Flight Performance of the AKARI Cryogenic System

Abstract

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 Akari Project: Legacy and Data Processing Status

“…All of the above science instruments were cooled with a unique cryogenic system, which consisted of cryogen (liquid He) and mechanical coolers (2-stage Stirling coolers; Nakagawa et al, 2007). With the help of cryocoolers, the instruments remained cooled in orbit for 550 days with relatively small amount (179 L) of liquid He.…”

An Extension of a Reliable Wavelength Coverage of the Akari Ng Grism Mode

“…Frey et al (2006) measured the refractive index of Ge at low temperatures down to 30 K. In Phases 1 and 2, the IRC was operated at a lower temperature than 30 K (< 7 K, Nakagawa et al (2007)). Therefore we interpolate the results tabulated in Frey et al (2006) with a cubic spline curve and scale it so that the direct light position does not change from the current wavelength calibration of the latest IRC toolkit.…”

Cooling system for the soft X-ray spectrometer onboard Astro-H

“…The first change is to replace the solid neon (17 K) with a 20 K-class cryocooler with a cooling power of 200 mW. This has already been demonstrated in orbit, by 2ST coolers on Akari [6]. According to thermal analysis, if we have four units of 2ST to cool the shield, the LHe lifetime is longer than 3 years in the normal case, and is still about 2 years even if one of the 2ST fails.…”

“…It is shut off using an electrically-driven cryovalve once the flow rate becomes small. A similar vent system was adopted by Akari and was demonstrated in orbit [6].…”

“…One of the critical parts of the SXS is the cooling system. The design is based on the experience and lessons of the XRS and also makes the maximum use of the heritage of the XRS, Akari (Astro-F; launched in 2006) [5,6], and SMILES [7,8,9], to satisfy the lifetime requirement of 3 years (goal: 5 years). As of June 2009, we are in the preliminary design phase.…”