Cryogenics 48 (2008) 258–266

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Cryogenics

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Development of mechanical cryocoolers for the Japanese IR space telescope SPICA

Hiroyuki Sugita a,*, Yoichi Sato a, Takao Nakagawa b, Hiroshi Murakami b, Hidehiro Kaneda b, Keigo Enya b,Masahide Murakami c, Shoji Tsunematsu d, Masayuki Hirabayashi d, SPICA Working Groupa Institute of Aerospace Technology, Japan Aerospace Exploration Agency, 2-1-1 Sengen Tsukuba, Ibaraki 305-8505, Japanb Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japanc Institute of Engineering Mechanics and Systems, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japand Sumitomo Heavy Industries, Ltd., Ehime Works, Niihama Factory, 5-2 Soubiraki-cho, Niihama, Ehime 792-8588, Japan

a r t i c l e i n f o

Article history:Received 19 July 2007Received in revised form 22 October 2007Accepted 6 March 2008

Keywords:F. Space cryogenicsE. StirlingE. Joule-Thomson coolersB. 3He

0011-2275/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.cryogenics.2008.03.007

* Corresponding author. Tel.: +81 29 868 4219; faxE-mail address: sugita.hiroyuki@jaxa.jp (H. Sugita)

a b s t r a c t

The next Japanese infrared space telescope SPICA features a large 3.5-m-diameter primary mirror and anoptical bench cooled to 4.5 K with advanced mechanical cryocoolers and effective radiant cooling insteadof using a massive and short-lived cryogen system. To obtain a sufficient thermal design margin for thecryogenic system, cryocoolers for 20 K, 4 K, and 1 K have been modified for higher reliability and highercooling power. The latest results show that all mechanical cryocoolers achieve sufficient cooling capacityfor the cooling requirement of the telescope and detectors on the optical bench at the beginning of life.Consequently, the feasibility of the SPICA cryogenic system concept was validated, while attempts toachieve higher reliability, higher cooling capacity and less vibration have continued for stable operationsat the end of life.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The space infrared telescope for cosmology and astrophysics(SPICA) mission [1,2], as illustrated in Fig. 1, has been proposedto the Japan aerospace exploration agency (JAXA) as the secondJapanese infrared (IR) space telescope to be launched in the mid-2010s, following the successful AKARI (previously called the AS-TRO-F) mission launched in 2006 [3]. The SPICA spacecraft,launched with an H-IIA launch vehicle, is to be transferred into ahalo orbit around the Sun–Earth L2 (second Lagrangian point),where effective radiant cooling is feasible owing to solar rays andradiant heat fluxes from the Earth constantly coming from thesame direction. That optimal thermal environment enables thisIR space telescope to be equipped with a large single-aperture pri-mary mirror, 3.5 m in diameter, cooled to 4.5 K with advancedmechanical cryocoolers and effective radiant cooling instead of amassive and short-lived cryogen. For that reason, the SPICA spacetelescope is expected to perform as a space observatory optimizedfor mid-IR and far-IR astronomy with unprecedented sensitivityand high spatial resolution during a long period of over five years.This paper describes the current development status of mechanicalcryocoolers used for the cryogenic system as a key technology forthe JAXA/SPICA mission.

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2. Cryogenic system design

The SPICA adopts a new concept of cryogenic system that usesno cryogen. The 4.5 K stage, consisting of the primary mirror andthe optical bench equipped with focal plane instruments (FPIs), isrefrigerated by the combined method of radiant cooling andmechanical cooling. Considering the cooling capacity of an existingspace-qualified 4 K-class cryocooler [4], the heat load of the 4.5 Kstage is restricted to less than 30 mW. Because the Joule heatingof the FPIs is estimated as 15 mW, the heat flow from the hotstages to the 4.5 K stage must remain less than 15 mW. The base-line design of the SPICA cryogenic system, shown in Figs. 2 and 3,was obtained through previous studies [5–7].

