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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
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NASA NTRS1995

Spectral diagnostics of high energy emission in lambda Eri

Multi-line observations of the optical spectrum of lambda Eri demonstrates that rapidly varying, low-velocity emissions occur in several He I lines even when H alpha shows no emission. A peculiar aspect of the He I emissions is that the ratio 5876/6678 is = 1. A theory of helium line formation generally admits two common emission mechanisms. The first is recombination/cascades, which is well known to give a ratio of greater than or equal to 3. The second is a non-LTE effect that occurs in hot (O-type) photospheres when resonance He I 584 radiation becomes transparent and drives single lines along into the emission. To accommodate a ratio of 5876/6678 = 1 may require that both processes sometimes operate at the same time, presumably in separate localities near the surface of this star.

Smith, Myron↗
NASA NTRS1997

X-Ray Flare Characteristics in Lambda Eridani

This proposal was for a joint X-ray/ultraviolet/ground-based study of the abnormal Be star lambda Eri, which has previously shown evidence of X-ray flaring from ROSAT observations in 1991. The X-ray component consisted of observations from both the ASCA and ROSAT satellites.

Smith, Myron A.↗
NASA NTRS1995

Arcs and Clumps in the Uranian Lambda Ring

Careful reprocessing of the Voyager images reveals that the Uranian lambda ring has marked longitudinal variations in brightness comparable in magnitude to those in Saturn's F ring and Neptune's Adams ring. The ring's variations show a dominant five-cycle (72-degree) periodicity, although additional structure down to scales of about 0.5 degree is also present. The ring's shape is defined by a small overall eccentricity plus a six-cycle (60-degree) sinusoidal variation of radial amplitude around 4 kilometers. Both of these properties can be explained by the resonant perturbations of a moon at a semimajor axis of 56,479 kilometers, but no known moon orbits at this location. Unfortunately, the mass required suggests that such a body should have been imaged by Voyager.

Showalter, Mark R.↗
NASA NTRS1997

X-Ray Flare Characteristics in lambda Eridani

This proposal was for a joint X-ray/ultraviolet/ground-based study of the abnormal Be star lambda Eri, which has previously shown evidence of X-ray flaring from ROSAT observations in 1991. The X-ray component consisted of observations from both the ASCA and ROSAT satellites.

Smith, Myron A.↗
NASA NTRS1994

Photospheric Activity in Selected Be STARS: lambda Eri and gamma Cas

Recent observations of rapid variations in optical He I lines, X-rays, and FUV wavelengths in the prototypical classical Be stars lambda Eri and star gamma Cas hint that the violent processes occur on the surfaces of these stars almost all the time. We suggest that of these phenomena show greater similarities with magnetic flaring than any other process through to occur on stars.

Smith, Myron A.↗
NASA NTRS1999

Detection of O VII Lambda 1522 in IUE Spectra of Planetary Nebula Nuclei and Other Hot Stars

We present the first detection of O VII lambda 1522 emission or absorption from archival IUE spectra in 14 planetary nebula nuclei and three PG 1159-type stars. The n = 5 approaching 6 transition of O VII was determined by Kruk & Werner and observed by them in the spectrum of the very hot PG 1159-type star H1504+65 from data obtained with the Hopkins Ultraviolet Telescope (HUT). Emission-line fluxes or absorption equivalent widths as well as radial velocities for the program stars are presented. The precise rest wavelength for the 5 approaching 6 transition requires further investigation.

Feibelman, Walter A.↗
NASA NTRS1999

Study of Flow of Superfluid He-II Very Near Tau(sub lambda)

We report here, preliminary data from an experiment studying flow of superfluid helium through a slit orifice (of sub-micron width) very close to T(sub lambda). Critical supercurrent (I(sub c)) data is obtained from a step function drive to the diaphragm in a Helmholtz resonator cell. The superfluid density (rho(sub s)) data can be obtained from the resonant frequency of the Helmholtz oscillator, as determined by transfer function of the resonator or from the free ringing after the step function excitation. Preliminary data shows that I(sub c) is proportional to (rho(sub s))(exp 1.27) and rho(sub s)) is proportional to tau(exp 0.73), where tau is the reduced temperature. However, the magnitude of I(sub c) is much larger than expected, indicating a possible parallel flow path. Further investigations are in progress. Keywords: superfluid; hydrodynamics; critical exponent

Mukharsky, Yury↗
NASA NTRS1999

0.4 Microns Spatial Resolution with 1 GHz (lambda = 30 cm) Evanescent Microwave Probe

In this article we describe evanescent field imaging of material nonuniformities with a record resolution of 0.4 microns at 1 GHz (lambda(sub g)/750000), using a resonant stripline scanning microwave probe. A chemically etched tip is used as a point-like evanescent field emitter and a probe-sample distance modulation is employed to improve the signal-to-noise ratio. Images obtained by evanescent microwave probe, by optical microscope, and by scanning tunneling microscope are presented for comparison. Probe was calibrated to perform quantitative conductivity measurements. The principal factors affecting the ultimate resolution of evanescent microwave probe are also discussed.

