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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,531 records · Page 85
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NASA NTRS1991

Growth and characterization of high current density, high-speed InAs/AlSb resonant tunneling diodes

InAs/AlSb double-barrier resonant tunneling diodes with peak current densities up to 370,000 A/sq cm and high peak-to-valley current ratios of 3.2 at room temperature have been fabricated. The peak current density is well-explained by a stationary-state transport model with the two-band envelope function approximation. The valley current density predicted by this model is less than the experimental value by a factor that is typical of the discrepancy found in other double-barrier structures. It is concluded that threading dislocations are largely inactive in the resonant tunneling process.

Soderstrom, J. R.↗
NASA NTRS1991

Nonlinear modal resonances in low-gravity slosh-spacecraft systems

Nonlinear models of low gravity slosh, when coupled to spacecraft vibrations, predict intense nonlinear eigenfrequency shifts at zero gravity. These nonlinear frequency shifts are due to internal quadratic and cubic resonances between fluid slosh modes and spacecraft vibration modes. Their existence has been verified experimentally, and they cannot be correctly modeled by approximate, uncoupled nonlinear models, such as pendulum mechanical analogs. These predictions mean that linear slosh assumptions for spacecraft vibration models can be invalid, and may lead to degraded control system stability and performance. However, a complete nonlinear modal analysis will predict the correct dynamic behavior. This paper presents the analytical basis for these results, and discusses the effect of internal resonances on the nonlinear coupled response at zero gravity.

Peterson, Lee D.↗
NASA NTRS1991

A 200 MHz surface acoustic wave resonator mass microbalance

The development of a 200 MHz SAW instrument based on an SAW resonator instead of a conventional delay line is reported. The instrument is small, compact, and uses two separate 200 MHz resonataor crystals which operate in the dual difference mode. The difference frequency stability is better than 5 x 10 to the -9th (+ or - 1 Hz) which is comparable to a 10 MHz bulk crystal and at least an order of magnitude better than other commercially available SAW delay lines that operate at comparable frequencies. The mass sensitivity is about 9 x 10 to the 10th Hz sq cm/g. Response to temperature change is found to be about 20 Hz/C from -30 C to 30 C. Response to pressure is found to be linear from 760 to 415 Torr at 3 Hz/Torr. Detection limits for HCl vapors were determined to be in the low ppb range using an SAW resonator coated with triethanolamine.

Bowers, W. D.↗
NASA NTRS1990

An electronically tuned, stable 8415 MHz dielectric resonator FET oscillator for space applications

A voltage-controlled 8415-MHz FET oscillator stabilized by a dielectric resonator is described. The oscillator provides over 3.2-MHz linear electronic tuning range with a flat power output equal to +1.8 dBm (27 C, nominal), a single-sideband noise-to-carrier ratio of -68 dBc/Hz at 1 kHz off carrier, and a frequency-temperature coefficient of 0.54 parts per million/C over a -24 C to 75 C range. The oscillator withstood 150 Krads (Si) of gamma radiation with no significant performance degradation. The overall performance of the FET oscillator is far superior in many ways to that of an equivalent bipolar oscillator for space applications. For space applications, the FET dielectric resonator oscillator (DRO) is preferred over bipolar DRO because of its lower DC power consumption, better thermal frequency stability, linear electronic tunability, and higher RF output power capability.

Mysoor, Narayan R.↗
NASA NTRS1991

HF radar observations of Pc 5 field line resonances in the midnight/early morning MLT sector

The technique of F region sounding by HF coherent radar is applied to the study of field line resonances equatorward of a region of shear flow in the early morning sector. The motions were predominantly in the geomagnetic east-west direction, indicating north-south electric fields. These oscillations had pronounced peaks in their latitudinal power distribution. For the pulsation at 1.95 mHz, a latitudinal phase shift of 180 deg was observed across the peaks in all the look directions of the radar, and a longitudinal wavelength corresponding to an m value of 3 was obtained. For the 2.6-mHz pulsation, the phase shifts across the peaks had a variation with a look direction that indicated a significant longitudinal as well as latitudinal variation; for this activity, an m value of about 16 is estimated. These features are interpreted in terms of the field line resonance theory, and the possible sources of the pulsation energy are discussed.

