DEVELOPMENT OF THE SECOND-GENERATION DOUBLE-STAGE STIRLING CRYOCOOLER FOR INNOVATIVE SPACE MISSIONS

Transcription

1 48 DEVELOPMENT OF THE SECOND-GENERATION DOUBLE-STAGE STIRLING CRYOCOOLER FOR INNOVATIVE SPACE MISSIONS Yoichi SATO 1), Hiroyuki SUGITA 1), Takao NAKAGAWA 2) Shoji TSUNEMATSU 3), Kiyomi OTSUKA 3), Katsuhiro NARASAKI 3) 1) Aerospace Research and Development Directorate, Japan Aerospace Exploration Agency, Sengen, Tsukuba, Ibaraki , Japan TEL.: , FAX: , 2) Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai, Chuo-ku, Sagamihara, Kanagawa , Japan 3) Sumitomo Heavy Industries, Ltd., Niihama Works, 5-2 Soubiraki-cho, Niihama, Ehime , Japan Abstract Mechanical cryocooler for space application is efficient way to cool down the optical detector, telescope and thermal shields to cryogenic temperature below about 100 K in the aspect of mass and size. The 20 K-class double-stage Stirling cycle cryocooler with cooling power of 200 mw at 20 K and lifetime of 1.5 years was originally developed for a cooling component of the Japanese IR telescope satellite AKARI launched in Based on this AKARI cryocooler, improvements with higher cooling performance and reliability with 1) the optimized 8-mm diameter displacer at second stage, 2) the flexure bearings for displacer supporting and 3) selection of low-outgassing materials and optimal baking process were investigated to develop the second-generation double-stage Stirling cryocooler for application to the next innovative astronomy mission such as ASTRO-H/SXS (2015) and SPICA (2022). The verification tests by using the EM (Engineering Model) were performed and maximum cooling power of 17.6 K with 200 mw at the 2nd cold stage and 96.1 K with 1000 mw at the 1st cold stage was obtained with margin. Mechanical performance test was also carried out and proved tolerability for mechanical environment of qualification level of ASTRO-H/SXS. Continuous running to verify specified lifetime of over 3 years is still under testing and hours (~ 560 days) in total has just achieved as of August Introduction The astronomical and earth observation mission generally needs cryogenic temperature for the detector, telescope and thermal shields to enhance observational sensitivity and resolution associated with noise reduction. The Stirling cycle cryocoolers have been widely used in the astronomical and earth observation mission of JAXA as summarized in Table 1. The 20 K-class double-stage Stirling cryocooler, which had originally developed for the IR telescope cryostat of AKARI (Ref. 1, 2), is splittype Stirling cycle cryocooler composed of a double-staged cold head, a linear compressor and a connecting tube. The compressor has two opposite pistons supported by the linear ball bearings, which achieves the piston clearance seal and long piston stroke with low drive frequency for high efficient regenerate. The contact seals are used for displacer seals in the cold head. The cold head has an active balancer opposite to the displacer as a counter weight to reduce vibration (Ref. 3). The AKARI cryocooler had been operated on orbit for more than 1.5 years of design lifetime. To meet c Japanese Rocket Society

