Peter J. Shirron

Enhanced magnetocaloric effects in R3(Ga1−xFex)5O12 (R=Gd, Dy, Ho; 0<x<1) nanocomposites

A series of R 3 (Ga 1-x Fe x ) 5 O 12 (R=Gd, Dy, Ho; 0<x<1) compounds for potential magnetic refrigerants were synthesized by chemical routes and characterized by X-ray diffraction, and SQUID magnetometry. Dy and Ho were chosen since they respectively possess increasing orbital contributions to the total angular magnetic moment of the atom over the zero value for Gd. X-ray data showed that garnet structures were obtained and that improvements over the Gd 3 (Ga 0.5 Fe 0.5 ) 5 O 12 compound, which was reported in 1992 as possessing enhanced magnetocaloric effects, may be achieved by equilibrating at 1473K for 15 h, rather than at 1173K for 15h as was done in the earlier studies. Magnetometry measurements showed that when Gd was substituted either by Dy or Ho, the material was superparamagnetic, possessing fine magnetic clusters resulting in enhanced magnetocaloric effects (ΔS m ) with respect to the basic paramagnetic garnet (i.e., x = 0). In addition, with variation in x, the optimal ΔS m was measured for the x = 0.5 compound, similar to that found for the Gd-containing garnet nanocomposites. The optimal ΔS m values for the Ho- and Dy-containing compounds, respectively, were found to be about the same or smaller than that for the optimal Gd-containing nanocomposite Gd 3 (Ga 0.5 Fe 0.5 ) 5 O 12 , despite the increased total angular moment. We interpret these results as indicating a reduction in the interaction strength between the rare-earth elements and the Fe as the Gd is replaced by Dy or Ho, and that Dy reduces this interaction strength faster than does Ho.

The Astro-H High Resolution Soft X-Ray Spectrometer

We present the overall design and performance of the Astro-H (Hitomi) Soft X-Ray Spectrometer (SXS). The instrument uses a 36-pixel array of x-ray microcalorimeters at the focus of a grazing-incidence x-ray mirror Soft X-Ray Telescope (SXT) for high-resolution spectroscopy of celestial x-ray sources. The instrument was designed to achieve an energy resolution better than 7 eV over the 0.3-12 keV energy range and operate for more than 3 years in orbit. The actual energy resolution of the instrument is 4-5 eV as demonstrated during extensive ground testing prior to launch and in orbit. The measured mass flow rate of the liquid helium cryogen and initial fill level at launch predict a lifetime of more than 4 years assuming steady mechanical cooler performance. Cryogen-free operation was successfully demonstrated prior to launch. The successful operation of the SXS in orbit, including the first observations of the velocity structure of the Perseus cluster of galaxies, demonstrates the viability and power of this technology as a tool for astrophysics.

Soft x-ray spectrometer (SXS): The high-resolution cryogenic spectrometer onboard ASTRO-H

We present the development status of the Soft X-ray Spectrometer (SXS) onboard the ASTRO-H mission. The SXS provides the capability of high energy-resolution X-ray spectroscopy of a FWHM energy resolution of < 7eV in the energy range of 0.3 – 10 keV. It utilizes an X-ray micorcalorimeter array operated at 50 mK. The SXS microcalorimeter subsystem is being developed in an EM-FM approach. The EM SXS cryostat was developed and fully tested and, although the design was generally confirmed, several anomalies and problems were found. Among them is the interference of the detector with the micro-vibrations from the mechanical coolers, which is the most difficult one to solve. We have pursued three different countermeasures and two of them seem to be effective. So far we have obtained energy resolutions satisfying the requirement with the FM cryostat.

A Multi-Stage Continuous-Duty Adiabatic Demagnetization Refrigerator

The design of a multi-stage adiabatic demagnetization refrigerator (ADR) that can provide continuous cooling at very low temperatures is presented. The ADR is being developed for use in x-ray, IR and sub-millimeter space astronomy missions which will employ large format detector arrays operating at 50 mK and lower and which may dissipate up to 10 μW. It is also being designed to reject heat slowly to a relatively warm heat sink (in the 6–10 K range), so that future missions may use mechanical cryocoolers instead of liquid helium for pre-cooling. The continuous nature of the device gives it a much higher cooling power per unit mass, allowing it to be much smaller and lighter than existing ADRs with comparable performance. Design details are discussed.

