Experimental study and theoretical analysis of photoelectric characteristics of AlxGa₁₋xAs–p-GaAs–n-GaAs-based photoconverters with relief interfaces

We studied experimentally the photoelectric characteristics of the AlxGa₁₋xAs–p-GaAs–n-GaAs structures with relief interfaces. A theoretical analysis of spectral dependences of internal quantum efficiency of short-circuit current in the above solar cells (SC) was performed. In particular, the low-energy spectral region (where absorption is weak) was considered. A comparison was made between the experimental and theoretical photocurrent spectral curves. From it, we determined a number of parameters of the AlxGa1–xAs and GaAs р-layers, as well as of the n-GaAs layers. Some recommendations concerning the ways to increase photocurrent and extend photosensitivity spectral region were developed for technologists. A theoretical analysis of a “spotty” model for open-circuit voltage formation in relief AlxGa₁₋xAs–p-GaAs–n-GaAs-based SC was made. This model enables one to give a qualitative explanation for decrease of open-circuit voltage in relief SC as compared to the case of flat interface

Characterization of nanoscaled films on flat and grating substrates as some elements of plasmonics

The optical properties of multilayer structures consisting of dielectric, conductivity-oxide and nanoscaled metal layers, deposited on the planar substrates (witness samples) and surface relief ones (diffraction gratings) with micro- and nanoscale sizes, are investigated by AFM, spectral ellipsometry (SE), and photometric techniques. The SE-measured parameters are related to actual characteristics of the layers when specified the model of their near-surface regions. Using a parametrization of the layer dielectric function versus the wavelength and a fitting procedure, the dielectric parameters are determined. It is shown that the optical constants are affected by both the substrate morphology and the adjacent medium. Preliminary data about the influence of isolated particle plasmon excitations in 2D-substrates with the top nanoscaled Au layer on its optical properties are presented

First and second order optimality conditions for optimal control problems of state constrained integral equations

This paper deals with optimal control problems of integral equations, with initial-final and running state constraints. The order of a running state constraint is defined in the setting of integral dynamics, and we work here with constraints of arbitrary high orders. First and second-order necessary conditions of optimality are obtained, as well as second-order sufficient conditions

A shooting algorithm for problems with singular arcs

In this article we propose a shooting algorithm for a class of optimal control problems for which all control variables appear linearly. The shooting system has, in the general case, more equations than unknowns and the Gauss-Newton method is used to compute a zero of the shooting function. This shooting algorithm is locally quadratically convergent if the derivative of the shooting function is one-to-one at the solution. The main result of this paper is to show that the latter holds whenever a sufficient condition for weak optimality is satisfied. We note that this condition is very close to a second order necessary condition. For the case when the shooting system can be reduced to one having the same number of unknowns and equations (square system) we prove that the mentioned sufficient condition guarantees the stability of the optimal solution under small perturbations and the invertibility of the Jacobian matrix of the shooting function associated to the perturbed problem. We present numerical tests that validate our method.Comment: No. RR-7763 (2011); Journal of Optimization, Theory and Applications, published as 'Online first', January 201

Kinetic Turbulence

The weak collisionality typical of turbulence in many diffuse astrophysical plasmas invalidates an MHD description of the turbulent dynamics, motivating the development of a more comprehensive theory of kinetic turbulence. In particular, a kinetic approach is essential for the investigation of the physical mechanisms responsible for the dissipation of astrophysical turbulence and the resulting heating of the plasma. This chapter reviews the limitations of MHD turbulence theory and explains how kinetic considerations may be incorporated to obtain a kinetic theory for astrophysical plasma turbulence. Key questions about the nature of kinetic turbulence that drive current research efforts are identified. A comprehensive model of the kinetic turbulent cascade is presented, with a detailed discussion of each component of the model and a review of supporting and conflicting theoretical, numerical, and observational evidence.Comment: 31 pages, 3 figures, 99 references, Chapter 6 in A. Lazarian et al. (eds.), Magnetic Fields in Diffuse Media, Astrophysics and Space Science Library 407, Springer-Verlag Berlin Heidelberg (2015

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares, inspired by RHESSI observations. The individual articles are to appear in Space Science Reviews (2011

Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics

We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions

Quadratic order conditions of a local minimum for singular extremals in a general optimal control problem

Abstract: Proceedings of AMS, "Differential Geometry and Control"

Investigation of electrical c haracteristics of heteroepitaxial structures as a function of microrelief and manufacturing technology features

Over a wide temperature range (77…400 K), we studied I-V curves of photoelectric converter (solar cell) prototypes made on the basis of p-AlxGa₁-xAs– p-GaAs–n-GaAs–n⁺-GaAs heteroepitaxial structures grown on smooth or microrelief n⁺-GaAs substrates using the standard liquid phase epitaxy (LPE) and the capillary LPE from a confined volume, as well as the effect of some external factors (thermal treatment and 60Со γ-irradiation) on the current flow mechanisms. At temperatures ≤ 200 K, the tunnel component of the forward current was predominant in all the smooth samples studied up to the voltages close to 1 V, while at room temperatures all three components (diffusion, recombination and tunnel) were of the same order. This is evidenced, in particular, by the dependencys of the effective ideality factor on the applied voltage. Predominance of the tunnel current component in a wide temperature range at small biases was observed for all the solar cells obtained on the textured n⁺-GaAs substrates. In this case, an additional factor favoring the increase of the tunnel current component was relief irregularities with small curvature radii

Necessary extremum conditions without a priori normality assumptions

In a linear space X, the following extremal problem is considered: f(x)tomin,; F_1(x)le 0,; F_2(x)=0,; xin C, where C is a closed set, the mappings F_1, F_2 have finite-dimensional images, and f, F_1, F_2 are twice smooth in any finite-dimensional subspace of X (i.e., with respect to the finite topology). No regularity assumptions are imposed on these mappings. The set C is approximated by using the cone of Mordukhovich. Second order necessary conditions for the local minimality in the finite topology are obtained