- Email: [email protected]
Development of double-stage ADR for future space missions
Cryogenics 50 (2010) 597–602
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Development of double-stage ADR for future space missions Keisuke Shinozaki a,b,*, Kazuhisa Mitsuda a, Noriko Y. Yamasaki a, Yoh Takei a, Kensuke Masui a,1, Kentaro Asano c,f, Takaya Ohashi c, Yuichiro Ezoe c, Yoshitaka Ishisaki c, Ryuichi Fujimoto d, Kosuke Sato d, Kenichi Kanao e, Seiji Yoshida e a
Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan Aerospace Research and Development Directorate, JAXA, 1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan Dept. of Phys., Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan d Faculty of Mathematics and Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan e Sumitomo Heavy Industries Ltd., Development Section, Quantum Equipment Division, Niihama works, 5-2 Soubiraki-cho, Niihama, Ehime 792-8588, Japan f Institute of Astronomy, University of Tokyo, 2-21-1 Ohsawa, Mitaka, Tokyo, Japan b c
a r t i c l e
i n f o
Article history: Received 25 June 2009 Received in revised form 14 February 2010 Accepted 22 February 2010
Keywords: E. Adiabatic demagnetization F. Cryostat-s F. Space cryogenics
a b s t r a c t We report a development of a portable dewar with a double-stage ADR in it, and its cooling test results. The purpose of this system is to establish a cooling cycle of double-stage adiabatic demagnetization from 4.2 K to 50 mK, which is strongly desired for future space science missions. In our test dewar, two units of ADR are installed in parallel at the bottom of a liquid He tank. We used 600 g of GGG (Gadolinium Gallium Garnet) for the higher temperature stage (4 Tesla) and 90 g of CPA (Chromic Potassium Alum) for the lower temperature stage (3 Tesla). A passive gas-gap heat switch (PGGHS) is used between these two stages, while a mechanical heat switch between the He tank and the GGG stage. Using this system, 50 mK was achieved, and various kinds of cooling cycles with different operating temperatures and different sequences of magnetization were tested. We also evaluated the performance of the PGGHS, and interference of the magnetic field with each other during a stable temperature control. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction One of the critical requirements for cooler systems for future space science missions is a high cooling power (1–2 lW) at very low temperatures (50–100 mK), to bring the best performance to cryogenic detectors. For example, X-ray microcalorimeters must be cooled down to below 100 mK, in order to achieve a high spectral resolution. An Adiabatic Demagnetization Refrigerator (ADR) has advantages as a space refrigerator, because (1) a solid cooling material is used, (2) its operation is not affected under micro-gravity, and (3) it has no moving parts. The XRS instrument onboard Japanese X-ray astronomy satellite Suzaku utilized a single-stage ADR as a cooler system, and achieved 60 mK in orbit for the first time, starting from 1.3 K of superfluid liquid He [1]. In the near future, a larger cooling power and a lower operation temperature are required for the cooler system. In addition, a cryogen-free cooler system will be the mainstream. In this case, 1.7 K (3He JT) or 4.5 K (4He JT) will be the heat sink temperature in orbit, * Corresponding author. Address: Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan. Tel.: +81 427 59 8135; fax: +81 427 59 8455. E-mail address: [email protected] (K. Shinozaki). 1 Present address: FUJIFILM Advanced Research Laboratories, Ushijima 577, Kaiseimachi, Ashigara-gun, Kanagawa 258-8577, Japan. 0011-2275/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2010.02.020
while 3–4 K on ground using a GM cryocooler. If a single-stage ADR is used like the XRS, however, all these make the ADR size very large and the magnetic field very strong. A multi-stage ADR has a capability to provide a high cooling power at a low operation temperature (10–50 mK) with a compact size and a magnetic field of a few Tesla [2]. However, the number of components becomes larger and its operation becomes complicated, compared to a single-stage ADR. This is a concern for space applications, from a reliability point of view. A double-stage ADR is a compromise solution. It achieves a higher cooling power and a lower attainable temperature with relatively small size and low magnetic field, compared to a single-stage ADR, while the number of components are not so large, and its operation is not so complicated, compared to a multistage ADR. The SXS (Soft X-ray Spectrometer) onboard Japanese X-ray satellite Astro-H [3,4] adopts a double-stage ADR, to achieve an operation temperature of 50 mK and a cooling power of 0.4 lW [5]. We constructed a portable test dewar with a double-stage ADR in it as a technology demonstration of double-stage magnetic cooling [6]. We performed cooling performance tests to understand its characteristics and operations, which provided useful results for future design and fabrication. In this paper, we describe following topics; a basic structure and a cooling cycle of a double-stage ADR
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in Section 2, the design of our double-stage ADR dewar in Section 3, and results of the cooling performance tests in Section 4. 2. Concept of double-stage ADR Fig. 1 shows a schematic drawing of a double-stage ADR, which consists of two ADR units (pills, magnets, and heat switches). The higher temperature stage provides a heat sink of an intermediate temperature for the lower temperature stage. The operation cycle can be divided into the following seven phases by the magnetic field strength and the heat switch status. 1. The higher temperature stage is at the intermediate operation temperature of T int (typically 0.4–1 K) with almost 0 Tesla and the lower temperature stage is at the operation temperature (50 mK) with 0.01–0.05 Tesla. Magnetization of the higher stage starts while both heat switches are off. 2. The higher stage reaches the heat sink temperature T sink (1.8– 4 K), and then the heat switch between the higher stage and the heat sink is turned on. 3. The higher stage is at T sink with the maximum magnetic field, then the heat switch between the higher stage and the heat sink is turned off. Magnetization of the lower stage starts. 4. The higher stage reaches T int . 5. The lower stage reaches T int , then the heat switch between the higher and the lower stage is turned on. 6. The lower stage is at T int with the maximum magnetic field, then the heat switch between the higher and the lower stage is turned off. 7. The lower stage reaches the operation temperature, with both heat switches off. In this sequence, the higher temperature stage is recycled while the operation temperature of the lower temperature stage is main-
Fig. 1. Schematic drawing of a double-stage ADR. Operating temperature of the higher temperature stage is between T sink and intermediate temperature T int , while the lower temperature stage is between T int and 50 mK.
