Shoji Tsunematsu

Flight Performance of the AKARI Cryogenic System

We describe the flight performance of the cryogenic system of the infrared astronomical satellite AKARI, which was successfully launched on 2006 February 21 (UT). AKARI carries a 68.5 cm telescope together with two focal plane instruments, Infrared Cameras (IRC) and Far Infrared Surveyor (FIS), all of which are cooled down to cryogenic temperature to achieve superior sensitivity. The AKARI cryogenic system is a unique hybrid system, which consists of cryogen (liquid helium) and mechanical coolers (2-stage Stirling coolers). With the help of the mechanical coolers, 179 L (26.0 kg) of super-fluid liquid helium can keep the instruments cryogenically cooled for more than 500 days. The on-orbit performance of the AKARI cryogenics is consistent with the design and pre-flight test, and the boil-off gas flow rate is as small as 0.32 mg/s. We observed the increase of the major axis of the AKARI orbit, which can be explained by the thrust due to thermal pressure of vented helium gas.

Development of Cryogenic System for Smiles

Superconducting Submillimeter‐Wave Limb‐Emission Sounder (SMILES) is to be operated aboard the Japanese Experiment Module (JEM) of the International Space Station (ISS) in 2007. SMILES uses two Superconductor‐insulator‐superconductor (SIS) mixers for submillimeter‐wave atmospheric observation, and they are cooled to 4 K levels by a cryogenic system with a two‐stage Stirling cooler, a Joule‐Thomson (JT) cycle cooler and a cryostat composed of three stages. The cooling capacity is designed as about 20 mW at 4.5 K, 200 mW at 20 K and 1 W at 100 K with the total input power of approximately 140 W. The proto‐flight model (PFM) of the cryogenic system has achieved such cooling capacity with significantly less input power, as well as mechanical capability required for launching conditions.

Performance of the helium dewar and cryocoolers of ASTRO-H SXS

The Soft X-ray Spectrometer (SXS) is a cryogenic high-resolution X-ray spectrometer onboard the ASTRO-H satellite, that achieves energy resolution better than 7 eV at 6 keV, by operating the detector array at 50 mK using an adiabatic demagnetization refrigerator. The cooling chain from room temperature to the ADR heat sink is composed of 2-stage Stirling cryocoolers, a 4He Joule-Thomson cryocooler, and super uid liquid He, and is installed in a dewar. It is designed to achieve a helium lifetime of more than 3 years with a minimum of 30 liters. The satellite was launched on 2016 February 17, and the SXS worked perfectly in orbit, until March 26 when the satellite lost its function. It was demonstrated that the heat load on the He tank was about 0.7 mW, which would have satisfied the lifetime requirement. This paper describes the design, results of ground performance tests, prelaunch operations, and initial operation and performance in orbit of the flight dewar and cryocoolers.

Development of Two‐Stage Stirling Cooler for ASTRO‐F

A two‐stage small Stirling cooler has been developed and tested for the infrared astronomical satellite ASTRO‐F that is planned to be launched by Japanese M‐V rocket in 2005. ASTRO‐F has a hybrid cryogenic system that is a combination of superfluid liquid helium (HeII) and two‐stage Stirling coolers. The mechanical cooler has a two‐stage displacer driven by a linear motor in a cold head and a new linear‐ball‐bearing system for the piston‐supporting structure in a compressor. The linear‐ball‐bearing supporting system achieves the piston clearance seal, the long piston‐stroke operation and the low frequency operation. The typical cooling power is 200 mW at 20 K and the total input power to the compressor and the cold head is below 90 W without driver electronics. The engineering, the prototype and the flight models of the cooler have been fabricated and evaluated to verify the capability for ASTRO‐F. This paper describes the design of the cooler and the results from verification tests including cooler perfo...

Vibration isolation system for cryocoolers of Soft X-ray Spectrometer (SXS) onboard ASTRO-H (Hitomi)

Soft X-ray Spectrometer (SXS) onboard ASTRO-H (named Hitomi after launch) is a microcalorimeter-type spectrometer, installed in a dewar to be cooled at 50 mK. The energy resolution of the SXS engineering model suffered from micro-vibration from cryocoolers mounted on the dewar. This is mitigated for the flight model by introducing vibration isolation systems between the cryocoolers and the dewar. The detector performance of the flight model was verified before launch of the spacecraft in both ambient condition and thermal-vac condition, showing no detectable degradation in energy resolution. The in-orbit performance was also consistent with that on ground, indicating that the cryocoolers were not damaged by launch environment. The design and performance of the vibration isolation system along with the mechanism of how the micro-vibration could degrade the cryogenic detector is shown.

Porous plug phase separator and superfluid film flow suppression system for the soft x-ray spectrometer onboard ASTRO-H

Suppression of super fluid helium flow is critical for the Soft X-ray Spectrometer onboard ASTRO-H (Hitomi). In nominal operation, a small helium gas flow of ~30 μg/s must be safely vented and a super fluid film flow must be sufficiently small <2 μg/s. To achieve a life time of the liquid helium, a porous plug phase separator and a film flow suppression system composed of an orifice, a heat exchanger, and knife edge devices are employed. In this paper, design, on-ground testing results and in-orbit performance of the porous plug and the film flow suppression system are described.

