M. J. DiPirro

The Space Infrared Interferometric Telescope (SPIRIT): High- resolution imaging and spectroscopy in the far-infrared

We report results of a recently-completed pre-Formulation Phase study of SPIRIT, a candidate NASA Origins Probe mission. SPIRIT is a spatial and spectral interferometer with an operating wavelength range 25 - 400 µm. SPIRIT will provide sub-arcsecond resolution images and spectra with resolution R = 3000 in a 1 arcmin field of view to accomplish three primary scientific objectives: (1) Learn how planetary systems form from protostellar disks, and how they acquire their inhomogeneous composition; (2) characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets of different types form; and (3) learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. Observations with SPIRIT will be complementary to those of the James Webb Space Telescope and the ground-based Atacama Large Millimeter Array. All three observatories could be operational contemporaneously.

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.

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.

Superfluid helium transfer flight demonstration using the thermomechanical effect

Abstract In this Paper the use of a thermomechanical pump is investigated for the transfer of superfluid helium in the microgravity of a Shuttle or Space Station. The concept is currently being tested in a ground based experiment and an associated Shuttle demonstration experiment is being designed.

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...

Fluid acquisition system for superfluid helium

To enable the resupply of liquid helium users on orbit, the technology for helium transfer in space is being developed. A key element of the resupply process is the fluid acquisition in the supply dewar; feeding the liquid to the pump. For the Superfluid Helium On-Orbit Transfer (SHOOT) flight experiment a number of fluid acquisition techniques have been examined. Subscale tests performed in one gravity on two of the candidates are described. The hope is that these results may be scaled to much larger systems in low gravity. These two types are a screened channel device and a capillary device. Flow rates versus negative head height are presented. Difficulties with each device and the test environment are explained. Some of the measurements to be made on orbit on the fluid acquisition for SHOOT are described.

Magnetocaloric Effect of Polycrystal GdLiF4 for Adiabatic Magnetic Refrigeration

Polycrystal gadolinium lithium fluorides (GdLiF4=GLF) have been investigated as magnetic materials for an adiabatic demagnetization refrigeration (ADR) system operating at 4 K. Because of difficulties in growing a single crystal of GLF, a polycrystalline sample was fabricated and investigated. The relative density of the fabricated polycrystal was approximately 95%. The measured magnetization data for temperatures above 2 K showed typical paramagnetic behavior. The heat capacity was measured between 0.5 K and 10 K at various magnetic fields. The data suggest that a peak temperature relating to magnetic ordering will be found at a temperature lower than 0.5 K. The calculated entropy data show that the entropy change in GLF at magnetic fields greater than 2 T are 20% to 60% larger than those of a typical magnetic refrigerant GGG. We conclude that GLF will be one of the most suitable magnetic materials for a 4 K ADR.

Suppression of superfluid film flow in the XRS helium dewar

Superfluid helium dewars for space missions use a porous plug phase separator to retain the bulk liquid while releasing boiloff vapor. Although the thermomechanical force is quite effective in holding the liquid against the internal vapor pressure, van der Waals forces will lead to a leakage of helium through the porous plug to feed the flow of a superfluid film in the vent line. Since the latent heat is generally not extracted from the remaining liquid, the film poses a serious problem for space missions such as the X-Ray Spectrometer (XRS) which have extremely low heat load budgets. A design for the XRS porous plug and vent system is presented which keeps film losses below 2 µg/s through a combination of a heat exchanger, to evaporate the film, and a knife-edge device. Tests of the vent have been conducted in a minus 1g configuration. At the expected on-orbit tank temperature of 1.22 K, film losses are negligible, but rise to 4 µg/s at the maximum tank temperature of 1.30 K. Design parameters and performance tests are discussed.

Thermal design of the XRS helium cryostat

Abstract The required lifetime for the Astro-E X-Ray Spectrometer (XRS) is 2 years, with a goal of 2.5 years. To meet this requirement, significant advances in state-of-the-art longlife cryogenic systems are required. The XRS system is a hybrid neon/helium system with a final stage of cooling provided by an adiabatic demagnetization refrigerator. The thermal design of the helium cryostat is described in this paper. To achieve a lifetime of 2.5 years with a helium volume of approximately 20 litres, the heat load on the helium must be of the order of 800 μW or less. The expected lifetime and sensitivity of the lifetime to changes in the design or external heat loads is modelled. Results of preliminary thermal conductivity measurements are presented and future tests are identified. A study of heat loads that were small enough to be neglected in previous designs of long-life cryogenic systems was undertaken. A summary of the findings is presented.

Flight performance of the SHOOT liquid acquisition devices

Abstract The Superfluid Helium On-Orbit Transfer (SHOOT) Flight Demonstration, launched on STS-57 in June 1993, demonstrated the technology required for the management and transfer of superfluid helium in low gravity, and was the first orbital experiment using liquid acquisition systems with cryogens. Its primary experimental objective was to test the performance of two different liquid acquisition devices (LADs) which use surface tension forces to gather the liquid and feed it to the superfluid pumps. One system consists of c-channels whose open sides are covered by fine mesh screens which face the tank walls. The other consists of a series of Mylar/TM vanes which extend radially from the centre of the tank. Several superfluid transfers were performed during relatively quiescent periods and during two transfers adverse orbiter accelerations of 0.034 and 0.069 m s −2 (3.5 × 10 −3 and 7.0 × 10 −3 g ) were generated to move liquid away from the pumps to test the ability of each LAD to work against relatively large hydrostatic heads. Discrete liquid/vapour discriminators placed strategically within the tanks were used to detect failure of the LADs to supply liquid to the pumps. One goal was to test whether, if such a failure occurs, the transfer process can spontaneously recover without operator intervention. Overall performance of both LADs during transfers with and without adverse accelerations is presented.