because of the limited capacity of the ADR salt pill used. During the first demagnetization with only 2 kg of aluminum we measured a large thermal
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decoupling at 50 mK between the ADR cold tip and the aluminum load. This decoupling was located into the aluminum itself because of its poor thermal conductivity at this temperature. Since aluminum becomes superconducting around 1 K, the conductivity remains high enough to avoid large thermal gradient into the 300 mK box, but it is problematic at the 50 mK stage. This gradient has been reduced by intercepting the three carbon fiber bipods with a copper bus. Fig. 6 displays the time constant and thermal gradient we obtained with the 6 kg load. We can see that the DT is drastically increasing from 80 mK to 50 mK and we measured about 7 mK between the copper strap end and the aluminum load. The temperatures have been measured using lakeshores RuO2 thermometers. One has been bought calibrated and the other one has been calibrated during the first 50 mK cooling. This remaining decoupling has not been investigated so far. The cooldown time to get below 100 mK is about 15 min, and regarding the cooling curves we estimate to 45 min the time to cool down the dummy load down to 50 mK. The copper thermal strap has been oversized and we can see that even during the cool down, the thermal gradient is essentially located in between the copper strap interface and the aluminum. For an instrument version, the cool down time and final temperature gradient could be reduced by adding a copper bus all over the aluminum piece. With this current thermal coupling we can expect about 75 min to warm up and cool down 10 kg of aluminum. Fig. 7 shows the operation of the cryochain demonstrator in a full mode. Because of the short ADR capacity we had to change some inputs settings. The recycling time of the sorption coolers has been reduced to 10 h by increasing the heat sinks cooling resources to 10 mW at 4.5 K and 4 mW at 1.7 K. The 20 h ADR hold time has been achieved by increasing the operating temperature to 140 mK. With a 2.5 h warm up and cool down ‘‘ADR/dummy 6 kg load’’ cycling time, the duty cycle of this demonstrator set is close to 89%.
Fig. 7. Complete BLISS cycle operating with 6 kg at 140 mK and sorption coolers running with 10 mW at 4.5 K and 4 mW at 1.7 K.
3.3. Large BLISS ADR recycling simulation Fig. 8 shows three BLISS recycling with simulation of a 2.4 J ADR recycling at 450 mK. For this ADR cycling load, and with 8 mW at 4.5 K and 3 mW at 1.7 K the sorption coolers are cycling in 24 h. The ADR salt magnetization heat has been simulated using a heater located at the small ADR cold tip. The power was 100 lW during 400 min. If we add the expected 75 min needed to make the 50– 300 mK thermal cycling round trip, we get a cycling time of
Fig. 8. Cycling of 3He coolers with 8 mW at 4.5 K and 3 m W at 1.7 K. When both coolers are cold, ADR cycling simulation with 100 lW at 450 mK for a total load of 2.4 J each time.
7.9 h. If we consider a 90% efficiency in the demagnetization process, we obtain 0.24 J at 50 mK. The heat capacity of the 10 kg instrument cooling from 320 mK, at the end of ADR thermalisation, to 50 mK, would take about 0.03 J. So, in the worst case, we can argue a cooling capacity of 0.24–0.03 = 0.21 J at 50 mK. With a sorption coolers cycling of 24 h, the power would be 2.43 lW with a duty cycle of 75%. If we recycle twice the sorption coolers in between each ADR cycling we get to 86% of duty cycle and 1.2 lW over 48 h observation runs.
4. Conclusion
Fig. 6. Cooling down of 6 kg of aluminum from 300 mK to 50 mK.