tained, then the lower temperature stage is recycled just after recycling of the higher temperature stage is finished. Other cycle operations are possible, and should be selected by comparing a recycle time, a total heat load to the heat sink, feasibility, a heat load to another stage during recycling, and required mass of a magnetic material of each stage. In particular, the magnetic material and the intermediate operation temperature of the higher temperature stage (GGG in our portable dewar) are critical for the cooling power of the lower temperature stage, cycle operation method, and total volume.
3. Portable double-stage ADR dewar We constructed a portable double-stage ADR for research and development, and studied the following issues. To establish recycle and temperature control of the double-stage ADR. Performance verification of the components, e.g., salt pills, heat switches. To understand how the magnetic field of the two ADRs affect each other. Cooling performance with different cycle method. Figs. 2 and 3 show a design drawing and a bottom view of our portable ADR, respectively. The design of this dewar is similar to that of the previously fabricated portable single-stage ADR [7], and is originally based on the system for the quantum calorimeter sounding rocket experiments [8]. Our new dewar has a liquid He tank of 7.4 ‘ in volume as a pre-cooler of the ADR, and there are double vapor-cooled radiation shields around the He tank instead of a LN2 tank. All the components for the double-stage ADR are installed in parallel at the bottom of the He tank. Specifications of this ADR are listed in Table 1. It is designed so that the ADR can be recycled with a heat sink temperature of 4.2 K, and no pumping of liquid He is needed2. This simplifies the cool down operation, and does not cause any microphonics due to a pump. We note that this heat sink temperature is compatible with 4 He JT cryocooler. As paramagnetic materials, we selected a combination of GGG (Gallium Gadolinium Garnet) and CPA (Chromium Potassium Alum), because GGG has a high cooling power from 4.2 K to 1.0 K. Operation temperature of the GGG stage and the maximum magnetic field are determined in consideration of a balance between the required cooling power and the unit size. The size of each stage is strongly dependent on the sizes of a super-conducting magnet and a SiFe magnetic shield. The required mass of GGG would be lower if larger magnetic field can be applied, which depends on a usable diameter of the magnet wire. SiFe passive magnetic shields are installed around each stage, but other shields (Pb, Al and Cryoperm) will be needed at the region close to the detector stage. Now the operation temperature of the GGG is 1–1.3 K and the maximum magnetic field Bmax is 4 Tesla. The heat flow is controlled by two heat switches. Between the He tank and the GGG stage, a mechanical heat switch is used. Between the GGG stage and the CPA stage, on the other hand, a passive gas-gap heat switch (PGGHS) was adopted as a technology demonstration. Fig. 4 shows the side view of the PGGHS. A charcoal getter is installed on the inner side of an OFC plate at the cold side (connected to the CPA stage), and the temperature of a transition from low to high thermal conductance is controlled by the amount of 3He gas sealed in the switch. When the CPA stage is below the 2 It is possible to pump down liquid He to achieve a lower heat-sink temperature, and hence, a longer hold time.
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Fig. 2. Design drawing of a double-stage ADR fabricated at ISAS/JAXA. Double-stage ADR is mounted on the bottom of the He tank.
Table 1 Design parameters of double-stage ADR test dewar. Materials Material mass Tope Bmax Heat switch Required hold time of each stage Heat load to each stage Required cooling power of each stage Required heat capacity at Tope Pre-cooler Total mass
Fig. 3. Assembly of the double-stage ADR in the test dewar. Two magnets and paramagnetic materials (left: CPA stage, right: GGG stage), PGGHS (front side) and mechanical heat switch (backside) are installed.
