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著者人数: 12名, Accepted: 2017-10-01, 資料番号: SA1170240000

The 4K Joule-Thomson (JT) cryocooler is a key cryogenic component for future astronomy missions such as ATHENA and LiteBIRD. It was originally developed for SMILES (2009) and upgraded for ASTRO-H/SXS (2016) and SPICA. The 20K two-stage Stirling cryocooler developed for AKARI (2006) was also upgraded and used as a precooler. The operational life is a critical factor in planning long-term missions. An engineering model of the 4K-JT cryocooler was built for continuous operation to verify its lifetime. Testing was done from 2010 to 2019 and successful three-year operation was demonstrated with an extended operation; this was beyond the design specification. This paper describes the overall history of the lifetime test of the 4K-JT cryocooler and an evaluation of the end-of-life cooling performance and performance changes during long-term operation.

To cool far infrared detectors for infrared observation or superconductor–insulator–superconductor (SIS) mixers for atmospheric observation, 1 K-class and 4 K-class coolers have been developed. These coolers consist of a two-stage Stirling cooler for pre-cooling and a Joule–Thomson (JT) cooler with a single JT valve. This paper presents descriptions of theoretical analyses based on enthalpy balance to elucidate the benefits of a two-stage JT valve type compared with those of a single JT valve type in a JT cooler. First, relational expressions for heat balance analysis of enthalpy for single-stage JT expansion are introduced. Then similar relational expressions for two-stage JT expansion are introduced under some assumptions. Results of heat balance analyses using several parameters demonstrated that, using two-stage JT expansion, the cooling capacity for a 1 K-class cooler is improved by 100%; that of a 4 K-class cooler is improved by about 30%.

Two-stage Stirling cryocoolers (2ST cooler) produced by Sumitomo Heavy Industries, Ltd. have been launched into orbit on three satellites: the “AKARI (ASTRO-F)” infrared astronomical satellite, JEM/SMILES on ISS, and the “HITOMI (ASTRO-H)” X-ray astronomical satellite. A 2ST cooler compressor has a linear-ball-bearing system as a piston-supporting structure. The linear ball bearing system is a key components to realize a lower drive frequency (15 Hz), a long piston stroke (30 mm). Its typical cooling power is 200 mW at 20 K for the second stage and 1000 mW at 100 K for the first stage, with 90 W electrical input power. During the test of the “HITOMI” engineering model, the energy resolution of the detector was found to be degraded when cryocoolers were in operation. After investigation, it was found that micro vibration from 2STs caused the degradation. The continuum in the vibration spectrum propagated into sub-Kelvin region and generated thermal noise. The continuum has origin in linear ball bearing in the compressor. In the case of “HITOMI”, vibration isolators were introduced to resolve this issue. For future mission, we are required to reduce microvibration of cryocooler itself. Therefore, the piston support mechanism in the compressor was modified from linear ball bearings to triangle shape flexure springs in order to reduce the continuum in vibration spectrum. In order to achieve a long piston stroke (±15 mm), the generated stress could be reduced to 400 MPa (Less than 1/2 of fatigue limit) or less even when the shape of the triangle shape flexure spring was devised and displaced by 15 mm. The typical vibration level has been reduced to 1 × 10−5 Nrms2 / Hz or less at a frequency of 200 Hz or less and 1/10 times or less at 200–600 Hz than that of a compressor with a linear ball bearing system. The cooling power is kept to 260 mW at 20 K, with 90 W electrical input power. This low vibration cooler is expected to be an improved cryogenic system for use in future projects with sub-Kelvin detectors.

著者人数: 17名, 資料番号: SA1004974000

著者人数: 18名, 資料番号: SA1004976000

著者人数: 14名, Accepted: 2012-01-04, 資料番号: SA1004214000

Superconducting Sub-millimeter-wave Limb-Emission Sounder (SMILES) is to be operated aboard the Japanese Experiment Module (JEM) of the International Space Station (ISS) in 2009. SMILES uses two superconductor–insulator–superconductor (SIS) mixers for sub-millimeter-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. Two-stage Stirling cooler precools the JT circuit and also cools radiation shields in the cryostat. JT circuit has three tube-in-tube type heat exchangers and an orifice for JT expansion in the cryostat. The cryogenic system is built, tested and delivered.

Accepted: 2010-02-22, 資料番号: SA1002677000

The AKARI satellite (formerly known as ASTRO-F) is Japan’s first infrared astronomical satellite. AKARI is equipped with the infrared camera (IRC) and the far-infrared surveyor (FIS), which are cooled below 7 K. The AKARI’s 68.5 cm telescope, which is made of SiC, is also cooled below 7 K. A unique feature of the AKARI cryostat is that it uses both cryogen and mechanical coolers. Using mechanical coolers, the helium lifetime can be greater than one year with 170 L of liquid helium. AKARI was launched on February 21, 2006 (UT), from the Uchinoura Space Center (USC). It has been performing successfully in orbit.

SPACE CRYOGENICS WORKSHOP 2003, 資料番号: SA1003440000

We have developed a simple and small thermal link for cooling a cylindrical cartridge, on which device under test is mounted. It consists of a crown-like ring with an inner diameter of 170 or 140 mm and a clamping belt, which is a metal spring or nylon. A thermal conduction of the thermal link is achieved as the crown-like ring clamps the disk-like stage of the cartridge with external force of the clamping belt. This link can be applied at various temperature ranges from 2 to 100 K. The measured thermal conductance of the 170 mm link is 1.7, 5.6 and 3.3 W K −1 for 4, 12, and 80 K stages, respectively. These values are consistent with a empirical calculation within 10% errors. This link is also effective to reduce mechanical vibration to be 6 μm (peak-to-peak) in the horizontal direction of the cartridge. This link is easily fabricated and is useful to various detectors which require to be cooled down.