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At least 1,045 records · Page 58

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NASA NTRS1979

Area scaling investigations of charging phenomena

The charging and discharging behavior of square, planar samples of silvered, fluorinated ethylene-propylene (FEP) Teflon thermal control tape was measured. The equilibrium voltage profiles scaled with the width of the sample. A wide range of discharge pulse characteristics was observed, and the area dependences of the peak current, charge, and pulse widths are described. The observed scaling of the peak currents with area was weaker than that previously reported. The discharge parameters were observed to depend strongly on the grounding impedance and the beam voltage. Preliminary results suggest that measuring only the return-current-pulse characteristics is not adequate to describe the spacecraft discharging behavior of this material. The seams between strips of tape appear to play a fundamental role in determining the discharging behavior. An approximate propagation velocity for the charge cleanoff was extracted from the data. The samples - 232, 1265, and 5058 square centimeters in area - were exposed at ambient temperature to a 1- to 2-nA/sq cm electron beam at energies of 10, 15, and 20 kilovolts in a 19-meter-long by 4.6-meter-diameter simulation facility at the Lewis Research Center.

Aron, P. R.↗

NASA NTRS1979

Investigation of electrostatic discharge phenomena on conductive and non-conductive solar reflectors

A sample composed of non conductive optical solar reflectors (OSR) was irradiated with low energy electrons at the DERTS facility to study the effects of charge build up on thermal control coating materials. The degration effects on this panel due to electrostatic discharges were investigated to find possible alternatives to limit the amount of damage. The following systems are evaluated: (1) non conductive OSR (non conductive adhesive) (2) non conductive OSR (conductive adhesive); and conductive OSR (conductive adhesive).

S J Bosma↗

NASA NTRS1979

Analysis of time dependent phenomena observed with the LPSP OSO-8 instrument

The dynamics of the solar photosphere and chromosphere are studied. Observations obtained by the Laboratorie de Physique Stellaire et Planetaire's (LPSP) ultraviolet spectrometer onboard the OSO-8 spacecraft are analyzed, and dynamic models of the chromosphere and the emitted resonance line spectrum are calculated. Some of the unpublished data analysis and theoretical modeling which are being prepared for publication are discussed. A discussion of the state of the theory of velocity fields in the solar atmosphere is also presented. An invited review presented at the OSO-8 Workshop on the topic of oscillatory motions in the quiet sun is included. The results of the OSO-8 data analysis prepared in close collaboration with LPSP scientists are presented. Material for two articles is also presented.

Leibacher, J. W.↗

NASA NTRS1979

Some recent developments in the prediction of shock interaction phenomena at hypersonic speeds

The shock strengths for which either Edney type I or type II shock interference patterns can occur when two oblique shocks of opposite families intersect were determined graphically at Mach 10 by using logarithmic shock polar diagrams. The theoretical region of overlap for the two types of interaction was investigated by observing in the Schlieren system of the Langley 15-inch hypersonic flow apparatus the intersection of oblique shocks generated by two sharp 10 degree wedges as the wedge angles of attack and their relative positions were altered. A range of shock strengths for which either of the two interference patterns can exist was demonstrated.

Crawford, D. H.↗

NASA NTRS1979

Contribution to the theory of photopic vision: Retinal phenomena

Principles of thermodynamics are applied to the study of the ultramicroscopic anatomy of the inner eye. Concepts introduced and discussed include: the retina as a three-dimensional sensor, light signals as coherent beams in relation to the dimensions of retinal pigments, pigment effects topographed by the conjugated antennas effect, visualizing lights, the autotropic function of hemoglobin and some cytochromes, and reversible structural arrangements during photopic adaptation. A paleoecological diagram is presented which traces the evolution of scotopic vision (primitive system) to photopic vision (secondary system) through the emergence of structures sensitive to the intensity, temperature, and wavelengths of the visible range.

Calvet, H.↗

NASA NTRS1978

Correlated photometric and polarimetric phenomena in AM Herculis

Via simultaneous multicolor photometry and polarimetry of AM Herculis, we find correlations among polarization, flux, and color in the V and I bands in periodic and nonperiodic (i.e., flickering) activity. The primary minimum observed in both bands is accompanied by a blueward shift of the (B - R) color and a decrease in the absolute value of the percent circular polarization in the V band. Outside primary minimum, peaks of flickering activity tend to be associated with an increase in the absolute value of the circular polarization and the concurrent reddening of the (B - R) and (R - I) colors. The visual light curve of AM Her can be explained by a flickering, circularly polarized light source which is obscured at primary minimum, and is redder than the total system color in the URVRI bands.