2 49 JSTS Vol. 28, No. 1 requirements of the next ASTRO-H/SXS and SPICA mission, longer lifetime and higher cooling power of the double-stage Stirling cryocooler were strongly required. This improvement also enhances cooling performance of the Joule-Thomson (JT) cycle cryocooler system because lower temperature at the second cold stage of the double-stage Stirling cryocooler is quite effective to increase the JT cooling power. Table 1 Applications of the Stirling cycle cryocooler to JAXA missions Mission Single-stage Stirling cryocooler ASTRO-EII/SUZAKU (X-ray Astronomy Satellite) SELENE/KAGUYA (Lunar Exploration Satellite) Planet-C/AKATSUKI (Venus Climate Satellite) GCOM-C1/SGLI (Global Change Observation Mission) ASTRO-H/SXI (X-ray Astronomy Satellite) Double-stage Stirling cryocooler ASTRO-F/AKARI (Infrared Astronomy Satellite) ISS/JEM/SMILES (Earth Atmosphere Observation) ASTRO-H/SXS (X-ray Astronomy Satellite) SPICA (Next Infrared Astronomy Satellite) ATHENA (X-ray Astronomy Satellite) Cooling objectives X-ray spectrometer Gamma ray spectrometer Near-infrared camera Infrared sensor X-ray CCD Telescope and detector Superconductive submilimeter mixer Microcalorimeter Telescope and detector TES microcalorimeter ASTRO-H is the Japanese X-ray astronomy satellite planned for launch in 2014 by JAXA with an H-IIA launch vehicle, following the successful SUZAKU mission (Ref. 4). The Soft X-ray Spectrometer (SXS), one of the scientific instruments onboard ASTRO-H, is being developed under extensive international collaboration between Japan and the US, with European participation. The SXS is a high resolution spectrometer utilizing an X-ray microcalorimeter array (Ref. 5). To achieve a high energy resolution of 7 ev at kev, the microcalorimeter is cooled down to 50 mk by the Adiabatic Demagnetization Refrigerator (ADR) with a 30-liter superfluid helium as heat sink. To extend a required lifetime of liquid helium over 3 years in orbit, two units of the upgraded doublestage Stirling cryocooler in redundant configuration are operated with half power of 2 50 W for the vapor-cooled shields in nominal. There is also a 4 K-class JT cryocooler to reduce parasitic heat load to the helium tank. Another two units of the upgraded double-stage Stirling cryocoolers are used as precoolers in this JT system. Since these are not redundant configuration, in the case of one precooler failure, a remaining one must be operated with max power to minimize the parasitic heat load through the failed cryocooler (Ref. 6). The schematics of the SXS cooling chain is shown in Fig. 1. The next-generation infrared telescope mission SPICA (the Space Infrared Telescope for Cosmology and Astrophysics) has been studied as a JAXA pre-project in collaboration with ESA to be launched in 2022 (Ref. 7). The SPICA has a large IR telescope with 3-m diameter and will cover midand far-ir astronomy in a range of µm with extremely high sensitivity and spatial resolution during a long period of over 5 years for goal. The SPICA telescope and focal plane instruments are cooled down to below 6 K by the radiant and mechanical cooling system without cryogen. Mechanical cryocooler system consists of the two sets of 4 K-class Joule-Thomson cryocooler and two sets of 1 K- class Joule-Thomson cryocooler for redundancy. Each JT system combined with one unit of the upgraded double-stage Stirling cryocooler as precooler through mechanical heat switches (Fig. 2). The heat switches will work to disconnect thermally with a failed precooler to minimize the parasitic heat load to the telescope stage (Ref. 8).

3 50 Fig. 1 Schematics for cooling chain of ASTRO-H/SXS: The upgraded double-stage Stirling cryocooler 2ST are used as dewer shield coolers and JT precoolers in the SXS cooling system. g TOB (Telescope Optical Bench) < 5.5K Baffle Telescope shell Sun Shield Telescope shell 30K Baffle16.5K Inner shield 55K IOB (Instrument Optical Bench) < 5.5K 5mW 1.7K Heat Switch 12K 5mW JT orifice BLISS SAFARI MIRACLE MIRMES FPI (Focal Plane Instruments) JT closed cycle loop MIRHES FPC SCI 20mW 4.5K 17K 20mW Sun Shield Inner Shield Middle Shield Outer Shield Cooler Radiator SAP Middle shield 82K Outer shield 121K Cooler baseplate 1K-JT cooler HP or 1K-JT LHP 90K JT compressors 2ST-Precooler 2ST-Precooler 1K-JT cooler 4K-JT cooler 1K-PC 1K-PC 1K-JT 4K-JT 90K 2ST-Precooler 2ST-Precooler 4K-PC 4K-PC 4K-JT cooler 4K-JT Cooler radiator 65 W 60 W 60 W 65 W 65 W 60 W 60 W 65 W Bus Module panel 1K JTD 1K PCD 1K JTD 4K-JTD 4K PCD 4K JTD Cooler Drive Electronics Bus Module Fig. 2 Configuration of SPICA mechanical cooling system: Each JT cryocooler has one unit of the upgraded double-stage Stirling cryocooler for precoler 2 Description of the second-generation double-stage Stirling cryocooler Several technical approaches were investigated to upgrade the AKARI cryocooler. Optimized diameter to 8 mm at the second displacer provided 62.5 % increased cooling power due to larger gas expansion volume at 20 K stage (Ref. 9). Seal performance of the displacer is also sensitive factor to a cooling performance. The current research revealed that mechanical wear of displacer seal material contributes approximately 76 % of total aging degradation of cooling power in the AKARI cryocooler. Thus, support mechanism with flexure bearings for the displacer was adopted because of less mechanical wear between displacer and its cylinder to sustain the seal performance for long-term operation. The flexure bearing is key mechanism for space cryocooler to provide higher reliability. It was originally utilized in the Single-stage Stirling cryocooler and continuous operation in orbit and ground tests for more than 5 years was achieved (Ref. 10). The flexure bearings were adopted not only to the displacer of the second-generation double-stage Stirling cryocooler but also to the compressors of the 1K and 4K-class JT cryocooler. Outgassing from component parts and materials that are used in

