Publications

The Critical Dynamics in Microgravity Experiment, DYNAMX, is under development for space flight at Sandia National Laboratories and the University of New Mexico with Dr. Rob Duncan as the Principal Investigator. This experiment will investigate the effects on the superfluid transition in 4He of currents generated by heat flow, measuring the thermal conductivity in the fluid as a function of applfed heat. DYNAMX will also take advantage of the weightless environment to measure the conductivity properties in the region of the interface between the two phases. Thus, DYNAMX represents an experiment that will explore a system driven far from equilibrium. The experiment development is sponsored by the Microgravity Science and Applications Division of NASA, with the Jet Propulsion Laboratory as the managing center. This paper will describe the science objectives, the current design of the experiment apparatus, the steps being taken to prepare this experiment for flight, and the results of ground-based feasibility demonstrations now underway.

The effect of a sidewall thermometry probe on heat transport measurements in liquid 4He very close to the superfluid transition is simulated numerically. The effective thermal position of the probe changes with closeness of approach to the superfluid transition. The radial heat flux induced by the sidewall probe is calculated for specific probe designs. © 1994.

A new generation of digital multimeters was used to compare the ratios of the resistances of wire-wound reference resistors and quantized Hall resistances. The accuracies are better than 0.1 ppM for ratios as large as 4:1 if the multimeters are calibrated with a Josephson array. 9 refs.

Microfabricated series-arrays of Josephson junctions have been developed which are capable of producing quantized voltage levels over a wide voltage range. These arrays have been used in a calibration system since February 10, 1987 to maintain the US Legal Volt at the National Institute of Standards and Technology (NIST, formerly NBS). A similar system within the Primary Standards Laboratory (which is operated for the Department of Energy, Albuquerque Operations Office by Sandia National Laboratories (SNL)) has been in operation since July, 1989. Measurements of the temperature dependence of the array's quantized voltage states and DC characteristics are reported here. 8 refs., 2 figs.

A refrigerated Dewar has been used successfully with the Josephson array voltage calibration system. It has been used to re-liquefy helium with no degradation of the calibration system's performance. The independence of the array voltage from temperature has been confirmed to three parts in 10{sup 8} per Kelvin using this Dewar. 6 refs., 3 figs.