Priedhorsky, W. C.↗

NASA NTRS1979

Non-linear phenomena in films of solar arrays

The paper assesses nonlinear effects in films which are associated with the instability of their shape as a result of contractions during longitudinal and transverse in-plane oscillations. The following problems are solved analytically: transverse in-plane oscillations, longitudinal oscillations in films of rotating solar sail, transverse in-plane oscillations in films of a rotating solar sail, and effect of damping in films of a rotating solar sail. The reason for damping lies in the loss of kinetic energy during absorption of the film particles by root wrinkles without reflection (absolutely inelastic shock).

Zak, M.↗

NASA NTRS1978

Insulator edge voltage gradient effects in spacecraft charging phenomena

Insulating surfaces on geosynchronous satellites have been charged by geomagnetic substorms to a point where discharges occur. The electromagnetic pulses from these discharges couple into satellite electronic systems disrupting operations. Laboratory tests conducted on insulator charging have indicated that discharges appear to be initiated at insulator edges where voltage gradients can exist. An experimental investigation has been conducted to measure edge voltage gradients on silvered Teflon samples as they are charged by monoenergetic electron beams. It has been found that the surface voltage at insulator edges can be approximated by an exponential expression based on an electron current density balance. Using this expression at known breakdown conditions results in a discharge voltage gradient down the insulator edge to ground of about 150,000 V/cm.

Stevens, N. J.↗

NASA NTRS1979

Dynamic cross correlation studies of wave particle interactions in ULF phenomena

Magnetic field observations made by satellites in the earth's magnetic field reveal a wide variety of ULF waves. These waves interact with the ambient particle populations in complex ways, causing modulation of the observed particle fluxes. This modulation is found to be a function of species, pitch angle, energy and time. The characteristics of this modulation provide information concerning the wave mode and interaction process. One important characteristic of wave-particle interactions is the phase of the particle flux modulation relative to the magnetic field variations. To display this phase as a function of time a dynamic cross spectrum program has been developed. The program produces contour maps in the frequency time plane of the cross correlation coefficient between any particle flux time series and the magnetic field vector. This program has been utilized in several studies of ULF wave-particle interactions at synchronous orbit.

Mcpherron, R. L.↗

NASA NTRS1979

Status of cavity noise phenomena measurement and suppression on the B-1 aircraft

During the B-1 aircraft development, an extensive program of weapons bay cavity noise measurement and suppression studies was performed using wind tunnel models, flight test measurements, and aircraft design modifications. Substantial cavity noise reduction was demonstrated during flight test operations. The unsuppressed cavity noise level of 170 dB was reduced to values less than 150 dB with external retractable spoilers upstream of the cavity opening.

Tipton, A. G.↗

NASA NTRS1979

Physical phenomena in mercury ion thrusters

Experimental tests results demonstrating that reductions in screen grid thickness enhance the performance of ion thruster grids are presented. Shaping of the screen hole cross section is shown on the other hand not to affect performance substantially. The effect of the magnetic field in the vicinity of the hollow cathode on cathode performance is studied and test results are presented that show reductions in keeper voltages of a few volts can be realized by judicious applications of fields on the order of 100 gauss. The plasma downstream of a SERT 2 thruster operating without high voltage is studied. A model describing electron escape from the thruster under these conditions is discussed. A model defining the performance of the baffle aperture of an ion thruster is refined and experimental verification of the model is undertaken.

Wilbur, P. J.↗

NASA NTRS1980

The opportunities offered by SCADM for the study of surface phenomena related to interior structure and dynamics

The physical processes to be probed by experiments may be grouped as large scale flows, oscillations, and chromospheric/coronal diagnostics. While the fundamental concerns and observational equipments are similar within each class, different investigations may tell different things about the Sun. Observational requirements are listed for experiments to study (1) plasma-magnetic field interactions; (2) interior structure via oscillations; (3) chromospheric and coronal tracers; (4) rotation, meridional flows, and giant cells; (5) the depth dependence of rotation; (6) EUV luminosity; (7) intensity fluctuations and tracers; and (8) diameter oscillations, the effects of noise and timestring on experiment results are assessed.