The 3.5-m-diameter primary mirror and the optical bench aresurrounded by a baffle, a telescope shell, and outer shields. Eachcomponent is thermally connected through structural supports ofthe carbon fiber reinforced plastics (CFRP) and wire harnesses ofManganin. Therefore, since the thermal radiation between coldstages is much smaller than that between hot stages because ofthe small absolute values of the temperatures, the heat flow be-tween cold stages is determined mainly by thermal conductionthrough these structural supports. In hot stages, the sun shieldand the shield #3 with the multi-layer insulation (MLI) block ther-mal radiation to colder components, while the radiators of theseshields damp most of the absorbed heat from the sun and thespacecraft bus module to deep space. The layout of these shieldsand the solar array paddles is determined to optimize radiant cool-ing. The results of heat flow analyses show that the total heat load

Fig. 1. Conceptual image of JAXA/SPICA.

H. Sugita et al. / Cryogenics 48 (2008) 258–266 259

into the 4.5 K stage is suppressed less than 30 mW because of thestable thermal environment at the Sun–Earth L2 and the efficientradiant cooling structure. Hence, assuming the cooling capacity

Fig. 2. Baseline configuratio

of the existing 4K-class cryocooler, the obtained configuration ofa cryogenic system barely meets the cooling requirement at the4.5 K stage by a small margin in the mission feasibility study [8].

n of a SPICA spacecraft.

Fig. 3. Schematic drawing of a SPICA cryogenic system.

260 H. Sugita et al. / Cryogenics 48 (2008) 258–266

Therefore, from the perspective of the mission success, it is clearthat robustness of the cryogenic system must be pursued foruncertainties in design and manufacturing, deterioration of com-ponents or materials, and accidental failures. One of promisingmeasures is an upgrade of the 4 K-class cryocooler in reliability

Table 1Specifications of cryocoolers for SPICA

Cooler type 20 K class 4 K class

Cooling object Cooling down and JT precooling Primary mirror an

Configuration 2-Stage Stirling 2ST + 4He-JTCooling

requirement200 mW at 20 K 30 mW at 4.5 K

Driving power <90 W <160 WR&D level Flight model onboard AKARI (launched on

February 2006)Flight model develaunched in 2009)

and cooling power, as described in the following section. On theother hand, other composite materials such as the aramid fiberreinforced plastics (AFRP) with lower thermal conduction usedfor structure supports between cold stages are also effective andworth investigating for reduction of the heat flow to the 4.5 K

2 K class 1 K class

d optical bench Unstressed Ge: Gadetector

Stressed Ge: Gadetector

2ST + 3 or 4 He-JT 2ST + 3He-JT10 mW at 2.5 K 10 mW at 1.7 K

<180 W <180 Wloped for JEM/SMILES (to be Laboratory

demonstrationLaboratorydemonstration

Fig. 4. Modification of supporting structure of the first displacer.

Fig. 5. Measurement of vibrational disturbance of crycoolers.

H. Sugita et al. / Cryogenics 48 (2008) 258–266 261

262 H. Sugita et al. / Cryogenics 48 (2008) 258–266

stage. Thermophysical properties such as the conductivity, the spe-cific heat and the coefficient of the thermal expansion (CTE) forthose materials have been continued to be measured and to beevaluated in the cryogenic temperature range.

3. Development of mechanical cryocoolers

3.1. Cryocooler requirements

The SPICA telescope system will have high sensitivity and longobservation time because of the 4.5 K stage, consisting of the pri-mary mirror and the optical bench, which are cooled by a 4 K-classJoule-Thomson (JT) cooler and a 20 K-class two-stage Stirling (2ST)cooler without using consumable cryogen. Some detectors of FPIson the optical bench require other cryocoolers for temperatures