Tabib-Azar, M.↗
NASA NTRS2000

Measurement of the Heat Capacity of He-II Under a Heat Current Near the Lambda Transition

We present preliminary measurements of the heat capacity of superfluid helium-4 under an applied heat current near the lambda transition. The calorimeter is a standard cylindrical thermal conductivity cell with a 0.6 mm gap between two copper endplates. The sidewall is made of stainless steel. A heat current density in the range of 1 to 4 microW/sq cm is applied through the helium sample while a pulse method is used to measure the heat capacity. Temperature changes are recorded with high-resolution thermometers (HRTs) located on the top and bottom endplates. Corrections are made to the readings of the HRTs to account for the Kapitza boundary resistance and the anomalous Kapitza boundary resistance. After the corrections, both the top and the bottom HRTs. give the same heat capacity values. The heat capacity is found to be much larger than the prediction of recent theories. We also plotted our data on a scaled plot to test the prediction of scaling by the theories. The result and its interpretation will be presented. The cell height was deliberately made to be thin to reduce the effects of gravity. Nonetheless, gravity is expected to have significant effects on the heat capacity data in the temperature range of our measurement. A space experiment would remove this unwanted gravity effect and allow the true physics to be examined. Moreover, in the absence of gravity, a deeper cell can be used allowing HRTs to be mounted on to the sidewall providing direct measurements of the helium temperature, unaffected by the anomalous Kapitza boundary resistance.

Harter, Alexa W.↗
NASA NTRS2003

Vortex Loops at the Superfluid Lambda Transition: An Exact Theory?

A vortex-loop theory of the superfluid lambda transition has been developed over the last decade, with many results in agreement with experiments. It is a very simple theory, consisting of just three basic equations. When it was first proposed the main uncertainty in the theory was the use Flory scaling to find the fractal dimension of the random-walking vortex loops. Recent developments in high-resolution Monte Carlo simulations have now made it possible to verify the accuracy of this Flory-scaling assumption. Although the loop theory is not yet rigorously proven to be exact, the Monte Carlo results show at the least that it is an extremely good approximation. Recent loop calculations of the critical Casimir effect in helium films in the superfluid phase T < Tc will be compared with similar perturbative RG calculations in the normal phase T > Tc; the two calculations are found to match very nicely right at Tc.

Williams, Gary A.↗
NASA NTRS1993

Dynamic Measurements Near the Lambda-point in a Low-G Simulator on the Ground

The properties of liquid helium very near the lambda-transition in the presence of a heat current has received recent theoretical and experimental attention. In this regime, gravity induced pressure effects place severe constraints on the types of experiments that can be performed. A new experiment is described which largely overcomes these difficulties by magnetostrictively canceling gravity influences in the helium sample with a suitable magnetic coil. Design limitations of the technique and a discussion of proposed experiments is presented.

Israelsson, U. E.↗
NASA NTRS1993

Dynamic Measurements Near the Lambda-point in a Low-gravity Simulator on the Ground

The properties of liquid helium very near the lambda-transition in the presence of a heat current has received recent theoretical and experimental attention. In this regime, gravity induced pressure effects place severe constraints on the types of experiments that can be performed. A new experiment is described which largely overcomes these difficulties by magnetostrictively canceling gravity influences in the helium sample with a suitable magnetic coil. Design limitations of the technique and a discussion of proposed experiments is presented.

Israelsson, U. E.↗
NASA NTRS1993

Design of a Low Gravity Simulator for Performing Non-Equilibrium Investigations near the Lambda Transition of ^4He

A design is presented of a low gravity simulator where a magnetic field gradient is employed to oppose the hydrostatic pressure effects of gravity. It appears feasible to reduce the effective gravity environment of the helium in the cell by about two orders of magnitude. The corresponding shift in transition temperature with vertical height would be reduced to 12.7 nK/cm. Methods for instrumenting the simulator to perform high resolution investigations of non-equilibrium phenomena near the lambda point are presented. The advantages of using a low gravity simulator in searching for the predicted change in character of the superfluid transition from continuous to first order in the presence of a heat current are also discussed.

Israelsson, U. E.↗
NASA NTRS1999

Second Sound Measurements Very Near the Lambda Point

The sound was generated by wire-wound heaters embedded in the end opposite the sensor in each cavity. The superfluid density was determined from second sound measurements and the critical exponent v was obtained from fits to the data. The results from the exponent were found to be very sensitive to the treatment of systematic effects in the data.

sound measurements Lambda point superfluid↗