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At least 217 records · Page 12

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NASA NTRS1985

Electromagnetic radiation and nonlinear energy flow in an electron beam-plasma system

It is shown that the unstable electron-plasma waves of a beam-plasma system can generate electromagnetic radiation in a uniform plasma. The generation mechanism is a scattering of the unstable electron plasma waves off ion-acoustic waves, producing electromagnetic waves whose frequency is near the local plasma frequency. The wave vector and frequency matching conditions of the three-wave mode coupling are experimentally verified. The electromagnetic radiation is observed to be polarized with the electric field parallel to the beam direction, and its source region is shown to be localized to the unstable plasma wave region. The frequency spectrum shows negligible intensity near the second harmonic of the plasma frequency. These results suggest that the observed electromagnetic radiation of type III solar bursts may be generated near the local plasma frequency and observed downstream where the wave frequency is near the harmonic of the plasma frequency.

Whelan, D. A.↗

NASA NTRS1991

System and component design and test of a 10 hp, 18,000 rpm AC dynamometer utilizing a high frequency AC voltage link, part 1

Hard and soft switching test results conducted with one of the samples of first generation MOS-controlled thyristor (MCTs) and similar test results with several different samples of second generation MCT's are reported. A simple chopper circuit is used to investigate the basic switching characteristics of MCT under hard switching and various types of resonant circuits are used to determine soft switching characteristics of MCT under both zero voltage and zero current switching. Next, operation principles of a pulse density modulated converter (PDMC) for three phase (3F) to 3F two-step power conversion via parallel resonant high frequency (HF) AC link are reviewed. The details for the selection of power switches and other power components required for the construction of the power circuit for the second generation 3F to 3F converter system are discussed. The problems encountered in the first generation system are considered. Design and performance of the first generation 3F to 3F power converter system and field oriented induction moter drive based upon a 3 kVA, 20 kHz parallel resonant HF AC link are described. Low harmonic current at the input and output, unity power factor operation of input, and bidirectional flow capability of the system are shown via both computer and experimental results. The work completed on the construction and testing of the second generation converter and field oriented induction motor drive based upon specifications for a 10 hp squirrel cage dynamometer and a 20 kHz parallel resonant HF AC link is discussed. The induction machine is designed to deliver 10 hp or 7.46 kW when operated as an AC-dynamo with power fed back to the source through the converter. Results presented reveal that the proposed power level requires additional energy storage elements to overcome difficulties with a peak link voltage variation problem that limits reaching to the desired power level. The power level test of the second generation converter after the addition of extra energy storage elements to the HF link are described. The importance of the source voltage level to achieve a better current regulation for the source side PDMC is also briefly discussed. The power levels achieved in the motoring mode of operation show that the proposed power levels achieved in the generating mode of operation can also be easily achieved provided that no mechanical speed limitation were present to drive the induction machine at the proposed power level.

Lipo, Thomas A.↗

NASA NTRS1987

A search for sinusoidal gravitational radiation in the period range 30-2000 seconds

Since condensed massive objects in binary systems should generate observable periodic gravitational radiation, a large fraction of which may appear in one harmonic of the orbital frequency, monochromatic Doppler frequency perturbations were searched for at about 9000 discrete frequencies in the 0.0005-0.0333-Hz band using predetection data taken with the Pioneer 11 spacecraft. The results of the search are presented, and formal sensitivities for the possible signals are discussed. The results are interpreted as observational thresholds for possible incident gravitational wave trains having polarization states and propagation directions that are a priori unknown.

Armstrong, J. W.↗

NASA NTRS1984

Effect of low-frequency ambient magnetic fields on the control unit and RF head of a commercial SQUID magnetometer

The control unit and RF head of the SHE model 330XRFSQUID system are shown to be sensitive to ambient ac magnetic fields below 1 HZ, which cause the appearance of false signals corresponding to a magnetometer signal of 0.000001 phi(0) per gauss of field applied. The control unit shows a sensitivity that is linear with frequency, suggesting that the signal is generated by Faraday induction. In contrast, the RF head response is independent of frequency and shows a strong second-harmonic coversion. This response may be due to the magnetic field sensitivity of the ferrite core inductor in the tuned amplifier of the RF head. These signals induced by ambient fields are a potential source of error in Stanford's Relativity Gyroscope experiment, which uses SQUID's on board a rolling satellite as part of the gyroscope readout system. The extent of the magnetic field sensitivity in these components necessitates the use of additional magnetic shielding aboard the satellite.

