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At least 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

Excitation of Continuous and Discrete Modes in Incompressible Boundary Layers

This report documents the full details of the condensed journal article by Ashpis & Reshotko (JFM, 1990) entitled "The Vibrating Ribbon Problem Revisited." A revised formal solution of the vibrating ribbon problem of hydrodynamic stability is presented. The initial formulation of Gaster (JFM, 1965) is modified by application of the Briggs method and a careful treatment of the complex double Fourier transform inversions. Expressions are obtained in a natural way for the discrete spectrum as well as for the four branches of the continuous spectra. These correspond to discrete and branch-cut singularities in the complex wave-number plane. The solutions from the continuous spectra decay both upstream and downstream of the ribbon, with the decay in the upstream direction being much more rapid than that in the downstream direction. Comments and clarification of related prior work are made.

Ashpis, David E.↗
NASA NTRS2001

Micromachined TWTs for THz Radiation Sources

The Terahertz (THz) region of the electromagnetic spectrum (about 300 - 3000 GHz in frequency or about 0.1 - 1 mm free space wavelength) has enormous potential for high-data-rate communications, spectroscopy, astronomy, space research, medicine, biology, surveillance, remote sensing, industrial process control, etc. It has been characterized as the most scientifically rich, yet under-utilized, region of the electromagnetic spectrum. The most critical roadblock to full exploitation of the THz band is lack of coherent radiation sources that are powerful (0.001 - 1.0 W continuous wave), efficient (> 1%), frequency agile (instantaneously tunable over 1% bandwidths or more), reliable, and comparatively inexpensive. To develop vacuum electron device (VED) radiation sources satisfying these requirements, fabrication and packaging approaches must be heavily considered to minimize costs, in addition to the basic interaction physics and circuit design. To minimize size of the prime power supply, beam voltage must be minimized, preferably 10 kV. Solid state sources satisfy the low voltage requirement, but are many orders of magnitude below power, efficiency, and bandwidth requirements. On the other hand, typical fast-wave VED sources in this regime (e.g., gyrotrons, FELs) tend to be large, expensive, high voltage and very high power devices unsuitable for most of the applications cited above. VEDs based on grating or inter-digital (ID) circuits have been researched and developed. However, achieving forward-wave amplifier operation with instantaneous fractional bandwidths > 1% is problematic for these devices with low-energy (< 15 kV) electron beams. Moreover, the interaction impedance is quite low unless the beam-circuit spacing is kept particularly narrow, often leading to significant beam interception. One solution to satisfy the THz source requirements mentioned above is to develop micromachined VEDs, or "micro-VEDs". Among other benefits, micro-machining technologies provide superior high frequency wall conductivity as a result of superior surface smoothness compared with conventional mechanical or electric discharge machining approaches. Micro-VED technologies are already being applied to the development of millimeter-wave klystrons at Stanford Linear Accelerator Center and submillimeter-wave klystrons at the University of Leeds. We are investigating the use of micro-machining technologies to develop THz regime TWTs, with emphasis on folded-waveguide TWTs. The folded-waveguide TWT (FW-TWT) has several features that make it attractive for THz-regime micro-VED applications. It is a relatively simple circuit to design and fabricate, it is amenable to precision pattern replication by micro-machining, and it is has been demonstrated capable of forward-wave amplification with appreciable bandwidth. We are conducting experimental and computational studies of micro-VED FW-TWTs to examine their feasibility for applications at frequencies from 200 - 1000 GHz.

