Shallow Angles Of Incidence Research Articles

Radiative transfer modelling of discontinuous tree canopies at microwave frequencies

Development of a first-order radiative transfer model for predicting backscatter from tree canopies has been underway at the University of Michigan Radiation Laboratory for some time. This model is known as the Michigan Microwave Canopy Scattering (MIMICS) model. This article presents the second-generation MIMICS model (MIMICS II) which accounts for canopies with discontinuous (open) crown layer geometries. MIMICS II models open crown layers by treating the location, size, and shape of the individual tree crowns as random variables. The backscattering coefficients for the canopy are then determined by introducing statistics derived from these parameters into the radiative transfer solution. Application of the radiative transfer equations to the discontinuous canopy geometry is presented. The resulting model is a robust fully polarimetric solution that is applicable over a wide variety of canopy architectures. Model simulations are compared to results generated with the continuous canopy model. The effect of the open crown geometry is found to be most significant at shallow incidence angles and at high frequencies for trees with well-developed crowns. Under these conditions, the gaps in the crown layer give rise to a notable increase in crown layer transmissivity which allows the radar to see through to the lower layers of the canopy more easily, thereby directly affecting the backscatter contribution of the trunks and ground.

Effect of substrate preparation on smectic liquid crystal alignment: A structural study

Octylcyanobiphenyl, a bilayer smectic A liquid crystal at room temperature, was deposited on gratings, grids, and flat surfaces, and was studied using high-resolution x-ray scattering at shallow angles of incidence. One surface of the film was in contact with air and the other was in contact with the glass or silicon substrate, which had been treated with one of two different silane solutions. At the air interface, surface tension forces caused the liquid crystal molecules to align perpendicularly with respect to the plane of the substrate. Competing with the liquid crystal–air interface, which is a strong aligner, a grating at the liquid crystal–substrate interface can produce distortions in the smectic layering which results in excess elastic energy and favors alignment parallel to the substrate and the grooves. Our results show that in films less than ∼30 μm thick, the homeotropic orientation was maintained throughout the film due to the constraint of perpendicular alignment at the air interface. However, for thicker films on gratings, Bragg scattering from molecules lying parallel to the grooves was observed. The free-air surface was thus the strongest aligning force, followed by the surface topology, while surface anchoring was not found to play a role.

Effects of neutral gas collisions on the power transmission factor at the divertor sheath

We show that charge-exchange and other ion-neutral collisions can reduce the power transmission factor of the plasma sheath, thereby lowering the ion impact energy and target plate sputtering. The power transmission factor relates the heat flux reaching the divertor target to the plasma density and temperature just in front of the surface: δ=Qsurf/Jewk Te. Experimental data from the DIII-D tokamak suggests that δ could be as low as 2–3 near the region of peak divertor particle flux, instead of the 7–8 expected from usual sheath theory. Several effects combine to allow ion-neutral interactions to be important in the divertor plasma sheath. The shallow angle of incidence of the magnetic field (1–3° in DIII-D) leads to the spatial extension of the sheath from approximately ρi∼1 mm normal to the plate [R. Chodura, Phys. Fluids 25 (1982) 1628] to several centimeters along the field lines. Ionization reduces the sheath potential, and collisions reduce the ion impact energy.

Energy deposition in iron pentacarbonyl ions undergoing surface-induced dissociation in a fourier transform mass spectrometer

Internal energy deposition into iron pentacarbonyl positive ions undergoing surface-induced dissociation (SID) in a Fourier transform mass spectrometer is estimated from the abundances and known critical energies of the product fragment ions. A narrow energy distribution, comparable to that reported in earlier BQ and tandem quadrupole SID studies of the same compound, is observed. As judged by the ratio of fragment ions to incident parent ions observed, SID of iron pentacarbonyl in the 3 T Fourier transform mass spectrometer is more efficient, but results in lower conversion of laboratory to internal energy. This may be a result of the more shallow collision incidence angle employed in the Fourier transform mass spectrometer measurements (a few degrees), which contrasts with the 32–60° collision angles used in the earlier BQ and tandem quadrupole mass spectrometry studies. Collision-induced dissociation with He under single collision conditions is also reported. Not unexpectedly, conversion of kinetic to internal energy was lower than found in a previous Fourier transform mass spectrometer study of the iron pentacarbonyl cation employing argon as collision gas under multiple collision conditions.

An in situ x-ray diffraction method for the structure of amorphous thin films using shallow angles of incidence

A technique for the structural characterization of thin amorphous films employing synchrotron radiation parallel beam x-ray optics at grazing angles of incidence is detailed. At incident angles near to the critical angle for total external reflection, sampling of specimens may be achieved via the evanescent mode. The parallel beam geometry allows the use of a technique in which a 2θ detector, incorporating a parallel plate collimator, scans diffraction data for a given incident angle. For a specified wavelength, the incident angle chosen will determine the penetration of the radiation into the sample (∼10–1000 Å). The data must be corrected for significant peak shifting resulting from x-ray refraction, as well as for the effects associated with conventional θ:2θ scans. Preliminary data resulting from the first application of this technique to amorphous hydrogenated silicon:carbon thin films, deposited onto crystalline silicon substrates, will be presented and discussed. Conventional θ:2θ powder diffraction data will also be presented as a comparative standard.