Brown, T. M.↗

NASA NTRS1980

Radar target for remotely sensing hydrological phenomena

An apparatus for remotely measuring and accessing water status relative to snow and glacial melt, surface runoff, rainfall, evaporation, flow rate, and soil moisture is described. A radar target located at a selected location on the surface of the Earth is designed to collect water and render its cross sectional area variable as a function of the height of the water level within the target. The target is remotely monitored by an orbiting or airborne synthetic aperature radar. The target appears as a bright spot embedded within the radar image. The target brightness is indicative of the height of the water level within the ground located target.

Sivertson, W. E., Jr.↗

NASA NTRS1980

Nighttime/daytime optical survey of lightning and convective phenomena experiment (NOSL)

The Nighttime/Daytime Optical Survey of Lightning experiment was selected for flight on Orbital Flight Test 2 of the Space Shuttle. The background and rationale for the experiment are presented and the experiment hardware, the operating procedures, and the data analysis technique are discussed. Some preliminary results from the prototype hardware development are also presented.

Vonnegut, B.↗

NASA NTRS1979

Analysis of time dependent phenomena observed with the LPSP OSO-8 instrument

Data obtained by the Laboratoire de Physique Stellaire et Planetaire's ultraviolet spectrometer onboard the OSO-8 spacecraft were analyzed in an effort to dynamically model the solar chromosphere as an aid in enhancing knowledge of the dynamical processes themselves and of spectral line formation in the dynamic chromosphere. Repeated spectral scans of strong, optically thick resonance lines formed in the solar chromosphere were examined for indications of oscillatory velocities and intensities among other indications of velocity which were studied, the blue peak is reasonably well defined, and the position of a parabolic filter fitted by the least squares method was used to define it. Observed chromospheric oscillation periods are discussed as well as the variations in altitude of the emitting region which result primarily from the motion up and down during the oscillation.

Leibacher, J. W.↗

NASA NTRS1980

Analysis of differential and active charging phenomena on ATS-5 and ATS-6

Spacecraft charging on the differential charging and artificial particle emission experiments on ATS 5 and ATS 6 were studied. Differential charging of spacecraft surfaces generated large electrostatic barriers to spacecraft generated electrons, from photoemission, secondary emission, and thermal emitters. The electron emitter could partially or totally discharge the satellite, but the mainframe recharged negatively in a few 10's of seconds. The time dependence of the charging behavior was explained by the relatively large capacitance for differential charging in comparison to the small spacecraft to space capacitance. A daylight charging event on ATS 6 was shown to have a charging behavior suggesting the dominance of differential charging on the absolute potential of the mainframe. Ion engine operations and plasma emission experiments on ATS 6 were shown to be an effective means of controlling the spacecraft potential in eclipse and sunlight. Elimination of barrier effects around the detectors and improving the quality of the particle data are discussed.

Olsen, R. C.↗

NASA NTRS1979

A reconsideration of nucleation phenomena in light of recent findings concerning the properties of small clusters, and a brief review of some other particle growth processes

The paper examines mechanisms of nucleation and growth by condensation and coagulation in the light of recent research on properties of small clusters. Homogeneous, hetero-molecular, and heterogeneous nucleation is analyzed, and expressions for the rate of formation of a stable condensed phase and evaluation of the free energy of formation of charged droplets are given. Application of high-pressure mass spectrometry which makes possible a direct determination of intensity spectra for cluster distributions, measurement of the thermodynamic properties of individual ion clusters and determination of cluster entropy and bond energy is discussed. Finally, coagulation of the condensed phase is considered, noting that concentration and mean particle size vary during coagulation, but the shape distribution is time independent, leading to the concept of a self-preserving aerosol size distribution.

Castleman, A. W., Jr.↗

NASA NTRS1979

On seasonal phenomena in Saturn's atmosphere - New observations of Saturn's 3nu3 methane band

Saturn's 3nu3 CH4 band has been reobserved during April-June 1976, using the same telescope and spectrographic set-up used to study this band during the 1970 apparition. Direct comparison of the ratio spectra Saturn/moon reveals greater absorption in the manifold cores during 1976, but the increase is not as pronounced as that for the stronger CH4 bands. The rotational temperature and effective pressure of this band did not change significantly. These results are interpreted in terms of a possibly seasonal settling of the upper boundary of Saturn's atmospheric haze layer.

Trafton, L.↗