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NASA NTRS1977

Primary aberrations for grazing incidence

Beginning with exact relations between object and image coordinates for a single reflective surface, a systematic analysis of general grazing incidence systems is presented. A complete set of primary aberrations for single-element and two-element systems is developed. The importance of a judicious choice for a coordinate system in showing field curvature to be clearly the predominant aberration for a two-element system is discussed. The validity of the theory is verified through comparisons with the exact ray-trace results for the case of a telescope.

Winkler, C. E.↗

NASA NTRS1976

Technical feasibility of a modular dish solar electric system

The paper presents the technical feasibility of generating electricity by using a gas turbine in conjunction with a paraboloid of revolution (dish) solar concentrator. A conceptual design of a dish concentrator is obtained by parametrically examining the significant optical parameters. The optical performance analysis was conducted using a Monte Carlo ray-trace simulation program. The performance of four candidate thermodynamic cycles utilizing two working fluids was analyzed; the cycles were the regenerative and nonregenerative Brayton cycles of both open and closed type. Air and helium were the working fluids for the open and closed type respectively. Heat transfer from the solar radiation to the gas flowing in metal tubes at the receiver was also analyzed. A paraboloid of revolution dish with a cavity receiver using an open air regenerative Brayton cycle turbine emerged as a technically feasible concept in the power range from 30 to 100 KWth per module.

Gupta, B. P.↗

NASA NTRS1977

The linear Fresnel lens - Solar optical analysis of tracking error effects

Real sun-tracking solar concentrators imperfectly follow the solar disk, operationally sustaining both transverse and axial misalignments. This paper describes an analysis of the solar concentration performance of a line-focusing flat-base Fresnel lens in the presence of small transverse tracking errors. Simple optics and ray-tracing techniques are used to evaluate the lens solar transmittance and focal-plane imaging characteristics. Computer-generated example data for an f/1.0 lens indicate that less than a 1% transmittance degradation occurs for transverse errors up to 2.5 deg. In this range, solar-image profiles shift laterally in the focal plane, the peak concentration ratio drops, and profile asymmetry increases with tracking error. With profile shift as the primary factor, the ninety-percent target-intercept width increases rapidly for small misalignments, e.g., almost threefold for a 1-deg error. The analytical model and computational results provide a design base for tracking and absorber systems for the linear-Fresnel-lens solar concentrator.

Cosby, R. M.↗

NASA NTRS1977

Spacecraft observations of man-made whistler-mode signals near the electron gyrofrequency

The reported investigation extends the range of whistler-mode wave observations to a wave frequency/electron gyrofrequency ratio of about 0.9, where an abrupt cutoff is observed. This cutoff can be explained entirely in terms of accessibility and hence, if there is damping, it must be limited to normalized frequencies above 0.9. In connection with a study of the behavior of the signal intensity, ray tracings were carried out at 80 kHz. The ray-tracing calculations were carried out with the aid of a computer program written by Walter (1969) and modified by Angerami (1970).

Dunckel, N.↗

NASA NTRS1977

Study of stratospheric-ionospheric coupling during thunderstorms and tornadoes

A continuous-wave-spectrum high-frequency Doppler sounder array with three transmitters at each of three sites was used to observe the dynamics of the coupling of energy between the stratosphere and the ionosphere. During times of severe weather activity wavelike disturbances have been detected on ground-based ionospheric sounding records as perturbations in electron densities. Infrasonic waves with wave periods of 3-7 min and with horizontal phase velocities of 600-800 m/s were observed when there was thunderstorm activity; gravity waves with wave periods of 10-15 min and horizontal phase velocities of 100-200 m/s were detected when there was tornado activity. Both triangulations from the cross correlation functions of the Doppler records based on an assumption of no background wind shear and ray-tracing computations including an assumed background wind shear indicate that the waves originated in the vicinity of the thunderstorms and tornadoes. A comparison of the wavelengths of the infrasonic and gravity waves observed at ionospheric heights and those in cloud-top pictures from satellites show that they are all of the order of 100-300 km.

Hung, R. J.↗

NASA NTRS1979

Mesospheric heating due to intense tropospheric convection

A series of rocket measurements made twice daily at Wallops Island, Va., revealed a rapid heating of the mesosphere on the order of 10 K on days when thunderstorms or squall lines were in the area. This heating is explained as the result of frictional dissipation of vertically propagating internal gravity waves generated by intense tropospheric convection. Ray-tracing theory is used to determine the spectrum of gravity wave groups that actually reach mesospheric heights. This knowledge is used in an equation describing the spectral energy density of a penetrative convective element to calculate the fraction of the total energy initially available to excite those waves that do reach the level of heating. This value, converted into a vertical velocity, is used as the lower boundary condition for a multilayer model used to determine the detailed structure of the vertically propagating waves. The amount of frictional dissipation produced by the waves is calculated from the solutions of the frictionless model by use of a vertically varying eddy viscosity coefficient. The heating produced by the dissipation is then calculated from the thermodynamic equation.

