Search

The critical-velocity behavior of oscillatory superfluid Helium-4 flow through a 2-micrometer by 2-micrometer aperture in a 0.1-micrometer-thick foil has been studied from 0.36 K to 2.10 K at frequencies from less than 50 Hz up to above 1880 Hz. The pressure remained less than 0.5 bar. In early runs during which the frequency remained below 400 Hz, the critical velocity was a nearly-linearly decreasing function of increasing temperature throughout the region of temperature studied. In runs at the lowest frequencies, isolated 2 Pi phase slips could be observed at the onset of dissipation. In runs with frequencies higher than 400 Hz, downward curvature was observed in the decrease of critical velocity with increasing temperature. In addition, above 500 Hz an alteration in supercritical behavior was seen at the lower temperatures, involving the appearance of large energy-loss events. These irregular events typically lasted a few tens of half-cycles of oscillation and could involve hundreds of times more energy loss than would have occurred in a single complete 2 Pi phase slip at maximum flow. The temperatures at which this altered behavior was observed rose with frequency, from ~ 0.6 K and below, at 500 Hz, to ~ 1.0 K and below, at 1880 Hz., Comment: 35 pages, 13 figures, prequel to cond-mat/0502039

The evolution of semicircular quantum vortex loops in oscillating potential flow emerging from an aperture is simulated in some highly symmetrical cases. As the frequency of potential flow oscillation increases, vortex loops that are evolving so as eventually to cross all of the streamlines of potential flow are drawn back toward the aperture when the flow reverses. As a result, the escape size of the vortex loops, and hence the net energy transferred from potential flow to vortex flow in such 2 Pi phase-slip events, decreases as the oscillation frequency increases. Above some aperture-dependent and flow-dependent threshold frequency, vortex loops are drawn back into the aperture. Simulations are preformed using both radial potential flow and oblate-spheroidal potential flow., Comment: 18 pages, 6 figures, sequel to cond-mat/0502037

UNKNOWN

Four teams incorporating scientists and engineers from more than 50 universities and 20 engineering firms assessed techniques for detecting and characterizing terrestrial planets orbiting nearby stars.

Four teams incorporating scientists and engineers from more than 50 universities and 20 engineering firms have assessed techniques for detecting and characterizing terrestrial planets orbiting nearby stars. The primary conclusion from the effort of the past two years is that with suitable technology investment starting now, a mission to detect terrestrial planets around 150 nearby stars could be launched within a decade.

Continuous-duty hydrogen sorption cryocoolers are being developed for the European Space Agency (ESA) Planck mission. To achieve an acceptable level of performance and robust operation with these hydride refrigerators during flight, detailed investigations have been performed on the sorbent materials and on critical hardware components.

Partial substitution of tin for nickel in the LaNi(sub 5) alloy greatly enhances the stability and durability of the hydride phase during both gas phase and electrochemical cycling. Effects observed included reduction in hydrogen storage capacity, decreases in the plateau pressures, increased slopes of the plateaus, and smaller hysteresis ratios.

Systematic studies of the hydriding behavior of LaNi(sub 5-x)Sn(sub x)H(sub z) alloys with tin contents in the range between 0.20 and 0.25 have revealed changes in the pressure-composition-temperature (PCT) isotherms measured after heating the hydrides above 450 K. These changes are indications of degradation processes and increased disorder within the alloy structure.

We have developed models for predicting the performance of helium-based Joule-Thomson continuous-flow cryocoolers using charcoal-pumped sorption compressors. The models take as inputs the number of compressors, desired heat-lift, cold tip temperature, and available precooling temperature and provide design parameters as outputs. Future laboratory development will be used to verify and improve the models. We will present a preliminary design for a two-stage vibration-free cryocooler that is being proposed as part of a mid-infrared camera on NASA's Next Generation Space Telescope. Model predictions show that a 10 mW helium-carbon cryocooler with a base temperature of 5.5 K will reject less than 650 mW at 18 K. The total input power to the helium-carbon stage is 650 mW. These models, which run in MathCad and Microsoft Excel, can be coupled to similar models for hydrogen sorption coolers to give designs for 2-stage vibration-free cryocoolers that provide cooling from approx. 50 K to 4 K.

We present a summary of cryocoolers that are presently available or in development and would be suitable for support of mid-infrared camera on the Next Generation Space Telescope.

Two continuous opertation 18K/20K sorption coolers are being developed by the Jet Propulsion Laboratory (JPL) as a NASA contribution to the European Space Agency (ESA) Planck mission that is currently planned for a 2007 launch.

We have developed models for the rapid design of helium-based Joule-Thomson cryocoolers using charcoal-pumped sorption compressors.

At cryogenic temperatures, TbDy alloys exhibit giant magnetostriction, which makes these materials interesting for engineering service in cryogenics actuators, valves, and positioners.

UNKNOWN

Magnetostrictive actuators based on TbDy alloys show promise for use as actuators on cryogenic devices. They can be made to deliver large forces and useful displacements below liquid nitrogen temperatures, with many advantages over piezoelectric actuators or motion feedthroughs from higher temperature actuators.

Hydrogen sorption cryocoolers baselined on Planck Surveyor cosmic microwave background mission.

We are developing metal-hydride based sorption cryocoolers for the Planck Surveyor mission.