This work was undertaken in the framework of the BLISS instrument. The goal was to demonstrate the cryochain operation with limited cooling powers available on the heat sinks on the SPICA satellite. The cryochain has been developed from 2009 to 2012. At this time the available powers for BLISS were 8 mW at 4.5 K and 3 mW at 1.7 K. This choice leads us to choose the cryo architecture we presented in this paper. We demonstrated the 300 mK continuous sorption cooler operating with success over a 24 h global recycling (12 h for each
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cooler) and we have shown that it has enough cooling capacity to cycle a 2.4 J ADR salt pill at 450 mK. The coupling of the sorption cooler to a pre-existing FAA ADR in order to run a dummy instrument, representative as possible of the real flight instrument (size and masses), has shown that it is possible to warm up to 450 mK and cool down again to 50 mK a 6 kg aluminum mass in 45 min. We have shown that the thermal coupling of the aluminum mass was not perfect and thus we expect to be able to do this thermal cycling in 1 h with 10 kg of aluminum at the 50 mK stage. According to what we measured and our mass scaling assumptions we can argue 75% of duty cycle for 24 h observation runs and 2.43 lW of cooling power at 50 mK. If we reduce the cooling power and extend the observation time by 12 h of one sorption cooler cycling duration, we can increase the duty cycle of the instrument. As an example, if we recycle twice the sorption coolers in between each ADR cycling a duty cycle of 86% is reached and 1.2 lW of net heat lift are provided over 48 h. The duty cycle can be improved this way as long as the ADR cooling power remains enough for the 50 mK stage cooling. It would be also possible to reduce the consumption at 4.5 K and 1.7 K and increase thereby the sorption cycling times. With the present stainless steel engineering version we are limited by the conductive heat loads which substantially increase the cycling time for small gains on the consumptions. In the case of a titanium alloy version, it would be possible to reduce the cryochain operating power demand more significantly.
National Aeronautics and Space Administration. We acknowledge Adrian Lee for lending the ADR magnet and salt pill used for this test.
Acknowledgements
[17] [18]
This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the
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Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
SPICA/BLISS cryo-chain demonstrator T. Prouvé c,a,⇑, L. Duband c, J. Hodis b, J.J. Bock a,b, C. Matt Bradford b,a, W. Holmes b a
Department of Physics and Astronomy, California Institute of Technology, Pasadena, CA 91106, United States Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, United States c Univ. Grenoble Alpes, CEA INAC-SBT, F-38000 Grenoble, France b
a r t i c l e
i n f o
Article history: Received 17 March 2015 Received in revised form 3 June 2015 Accepted 4 June 2015 Available online 11 June 2015 Keywords: Astrophysics Sub-Kelvin Space SPICA Sorption-cooler ADR
a b s t r a c t The Background Limited Infrared Submillimeter Spectrometer (BLISS) is an instrument proposed for SPICA, the Japanese–European space-borne telescope mission under study for a possible launch in the next decade. The BLISS concept is a suite of aluminum spectrometer modules totaling 10 kg cooled to 50 mK. Cooling this ambitious instrument with high-duty cycle within the stringent heat-rejection allocations envisioned for SPICA is a challenge. We have developed a solution consisting of two stages: (1) a continuous 300 mK intercept stage provided by two 3He sorption coolers operated sequentially, and (2) a 50 mK adiabatic demagnetization refrigerator (ADR) operated in single-shot mode. We have built a prototype cooler and demonstrated it in a dedicated SPICA-like thermal testbed with regulated stages enabling measurement of rejected heat at 1.7 K and 4.5 K. The approach offers lower mass than a dual-stage ADR, and lower rejected power to 1.7 K and 4.5 K than a comparable single-shot 300 mK system, while insuring a high duty cycle. As a demonstration of feasibility for SPICA and future cryogenic missions, we show long-term cooling with flight-like parasitics at 50 mK and 300 mK requiring only 3 mW and 8 mW rejected at 1.7 K and 4.5 K, respectively. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction SPICA is a proposed 3 m class space telescope optimized for mid- and far-IR (5–500 lm) observations of galaxies throughout the age of the Universe and star-formation sites in our own Galaxy [1]. It will be actively cooled to approximately 6 K in order to reduce the thermal emission from the telescope [2,3]. This low-background platform offers the potential for 2–4 orders of magnitude improvement in sensitivity in the important but relatively unexplored far-IR waveband. To capitalize on this, SPICA’s greatest science return will come from cryogenic instruments with arrays of low-background detectors. In the far-IR beyond 40 lm, high sensitivity requires bolometers with arrays of direct detectors such as transition-edge-sensed (TES) bolometers designed for the low-backgrounds, cooled to approximately 50 mK. SPICA must cool both the large telescope and provide heat lift to support this sub-Kelvin instrumentation. The SPICA spacecraft will use passive cooling available at L2, [7], and 4 K and 1.7 K closed-cycle Joule–Thompson coolers filled with 4He and 3He, respectively, working fluids. Heat lift at 4 K is used to both cool the telescope and provide a heat sink for the instrument suite. ⇑ Corresponding author at: Univ. Grenoble Alpes, CEA INAC-SBT, F-38000 Grenoble, France. http://dx.doi.org/10.1016/j.cryogenics.2015.06.001 0011-2275/Ó 2015 Elsevier Ltd. All rights reserved.