Taylor, L. L.↗

NASA NTRS1980

Computer prediction of large reflector antenna radiation properties

A FORTRAN program for calculating reflector antenna radiation patterns was rewritten and extended to include reflectors composed of a number of panels. These individual panels must be analytic surfaces. The theoretical foundation for the program is as follows: Geometrical optics techniques are used to trace rays from a feed antenna to the reflector surface and back to a mathematical plane just in front of the reflector. The resulting tangential electric field distribution, assumed to be the only source of forward radiation, is integrated numerically to calculate the radiation pattern for a desired set of angles. When the reflector is composed of more than one panel, each panel is treated as a separated antenna, the ray-tracing procedure and integration being repeated for each panel. The results of the individual aperture plane integrations are stored and summed to yield the relative electric field strength over the angles of interest. An example and several test cases are included to demonstrate the use of the program and verify the new method of computation.

Botula, A.↗

NASA NTRS1980

Gravity waves and GOES IR data study of an isolated tornadic storm on 29 May 1977

Enhanced convection-initiated gravity waves associated with an isolated tornado in the absence of a squall line are investigated. Ray-tracing computations based on data observed on May 29, 1977 indicated that the wave sources were located in north-central Oklahoma. Comparison with a radar echo map during the time period when the waves were excited showed that the waves were generated by an isolated cloud with enhanced convection. GOES infrared digital data during the time period from wave excitation to tornado touchdown were analyzed. Results showed that the cloud where the gravity waves were excited was characterized by both a very low temperature at the cloud top and a very high expansion rate of the cold cloud-top area. The lead time between the excitation of the gravity waves and the tornado touchdown is discussed in conjunction with the growth rate of the clouds associated with the tornado.

Hung, R. J.↗

NASA NTRS1982

Slab-geometry Nd:glass laser performance studies

It is noted that slab-geometry solid-state lasers potentially provide significant performance improvements relative to conventional rod-geometry lasers. Experimental measurements that use an Nd:glass test-bed slab laser are presented. A comparison is made between the results and computer-model predictions of the slab-geometry approach. The computer model calculates and displays the temperature and stress fields in the slab, and on the basis of these predicts birefringence and index-of-refraction distributions. The effect that these distributions have on optical propagation is determined in a polarization-sensitive ray-tracing section of the model. Calculations are also made of stress-induced surface curvature and the resulting focusing effects. The measurements are found to be in good agreement with the computer-model predictions. It is concluded that the slab configuration offers significant laser-performance advantages in comparison with the traditional rod-laser geometry.

Eggleston, J. M.↗

NASA NTRS1982

Conceptual support design of the high resolution mirror assembly for the Advanced X-ray Astrophysics Facility (AXAF)

The performance goals of the Advanced X-ray Astrophysics Facility (AXAF) require that the Wolter I optics support system be insensitive to temperature fluctuations and the effects of gravity during ground testing. Previous work has shown that acceptable concepts are achievable but require much attention to design details. This paper discusses a new support system wherein each optic is held by a single circumfererential ring near its center of mass rather than a multipoint support as previously analyzed. The thermal sensitivity of this system is considerably lower than any concept yet studied. This advanced support concept allows the use of a wider variety of mateials and simple design details. Structural analysis utilizing the finite element method and optical ray-trace analysis are used to predict the system performance.

Cohen, L. M.↗

NASA NTRS1983

A theoretical study of plasmaspheric hiss generation

An investigation of the generation of plasmaspheric hiss by the whistler mode instability has been carried out. Using combined ray-tracing and growth rate calculations, the path-integrated growth rates of a large number of waves have been calculated using a distribution function. In general, the waves damp out before being magnetospherically reflected, or soon after. This is true of all the waves traced, independent of the distribution functions used for the energetic (resonant) electrons. Thus the growth rates are generally low, since the waves undergo few equatorial transits. A comparison of these results with observed spectra indicates that while some spectral characteristics are in qualitative agreement, there is a major discrepancy in the intensity levels.

Huang, C. Y.↗

NASA NTRS1984

Parametric instabilities during electron cyclotron heating of tandem mirrors

Electron cyclotron resonance heating is one of the most commonly used methods of heating electrons in the plugs and in the thermal barriers of tandem mirrors. The intense coherent electromagnetic waves used for such heating are susceptible to parametric decay into other modes. Significant growth rates are found for the decay of either ordinary or extraordinary waves into two magnetized electron plasma waves. This and related effects may result in electron heating mechanisms rather different than those assumed in linear ray-tracing calculations. These results may help explain the unusual effects observed during heating of the Phaedrus tandem mirror device. In the general case, these instabilities may be strongly inhibited by density gradients.