4K Stage

Bypass Valve for Precooling

Orifice

Cold Head

P1in

2nd Thermal Shield

1st Thermal Sh

PJ

1905 kPa

121.0 kPa

Filter

2-Stage Sti

Power consu

292.0 K 298.7 K

102.7 K

90.26 K

89.99 K

20.95 K

17.52 K

18.10 K

4.86 K

50.1 mW

4.42 K

17.69 K

88.70 K

Eva

Heat Exchanger 1

Heat Exchanger 2

Heat Exchanger 3

2nd-St

1st-St

Fig. 6. Schematic drawing of the 4 K-clas

lower than 4.5 K. Specifications of cryocoolers for the SPICA arelisted in Table 1. Previous studies [4,8,9] have shown the 4 K-classand 1 K-class JT coolers to achieve the required cooling power inthe laboratory, while the 20 K-class 2ST cooler of the AKARI hasbeen running on orbit for 1.5 years as of August, 2007 [10–12].Therefore, it is noteworthy that the cooling requirement for cryo-coolers is satisfied at the beginning of life (BOL). However, to oper-ate these cryocoolers stably in space for more than five years,higher reliability and sufficient cooling power at the end of life(EOL) must be investigated and verified on the ground.

Furthermore, because the dynamic vibration induces mechani-cal resonance and deterioration of the pointing stability of FPIs,the reduction of vibrational disturbance by mechanical cryocoolersis an important issue on the cryocooler development for the SPICAmission.

Evacuation

FJH

Compressor

JTC2

P1out

P2out

ield

JT Compressor

Power consumption 55.9W

628.0 kPa

1999 kPa4He: 8.660 mg/sec

rling Cooler

mption 89.2W

cuated vessel

age

age

JTC1

s crycooler and experimental results.

Fig. 7. Modification of compressor for the 4 K-class JT cooler.

Fig. 8. 4 K-class JT compressor with flexure springs.

H. Sugita et al. / Cryogenics 48 (2008) 258–266 263

3.2. 20 K-class two-stage Stirling cooler (2ST)

The SPICA spacecraft is equipped with four or more 20 K-class2ST coolers as a part of JT coolers for 1.7 K, 2.5 K, and 4.5 K andan additional precooler to reduce the cooling time of the 4.5 Kstage at the initial operation. Therefore, this common 20 K-classcooler, based on the 2ST cooler onboard the AKARI [10], must have

high reliability for continuous operation during more than fiveyears to complete the SPICA mission. At the same time, the in-crease of cooling capacity of the 20 K-class cooler must be pursued,because it greatly contributes to the increased cooling capacity ofthe JT coolers. Therefore, the 2ST cooler has been investigated forits reliability and was modified to provide cooling capacity of325 mW at 20 K with the extended 8-mm-diameter second dis-placer [8].

On the other hand, reduction of outgases inside the 2ST coolerhas been investigated, because the previous study [8] revealedthat degradation of the working gas purity remarkably deterio-rated the cooling performance and finally led to irregular motionof the cold head. As a result of revaluation of materials composing2ST for over-5-year operation, outgases such as H2O, CO2 and CH4

were estimated to be emitted from permanent magnets, movingcoils and glue inside the cooler. Species and amount of outgasesfrom these test pieces and the baking time were studied withthe atmospheric pressure ionization mass spectrometric (API–MS) method and the gas chromatograph mass spectrometric(GC–MS) method. Moreover, Fig. 4a shows that the seal of the dis-placer in the cold head cannot avoid contact with the cylinder.Displacer abrasion might increase the displacer clearance, engen-dering working gas contamination after running for many years.In addition, Fig. 4b shows that, to realize a no-contact structurebetween the first-stage displacer and the cylinder, flexure springsare installed. Then, the clearance of the second-stage displacer isset 2.5 times wider for abrasion reduction, because the coolingperformance is unchanged. The cooling tests for the modifiedcooler improve the cooling capacity that 0.2 W at 16.0 K at thesecond-stage and 1 W at 83.6 K at the first-stage in the conditionof the input power of AC 90 W, the working gas pressure of

264 H. Sugita et al. / Cryogenics 48 (2008) 258–266

0.9 MPa (nominal 1.0 MPa), the driving frequency of 15 Hz, andthe voltage phase difference between the compressor and the coldhead of 170 �C (nominal 180 �C). Based on obtained data, an up-graded engineering model of 2ST with low-outgas materials andless amount of glue is to be fabricated and validated at a long lifetest.

