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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

High Current Hollow Cathode Plasma Plume Measurements

Plasma plume measurements are reported for a hollow cathode assembly (HCA) oper-ated at discharge currents of 50, 70, and 100 A at xenon ow rates between 19 - 46 sccm.The HCA was centrally mounted in the annulus of the NASA-300MS Hall Thruster andwas operated in the spot and plume modes with additional data taken with an appliedmagnetic eld. Langmuir probes, retarding potential analyzers, and optical emission spec-troscopy were employed to measure plasma properties near the orice of the HCA and toassess the charge state of the near-eld plasma. Electron temperatures (2-6 eV) and plasmapotentials are consistent with probe-measured values in previous investigations. Operationwith an applied-eld yields higher discharge voltages, increased Xe III production, andincreased signals from the 833.5 nm C I line. While operating in plume mode and with anapplied eld, ion energy distribution measurements yield ions with energies signicantlyexceeding the applied discharge voltage. These ndings are correlated with high-frequencyoscillations associated with each mode.

Plasma diagnostics↗
NASA NTRS2014

Development Status of Adjustable X-Ray Optics with 0.5 Arcsecond Resolution

We report on the continuing development of adjustable, grazing incidence X-ray optics for 0.5 arcsec telescopes. Adjustable X-ray optics offer the potential for achieving sub-arcsecond imaging resolution while sufficiently thin and light-weight to constitute a mirror assembly with several square meters collecting area. The adjustable mirror concept employs a continuous thin film of piezoelectric material deposited on the back of the paraboloid and hyperboloid mirror segments. Individually addressable electrodes on the piezoelectric layer allow the introduction of deformations in localized "cells" which are used to correct mirror figure errors resulting from fabrication, mounting and aligning the thin mirrors, residual gravity release and temperature changes. We describe recent results of this development. These include improving cell yield to approx. 100 per cent, measurements of hysteresis and stability, comparisons of modeled and measured behavior, simulations of mirror performance, and the development and testing of conical Wolter- I mirror segments. We also present our plans going forward toward the eventual goal of achieving TRL 6 prior to the 2020 Decadal Review.

Reid, P. B.↗
NASA NTRS2014

A Framework for Modelling Connective Tissue Changes in VIIP Syndrome

Insertion of astronauts into microgravity induces a cascade of physiological adaptations, notably including a cephalad fluid shift. Longer-duration flights carry an increased risk of developing Visual Impairment and Intracranial Pressure (VIIP) syndrome, a spectrum of ophthalmic changes including posterior globe flattening, choroidal folds, distension of the optic nerve sheath, kinking of the optic nerve and potentially permanent degradation of visual function. The slow onset of changes in VIIP, their chronic nature, and the similarity of certain clinical features of VIIP to ophthalmic findings in patients with raised intracranial pressure strongly suggest that: (i) biomechanical factors play a role in VIIP, and (ii) connective tissue remodeling must be accounted for if we wish to understand the pathology of VIIP. Our goal is to elucidate the pathophysiology of VIIP and suggest countermeasures based on biomechanical modeling of ocular tissues, suitably informed by experimental data, and followed by validation and verification. We specifically seek to understand the quasi-homeostatic state that evolves over weeks to months in space, during which ocular tissue remodeling occurs. This effort is informed by three bodies of work: (i) modeling of cephalad fluid shifts; (ii) modeling of ophthalmic tissue biomechanics in glaucoma; and (iii) modeling of connective tissue changes in response to biomechanical loading.

microgravity↗
NASA NTRS2019

High Performance GaN/AlGaN Ultraviolet Avalanche Photodiode Detector Technologies

Detection of ultraviolet (UV) bands provides distinct advantages for NASA, defense, and commercial applications, including increased spatial resolution, small pixel sizes, and large format arrays. AlxGa1-xN semiconductor alloys have attracted great interest for detection in the UV spectral region because of their potential for high optical gain, high sensitivity, and low dark current performance in ultraviolet avalanche photodiodes (UV-APDs). We are developing GaN/AlGaN UV-APDs that demonstrate consistent and reliable UV-APD performance and operation. For these UV detectors we have measured gains of above 5×10(exp 6) and high quantum efficiencies at ~350 nm enabled by a strong avalanche multiplication process. These UV-APDs are fabricated through high quality metal organic chemical vapor deposition (MOCVD) growth on lattice-matched, low dislocation density GaN substrates with optimized GaN/AlGaN UV-APD material growth and doping parameters. The high performance, variable-area GaN/AlGaN UV-APD detectors and arrays can be customized to a wide variety of sizes including large-area formats to enable sensing and high-resolution detection over UV bands of interest.