Booske, John H.↗
NASA NTRS2002

Diurnal Cycle of Convection in the East Pacific ITCZ during EPIC-2001

During the last three weeks of September 2001, the EPIC-2001 intensive field campaign focused on studies of deep convection in the ITCZ over the Mexican warm pool region (10N, 95W) of the East Pacific. This study focuses on the pronounced observed diurnal cycle of environmental and convective parameters within the experiment domain. Data from three primary sources are examined: the R/V Ronald H. Brown C-band weather radar, 4-hourly soundings from the Brown and the Global Atmospherics, Inc. National Lightning Detection Network (long range product). Satellite data from TRMM, GOES and OV-1 are also used. The domain boundary layer shows a robust daily evolution of moist enthalpy (as reflect by equivalent potential temperature, theta-e, or wet bulb potential temperature, theta-w), with contributions from changes in both dry and moist entropy. Peak theta-w is found after local nightfall; the average diurnal range of theta-w is approximately 1 deg C. A composite diurnal cycle of convective properties was derived from the C-band volume scans, sampled continuously through the experiment at 10 minute updates. Products derived from the volumetric data include a surface PPI, 15 and 30 dBZ echo top height, vertically integrated liquid, and 6 km (mixed phase region) reflectivity CAPPIs. For almost all products, the parameter means showed virtually no diurnal cycle. However, for the upper-level products, the parameter spectra showed a clear peak in the occurrence of deep/vigorous convection (the "tail end of the distribution") between 7-9 UTC (1-3 AM local), while overall frequency of occurrence peaked later, from 12-15 UTC (6-9 AM local). This represents a daily "outbreak" of isolated deep cells a couple of hours after sunset and subsequent growth, organization and decay through the nighttime hours. The coherence of the diurnal cycle of the convective spectrum is impressive given the wide variety of convective organization observed during the experiment, and given the modulation by passage of 3-5 day easterly waves. While earlier satellite OLR composites suggested an offshore coastal migration of storms into the domain at night, examination of the 150 km and 300 km range radar products showed little evidence of such organization; almost all convection developed "in-place" within the analysis domain. Consistent with the diurnal thermodynamic and microphysical evolution, a clear cycle in cloud-to-ground (CG) lightning occurrence was observed. The local CG diurnal cycle is significantly stronger than the satellite-derived tropical ocean diurnal cycle of total (IC+CG) lightning. Flash rates of 3-4 fl/min were often visually observed after nightfall; these are fairly 'healthy' flash rates for tropical ocean storms, and the domain was qualitatively noted to be unusually lightning-productive by the R/V Brown crew (also consistent with satellite-based climatologies).

Boccippio, Dennis J.↗
NASA NTRS2003

Delaminations Investigated With Ultrasonic Spectroscopy

A previous study suggested that the ultrasonic spectroscopy technique identified possible disbonds or delaminations in polymer matrix composite (PMC) rings sectioned from flywheel rotors (ref. 1). These results went unsubstantiated by other nondestructive evaluation (NDE) methods. To explain the results, PMC rings were further investigated with ultrasonic spectroscopy (ref. 2). The ultrasonic spectroscopy system utilizes a continuous-swept sine waveform as the input. After the swept sine wave traverses the material, the captured waveform is subjected to two fast Fourier transforms. The second fast Fourier transform along with equalization of the frequency spectrum, allows for evaluation of the fundamental resonant frequency. The full-thickness resonance, the resonance corresponding to the location of the intentional disbond, and the frequency spectrum were examined in an effort to characterize the sensitivity of the NDE method to various delamination conditions.

Cosgriff, Laura M.↗
NASA NTRS2013

Technology Development Roadmap: A Technology Development Roadmap for a Future Gravitational Wave Mission

Humankind will detect the first gravitational wave (GW) signals from the Universe in the current decade using ground-based detectors. But the richest trove of astrophysical information lies at lower frequencies in the spectrum only accessible from space. Signals are expected from merging massive black holes throughout cosmic history, from compact stellar remnants orbiting central galactic engines from thousands of close contact binary systems in the Milky Way, and possibly from exotic sources, some not yet imagined. These signals carry essential information not available from electromagnetic observations, and which can be extracted with extraordinary accuracy. For 20 years, NASA, the European Space Agency (ESA), and an international research community have put considerable effort into developing concepts and technologies for a GW mission. Both the 2000 and 2010 decadal surveys endorsed the science and mission concept of the Laser Interferometer Space Antenna (LISA). A partnership of the two agencies defined and analyzed the concept for a decade. The agencies partnered on LISA Pathfinder (LPF), and ESA-led technology demonstration mission, now preparing for a 2015 launch. Extensive technology development has been carried out on the ground. Currently, the evolved Laser Interferometer Space Antenna (eLISA) concept, a LISA-like concept with only two measurement arms, is competing for ESA's L2 opportunity. NASA's Astrophysics Division seeks to be a junior partner if eLISA is selected. If eLISA is not selected, then a LISA-like mission will be a strong contender in the 2020 decadal survey. This Technology Development Roadmap (TDR) builds on the LISA concept development, the LPF technology development, and the U.S. and European ground-based technology development. The eLISA architecture and the architecture of the Mid-sized Space-based Gravitational-wave Observatory (SGO Mid)-a competitive design with three measurement arms from the recent design study for a NASA-led mission after 2020-both use the same technologies. Further, NASA participation in an ESA-led mission would likely augment the eLISA architecture with a third arm to become the SGO Mid architecture. For these reasons, this TDR for a future GW mission applies to both designs and both programmatic paths forward. It is adaptable to the different timelines and roles for an ESA-led or a NASA-led mission, and it is adaptable to available resources. Based on a mature understanding of the interaction between technology and risk, the authors of this TDR have chosen a set of objectives that are more expansive than is usual. The objectives for this roadmap are: (1) reduce technical and development risks and costs; (2) understand and, where possible, relieve system requirements and consequences; (3) increase technical insight into critical technologies; and (4) validate the design at the subsystem level. The emphasis on these objectives, particularly the latter two, is driven by outstanding programmatic decisions, namely whether a future GW mission is ESA-led or NASA-led, and availability of resources. The relative emphasis is best understood in the context of prioritization.

