Aristides T. Serlemitsos

Design of a spaceworthy adiabatic demagnetization refrigerator

Abstract Goddard Space Flight Center (GSFC) is presently developing an X-ray spectrometer (XRS) to be flown aboard the Advanced X-ray Astrophysics Facility (AXAF). 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 under development at GSFC for almost a decade. When XRS because a viable AXAF candidate, the development effort intensified. A baseline model heat switch has been tested extensively with an on/off ratio of ∼ 10 000 and a parasitic heat leak of 10 μW. In the same time period various salt pills have been built employing different methods of thermal contact and of crystal growth. We are presently in the process of building the engineering model of the ADR. Using the data obtained from the breadboard models we have designed an ADR with improved structural integrity while, at the same time, improving on its efficiency and performance. The new ADR design will be reported in detail.

The XRS low temperature cryogenic system: Ground performance tests results

Abstract The X-Ray Spectrometer (XRS) instrument is part of the Astro-E mission scheduled to launch early in 2000. Its cryogenic system is required to cool a 32-element array of X-ray microcalorimeters to 60–65 mK over a mission lifetime of at least 2 years. This is accomplished using an adiabatic demagnetization refrigerator (ADR) contained within a two-stage superfluid helium/solid neon cooler. Goddard Space Flight Center is providing the ADR and helium dewar. The flight helium dewar was assembled in Sept. 1997 and subjected to extensive thermal performance tests. This paper presents test results at both the subsystem and component levels. In addition, results of the low temperature topoff performed in Japan with the engineering unit neon and helium dewars are discussed.

Final Design of the XRS/Astro-E ADR

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.

Flight worthy infrared bolometers with high throughput and low NEP

The Far Infrared Absolute Spectrometer (FIRAS) and Diffuse Infrared Background Experiment (DIRBE) instruments on the Cosmic Background Explorer (COBE)1 mission require bolometers with a relatively short time constant, high throughput, and low NEP. At the same time these detectors must be flight worthy and have very small microphonic response. None of the bolometers available at the beginning of the program, either monolithic or composite, met all of the above requirements. Therefore, the Goddard Space Flight Center developed composite bolometers in-house that meet the stringent requirements of COBE. Small chips of doped and compensated silicon are used as sensing elements. The absorbing substrate consists of diamond wafers coated with thin layers of chromium and gold. Maximum IR absorptance is accomplished by controlling the total surface resistance of these layers. The throughput for the FIRAS bolometers is 1.28 sr-cm2, while for DIRBE it is 0.21 sr-cm2. At 1.6 K, the time constants range from 3 to 40 ms and their NEP range from 4x10-15 to 1x10-14 W/Hz1/2. The noise spectra of these bolometers are flat above 1 to 2 Hz.

Magnetic Shielding for a Spaceborne Adiabatic Demagnetization Refrigerator (ADR)

The Goddard Space Flight Center has studied magnetic shielding for an adiabatic demagnetization refrigerator. Four types of shielding were studied: active coils, passive ferromagnetic shells, passive superconducting coils, and passive superconducting shells. The passive superconducting shells failed by allowing flux penetration. The other three methods were successful, singly or together.

Spaceworthy ADR: recent developments

Recent developments concerning the performance and reliability of a spaceworthy adiabatic demagnetization refrigerator (ADR) for the AXAF X-ray spectrometer are considered. They include a procedure for growing the salt pill around a harness made up of 6080 gold-plated copper wires, a totally modular gas gap heat switch, and a suspension system utilizing Kevlar fibers.

Performance of the XRS/ASTRO-E engineering model ADR

Abstract NASAs Goddard Space Flight Center (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 Advanced X-ray Astrophysics Facility (AXAF) [2] . Budgetary constraints necessitated the move of the XRS to the ASTRO-E program and this resulted in 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 270 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. For a LHe bath temperature of 1.3 K the gas gap heat switch presently used has an on/off ratio of 22 000 and a parasitic heat leak of 2.9 μW/K.

The AXAF/XRS test dewar - A versatile design

The X-Ray Spectrometer (XRS)1,2, to be flown on the Advanced X-ray Astrophysics Facility (AXAF)3 is being developed at NASA’s Goddard Space Flight Center (GSFC). XRS consists of an array of microcalorimeters and an Adiabatic Demagnetization Refrigerator (ADR)4. The ADR provides an operating temperature of 0.065 to 0.100 K. To support extensive development testing of the ADR and the detector array a test dewar has been designed and built at GSFC. Performance specifications say that this dewar must provide an adequate hold time of at least 24 hours with a bath temperature of 1.5 K. The design should minimize microphonic noise to the very sensitive detectors. It also must be flexible and modular to allow quick access to the experiment.

The Cryogenic Subsystem For The X-Ray Spectrometer On The Advanced X-Ray Astrophysics Facility (AXAF)

The X-Ray Spectrometer (XRS) instrument on the Advanced X-Ray Astrophysics Facility (AXAF) will use X-ray detectors that operate at 0.1 K. The detectors will be maintained at 0.1 K by an Adiabatic Demagnetization Refrigerator (ADR) that operates inside a liquid helium dewar. The ADR rejects approximately 2 mW of heat to the stored liquid helium. With this low instrument heat load, the liquid helium dewar will have a long lifetime if the parasitic heat load on the helium from the surrounding, warm facility is minimized. Spaceborne helium dewars typically use up to 3 vapor cooled shields to intercept the parasitic heat load. The XRS will add mechanical coolers to provide additional cooling to the outer vapor cooled shield. The cryogenic system consists of an ADR, a liquid helium dewar, mechanical coolers, and a thermal strap to connect the coolers to the dewar. The lifetime of the stored cryogen is calculated to be up to 5 years. This cryogenic system is described, with particular attention given to the dewar, mechanical cooler, and ADR design, testing, and trade studies. A breadboard ADR is presently being fabricated and tested. The status of the construction and testing of this breadboard will be described.