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At least 325 records · Page 18

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NASA NTRS2013

Microwave Kinetic Inductance Detector with Selective Polarization Coupling

A conventional low-noise detector requires a technique to both absorb incident power and convert it to an electrical signal at cryogenic temperatures. This innovation combines low-noise detector and readout functionality into one device while maintaining high absorption, controlled polarization sensitivity, and broadband detection capability. The resulting far-infrared detectors can be read out with a simple approach, which is compact and minimizes thermal loading. The proposed microwave kinetic inductance detector (MKID) consists of three basic elements. The first is the absorptive section in which the incident power is coupled to a superconducting resonator at far-infrared frequency above its superconducting critical frequency (where superconductor becomes normal conductor). This absorber's shape effectively absorbs signals in the desired polarization state and is resonant at the radio frequency (RF) used for readout of the device. Control over the metal film used in the absorber allows realization of structures with either a 50% broadband or 100% resonance absorptance over a 30% fractional bandwidth. The second element is a microwave resonator - which is realized from the thin metal films used to make the absorber as transmission lines - whose resonance frequency changes due to a variation in its kinetic inductance. The resonator's kinetic inductance is a function of the power absorbed by the device. A low-loss dielectric (mono-crystalline silicon) is used in a parallel-plate transmission line structure to realize the desired superconducting resonators. There is negligible coupling among the adjacent elements used to define the polarization sensitivity of each detector. The final component of the device is a microwave transmission line, which is coupled to the resonator, and allows detection of changes in resonance frequency for each detector in the focal plane array. The spiral shape of the detector's absorber allows incident power with two polarizations to couple to the detector equally. A stepped impedance resonator was used that allows the incident power absorbed in the detecting membrane area to be uniformly distributed in the detector's transmission line at the RF readout frequency. This maximizes the sensitivity of the detector. The signal is read out via a frequency multiplexing technique that requires a minimum number of interface transmission lines for readout. This reduces the packaging complexity and coupling to the device's thermal environment.

Wollack, Edward↗

NASA NTRS1985

Iron impurities in Si3N4 processing

The atomic environment of iron impurities is investigated during the processing cycle of reaction-bonding silicon nitride (RBSN). Several analysis techniques are utilized, including X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and electron spin resonance (ESR), to examine iron impurities in the starting silicon powder, in sintered silicon compacts, and in RBSN materials. Results indicate that iron impurities in as-received metallurgical grade silicon powder are incorporated in the silicon bulk as a highly distorted FeSi2 compound. No surface iron or iron-based particulate is observed in the starting material. Upon sintering, the iron environment becomes an ordered FeSi2 structure. In the RBNS material, the FeSi2 structure is again distorted, as observed by both EXAFS and ESR.

Bouldin, C. E.↗

NASA NTRS2005

Resonant Wingbeat Tuning Circuit using Strain-Rate Feedback for Ornithoptic Micro Aerial Vehicles

A resonant wingbeat tuning circuit automatically tunes the frequency of an actuating input to the resonant frequency of a flexible wing structure. Through the use of feedback control, the circuit produces the maximum flapping amplitude of a mechanical ornithoptic system, tracking the resonant frequency of the vibratory flapping apparatus as it vanes in response to change in flight condition, ambient pressure, or incurred wing damage.

David L Raney↗

NASA NTRS1999

Deblurring

In most instances, traditional EEG methodology provides insufficient spatial detail to identify relationships between brain electrical events and structures and functions visualized by magnetic resonance imaging or positron emission tomography. This article describes a method called Deblurring for increasing the spatial detail of the EEG and for fusing neurophysiologic and neuroanatomic data. Deblurring estimates potentials near the outer convexity of the cortex using a realistic finite element model of the structure of a subject's head determined from their magnetic resonance images. Deblurring is not a source localization technique and thus makes no assumptions about the number or type of generator sources. The validity of Deblurring has been initially tested by comparing deblurred data with potentials measured with subdural grid recordings. Results suggest that deblurred topographic maps, registered with a subject's magnetic resonance imaging and rendered in three dimensions, provide better spatial detail than has heretofore been obtained with scalp EEG recordings. Example results are presented from research studies of somatosensory stimulation, movement, language, attention and working memory. Deblurred ictal EEG data are also presented, indicating that this technique may have future clinical application as an aid to seizure localization and surgical planning.

Non-NASA Center↗

NASA NTRS1993

Self-organization in neural networks - Applications in structural optimization

The present paper discusses the applicability of ART (Adaptive Resonance Theory) networks, and the Hopfield and Elastic networks, in problems of structural analysis and design. A characteristic of these network architectures is the ability to classify patterns presented as inputs into specific categories. The categories may themselves represent distinct procedural solution strategies. The paper shows how this property can be adapted in the structural analysis and design problem. A second application is the use of Hopfield and Elastic networks in optimization problems. Of particular interest are problems characterized by the presence of discrete and integer design variables. The parallel computing architecture that is typical of neural networks is shown to be effective in such problems. Results of preliminary implementations in structural design problems are also included in the paper.