Sood, Ashok K.↗
NASA NTRS1996

Multiphoton Process and Anomalous Potential of Cell Membrane by Laser Radiation

In this paper, by the use of quantum biology and quantum optics, the laser induced potential variation of cell membrane has been studied. Theoretically, we have found a method of calculating the monophoton and multiphoton processes in the formation of the anomalous potential of cell membrane. In contrast with the experimental results, our numerical result is in the same order. Therefore, we have found the possibility of cancer caused by the laser induced anomalous cell potential.

Zhang, Kaixi↗
NASA NTRS2013

Engineering Specification for Large-aperture UVO Space Telescopes Derived from Science Requirements

An advanced large aperture UV/optical UVO space telescope is required for the next generation of astrophysics and exoplanet science. The science requirements of proposed exoplanet and astrophysics missions were used to determine the encircled energy, point spread function stability and thermal environment requirements. These requirements then determine the optical wavefront specification for potential telescope assemblies which can fit inside current and planned launch vehicles. The optical wavefront specification becomes the top level of the error budget that is split into various sources that control the structural, thermal and optical design.

Stahl, H. Philip↗
NASA NTRS1985

Precision space structures

NASA large space structures efforts to date aimed towards large, flexible antenna-like structures (30-100M) with relatively long wavelengths (1-30cm) and moderate disturbances leading to some structure-control interaction. Potential missions in the OPTICS regime require small reflectors/mirrors, short wavelengths (visible to 100 microns), very tight tolerances in surface, alignment, pointing stability, as well as the potential of considerable on-board disturbances. For optics systems: figure/surface control can be quasi-static, active/passive isolation schemes are possible, vibration control is necessary, and the attitude control system can be low BW - Ground testing of reflectors and mirrors is more feasible than with antennas.

Soosaar, K.↗
NASA NTRS2010

The Invigoration of Deep Convective Clouds Over the Atlantic: Aerosol Effect, Meteorology or Retrieval Artifact?

Associations between cloud properties and aerosol loading are frequently observed in products derived from satellite measurements. These observed trends between clouds and aerosol optical depth suggest aerosol modification of cloud dynamics, yet there are uncertainties involved in satellite retrievals that have the potential to lead to incorrect conclusions. Two of the most challenging problems are addressed here: the potential for retrieved aerosol optical depth to be cloud-contaminated, and as a result, artificially correlated with cloud parameters; and the potential for correlations between aerosol and cloud parameters to be erroneously considered to be causal. Here these issues are tackled directly by studying the effects of the aerosol on convective clouds in the tropical Atlantic Ocean using satellite remote sensing, a chemical transport model, and a reanalysis of meteorological fields. Results show that there is a robust positive correlation between cloud fraction or cloud top height and the aerosol optical depth, regardless of whether a stringent filtering of aerosol measurements in the vicinity of clouds is applied, or not. These same positive correlations emerge when replacing the observed aerosol field with that derived from a chemical transport model. Model-reanalysis data is used to address the causality question by providing meteorological context for the satellite observations. A correlation exercise between the full suite of meteorological fields derived from model reanalysis and satellite-derived cloud fields shows that observed cloud top height and cloud fraction correlate best with model pressure updraft velocity and relative humidity. Observed aerosol optical depth does correlate with meteorological parameters but usually different parameters from those that correlate with observed cloud fields. The result is a near-orthogonal influence of aerosol and meteorological fields on cloud top height and cloud fraction. The results strengthen the case that the aerosol does play a role in invigorating convective clouds.