Ruohoniemi, J. M.↗
NASA NTRS1991

The inability of the resonant Compton upscattering model of gamma-ray bursts to produce a third cyclotron harmonic

The inability of the resonant Compton upscattering model to produce a third cyclotron resonance, which provides a strong observational test of this model, is discussed. The relevant features of the Compton upscattering model and the thermal cross sections responsible for the cyclotron lines are examined. The impossibility of producing a third cyclotron feature for all realistic temperatures of the line-forming plasma is addressed.

Brainerd, J. J.↗
NASA NTRS1992

A scheme to alter the resonant frequency of the microstrip patch antenna

Simple schemes are presented for altering the resonant frequency of a rectangular patch antenna without the need to change its size. In particular, by placing a perturbance below the patch it is shown that as much as 20 percent increase and 30 percent decrease from the resonant frequency of the unperturbed patch can be achieved. The specific configurations considered in this letter include a cavity-backed, aperture-backed, and protrusion-backed patch, and for each case design curves are presented.

Volakis, John L.↗
NASA NTRS1991

Resonant Acoustic Determination of Complex Elastic Moduli

A simple, inexpensive, yet accurate method for measuring the dynamic complex modulus of elasticity is described. Using a 'free-free' bar selectively excited in three independent vibrational modes, the shear modulus is obtained by measuring the frequency of the torsional resonant mode and the Young's modulus is determined from measurement of either the longitudinal or flexural mode. The damping properties are obtained by measuring the quality factor (Q) for each mode. The Q is inversely proportional to the loss tangent. The viscoelastic behavior of the sample can be obtained by tracking a particular resonant mode (and thus a particular modulus) using a phase locked loop (PLL) and by changing the temperature of the sample. The change in the damping properties is obtained by measuring the in-phase amplitude of the PLL which is proportional to the Q of the material. The real and imaginary parts or the complex modulus can be obtained continuously as a function of parameters such as temperature, pressure, or humidity. For homogeneous and isotropic samples only two independent moduli are needed in order to characterize the complete set of elastic constants, thus, values can be obtained for the dynamic Poisson's ratio, bulk modulus, Lame constants, etc.

Brown, David A.↗
NASA NTRS1991

Charged dust dynamics - Orbital resonance due to planetary shadows

The dynamics of a weakly charged dust grain orbiting in the equatorial plane of a planet surrounded by a rigidly corotating magnetospehre is examined. It is shown that an introduction of an effectilve 1D potential causes a perturbation due to electrostatic forces, which induces a motion of the pericenter, similar to the effect of the planetary oblateness. A case is examined where the charge varies periodically due to the modulation of the photoelectron current occurring as the grain enters and leaves the planetary shadow, causing the electromagnetic perturbation to resonate with the orbital period and to modify the size and eccentricity of the orbit. This effect is demonstrated both numerically and analytically for small grains comprising the Jovian ring, showing that their resulting changes are periodic, and their amplitude is much larger than that of the periodic changes due to light-pressure perturbation or the secular changes due to resonant charge variations that develop over a comparable time span.

Horanyi, M.↗
NASA NTRS1991

The lower subsidiary diffuse plasma resonances and the classification of radio emissions below the plasma frequency

Using previusly published data and newly scaled ionograms from the Alouette 2 and ISIS 1 experiments, the diffuse ionospheric resonances Dn, stimulated by topside sounders Dn, are classified. The classification also includes the lower subsidiary resonances Dn(-) (n = 1, 2, 3, and 4). It is shown that the Dn(-) frequencies are related to f(Dn) and f(H) by the expression f(Dn)- = sq rt of (f(Dn)-squared - f(H)-squared).

Osherovich, Vladimir A.↗
NASA NTRS1991

Tidal origin of the Laplace resonance and the resurfacing of Ganymede

In the present scenario for the Laplace resonance's tidal origin, the Laplace relation for the three inner Galilean satellites Io, Europa, and Ganymede may have been established within a previous three-body resonance which excited Ganymede's orbital eccentricity to a high value. The tidal heating enhancement associated with that high-eccentricity period furnishes a plausible explanation for the Ganymede surface, which appears to be geologically younger than that of Callisto.

