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NASA NTRS2013

Terrestrial Analogs for Clay Minerals at Yellowknife Bay, Gale Crater, Mars

Sediments of the Sheepbed unit, Gale Crater, were analyzed by the CheMin X-ray diffraction instrument on the Curiosity Rover. The sediments consist of typical basalt minerals (Fe-forsterite, augite, pigeonite, plagioclase), as well as Fe oxide/hydroxides, Fesulfides, amorphous material, and a phyllosilicate. The phyllosilicate has a broad 001 peak at approx 1.0 nm, consistent with a poorly ordered smectite. However, in the absence of diagnostic tests possible on Earth, its identity is not clear. The position of the 06L diffraction band is generally used to distinguish dioctahedral from trioctahedral smectite, but it is beyond CheMin's range of 2 Theta. The measured position of the 02L diffraction band (approx 22.5deg 2 Theta by CheMin), implies that the smectite is trioctahedral. The exact position and shape of the 02L band is determined by the cations in the 'M' sites of the smectite; to constrain those cations, we sought analogs among terrestrial smectites, emphasizing those developed from basaltic precursors. A potential analog for the Sheepbed smectite is 'griffithite,' a variety of trioctahedral smectite in altered basalt of the Topanga formation, Griffith Park, Los Angeles. 'Griffithite' has an 02L diffraction band that is close in position and shape to that of the Sheepbed smectite, although 'griffithite' has a very sharp 001 peak, indicating a high degree of layer ordering not seen in the Sheepbed smectite. A typical chemical formula for 'griffithite,' determined by electron microprobe, is (Ca0.59 Na0.03) (Mg4.28 Fe1.83) (Si6.64 Al1.36) O20 (OH)4, normalized to Si+Al=8. This formula is consistent with a fully trioctahedral Fe-Mg smectite with Ca and Na as interlayer cations. In the Topanga basalt, four types of 'griffithite' are present: fine-grained, filling cracks and vesicles; coarse-grained, filling vesicles; coarse-grained, replacing olivine phenocrysts; and coarse-grained, replacing glassy mesostasis. The fine-grained 'griffithite' formed first, and the last three varieties may be contemporaneous. One sample shows agate (alpha- quartz) that was precipitated between the episodes of deposition of the fine-grained and coarse-grained 'griffithite.' 'Griffithite' is not unique as a possible terrestrial analog - some clay minerals from the Doushantou formation, China, have similar 02L diffraction bands, and many basalts contain smectites in vesicles and as replacements after olivine. Similar trioctahedral smectites occur also in the nakhlite martian meteorites - as veinlets and replacements of olivine. By understanding the formation of these terrestrial clays, we hope to constrain the nature and mechanism of formation of the Sheepbed clay mineral.

Treiman, Allan H↗
NASA NTRS1973

The radiation from apertures in curved surfaces

The geometrical theory of diffraction is extended to treat the radiation from apertures or slots in convex, perfectly-conducting surfaces. It is assumed that the tangential electric field in the aperture is known so that an equivalent, infinitesimal source can be defined at each point in the aperture. Surface rays emanate from this source which is a caustic of the ray system. A launching coefficient is introduced to describe the excitation of the surface ray modes. If the field radiated from the surface is desired, the ordinary diffraction coefficients are used to determine the field of the rays shed tangentially from the surface rays. The field of the surface ray modes is not the field on the surface; hence if the mutual coupling between slots is of interest, a second coefficient related to the launching coefficient must be employed. In the region adjacent to the shadow boundary, the component of the field directly radiated from the source is presented by Fock-type functions. In the illuminated region the incident radiation from the source (this does not include the diffracted field components) is treated by geometrical optics. This extension of the geometrical theory of diffraction is applied to calculate the radiation from slots on elliptic cylinders, spheres and spheroids.

Pathak, P. H.↗
NASA NTRS1978

Infrared Astronomical Satellite (IRAS) analysis, introduction

Stray light transmittance analysis on a design of the Infrared Astronomical Satellite (IRAS) is presented. The system was evaluated for scattered radiation propagation with the use of the APART (Arizon's Paraxial Analysis of Radiation Transfer), and the propagation of diffracted energy with both PADE (Paraxial Analysis of Diffracted Energy) and the GUERAP II programs. The scattered radiation analysis was performed by using both a 5% diffuse black on the baffle and vane surfaces and by using a mathematical model of the Martin Black scattering characteristics. The sources of infrared radiation and of radiation in the visible wavelengths are identified. Diffraction effects are dominant only at the large off-axis angles, except for a few specific cases and smaller angles. At the longer wavelengths the diffraction contributions go up significantly. It is the comparative increase which determines which propagation process predominates. Combined transmittance values are presented.

