24 research outputs found
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.Comment: 19 pages, 10 figures, and 6 tables. Accepted for publication in the
AKARI special issue of the Publications of the Astronomical Society of Japa
Porous Plug Phase Separator and Superfluid Film Flow Suppression System for the Soft X-Ray Spectrometer Onboard Hitomi
When using superfluid helium in low gravity environments, porous plug phase separators are commonly used to vent boiloff gas while confining the bulk liquid to the tank. Invariably, there is a flow of superfluid film from the perimeter of the porous plug down the vent line. For the Soft X-ray Spectrometer onboard ASTRO-H (Hitomi), its approximately 30-liter helium supply has a lifetime requirement of more than 3 years. A nominal vent rate is estimated as ~ 30 ug/s, equivalent to ~ 0.7 mW heat load. It is therefore critical to suppress any film flow whose evaporation would not provide direct cooling of the remaining liquid helium. That is, the porous plug vent system must be designed to both minimize film flow and to ensure maximum extraction of latent heat from the film. The design goal for Hitomi is to reduce the film flow losses to <2 ug/s, corresponding to a loss of cooling capacity of <40 uW. The design adopts the same general design as implemented for Astro-E and E2, using a vent system composed of a porous plug, combined with an orifice, a heat exchanger, and knife-edge devices. In this paper, design, on-ground testing results and in-orbit performance are described