Laser Interferometry Space Antenna↗
NASA NTRS1975

Broadband noise generated by turbulent inflow to rotor or stator blades in an annular duct

The Green's function relating the radiated pressure field to the fluctuating forces on rotor or stator blades is developed in the presence of dissipation due to turbulent velocity fluctuations and sound speed fluctuations. The resonances in the output power spectrum which would occur at the cut-off frequencies in the absence of dissipation should be removed and smeared out by the incorporation of dissipation. Wave number dependence is developed for an effective eddy viscosity due to the aforementioned fluctuations in the background medium. The space-time correlation function for blade-normal velocity fluctuations on a single or on two different blades is developed in terms of the velocity correlation tensor for the inflow under the assumptions of isotropy and (Taylor) frozen behavior. The correlation function is then simplified under certain approximations and the behavior of the blade-force correlation function is inferred.

Lnae, F.↗
NASA NTRS1985

Energetic ions upstream of Jupiter's bow shock

In an analysis of the Jovian ion events, Baker et al. (1984) have found that there appears to be a spectral hardening as the interplanetary magnetic field (IMF) changed orientation from the postnoon to a prenoon connection point. It was also observed that ion events were accompanied by long-period (approximately 10 min) waves whose presence was weakly related to a decrease in the ion anisotropy. The present investigation examines in substantial detail the development of the ion energy spectrum over the full range (approximately 30 keV to approximately 4 MeV). The results of the investigation show that the energy spectra are dominated by heavy ions (presumed to be oxygen and sulfur) at higher (approximately equal to or greater than 300 keV) energies.

Krimigis, S. M.↗
NASA NTRS1995

Tunable CW diode-pumped Tm,Ho:YLiF4 laser operating at or near room temperature

A conversion efficiency of 42% and slope efficiency of 60% relative to absorbed pump power are obtained from a continuous wave diode-pumped Tm,Ho:YLiF4 laser at 2 microns with output power of 84 mW at a crystal temperature of 275 K. The emission spectrum is etalon tunable over a range of7 nm (16.3/cm) centered on 2.067 microns with fine tuning capability of the transition frequency with crystal temperature at a measured rate of -0.03/(cm)K. The effective emission cross-section is measured to be 5 x 10(exp -21) cm squared. These and other aspects of the laser performance are disclosed in the context of calculated atmospheric absorption characteristics in this spectral region and potential use in remote sensing applications. Single frequency output and frequency stabilization are achieved using an intracavity etalon in conjunction with an external reference etalon.