Source record↗
NASA NTRS1986

X-ray investigations related to the shock history of the Shergotty achondrite

The shock stress suffered by naturally shocked materials from the Shergotty achondrite was studied using X-ray diffraction techniques and experimentally shocked augite and enstatite as standards. The Shergotty pyroxenes revealed the formation of continuous diffraction rings, line broadening, preferred orientation of small scale diffraction domains, and other evidence of substantial lattice disorders. As disclosed by the application of Debye-Scherrer techniques, they are hybrids between single crystals and fine-grained random powders. The pyroxene lattice is very resistant to shock damage on smaller scales. While measurable lattice disaggregation and progressive fragmentation occur below 25 GPa, little additional damage is suffered from application of pressures between 30 to 60 GPa, making pressure calibration of naturally shocked pyroxenes via X-ray methods difficult. Powder diffractometer scans on pure maskelynite fractions of Shergotty revealed small amounts of still coherently diffracting plagioclase, which may contribute to the high refractive indices of the diaplectic feldspar glasses of Shergotty.

Horz, F.↗
NASA NTRS1986

Electromagnetic backscattering by corner reflectors

The analysis of the backscatter cross section of a dihedral corner reflector, using Geometrical Theory of Diffraction (GTD) and Physical Theory of Diffraction (PTD), is completed in the azimuthal plane, and very good agreement with experimental results is obtained. The advantages and limitations of the GTD and PTD techniques are discussed specifically for radar cross section applications. The utilization of GTD and PTD in oblique incidence diffraction from conducting targets is discussed. Results for equivalent current off-axis diffraction from the flat rectangular plate are presented using the equivalent currents of Knott, Senior, and Michaeli. The rectangular subdivision technique of Sikta, and its extension by Sunatara, alleviate some of the limitations of the equivalent techniques. As yet, neither technique can be used in bistatic scattering or for multiple scattering of a complex target.

Balanis, C. A.↗
NASA NTRS1986

Electromagnetic backscattering by corner reflectors

The Geometrical Theory of Diffraction (GTD), which supplements Geometric Optics (GO), and the Physical Theory of Diffraction (PTD), which supplements Physical Optics (PO), are used to predict the backscatter cross sections of dihedral corner reflectors which have right, obtuse, or acute included angles. These theories allow individual backscattering mechanisms of the dihedral corner reflectors to be identified and provide good agreement with experimental results in the azimuthal plane. The advantages and disadvantages of the geometrical and physical theories are discussed in terms of their accuracy, usefulness, and complexity. Numerous comparisons of analytical results with experimental data are presented. While physical optics alone is more accurate and more useful than geometrical optics alone, the combination of geometrical optics and geometrical diffraction seems to out perform physical optics and physical diffraction when compared with experimental data, especially for acute angle dihedral corner reflectors.

Balanis, C. A.↗
NASA NTRS1987

Electromagnetic backscattering by plates and disks

With the recent development of diffraction coefficients for imperfectly conducting half-planes, it has become possible to analyze a wide variety of problems for which the impedance surface boundary condition applies. This impedance boundary condition, while approximate, was utilized to extend the usefulness of the Uniform Geometrical Theory of Diffraction (UTD) beyond the perfectly conducting geometries. These half-plane diffraction coefficients are used to analyze patterns of an antenna in the presence of an imperfectly conducting flat polygonal plate. The Geometrical Theory of Diffraction (GTD) techniques were also used to investigate the backscattering from perfectly conducting plates. To further improve the soft polarization results for wide angles, a model for the creeping wave or circulating current on the edge of the disk was obtained and used to find an additional component of the backscattered field. The backscattering from a square plate was then analyzed using GTD. Backscattering in both the principal and off-principal planes was examined.