transition temperature, the gas is adsorbed onto the getter and the thermal conductivity of the switch is determined by the conduc-
GGG (higher)/CPA (lower) 600 g/90 g (0.593 mol)/(0.20 mol) 1 K/50 mK 4 Tesla/3 Tesla Mechanical (GGG stage) Passive gas-gap (CPA stage) >15 h/>10 h 10 lW/1.0 lW 20 lW/0.4 lW 1.08 J/0.0504 J Liquid He (4.2 K, 7.4 ‘) 55 kg
tance of the shell. When it is above the transition temperature, the gas desorb and fills the cylinder and the switch has a high thermal conductance. As a result, PGGHS needs no active getter operation. A multi-stage ADR needs many heat switches, and hence, PGGHS is very useful to make the multi-stage ADR operation more simple [9,10]. A disadvantage is that its transition temperature is
600
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Fig. 4. Passive gas-gap heat switch test piece
fixed, and flexible operation is not possible, in case that something unexpected happens. 4. Cooling cycle performance test First, we measured the thermal conductance of the heat switches. For the mechanical heat switch between the GGG stage and the He tank, the on-conductance was 10 mW/K at 4.2 K and 2 mW/K at 2 K, while for the PGGHS, it was 1 mW/K when the cold side temperature was 2.0 K (on) and 1 lW/K when it was 1 K (off). Then, we operate the double-stage ADR in a cooling cycle shown in Fig. 5. After the cooling cycle of the GGG stage, the GGG stage reached 0.9 K, while the CPA stage reached 1.5 K just after the GGG stage cycle, and 2.2 K just before demagnetization of the CPA stage. Then 50 mK was achieved after the CPA cooling cycle. Next, we changed the 3He gas pressure in the switch, and adjusted its transition temperature to 0.9 K, so that the CPA stage can be further precooled during the recycle of the GGG stage. However, the temperature of the CPA stage was 1.2 K, even after 1 h of
Temperature (K)
12
GGG stage
10
4.1. Rejected heat from CPA stage to GGG
8 6 4 2 0 71
CPA stage
72
73
74
75
76
77
78
79
Time (hour) 4
Magnetic field(Tesla)
GGG stage’s cooling cycle. This is probably because the conductance of a thermal link between the CPA salt pill and the PGGHS is not large enough. By changing this link, the performance could be improved. Unfortunately, the CPA salt pill was degraded due to dehydration caused by a leakage of the SUS cylinder, during thermal cycles. After this degradation, 50 mK could not be reached. The current performance of the test ADR is summarized in Table 2. Hold time of liquid He is as long as 2 days. We measured the heat capacity of GGG between 0.7 K and 7 K, and these residuals between measured value and theoretical values for 600 g of GGG [11] are smaller than 10%, while most of measured values are slightly larger than theoretical values. The lowest temperature is 0.9 K under the situation of 4.5 K heat sink and 3.5 Tesla before demagnetization, while the expected temperature based on the literature values is 0.8 K. The entropy loss of 600 g of an ideal crystal of GGG is 1.5 J/K. The expected hold time from these results is 12 h at 1.2 K and 23 h at 1.3 K, assuming a rejected heat from 90 g of CPA at each temperature, as described in Section 4.1. The expected hold time is shorter than required hold time, since larger heat was absorbed during magnetization of the CPA stage. The performance of the CPA stage is also summarized in Table 2. After the degradation of the CPA salt pill, the expected hold time of 100 mK is 12 h under the situation of 1.5 K heat sink and 2.6 Tesla before demagnetization and 1 lW of heat load. However, the heat load to the CPA stage is 3–3.5 times larger than expected value (1 lW), and the hold time of the CPA stage is short. We measured again the thermal conductance of PGGHS when it was turned off, and 2.5–3 lW/K at 1.0 K was measured, This conductance was large enough to explain the measured heat load to the CPA. We assume that some materials (e.g. charcoal) thermally shorten the gap between the hot side and the cold side in the PGGHS. Temperature control of each stage should not be affected by the magnetic field of the other stage. Each requirements of CPA/GGG stage are 10/400 Gauss in order not to affect temperature controls of each stage from other stage. In our design, expected maximum magnetic fields of CPA/GGG stage during temperature control are 0.18/1.6 Tesla, and estimated magnetic fields of CPA/GGG stage are lower than 10/1 Gauss. In our temperature control measurement, no magnetic field effects from other stage were measured, and the temperature stability of 210 lK rms for the GGG stage and 11 lK rms for the CPA stage was achieved.
3.5 3
GGG stage
W¼
CPA stage
2.5
Z 0
2
t
@S TðtÞ dt @t B
ð1Þ
in this equation, we used the magnetic entropy formula represented in [12],
1.5 1 0.5 0 71
We measured the temperature rise of the GGG stage during magnetization of the CPA stage, in order to obtain the rejected heat from the CPA to the GGG stage. In this measurement, the temperature control of the GGG stage was not done, because the GGG’s total absorbed heat can be estimated from the entropy loss, which is calculated from S—T relation (entropy and temperature) without an external magnetic field. The total magnetized heat W from the CPA was also estimated from the equation,
72
73
74
75
76
77
78
79
Time (hour) Fig. 5. Top: Temperature behavior during a typical double-stage recycle test. GGG stage (solid black), CPA stage (dashed black), He tank bottom (solid gray) and He tank top (dashed gray) are shown, respectively. Bottom: magnetic field regulation (black) of each stage.
Z B @M SðT; BÞ ¼ NkB lnð2J þ 1Þ þ dB0 @T B0 0 2J þ 1 2J þ 1 1 x MðT; BÞ ¼ Ng lB J coth x coth 2J 2J 2J 2J
ð2Þ ð3Þ
B is the magnetic field which contributes to the spin, and is represented by the superposition of the internal spin b and the external field B0 , as
K. Shinozaki et al. / Cryogenics 50 (2010) 597–602 Table 2 Current performance of the test ADR. T start : the temperature before demagnetization. Hold time of liquid He
50 h
Typical parameters of GGG stage (Tstart 4.5 K, Bmax 3.5 Tesla) Lowest temperature Entropy loss Expected hold time (30 lW)
0.9 K (0.8 K in principle) 1.4 J/K (600 g of GGG) 2.2 h (1.1 K), 12 h (1.2 K), 23 h (1.3 K)
PGGHS Thermal conductance
1 mW/K (2.0 K), 1 lW/K (1.0 K)a
Typical parameters of CPA stage (Tstart 2.2 K, Bmax 2.6 Tesla)b Lowest temperature Entropy loss Expected hold time (1.4 lW) a b
50 mK (40 mK in principle) 1.5 J/K (90 g of CPA) 11.6 h (100 mK), 6.2 h (80 mK), 1.1 h (50 mK)
Before improvement of PGGHS. Before degradation of CPA stage.
B¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 b þ B20
ð4Þ
There is an uncertainty in a consideration of b for CPA. However, it is not so important, since the contribution of b to estimate the total magnetized heat is very small. We used 500 Gauss for b of CPA. The total absorbed heat measured was 3.6 J, which was about 1.2 J (50%) larger than the expected value. This difference is large, even if we consider an uncertainty in the S—T formula of paramagnetic material at low temperatures. Each expected values of total heats of the heat load from higher temperature region to GGG stage (<0.05 J) as well as eddy current heating (<0.1 J) are very small. If a larger eddy current heating is generated, a temperature rise of the GGG stage would be observed also during a demagnetization of CPA. However, it was very small in the experiment. Furthermore, GGG is single crystal and it is not likely that 1/3 of GGG is not functional as a cooling material. The entropy loss of
Temperature (K)
8 7 6 5
GGG at the magnetization of CPA stage must be crucial to optimize the operation temperature of GGG, which directly determines the hold time of CPA. Further studies are needed under different conditions, e.g., changing the maximum magnetic field applied to the CPA, controlling the CPA temperature so as not to exceed a certain temperature, in order to find the cause of the difference and to optimize the double-stage cooling cycle. We also performed the double-stage cycle with a different sequence, as shown in Fig. 6. In this operation, both stages are magnetized at the same time under the condition of both heat switches turned on, and each stage is then demagnetized in turn. In this cooling cycle, total heat rejected from CPA to GGG is different from that of the cycle shown in Fig. 5. Since the heat balance becomes better in principle, at least 10% longer of CPA hold time is expected compared to the cycle shown in Fig. 5. In our demonstration, this cooling operation was safely completed and lower than 70 mK was achieved. However, the temperature of the CPA stage decreased very slowly during the demagnetization of the GGG stage, which was caused by the low thermal conductance between the GGG and CPA stage. To further study the actual heat balance between GGG and CPA stage, temperature behavior of the two stages under several operations should be measured. It will also provides valuable data for a design of multi-stage ADRs. 5. Conclusions We constructed a portable double-stage ADR dewar. We installed two ADR units including SiFe magnetic shields and heat switches, and achieved the double-stage cooling cycle from 4.2 K to 50 mK. In our measurement, the GGG stage had a good cooling performance, which was consistent with the expectation. The CPA stage achieved 50 mK with a moderately good performance. After several thermal cycles, however, CPA salt was degraded because of dehydration by a leakage. The transition temperature of the PGGHS was tuned to 0.9 K and cooling cycle of the CPA stage became favorable. In the temperature control test, stable temperature was maintained with no effect of magnetic field interference with each other. Two different types of cooling cycle magnetization were performed, and results were described. Detailed cooling performances including a rejected heat from the CPA to GGG stage are under investigation.
4
References
3 2 1 0
46.5
47
47.5
48
48.5
49
Time (hour) 3.5
Magnet Current (A)
601
GGG stage
3 2.5
CPA stage
2 1.5 1 0.5 0
46.5
47
47.5
48
48.5
49
Time (hour) Fig. 6. Temperature behavior during a typical double-stage recycle test (both magnetize cycle). CPA stage (black), He tank bottom (solid gray), He tank top (dashed gray), and High T c cold end (dotted gray) are shown, respectively, while GGG stage could not be monitored. Bottom: magnetic field regulation (black) of each stage.
[1] Kelley RL, Mitsuda K, Christine AA, Petar A, Audley Michall D, Bialas TG, et al. The Suzaku high resolution X-ray spectrometer. Publication of the Astronomical Society of Japan, vol. 59; 2007. p. 77–112. [2] Shirron P, Canavan E, DiPirro M, Francis J, Jackson M, Tuttle J, et al. Development of a cryogen-free continuous ADR for the constellation-X mission. Cryogenics 2004;44:581–8. [3] Takahashi T. The NeXT X-ray mission to explore nonthermal universe. In: Proc SPIE, 7011-98, Space telescopes and instrumentation II ultraviolet to gamma ray; 2008. These proceedings. [4] Mitsuda K, Yamasaki NY, Shinozaki K, Nakagawa T, Sugita H, Satoh Y, et al. The X-ray microcalorimeter on the NeXT mission. In: Proc SPIE, 7011-98, Space telescopes and instrumentation II ultraviolet to gamma ray; 2008. These proceedings. [5] Shirron P, Wegel D, Miller F. ADR design for the soft X-ray spectrometer (SXS) instrument on the astro-H mission. In: Proceedings in the space cryogenics workshop; 2009. [6] Shinozaki K, Mitsuda K, Yamasaki NY, Takei Y, Dipirro M, Ezoe Y, et al. Design of the two-stage series adiabatic demagnetization refrigerator for the NeXT and spectrum-RG missions. In: Proc SPIE, 7011-140, Space telescopes and instrumentation II ultraviolet to gamma ray; 2008. [7] Shinozaki K, Hoshino A, Ishisaki Y, Morita U, Ohashi T, Mihara T, et al. Soft Xray measurement of the toroidal pinch experiment RX reversed field pinch plasma using transition edge sensor calorimeter. Rev Sci Inst 2006;77(April):043104. [8] McCammon D, Almy R, Apodaca E, Bergmann Tiest W, Cui W, Deiker S, et al. A high spectral resolution observation of the soft X-Ray diffuse background with thermal detectors. Astrophys J 2002;576(September):188–203.