Conceptual design of a cryogenic system for the next-generation infrared space telescope SPICA

The conceptual design of the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) has been studied as a pre-project of the Japan Aerospace Exploration Agency (JAXA) in collaboration with ESA to be launched in 2018. The SPICA is transferred into a halo orbit around the second Lagrangian point in the Sun-Earth system, where radiant cooling is available effectively. The SPICA has a large IR telescope 3 m in diameter, which is cooled without cryogen to below 6 K by the radiant and mechanical cooling system. Therefore, the SPICA mission will cover mid- and far-IR astronomy with high sensitivity and spatial resolution during a long period of over 5 years for goal. Most heat radiation from the sun and spacecraft is blocked by the Sun Shield and thermal radiation shields covered with Multi-Layer Insulator (MLI) to limit heat radiation to the Scientific Instrument Assembly (SIA). The SIA, which is composed of the primary mirrors and optical benches equipped with Focal Plane Instruments (FPIs), is refrigerated to below 6 K by two sets of 4K-class Joule-Thomson (JT) cooler with a cooling power of 40 mW at 4.5 K. The Far-IR detector is refrigerated to 1.7 K by two sets of 1K-class JT coolers with a cooling power of 10 mW at 1.7 K. Improvements for the higher reliability and sufficient cooling performance are required in the development of SPICA mechanical cryocoolers. Thermal analysis indicates that the SPICA cryogenic system works effectively to limit the total heat load on the SIA to 41.2 mW. This paper describes the conceptual design of the SPICA cryogenic system, which was established with thermal feasibility for nominal operation mode.

Cryogenic system for the infrared space telescope SPICA

The SPICA mission has been proposed to JAXA as the second Japanese IR space telescope to be launched in 2017. The SPICA spacecraft, launched with an H-IIA launch vehicle, is to be transferred into a halo orbit around the Sun-Earth L2, where effective radiant cooling is feasible owing to solar rays and radiant heat fluxes from the Earth constantly coming from the same direction. That optimal thermal environment enables this IR space telescope to use a large 3.5-mdiameter- single-aperture primary mirror cooled to 4.5 K with advanced mechanical cryocoolers and effective radiant cooling instead of a massive and short-lived cryogen. As a result of thermal and structural analyses, the thermal design of cryogenic system was obtained. Then, mechanical cryocoolers have been developed to meet cooling requirement at 1.7 K, 4.5 K and 20 K. The latest results of upgrading of the 20 K-class two-stage Stirling cooler, the 4K-class JT cooler, and the 1K-class JT cooler indicate that all cryocoolers gain a sufficient margin of cooling capacity with unprecedentedly low power consumption for the cooling requirement. It is concluded that the feasibility of the SPICA mission was confirmed for the critical cryogenic system design, while some attempts to achieve higher reliability, higher cooling capacity and less vibration have been continued for stable operations throughout the entire mission period.

Mechanical cooler and cryostat for submillimeter SIS mixer receiver in space

This paper reports on a space-qualified cooling system for submillimeter SIS mixer receiver (SIS: superconductor- insulator-superconductor). Designed cooling capacity of the system is 20 mW at 4.5 K, 200 mW at 20 K, and 1000 mW at 100 K. The combination of two-stage Stirling cooler and Joule- Thomson one has demonstrated the capacity with a power consumption of less than 300 W, including losses of drive electronics. The cryostat has a thermal insulation structure of S2-GFRP straps to fasten its 100 K stage. 20 K stage of the cryostat is held with GFRP pipes on the 100 K stage, while 4 K stage is supported with CFRP pipes on the 20 K stage. The cooling system accommodates two SIS mixers at 4.5 K, two IF amplifiers at 20 K, and two more IF amplifiers at 100 K. The mass of the cooling system is 40 kg for the mechanical cooler itself, 26 kg for the cryostat, and 24 kg for the driver electronics. The system has been developed for a 640 GHz receiver for an atmospheric limb-emission sounder SMILES, which is to be aboard the International Space Station in 2005. The engineering model of the system has been built and tested successfully.

Spaceborne 640-GHz SIS receiver based on a 4-K mechanical cooler

An engineering model has been built for a space-borne 640- GHz SIS receiver. It is the key component of Superconducting Submillimeter-Wave Limb-Emission Sounder, which is to be operated aboard the Japanese Experiment Module of the International Space Station in 2005. The receiver includes two Superconductor-Insulator-Superconductor (SIS) mixers cooled at 4.5 K, as well as four High-Electron-Mobility- Transistor (HEMT) amplifiers, two of which cooled at 20 K and the other two at 100 K. These components are integrated in a compact cryostat with two-stage Stirling and Joule- Thomson refrigerators. The receiver components has been successfully cooled and the cryostat has survived random vibration tests. The 640-GHz SIS mixer, which uses a pair of Nb/AlOx/Nb junctions connected in parallel, is built so that a broad RF matching be achieved without mechanical tuners. It is followed by cooled low noise HEMT amplifiers with a noise temperature of less than 17 K. The total receiver noise temperature has been measured around 180 - 220 K over the bandwidth of 5.5 GHz.