Susan R. Breon, Cryogenics & Fluids Branch, Code 552, NASA/Goddard Space Flight Center, Greenbelt, MD 20771
Richard A. Hopkins 2M.S. RA-5 Ball Electro-Optics/Cryogenics Division Boulder, CO 80306
The Advanced X-Ray Astrophysics Facility (AXAF) program was restructured in 1992. The most significant change was placing the instruments on two separate spacecraft: AXAF-S devoted to high resolution spectroscopy and AXAF-I devoted primarily to x-ray imaging. The sole instrument on the AXAF-S spacecraft is the XRS, which achieves a resolution of ~12 eV over the energy range of 0.3 to 10 keV by using calorimeters cooled to less than 0.1 Kelvin. These low temperatures are produced by an adiabatic demagnetization refrigerator, which in turn is cooled by superfluid helium. Stirling-cycle mechanical refrigerators are used to cool the outermost vapor-cooled shield of the helium dewar, thus reducing the parasitic heat load on the cryogen. Work on this complex cryogenic system has been ongoing for over 6 years. As a result of the AXAF restructuring, interfaces are being redefined, new requirements have been levied, and an entirely new AXAF-S Project management structure has been created. This has produced an unusually dynamic environment for revising the design of the XRS instrument for flight on the new AXAF-S mission.
Advances in Cryogenic Engineering 39, 187 (1994).