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[9] DiPirro MJ, et al. Design and test of passively operated heat switches for 0.2– 15 K. In: American Institute of Physics, Advances in cryogenic engineering: transactiions of the cryogenic engineering conference – CEC, vol. 49; 2004 [436+]. [10] Shirron PJ, et al. Passive gas-gap heat switches for use in adiabatic demagnetization refrigerators. In: Proc cryogenic engineering conference, Advances in cryogenic engineering, vol. 47. American Institute of Physics; 2002 [1175+].
[11] Fisher RA, Brodale GE, Hornung EW, Giauque WF. Magnetothermodynamics of gadolinium gallium garnet. I. Heat capacity, entropy, magnetic moment from 0.5 to 4.2 K, with fields to 90 kG along the [1 0 0] axis. J Chem Phys 1973;59(November):4652–63. [12] Hoshino A, Shinozaki K, Ishisaki Y, Mihara T. Improved PID method of temperature control for adiabatic demagnetization refrigerators. Nucl. Instrum. Methods A 2006;558(March):536–41.
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Development of double-stage ADR for future space missions Keisuke Shinozaki a,b,*, Kazuhisa Mitsuda a, Noriko Y. Yamasaki a, Yoh Takei a, Kensuke Masui a,1, Kentaro Asano c,f, Takaya Ohashi c, Yuichiro Ezoe c, Yoshitaka Ishisaki c, Ryuichi Fujimoto d, Kosuke Sato d, Kenichi Kanao e, Seiji Yoshida e a
Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan Aerospace Research and Development Directorate, JAXA, 1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan Dept. of Phys., Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan d Faculty of Mathematics and Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan e Sumitomo Heavy Industries Ltd., Development Section, Quantum Equipment Division, Niihama works, 5-2 Soubiraki-cho, Niihama, Ehime 792-8588, Japan f Institute of Astronomy, University of Tokyo, 2-21-1 Ohsawa, Mitaka, Tokyo, Japan b c
a r t i c l e
i n f o
Article history: Received 25 June 2009 Received in revised form 14 February 2010 Accepted 22 February 2010
Keywords: E. Adiabatic demagnetization F. Cryostat-s F. Space cryogenics
a b s t r a c t We report a development of a portable dewar with a double-stage ADR in it, and its cooling test results. The purpose of this system is to establish a cooling cycle of double-stage adiabatic demagnetization from 4.2 K to 50 mK, which is strongly desired for future space science missions. In our test dewar, two units of ADR are installed in parallel at the bottom of a liquid He tank. We used 600 g of GGG (Gadolinium Gallium Garnet) for the higher temperature stage (4 Tesla) and 90 g of CPA (Chromic Potassium Alum) for the lower temperature stage (3 Tesla). A passive gas-gap heat switch (PGGHS) is used between these two stages, while a mechanical heat switch between the He tank and the GGG stage. Using this system, 50 mK was achieved, and various kinds of cooling cycles with different operating temperatures and different sequences of magnetization were tested. We also evaluated the performance of the PGGHS, and interference of the magnetic field with each other during a stable temperature control. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction One of the critical requirements for cooler systems for future space science missions is a high cooling power (1–2 lW) at very low temperatures (50–100 mK), to bring the best performance to cryogenic detectors. For example, X-ray microcalorimeters must be cooled down to below 100 mK, in order to achieve a high spectral resolution. An Adiabatic Demagnetization Refrigerator (ADR) has advantages as a space refrigerator, because (1) a solid cooling material is used, (2) its operation is not affected under micro-gravity, and (3) it has no moving parts. The XRS instrument onboard Japanese X-ray astronomy satellite Suzaku utilized a single-stage ADR as a cooler system, and achieved 60 mK in orbit for the first time, starting from 1.3 K of superfluid liquid He [1]. In the near future, a larger cooling power and a lower operation temperature are required for the cooler system. In addition, a cryogen-free cooler system will be the mainstream. In this case, 1.7 K (3He JT) or 4.5 K (4He JT) will be the heat sink temperature in orbit, * Corresponding author. Address: Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan. Tel.: +81 427 59 8135; fax: +81 427 59 8455. E-mail address: [email protected] (K. Shinozaki). 1 Present address: FUJIFILM Advanced Research Laboratories, Ushijima 577, Kaiseimachi, Ashigara-gun, Kanagawa 258-8577, Japan. 0011-2275/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2010.02.020
while 3–4 K on ground using a GM cryocooler. If a single-stage ADR is used like the XRS, however, all these make the ADR size very large and the magnetic field very strong. A multi-stage ADR has a capability to provide a high cooling power at a low operation temperature (10–50 mK) with a compact size and a magnetic field of a few Tesla [2]. However, the number of components becomes larger and its operation becomes complicated, compared to a single-stage ADR. This is a concern for space applications, from a reliability point of view. A double-stage ADR is a compromise solution. It achieves a higher cooling power and a lower attainable temperature with relatively small size and low magnetic field, compared to a single-stage ADR, while the number of components are not so large, and its operation is not so complicated, compared to a multistage ADR. The SXS (Soft X-ray Spectrometer) onboard Japanese X-ray satellite Astro-H [3,4] adopts a double-stage ADR, to achieve an operation temperature of 50 mK and a cooling power of 0.4 lW [5]. We constructed a portable test dewar with a double-stage ADR in it as a technology demonstration of double-stage magnetic cooling [6]. We performed cooling performance tests to understand its characteristics and operations, which provided useful results for future design and fabrication. In this paper, we describe following topics; a basic structure and a cooling cycle of a double-stage ADR