The 1.7 K lift is an additional support for the sub-Kelvin instrumentation. Cooling power is limited, so careful integration of the full system will be the key to SPICA’s success [6]. Stringent limits are placed on total instrument mass staged at 4 K, since 4 K mass drives total observatory parasitic load, and on the heat lift at 4 K and 1.7 K for the instruments. Two sub-K far-IR instruments have received substantial study for SPICA: a Fourier-transform spectrometer (FTS) named SAFARI is proposed by a European team led by SRON [4], and a sensitive grating spectrometer concept named BLISS has been developed by a team centered at JPL/Caltech [5]. Both studies have sought a 50 mK cooling scheme which is sufficiently low mass and high Carnot efficiency for lifting 50 mK power and rejecting it at 1.7 K and 4.5 K. 1.1. Approach Thus far, the least-massive solution to stage from 1.7 K to an ultimate 50 mK is a combination including a 3He sorption cooler and an ADR [9–11]. Both BLISS and SAFARI are using this hybrid approach. For BLISS, however, we have incorporated the additional advance of a continuous 300 mK intercept stage using two identical sorption refrigerators cycled sequentially. This approach, shown schematically in Fig. 1, offers three key advantages. First,
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Fig. 1. Schematic of the BLISS cryogenic chain design.
and most important, the 300 mK cycle time is removed from the duty cycle; a substantial gain since cycling a single-shot sorption fridge cycle can require about 10 h when limited to the 10 mW at 4.5 K and 5 mW at 1.7 K on current allocations for far-IR instruments. Second, both the 50 mK cold stage (10 kg of aluminum spectrometers for BLISS) and the intercept stage (5 kg of structure and shields) are maintained under 300–400 mK. This eliminates the reduction of efficiency from warming and cooling the >15 kg of instrument mass from 1.7 K to 300 mK each cycle which would require 1 J of cooling energy. Third, the thermalisation time associated with cycling up to 1.7 K is eliminated. For an instrument similar to BLISS called Zspec, these thermal transients were 8 h [8]. This approach offers a high duty cycle and thermodynamic efficiency which would otherwise be impossible under the stringent requirements for instantaneous heat lift at 4.5 K and 1.7 K. The cycling phases, shown in Fig. 2 are as follows. One of the two 3He coolers is used to maintain 300 mK while the other is recycling, dumping heat at 1.7 K and 4.5 K. Since the recycle time can be long, essentially the hold time of one of the coolers in light of the parasitics, the instantaneous loads to 1.7 K and 4.5 K, essentially due to the cooling and condensation of the 3He gas, can be low. The 50 mK stage is still a single-shot ADR, but its duty cycle can be in excess of 80%. The cold stage is cooled to 50 mK for an integer number of 3He cooler cycling times. 2. Demonstrator design The basic design of the CEA–SBT sorption cooler has been presented in a previous paper [9]. Two 6-l 3He adsorption coolers provide the continuous 300 mK cooling. The constructed prototypes are nearly identical to the flight units made for Herschel [12] except that the titanium pump tubes have been replaced with stainless steel. The replacement of the stainless-steel tubes by TA6V tubes would improve the system at two levels:
could be recycle with lower ultimate resource at 4.5 K. This gain is about 30%. Of course the duty cycle would be impacted. But for a double stage system, the duty cycle is not really an issue as long as both stages are tuned properly. Low T phase: Going to titanium will of course lower the load on the sorption stages (by 30% if we do not include ADR cycling). Meaning that a redesign toward a smaller system could be made. Smaller system means less energy to be dissipated at 1.7 and 4.5 K and thus we could either lower the dissipation and keep the same duty cycle, or keep the same dissipation and gain on the duty cycle (if needed). The general architecture is shown schematically in Fig. 1. Cooler 1 (C1) and cooler 2 (C2) are operated inside a 1.7 K aluminum box. A copper bus reaches out of the box to interface with the 4.5 K heat sink. The charcoal pumps and evaporators are shielded. In this way the two coolers do not radiatively couple to one another when one is cycling. Heat Switch one (HS1) and HS4 are charged with 4He, while the other heat switches are charged with 3He. HS2 and HS5 connect the evaporators to 1.7 K when the coolers are recycling. HS3 and HS6 connect the evaporators to the 300 mK stage when the coolers are cooling at low temperature. HS7 connects the ADR salt pill during the magnetization phase to extract the heat of magnetization. Initial operation of the fridge indicated that HS3 and HS6 were difficult to turn OFF when a cooler needed to be recycled [9]. This led to large temperature oscillations of the 300 mK stage and to an
