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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Advancing Metasurfaces Towards New Frontiers: Nonvolatile Reconfigurable Optics

The applications of adaptive optics extend across multiple sectors, encompassing areas such as LiDAR, biological and chemical sensing, and free-space communications. In this study, we report on the design, fabrication, testing, and modeling of electrically reconfigurable metasurfaces using a low-loss high contrast phase change material, Ge2Sb2Se4Te integrated with an IR-transparent silicon microheater. Through this work, we introduce a reliable architecture for switching PCM-based metasurfaces within an integrated circuit configuration which is compatible with standard foundry fabrication processes. We demonstrate the capability of controlling the transmission of electromagnetic waves through the precise stimulation of PCM-based pixels, each spanning a few hundred microns, over numerous cycles. By leveraging PCM-based pixels, we unlock the potential to create metasurfaces encompassing a diverse range of functionalities such as dielectric filters, metalens, or beam steering devices, which is governed by the design of the meta-atoms. Further, we perform an in-depth investigation into the failure mechanism utilizing techniques such as Fourier transform IR spectroscopy (FTIR), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and thermal modeling. According to our results, due to a sharp temperature rise at the PCM/heater interface, we observe severe delamination of the PCM from the heater. More uniform PCM deposition, better adhesion at the PCM/heater interface, and a more uniform temperature distribution in the device could potentially mitigate the failure and lead to a longer life-time. By addressing these challenges, we aim to unlock the full potential of PCM-based devices and advance the field of adaptive optics.

Phase change material↗
NASA NTRS1988

Growth of electron plasma waves above and below f(p) in the electron foreshock

This paper investigates the conditions required for electron beams to drive wave growth significantly above and below the electron plasma frequency, f(p), by numerically solving the linear dispersion equation. It is shown that kinetic growth well below f(p) may occur over a broad range of frequencies due to the beam instability, when the electron beam is slow, dilute, and relatively cold. Alternatively, a cold or sharp feature at low parallel velocities in the distribution function may drive kinetic growth significantly below f(p). Kinetic broadband growth significantly above f(p) is explained in terms of faster warmer beams. A unified qualitative theory for the narrow-band and broad-band waves is proposed.

Cairns, Iver H.↗
NASA NTRS1993

A new method for the identification of non-Gaussian line profiles in elliptical galaxies

A new parameterization for the line profiles of elliptical galaxies, the Gauss-Hermite series, is proposed. This approach expands the line profile as a sum of orthogonal functions which minimizes the correlations between the errors in the parameters of the fit. This method also make use of the fact that Gaussians provide good low-order fits to observed line profiles. The method yields measurements of the line strength, mean radial velocity, and the velocity dispersion as well as two extra parameters, h3 and h4, that measure asymmetric and symmetric deviations of the line profiles from a Gaussian, respectively. The new method was used to derive profiles for three elliptical galaxies which all have asymmetric line profiles on the major axis with symmetric deviations from a Gaussian. Results confirm that elliptical galaxies have complex structures due to their complex formation history.

Van Der Marel, Roeland P.↗
NASA NTRS1993

Angular momentum regulation in low-mass young stars surrounded by accretion disks

From study of a sample of 34 T Tauri stars with photometrically derived rotation periods and spectral types later than KS, we find that the observed periods appear to be related to the presence or absence of an accretion disk. Those stars which we infer to be surrounded by accretion disks have rotation periods P(rot) over 4 days with a most probable P(rot) of about 8.5 days, while those stars which lack accretion disk signatures cover a wide range of P(rot) from 1.5 to 16 days, including a significant number of objects with P(rot) less than 4 days. This suggests the possibility that the 'initial' angular momentum of a star is not established until it dissipates its circumstellar accretion disk. During the disk accretion phase, the stellar angular velocity appears to be regulated at a low value, countering the tendency of the star to spin up both from contraction toward the main sequence and from the accretion of inner disk material of high specific angular momentum. When the accretion disk is dissipated, this regulation mechanism will cease to function. At this point, the star is no longer maintained at a low angular velocity, but is 'free' to conserve its angular momentum, and thus to increase its angular velocity in response to contraction and changes in moment of inertia. This hypothesis, combined with a spread in disk dispersal time scales, provides a context for explaining the observed distribution of stellar rotational velocities for stars on the ZAMS in young clusters.

