ALBERT

Description: The Soft X-ray Spectrometer instrument on the Astro-H observatory contains a 6x6 array of x-ray microcalorimeters that is cooled to 50 mK by an adiabatic demagnetization refrigerator (ADR). The ADR consists of three stages in order to provide stable detector cooling using either a 1.2 K superfluid helium bath or a 4.5 K Joule-Thomson (JT) cryocooler as its heat sink. When liquid helium is present, two of the ADR's stages are used to single-shot cool the detectors while rejecting heat to the helium. After the helium is depleted, all three stages are used to continuously cool the helium tank (to about 1.5 K) and single-shot cool the detectors (to 50 mK), using the JT cryocooler as its heat sink. The Astro-H observatory, renamed Hitomi after its successful launch in February 2016, carried approximately 36 liters of helium into orbit. On day 5, the helium had cooled sufficiently (〈1.4 K) to allow operation of the ADR. This paper describes the design, operation and on-orbit performance of the ADR, and the use of the ADR's heat rejection as a tool for mass gauging the helium tank.

Description: Lynx is a proposed NASA X-Ray telescope flight mission aimed at achieving state-of-the-art angular and energy resolution with a 100 kilopixel array to probe the hot energetic young universe in unprecedented detail. To achieve these goals, our team plans on leveraging our current work in development of the focal plane for the Athena X-Ray Integral Field Unit (X-IFU) while advancing the state-of-the-art in transition edge sensor (TES) X-ray detector technology. The TES is an optimal technology for achieving both high energy and fine angular resolution at the same time because pixel features can be made extremely small and the absorber which dominates the heat capacity can be tuned to meet resolution requirements. Specifically, the proposed mission concept calls for a hybrid detector of three different arrays fabricated in the same planar process in one focal plane and optimized for different science goals. The main arrays consist of 5x5 hydras, 25 pixels of 4 micron thick Au absorbers each with a different thermal link to one common TES. The outer array has absorbers on a 50-micron pitch for most of the 5 arc-minute field-of-view, and the inner array has 25-micron absorbers for the central 1 arc-minute region. A high resolution array consisting of single pixel 1 micron thick Au absorbers on 50-micron pitch will lie off to the side. Reading out an array of this magnitude will likely require improvements in indium bump bonding to superconducting flexible wiring. Fabrication of absorbers of two different sizes requires electroplating through a photoresist mold by careful tuning of the current density to achieve uniform flat absorbers on a fine pitch scale, followed by ion milling to yield narrow streets separating the pixels while preserving high quantum efficiency. We report on progress made at fabricating the hybrid array with different absorber sizes and thicknesses. Further, we also report on ongoing work to adequately heat sink the pixels with backside wire bonding and copper coating. We also report on work to improve detector pixel yield and top side indium bump bonding to flexible wiring.

Description: The Soft X-ray Spectrometer (SXS) instrument that flew on the Astro-H observatory was designed to perform imaging and spectroscopy of x-rays in the energy range of 0.2 to 13 keV with a resolution requirement of 7 eV or better. This was accomplished using a 6x6 array of x-ray microcalorimeters cooled to an operating temperature of 50 mK by an adiabatic demagnetization refrigerator (ADR). The ADR consisted of three stages in order to operate using either a 1.2 K superfluid helium bath or a 4.5 K Joule-Thomson (JT) cryocooler as its heat sink. The design was based on the following operating strategy. After launch, while liquid helium was present (cryogen mode), two of the ADRs stages would be used to single-shot cool the detectors, using the helium as a heat sink. When the helium was eventually depleted (cryogen-free mode), all three ADR stages would be used to continuously cool the helium tank to about 1.5 K, and to single-shot cool the detectors (to 50 mK), using the JT cryocooler as a heat sink. The Astro-H observatory, renamed Hitomi after its successful launch in February 2016, carried approximately 36 liters of helium into orbit. Based on measurements during ground testing, the average heat load on the helium was projected to be 0.66 mW, giving a lifetime of more than 4 years. On day 5, the helium had cooled to 〈1.4 K and ADR operation began, successfully cooling the detector array to 50 mK. The ADRs hold time steadily increased to 48 hours as the helium cooled to a temperature of 1.12 K. As the commissioning phase progressed, the ADR was recycled (requiring approximately 45 minutes) periodically, either in preparation for science observations or whenever the 50 mK stage approached the end of its hold time. In total, 18 cycles were completed by the time an attitude control anomaly led to an unrecoverable failure of the satellite on day 38. This paper presents the design, operation and on-orbit performance of the ADR in cryogen mode as the foreshortened mission did not provide an opportunity to test cryogen-free mode.