50th International Conference on Environmental Systems ICES-2021-188 12-15 July 2021

Thermal Design of SPICA Cryogenic Cooling System

Masaru Saijo1, Takao Nakagawa2, Keisuke Shinozaki1, Kenichiro Sawada1, Hiroyuki Ogawa3, Hideo Matsuhara2, Chihiro Tokoku4, Naoki Isobe5, and Toyoaki Suzuki6 Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 252-5210

This paper presents a thermal design of a cryogenic cooling system for the Space Infrared Telescope for Cosmology and Astrophysics (SPICA), which is an infrared astronomy mission with a 2.5-m telescope. SPICA is a joint space mission between ESA and JAXA and was one of the three candidates for the ESA's Cosmic Vision Medium Class M5 mission. One of the challenging tasks of SPICA is to cool the telescope and all instruments to cryogenic temperature, below 8 K, for highly sensitive, high-resolution observations. Two possible configurations, vertical and horizontal ones, were proposed for SPICA, and the vertical configuration was chosen mainly from system points of view. This paper discusses the potential difficulties in thermal design with particular emphasis on the vertical configuration compared to the horizontal configuration. We show the latest thermal design and the steady- state thermal analysis results especially for the vertical configuration.

Nomenclature 2ST = 2 Stage Stirling Cooler CRYO = Cryogenic Assembly FPIs = Focal Plane Instruments JT = Joule-Thomson Cooler MCS = Mechanical Cooler system MLI = Multi-Layer Insulation SIA = Science Instrument Assembly SPICA = Space Infrared Telescope for Cosmology and Astrophysics SVM = Service Module TIRCS = Thermal Insulation and Radiation Cooling System TS = Telescope Shield TSM = Truss Separation Mechanism

I. Introduction SPICA (the SPace Infrared telescope for Cosmology and Astrophysics) is the joint European-Japanese infrared space telescope that was one of the three remaining candidates for the ESA's Cosmic Vision Medium Class M5 mission but was canceled in October 2020. The SPICA spacecraft is designed for mid- and far-infrared astronomy, and its objective is to reveal the evolutionary history of the universe. For further details of the SPICA mission, see Roelfsema et al.1,2.

1 Engineer, Research Unit 2, Research and Development Directorate, 2-1-1 Sengen. 2 Professor, Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Chuo-ku. 3 Professor, Department of Space Flight Systems, Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Chuo-ku. 4 Engineer, Department of Space Flight Systems, Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Chuo-ku. 5 Assistant Professor, Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Chuo-ku. 6 Associate Professor, Institute of Space and Astronautical Science, Department of Space Astronomy and Astrophysics, 3-1-1 Yoshinodai, Chuo-ku. One of the most challenging tasks from a design perspective is to cool all the science instruments and the telescope to a cryogenic temperature (< 8K) for highly sensitive, high-resolution infrared observations. Figure 1 gives an overview of the latest SPICA configuration. The observatory is composed of the PayLoad Module (PLM) and the SerVice Module (SVM). PLM is composed of the Science Instrument Assembly (SIA) and the CRYOgenic assembly (CRYO), which cools SIA using mechanical cryocoolers and a passive radiative cooling system. JAXA is in charge of the design and development of CRYO, which is a key technology for the success of the mission.

Cryogenic Assembly

(CRYO)

(SIA)

(PLM)

Payload Module Payload

Science InstrumentAssembly Science (SVM)

Service Module Service

Figure 1. SPICA latest configuration overview.

We had performed for several years the conceptual design study of SPICA spacecraft in the horizontal configuration, in which the telescope's axis is perpendicular to the spacecraft axis3,4. However, in recent years, the vertical configuration, in which the telescope's axis is parallel to the spacecraft axis, was proposed. The vertical configuration has potential advantages such as mass reduction, increase in the power generation, simplifying the SIA support structure, and is considered to be more promising than the horizontal configuration in the current study. For the reasons above, mainly from viewpoints other than thermal design, the SPICA system teams decided to employ the vertical configuration. The configuration change requires a drastic change in thermal design. Hence, a significant impact on the thermal feasibility of CRYO could exist for the current configuration. This paper discusses the possible difficulties in thermal design with particular emphasis on the vertical configuration compared to the horizontal configuration. We then show the latest thermal design and analysis result of the cryogenic cooling system of SPICA PLM in the vertical configuration.