Hajela, Prabhat↗

NASA NTRS1987

Nonlinear absorption in AlGaAs/GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition

This paper reports the study of growth conditions for achieving the sharp exciton resonances and low-intensity saturation of these resonances in AlGaAs-GaAs multiple quantum well structures grown by metalorganic chemical vapor deposition. Low growth temperature is necessary to observe this sharp resonance feature at room temperature. The optimal growth conditions are a tradeoff between the high temperatures required for high quality AlGaAs and low temperatures required for high-purity GaAs. A strong optical saturation of the excitonic absorption has been observed. A saturation density as low as 250 W/sq cm is reported.

Lee, H. C.↗

NASA NTRS2011

Conformational Structure of Tyrosine, Tyrosyl-Glycine, and Tyrosyl-Glycyl-Glycine by Double Resonance Spectroscopy

We investigated the variation in conformation for the amino acid tyrosine (Y), alone and in the small peptides tyrosine-glycine (YC) and tyrosine-glycine-glycine (YGG), in the gas phase by using UV-UV and IR-UV double resonance spectroscopy and density functional theory calculations. For tyrosine we found seven different conformations, for YG we found four different conformations, and for YGG we found three different conformations. As the peptides get larger, we observe fewer stable conformers, despite the increasing complexity and number of degrees of freedom. We find structural trends similar to those in phenylalanine-glycine glycine (FGG) and tryptophan-glycine-glycine (WGG)j however) the effect of dispersive forces in FGG for stabilizing a folded structure is replaced by that of hydrogen bonding in YGG.

Abo-Riziq, Ali↗

NASA NTRS1986

Dynamic loads on twin jet exhaust nozzles due to shock noise

Acoustic near field data were collected with model single and twin jet nozzles to determine if closely spaced nozzles produce higher acoustic loading than do single nozzles. The tests were spurred by structural failure of the B-1 exhaust nozzle external flaps and similar damage on the F-15. The test was performed using two 5/8 in. ID pipes machined and placed side-by-side to mimic B-1 nozzles. A microphone mounted on the internozzle fairing measured acoustic levels near the nozzle exit plane. The nozzles oscillated significantly more than did a single nozzle over a wide range of nozzle pressure ratios. Acoustic levels in the dual jets exceeded single jet noise by as much as 20 dB, making acoustic resonance a definite candidate for structural damage in the twin jet configuration.

Norum, T. D.↗

NASA NTRS2002

Slow Light in Coupled Resonator Optical Waveguides

Recently, we discovered that a splitting of the whispering gallery modes (WGMs) occurs in coupled resonator optical waveguides (CROWs), and that these split modes are of a higher Q than the single-resonator modes, leading to enormous circulating intensity magnification factors that dramatically reduce thresholds for nonlinear optical (NLO) processes. As a result of the enhancements in Q, pulses propagating at a split resonance can propagate much slower (faster) for over (under)-coupled structures, due to the modified dispersion near the split resonance. Moreover, when loss is considered, the mode-splitting may be thought of as analogous to the Autler-Townes splitting that occurs in atomic three-level lambda systems, i.e., it gives rise to induced transparency as a result of destructive interference. In under- or over-coupled CROWs, this coupled resonator induced transparency (CRIT) allows slow light to be achieved at the single-ring resonance with no absorption, while maintaining intensities such that NLO effects are maximized. The intensity magnification of the circulating fields and phase transfer characteristics are examined in detail.

Chang, Hongrok↗

NASA NTRS1993

Mode Profiles in Waveguide-Coupled Resonators

Surface acoustic wave (SAW) waveguide-coupled resonators are of considerable interest for narrow-band filter applications, though to date there has been very little published on the acoustic details of their operation. As in any resonator, one must fully understand its mode structure and herein we study the SAW mode profiles in these devices. Transverse mode profiles in the resonant cavity of the device were measured at various frequencies of interest using a knife-edge laser probe. In addition we predict the mode profiles for the device structure by two independent methods. One is a stack-matrix approach adapted from integrated optics and the other is a conventional analytical eigenmode analysis of the Helmholtz equation. Both modeling techniques are in good agreement with the measured results.

Hunt, William D.↗

NASA NTRS1982

The dominant coherent structure of the circular jet organized by controlled perturbation

The dominant large-scale coherent structure of a circular jet has been organized through controlled perturbation at the jet Strouhal number of 0.3, induced acoustically in two air jets through cavity resonance. Selected phases of the coherent structure were studied for three Reynolds numbers (25,000; 55,000; and 110,000), and two limiting initial conditions; laminar and fully turbulent initial boundary layers. Distributions of phase-average structure properties, i.e., longitudinal and lateral velocities, coherent vorticity and Reynolds stress, phase-random turbulence intensities and Reynolds stress, and streamline and pseudo-streamfunction patterns are obtained. The phase-random turbulence intensity is greatest at the structure center where the coherent vorticity is also the maximum. Some properties of the structure show mild but systematic dependence on Reynolds number.