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in Section 2, the design of our double-stage ADR dewar in Section 3, and results of the cooling performance tests in Section 4. 2. Concept of double-stage ADR Fig. 1 shows a schematic drawing of a double-stage ADR, which consists of two ADR units (pills, magnets, and heat switches). The higher temperature stage provides a heat sink of an intermediate temperature for the lower temperature stage. The operation cycle can be divided into the following seven phases by the magnetic field strength and the heat switch status. 1. The higher temperature stage is at the intermediate operation temperature of T int (typically 0.4–1 K) with almost 0 Tesla and the lower temperature stage is at the operation temperature (50 mK) with 0.01–0.05 Tesla. Magnetization of the higher stage starts while both heat switches are off. 2. The higher stage reaches the heat sink temperature T sink (1.8– 4 K), and then the heat switch between the higher stage and the heat sink is turned on. 3. The higher stage is at T sink with the maximum magnetic field, then the heat switch between the higher stage and the heat sink is turned off. Magnetization of the lower stage starts. 4. The higher stage reaches T int . 5. The lower stage reaches T int , then the heat switch between the higher and the lower stage is turned on. 6. The lower stage is at T int with the maximum magnetic field, then the heat switch between the higher and the lower stage is turned off. 7. The lower stage reaches the operation temperature, with both heat switches off. In this sequence, the higher temperature stage is recycled while the operation temperature of the lower temperature stage is main-
Fig. 1. Schematic drawing of a double-stage ADR. Operating temperature of the higher temperature stage is between T sink and intermediate temperature T int , while the lower temperature stage is between T int and 50 mK.
tained, then the lower temperature stage is recycled just after recycling of the higher temperature stage is finished. Other cycle operations are possible, and should be selected by comparing a recycle time, a total heat load to the heat sink, feasibility, a heat load to another stage during recycling, and required mass of a magnetic material of each stage. In particular, the magnetic material and the intermediate operation temperature of the higher temperature stage (GGG in our portable dewar) are critical for the cooling power of the lower temperature stage, cycle operation method, and total volume.
3. Portable double-stage ADR dewar We constructed a portable double-stage ADR for research and development, and studied the following issues. To establish recycle and temperature control of the double-stage ADR. Performance verification of the components, e.g., salt pills, heat switches. To understand how the magnetic field of the two ADRs affect each other. Cooling performance with different cycle method. Figs. 2 and 3 show a design drawing and a bottom view of our portable ADR, respectively. The design of this dewar is similar to that of the previously fabricated portable single-stage ADR [7], and is originally based on the system for the quantum calorimeter sounding rocket experiments [8]. Our new dewar has a liquid He tank of 7.4 ‘ in volume as a pre-cooler of the ADR, and there are double vapor-cooled radiation shields around the He tank instead of a LN2 tank. All the components for the double-stage ADR are installed in parallel at the bottom of the He tank. Specifications of this ADR are listed in Table 1. It is designed so that the ADR can be recycled with a heat sink temperature of 4.2 K, and no pumping of liquid He is needed2. This simplifies the cool down operation, and does not cause any microphonics due to a pump. We note that this heat sink temperature is compatible with 4 He JT cryocooler. As paramagnetic materials, we selected a combination of GGG (Gallium Gadolinium Garnet) and CPA (Chromium Potassium Alum), because GGG has a high cooling power from 4.2 K to 1.0 K. Operation temperature of the GGG stage and the maximum magnetic field are determined in consideration of a balance between the required cooling power and the unit size. The size of each stage is strongly dependent on the sizes of a super-conducting magnet and a SiFe magnetic shield. The required mass of GGG would be lower if larger magnetic field can be applied, which depends on a usable diameter of the magnet wire. SiFe passive magnetic shields are installed around each stage, but other shields (Pb, Al and Cryoperm) will be needed at the region close to the detector stage. Now the operation temperature of the GGG is 1–1.3 K and the maximum magnetic field Bmax is 4 Tesla. The heat flow is controlled by two heat switches. Between the He tank and the GGG stage, a mechanical heat switch is used. Between the GGG stage and the CPA stage, on the other hand, a passive gas-gap heat switch (PGGHS) was adopted as a technology demonstration. Fig. 4 shows the side view of the PGGHS. A charcoal getter is installed on the inner side of an OFC plate at the cold side (connected to the CPA stage), and the temperature of a transition from low to high thermal conductance is controlled by the amount of 3He gas sealed in the switch. When the CPA stage is below the 2 It is possible to pump down liquid He to achieve a lower heat-sink temperature, and hence, a longer hold time.