4 51 JSTS Vol. 28, No. 1 the compressor and cold head is critical issue to improve lifetime. The molecular impurity accumulated in the helium gas for long-term operation could be solidified at the cold area of the cold head. And after all it will bring cooling power deterioration due to insufficient regeneration and unsteady motion of the displacer. Especially in the JT system, there occurred a choked in the heat exchanger by solidified carbon dioxide (CO 2 ) at precooling phase. In the development of the upgraded double-stage Stirling cryocooler, titanium nitride coating to the magnet, reduction of glue amount for magnet fixture and optimal baking process were applied to reduce outgas. Moreover, the current outgas measurements proved that total outgas is affected by storage history of the materials, especially CO 2 gas in ambient atmosphere is likely to be absorbed by the GFRP materials. Thus, atmospheric exposure time of the materials in the Flight Model (FM) manufacturing and assembly process is limited to within 120 hours. Total CO 2 gas amount after 5 years operation is estimated below acceptable level of 500 ppm (Ref. 9). The Engineering Model (EM) with these modifications was fabricated for verification tests. Design specifications of the second-generation double-stage Stirling cryocooler are summarized in Table 2 and a cross-sectional view of the second-generation doublestage Stirling cryocooler is shown as Fig. 3. Table 2 Design specifications of the second-generation double-stage Stirling cryocooler Items Specifications Cooling power (EOL) 200 mw at 20 K, 1000 mw at 100 K Life time Requirement: 3 years, Goal: 5 years Power consumption 90 W Environment temperature -70~30 C Drive frequency 15 Hz Mass 9.5 kg Working gas 4 He Gas pressure 1.0 MPa(G) Flexure bearing Fig. 3 Cross sectional view of the second-generation double-stage Stirling cryocooler

5 52 3 Development tests 3.1 Cooling performance test Net cooling power was evaluated with nominal heat inputs of 200 mw at the 2nd cold stage and 1000 mw at the 1st cold stage. The cold stages of displacer were placed in a vacuum chamber, while the compressor and cold head were at room temperature in atmospheric environment. Helium gas pressure was 1.0 MPa (G) and drive frequency was 15 Hz as nominal test condition. As a performance at begging of life (BOL), maximum cooling power of 17.6 K with 200 mw at the 2nd cold stage and 96.1 K with 1000 mw at the 1st cold stage was obtained with 90 W power input, while minimum temperature of 9.6 K at the 2nd cold stage and 67.0 K at the 1st cold stage was obtained without heat load input. Temperature margin from specification will be consumed for aging degradation for longterm operation (Ref. 12). It was also confirmed in the thermal vacuum test that nominal cooling power was obtained even at lower limit of the operation temperature of -70 C (Ref. 11). 30 2nd stage temperature [K] W Gas pressure : 1.0MPa(G) Frequency : 15Hz Environment : Room temperature Power input 50W 60W Heat load at 2nd stage 0W 0W 1W Heat load at 1st stage st stage temperature [K] 0.2W Fig. 4 Diagrams of nominal cooling performance of the second-generation double-stage Stirling cryocooler 3.2 Mechanical performance test Mechanical performance test was carried out to verify that the upgraded double-stage Stirling cryocooler can survive in the launch vibration environment as a component specification. The unit of compressor and cold head were separately evaluated for tolerability to pyro shock, random vibration and sine wave vibration. Test conditions were based on the Qualification Test (QT) level of the environment-proof design criteria for on-board equipment of ASTRO-H. Especially for the sine wave vibration test, an appropriate value of test acceleration load level according to the FEM (Finite Element Method) structural analysis of the SXS dewar was used to avoid from overloading to the resonance mechanism in the cryocooler. It was confirmed that all test conditions were satisfied with the criteria and no degradation of cooling performance appeared after these mechanical performance tests. Fig. 5 shows test load level of QT mechanical environment (Ref. 12).