A compact, high-performance continuous magnetic refrigerator for space missions

Abstract We present test results of the first adiabatic demagnetization refrigerator (ADR) that produce true continuous cooling at sub-kelvin temperatures. This system uses multiple stages that operate in sequence to cascade heat from a “continuous” stage up to a heat sink. Continuous operation avoids the usual constraints of long hold times and short recycle times that lead to the generally large mass of single-shot ADRs, and allows us to achieve much higher cooling power per unit mass. Our design goal is 10 μW of cooling at 50 mK while rejecting heat to a 6–10 K heat sink. The total cold mass is estimated to be less than 10 kg, including magnetic shielding of each stage. These parameters envelop the requirements for currently planned astronomy missions. The relatively high temperature heat rejection capability allows it to operate with a mechanical cryocooler as part of a cryogen-free, low temperature cooling system. This has the advantages of long mission life and reduced complexity and cost. At present, we have assembled a three-stage ADR that operates with a superfluid helium bath. Additional work is underway to develop magnetocaloric materials that can extend its heat rejection capability up to 10 K. Design, operation and performance of the ADR are discussed.

HAWC: a far-infrared camera for SOFIA

When SOFIA enters operation, it will be the largest far- infrared telescope available, so it will have the best intrinsic angular resolution. HAWC (High-resolution Airborne Wideband Camera) is a far-infrared camera designed to cover the 40 - 300 micron spectral range at the highest possible angular resolution. Its purpose is to provide a sensitive, versatile, and reliable facility-imaging capability for SOFIAs user community during its first operational use.

On-orbit superfluid transfer: preliminary results from the SHOOT flight demonstration

Abstract Preliminary results from the Superfluid On-Orbit Transfer (SHOOT) Flight Demonstration which flew on Space Shuttle STS-57 in June 1993 are presented. SHOOT demonstrated the technology required to transfer superfluid helium between Dewars in low gravity. In addition a number of components developed for SHOOT were flight proven and are now available for use on other payloads. Included in this paper are a description of the transfer process, the transfer rates and losses, observations of the differences between ground transfers and those done on-orbit, and the performance of a number of components. Among these components are liquid acquisition devices, phase separators, liquid/vapour discriminators, thermomechanical pumps, three types of valves, venturi flow meters and a heat pulse mass gauging system.

Passive gas-gap heat switches for use in adiabatic demagnetization refrigerators

We have designed, built, and tested a gas-gap heat switch that turns on and off passively, without the need for a separate, thermally activated getter. This switch uses 3He condensed as a thin film on alternating plates of copper. The switch is thermally conductive at temperatures above about 0.2 K, and is insulating if either end of the switch cools below about 0.15 K. The “on” conductance (7 mW/K at 0.25 K) is limited by the surface area and gap between the copper leaves, the saturated vapor pressure of the 3He, and the Kapitza boundary resistance between the 3He and the copper. The “off” conductance is determined by the helium containment shell which physically supports the two conductive ends. We have also designed and are building passive gas-gap heat switches that will passively turn off near 1 K and near 4 K. For these switches we rely on the strong temperature dependence of the vapor pressure of 4He adsorbed onto neon or copper substrates, respectively, when the coverage is less than one monolayer...

In-orbit operation of the ASTRO-H SXS

We summarize all the in-orbit operations of the Soft X-ray Spectrometer (SXS) onboard the ASTRO-H (Hit- omi) satellite. The satellite was launched on 2016/02/17 and the communication with the satellite ceased on 2016/03/26. The SXS was still in the commissioning phase, in which the setups were progressively changed. This article is intended to serve as a reference of the events in the orbit to properly interpret the SXS data taken during its short life time, and as a test case for planning the in-orbit operation for future micro-calorimeter missions.

The x-ray microcalorimeter on the NeXT mission

The Soft X-ray Spectrometer (SXS) onboard the NeXT (New exploration X-ray Telescope) is an X-ray spectrometer utilizing an X-ray microcalorimeter array. Combined with the soft X-ray telescope of 6 m focal length, the instrument will have a ~ 290cm2 effective at 6.7 keV. With the large effective area and the energy resolution as good as 6 eV (FWHM), the instrument is very suited for the high-resolution spectroscopy of iron K emission line. One of the major scientific objectives of SXS is to determine turbulent and/or macroscopic motions of the hot gas in clusters of galaxies of up to z ~ 1. The instruments will use 6 × 6 or 8 × 8 format microcalorimeter array which is similar to that of Suzaku XRS. The detector will be cooled to a cryogenic temperature of 50 mK by multi-stage cooling system consisting of adiabatic demagnetization refrigerator, super fluid He, a 3He Joule Thomson cooler, and double-stage stirling cycle cooler.