Marcus, C. M.↗

NASA NTRS1993

Five years of Project META - An all-sky narrow-band radio search for extraterrestrial signals

We have conducted a five-year search of the northern sky (delta between 30 and 60 deg) for narrow-band radio signals near the 1420 MHz line of neutral hydrogen, and its second harmonic, using an 8.4 x 10 exp 6 channel Fourier spectrometer of 0.05 Hz resolution and 400 kHz instantaneous bandwidth. The observing frequency was corrected both for motions with respect to three astronomical inertial frames, and for the effect of Earth's rotation, which provides a characteristic changing Doppler signature for narrow-band signals of extraterrestrial origin. Among the 6 x 10 exp 13 spectral channels searched, we have found 37 candidate events exceeding the average detection threshold of 1.7 x 10 exp -23 W/sq m, none of which was detected upon reobservation. The strongest of these appear to be dominated by rare processor errors. However, the strongest signals that survive culling for terrestrial interference lie in or near the Galactic plane. We describe the search and candidate events, and set limits on the prevalence of supercivilizations transmitting Doppler-precompensated beacons at H I or its second harmonic. We conclude with recommendations for future searches, based upon these findings, and a description of our next-generation search system.

Horowitz, Paul↗

NASA NTRS1999

A Mode Detection Method Using the Azimuthal Directivity of a Turbofan Model

The azimuthal, far field directivity of a scale fan model was measured in high resolution. The model is a 12 inch diameter rotor with 16 blades followed by 40 stator vanes. The tests were conducted at the nominal 100% speed corresponding to a tip speed of 905 ft/sec. Measurement of the radiated sound field, forward of the fan, was made in an anechoic chamber with an inflow control device and a baffle separating the aft and forward radiated interaction noise. The acoustic field was surveyed with a circular hoop array of 16 microphones which was moved to 14 axial stations. At each axial station the hoop was rotated in half-degree increments to take 736 points in the azimuthal angle. In addition to sound pressure level, the phase angle relative to a reference microphone was measured at each point. The sound pressure level is shown to vary in patterns by 10-15 dB especially for the fundamental tone but also for the first and second harmonic. A far field mode detection method has been developed and used with the data which determines the modes generated by the fan and which then interact to form the azimuthal directivity.

Thomas, R. H.↗

NASA NTRS2003

Temperature, stress, and corrosive sensing apparatus utilizing harmonic response of magnetically soft sensor element (s)

A temperature sensing apparatus including a sensor element made of a magnetically soft material operatively arranged within a first and second time-varying interrogation magnetic field, the first time-varying magnetic field being generated at a frequency higher than that for the second magnetic field. A receiver, remote from the sensor element, is engaged to measure intensity of electromagnetic emissions from the sensor element to identify a relative maximum amplitude value for each of a plurality of higher-order harmonic frequency amplitudes so measured. A unit then determines a value for temperature (or other parameter of interst) using the relative maximum harmonic amplitude values identified. In other aspects of the invention, the focus is on an apparatus and technique for determining a value for of stress condition of a solid analyte and for determining a value for corrosion, using the relative maximum harmonic amplitude values identified. A magnetically hard element supporting a biasing field adjacent the magnetically soft sensor element can be included.

Grimes, Craig A.↗

DOE OSTI2024

Unconventional spin-orbit torques from sputtered MoTe 2 films

Materials with strong spin-orbit coupling and low crystalline symmetry are promising for generating large unconventional spin-orbit torques (SOTs), such as in-plane fieldlike (FL) torques and out-of-plane dampinglike (DL) torques, which can effectively manipulate and deterministically switch an out-of-plane magnetization without the need for additional external in-plane magnetic fields. Here, we report SOTs generated by magnetron-sputtered 1⁢T' MoTe 2 /Permalloy (Py; Ni 80 ⁢Fe 20 )/MgO heterostructures using both spin-torque ferromagnetic resonance (ST-FMR) and second harmonic Hall measurements. We observed unconventional FL and DL torques in our samples due to spins polarized normal to the interface of MoTe 2 and Py layers, and studied the influence of crystallographic order and MoTe 2 layer thickness on the SOTs. By comparing the Raman spectra of 1⁢T' MoTe 2 samples prepared in different ways, we found a tensile strain in sputtered MoTe 2 films, which might further enhance the generation of unconventional torques by reducing the symmetry of 1⁢T' MoTe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS2011

Comment on "An Analysis of VLF Electric Field Spectra Measured in Titan's Atmosphere by The Huygens Probe" By J. A. Morente et al.

