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.

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.

A series-array voltage standard containing 2,076 Josephson junctions has been operated in a liquid helium bath maintained within a refrigerated Dewar over the temperature range 1.56K ≤ T ≤ 4.54K. No systematic variation in the array voltage near 1.018V with temperature was detected over this entire range, indicating that any temperature coefficient of the array voltage must be less than 2×10-8 V/K. The critical current and gap energy of the weakest junction within the array in the absence of millimeter wave power were measured as a function of temperature over this same range. The noise sensitivity of the array was observed to change abruptly but by a small amount as the helium bath was pumped superfluid. Voltage calibrations were unaffected by the bath's superfluid transition. © 1990.

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.