Zaman, K. B. M. Q.↗

NASA NTRS1970

Properties of nonaqueous electrolytes

Physical property measurements and structural studies conducted in aprotic solvents using various solutes are applicable to the further development of lithum batteries. Structural studies utilize nuclear magnetic resonance and electron paramagnetic resonance techniques.

Foster, J. N.↗

NASA NTRS1974

Beam maser measurements of CH3OH rotational transitions

Precise measurements of rotational transitions in methanol are reported that were made by means of beam maser spectrometers. No hyperfine structure was resolved at a resonance line width of 8 kHz. Accurate center frequencies for the transitions measured are useful for determining Doppler shifts for observed interstellar lines.

Gaines, L.↗

NASA NTRS1976

Hybrid theory and calculation of e-N2 scattering

A theory of electron-molecule scattering is developed which is a synthesis of close-coupling and adiabatic-nuclei theories. Specifically, the theory is close-coupling with respect to vibrational degrees of freedom and adiabatic-nuclei with respect to rotation. It can be applied to any number of partial waves required; the remaining ones can be calculated purely in one or the other approximation. A theoretical criterion based on fixed-nuclei calculations is given which indicates those partial waves and energy domains requiring the various approximations. The theory allows all cross sections (pure rotational, vibrational, simultaneous vibration-rotation, differential, and total) to be calculated, and explicit formulas for all these cross sections are given. The theory is applied to low-energy e-N2 scattering. The fixed-nuclei results are such that the criterion shows clearly that vibrational close coupling is necessary, but only for the Pi sub g partial wave. It is found that the close-coupling calculation for this wave gives rise to the substructure as well as the gross structure of the 2.4-eV resonance and that vibrational excitation cross sections are about twice as large as previously inferred.

Chandra, N.↗

NASA NTRS1977

Load and stability measurements on a soft-inplane rotor system incorporating elastomeric lead-lag dampers

An experimental investigation was conducted of the dynamic response and inplane stability associated with a new soft-inplane helicopter rotor. The unique feature of this rotor was the use of an internal elastomeric damper to restrain the blade inplane motion about the lead-lag hinge. The properties of the elastomer were selected to provide both a nominal first inplane frequency ratio of 0.65 and sufficient damping to eliminate the need for additional external damping sources to prevent ground resonance on a typical fuselage structure. For this investigation a 1/5-scale aeroelastic model was used to represent the rotor. The four-blade model had a diameter of 3.05 m (10 ft) and a solidity of 0.103. The first out-of-plane frequency ratio was 1.06. The model was tested in hover and in forward flight up to an advance ratio of 0.45. At each forward speed the rotor lift was varied up to simulated maneuver conditions. The measured rotor loads and response were within acceptable limits, and no adverse response qualities were observed. Moderate out-of-plane hub moments were measured, even for zero lift, to indicate the beneficial control power available for this design. Blade inplane stability testing indicated that the rotor system damping remained at moderate levels throughout the operating envelope.

Weller, W. H.↗

NASA NTRS1977

Acoustoelasticity

Sound or pressure variations inside bounded enclosures are investigated. Mathematical models are given for determining: (1) the interaction between the sound pressure field and the flexible wall of a Helmholtz resonator; (2) coupled fluid-structural motion of an acoustic cavity with a flexible and/or absorbing wall; (3) acoustic natural modes in multiple connected cavities; and (4) the forced response of a cavity with a flexible and/or absorbing wall. Numerical results are discussed.

Dowell, E. H.↗

NASA NTRS1978

MOD-1 WTG dynamic analysis

An analysis of the MOD-1 2000 kW horizontal axis wind turbine was given. The MOD-1 design was briefly described, and the analysis used to evaluate the dynamic loads and structural interactions is discussed. The resonant frequency placement, the treatment of unsteady wind loading, and the dynamic load sensitivity to frequency shifts were reviewed for the design.

Stahle, C. V., Jr.↗

NASA NTRS1983

Solar system history as recorded in the Saturnian ring structure

Holberg's analysis of the Voyager Saturn photographs in reflected and transparent light, and occultation data of stars seen through the rings are discussed. A hyperfine structure, with 10,000 ringlets can be explained by the Baxter-Thompson negative diffusion. This gives the ringlets a stability which makes it possible to interpret them as fossils, which originated at cosmogonic times. It is shown that the bulk structure can be explained by the combined cosmogonic shadows of the satellites Mimas, Janus and the Shepherd satellites. This structure originated at the transition from the plasma phase to the planetesimal phase. The shadows are not simple void regions but exhibit a characteristic signature. Parts of the fine structure, explained by Holberg as resonances with satellites, are interpreted as cosmogonic shadow effects. However, there are a number of ringlets which can neither be explained by cosmogonic nor by resonance effects. Analysis of ring data can reconstruct the plasma-planetesimal transition with an accuracy of a few percent.

Alfven, H.↗