Mcguckin, Brendan T.↗
NASA NTRS2004

On the Origin of Whistler Mode Radiation in the Plasmasphere

The origin of whistler mode radiation in the plasmasphere is examined from three years of plasma wave observations from the Dynamics Explorer and three years from the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. These data are used to construct plasma wave intensity maps of whistler mode radiation in the plasmasphere. The highest average intensities of the radiation in the wave maps show source locations and/or sites of wave amplification. Each type of emission is classified based on its magnetic latitude and longitude rather than any spectral feature. Equatorial electromagnetic (EM) emissions (approx. 30-330 Hz), plasmaspheric hiss (approx. 330 Hz - 3.3 kHz), chorus (approx. 2 kHz - 6 kHz), and VLF transmitters (approx. 10-50 kHz) are the main types of waves that are clearly delineated in the plasma wave maps. Observations of the equatorial EM emissions show that the most intense region is on or near the magnetic equator in the afternoon sector and that during times of negative B(sub z) (interplanetary magnetic field),the maximum intensity moves from L values of 3 to less than 2. These observations are consistent with the origin of this emission being particle-wave interactions in or near the magnetic equator. Plasmaspheric hiss shows high intensity at high latitudes and low altitudes (L shells from 2 to 4) and in the magnetic equator over L values from 2 to 3 in the early afternoon sector. The longitudinal distribution of the hiss intensity (excluding the enhancement at the equator) is similar to the distribution of lightning: stronger over continents than over the ocean, stronger in the summer than winter, and stronger on the dayside than nightside. These observations strongly support lightning as the dominant source for plasmaspheric hiss, which through particle-wave interactions, maintains the slot region in the radiation belts. The enhancement of hiss at the magnetic equator is consistent with particle-wave interactions. The chorus emissions are most intense on the morning side as previously reported. At frequencies from approx. 10-50 kHz VLF transmitters dominate the spectrum. The maximum intensity of the VLF transmitters is in the late evening or early morning with enhancements all along L shells from 1.8 to 3.

Green, James L.↗
NASA NTRS2019

LiteBIRD: An All-Sky Cosmic Microwave Background Probe of Inflation

The LiteBIRD mission will map polarized fluctuations in the cosmic microwave background (CMB) to search for the signature of gravitational waves from inflation, potentially opening a window on the Universe a fraction of a second after the Big Bang. CMB measurements from space give access to the largest angular scales and the full frequency range to constrain Galactic foregrounds, and LiteBIRD has been designed to take best advantage of the unique window of space. LiteBIRD will have a powerful ability to separate Galactic foreground emission from the CMB due to its 15 frequency bands spaced between 40 and 402 GHz and sensitive 100-mK bolometers. LiteBIRD will provide stringent control of systematic errors due to the benign thermal environment at the second Lagrange point, L2, 20-K rapidly rotating half-wave plates on each telescope, and the ability to crosscheck its results by measuring both the reionization and recombination peaks in the B-mode power spectrum. LiteBIRD would be the next step in the series of CMB space missions, COBE, WMAP, and Planck, each of which has given landmark scientific discoveries.

Lee, Adrian T.↗
NASA NTRS2019

The CALorimetric Electron Telescope (CALET) on the International Space Station: Results from the First Two Years On Orbit

The CALorimetric Electron Telescope (CALET) is a high-energy astroparticle physics space experiment installed on the International Space Station (ISS), developed and operated by Japan in collaboration with Italy and the United States. The CALET mission goals include the investigation of possible nearby sources of high-energy electrons, of the details of galactic particle acceleration and propagation, and of potential signatures of dark matter. CALET measures the cosmic-ray electron+positron flux up to 20 TeV, gamma-rays up to 10 TeV, and nuclei with Z=1 to 40 up to 1,000 TeV for the more abundant elements during a long-term observation aboard the ISS. Starting science operation in mid-October 2015, CALET performed continuous observation without major interruption with close to 20 million triggered events over 10 GeV per month. Based on the data taken during the rst two-years, we present an overview of CALET observations: 1) Electron+positron energy spectrum, 2) Nuclei analysis, 3) Gamma-ray observation including a characterization of on-orbit performance. Results of the electromagnetic counterpart search for LIGO/Virgo gravitational wave events are discussed as well.

Y Asaoka↗
NASA NTRS1979

Plasma wave turbulence at the magnetopause - Observations from ISEE 1 and 2

An investigation of plasma wave electric and magnetic fields in the vicinity of the magnetopause using measurements from the ISEE 1 and 2 spacecraft is presented. Strong electric and magnetic field turbulence is often observed at the magnetopause; the electric field spectrum of this turbulence extends from less than a few hertz to over 100 kHz, and the magnetic field from a few hertz to about 1 kHz. Similar turbulence spectra are observed in association with flux transfer events and possible 'inclusions' of boundary layer plasma in the magnetosphere. Two possible plasma instabilities, the electrostatic ion-cyclotron and the lower-hybrid-drift instability, should explain the broad-band electric field turbulence; the narrow-band electrostatic emissions near the local electron plasma frequency are believed to be plasma oscillations or electrostatic waves near the upper-hybrid-resonance frequency.