Malhotra, Renu↗
NASA NTRS1991

Helicopter air resonance modeling and suppression using active control

A coupled rotor/fuselage helicopter analysis with the important effects of blade torsional flexibility, unsteady aerodynamics, and forward flight is presented. Using this mathematical model, a nominal configuration is selected with an air resonance instability throughout most of its flight envelope. A multivariable compensator is then designed using two swashplate inputs and a single-body roll rate measurement. The controller design is based on the linear quadratic Gaussian technique and the loop transfer recovery method. The controller is shown to suppress the air resonance instability throughout a wide range of helicopter loading conditions and forward flight speeds.

Takahashi, M. D.↗
NASA NTRS1992

The introduction of spurious models in a hole-coupled Fabry-Perot open resonator

A hemispherical open resonator has previously been used to make relative comparisons of the surface resistivity of metallic thin-film samples in the submillimeter wavelength region. This resonator is fed from a far-infrared laser via a small coupling hole in the center of the concave spherical mirror. The experimental arrangement, while desirable as a coupling geometry for monitoring weak emissions from the cavity, can lead to the introduction of spurious modes into the cavity. Sources of these modes are identified, and a simple alteration of the experimental apparatus to eliminate such modes is suggested.

Cook, Jerry D.↗
NASA NTRS1992

Partially coherent scattering in stellar chromospheres. I - Effects on resonance line thermalization

The conditions necessary for partially coherent scattering to influence the thermalization depth are investigated, and the effects of partial redistribution in homogeneous slab atmospheres are approximated using several common resonance lines as examples. For electron densities above 10 exp 10/cu cm, it is concluded that even when coherent scattering dominates the escape process, the thermalization depths of the strongest resonance lines of H, Ca II, and Mg II agree roughly with the Doppler diffusion in frequency. No elastic scattering effects on the thermalization of Mg II h and k and Ca II H and K are found. At lower densities, such as for giant star chromospheres and QSO models, the results deviate strongly from the Doppler complete redistribution case.

Gayley, Kenneth G.↗
NASA NTRS1992

Aerosol-induced laser breakdown thresholds - Effect of resonant particles

Laser intensity thresholds for the onset of stimulated Raman scattering and the breakdown in resonant micron-sized droplets are reduced to below those for nonresonant droplets by a factor of about 3. This reduction is most likely caused by the enhancement of electromagnetic energy (photon) densities within the droplets over and above that in nonresonant droplets. The magnitude of the threshold reduction for breakdown is consistent with the assertion that: (1) input (pump) wavelength resonances that initiate plasma have cavity Qs of about 10 exp 4; and (2) finite regions of high-electromagnetic-energy density within the droplet, with dimensions of the order of the Debye length, are required to initiate plasma.

Pinnick, R. G.↗
NASA NTRS1992

Calculation of the quasi-energies and resonances behavior of the hydrogen Lyman-alpha problem

Recently, Bakshi and Kalman presented numerical results for the quasi-energies of the n = 2 multiplet in the hydrogen Lyman-alpha transition for a plasma in which both strong static and oscillating electric fields are present. Recent work on related magnetic and optical resonance problems provides a simplified mathematical treatment, as well as greater insight into the complex resonance behavior of this interaction.

Ruyten, Wilhelmus M.↗
NASA NTRS1992

Lorentz resonances and the vertical structure of dusty rings - Analytical and numerical results

The Schaffer and Burns (1987) linear theory of Lorentz resonances (LRs) in planetary rings is extended in order to accurately compute LR locations and to elucidate the nature of grain trajectories within the LR zones. Using the perturbation theory and energy arguments, it is shown that an increase in the inclination or eccentricity of a grain must be accompanied by a shift in the mean orbital radius of the particle. This shift alters the epicyclic frequencies in such a way that the infinite response of the linear resonance theory is suppressed. Chaotic motion is found for the range of charge-to-mass ratios that cause the vertical and horizontal LRs to overlap.

Schaffer, Les↗