Nicholson, D. R.↗

NASA NTRS1984

The Io decametric emission cone

The Voyager 1 and 2 Planetary Radio Astronomy observations of Jovian decametric radiation (DAM) were transformed into a fixed Io-Jupiter coordinate system for Io longitudes every 10 deg apart starting from 200 to 260 deg (seven plots per spacecaft). An analysis of the data in this coordinate system has shown that the Io-dependent DAM emissions are generated in hollow cones at all frequencies over the fixed Io longitudes studied. The edges of the emission cones are, in many cases, broad in longitude (30 deg or more), with distinct differences found between the characteristics of DAM emission at the edges of the emission cones. The shapes of the DAM emission cones are explained by using three-dimensional ray-tracing calculations within a model Jovian atmosphere.

Green, J. L.↗

NASA NTRS1983

Optical, thermal, and electrical performance of low-CR solar arrays

This paper describes the analysis and testing of a photovoltaic low-CR concentrator shaped like a truncated pyramid with an aperture of 0.5 m on a side and a geometric concentration ratio of six. The truncated base plane is covered by either silicon (Si) or gallium arsenide (GaAs) solar cells. Ray-trace analysis of the concentrator predicts a peak optical efficiency of 0.77, which falls off only gradually with pointing error. A coupled thermal-electrical analysis of the system shows that the moderately nonuniform illumination produced by the concentrator does not result in significant mismatch losses, provided the solar cells are connected in parallel groups. The results of ground tests involving a full-scale prototype concentrator conform well with theoretical predictions.

French, E. P.↗

NASA NTRS1984

Effect of parallel refraction on magnetospheric upper hybrid waves

Large amplitude (not less than 10 mV/m) electrostatic plasma waves near the upper hybrid (UH) frequency have been observed from 0 to 50 deg magnetic latitude (MLAT) during satellite plasma-pause crossings. A three-dimensional numerical ray-tracing calculation, based on an electron distribution measured during a GEOS 1 dayside intense upper-hybrid wave event, suggests how UH waves might achieve such large amplitudes away from the geomagnetic equator. Refractive effects largely control the wave amplification and, in particular, the unavoidable refraction due to parallel geomagnetic field gradients restricts growth to levels below those observed. However, a cold electron density gradient parallel to the field can lead to upper hybrid wave growth that can account for the observed emission levels.

Engel, J.↗

NASA NTRS1985

Long range downwind propagation of low-frequency sound

The propagation of low-frequency noise outdoors was studied using as the source a large (80-m diameter) 4-megawatt horizontal axis wind turbine. Acoustic measurements were made with low-frequency microphone systems placed on the ground at five downwind sites ranging from 300 m to 10,000 m (6.3 mile) away from the wind turbine. The wind turbine fundamental was 1 Hz and the wind speed was generally 12 - 15 m/s at the hub height (80 m). The harmonic levels, when plotted versus propagation distance, exhibit a 3 dB per doubling of distance divergence. Two plausible explanations identified for this cylindrical spreading behavior were propagation of the low frequency wind turbine noise via a surface wave and downwind refraction. Surface was amplitude predictions were found to be more than 20 dB smaller than the measured levels. Ray-tracing results were used to qualitatively explain measured trends. A normal mode approach was identified as a candidate method for low-frequency acoustic refraction prediction.

Willshire, W. L., Jr.↗

NASA NTRS1983

Near-infrared mapping spectrometer optical subsystem development and testing

The Near-Infrared Mapping Spectrometer (NIMS) is one of the four remote-sensing science instruments of the Galileo Orbiter scientific payload. The NIMS scientific objectives require operating the detector and optical subsystem at cryogenic temperatures. The necessity of assembling, aligning, and testing the optics at room temperature and meeting design specifications at the cryogenic operating temperature (130 K) presented a set of challenging technical problems. A systematic approach to the development of athermalized mounts and supporting structures for optical components is described. A technique utilizing the visible spectral range and supplementary ray-trace information for alignment of an infrared instrument is presented. The optical subsystem point-spread function and spatial and spectral resolution were determined at room temperature using selected spectral and spatial targets. Based on thermal-distortion analyses of the structure and mounts, compensators were selected, implemented, and verified at cryogenic temperatures. The selection of the compensator and the overall system performance were verified in a thermal vacuum chamber. Various external and internal calibration targets were used.

Macenka, S. A.↗

NASA NTRS1985

Geometric effects of global lateral heterogeneity on long-period surface wave propagation

The present investigation has the objective to document examples of anomalous long-period surface wave amplitude behavior and to provide a preliminary appraisal of the effects of global lateral heterogeneity on surface wave propagation from a ray theory perspective. Attention is given to remarkable long-period surface wave anomalies described in literature, an equidistance azimuthal plot centered on the Iranian source region, Rayleigh wave and Love wave spectra for the 256-s period arrivals for the Tabas earthquake, constrained moment tensor and fault model inversion solutions ofr Iranian earthquakes, aspects of surface wave ray tracing, and a table of Rayleigh wave amplitude anomalies for Iranian earthquakes. Surface wave ray-tracing calculations for models of global phase velocity variations proposed by Nakanishi and Anderson (1984) are found to show that large-amplitude anomalies will be observed for Love and Rayleigh waves with periods of 100-250 s.

Lay, T.↗