Edwards, Suzan↗
NASA NTRS2000

Float-Zone Growth of Gel-xSix (x ? 10 at .%) Single Crystals: Influence of Thermocapillary and Solutocapillary Convection

Gel-xSix (x ? 10 at.%) single crystals have been grown with the float-zone technique applied within a monoellipsoid mirror furnace. The feed crystal consisted of synthesized GeO.95siO.05 polycrystalline material. Two Boron-doped (much much more than 1-2 (raised dot) 1017 at/cm3) crystals were grown under identical conditions using <100> Ge seeds. One Ge:Ga-doped (much much more than 1018 at/cm3) reference crystal was also grown in the same furnace. Compositional profiles were measured with an energy dispersive analysis by X-ray (EDAX). The maximum silicon concentration attained was 10 at.%. Micrographs of the etched crystals show sharp changes in interface curvature at the crystal edges. These are considered to be caused by solutal Marangoni convection, whose strength is assumed to be a direct function of silicon concentration, Theoretical considerations show that the flow direction, velocity, and strength varies significantly from a solutal Marangoni convection regime directly in front of the solid-liquid interface to a thermal Marangoni convection regime within the bulk melt.

Campbell, Tom A.↗
NASA NTRSDate not supplied

Particle Loading Tests on HEPA Flat Sheet Media at Sub-Ambient Pressures Using a Lunar Dust Simulant

When humans return to the moon under the NASA Artemis program, their activities on the lunar surface will inevitably lead to the intrusion of some level of lunar dust into the lander cabin. Therein the crew would be exposed to the potential hazards of lunar dust and the possibility of subsequent transfer into orbiting segments after docking. The spacecraft’s cabin filtration system will need to be effective at removing the airborne lunar dust to properly purify the breathable cabin air and minimize the spreading of the dust throughout the vehicle and orbital segments through the mission duration. The fine nature of the lunar dust will require high efficiency filtration, such as HEPA. A series of tests were performed in a specially designed recirculating sealed flow loop, for testing filter media and filter elements at the NASA Glenn Research Center. The flow loop was used to assess the performance and capacity of flat sheet filter media at representative cabin pressures using JSC-1AF lunar dust simulant and at high rates of particle loading. The pressure drop across the filter media was measured as a function of accumulated particle mass load at ambient pressure and at two sub-ambient pressures, 0.0703MPa and 0.0565MPa, and at a media velocity that was scaled relative to its pleated configuration. The challenge particle flows were generated by a custom designed particle generator that introduces dispersed particles of the lunar simulant at high concentrations. An optical particle counting instrument provided filter efficiency measurements within the sealed environment. The pressure drop was found to increase linearly with the amount of dust load on the media, for all test conditions, while the starting pressure drop was found to be lower at the lowest sub-ambient pressure case. High filter efficiency was maintained after high particle loads on the media.

Sub-ambient↗
NASA NTRS1976

Measurements on wave propagation characteristics of spiraling electron beams

Dispersion characteristics of cyclotron-harmonic waves propagating on a neutralized spiraling electron beam immersed in a uniform axial magnetic field are studied experimentally. The experimental setup consisted of a vacuum system, an electron-gun corkscrew assembly which produces a 110-eV beam with the desired delta-function velocity distribution, a measurement region where a microwave signal is injected onto the beam to measure wavelengths, and a velocity analyzer for measuring the axial electron velocity. Results of wavelength measurements made at beam currents of 0.15, 1.0, and 2.0 mA are compared with calculated values, and undesirable effects produced by increasing the beam current are discussed. It is concluded that a suitable electron beam for studies of cyclotron-harmonic waves can be generated by the corkscrew device.

Singh, A.↗
NASA NTRS1973

Molecular attenuation and phase dispersion between 40 and 140-GHz for path models from different altitudes

Radio wave propagation in the 40 to 140 GHz band through the first hundred kilometers of the atmosphere is strongly influenced by the microwave spectrum of oxygen (O2-MS). A unified treatment of molecular attenuation and phase dispersion is formulated. Results of molecular physics are translated into frequency, temperature, pressure, and magnetic field dependencies of a complex refractive index. The intensity distribution of the O2-MS undergoes several changes with increasing altitude. The influence of water vapor is discussed. Examples of computer plots are given as a function of altitude for homogeneous, zenith, and tangential path geometries. Molecular resonances of minor atmospheric gases are discussed briefly.

