We have made heat capacity measurements of superfluid 4He at temperatures very close to the lambda point, T λ, in a constant heat flux, Q, when the helium sample is heated from above. In this configuration the helium enters a self-organized (SOC) heat transport state at a temperature T soc(Q), which for Q≥100 nW/cm2 lies below T λ. At low Q we observe little or no deviation from the Q=0 heat capacity up to T SOC(Q); beyond this temperature the heat capacity appears to be sharply depressed, deviating dramatically from its bulk behaviour. This marks the formation and propagation of a SOC/superfluid two phase state, which we confirm with a simple model. The excellent agreement between data and model serves as an independent confirmation, of the existence of the SOC state. As Q is increased (up to 6 µW/cm2) we observe a Q dependent depression in the heat capacity that occurs just below T SOC(Q), when the entire sample is still superfluid, This is due to the emergence of a large thermal resistance in the sample, which we have measured and used to model the observed heat capacity depression. Our measurements of the superfluid thermal resistivity are a factor of ten larger than previous measurements by Baddar et al.

New paramagnetic susceptibility thermometers have been developed for use in fundamental physics missions in earth orbit. These devices use a SQUID magnetometer to measure the variation in the dc magnetization of a thermometric element that consists of a dilute concentration of manganese in a palladium matrix. Near 2.2 K these new PdMn thermometers have demonstrated a temperature resolution of better than 100 pK/rootHz and a time constant of 50 ms when operated with a 50 K/W thermal resistance to the liquid helium sample. These thermometers have been observed to be remarkably stable, with a drift of less than 10 fK/s. The observed power spectral density of the noise from these thermometers is consistent with separate measurements of the device's time constant and thermal standoff from the bath. Recently these PdMn materials have been made into thin films and microstructures for use in future studies of quantum liquids, and for possible use in a new class of bolometers and radiometers. These thermometers have been integrated into an experimental cell and thermal isolation network that are adequate to keep stray heats stable to within a few picowatts, with no systematic temperature errors greater than 60 pK, over the course of a planned fundamental physics experiment on Earth orbit.

Critical Dynamics in Microgravity (DYNAMX) has been selected to be one of the first two fundamental physics experiments to deploy to the International Space Station (ISS) on board the Low Temperature Microgravity Physics Experiments Facility (LTMPEF) M1 mission in 2005. DYNAMX will make very precise heat flow measurements across a column of liquid /sup 4/He as the sample transitions from its superfluid to its normal fluid states while supporting a heat flux Q. These measurements will permit the study of dynamical effects on this phase transition to long-range quantum order for the first time.

We report on the design of a new prototype flight instrument that will be used to repeat previous Earth-based measurements of nonlinear heat transport near the superfluid transition in the microgravity laboratory. Since this nonlinear conductivity is associated with dynamic limitations to the divergent correlation length, and since gravitational acceleration also limits the correlation length's divergence, we anticipate that the nonlinear conductivity will depend strongly on gravitational acceleration. The apparatus, data taking procedure, systematic corrections, and error sources are discussed here.

The Critical Dynamics in Microgravity Project (DYNAMX), which is in competition for flight aboard the International Space Station in the year 2004, will study heat transport through a column of liquid He-4 as the liquid helium transitions from its superfluid to its normal-fluid state. Stray heat will be controlled to within 10 pW, and temperatures will be measured with a resolution of better than 300 pK, over the course of the measurements on orbit. The most closely controlled heat conduction measurements in physics have been performed on Earth to support these planned flight measurements (Day, 1998; Moeur, 1997). The alignment of the gravitational acceleration g with the heat flux Q in the Earth-based laboratory is essential in order to make the experiment a pseudo one-dimensional system that is tractable to analysis (Duncan, 1998). This alignment must be achieved to within two mrad in an absolute sense, and it should be held constant to the best degree possible throughout the duration of the measurements. The work described in this paper includes the design, installation, and testing of a vibration isolation and leveling system for a IO-inch diameter neck liquid helium dewar system. This dewar can be used for testing for DYNAMX, or for other projects requiring the same or a better isolation and level control. The design, installation, and performance evaluation of an active leveling system, which includes a feedback and control sub-system, to achieve control of the level of the dewar system to a few rad, is also discussed.

Measurements of heat transport at the transition from perfect thermal superconductivity to nonlinear heat diffusion in pure 4 He provide a very sensitive probe of matter wave coherence. Superfluid heat transport is proportional to the product of the superfluid density and the superfluid velocity, which are both directly related to the superfluid order parameter. From dynamic scaling theory, the correlation length near the superfluid transition provides a measure of the length over which phase fluctuations of the order parameter persist. Our measurements suggest that both the hydrostatic pressure variation within the liquid helium column, together with the heat flux Q, limit the otherwise divergent correlation length near the superfluid transition. Future measurements planned for the microgravity laboratory will provide the fast extensive experimental test of a renormalized, field theoretic description of heat transport near the superfluid transition. It will also provide a conclusive experimental study of the influence of hydrostatic pressure effects and dynamical effects on the correlation length. A new class of microgravity experiments is proposed that will permit measurements to within 10 pK of the superfluid transition temperature, allowing an entirely new class of ultra-accurate scientific investigations to be performed.