Fig. 5 shows the vibrational force measurement system forcryocoolers. Four three-axis piezoelectric dynamic force sensorsPCB-M260M21 with resolution of 0.001 N-rms are installed underthe cooler mounting plate to detect the dynamic force transferredfrom the cryocooler on the mounting plate. An inertial shakervibration source is also introduced to calibrate this measurementsystem. As the first approach to vibration reduction of the coldhead, the active balancer, opposed to the displacer in the cold headas depicted in Fig. 4a, works as a counterweight that is synchro-nized with the driving frequency to cancel out the vibration of

1K Stage

Bypass Valve for Precooling

Orifice

Cold Head

P1in

2nd Thermal Shield

1st Thermal Sh

PJ

866 kPa

7.30 kPa

Filter

2-Stage Sti

Power consu

296.1 K 301.2 K

107.65 K

93.02 K

92.75 K

13.93 K

11.81 K

11.79 K

1.66 K

16.0 mW

1.69 K

11.77 K

91.76 K

Eva

Heat Exchanger 1

Heat Exchanger 2

Heat Exchanger 3

2nd-St

1st-St

Fig. 9. Schematic drawing of the 1 K-clas

the displacer. In addition, to reduce disturbance force of funda-mental frequency of 15 Hz and higher harmonics at the same time,the compressor and the active balancer are driven by electricpower with adjusted waveform including higher frequency ele-ments. Preliminary experiments imply that the dynamic distur-bance force of 1st–10th harmonics can be reduced to less than0.1 N in the direction of the driving axis. Further investigation is re-quired for the precise measurement and establishment of a practi-cal and reliable method for vibration reduction.

3.3. 4 K-class JT cryocooler

The temperature of 4.5 K is obtained using the JT circuit in com-bination with the 2ST cooler as a precooler to 10–20 K. The 4 K-class JT cooler has also been modified for higher reliability duringthe more than 5-year operational life and higher cooling capacity.

Evacuation

FJH

Compressor

JTC2

P1out

P2out

ield

JT Compressor

Power consumption 76.6W

78.9 kPa

589.6 kPa3He: 2.647 mg/sec

rling Cooler

mption 89.0W

cuated vessel

age

age

JTC1

s crycooler and experimental results.

Table 2Comparisons between the latest and the previous tests for the 1 K-class cryocooler

Latest test (2007) Previous test (2004)

JT orifice size 24 lm / 20 lm /Cooling power 16 mW at 1.7 K 12 mW at 1.7 K2ST temperature 93.02 K at the 1st stage 86.45 K at the 1st stage

11.81 at the 2nd stage 12.40 at the 2nd stageDriving power 89.0 W for 2ST 89.1 W for 2ST

76.6 W for 3He-JT 73.2 W for 3He-JTJT mass flow rate 2.647 mg/s 2.275 mg/s4-Stage compression 7.30–>590 kPa 6.65–>576 kPa

H. Sugita et al. / Cryogenics 48 (2008) 258–266 265

The existing 4 K-class JT cryocooler has 30 mW cooling capacity; itwas developed for the superconducting submillimeter-wave limb-emission sounder (SMILES) mission at the Japanese experimentmodule exposed facility (JEM-EF) of the international space station(ISS) to be launched in 2009 [4].

Improvement of the 4 K-class cryocooler has an especiallystrong effect on securing the thermal design margin of the cryo-genic system. The 4 K-class JT cooler is examined by using newheat exchangers of coaxial double tubes with low pressure lossand by combining the modified 2ST cooler with the extended dis-placer, as depicted in Fig. 6. Those results show that the maximumcooling power of 50.1 mW was efficiently obtained with an electric

Coo

ling

capa

city

[m

W]

1K-s

tage

tem

pera

ture

[K

]