Gurnett, D. A.↗
NASA NTRS1985

Submillimeter wave absorption of n-type InSb at low temperatures

The absorption coefficient of two high-purity n-InSb samples is measured in the 10-40 per cm range using Fourier transform spectroscopy. The absorption coefficient spectrum is presented for both samples at 4.2 K. It is also shown for the lower resistance sample cooled to 2.2 K and heated by dc bias to elevated electron gas temperatures of 7.5 and 17.9 K. ac Drude theory gives rather poor agreement with experiment at 2.2 and 4.2 K but does much better when the sample electron gas is heated. In contrast, a simple quantum mechanical theory of absorption based on inverse Bremsstrahlung yields promising agreement at the lower temperatures although its applicability is questionable. The non-Drudian absorption is shown to have a favorable effect on the performance of InSb hot-electron bolometers.

Brown, E. R.↗
NASA NTRS1986

Nondestructive evaluation of adhesive bond strength using the stress wave factor technique

Acousto-ultrasonic nondestructive evaluation has been conducted to evaluate the adhesive bond strength between rubber and steel plates using the stress wave factor (SWF) measurement technique. Specimens with different bond strength were manufactured and tested using the SWF technique. Two approaches were used to define the SWF. One approach defines the SWF as the signal energy and the other approach defines the SWF as the square root of the zero moment of the frequency spectrum of the received signal. The strength of the rubber-steel adhesive joint was then evaluated using the destructive peel strength test method. It was observed that in both approaches higher values of the SWF measurements correspond to higher values of the peel strength test data. Therefore, these results show that the stress wave factor technique has the potential of being used in quality assurance of the adhesive bond strength between rubber and steel substrates.

Dos Reis, Henrique L. M.↗
NASA NTRS1981

Observations of dynamical phenomena in sunspots

A preliminary report of the results of one observing run based on data from one spectral line, the photospheric magnetic line Fe 6303, is presented as part of a series of observations of dynamical phenomena in sunspots using photographic spectra with the SPO vacuum tower telescope and echelle spectrograph. The ejection of a magnetic feature from the outer edge of the penumbra was observed. The initial total field strength of the feature was about 1000 gauss, which appeared to decrease as the feature moved away from the sunspot. The proper motion was about 2 km/s, and the velocity field measured in the V profile showed a downflow of 400 m/s on the spotward side of the moving magnetic feature. Umbral oscillations at the photospheric level with a herringbone structure characteristic of horizontally propagating waves, suggesting some overtone mode of membrane oscillation in the umbra, were seen. The peak amplitude of the oscillation was about 200 m/s, and the mean power spectrum had several clear peaks.

Nye, A. H.↗
NASA NTRS1989

Study of optical output couplers for submillimeter wavelength backward-wave oscillators (BWO's)

Several scaled experiments of optical output couplers for submillimeter backward-wave oscillators (BWOs). Various designs of planar antennas (Vivaldi horns) lens-feed systems (hyperhemispherical lens) were constructed and tested between 20 and 100 GHz using a spectrum analyzer. The lens system was also tested at 337 GHz using a CO2 pumped FIR laser. It is found that Vivaldi horns have unsatisfactory resonances, perhaps because the horns studied were relatively short. Several techniques to maximize and flatten the frequency response of these horns are presented. The results suggest that alternate coupling schemes are superior to Vivaldi horns.

Cook, Jerry D.↗
NASA NTRS1990

Rough surface scattering results based on bandpass autocorrelation forms

Surface-height autocorrelation forms such as Gaussian and exponential are often used in studies of near-normal incidence rough-surface scattering. Such models require the existence of a constant, or DC, value in the spectrum. The consequences of autocorrelation forms that correspond to spectral processes that are essentially bandpass in nature are examined. One such process is that of ocean wind waves. In this case, the spectral components do not extend down to zero frequency. The physical optics backscatter theory is reexamined relative to such autocorrelation functions. Experimental results obtained from a wavetank are compared to the autocorrelation model used in the analysis. The analysis indicates that Gaussian correlation length or mean-square slope is not an appropriate parameter for narrowband conditions and that significant slope is a more relevant parameter. Inherent in the deep-phase assumption is some form of slope dependency. The analysis given (and variants thereof) can be used to provide insight into the physical effects of separate spectral components and of spectral directionality.

Miller, Lee S.↗