Comparison of light transport models based on finite element and the discrete ordinates methods in view of optical tomography applications

Numerical tests are used to evaluate the accuracy of two finite element formulations associated with the discrete ordinates method for solving the radiative transfer equation: the Least Square and the Discontinuous Galerkin finite element formulations. The results show that the use of a penalization method to set the Dirichlet boundary conditions leads to a more accurate solution than the weakly type setting where the Least Square method is seen to be more sensitive. Convergence in mesh size shows that, while both methods give accurate results, the Discontinuous Galerkin formulation uses five times more degrees of freedom than the Least Square formulation, which may lead to large systems to handle when the number of mesh elements is large. The comparison of both methods using the S_n and the T_n angular quadratures has shown that the Discontinuous Galerkin gives more accurate solutions, as expected, for problems with strong discontinuities, but may exhibit some oscillations due to the Galerkin procedure. A last test featuring a collimated irradiation shows that both methods give the same accuracy due to the separation of the radiative intensity into transmitted and scattered components, which removes the discontinuities in the implementation of the boundary conditions.     

Electron acceleration driven by ultrashort and nonparaxial radially polarized laser pulses

Exact closed-form solutions to Maxwell's equations are used to investigate the acceleration of electrons in vacuum driven by ultrashort and nonparaxial radially polarized laser pulses. We show that the threshold power above which significant acceleration takes place is greatly reduced by using a tighter focus. Moreover, electrons accelerated by tightly focused single-cycle laser pulses may reach around 80% of the theoretical energy gain limit, about twice the value previously reported with few-cycle paraxial pulses. Our results demonstrate that the direct acceleration of electrons in vacuum is well within reach of current laser technology.     

On the dimensioning of cellular OFDMA networks

In this paper, we address the issue of cellular OFDMA network dimensioning. Network design consists of evaluating cell coverage and capacity and may involve many parameters related to environment, system configuration, and quality of service (QoS) requirements. In order to quickly study the impact of each of these parameters, analytical formulas are needed. The key function for network dimensioning is the Signal to Interference Ratio (SIR) distribution. We thus analyze in an original way the traditional issue of deriving outage probabilities in OFDMA cellular networks. Our study takes into account the joint effect of path-loss, shadowing, and fast fading effects. Starting from the Mean Instantaneous Capacity (MIC), we derive the effective SIR distribution as a function of the number of sub-carriers per sub-channel. Our formula, based on a fluid model approach, is easily computable and can be obtained for a mobile station (MS) located at any distance from its serving base station (BS). We validate our approach by comparing all results to Monte Carlo simulations performed in a hexagonal network, and we show how our analytical study can be used to analyze outage capacity, coverage holes, and network densification. The proposed framework is a powerful tool to study performances of cellular OFDMA networks (e.g. WiMAX, LTE).     

Filament-induced breakdown remote spectroscopy in a polar environment

We demonstrate the feasibility of filament-induced breakdown spectroscopy (FIBS) for remote sensing of solid samples in a polar environment. FIBS spectra from an aluminum target induced by 800-nm laser pulses propagating in air were probed. The air visibility in an open winter field was as low as 3.2 km fluctuating with precipitation, pressure and relative humidity. Under such polar condition, clean spectral Al I lines from an aluminum target located at a distance of 60 m were obtained. This shows the technique FIBS could be potentially useful for sensing remote targets in a variety of polar environments.     

New developments in frequency domain optical tomography. Part I: Forward model and gradient computation

This two part study introduces new developments in frequency domain optical tomography to take into account the collimated source direction in the computation of both the forward and the adjoint models. The solution method is based on the least square finite element method associated to the discrete ordinates method where no empirical stabilization is needed. In this first part of the study, the solution method of the forward model is highlighted with an easy handling of complex boundary condition through a penalization method. Gradient computation from an adjoint method is developed rigorously in a continuous manner through a lagrangian formalism for the deduction of the adjoint equation and the gradient of the objective function. The proposed formulation can be easily generalized to stationary and time domain optical tomography by keeping the same expressions.     

Transformations of gamma-alumina in aqueous suspensions 1. Alumina chemical weathering studied as a function of pH

Hydration of gamma-Al2O3 is often reported to occur via the superficial transformation of the alumina surface into aluminum hydroxide-like layers. However, very little evidence has been given so far to support this hypothesis. It is demonstrated here by X-ray diffraction, TEM, electron diffraction, and solubility studies that a second process of hydration takes place that involves the dissolution of alumina and subsequent precipitation of well-shaped Al(OH)3 particles from supersaturated alumina aqueous solution. This process can be observed on a macroscopic scale (XRD, TEM) for any pH5, provided that the contact time between alumina and water exceeds 10 h. The least thermodynamically stable phase of aluminum hydroxide, bayerite, becomes favored compared with gibbsite when the pH of the solution is increased. It is assumed that the rate of formation of bayerite germs is greater than that of gibbsite due to variations in aluminum speciation in solution as a function of pH.     

Sorption of Cu(II) onto vineyard soils: macroscopic and spectroscopic investigations

The sorption of Cu on five vineyard soils was examined via macroscopic and spectroscopic investigations. The composition of the soils was previously determined using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). X-ray absorption spectroscopy (XAS) was employed to determine the metal environment with regard to the identity and interaction of the nearest atomic neighbors, the bond distances, and the coordination numbers. The five soils present similar sorption properties and there is no XAS evidence that the nature of the soil samples affects the local chemical environment of Cu(II). The kinetics of the Cu sorption reactions is rapid, with the equilibrium loading of Cu on the surface achieving approximately 200 mumol g(-1), i.e., 12.7 mg g(-1). The XAS data indicate that Cu is adsorbed in the form of inner-sphere complexes with first shell CuO parameters of four equatorial CuO bonds equal to 1.93 A and two axial CuO bonds at 2.43 A. This is in accordance with a Jahn-Teller distorted octahedron environment around copper. Our results provide evidence of the complexation of Cu(II) onto soil organic matter coated with an inorganic surface (quartz, clay, and goethite).