Nonlinear heat conduction has recently been measured near the superfluid transition in pure 4He at very low heat flux Q. Since both dynamic effects and gravity limit the divergence of the superfluid correlation length near the transition at low-Q, these measurements must be repeated in the microgravity environment in order to observe the dynamic effects in isolation. Comparison of the microgravity data to similar data obtained on Earth will provide experimental insight into the effect of gravity on this nonlinear conduction region at low heat flux where theoretical predictions are lacking. While some measurement advantages exist in the microgravity laboratory, it is the study of the direct effect of gravity on the nonlinear conduction measurements that motivate the microgravity need.

The second operational test of the String Thermionic Assembly Research Testbed-Re-START-occurred over the period from June 9 to June 14, 1997. This test series was designed to help qualify and validate the designs and test methods proposed for the Integrated Solar Upper Stage (ISUS) power converters for use during critical evaluations of the complete ISUS bi-modal system during the Engine Ground Demonstration (EGD). The test article consisted of eight ISUS prototype thermionic converter diodes electrically connected in series, Results demonstrated the high temperature structural performance of the re-engineered diode mounting assembly, measurable degradation in electrical performance of seven of the test diodes, and the susceptibility of the diode array to load conditions during fast heat up ramps.

In 1993 and 1994, the Russian Scientific Research Institute NII NPO “LUCH” and Space Power, Inc. (SPI), of San Jose, California, developed a prototype of the single-cell thermionic fuel element (TFE) for the SPACE-R space nuclear power system (NPS). The SPACE-R system was designed as a part of the US Department of Energy's (DOE) Space Reactor Development Program to develop a long life, space reactor system capable of supplying up to 40 kW output power. The jointly developed SC-320 TFE is a prototype of the next generation thermionic converter for nuclear applications in space. This paper presents the results of the initial demonstration tests and subsequent parametric evaluations conducted on the SC-320 TFE as compared to the calculated performance characteristics. The demonstration tests were conducted jointly by Russian and American specialists at the Thermionic Evaluation Facility (TEF) at the New Mexico Engineering Research Institute (NMERI) of the University of New Mexico in Albuquerque

This paper presents the results of nearly 8000 hours of testing of a fully developed cesium vapor source on the integrated TOPAZ II Ya‐21U thermionic space power system. The test period included 4000 hours of system thermal vacuum operation and evaluation by Russian specialists at the Central Design Bureau for Machine Building (CDBMB), St. Petersburg, Russia; nearly 4000 hours of thermal vacuum tests at the Thermionic Systems Evaluation Test (TSET) laboratory; and mechanical tests at the Sandia National Laboratories (SNL), Albuquerque, NM. Testing of the non‐nuclear Ya‐21U system provided significant information for evaluation and characterization of the cesium vapor source that could not be obtained by development and qualification testing of only components. The Ya‐21U system and cesium vapor source were subjected to excessive, unplanned stress levels during the system evaluation tests which resulted in leakage of oxygen into the cesium subsystem and cesium vapor from the TFEs. The information and experience gained during the thermionic system evaluation test are useful for improvement of future cesium vapor subsystem designs, test support equipment, and system test procedures.

Five years ago, during the 8th Symposium on Space Nuclear Power Systems, in Albuquerque, NM, Academician Nikolai Nikolaevich Ponomarev‐Stepnoi, First Deputy Director of the Russian Research Center, Kurchatov Institute, proposed the sale of the Soviety Union’s TOPAZ II technology to the United States. This proposal, made at great personal risk, was initially viewed with much skepticism by most Americans attending that conference since the Cold War was still in full swing. There were, however, a few visionaries, some would say fanatics, that set about to make this sale possible. Even these visionaries did not anticipate the collapse of the Soviet Union or the subsequent efforts by the U.S. and other Western powers to help the Newly Independent States transition to a market economy.Little did these visionaries know that the formation of the ‘‘TOPAZ II Program,’’ using former military space power technology of the Soviet Union, would become the preeminent example of technology cooperation between two former adversaries. A unique teaming arrangement formed in New Mexico, called the New Mexico Strategic Alliance and consisting of the Air Force Phillips Laboratory, Sandia National Laboratories, the University of New Mexico, and Los Alamos Nationalo Laboratory, was a key ingredient in making this program a success. A brief summary of some of the highlights of this technology partnership is given to explain how international patnerships of this type can enable commercialization and technology transfer.