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Fig. 2. Design drawing of a double-stage ADR fabricated at ISAS/JAXA. Double-stage ADR is mounted on the bottom of the He tank.
Table 1 Design parameters of double-stage ADR test dewar. Materials Material mass Tope Bmax Heat switch Required hold time of each stage Heat load to each stage Required cooling power of each stage Required heat capacity at Tope Pre-cooler Total mass
Fig. 3. Assembly of the double-stage ADR in the test dewar. Two magnets and paramagnetic materials (left: CPA stage, right: GGG stage), PGGHS (front side) and mechanical heat switch (backside) are installed.
transition temperature, the gas is adsorbed onto the getter and the thermal conductivity of the switch is determined by the conduc-
GGG (higher)/CPA (lower) 600 g/90 g (0.593 mol)/(0.20 mol) 1 K/50 mK 4 Tesla/3 Tesla Mechanical (GGG stage) Passive gas-gap (CPA stage) >15 h/>10 h 10 lW/1.0 lW 20 lW/0.4 lW 1.08 J/0.0504 J Liquid He (4.2 K, 7.4 ‘) 55 kg
tance of the shell. When it is above the transition temperature, the gas desorb and fills the cylinder and the switch has a high thermal conductance. As a result, PGGHS needs no active getter operation. A multi-stage ADR needs many heat switches, and hence, PGGHS is very useful to make the multi-stage ADR operation more simple [9,10]. A disadvantage is that its transition temperature is
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Fig. 4. Passive gas-gap heat switch test piece
fixed, and flexible operation is not possible, in case that something unexpected happens. 4. Cooling cycle performance test First, we measured the thermal conductance of the heat switches. For the mechanical heat switch between the GGG stage and the He tank, the on-conductance was 10 mW/K at 4.2 K and 2 mW/K at 2 K, while for the PGGHS, it was 1 mW/K when the cold side temperature was 2.0 K (on) and 1 lW/K when it was 1 K (off). Then, we operate the double-stage ADR in a cooling cycle shown in Fig. 5. After the cooling cycle of the GGG stage, the GGG stage reached 0.9 K, while the CPA stage reached 1.5 K just after the GGG stage cycle, and 2.2 K just before demagnetization of the CPA stage. Then 50 mK was achieved after the CPA cooling cycle. Next, we changed the 3He gas pressure in the switch, and adjusted its transition temperature to 0.9 K, so that the CPA stage can be further precooled during the recycle of the GGG stage. However, the temperature of the CPA stage was 1.2 K, even after 1 h of
Temperature (K)
12
GGG stage
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4.1. Rejected heat from CPA stage to GGG
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CPA stage
72
73
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75
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77
78
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Magnetic field(Tesla)
GGG stage’s cooling cycle. This is probably because the conductance of a thermal link between the CPA salt pill and the PGGHS is not large enough. By changing this link, the performance could be improved. Unfortunately, the CPA salt pill was degraded due to dehydration caused by a leakage of the SUS cylinder, during thermal cycles. After this degradation, 50 mK could not be reached. The current performance of the test ADR is summarized in Table 2. Hold time of liquid He is as long as 2 days. We measured the heat capacity of GGG between 0.7 K and 7 K, and these residuals between measured value and theoretical values for 600 g of GGG [11] are smaller than 10%, while most of measured values are slightly larger than theoretical values. The lowest temperature is 0.9 K under the situation of 4.5 K heat sink and 3.5 Tesla before demagnetization, while the expected temperature based on the literature values is 0.8 K. The entropy loss of 600 g of an ideal crystal of GGG is 1.5 J/K. The expected hold time from these results is 12 h at 1.2 K and 23 h at 1.3 K, assuming a rejected heat from 90 g of CPA at each temperature, as described in Section 4.1. The expected hold time is shorter than required hold time, since larger heat was absorbed during magnetization of the CPA stage. The performance of the CPA stage is also summarized in Table 2. After the degradation of the CPA salt pill, the expected hold time of 100 mK is 12 h under the situation of 1.5 K heat sink and 2.6 Tesla before demagnetization and 1 lW of heat load. However, the heat load to the CPA stage is 3–3.5 times larger than expected value (1 lW), and the hold time of the CPA stage is short. We measured again the thermal conductance of PGGHS when it was turned off, and 2.5–3 lW/K at 1.0 K was measured, This conductance was large enough to explain the measured heat load to the CPA. We assume that some materials (e.g. charcoal) thermally shorten the gap between the hot side and the cold side in the PGGHS. Temperature control of each stage should not be affected by the magnetic field of the other stage. Each requirements of CPA/GGG stage are 10/400 Gauss in order not to affect temperature controls of each stage from other stage. In our design, expected maximum magnetic fields of CPA/GGG stage during temperature control are 0.18/1.6 Tesla, and estimated magnetic fields of CPA/GGG stage are lower than 10/1 Gauss. In our temperature control measurement, no magnetic field effects from other stage were measured, and the temperature stability of 210 lK rms for the GGG stage and 11 lK rms for the CPA stage was achieved.