Koren, Ilan↗
NASA NTRS1998

Optical Experiments With Manganese Doped Yttrium Orthoaluminate, a Potential Material for Holographic Recording and Data Storage

The YAlO3 host crystal has a distorted perovskite structure that belongs to the orthorhombic centrosymmetric Pbnm space group. The cationic sites in the structure available for Mn substitution are the relatively large strongly distorted YO12 polyhedral (Y3+ ionic radius R(sub Y) = 1.02 A) and the smaller nearly ideal AlO6 octahedra R(sub Al) = 0.53 A). Manganese may enter YAlO3 in the form of Mn2+ ions (R(sub Mn)= 0.96 A), substituting most likely Y3+ ions, and Mn3+ ions (R(sub Mn) = 0.65 A) or Mn4+ ions (R(sub Mn) = 0.53 A) substituting Al3+ ions. The latter substitution is most probable because of dimensional parameters. Point defects, which are common in YAl03, may provide the charge compensation required for substitution.

Warren, Matthew E.↗
NASA NTRS2011

Initial Characterization of Optical Communications with Disruption-Tolerant Network Protocols

Disruption-tolerant networks (DTNs) are groups of network assets connected with a suite of communication protocol technologies designed to mitigate the effects of link delay and disruption. Application of DTN protocols to diverse groups of network resources in multiple sub-networks results in an overlay network-of-networks with autonomous data routing capability. In space environments where delay or disruption is expected, performance of this type of architecture (such as an interplanetary internet) can increase with the inclusion of new communications mediums and techniques. Space-based optical communication links are therefore an excellent building block of space DTN architectures. When compared to traditional radio frequency (RF) communications, optical systems can provide extremely power-efficient and high bandwidth links bridging sub-networks. Because optical links are more susceptible to link disruption and experience the same light-speed delays as RF, optical-enabled DTN architectures can lessen potential drawbacks and maintain the benefits of autonomous optical communications over deep space distances. These environment-driven expectations - link delay and interruption, along with asymmetric data rates - are the purpose of the proof-of-concept experiment outlined herein. In recognizing the potential of these two technologies, we report an initial experiment and characterization of the performance of a DTN-enabled space optical link. The experiment design employs a point-to-point free-space optical link configured to have asymmetric bandwidth. This link connects two networked systems running a DTN protocol implementation designed and written at JPL for use on spacecraft, and further configured for higher bandwidth performance. Comparing baseline data transmission metrics with and without periodic optical link interruptions, the experiment confirmed the DTN protocols' ability to handle real-world unexpected link outages while maintaining capability of reliably delivering data at relatively high rates. Finally, performance characterizations from this data suggest performance optimizations to configuration and protocols for future optical-specific DTN space link scenarios.

Space networking↗
NASA NTRS1984

Optical Information Processing for Aerospace Applications 2

Current research in optical processing, and determination of its role in future aerospace systems was reviewed. It is shown that optical processing offers significant potential for aircraft and spacecraft control, pattern recognition, and robotics. It is demonstrated that the development of optical devices and components can be implemented in practical aerospace configurations.

Stermer, R. L.↗
NASA NTRS1999

Aerogel Composites: Strong and Waterproof

Aerogels are exotic materials having superior thermal and physical properties with great potential for both space and industrial uses. Although aerogels are excellent low-density insulators with unique acoustic and optical properties, their commercialization potential is currently limited by moisture absorption, fragility, and cost. This paper describes useful, easily scaled-up solutions to the first two of these three problems. The waterproofing and water-repellent method described here is a cheaper and simpler improvement over previous permanent methods.

White, Susan↗
NASA NTRS1989

The divergence characteristics of constrained-sheath optics systems for use with 5-eV atomic oxygen sources

The potential usefulness of the constrained sheath optics concept as a means of controlling the divergence of low energy, high current density ion beams is examined numerically and experimentally. Numerical results demonstrate that some control of the divergence of typical ion beamlets can be achieved at perveance levels of interest by contouring the surface of the constrained sheath properly. Experimental results demonstrate that a sheath can be constrained by a wire mesh attached to the screen plate of the ion optics system. The numerically predicted beamlet divergence characteristics are shown to depart from those measured experimentally, and additional numerical analysis is used to demonstrate that this departure is probably due to distortions of the sheath caused by the fact that it attempts to conform to the individual wires that make up the sheath constraining mesh. The concept is considered potentially useful in controlling the divergence of ion beamlets in applications where low divergence, low energy, high current density beamlets are being sought, but more work is required to demonstrate this for net beam ion energies as low as 5 eV.