Recycling phase: The gain on the duty cycle is not significant since it only impacts the heating phase of the pump. However when the pump is at 30 K the power dissipated thru the various links between the pump and the 4.5 K interface is lower with Titanium; meaning the cooler
Fig. 2. Schematic showing the BLISS refrigerator operation approach. (A) Cycling of one sorption cooler at a time, (B) cycling of the ADR.
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increase in the 3He consumption of the cold cooler. It was realized that because the HS3 and HS6 cold ends are maintained at 300 mK even when their respective evaporators are warmed, the cold ends were adsorbing 3He at rate comparable to the pumping speed of the mini pump, thus preventing complete evacuation of the heat switch. Small temperature variations on the continuous 300 mK stage lead to 3He gas bursts which turn HS on briefly, thus creating the oscillations. The solution [13] was to increase the pumping speed of the HS mini pump significantly above the rate of 3He adsorption rate. To do this, the long pumping tubes have been replaced by a 500 times more conductive pumping tube. The new low temperature gas gap HS design is presented in Fig. 3. For the ADR, we used an existing FAA salt pill [14], which was adapted to the BLISS cryo-chain demonstrator. The salt is supported from the 1.7 K superconducting magnet with a Kevlar suspension, which includes an intercept cooled by the 300 mK stage. While HS7 might be prone to the same problem of gas adsorption as HS 3 and HS 6, in practice it has no problem turning OFF. As the salt is demagnetized, the HS7 corresponding end is cooled accordingly. This naturally condenses the remaining 3He gas. At T < 200 mK the gas conduction in the heat switch was measured to be insignificant compared to other parasitics to the FAA salt pill. 2.1. 300 mK and ADR cycling control An important aspect of operating this integrated system is to tune the cycling parameters of the sorption refrigerators. Specifically, a higher desorption temperature for the pump yields a larger a longer autonomy and/or cooling power (more 3He condensed), but creates larger, possibly unacceptable, heat loads on the 4.5 K and 1.7 K stages. For our BLISS/SPICA prototype, the pump is heated to 30 K, this provides a desorbing fraction above 80% with a condensation point at 1.7 K. Higher temperatures would overload the 4.5 K stage with conductive load down the pump tube. The pumps heater is regulated with PID controllers which can be operated so as to not exceed desired thermal allocations at 4.5 K and 1.7 K. To cool down one of the evaporators, the pump is cooled by turning ON the pump heat switch (HS1 or HS4). As the latent heat of the pump is extracted, the heat switch is used as a clutch to insure that the load to 4.5 K does not exceed the allocation. The heat switch conductance is controlled by changing the heat flow to its miniature pump; it is servoed to maintain a constant heat flow to 4.5 K. The ADR magnetization occurs when both 3He coolers are at 300 mK. The current ramp rate is also regulated by a PID, in this case targeting either maximum 4.5 K power consumption or a salt pill temperature profile. The PID control allows optimization of the cycle time (thus duty cycle) and can provide robustness to degradations or failures of the satellite infrastructure. For example, the cycle could be slowed to accommodate a smaller maximum load to 4.5 K. 2.2. 300 mK and 50 mK thermal loads We have coupled the 300 mK and 50 mK coolers to aluminum dummy thermal ballast masses which mimic the masses, approximate physical aspect ratios, and parasitics of the shields and spectrometer designs for BLISS. The thermal demonstrator assembly is shown in Fig. 4. The 300 mK box is made with three trapezoidal shaped belts, which ensure the stiffness of the box, and 1 mm thick aluminum sheets cover the box. It is 450 mm high by 200 mm thick with 250 mm and 350 mm long trapezoid parallels. In total it consists of 5.4 kg of 6061 aluminum and 1.1 kg of copper including heat straps. It is supported by three carbon-fiber bipods [18], each
