Ulf E. Israelsson

Dilution refrigeration for space applications

Dilution refrigerators are presently used routinely in ground based applications where temperatures below 0.3 K are required. The operation of a conventional dilution refrigerator depends critically on the presence of gravity. To operate a dilution refrigerator in space many technical difficulties must be overcome. Some of the anticipated difficulties are identified in this paper and possible solutions are described. A single cycle refrigerator is described conceptually that uses forces other than gravity to function and the stringent constraints imposed on the design by requiring the refrigerator to function on the earth without using gravity are elaborated upon.

Liquid/vapour phase separation in 4He using electric fields

Abstract In space, a replacement must be found for gravity to physically control and, in certain instances, contain cryogenic liquids. A programme has been started at the Jet Propulsion Laboratory to study the use of electric field generated forces to establish the required orienting effects. We present measurements which show that it is possible to apply strong enough electric fields to a liquid/vapour interface of 4 He to obtain an orienting force comparable to gravity. Our measurements span the temperature range 1.7–4.2 K and demonstrate the applicability of Pashens law for maximum attainable field before breakdown occurs. Some advantages of the electric field separator as opposed to passive surface tension devices are identified.

The successful return of the low temperature platform facility (LTPF) to space

The Low Temperature Platform Facility (LTPF) flew on STS-87 from November 19 to December 5, 1997 as part of the fourth United States Microgravity Payload (USMP-4). During this flight, the LTPF contained the Confined Helium eXperiment (CHeX). The CHeX mission was a success because the LTPF functioned as expected, and the LTPF dewar provided a stable, long lived cooling source for the CHeX instrument. The LTPF is a NASA/JPL facility that is designed to accommodate space shuttle based science experiments or instruments that need cryogenic temperatures near 2 K. The LTPF had flown two times prior to this most recent flight: in 1984 as the Superfluid Helium Experiment on Space Lab-2 and in 1992 as the Lambda Point Experiment (LPE) on USMP-1. The facility has been substantially upgraded between every flight. For the CHeX mission, the improvements to the facility were driven by the need to meet updated Space Shuttle requirements and by the desire to increase the scientific return from the mission. These improvements involved changes to both the cryostat and the facility electronics. We report on the flight performance of the LTPF during the most recent mission (CHeX) and on some of the lessons learned while preparing for this mission.

High resolution thermometry for the confined helium experiment

We report results with a newly designedCu(NH4)2Br4·2H2O paramagnetic salt high resolution thermometry that will be used in the confined helium experiment to be flown on space Shuttle. The improved thermometry has a fast response time and can measure temperature with a resolution of 0.3 nK in a 10 Hz bandwidth. Our measurements have shown that the effect of cosmic ray heating observed in the lambda point experiment in space can be reduced to a negligible level. The current thermometry appears capable of operating in space at the limit imposed by thermodynamic fluctuations.

The Lambda-Point Experiment: Helium Cryostat, Cryo-Servicing, Functions, and Performance

The Lambda-Point Experiment (LPE) flew on the USMP-1 (U.S. Microgravity Payload) mission on Space Shuttle Columbia. The launch occurred on October 22, 1992. The goal of the experiment was to measure the singularity in the specific heat of helium around the lambda transition at 2.177 K, with nano-Kelvin resolution. The instrument was developed by Professor J. Lipa and his team at Stanford University. The flight instrument was built under a contract to Ball Aerospace in Colorado. The Jet Propulsion Laboratory (JPL) provided the cryostat facility and overall management of the experiment. The cryogenic system, functions, and performance are described in detail below. Before flight, over 100 cryo-servicing operations, covering a 20-month period from the first cool-down, were performed. The cryostat’s performance in space exceeded expectations, primarily because of the low outside-shell temperature. The temperature stability of the cryostat was maintained by passive control with a liquid/vapor phase separator.

4He Experiments near Tλ in a Low-gravity Simulator

We report on our latest measurements of gravity cancellation in the low-gravity simulator using a magnetostrictive force. We made these measurements using a new thermal conductivity cell design that is 0.5cm in diameter and 0.5cm in height. Gravity cancellation was verified by measuring both the reduction in the Tλvariation across the cell and the suppression of thermal convection as a function of the magnetic field. Full gravity cancellation was achieved in the simulator with B(dB/dz) ≈ 21 T2/cm, agreeing well with the calculated value and the valve found from levitating drops of helium.

Design and Testing of an Improved Cryopump for the Confined Helium Experiment

A cryopump flown on the space shuttle for the Lambda Point Experiment1 (LPE) was improved and redesigned for use in the Confined Helium Experiment (CHeX). The successful Lambda Point Experiment (LPE) flew on the space shuttle in October 1992. The Confined Helium Experiment (CHeX) will launch in October 1997. CHeX uses high resolution thermometers2 to measure the heat capacity of liquid helium when confined within finely spaced parallel disks. To eliminate heat flow between the calorimeter and the cryogenic bath, a cryopump is used to reduce the pressure in the experimental probe. The LPE cryopump trapped activated charcoal behind a nuclepore filter. The new cryopump uses activated charcoal glued to copper plates to eliminate the nuclepore filter. Vibration tests verified that the cryopump does not generate charcoal dust contamination. Early performance tests indicated that the probe vacuum is significantly lower with the new pump, and that the pressure falls more rapidly than with the previous design. Recent tests with the CHeX experimental probe show a significant reduction in the heat leak between the calorimeter and the cryogenic bath.

Reducing gravity at the superfluid transition in helium-4

There are two intrinsic experimental limitations in measurements near Tλ for traditional ground based experiments: the gravitationally induced pressure variations present in any macroscopic helium sample limit how closely the transition can be approached, and the onset of convection in a sample with T>Tλ limits the range of heat values that can be used. To overcome these limitations, we have built a low gravity simulator consisting of a superconducting magnet with a magnetic field profile shaped to provide a magnetic force opposite that of gravity. Preliminary measurements have shown a decrease in the range of reduced temperatures across a sample but not the expected suppression of the onset of convection.

Heat capacity experiment for very high resolution tests of the theory of confined materials

Abstract We report the current status of an experiment to measure the heat capacity of helium confined within a stack of evenly spaced silicon plates at temperatures very close to the superfluid transition. Newly developed high-resolution thermometry has substantially improved our ability to look into regions where three-dimensional crosses over to two-dimensional behaviour and where two-dimensional behaviour dominates. These regions have been of interest to theorists and experimentalists for decades. The main part of the apparatus consists of a high-purity copper calorimeter containing a stack of 408 silicon plates spaced 57 μm apart and a pair of high-resolution, fast response, paramagnetic salt thermometers. The thermometers have been shown to have a noise level of 10 −10 K with 1 Hz bandwidth. The expected resolution of the heat capacity measurements is 10 −9 K. To avoid the smearing effects of gravity in the 4.5-cm high helium sample, the measurements will be performed on the Space Shuttle. The results from the experiment can be combined with supplementary ground measurements on smaller length scales to perform additional tests of the renormalization group predictions for crossover to lower-dimensional behaviour.

Control of the Interface between 3He-Rich and 4He-Rich Phases Using Electric Fields

A program to experimentally and theoretically study the feasibility of operating a dilution refrigerator in a microgravity environment has been started at the Jet Propulsion Laboratory. One of the major obstacles to be overcome before the ultimate goal can be realized is to find a replacement for gravity in orienting the boundary interface between the 3He-rich and 4He-rich phases of the liquid in the mixing chamber of the refrigerator.We present preliminary measurements showing that moderate electric fields in a suitable geometry can be used to generate the required orienting forces.