1K-stage temperature

Mass flow rate

Cooling capacity

Input power to JT compressor

JT supply pressu

Cooling capacity

a

b

Fig. 10. Cooling tests of th

input power of AC 55.9 W for the JT compressors and AC 89.2 W forthe 2ST cooler. The remarkable improvement of the cooling powerat the 4.5 K stage is attributed to the increase of the mass flow rate,because of high-power precooling at 17.5 K by the modified 2STcooler with the extended 8-mm-diameter second displacer. Inaddition, Figs. 7 and 8 show that the linear JT compressors aremodified for higher efficiency and less vibration by replacing thelinear ball bearings supporting the drive shafts with the flexuresprings. The cooling capacity of the 4 K-class JT cooler is expectedto increase further by using the new JT compressors dischargingthe He gas with higher supply pressure and higher mass flow rate.

The outgases from components inside the 4 K-class JT coolerwere evaluated for long-term operation similarly to the evaluationfor the 2ST cooler. In addition, the vibration control has been con-tinued to be investigated. A long life test will be undertaken withan upgraded engineering model with higher reliability and highercooling power.

3.4. 1 K-class 3He-JT cryocooler

Because the far-IR detector on board the SPICA space telescoperequires the extremely low temperature of 1.7 K, the 3He-JT cryo-cooler was developed for higher cooling capacity, higher reliability,and less vibration, achieving the required cooling capacity of10 mW with the small input of AC 162 W [8,9].

Inpu

t pow

er to

JT

com

pres

sor

[W]

Mas

sflo

w r

ate

[mg/

sec]

re [kPa]

[mW]

e 1 K-class JT cooler.

266 H. Sugita et al. / Cryogenics 48 (2008) 258–266

Fig. 9 shows that the 1 K-class cryocooler was studied using themodified 2ST cooler with the extended 8-mm-diameter second dis-placer. The results presented in Table 2 and Fig. 9 show that theimproved cooling power of 16.0 mW is successfully obtained withan efficient input power of AC 76.6 W for the JT compressors andAC 89.0 W for the powerful 2ST cooler. Higher mass flow rate,caused by cooling the 3He gas in the JT circuit at 11.8 K by the mod-ified 2ST cooler with the extended 8-mm-diameter second displac-er, drastically increases the cooling capacity at 1.7 K. Moreover,cooling tests for the 1 K-class 3He-JT cooler were carried out atthe maximum cooling conditions for the JT supply pressure. The re-sults are shown in Fig. 10.

The outgases from components inside the 1 K-class JT coolerwere evaluated for long-term operation similarly to the 2ST coolerand the 4 K-class JT cooler. In addition, vibration control has beencontinued to be investigated. A long life test will be undertakenusing an upgraded engineering model with higher reliability andhigher cooling power.

4. Conclusion

The next Japanese infrared space telescope SPICA with advancedmechanical cryocoolers is expected to perform as a space observa-tory optimized for mid-IR and far-IR astronomy with unprece-dented sensitivity and high spatial resolution during a long periodof over five years. This paper described the upgrading status ofthe 20 K-class two-stage Stirling cooler, the 4 K-class JT cooler,and the 1 K-class JT cooler for the cryogenic infrared telescope atthe concept study phase. The latest results indicated that allmechanical cryocoolers gained a sufficient margin of cooling capac-ity with unprecedentedly low power consumption for the coolingrequirement of the large telescope and FPIs on the optical benchat least at the BOL. It is concluded that the feasibility of the SPICAmission was validated for the critical cryogenic system design,although some attempts to achieve higher reliability, higher coolingcapacity and less vibration have been continued for stable opera-tions during the entire mission period of over five years. Long lifetests of cryocoolers must be carried out to verify their reliability.The heat-rejection system from compressors of cryocoolers willalso be designed, fabricated and examined in detail.

Acknowledgements

The authors appreciate Prof. Mitsuda of ISAS/JAXA for fruitfuldiscussions related to cryocooler reliability. The authors also thankDr. Iwata and Prof. Tsuneta of the national astronomical observa-tory of Japan (NAOJ), and Dr. Uchida and Prof. Komatsu of IAT/JAXAfor cooperation in research on a mechanical cryocooler vibrationcontrol.

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