Morente et al. have recently revisited the VLF electric field measurements made with the Permittivity, Wave and Altimetry (PWA) instrument during the descent of the Huygens Probe through the atmosphere of Titan. They assert that they have identified several harmonics of the transverse resonance mode of the surface?]ionosphere cavity, which would prove the existence of an electrical activity in the atmosphere of the largest satellite of Saturn. We refute this finding on the basis that it results from an artifact due to an improper analysis of the data set. [2] The investigators of the Permittivity, Wave and Altimetry (PWA) experiment on the Huygens Probe have reported the extremely low frequency (ELF) and very low frequency (VLF) electric signals recorded during the descent through the atmosphere of Titan. The PWA data are archived in the Planetary Science Archive (PSA) of ESA, and an extensive description of the instrument is at the disposal of the scientific community. Morente and his coworkers have revisited this data set and reported the results of their investigations in two papers. In a first paper, they claim that they have detected in the ELF range (0.100 Hz) several harmonics of a global resonance allegedly generated by lightning activity in the spherical cavity guide formed by the surface of Titan and the inner boundary of the ionosphere, a phenomenon similar to the Schumann resonance observed at EartH In the second paper dedicated to the VLF electric signal recorded by PWA, in the range 0.10 kHz, they argue that they can also bring out the transverse resonance and its harmonics, a more local phenomenon that develops around the excitation source and whose frequency is controlled by the separation between Titan?fs surface and the inner ionospheric boundary. [3] The PWA investigators have analyzed the narrowband ELF signal at about 36 Hz effectively observed during the entire descent. They have not endorsed, however, the alternative approach of Morente et al. that discloses additional ELF narrow-band signals and numerous related harmonics. Hamelin et al. and Beghin et al. have questioned the work of Morente and his coworkers and demonstrated unambiguously that their findings are mere artifacts entirely due to a faulty procedure. Morente et al. refuted this critical analysis of their ELF paper in a rebuttal. The present comment draws a parallelism between the ELF and VLF papers and demonstrates that the VLF signal carries no evidence whatever of any transverse resonance. The analyses of the ELF and VLF signals by Morente and his coworkers are flawed, and their conclusions are shown here to be invalid. A more comprehensive and thorough assessment of the numerical approaches proposed by Morente et al. is given by Berthelin et al..

Grard, Rejean↗

NASA NTRS2016

Light Diffraction by Large Amplitude Ultrasonic Waves in Liquids

Light diffraction from ultrasound, which can be used to investigate nonlinear acoustic phenomena in liquids, is reported for wave amplitudes larger than that typically reported in the literature. Large amplitude waves result in waveform distortion due to the nonlinearity of the medium that generates harmonics and produces asymmetries in the light diffraction pattern. For standing waves with amplitudes above a threshold value, subharmonics are generated in addition to the harmonics and produce additional diffraction orders of the incident light. With increasing drive amplitude above the threshold a cascade of period-doubling subharmonics are generated, terminating in a region characterized by a random, incoherent (chaotic) diffraction pattern. To explain the experimental results a toy model is introduced, which is derived from traveling wave solutions of the nonlinear wave equation corresponding to the fundamental and second harmonic standing waves. The toy model reduces the nonlinear partial differential equation to a mathematically more tractable nonlinear ordinary differential equation. The model predicts the experimentally observed cascade of period-doubling subharmonics terminating in chaos that occurs with increasing drive amplitudes above the threshold value. The calculated threshold amplitude is consistent with the value estimated from the experimental data.

Adler, Laszlo↗

NASA NTRS1978

Variation of fan tone steadiness for several inflow conditions

An amplitude probability density function analysis technique for quantifying the degree of fan noise tone steadiness has been applied to data from a fan tested under a variety of inflow conditions. The test conditions included typical static operation, inflow control by a honeycomb/screen device and forward velocity in a wind tunnel simulating flight. The ratio of mean square sinusoidal-to-random signal content in the fundamental and second harmonic tones was found to vary by more than an order-of-magnitude. Some implications of these results concerning the nature of fan noise generation mechanisms are discussed.