Balanis, Constantine A.↗
NASA NTRS1987

New main reflector, subreflector and dual chamber concepts for compact range applications

A compact range is a facility used for the measurement of antenna radiation and target scattering problems. Most presently available parabolic reflectors do not produce ideal uniform plane waves in the target zone. Design improvements are suggested to reduce the amplitude taper, ripple and cross polarization errors. The ripple caused by diffractions from the reflector edges can be reduced by adding blended rolled edges and shaping the edge contour. Since the reflected edge continues smoothly from the parabola onto the rolled surface, rather than being abruptly terminated, the discontinuity in the reflected field is reduced which results in weaker diffracted fields. This is done by blending the rolled edges from the parabola into an ellipse. An algorithm which enables one to design optimum blended rolled edges was developed that is based on an analysis of the continuity of the surface radius of curvature and its derivatives across the junction. Futhermore, a concave edge contour results in a divergent diffracted ray pattern and hence less stray energy in the target zone. Design equations for three-dimensional reflectors are given. Various examples were analyzed using a new physical optics method which eliminates the effects of the false scattering centers on the incident shadow boundaries. A Gregorian subreflector system, in which both the subreflector and feed axes are tilted, results in a substantial reduction in the amplitude taper and cross polarization errors. A dual chamber configuration is proposed to eliminate the effects of diffraction from the subreflector and spillover from the feed. A computationally efficient technique, based on ray tracing and aperture integration, was developed to analyze the scattering from a lossy dielectric slab with a wedge termination.

Pistorius, C. W. I.↗
NASA NTRS1988

Scattering patterns of dihedral corner reflectors with impedance surface impedances

The radar cross section patterns of lossy dihedral corner reflectors are calculated using a uniform geometrical theory of diffraction for impedance surfaces. All terms of up to third order reflections are considered for patterns in the principal plane. The surface waves are included whenever they exist for reactive surface impedances. The dihedral corner reflectors examined have right, obtuse, and acute interior angles, and patterns over the entire 360 deg azimuthal plane are calculated. The surface impedances can be different on the four faces of the dihedral corner reflector; however, the surface impedance must be uniform over each face. Computed cross sections are compared with a moment method technique for a dielectric/ferrite absorber coating on a metallic corner reflector. The analysis of the dihedral corner reflector is important because it demonstrates many of the important scattering contributors of complex targets including both interior and exterior wedge diffraction, half-plane diffraction, and dominant multiple reflections and diffractions.

Balanis, Constantine A.↗
NASA NTRS1990

Variable magnification variable dispersion glancing incidence imaging x ray spectroscopic telescope

A variable magnification variable dispersion glancing incidence x ray spectroscopic telescope capable of multiple high spatial revolution imaging at precise spectral lines of solar and stellar x ray and extreme ultraviolet radiation sources includes a primary optical system which focuses the incoming radiation to a primary focus. Two or more rotatable carriers each providing a different magnification are positioned behind the primary focus at an inclination to the optical axis, each carrier carrying a series of ellipsoidal diffraction grating mirrors each having a concave surface on which the gratings are ruled and coated with a multilayer coating to reflect by diffraction a different desired wavelength. The diffraction grating mirrors of both carriers are segments of ellipsoids having a common first focus coincident with the primary focus. A contoured detector such as an x ray sensitive photographic film is positioned at the second respective focus of each diffraction grating so that each grating may reflect the image at the first focus to the detector at the second focus. The carriers are selectively rotated to position a selected mirror for receiving radiation from the primary optical system, and at least the first carrier may be withdrawn from the path of the radiation to permit a selected grating on the second carrier to receive radiation.

Hoover, Richard↗
NASA NTRS1989

Infrared spectra of crystalline and glassy silicates and application to interstellar dust

The infrared spectra of crystalline minerals predicted in theoretical condensation sequences do not match the astronomical observations. Since the astronomical spectra are a closer match to glassy silicates, the authors undertook a study to measure the infrared spectra of glassy silicates that have compositions similar to silicate minerals predicted in theoretical condensation sequences. The data should support observations aimed at elucidating condensation chemistry in dust forming regions. The authors measured the mass absorption coefficients, from 2.5 to 25 microns, of ground samples of olivine, diopside, and serpentine and also smoke samples that were prepared from these minerals. The smoke samples prepared in this way are predominantly glassy with nearly the same composition as the parent minerals. The crystalline samples consisted of pure olivine ((Fe(0.1)Mg(0.9))(2)SiO(4)), serpentine, diopside. Sample purity was confirmed by x ray diffraction. Each mineral was ground for 10 hours and a measured mass of the powder was mixed with KBr powder for absorption measurements using the method of Borghesi et a. (1985). The smoke samples were prepared from the same samples used for grinding by vaporizing the minerals using pulsed laser radiation in air. The smoke samples formed by condensation of the resulting vapor. The smoke settled onto infrared transparent KRS-5 substrates and onto a quartz crystal microbalance used to obtain mass measurements. A description of the preparation method is given in Stephens (1980). The glassy diopside showed only diffuse electron diffraction peaks and hence was nearly amorphous, while the serpentine smoke showed a weak diffraction pattern corresponding to MgO. The smoke from olivine showed a weak diffraction pattern corresponding to Fe2O3 and/or Fe3O4. The mass absorption coefficients, from 2.5 to 25 microns, of crystalline diopside, olivine, and serpentine and their corresponding smoke samples are shown in figures.