Liebe, H. J.↗
NASA NTRS1981

Linear theory of tearing in a high-beta plasma

The linear dispersion relation for a collisionless plasma in a sheared one-dimensional current sheet is calculated with reference to conditions in the daytime magnetopause. Calculations are extended to include plasmas with beta approximately equal to 1. It is found that the tearing mode eigenstructure and temporal growth rate are a sensitive function of the ratios l sub s/l sub G, l sub s/l sub T, and l sub s/l sub n, where l sub s is the shearing length of the magnetic field, l sub G is the gradient scale length, and l sub T is the temperature gradient scale length. In particular, if beta is approximately equal to 1, and l sub s is less than l sub G, l sub n, and l sub T, then the thickness of the layer over which particles are resonantly accelerated by the induction magnetic field is approximately rho, a thermal gyroradius. If the above conditions are not satisfied, plasma gradients may electrostatically stabilize the mode.

Quest, K. B.↗
NASA NTRS1993

Skin friction measurement with partially exposed polymer dispersed liquid crystals

Partially exposed polymer dispersed liquid crystal thin film (10-25 microns) deposited on a flat glass substrate has been used for the first time to measure skin friction. Utilizing the shear-stress-induced director reorientation in the partially exposed liquid-crystal droplets, optical transmission under crossed polarization has been measured as a function of the air flow differential pressure. Direct measurement of the skin friction with a skin friction drag balance, under the same aerodynamic conditions, lets us correlate the skin friction with optical transmission. This provides a unique technique for the direct measurement of skin friction from the transmitted light intensity. The results are in excellent agreement with the model suggested in this paper.

Parmar, D. S.↗
NASA NTRS2000

Radar Observations of Asteroid 10115 (1992 SK)

The approach of 1992 SK to within 0.056 AU of Earth on 1999 March 26 provided an excellent opportunity for radar observations. We observed it daily between March 22-26 at Goldstone at a transmitter frequency of 8560 MHz (3.50 cm). Weighted, optimally smoothed sums of cw (Doppler-only) echoes achieve a signal-to-noise ratio of about 300. Our highest resolution cw setup places up to approx. 30 0.5-Hz Doppler cells on the target at echo powers exceeding two standard deviations of the noise. Our highest resolution imaging setup places about 20 0.25-us (37.5 m) x 2.0 Hz pixels on 1992 SK at echo powers above the same threshold. We completed nearly 300 runs with each setup and obtained thorough rotational phase coverage (period = 7.320 h, P. Pravec, pers. comm.). The dispersion of the echoes in time delay indicates a lower bound on 1992 SK's maximum pole-on breadth of approx. 0.4 km that is consistent with the echo bandwidth and rotation period. Variations in the cw spectral bandwidths and shapes and in the delay-Doppler images as a function of rotation phase are evident on each day, are consistent in bandwidth with each other and with the rotation period, and indicate that the asteroid is asymmetric and modesty elongated.

Benner, L. A. M.↗
NASA NTRS1997

GRB Diversity vs. Utility as Cosmological Probes

Recent detections of apparent gamma-ray burst (GRB) counterparts in optical and radio wavebands strongly favor the cosmological distance scale, at least for some GRBs, opening the possibility of GRBs serving as cosmological probes. But GRBs exhibit great diversity: in total duration; in number, width and pulse configuration; and in pulse and overall spectral evolution. However, it is possible that a portion of this behavior reflects a luminosity distribution, and possible that evolution of with cosmic time introduces dispersion into the average GRB characteristics -- issues analogous to those encountered with quasars. The temporal domain offers a rich avenue to investigate this problem. When corrected for assumed spectral redshift, time dilation of event durations, pulse widths, and intervals between pulses must yield the same time-dilation factor as a function of peak flux, or else a luminosity distribution may be the cause of observed time dilation effects. We describe results of burst analysis using an automated, Bayesian-based algorithm to determine burst temporal characteristics for different peak flux groups, and derived constraints on any physical process that would introduce a luminosity distribution.

Norris, J. P.↗
NASA NTRS2003

Bose Condensation at He-4 Interfaces

Path Integral Monte Carlo was used to calculate the Bose-Einstein condensate fraction at the surface of a helium film at T = 0:77 K, as a function of density. Moving from the center of the slab to the surface, the condensate fraction was found to initially increase with decreasing density to a maximum value of 0.9, before decreasing. Long wavelength density correlations were observed in the static structure factor at the surface of the slab. A surface dispersion relation was calculated from imaginary-time density-density correlations. Similar calculations of the superfluid density throughout He-4 droplets doped with linear impurities (HCN)(sub n) are presented. After deriving a local estimator for the superfluid density distribution, we find a decreased superfluid response in the first solvation layer. This effective normal fluid exhibits temperature dependence similar to that of a two-dimensional helium system.