II. Thermal design requirements and design drivers This section describes the design requirements and design drivers for thermal design. One of the SPICA thermal design requirements is to cool the telescope below 8K to reduce the photon noise caused by thermal radiation from the telescope below the natural background radiation. The natural background noise limit enables astronomers to make infrared astronomical observations with unprecedented sensitivity. Another requirement is to cool FPIs for the superior sensitivity required to make the best use of the low background of the 8 K telescope. To meet the requirements, CRYO provides FPIs with two temperature stages: 4.8 K (4K-JT stage) and 1.8 K (1K-JT stage). There are two primary heat load sources to SIA. One is the thermal load from SVM, which is at the room temperature, and the other is the radiative thermal load by the direct sun illumination. SPICA is expected to be launched by a Japanese H3 rocket into an orbit around the second libration point (L2) of the Sun-Earth system. The 2 International Conference on Environmental Systems

orbit is 1.5 million km away from the Earth. In the orbit, the solar and earth radiations come from almost the same direction. Since the telescope's boresight must be moved toward the target, SPICA needs structures to avoid exposing SIA to the direct solar illumination during operation. Also, three focal plane instruments (FPIs, composed of SAFARI1 – SpicA FAR-infrared Instrument, SMI5 – SPICA Mid-Infrared Instrument, and B-BOP6 – magnetic field (B) explorer with BOlometric Polarimeter) on SIA dissipate the heat based on their operation modes. SIA needs to be cooled to a cryogenic temperature with these heat dissipations.

III. A conceptual study on the spacecraft configuration for thermal design We discuss in this section the conceptual thermal trade-off study on the spacecraft configurations. Figure 2 shows schematic overviews of the two configurations. The left panel shows the vertical configuration, in which the sun shields are to be placed at the right side (solar direction) of SIA, while the other set of thermal shields are to be placed between PLM and SVM on the bottom side. On the other hand, the right panel shows the horizontal configuration, in which the sun shields and thermal shields are to be placed at the bottom side (solar direction) of SIA. The solar array panel is mounted without a deploying mechanism at the bottom of SVM.

Figure 2. Schematic view of the vertical configuration (left) and horizontal configuration (right)

From the thermal design point of view, we have two important issues: one is how to reduce the heat flows to cryogenic temperature stages, and the other is how to increase the view factors to deep space to enable effective radiative cooling. The most significant difference between the two configurations is the directions of heat flows to cryogenic temperature stages. In the vertical configuration, the heat comes from two directions (warm SVM and sun shields), while in the horizontal configuration, the heat comes only from one direction (warm SVM). This makes the implementation of effective thermal shielding easier in the horizontal configuration than in the vertical configuration. Since the radiative heat transfer dominates the heat flows near the ambient temperature, the amount of heat that we must control is larger in the vertical configuration than in the horizontal configuration. Another important factor for the design of CRYO is how to keep enough view factors for effective radiative cooling. Figure 2 also shows that the horizontal configuration has a wider solid angle toward deep space for radiative cooling than the vertical configuration has. Hence the horizontal configuration can allocate larger view factors for effective radiative cooling than the vertical configuration. Therefore, in principle, the horizontal configuration is a better choice from the viewpoint of thermal design than the vertical configuration. However, mainly from system points of view, the vertical configuration was chosen for SPICA in the winter of 2020. Hence, to assess the thermal feasibility of the vertical configuration, we performed a preliminary thermal analysis with a simple thermal mathematical model. We found feasible thermal solutions not only for the horizontal configuration but also for the vertical configuration. However, we found that the vertical configuration is more challenging than the horizontal configuration due to the direction of heat input and the field of view to deep space.