Balombin, J. R.↗

NASA NTRS1978

Variation of fan tone steadiness for several inflow conditions

Balombin, J. R.↗

NASA NTRS1993

Beamforming in an acoustic shadow

The sound field deep within an acoustic shadow region is less well understood than that outside the shadow region. Signal levels are substantially lower within the shadow, but beamforming difficulties arise for other reasons such as loss of spatial coherence. Based on analysis of JAPE-91 data, and other data, three types of characteristic signals within acoustic shadow regions are identified. These signal types may correspond to different, intermittent signal propagation conditions. Detection and classification algorithms might take advantage of the signal characteristics. Frequency coherence is also discussed. The extent of coherence across frequencies is shown to be limited, causing difficulties for source classification based on harmonic amplitude relationships. Discussions emphasize short-term characteristics on the order of one second. A video presentation on frequency coherence shows the similarity, in the presence of atmospheric turbulence, between the received signal from a stable set of harmonics generated by a loudspeaker and that received from a helicopter hovering behind a hill.

Havelock, David↗

NASA NTRS2019

Multi-Layer Wireless Sensor Construct for Use at Electrically-Conductive Material Surfaces

A multi-layer wireless sensor construct is provided. The construct includes a first dielectric layer adapted to be attached to a portion of a first surface of an electrically-conductive material. A layer of mu metal is provided on the first dielectric layer. A second dielectric layer is provided on the layer of mu metal. An electrical conductor is provided on the second dielectric layer wherein the second dielectric layer separates the electrical conductor from the layer of mu metal. The electrical conductor has first and second ends and is shaped to form an unconnected open-circuit that, in the presence of a time-varying magnetic field, resonates to generate a harmonic magnetic field response having a frequency, amplitude and bandwidth.

Dudley, Kenneth L.↗

NASA NTRS1980

Nonlinear interaction of two waves in boundary-layer flows

First-order nonlinear interactions of Tollmien-Schlichting waves of different frequencies and initial amplitudes in boundary-layer flows are analyzed using the method of multiple scales. Numerical results for flow past a flat plate show that the spatial detuning wipes out resonant interactions unless the initial amplitudes are very large. Thus, a wave having a moderate amplitude has little influence on its subharmonic although it has a strong influence on its second harmonic. Moreover, two waves having moderate amplitudes have a strong influence on their difference frequency. The results show that the difference frequency can be very unstable when generated by the nonlinear interaction, even though it may be stable when introduced by itself in the boundary layer.

Nayfeh, A. H.↗

NASA NTRS1990

Coherence properties of a doubly resonant monolithic optical parametric oscillator

A doubly resonant optical parametric oscillator (DRO) pumped with the second harmonic of a narrow-linewidth Nd:YAG laser is described. The linewidth of the DRO signal was less than 13 kHz, the DRO was shown to generate a phase-locked subharmonic of the pump at degeneracy, and the signal and the idler were shown to be mutually coherent with the pump and to be phase-anticorrelated with each other away from degeneracy. The signal-idler heterodyne linewidth was 500 Hz, and pump phase modulation was shown to transfer to the DRO phase at degeneracy.

Nabors, C. D.↗

NASA NTRS1975

Propagation of high amplitude higher order sounds in slightly soft rectangular ducts, carrying mean flow

The resonance expansion method, developed to study the propagation of sound in rigid rectangular ducts is applied to the case of slightly soft ducts. Expressions for the generation and decay of various harmonics are obtained. The effect of wall admittance is seen through a dissipation function in the system of nonlinear differential equations, governing the generation of harmonics. As the wall admittance increases, the resonance is reduced. For a given wall admittance this phenomenon is stronger at higher input intensities. Both the first and second order solutions are obtained and the results are extended to the case of ducts having mean flow.

Wang, K. S.↗

NASA NTRS2002

Further Examination of the Vibratory Loads Reduction Results from the NASA/ARMY/MIT Active Twist Rotor Test

The vibration reduction capabilities of a model rotor system utilizing controlled, strain-induced blade twisting are examined. The model rotor blades, which utilize piezoelectric active fiber composite actuators, were tested in the NASA Langley Transonic Dynamics Tunnel using open-loop control to determine the effect of active-twist on rotor vibratory loads. The results of this testing have been encouraging, and have demonstrated that active-twist rotor designs offer the potential for significant load reductions in future helicopter rotor systems. Active twist control was found to use less than 1% of the power necessary to operate the rotor system and had a pronounced effect on both rotating- and fixed-system loads, offering reductions in individual harmonic loads of up to 100%. A review of the vibration reduction results obtained is presented, which includes a limited set of comparisons with results generated using the second-generation version of the Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics (CAMRAD II) rotorcraft comprehensive analysis.

Wilbur, Matthew L.↗