Stephens, John R.↗
NASA NTRS1990

Analysis of blended rolled edge reflectors using numerical UTD

The uniform geometrical theory of diffraction (UTD) concept is used to predict the scattered fields in the target zones of compact-range blended rolled-edge reflectors. Since the necessary diffraction coefficients are not known in a closed form, a numerical method to calculate the diffraction coefficients is described. In the numerical method, the problem is reduced to two dimensions, and physical optics line integration is used to compute the diffraction coefficients. Thus, the method is computationally efficient. The method is used to analyze two compact-range reflectors. The results obtained using the numerical UTD show good agreement with the scattered fields obtained using a corrected physical optics surface integration.

Ellingson, Steven W.↗
NASA NTRS1991

Transient scattering analysis for a circular disk

The backscattered fields of a perfectly conducting circular disk are analyzed from a transient signature viewpoint. The significant dominant scattering mechanisms are identified for both principal polarizations at a variety of angles. Particular attention is given to the edge wave. The backscattered field behavior due to an incident plane wave on a perfectly conducting disk is presented. Good agreement was obtained between the eigenfunction and geometric theory of diffraction solutions. The expected mechanisms from first-, second-, and third-order diffractions with an accurate edge wave representation are demonstrated through the use of transient signatures. The most significant error in the geometric theory of diffraction (GTD) solution occurs in time where the nonprincipal plane double diffraction term exists.

Dominek, Allen K.↗
NASA NTRS1992

High-frequency techniques for RCS prediction of plate geometries

The principal-plane scattering from perfectly conducting and coated strips and rectangular plates is examined. Previous reports have detailed Geometrical Theory of Diffraction/Uniform Theory of Diffraction (GTD/UTD) solutions for these geometries. The GTD/UTD solution for the perfectly conducting plate yields monostatic radar cross section (RCS) results that are nearly identical to measurements and results obtained using the Moment Method (MM) and the Extended Physical Theory of Diffraction (EPTD). This was demonstrated in previous reports. The previous analysis is extended to bistatic cases. GTD/UTD results for the principal-plane scattering from a perfectly conducting, infinite strip are compared to MM and EPTD data. A comprehensive overview of the advantages and disadvantages of the GTD/UTD and of the EPTD and a detailed analysis of the results from both methods are provided. Several previous reports also presented preliminary discussions and results for a GTD/UTD model of the RCS of a coated, rectangular plate. Several approximations for accounting for the finite coating thickness, plane-wave incidence, and far-field observation were discussed. Here, these approximations are replaced by a revised wedge diffraction coefficient that implicitly accounts for a coating on a perfect conductor, plane-wave incidence, and far-field observation. This coefficient is computationally more efficient than the previous diffraction coefficient because the number of Maliuzhinets functions that must be calculated using numerical integration is reduced by a factor of 2. The derivation and the revised coefficient are presented in detail for the hard polarization case. Computations and experimental data are also included. The soft polarization case is currently under investigation.

Balanis, Constantine A.↗
NASA NTRS1993

High-frequency techniques for RCS prediction of plate geometries and a physical optics/equivalent currents model for the RCS of trihedral corner reflectors

Part 1 of this report continues the investigation, initiated in previous reports, of scattering from rectangular plates coated with lossy dielectrics. The hard polarization coefficients given in the last report are incorporated into a model, which includes second- and third-order diffractions, for the coated plate. Computed results from this model are examined and compared to measured data. A breakdown of the contribution of each of the higher-order terms to the total radar cross section (RCS) is given. The effectiveness of the uniform theory of diffraction (UTD) model in accounting for the coating effect is investigated by examining a Physical Optics (PO) model which incorporates the equivalent surface impedance approximation used in the UTD model. The PO, UTD, and experimental results are compared. Part 2 of this report presents a RCS model, based on PO and the Method of Equivalent Currents (MEC), for a trihedral corner reflector. PO is used to account for the reflected fields, while MEC is used for the diffracted fields. Single, double, and triple reflections and first-order diffractions are included in the model. A detailed derivation of the E(sub theta)-polarization, monostatic RCS is included. Computed results are compared with finite-difference time-domain (FDTD) results for validation. The PO/MEC model of this report compares very well with the FDTD model, and it is a much faster model in terms of computational speed.