3 International Conference on Environmental Systems

IV. Latest thermal design overview Considering the conceptual study of the vertical configuration above, we performed the thermal design for CRYO, which we'll present in this section. The thermal subsystem design meets this requirement by employing a combination of active cooling (mechanical cryocoolers) and passive cooling (a radiative cooling system). A conventional cryogenic system is cooled with cryogen, such as liquid Helium. Such a system needs a huge vacuum vessel for storage which would be very massive and occupy a considerable volume. Thus, the conventional cooling method would not leave room for a large telescope. Therefore, we have selected a hybrid cooling system for the large aperture telescope that does not need cryogen7,8,9. Figure 3 shows the cooling chain in CRYO together with the SIA components to be cooled below 8 K. The SPICA Mechanical Cooler system (MCS) uses three types of cryocoolers: double-stage Stirling coolers (2ST), 4 K Joule- Thomson coolers (4K-JT), and 1 K Joule-Thomson coolers (1K-JT)10: 1) 2ST - cool TS below 30 K and also act as precoolers for the Joule-Thomson coolers. - we expect to have a cooling power of 320 mW @26 K at EOL for TS = 30 K (after taking the 50 % system margin for cooling power and 4K margin (including delta T=2 K through thermal strap) for interface temperature).

2) 4K-JT - cool the telescope below 8 K and provide the focal plane instruments (FPIs) with a 4.8 K stage. - we expect to have a cooling power of 30 mW @ 4.8 K at EOL (after taking the 33 % system margin for cooling power and 0.3 K margin for interface temperature). 3) 1K-JT - provide FPIs with the 1.8 K temperature stage. Two FPIs, which require a lower temperature for their detectors, have dedicated sub-K coolers. - we expect to have a cooling power of 10 mW @ 1.8 K at EOL (after taking the 33 % system margin for cooling power and 0.3 K margin for interface temperature). All the compressors and displacers are mounted on the cooler plates located at the side of SVM, while the cooler driver electronics are mounted in SVM, as are the other electronic boxes. Since the 2ST stage (TS) is the boundary for SIA, and most of the SIA components are connected to the 4K-JT stage (thick green lines in Figure 3), the most critical part of the design is the feasibility of the thermal budget at the 2ST stage and 4K-JT stage. In the next section, we discuss the system's thermal balance with particular attention to consolidating the heat budget at the 2ST stage and 4K-JT stage in the current analysis.

Figure 3. Cooling chain of the SPICA cryogenic system. 4 International Conference on Environmental Systems

To minimize the heat reaching SIA and provide the low-temperature stage required for the cryocoolers operation, CRYO has a Thermal Insulation and Radiation Cooling System (TIRCS). TIRCS is composed of a truss structure and multilayer shields. Figure 4 shows the main conductive heat paths from SVM to SIA through CRYO. We have three primary conductive paths: (A) truss structure, (B) cryoharness, and (C) cooler pipes from CRYO to SIA. The truss structure is composed of three sets of low-conductivity CFRP bipods. SIA is supported by the lower truss structure connected to SVM via D, as shown in Figure 4. Since it is critical to reducing the heat load and vibration conducted from SVM to SIA, we propose the Truss Separation Mechanism (TSM)11,12. TSM consists of tubes with low thermal conductivity and a CFRP separation spring installed between the truss structure and SIA. Lengthening the thermal path and minimizing the area limits the amount of conductive heat transfer to less than 2 mW. Diameter and material for cryoharness are based on the information from each instrument team. We included the 33% margin for harness thermal conductivity.

Figure 4. Conductive heat paths in the orbit configuration with major paths in red. The conductive heat paths to SIA include (A) truss structure, (B) cryoharness, and (C) cooler pipes.

