J. G. Tuttle

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.

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...

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.

Mass Gauging and Thermometry on the Superfluid Helium on-Orbit Transfer Flight Demonstration

The Superfluid Helium On-Orbit Transfer (SHOOT) Flight Demonstration, launched in June, 1993 was a shuttle attached payload designed to demonstrate the technology needed to resupply superfluid helium dewars in space. One of the technology items demonstrated was the ability to determine the mass in either supply or receiver dewar. Liquid quantity was measured using heat pulse mass gauging. For SHOOT the algorithm used to compute the mass from temperature measurements has been improved and the resolution and accuracy of the thermometry has been increased. The resulting mass resolution is 1 to 3%. In orbit the heat pulse mass gauging result is compared to results obtained using liquid/vapor discriminators when the liquid is purposely settled to one end of the dewars. Mass gauging results are also be compared to integrated flow meter results. The electronics for thermometer readout are also be briefly described.

Response of a MEMS Microshutter Operating at 60 K to Ionizing Radiation

Total ionizing dose (TID) measurements at low temperature (60 K) of a micro-electro-mechanical system (MEMS) microshutter array (MSA) indicate that exposing the MSA to ionizing radiation causes some of the shutters to stop operating properly. The number of non-functional shutters depends on the applied bias. With increasing dose, the number of micro-shutters that become non-functional increases.

Thermal Conductivity and Specific Heat Measurements of Candidate Structural Materials for the JWST Optical Bench

The James Webb Space Telescope will include an optical bench known as the integrated science instrument module (ISIM). Candidate structural materials for the ISIM must have low density, high stiffness, and low thermal expansion coefficient at the operating temperature of 30 Kelvin. The thermal conductivity and specific heat are important in modeling the on‐orbit cooldown. We built two different systems for measuring the thermal conductivity and specific heat of samples between 4 Kelvin and 290 Kelvin. Both experiments were carefully designed to minimize potential errors due to radiative heat transfer. We chose the cooling system and instrumentation to allow long‐term unattended operation. Software was developed to automate each experiment. It used an algorithm designed to ensure that each system was in steady state before a measurement was taken. We describe the two experiments and present the data.

Progress on a 4 K to 10 K Continuously Operating Adiabatic Demagnetization Refrigerator

An adiabatic demagnetization refrigerator (ADR) is under development for use in cooling relatively large loads (10–100 mW) at 4 K and rejecting that heat to a cryocooler operating at 10 K. ADRs can operate in this temperature range with an efficiency of 75% of Carnot, saving as much as 2/3 of the required overall input power. In addition this ADR can provide cooling down to 0.4 K. The ADR magnet consists of 8 short coils wired in series and arranged in a toroid to provide self‐shielding of its magnetic field. Eliminating passive or active shields saves 30% of the mass of the system. The average field is 3 Tesla using 5 amps. In the first model the coils are wound with ordinary NbTi superconducting wire and operated at 4 K. A second version will then use fine Nb3Sn wire to provide complete 10 K operation. As a refrigerant for this temperature range we are using readily available gadolinium gallium garnet (GGG) crystals, which provide suitable performance. In the future we will switch to either GdLiF4 or Gd...

A High Tc Superconducting Current Lead Assembly for the XDS Helium Cryostat

The X-ray Spectrometer Detector System (XDS) helium cryostat consists of a tank of pumped liquid helium at about 1.3 kelvin suspended inside a seventeen kelvin cylindrical support structure. The tank is a heat sink for an adiabatic demagnetization refrigerator (ADR) and its superconducting magnet. The cryostat’s small initial helium volume and mission lifetime goal of 2.5 years require that the average total heat load to the helium be less than about 800 microwatts. During the mission the superconducting magnet requires a current of 2 amps with a three percent duty cycle. In addition, wires capable of carrying up to 1 amp are needed for cryogenic valve operations during the cryostat’s ground servicing. The best optimized conventional current leads between the 17 kelvin stage and the magnet and valves would contribute an average heat load to the helium of about 3 milliwatts. An assembly of superconducting YBaCuO fibers bonded to a fiberglass tube and suspended by a Kevlar* braid was developed to conduct the current from the 17 kelvin support structure to a vapor-cooled 4 kelvin stage. NbTi wires provide a superconducting path from the 4 kelvin stage to the magnet and valves on the 1.3 kelvin helium tank. This paper describes the assembly’s fabrication and suspension and presents the results of its performance and vibration tests.