3.5 3
GGG stage
W¼
CPA stage
2.5
Z 0
2
t
@S TðtÞ dt @t B
ð1Þ
in this equation, we used the magnetic entropy formula represented in [12],
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We measured the temperature rise of the GGG stage during magnetization of the CPA stage, in order to obtain the rejected heat from the CPA to the GGG stage. In this measurement, the temperature control of the GGG stage was not done, because the GGG’s total absorbed heat can be estimated from the entropy loss, which is calculated from S—T relation (entropy and temperature) without an external magnetic field. The total magnetized heat W from the CPA was also estimated from the equation,
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79
Time (hour) Fig. 5. Top: Temperature behavior during a typical double-stage recycle test. GGG stage (solid black), CPA stage (dashed black), He tank bottom (solid gray) and He tank top (dashed gray) are shown, respectively. Bottom: magnetic field regulation (black) of each stage.
Z B @M SðT; BÞ ¼ NkB lnð2J þ 1Þ þ dB0 @T B0 0 2J þ 1 2J þ 1 1 x MðT; BÞ ¼ Ng lB J coth x coth 2J 2J 2J 2J
ð2Þ ð3Þ
B is the magnetic field which contributes to the spin, and is represented by the superposition of the internal spin b and the external field B0 , as
K. Shinozaki et al. / Cryogenics 50 (2010) 597–602 Table 2 Current performance of the test ADR. T start : the temperature before demagnetization. Hold time of liquid He
50 h
Typical parameters of GGG stage (Tstart 4.5 K, Bmax 3.5 Tesla) Lowest temperature Entropy loss Expected hold time (30 lW)
0.9 K (0.8 K in principle) 1.4 J/K (600 g of GGG) 2.2 h (1.1 K), 12 h (1.2 K), 23 h (1.3 K)
PGGHS Thermal conductance
1 mW/K (2.0 K), 1 lW/K (1.0 K)a
Typical parameters of CPA stage (Tstart 2.2 K, Bmax 2.6 Tesla)b Lowest temperature Entropy loss Expected hold time (1.4 lW) a b
50 mK (40 mK in principle) 1.5 J/K (90 g of CPA) 11.6 h (100 mK), 6.2 h (80 mK), 1.1 h (50 mK)
Before improvement of PGGHS. Before degradation of CPA stage.
B¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 b þ B20
ð4Þ
There is an uncertainty in a consideration of b for CPA. However, it is not so important, since the contribution of b to estimate the total magnetized heat is very small. We used 500 Gauss for b of CPA. The total absorbed heat measured was 3.6 J, which was about 1.2 J (50%) larger than the expected value. This difference is large, even if we consider an uncertainty in the S—T formula of paramagnetic material at low temperatures. Each expected values of total heats of the heat load from higher temperature region to GGG stage (<0.05 J) as well as eddy current heating (<0.1 J) are very small. If a larger eddy current heating is generated, a temperature rise of the GGG stage would be observed also during a demagnetization of CPA. However, it was very small in the experiment. Furthermore, GGG is single crystal and it is not likely that 1/3 of GGG is not functional as a cooling material. The entropy loss of
Temperature (K)
8 7 6 5
GGG at the magnetization of CPA stage must be crucial to optimize the operation temperature of GGG, which directly determines the hold time of CPA. Further studies are needed under different conditions, e.g., changing the maximum magnetic field applied to the CPA, controlling the CPA temperature so as not to exceed a certain temperature, in order to find the cause of the difference and to optimize the double-stage cooling cycle. We also performed the double-stage cycle with a different sequence, as shown in Fig. 6. In this operation, both stages are magnetized at the same time under the condition of both heat switches turned on, and each stage is then demagnetized in turn. In this cooling cycle, total heat rejected from CPA to GGG is different from that of the cycle shown in Fig. 5. Since the heat balance becomes better in principle, at least 10% longer of CPA hold time is expected compared to the cycle shown in Fig. 5. In our demonstration, this cooling operation was safely completed and lower than 70 mK was achieved. However, the temperature of the CPA stage decreased very slowly during the demagnetization of the GGG stage, which was caused by the low thermal conductance between the GGG and CPA stage. To further study the actual heat balance between GGG and CPA stage, temperature behavior of the two stages under several operations should be measured. It will also provides valuable data for a design of multi-stage ADRs. 5. Conclusions We constructed a portable double-stage ADR dewar. We installed two ADR units including SiFe magnetic shields and heat switches, and achieved the double-stage cooling cycle from 4.2 K to 50 mK. In our measurement, the GGG stage had a good cooling performance, which was consistent with the expectation. The CPA stage achieved 50 mK with a moderately good performance. After several thermal cycles, however, CPA salt was degraded because of dehydration by a leakage. The transition temperature of the PGGHS was tuned to 0.9 K and cooling cycle of the CPA stage became favorable. In the temperature control test, stable temperature was maintained with no effect of magnetic field interference with each other. Two different types of cooling cycle magnetization were performed, and results were described. Detailed cooling performances including a rejected heat from the CPA to GGG stage are under investigation.
4
References
3 2 1 0
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Time (hour) Fig. 6. Temperature behavior during a typical double-stage recycle test (both magnetize cycle). CPA stage (black), He tank bottom (solid gray), He tank top (dashed gray), and High T c cold end (dotted gray) are shown, respectively, while GGG stage could not be monitored. Bottom: magnetic field regulation (black) of each stage.
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