Figure 55 and Figure 66 show the thermal design overview. TIRCS has an outer/inner sunshield and two SVM shield layers for radiative cooling and insulation. This configuration was introduced to minimize the heat from sunlight and radiation from the warmer SVM, respectively. The outer sunshield has hinge structures attached to the two points on SVM and is deployable with a 10 deg tilt angle in orbit to increase the amount of heat dumped into deep space. The tilt angle was determined by the dependence analysis that showed the sensitivity for the deployable angle above 10 deg is not strong. The inner sunshield is divided into upper and lower parts at the height of the SVM shield 2. The aluminum sheets of the upper and lower inner sunshields are separated, although they share a common support structure, to minimize the heat transfer from higher temperature structure to lower temperature structure. The lower inner sunshield is connected to SVM by the support pipes made of GFRP with low thermal conductivity, through which the lower inner sunshield is conductively linked to SVM. TS is placed between warmer structures (sunshield, SVM shields) and the telescope baffle and is actively cooled by 2ST to 30K. The sunshields and TS are designed to 5 International Conference on Environmental Systems

protect SIA from sunlight in all observation attitudes and avoid the hotter shield for a view of the telescope baffle. The values of temperature-dependent thermal properties for the primary materials such as CFRP and Aluminum used in the thermal analysis model are based on data we measured in the low-temperature sample test campaign. Since the detailed design of the CFRP material used for the main truss is not entirely fixed yet, which could affect the thermal property, we considered the 100% margin on the measured value of CFRP thermal conductivity.

Outer SunShield Al honeycomb sandwich panel with Al bar SIA (Baffle, FPIs, etc.) (Outer) MLI (ε = 0.80 (Ag/FEP), a= 0.30,

8K boundary εeff = 0.0031,) (Outer) (Al) (Inner) Al (ε = 0.05, ρ =0.5)

Upper Inner SunShield (above SVM shield 2) Al pipe lattice structure with Al skin

(Outer) MLI (ε = 0.05, εeff = 0.01) (Inner) Al (ε = 0.05, ρ = 0.5) Telescope Shield (Radiator on edge) (ε = 0.8) 30K boundary (Outer) Al (ε = 0.05, ρ = 0.5) (Inner) Al (ε = 0.05, ρ = 0.5) Lower Inner SunShield (below SVM shield 2) (Radiator on edge) (ε = 0.8) Al pipe lattice structure with Al skin (Outer) MLI (ε = 0.05, εeff = 0.0031) (Inner) Al (ε = 0.05, ρ = 0.5) (Radiator on edge) (ε = 0.8)

Figure 5. The thermal design overview (1/2). Upper Truss Pipe (CFRP, φ=100 mm, t=1.0 mm)

Stiffener Square pipe (CFRP, t4.0mm) (Outer) CFRP (ε = 0.8)

Truss Separation Mechanism (TSM) Three sets of CFRP springs C=0.17 mW/K (8 K-30 K)

Lower Truss Pipe (CFRP, φ100mm×t1.0mm) (Outer) CFRP (ε = 0.8)

SVM shield 2 (Outer) Al (ε = 0.05, ρ = 0.5) SVM (Inner) Al (ε = 0.05, ρ = 0.5) SVM shield 1 273K boundary (Outer) Al (ε = 0.05, ρ = 0.5) (top) MLI (ε = 0.05, εeff = 0.0031) (Inner) MLI (ε = 0.05), ε = 0.0031) eff

Figure 6. The thermal design overview (2/2). 6 International Conference on Environmental Systems

V. Thermal analysis results This section shows the results of the thermal analysis of CRYO. Figure 7 shows an overview and temperature distribution in the static thermal model. We fix the temperature of SVM at 273 K as the interface boundary condition. SVM is covered by Multi-Layer Insulation (MLI) to be thermally isolated from the thermal shields. Figure 8 is the heat flow map resulting from our thermal analysis. Red lines show radiative heat flows, and blue lines show conductive flows. Green lines show the fixed heat input to the 4K-JT stage, including FPIs heat dissipations, conductive/radiative heat input from the structure, and environment heat input. A significant fraction of the heat from the sun is radiated into space and reduces the heat flow from the outer sunshield to the inner sunshield. Similarly, most of the heat from SVM is radiated into space, and the heat input to the lower-temperature stage is minimized. The conductive heat transfer through TSM to the stiffener, stable truss structure for the telescope is limited to a few mW. The figure shows that the heat flow to the 4K-JT is 29.5mW, which is compatible with the cooling capability allocation of 30mW at the 4K-JT stage. The figure also shows that the heat flow from TS to the 2ST is 310 mW, which is compatible with the cooling capability allocation of 320 mW for TS temperature of 30 K. We hence conclude that the current thermal model meets the heat budget requirements both at the 4K-JT stage and at TS.