6 53 JSTS Vol. 28, No Test level : QT For 3-axis of the STC and STH 10 0 Test level : QT Test time : 80 sec 10 2 Test level : QT on the SXS dewar Sweep rate : 2.0 oct/min SRS [G] Frequency [Hz] PSD [G 2 /Hz] STH-Z, STC-Y STH-X,Y, STC-X,Z Frequency [Hz] Acceleration [G] 10 1 STH-Z, STC-X,Y STH-X,Y, STC-Z Frequency [Hz] (a) Pyro shock (b) Random vibration (c) Sine wave vibration Fig. 5 Test load level of QT mechanical environment: The directions for STC-Z and STH-Z are parallel to drive-axis of the compressor and the cold head, while the directions for STC-X, Y and STH-X, Y are perpendicular to drive-axis of the compressor and the cold head, respectively. 3.3 EMC measurements Electromagnetic noise was also evaluated to compare with EMC (Electro-Magnetic Compatibility) level of the ASTRO-H electrical design criteria. The conductive emission (CE) level at each power harness line of the compressor, displacer and balancer was measured by a current probe and confirmed to be lower than criteria except for peaks noise at drive frequency and its harmonics. The radiative emission of magnetic field (RE01) was measured by a loop antenna at a 7 cm-distance from the outer surface of the cryocooler and electrical field (RE02) was measured by a set of rod, biconical and horn antennas at a 1 m-distance from the outer surface of cryocooler. Both of RE01 and RE02 were below criteria in entire measurement range. 3.4 Lifetime test Major causes dominating a mechanical cryocooler lifetime is degradation of seal performance and accumulation of outgassing. Since these phenomena are coupled each other, continuous running on actual time is a practical way to verify its lifetime. Fig. 6 is a schematic of configuration for the 4Kclass JT cryocooler lifetime test. The second-generation double-stage Stirling cryocooler is used as a precooler combined with the JT closed cycle loop and JT helium gas flow at supply pressure is cooled down at 20 K and 100 K stages. Then, at the JT orifice with µm in diameter, helium is expanded in isenthalpic process associated with Joule-Thomson effect to provide cooling power at 4.5 K. The laboratory GM (Gifford-McMahon) refrigerator is installed to cool the outer radiation shield surrounding the 100 K stage to provide a test environment temperature. Fig. 7 shows a current test result of lifetime test. From September 2010 to January 2011, the GM refrigerator failed and test heat load on the JT cold stage had to be reduced to 10 mw from a nominal value of 22 mw in order to keep same level of power consumption. From May 2011, a new laboratory GM refrigerator was installed and test was restarted with a nominal heat load. Total running time of hours (~560 days) was achieved as of August The cooling power of 22 mw at 4.5 K is able to be sustained without any remarkable degradation in progress of continuous operation.

