D. R. Swanson

Lambda point experiment in microgravity

Abstract In October 1992 a low temperature experiment was flown on the Space Shuttle in low earth orbit, using the JPL low temperature reserarch facility. The objective of the mission was to measure the heat capacity and thermal relaxation of helium very close to the lambda point with the smearing effect of gravity removed. We describe the experiment with emphasis on the high resolution thermometry and the thermal control system. We also report preliminary results from the measurements made during the flight and compare them with related measurements performed on the ground. The sample was a sphere 3.5cm in diameter contained within a copper calorimeter of very high thermal conductivity. The calorimeter was attached to a pair of paramagnetic salt thermometers with noise levels in the 10 −10 range. During the mission we found that the resolution of the thermometers was degraded somewhat, due to the impact of charged particles. This effect limited the useful resolution of the measurements to ≈2 nK from the lambda point.

Preliminary Results from a Shuttle Mission to Measure the Heat Capacity of Confined Helium near the Lambda Point

We describe the preliminary results from a Shuttle mission to measure the heat capacity of helium confined within a stack of evenly spaced silicon plates at temperatures very close to the superfluid transition. Recently developed high resolution thermometry has substantially improved our ability to study dimensional cross-over effects in well-defined geometries. These effects have been of interest to theorists and esperimentalists for decades. The main part of the apparatus consists of a high purity copper calorimeter containing a stack of 408 silicon plates spaced 57 microns apart, and a pair of high resolution, fast response, paramagnetic salt thermometers with a noise level of < 10−10K in a 1 Hz bandwidth. The resolution of the heat capacity measurements was about 5 × 10−9K, allowing the finite size peak to be mapped in detail. In addition, wide range data containing information on the behavior of the surface specific heat was collected. The preliminary analysis shows fair agreement with theory. The results can also be combined with supplementary ground measurements on smaller length scales to perform additional tests of scaling predictions for cross-over to lower dimensional behavior.

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.

Testing critical point universality along the λ-line

We are currently building a prototype for a new test of critical-point universality at the lambda transition in 4He, which is to be performed in microgravity conditions. The flight experiment will measure the second-sound velocity as a function of temperature at pressures from 1 to 30 bars in the region close to the lambda line. The critical exponents and other parameters characterizing the behavior of the superfluid density will be determined from the measurements. The microgravity measurements will be quite extensive, probably taking 30 days to complete. In addition to the superfluid density, some measurements of the specific heat will be made using the low-g simulator at the Jet Propulsion Laboratory. The results of the superfluid density and specific heat measurements will be used to compare the asymptotic exponents and other universal aspects of the superfluid density with the theoretical predictions currently established by renormalization group techniques.

The velocity of second sound near the lambda transition in superfluid4He

Abstract We report new high precision data on the velocity of second sound close to the lambda transition of 4 He at the saturated vapor pressure. The second sound wave is detected by a powdered salt pill of copper ammonium bromide in direct contact with the liquid helium. With a thermometer noise of ≤10 -10 K/√Hz we are able to make measurements in the linear regime to within 20 nanokelvin of the lambda point. We have obtained reasonable agreement with renormalization group theory predictions after allowance for the effect of gravity. The results are also compared with previous experiments.

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.

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.

Specific heat of helium confined to micron-scale geometries near the lambda point

Abstract Most experiments to date on the effects of confinement on helium have been restricted to the submicron regime. However, using very high-resolution thermometry techniques, it has become possible to explore the region extending up to about 100 μm . This development dramatically increases the range over which length scaling can be tested and eases the problem of uncontrolled surface effects. At the upper end of this length scale, experiments in space are needed to reduce the transition broadening due to hydrostatic compression. We present the most recent results from a measurement of the specific heat of helium confined to 57 μm thick planes and compare them with theoretical predictions. We also describe some recent results obtained with helium confined to cylindrical channels with diameters of 0.26 and 8 μm

Superfluid density as a function of temperature and pressure near the lambda line

Abstract We report new results for the superfluid density in the region close to the lambda line at pressures up to 25 bars. The data were obtained from second sound velocity measurements made to within 0.9 μK of the transition by a resonance technique. A superconducting pressure gauge was used to allow the measurements to be performed along a constant pressure path. From these data the temperature and pressure dependence of the superfluid density can be derived to test for critical exponent universality. We also report the temperature and pressure dependence of the resonator quality factor.

Single bubble nucleation and collapse in superfluid helium

In a Berthelot tube, a homogeneous liquid is confined to a constant volume while its temperature is varied. In a normal fluid the temperature is lowered until a it enters the metastable state and a bubble is nucleated, the nucleation typically being governed by heterogeneous nucleation. Upon warming the liquid, the bubble will collapse at a well defined temperature somewhat warmer than the nucleation point. The collapse however is accelerated by pressure due to surface tension. In superfluid helium a density maximum occurs near the lambda point so a bubble is nucleated by warming the helium. We report on measurements taken with high resolution thermometers in the Lambda Point Experiment calorimeter both on the ground and in microgravity as well as measurements taken in the CHeX calorimeter.