Hydrodynamic target response to an induced spatial incoherence-smoothed laser beam

One of the critical elements for high-gain target designs is the high degree of symmetry that must be maintained in the implosion process. The induced spatial incoherence (ISI) concept has some promise for reducing ablation pressure nonuniformities to ≊1%. The ISI method produces a spatial irradiance profile that undergoes large random fluctuations on picosecond time scales but is smooth on long time scales. The ability of the ISI method to produce a nearly uniform ablation pressure is contingent on both temporal smoothing and thermal diffusion. In the start-up phase of a shaped reactorlike laser pulse, the target is directly illuminated by the laser light and thermal diffusion is not effective at smoothing residual nonuniformities in the laser beam. During this period in the laser pulse, the target response is dominated by the initial shock generated by the laser pulse and the results indicate that this first shock can be the determining factor in the success or failure of the implosion process. The results of numerical simulations of several target/laser pulse designs which were investigated in an attempt to mitigate the impact of the initial shock structure stemming from the early temporal phase of an ISI-smoothed laser beam are presented. It is shown that ‘‘foamlike’’ layers, multiple laser wavelengths, and shallow angles of incidence can sharply reduce the perturbation level stemming from the first shock.

A Marine Vehicle Concept

A novel type of marine vehicle that operates above the water surface is proposed and its feasibility experimentally investigated. Viscous drag is eliminated by this above-surface operation. Lift and thrust are developed by the impact of special wheels upon the water surface, a phenomenon similar to the bouncing-off that occurs when a stone is thrown into water at a shallow angle of incidence. Some of the parameters that affect the performance of the proposed design are determined.

Molecular-dynamics simulation of low-energy beam deposition of silicon

The mechanisms controlling low-energy (10–100 eV) beam deposition of silicon onto a relaxed (111) silicon substrate have been studied using a molecular dynamics technique. The atomic interaction was modeled using a many-body empirical potential so that the effects of the covalent Si–Si bonding could be accurately included. Silicon atoms (10 eV) with near-perpendicular incidence were studied to determine the energy-loss mechanism resulting in capture and the local lattice excitation near and subsequent diffusion of excess vibrational energy away from the impact point. Shallow angles of incidence (5°–30°) were studied for beam energies of 20–100 eV. For incidence angles less than an energy- and orientation-dependent critical value, a new phenomenon of ‘‘surface channeling’’ is observed, in which the trajectory of the incoming particle is steered parallel to, and roughly 2 Å above, the surface of the substrate by inelastic interaction with the surface atoms. The phenomena observed in low energy beam deposition offer considerable promise for precision control of the growth of nonequilibrium semiconductor structures.

Techniques of Insulator/Semiconductor Heterostructure Specimen Preparation

ABSTRACTTechniques for the preparation of specimens for Transmission Electron Microscopy analysis are described. Cross-sectional specimens of insulator/semiconductor heterostructures have been successfully prepared. The problem of differential thinning rates and interface amorphization during argon ion-milling have been overcome using low argon ion accelerating voltages and shallow angles of incidence. Techniques for preparation of plan view specimens include the preparation of silicon substrates for in-situ crystal growth in an ultrahigh vacuum Transmission Electron Microscope.

Measurements of ocean surface spectrum from an aircraft using the two-frequency microwave resonance technique

The present investigation is concerned with the results of a two-frequency (Delta k) microwave radar experiment conducted from an aircraft and aimed primarily at the development of remote sensing techniques to measure ocean surface wave spectral characteristics. The experiment was conducted as part of the Maritime Remote Sensing (MARSEN) project in the North Sea during the autumn of 1979. The objective was to demonstrate the feasibility of and study the performance of the Delta k technique from a higher altitude platform, at shallower incidence angles, and at higher Doppler velocities than earlier stationary platform experiments allowed. A quantitative engineering evaluation of the results of two comprehensive flights is provided, and the qualitative significance of the results is discussed from a geophysical point of view in terms of the existing theory.

Accuracy and Optimization of the Two Polarization Technique for Obtaining the Optical Constants from Attenuated Total Reflection Measurements

The relative error of the optical constants determined by attenuated total reflectance at two polarizations and the same incidence angle is calculated as a function of the reflectance errors by a perturbation method. The variations in the error as a function of the incidence angle are then plotted for various values of the optical constants. The experimental accuracy is optimized by either shallow incidence angles and low index optics or by high index optics in the close vicinity of the critical angle. In either case, the number of reflections should be chosen to give strong attenuation. Under optimum conditions, the method gives reasonably good accuracies.