3 mm in diameter and 100 mm long. The high temperature ends of the bipods are decoupled from the 1 K plate, by graphite spacers, to be regulated to 1.7 K with 0.25 mW. The thermal budget for holding the 300 K box is estimated to 15 lW. The 50 mK stage consists of five massive 6061 aluminum pieces, 2 kg each. The central one features holes for the thermal link to the ADR, support and thermometers. The support is made by three carbon fiber bipods, each 1.5 mm in diameter by 30 mm long. The copper heat strap is going through the largest face of the 300 mK box inside a copper tube acting as a light baffle. The parasitic heat leak of the mechanical supports of the 50 mK box is estimated to 0.3 lW. 2.3. SPICA cryogenic testbed To perform the cryogenic tests, a dedicated cryostat has been developed at JPL. This cryostat is designed to mimic the SPICA 1.7 K and 4.5 K interfaces and to allow a demonstration of our refrigerator’s heat rejections at these two interfaces. It consists of a large 4 K cryostat built by High-Precision Devices [17] featuring a Cryomech PT 415. It has been equipped with a closed-cycle 1 K 4 He Joule–Thomson stage develop in house to provide cooling power at 1.7 K [15]. Both 4.5 K and 1.7 K heat sink stages are mechanically mounted to the 3.5 K and 1 K stages (respectively) with weak links, which are thermally regulated on both ends. The 4.5 K stage provides 45 mW of cooling power. The cold end of the 4.5 K stage is decoupled from the 3.5 K plate by a thin brass washer and regulated at 3.7 K using 4 mW. The 1.7 K stage is connected directly to the 1 K pot. A heat lift of 9 mW is available on this stage while the 1 K pot is regulated at 1.1 K with 5 mW. Fig. 4 gives an overview of the sub-Kelvin BLISS demonstrator. Precooling of this system from room temperature is a challenge due to the large heat capacity and necessarily low thermal conductance mechanical supports. The massive 10 kg 1 K stage, containing all the sub Kelvin cryogenics, is precooled with the 4 K pulse tube stage via two passive graphite heat switches [15] which have a thermal conductivity that decreases by a factor >105 from room temperature to 1 K. The measured cooled down time of the 1 K stage components to 1 K is 1 day. Once cold the net parasitic to the 1 K stage is <1 mW, because the graphite heat switches temperatures from 4 K to 1 K lead to very low thermal conductivity. More details are given in the paper [15]. The thermal connection of the 300 mK box and the 50 mK stage to the 1 K stage is required to be so small that passive heat switches are not feasible. Furthermore the 50 mK mass is only connected to 300 mK by HS7. To precool these stages from room temperature, we use two mechanical heat switches provided by HPD. These heat switches remove heat to the 3.5 K plate which is cooled by the pulse tube. The heat switches are actuated with room-temperature motors outside the cryostat. The 300 mK box has two copper links going through the 1 K plate, into the copper jaws of the mechanical heat switches. The sub Kelvin stages must be precooled to <10 K before the JT system can be activated. After which the 1 K stage and the sub Kelvin stages cool down to 1 K within 30 min. The cool down time of the sub Kelvin system is 2 days when no mass is cool down on the 50 mK stage and 4 days when 6 kg of aluminum are loaded. 3. Operating of the cryochain 3.1. Sub Kelvin heat switch operation In Fig. 5, the temperature of the 300 mK stage and the power consumption on the evaporators during 2 cycles of each cooler (with 8 mW at 4.5 K and 3 mW at 1.7 K) is presented. Peak heat load of 40 lW to the 300 mK stage occurs during the switching over from one 3He cooler to the other. This is due to the HS desorbing phase of the cycled cooler. The miniature pump efficiency
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Fig. 3. Low temperature gas gap HS design.