Outer Sunshield

Inner Sunshield SIA Telescope Shield

Telescope shield

Lower truss

SVM shield 2 SVM shield 1 SVM Top surface of SVM

Figure 7. Overview and temperature distribution in the steady- state thermal model.

7 International Conference on Environmental Systems

Figure 8. Heat-flow diagram (in mW) of the static thermal model. Red lines show radiative heat flows, blue lines show conductive heat flows, and green lines show the fixed heat input

VI. Conclusion SPICA cools its telescope to cryogenic temperature (<8K) to achieve unprecedented sensitivity for infrared astronomy. The SPICA system teams decided to employ the vertical configuration instead of the horizontal configuration from the system point of view. We performed the conceptual study on the spacecraft configuration for thermal design, and we found that the vertical configuration is more challenging than the horizontal configuration due to the direction of heat input and the field of view to deep space, yet it is feasible from the thermal design point of view. Then, to meet the cooling requirement for SPICA, we employed a combination of mechanical cryocoolers and passive radiative cooling system, and designed the cooling system in the vertical configuration. We performed a steady-state thermal analysis for the latest design, and the result shows that the current design meets the heat budget requirements.

Acknowledgments The authors would like to gratefully acknowledge Sumitomo Heavy Industries, Ltd. (SHI) and NEC Corporation for the dedicated contribution to the SPICA project.

8 International Conference on Environmental Systems

References 1Roelfsema, P.R., et al., "SPICA - A Large Cryogenic Infrared Space Telescope: Unveiling The Obscured Universe," Pub. Astro. Soc. Australia, 35, 30 (2018). 2Roelfsema, P.R., et al., "The joint infrared space observatory SPICA: Unveiling the obscured universe," Proc. SPIE 11443, 11443-25 (2020). 3Shinozaki, K., et al. "Mechanical cooler system for the next-generation infrared space telescope," Proc. SPIE 9904, 99043W (2016). 4 Ogawa, H., Shinozaki, K., and Nakagawa, T., "SPICA Cryogenic Infrared Telescope Thermal Design," Proc. International Conference on Environmental Systems, ICES-2016-305 5Kaneda, H., et al., "SPICA mid-infrared instrument (SMI): conceptual design and feasibility studies," Proc. SPIE 10698, 106980C (2018) 6André, P., et al., "Probing the cold magnetized Universe with SPICA-POL (B-BOP)," Pub. Astro. Soc. Australia, 36, 29 (2019). 7Sugita, H., et al., "Cryogenic system design of the next-generation infrared space telescope," Cryogenics 50, 566–571 (2010). 8Ogawa, H., et al., "New cryogenic system of the next-generation infrared astronomy mission SPICA," Proc. SPIE 9904, 9904- 82 (2016). 9Nakagawa, T., et al., "Cryogenic System of the infrared space mission SPICA," Proc. SPIE 11443, 1144328 (2020) 10Shinozaki, K., et al., "Mechanical cooler system for the infrared space mission SPICA," Proc. SPIE 11443, 1144329 (2020) 11Mizutani, T., et al., "Preliminary structural design and key technology demonstration of cryogenic assembly in the next- generation infrared space telescope SPICA," JATIS 1, 027001 (2015). 12Matsumoto, J., et al., "Conceptual Study of Truss Separation Mechanism for Space Infrared Telescope SPICA" Proc. of the Space Sciences and Technology Conference (2020)

9 International Conference on Environmental Systems