Draeger, E. W.↗
NASA NTRS2011

A Multifunctional Coating for Autonomous Corrosion Control

Nearly all metals and their alloys are subject to corrosion that causes them to lose their structural integrity or other critical functionality. Protective coatings are the most commonly used method of corrosion control. However, progressively stricter environmental regulations have resulted in the ban of many commercially available corrosion protective coatings due to the harmful effects of their solvents or corrosion inhibitors. This work concerns the development of a multifunctional smart coating for the autonomous control of corrosion. This coating is being developed to have the inherent ability to detect the chemical changes associated with the onset of corrosion and respond autonomously to indicate it and control it. The multi-functionality of the coating is based on microencapsulation technology specifically designed for corrosion control applications. This design has, in addition to all the advantages of existing microcapsulation designs, the corrosion controlled release function that triggers the delivery of corrosion indicators and inhibitors on demand, only when and where needed. Microencapsulation of self-healing agents for autonomous repair of mechanical damage to the coating is also being pursued. Corrosion indicators, corrosion inhibitors, as well as self-healing agents, have been encapsulated and dispersed into several paint systems to test the corrosion detection, inhibition, and self-healing properties of the coating. Key words: Corrosion, coating, autonomous corrosion control, corrosion indication, corrosion inhibition, self-healing coating, smart coating, multifunctional coating, microencapsulation.

Calle, Luz M.↗
NASA NTRS1980

Cusp proton signatures and the interplanetary magnetic field

The variation of proton average energy with latitude in the cusp has been suggested as an indicator of the means of particle entry. If magnetic merging is the principal means of particle entry, the proton average energy should fall with increasing latitude; if diffusion is the principal means, the average energy should first fall and then briefly rise as a function of latitude, showing a 'V' signature. In the present work, 60 selected cusp passes of the AE-D satellite for which IMP-J interplanetary or magnetosheath magnetic-field data were available were examined. About one-third of these passes showed clear or likely merging-type energy dispersions; a third showed clear or likely V-type energy dispersions, and a third showed unclear or no energy dispersions. The results are strongly correlated with the IMF - the merging signatures are associated with southward IMF and the V signatures with northward IMF. Unclear cases are associated with unsteady or weakly northward IMF.

Reiff, P. H.↗
NASA NTRSDate not supplied

Hydrodynamics of Two-Phase Flows through Porous Media in Microgravity: Packed Bed Reactor Experiment onboard of the International Space Station

The objective of the Packed Bed Reactor Experiment was to develop a fundamental understanding of the hydrodynamics of two-phase flow with no phase transition through porous media in microgravity. This work describes two experimental campaigns flown on the International Space Station (ISS) that were designed to achieve the objective. This work presents the results on flow patterns observed, two-phase flow pressure drop in porous media, and the impact of capillary effects on pressure drop at various gas and liquid flow rates. This work is the first to present predictive correlations of the two-phase friction factor for the different regimes identified based on the superficial liquid and gas velocity in microgravity, the first to hypothesize the different regimes based on the change in slope of the pressure gradient versus flow rate plots, the first to address the effects of the capillarity on the pressure gradient and the first to measure column holdup and assess the effective porosity based on the packing and the trapped gas bubbles. Experimental data on pressure drop in gas (N2) – liquid (water) flow show the presence of four different flow regimes in microgravity. Dispersed bubble flow (DB) and pulse (P) flow regimes are detected at high liquid flow rates, whereas at low liquid flow rates “large bubble” or elongated bubble (LB) and “gas channeling” (GC) regimes are observed. For these different flow regimes, different two-phase flow friction factor and pressure gradient correlations are presented for the first time as a function of the gas and liquid modified Reynolds numbers and Suratman number. Within the viscous-capillary (V-C) regime, it is found that the capillary contribution is the dominant force that contributes to the pressure drop for the wetting case (glass). However, for the non-wetting packing (Teflon), the viscous contribution dominates. It was found that the gas hold-up and pressure drop are functions of bed history at low liquid and gas flow rates with the magnitude of the hysteresis decreasing with increasing flow rates. PBRE-2 results show that the capillary force is a strong function of the superficial liquid velocity but is a much weaker function of the superficial gas velocity and varies inversely with the particle diameter. Within the Viscous-Capillary (V-C) regime, over 90% of the pressure gradient is attributed to the capillary contribution in the gas continuous regime. However, in the large bubble regime, the viscous and capillary contributions were comparable. After the completion of PBRE-2 with glass spherical beads, another bed packed with alumina was installed. Pressure gradient data for the alumina packed bed (PBRE-Water Recovery) were transferred to the PI and were not analyzed by the authors of this work. The Packed Bed Reactor Experiment concluded its on-orbit operation after two successful campaigns of testing in 2017 and in 2021. The flight hardware was brought back from orbit and is being reconditioned for a series of future experiments referred to as PBRE-WRS in support of water recovery (WRS). The WRS series consists of testing different two-phase fluid system components and packed beds to assess their pressure gradient characteristics, which will be used for designing packed bed reactors for various applications relevant to life support. Although there is so much relevance of the experimental results obtained from the two PBRE campaign in microgravity to life support, these findings also apply to other applications that involve two phase flow in porous medium such as fuel cells, transport of nutrient to plants in space and other chemical and materials processing that involve two phase flows. These systems operate differently in microgravity because, due to the lack of buoyancy, the density difference between the phases becomes irrelevant and no longer leads to phase separation.