Balanis, Constantine A.↗
NASA NTRS1991

Variable sensitivity moire interferometry

This paper demonstrates that optical microlithography can be used to produce a crossed grating which diffracts light into multiple orders sufficient to record moire interferograms with sensitivities ranging from 2.0 to 0.285 micron/fringe. The grating profile produced by the method is analyzed to establish the diffraction efficiency in each diffraction order, and generalized expressions are given for variable sensitivity moire interferometry. Experimental tests are conducted to verify analytical arguments. In one of these tests, two different diffraction order pairs are used simultaneously to verify that surface displacement can be measured at different sensitivities.

Johnson-Cole, Helen↗
NASA NTRS1994

Structure and stability of hydrous minerals at high pressure

The presence of even small amounts of hydrogen in the Earth's deep interior may have profound effects on mantle melting, rheology, and electrical conductivity. The recent discovery of a large class of high-pressure H-bearing silicates further underscores the potentially important role for hydrous minerals in the Earth's mantle. Hydrogen may also be a significant component of the Earth's core, as has been recently documented by studies of iron hydride at high pressure. In this study, we explore the role of H in crystal structures at high pressure through detailed Raman spectroscopic and x ray diffraction studies of hydrous minerals compressed in diamond anvil cells. Brucite, Mg(OH)2, has a simple structure and serves as an analogue for the more complex hydrous silicates. Over the past five years, this material has been studied at high pressure using shock-compression, powder x ray diffraction, infrared spectroscopy, Raman spectroscopy, and neutron diffraction. In addition, we have recently carried out single-crystal synchrotron x-ray diffraction on Mg(OH)2 and Raman spectroscopy on Mg(OD)2 at elevated pressure. From all these studies, an interesting picture of the crystal chemical behavior of this material at high pressure is beginning to emerge. Some of the primary conclusions are as follows: First, hydrogen bonding is enhanced by the application of pressure. Second, layered minerals which are elastically anisotropic at low pressure may not be so at high pressure. Furthermore, the brucite data place constraints on the effect of hydrogen on seismic velocities and density at very high pressure. Third, the stability of hydrous minerals may be enhanced at high P by subtle structural rearrangements that are difficult to detect using traditional probes and require detailed spectroscopic analyses. Finally, brucite appears to be unique in that it undergoes pressure-induced disordering that is confined solely to the H-containing layers of the structure.

Duffy, T. S.↗
NASA NTRS1994

Satellite Communications Applications Research

This report describes work performed by SDL in the development of monolithically-integrated master oscillator power amplifiers under the NASA-SCAR research contract. The goals of the program were to demonstrate high power diffraction-limited operation from a semiconductor laser with the following characteristics: (1) Output power of 0.5 to 1.0 W continuous-wave (cw); (2) Single spectral mode with greater than 20 dB of side mode suppression; (3) Single spatial mode operation; (4) Modulation capability of 0.5 GHz; and (5) Operating wavelength between 830-860 nm. Cw diffraction-limited output powers as high as 2.2 W were achieved while maintaining a single longitudinal mode. Side mode suppression ratios greater than 20 dB are typically obtained. The single spatial mode output has a Strehl ratio estimated at 0.85, and a high-speed modulation capability was demonstrated with a small-signal 3-dB roll-off at 1.0 GHz. Finally, all of this performance was obtained in the 860 nm wavelength band, which corresponds to the peak sensitivity of Si detectors. During the first option period, three deliverables were fabricated and delivered that included 1 distributed Bragg reflector (DBR) laser operating at 970 nm, 1 DBR laser operating at 860 nm, and one monolithic flared amplifier master oscillator power amplifier (MFA-MOPA) providing 0.5 W cw diffraction-limited output. During the second option period, a total of 5 MFA-MOPA devices were delivered, each operating to better than 1 W cw diffraction-limited at 860 nm.

Lang, Robert J.↗