Fig. 4. The BLISS cryogenic demonstrator.
Fig. 5. Left: 300 mK HS4 and HS6 operating, right: turning OFF of HS7.
improvements are significant compare with previous results with the old heat switch design (1). The 300 mK temperature during the hand off increases from the nominal steady state 315 mK to a maximum of 340 mK for no more than a few minutes. The heat switch shut down energy on the 300 mK cooler is measured to be 80 mJ. The operating of HS7 is shown on the right plot of Fig. 5 by representing the 300 mK heat load variation during the ADR cooling. The HS7 shut down energy is lower than 5 mJ (which is 2% of the cooling energy of the ADR FAA salt pill). The total parasitics conductive heat load on the 50 mK stage is about 1 lW. This heat load
is dominated by the contribution from the stainless steel tube of the HS7 demonstrator model, estimated to 0.26 lW. The heat load would be reduced to 0.038 lW if the HS7 tube were made of titanium alloy TA6V or less than 8 nW with alloy 15-3-3-3 [16]. 3.2. ADR operating with a 6 kg dummy aluminum load The masse of the dummy 50 mK stage was loaded with 6 kg of aluminum at its maximum (and not 10 kg) because of the limited capacity of the ADR salt pill used. During the first demagnetization with only 2 kg of aluminum we measured a large thermal
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decoupling at 50 mK between the ADR cold tip and the aluminum load. This decoupling was located into the aluminum itself because of its poor thermal conductivity at this temperature. Since aluminum becomes superconducting around 1 K, the conductivity remains high enough to avoid large thermal gradient into the 300 mK box, but it is problematic at the 50 mK stage. This gradient has been reduced by intercepting the three carbon fiber bipods with a copper bus. Fig. 6 displays the time constant and thermal gradient we obtained with the 6 kg load. We can see that the DT is drastically increasing from 80 mK to 50 mK and we measured about 7 mK between the copper strap end and the aluminum load. The temperatures have been measured using lakeshores RuO2 thermometers. One has been bought calibrated and the other one has been calibrated during the first 50 mK cooling. This remaining decoupling has not been investigated so far. The cooldown time to get below 100 mK is about 15 min, and regarding the cooling curves we estimate to 45 min the time to cool down the dummy load down to 50 mK. The copper thermal strap has been oversized and we can see that even during the cool down, the thermal gradient is essentially located in between the copper strap interface and the aluminum. For an instrument version, the cool down time and final temperature gradient could be reduced by adding a copper bus all over the aluminum piece. With this current thermal coupling we can expect about 75 min to warm up and cool down 10 kg of aluminum. Fig. 7 shows the operation of the cryochain demonstrator in a full mode. Because of the short ADR capacity we had to change some inputs settings. The recycling time of the sorption coolers has been reduced to 10 h by increasing the heat sinks cooling resources to 10 mW at 4.5 K and 4 mW at 1.7 K. The 20 h ADR hold time has been achieved by increasing the operating temperature to 140 mK. With a 2.5 h warm up and cool down ‘‘ADR/dummy 6 kg load’’ cycling time, the duty cycle of this demonstrator set is close to 89%.
