Masayuki Hirabayashi

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.

The X-Ray Spectrometer (XRS): a multi-stage cryogenic instrument for the Astro-E X-ray astrophysics mission

Abstract The XRS cryogenic system has undergone numerous system configuration changes since its inception in the early 1980s. The Astro-E XRS is a high precision X-ray spectrometer with better than 20 eV resolution between 0.3 and 10 keV. It is a single photon counting solid-state calorimeter with pixel elements cooled to ~ 0.065 K by a unique three-stage cooling system. The low temperature is produced by an adiabatic demagnetization refrigerator (ADR) operating between 0.065 K and 1.3 K. The 1.3 K temperature is maintained by a 33 litre helium tank. Graphite/epoxy straps suspend the helium tank from an outer cryogen tank containing 120 litres of solid neon at ~17 K. The neon tank is in turn suspended from the dewar mainshell by low-conductivity straps. The system has a minimum design lifetime of 2 years, with a 2.5-year goal. The instrument design is severely constrained by mass limitations (≤ 400 kg) and high launch loads. The Astro-E will fly in 2000 on an ISAS M-V solid rocket. The M-V is a new launch vehicle, with the first launch scheduled for 1996.

On-orbit thermal behaviour of the IRTS cryogenic system

Abstract The IRTS (Infrared Telescope in Space) was launched at 08:01 on 18 March 1995 UT by the H-II rocket as a part of the Space Flyer Unit (SFU), which is a re-usable free flyer designed as a multi-purpose common facility for scientific and engineering experiments. The He II tank was estimated to be approximately 85% full at 1.84 K at the moment of launch, 11 h after pumping was disconnected and the tank was sealed. The vapour evacuation through a porous plug was resumed 28 min after the launch. The thermal state reached the anticipated steady state at 1.91 K on the third day without any anomalous thermal behaviour of the porous plug in the initial zero- g state. Over the course of 10 days the SFU transferred to a low earth orbit (LEO) at 486 km from the initial phase orbit at 340 km with the telescope aperture lid ejecting on day 11 after launch. A 3 He refrigerator was then activated to decrease the temperature of some bolometer detectors down to 302 mK for observations in the far IR region. Every detector worked quite satisfactorily. The stable thermal state of He II at 1.91 K was maintained until the depletion of He II at 09:30 on 24 April UT. The cold life is found to be fairly close to the prediction on the basis of the ground test data. On April 27 the sun shield was jettisoned for the safe retrieval by a US shuttle in January 1996, and the IRTS mission was completed because of the regular shift of the SFU mission time-line to another experiment. In this report the flight data of the thermal performance of the cryogenic system are discussed.

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.

A new generation of large SIC telescopes for space applications

Large Space based IR are presently under development. These telescopes are placed on the L2 Lagrangian point and will operate in far infrared range. EADS-ASTRIUM is manugacturing HERSCHEL telescope and will extend its technology to the SPICA Telescope. HERSCHEL operates in the spectral range between 80 and 670 μm wavelength and is devoted to astronomical investigations in the far-infrared, sub-millimetre and millimetre wavelength range. ASTRIUM has been awarded by ESA to manufacture tgeh 3,5m all SiC telescope. The concept for the HERSCEL telescope is based on an axisymetric, 3,5-m-diameter Cassegrain design. The driving requirements are the large diameter (3,5m) especially for the manufacturing aspects, the WFE which has to be kept below 6μrms, the operational temperatuer (70k) which brings distortionas wrt ambient environment, and finally the mass to keep below 300kg. This Development is part of the ESA HERSCHEL PLANK program. SPICA Telescope driving requirements are also the large diameter (3,5m) especially critical for the manufacturing aspects, the WFE which has to be kept below 350nmrms, and the operational temperature (4,5K) which requires to master the distortions wrt ambient environment. Telescope will operate in the 5 to 200 μm wavelength range. ASTRIUM has been awarded by Sumitomo and ISAS to study the faisability of teh 3,5m all SiC telescope. The main features developed in this paper are: The final design and the recent manufacturing developments of the HERSHEL telescope and the expected performances of such a telescope in space environment The preliminary design of the SPICA telescope and teh predicted performances which are taking advantage from the Silicone Carbide properties developed for HERSCHEL telescope, especially considering the homogeneity inside the structure its stability from abient to the operational temperature range (4,5K). The study shows that the Silicone Carbide Telescope design can fulfil the mechanical and optical requirements, in a passive way without actuators.

Thermal and Mechanical Performance of Superfluid Helium Dewar for IRTS

The Infrared Telescope in Space (IRTS) is a small Hell cooled infrared telescope, launched by the H-II rocket and retrieved by the US Space Shuttle. A 100 L HeII dewar as an engineering model (EM) has been manufactured and tested to evaluate: (1) the heat leak into the cryostat and the temperature distribution, and (2) the mechanical strength and stiffness of the cryostat at both 2 K and 300 K. It is assured from the thermal test that the cryostats meet the thermal requirements for astrophysical observation and the cooling life time. The test results are reflected in the design of the proto-flight model (PFM) currently in progress.

Design of the two-stage series adiabatic demagnetization refrigerator for the NeXT and Spectrum-RG missions

The SXS (Soft X-ray Spectrometer) onboard the coming Japanese X-ray satellite NeXT (New Exploration Xray Telescope) and the SXC (Spectrum-RG X-ray Calorimeter) in Spectrum-RG mission are microcalorimeter array spectrometers which will achieve high spectral resolution of ~ 6 eV in 0.3-10.0 keV energy band. These spectrometers are well-suited to address key problems in high-energy astrophysics. To achieve these high spectral sensitivities, these detectors require to be operated under 50 mK by using very efficient cooling systems including adiabatic demagnetization refrigerator (ADR). For both missions, we propose a two-stage series ADR as a cooling system below 1 K, in which two units of ADR consists of magnetic cooling material, a superconducting magnet, and a heat switch are operated step by step. Three designs of the ADR are proposed for SXS/SXC. In all three designs, ADR can attain the required hold time of 23 hours at 50 mK and cooling power of 0.4μW with a low magnetic fields (1.5/1.5 Tesla or 2.0/3.0 Tesla) in a small configuration (180 mmφ× 319 mm in length). We also fabricated a new portable refrigerator for a technology investigation of two-stage ADR. Two units of ADR have been installed at the bottom of liquid He tank. By using this dewar, important technologies such as an operation of two-stage cooling cycle, tight temperature control less than 1 μK (in rms) stability, a magnetic shielding, saltpills, and gas-gap heat switches are evaluated.

Thermal Design and Test of IRTS Cryostat

Design study of a small, cooled infrared telescope (IRTS) has been made, which will be launched in 1994 as a payload of the Space Flyer Unit (SFU). It is cooled down to 1.8 K by stored superfluid helium of 100 dm3 for about one month, being capable of IR astrophysical observation even in far infrared region. Minimization of heat leak to the lowest temperature level is one of the primary design goal under various constraints. The application of the reflecting, Winston cone type, forebarrel and the black baffle system composed of the mirrors and the aftbarrel is considered for this purpose as well as for extremely low background. A BBM has been manufactured on the basis of the design study, which is primarily dedicated to a thermal test. The test is under way to obtain the design data for the PFM.