7 54 Fig. 6 Configuration for lifetime test of the 4 K-class JT cryocooler Temperature [K], Power [W}, Heat load [mw] K Stage Temp. JT Stage Heat Load 2ST Precooler Power JT Compressors Power K Stage Temp /1/1 2010/4/2 2010/7/2 2010/10/1 2011/1/1 2011/4/2 2011/7/2 2011/10/1 2012/1/1 2012/4/1 2012/7/1 2012/9/30 4K Stage Temp. Temperature [K] Time (JST) Fig. 7 Test result of lifetime test with history of stage temperatures and power consumptions

8 55 JSTS Vol. 28, No. 1 4 Conclusion This paper describes development of the second-generation double-stage cryocooler, which was upgraded for higher cooling performance and reliability based on the heritage of AKARI cryocooler corresponding to requirements of the next innovative mission such as ASTRO-H/SXS and SPICA. In the EM development tests, maximum BOL cooling power of 17.6 K with 200 mw at the 2nd cold stage and 96.1 K with 1000 mw at the 1st cold stage was obtained with temperature margin from specification values. Mechanical performance test proved tolerability for pyro shock and vibration environment of the ASTRO-H/SXS criteria. It was also confirmed that electromagnetic noise levels for both conductive and radiative emission were below the ASTRO-H criteria. Continuous running for lifetime verification is now under operation and total running time of hours (~560 days) was achieved as of August 2012 without any remarkable degradation. Acknowledgements The authors appreciate deeply all members the SXS/ASTRO-H project team, SPICA pre-project team and the cryogenic group at Niihama division of Sumitomo Heavy Industry, Ltd. for their support to this work. Reference [1] Murakami, H., et al. The infrared astronomical mission AKARI. Publ Astron Soc Jpn 2007;59: [2] Nakagawa, T., Enya, K., Hirabayashi, M., Kaneda, H., Kii, T., Kimura, Y., Matsumoto, T., Murakami, H., Murakami, M., Narasaki, K., Narita, M., Ohnishi, A., Tsunematsu, S., Yoshida, S., Flight performance of the AKARI cryogenic system. Publ Astron Soc Jpn 2007;59: [3] Narasaki, K., Tsunematsu, S., Ootsuka, K., Kyoya, M., Matsumoto, T., Murakami, H., Nakagawa, T., Development of two-stage stirling cryocooler for ASTRO-F. In: Advances in cryogenic engineering: transactions of the cryogenic engineering conference CEC, vol. 49; p [4] Takahashi, T., Mitsuda, K., et al. The ASTRO-H mission. Proc SPIE 2010;7732:7320Z. [5] Mitsuda, K., et al. The high-resolution X-ray microcalorimeter spectrometer system for the SXS on ASTRO-H. Proc SPIE 2010;7732: [6] Fujimoto, R., Mitsuda, K., Yamasaki, N., Takei, Y., Tsujimoto, M., Sugita, H., Sato, Y., Shinozaki, K., Ohashi, T., Ishisaki, Y., Ezoe, Y., Murakami, M., Kitamoto, S., Murakami, H., Tamagawa, T., Kawaharada, M., Yamaguchi, H., Sato, K., Kanao, K., Yoshida, S., DiPirro, M., Shirron, P., Sneiderman, G., Kelley, R. L., Porter, F. S., Kilbourne, C. A., Crow, J., Mattern, Andrea., Kashani, A., McCammon, Dan., den Herder, J. W., Cooling system for the Soft X-ray Spectrometer onboard Astro-H. Cryogenics 2010;50: [7] Nakagawa, T., SPICA team, The next-generation space infrared astronomy mission SPICA. SPIE 2010;7731:77310O. [8] Sato, Y., Sugita, H., Shinozaki, K., Okamoto, A., Yamawaki, T., Komatsu, K., Nakagawa, T., Murakami, H., Matsubara, H., Murakami, M., Takada, M., Takai, S., Okabayashi, A., Kanao, K., Tsunematsu, S., Otsuka, K., Narasaki, K., Conceptual design of cryogenic system for the nextgeneration infrared space telescope SPICA. SPIE , Proceedings of SPIE Volume 7731 (2010) [9] Sato, Y., Sugita, H., Komatsu, K., Shimizu, R., Uchida, H., Nakagawa, T., Murakami, H., Mitsuda, K., Murakami, M., Iwata, I., Tsuneta, S., Tsunematsu, S., Kanao, K., Ootsuka, K., Hirabayashi, M., Development of advanced two-stage stirling cryocooler for next space missions. Cryocooler 2009;15:13 21.

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