Packed Bed↗
NASA NTRS2007

Primary Objective Grating Astronomical Telescope

It has been 370 years since a seventeenth century French mathematician, Mersenne, presciently sketched out an astronomical telescope based on dual parabolic reflectors. Since that time the concept of the primary objective has been virtually unchanged. Now a new class of astronomical telescope with a primary objective grating (POG) has been studied as an alternative. The POG competes with mirrors, in part, because diffraction gratings provide the very chromatic dispersion that mirrors defeat. The resulting telescope deals effectively with long-standing restrictions on multiple object spectroscopy (MOS). Other potential benefits include unprecedented apertures and collection areas. The new design also favors space deployment as a gossamer membrane. The inventor, Tom Ditto, first discovered that higher-order diffraction images contain hidden depth cues, for which he was granted a seminal range finding patent in 1987. Subsequently, he invented and patented 3D localizers, profilometers and microscopes using POGs. The POG telescope was placed in the public domain to expedite research. The function of a telescopes primary objective is to collect flux and to deliver images. Both functions dictate that size matters, and bigger is better. For that reason, there has been a steady push over the past century to ramp up the size of the primary mirror. However, for every doubling of mirror diameter, the elapsed time between initial effort and first light has also doubled. Meanwhile, costs escalated beyond the mirror alone, because larger instruments required larger enclosures and better pointing mechanisms. One key catalog of observation, spectrographic data, is far more difficult to amass than two-dimensional imagery. While the number of observable objects has increased with mirror size, the capacity to take spectra has not increased proportionately. In the best of circumstances, spectrograms are available for one per cent of the all objects surveyed. Spectroscopy was a historical afterthought introduced in the nineteenth century shortly after the invention of the diffraction grating and over a century after Newtons 1670 telescope. Spectroscopy is generally accomplished using a diffraction grating as the disperser in the secondary. The light being delivered to the spectrograph is first captured by a primary mirror which provides no chromatic magnification by itself. Sizeable spectrographs could not be deployed while diffraction gratings were rare commodities scribed using mechanical ruling engines that produced one grating line at a time. Today diffraction gratings are commonplace. Their recent availability is a product of both the invention of holography and the mass replication of surface microstructures. Holography permits all lines in a grating to be made simultaneously in a single photographic exposure. Holograms can then be reproduced by embossing processes. The improvement in replication is analogous to how Gutenberg changed the availability of books. The masters may be expensive, but the copies are not. Computer science is another technology that emerged in the second half of the twentieth century without which our proposed spectrographic instrument could not function due to the complexity of image processing required in data reduction. The employment of very large diffraction gratings as primary objectives for astronomical telescopes requires a novel

spectroscopy↗
NASA NTRS2005

Optical and Thermo-optical Properties of Polyimide-Single-Walled Carbon Nanotube Films: Experimental Results and Empirical Equations

The incorporation of single-walled carbon nanotubes (SWNTs) into the bulk of space environmentally durable polymers at loading levels greater than or equal to 0.05 wt % has afforded thin films with surface and volume resistivities sufficient for electrostatic charge mitigation. However, the optical transparency at 500 nm decreased and the thermo-optical properties (solar absorptivity and thermal emissivity) increased with increaed SWNT loading. These properties were also dependent on film thickness. The absorbance characteristics of the films as a function of SWNT loading and film thickness were measured and determined to follow the classical Beer-Lambert law. Based on these results, an empirical relationship was derived and molar absorptivities determined for both the SWNTs and polymer matrix to provide a predictive approximation of these properties. The molar absorptivity determined for SWNTs dispersed in the polymer was comparable to reported solution determined values for HiPco SWNTs.

Smith, Joseph G., Jr.↗