Anderson, John R.↗
NASA NTRS2000

High-Q Microsphere Cavity for Laser Stabilization and Optoelectronic Microwave Oscillator

With submillimeter size and optical Q up to approximately 10 (exp 10), microspheres with whispering-gallery (WG) modes are attractive new component for fiber-optics/photonics applications and a potential core in ultra-compact high-spectral-purity optical and microwave oscillators. In addition to earlier demonstrated optical locking of diode laser to WG mode in a microsphere, we report on microsphere application in the microwave optoelectronic oscillator, OEO. In OEO, a steady-state microwave modulation of optical carrier is obtained in a closed loop including electro-optical modulator, fiber-optic delay, detector and microwave amplifier. OEO demonstrates exceptionally low phase noise (-140 dBc/Hz at l0kHz from approximately 10GHz carrier) with a fiber length approximately 2km. Current technology allows to put all parts of the OEO, except the fiber, on the same chip. Microspheres, with their demonstrated Q equivalent to a kilometer fiber storage, can replace fiber delays in a truly integrated device. We have obtained microwave oscillation in microsphere-based OEO at 5 to 18 GHz, with 1310nm and 1550nm optical carrier, in two configurations: 1) with external DFB pump laser, and 2) with a ring laser including microsphere and a fiber optic amplifier. Also reported is a simple and efficient fiber coupler for microspheres facilitating their integration with existing fiber optics devices.

Ilchenko, Vladimir S.↗
NASA NTRS1999

High-Q Microsphere Cavity for Laser Stabilization and Optoelectronic Microwave

With submillimeter size and optical Q up to ~ 10(sup 10), microspheres with whispering-gallery (WG) modes are attractive new component for fiber-optics/photonics applications and a potential core in ultra-compact high-spectral-purity optical and microwave oscillators.

microspheres fiber optics microwave photonics↗
NASA NTRS1986

Aerodynamics of seeing on large transport aircraft

Data were obtained in the full scale flight environment of the Kuiper Airborne Observatory (KAO) on the nature of turbulent shear layer over the open cavity. These data were used to verify proposed aerodynamic scaling relationships to describe the behavior of the turbulent layers and to estimate the optical performance of systems of various wavelengths operating within the KAO environment. These data and wind tunnel data are used to scale the expected optical effects for a potential stratospheric observatory for infrared astronomy (SOFIA) in which a telescope approximately 3.5 times larger than that on the KAO is envisioned. It appears that the use of combinations of active and passive aeromechanical flow control techniques can improve the optical behavior of systems in the SOFIA environment. Experiments to verify these potential improvements can be performed on the KAO with sufficient modifications to the cavity and aero-mechanical technique installations.

Rose, W. C.↗
NASA NTRS1985

Measuring plant canopy structure

The potential of three systems to quantify plant canopy structure, key input data to canopy reflectance models is examined. The results show an optical radar system is potentially capable of providing the most fundamental type of structural data, the area, and the direction of the normal of each small piece of foliage in each cube of space in the canopy. To decrease the size of the data set of canopy measurements needed to statistically characterize the structure of a plant canopy, it is proposed that the statistical methods of bootstrapping be employed and that plans of large caopies be measured one at a time.

Vanderbilt, V. C.↗
NASA NTRS1986

Overview of advanced components for fiber optic systems

The basic operating principles and potential performance of several state-of-the-art fiber-optic devices are illustrated with diagrams and briefly characterized. Technologies examined include high-birefringence polarization-maintaining fibers and directional couplers, single-mode fiber polarizers and cut-off polarizers, optical-fiber modulators with radially poled piezoactive polymer (PVF2) jackets, and piezoelectric-squeezer polarization modulators. The need for improved manufacturing techniques to make such fiber-optic devices cost-competitive with their thin-film integrated-optics analogs is indicated.

Depaula, Ramon P.↗