SPICA/SAFARI Sub-Kelvin Cryogenic Chain
Duval, J. M.
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SPICA, a Japanese led mission, is part of the JAXA future science program and is planned for launch in 2018. SPICA will perform imaging and spectroscopic observations in the 5 to 210 mm waveband. The SPICA payload features three instruments, one of which, SAFARI, is developed by a European based consortium. SPICA’s distinctive feature is to use an actively cooled telescope down to 4 K. In addition SPICA is a cryogen-free satellite and all the cooling will be provided by radiative cooling (L2 orbit) down to 30 K and by mechanical coolers for lower temperatures. The satellite will be launched warm and slowly reach its operating temperatures once in orbit. This warm launch approach allows to suppress any large liquid cryogen tank and to use the mass saved to launch a large diameter telescope (3.5 meters). This 4 K cooled telescope allows significantly reduced thermal radiation, offering superior sensitivity in the infrared region. The cryogenic system that enables this warm launch/cooled telescope concept is a key issue of the mission. This cryogenic chain features a number of cooling stages comprising passive radiators, Stirling coolers and several Joule Thomson loops, offering cooling powers at typically 20, 4.5, 2.5 and 1.7 K. The SAFARI detectors require cooling to temperatures as low as 50 mK, and thus the SAFARI instrument cooler will be operated from these heat sinks. It is composed of a small adiabatic demagnetization refrigerator (ADR) pre-cooled by a sorption cooler. This hybrid architecture allows a lower weight cooler able to reach 50 mK. Because the sorption cooler/ADR combination is probably the lightest solution to produce sub-Kelvin temperatures, it allows the stringent SAFARI mass budget to be met.