Fig. 7. Complete BLISS cycle operating with 6 kg at 140 mK and sorption coolers running with 10 mW at 4.5 K and 4 mW at 1.7 K.
3.3. Large BLISS ADR recycling simulation Fig. 8 shows three BLISS recycling with simulation of a 2.4 J ADR recycling at 450 mK. For this ADR cycling load, and with 8 mW at 4.5 K and 3 mW at 1.7 K the sorption coolers are cycling in 24 h. The ADR salt magnetization heat has been simulated using a heater located at the small ADR cold tip. The power was 100 lW during 400 min. If we add the expected 75 min needed to make the 50– 300 mK thermal cycling round trip, we get a cycling time of
Fig. 8. Cycling of 3He coolers with 8 mW at 4.5 K and 3 m W at 1.7 K. When both coolers are cold, ADR cycling simulation with 100 lW at 450 mK for a total load of 2.4 J each time.
7.9 h. If we consider a 90% efficiency in the demagnetization process, we obtain 0.24 J at 50 mK. The heat capacity of the 10 kg instrument cooling from 320 mK, at the end of ADR thermalisation, to 50 mK, would take about 0.03 J. So, in the worst case, we can argue a cooling capacity of 0.24–0.03 = 0.21 J at 50 mK. With a sorption coolers cycling of 24 h, the power would be 2.43 lW with a duty cycle of 75%. If we recycle twice the sorption coolers in between each ADR cycling we get to 86% of duty cycle and 1.2 lW over 48 h observation runs.
4. Conclusion
Fig. 6. Cooling down of 6 kg of aluminum from 300 mK to 50 mK.
This work was undertaken in the framework of the BLISS instrument. The goal was to demonstrate the cryochain operation with limited cooling powers available on the heat sinks on the SPICA satellite. The cryochain has been developed from 2009 to 2012. At this time the available powers for BLISS were 8 mW at 4.5 K and 3 mW at 1.7 K. This choice leads us to choose the cryo architecture we presented in this paper. We demonstrated the 300 mK continuous sorption cooler operating with success over a 24 h global recycling (12 h for each
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cooler) and we have shown that it has enough cooling capacity to cycle a 2.4 J ADR salt pill at 450 mK. The coupling of the sorption cooler to a pre-existing FAA ADR in order to run a dummy instrument, representative as possible of the real flight instrument (size and masses), has shown that it is possible to warm up to 450 mK and cool down again to 50 mK a 6 kg aluminum mass in 45 min. We have shown that the thermal coupling of the aluminum mass was not perfect and thus we expect to be able to do this thermal cycling in 1 h with 10 kg of aluminum at the 50 mK stage. According to what we measured and our mass scaling assumptions we can argue 75% of duty cycle for 24 h observation runs and 2.43 lW of cooling power at 50 mK. If we reduce the cooling power and extend the observation time by 12 h of one sorption cooler cycling duration, we can increase the duty cycle of the instrument. As an example, if we recycle twice the sorption coolers in between each ADR cycling a duty cycle of 86% is reached and 1.2 lW of net heat lift are provided over 48 h. The duty cycle can be improved this way as long as the ADR cooling power remains enough for the 50 mK stage cooling. It would be also possible to reduce the consumption at 4.5 K and 1.7 K and increase thereby the sorption cycling times. With the present stainless steel engineering version we are limited by the conductive heat loads which substantially increase the cycling time for small gains on the consumptions. In the case of a titanium alloy version, it would be possible to reduce the cryochain operating power demand more significantly.
National Aeronautics and Space Administration. We acknowledge Adrian Lee for lending the ADR magnet and salt pill used for this test.
Acknowledgements
[17] [18]
This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the
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