Final Design of the XRS/Astro-E ADR

Abstract

GSFC has developed an X-Ray Spectrometer (XRS) to be flown aboard Astro-E, in cooperation with the Japanese Institute of Space and Astronomical Science (ISAS). XRS uses an array of 32 microcalorimeters capable of detecting X-rays in the energy range of 0.3 – 10 keV with a resolution of 12 eV. In order to accomplish this, the detectors must be operated at a temperature of 0.065 K. In space, an Adiabatic Demagnetization Refrigerator (ADR) must be used to cool the detectors to that temperature. A spaceworthy ADR has been developed at GSFC to be used in the XRS. Originally, the ADR was developed to be flown aboard the AXAF1,2. Budgetary constraints necessitated the move of the XRS to the Astro-E program and this resulted in a considerable reduction in weight and a much tighter thermal specifications for the ADR. The allowable average thermal load of the ADR to the LHe dewar was changed from 2.6 mW to 250 µW. Time constraints did not allow a complete redesign of the ADR. The original shape and size were left unchanged and the new specifications were met by streamlining the heat switch and lengthening the salt pill magnetization cycle time. Size and weight contraints forced us to abandon the original redundancy plan of having a heat switch on either side of the salt pill. We experimented to provide redundancy by replacing a single heat switch with two much slimmer heat switches occupying the same volume, each with almost half the capacity but also half the parasitic heat leak. They were independent from each other, thus providing redundancy, but could be operated simultaneously so as not to reduce the heat switch capacity significantly. During vibration testing we discovered that the considerable reduction in stiffness made these heat switches vulnerable to shorting. It was, therefore, decided that it was preferable to proceed with a single heat switch without redundancy. For a LHe bath temperature of 1.3 K the gas gap heat switch presently used has an on/off ratio of ~28000 and a parasitic heat leak of 2.4 µW/K.