AlGaN-based quantum-well heterostructures for deep ultraviolet light-emitting diodes grown by submonolayer discrete plasma-assisted molecular-beam epitaxy

Features of plasma-assisted molecular-beam epitaxy of AlGaN compounds at relatively low temperatures of the substrate (no higher than 740°C) and various stoichiometric conditions for growth of the nitrogen- and metal-enriched layers are studied. Discrete submonolayer epitaxy for formation of quantum wells and n-type blocking layers without varying the fluxes of components was used for the first time in the case of molecular- beam epitaxy with plasma activation of nitrogen for the nanostructures with the Al _x Ga_{1 ? x} N/Al _y Ga_{1 ? y} N quantum wells. Structural and optical properties of the Al _x Ga_{1 ? x} N layers in the entire range of compositions (x = 0–1) and nanostructures based on these layers are studied; these studies indicate that there is photoluminescence at room temperature with minimum wavelength of 230 nm. Based on the analysis of the photoluminescence spectra for bulk layers and nanoheterostructures and their temperature dependences, it is concluded that there are localized states in quantum wells. Using the metal-enriched layers grown on the c-Al₂O₃ substrates, heterostructures for light-emitting diodes with Al _x Ga_{1 ? x} N/Al _y Ga_{1 ? y} N quantum wells (x = 0.4–0.5, y = x + 0.15) were obtained and demonstrated electroluminescence in the ultraviolet region of the spectrum at the wavelength of 320 nm.     

Implementation of adaptive critic-based neurocontrollers for turbogenerators in a multimachine power system

This paper presents the design and practical hardware implementation of optimal neurocontrollers that replace the conventional automatic voltage regulator (AVR) and the turbine governor of turbogenerators on multimachine power systems. The neurocontroller design uses a powerful technique of the adaptive critic design (ACD) family called dual heuristic programming (DHP). The DHP neurocontrollers' training and testing are implemented on the Innovative Integration M67 card consisting of the TMS320C6701 processor. The measured results show that the DHP neurocontrollers are robust and their performance does not degrade unlike the conventional controllers even when a power system stabilizer (PSS) is included, for changes in system operating conditions and configurations. This paper also shows that it is possible to design and implement optimal neurocontrollers for multiple turbogenerators in real time, without having to do continually online training of the neural networks, thus avoiding risks of instability.     

Distributed Transmit Antenna Selection (DTAS) Under Performance or Energy Consumption Constraints

Motivated by the transmit antenna selection (TAS) concept, used in Multiple-Input-Multiple-Output systems, we argue for distributed transmit antenna selection (DTAS), which corresponds to a method of selecting a subset of available relays in cooperative diversity systems. Assuming amplify and forward relays, the proposed selection method represents a low-complexity tool for determining the optimum relaying set. Two optimization problems are studied: the error probability minimization subject to total energy consumption constraints, and the dual one, the total energy consumption minimization under error performance constraints. Numerical examples verify the advantage of the proposed method in adapting the number of relaying terminals to the desired performance-consumption tradeoff.     

Laminar-turbulent transition in an electromagnetically levitated droplet

During experiments on the MSL-1 (first microgravity science laboratory) mission of the space shuttle (STS-83 and STS-94, April and July 1997), a droplet of palladium-silicon alloy was electromagnetically levitated for viscosity measurements. For the nondeforming droplet, the resultant magnetohydrodynamic (MHD) flow inside the drop can be inferred from motion of impurity particulates on the surface. In the experiments, subsequent to melting, Joule heating produces a continuous reduction of viscosity of the fluid resulting in an acceleration of the flow with time. These observations indicate formation of a pair of co-rotating toroidal flow structures inside the spheroidal drop that undergo flow instabilities. As the fluid temperature rises, the amplitude of the secondary flow increases, and beyond a point, the tracers exhibit noncoherent chaotic motion signifying emergence of turbulence inside the drop. Assuming that the observed laminar-turbulent transition is shear-layer type, the internal structure of the toroidal loops is used to develop a semiempirical correlation for the onset of turbulence. Our calculations indicate that the suggested correlation is in modest agreement with the experimental data, with the transition occurring at a Reynolds number of 600.     

Measurement of target coordinates in three-dimensional bistatic forward-scattering radar

The maximum-likelihood technique is applied to determine the coordinates of moving targets in a three-dimensional bistatic forward-scattering radar. The potential accuracy of the coordinates’ determination is estimated. Simulation results are presented.     

Closed-form error analysis of the non-identical Nakagami-m relay fading channel

We present closed-form expressions for the average bit error probability (ABEP) of BPSK, QPSK and M-QAM of an amplify-and-forward average power scaling dual-hop relay transmission, over non-identical Nakagami-m fading channels, with integer values of m. Additionally, we evaluate in closed-form the ABEP under sufficiently large signal-to-noise ratio for the source-relay link, valid for arbitrary rn. Numerical and simulation results show the validity of the proposed mathematical analysis and point out the effect of the two hops unbalanced fading conditions on the error performance.     

Structural and Superconducting Property Variations with Nominal Mg Non‐Stoichiometry in MgxB2 and Its Enhancement of Upper Critical Field

By applying a combination of characterisation tools, changes in structural and superconducting properties with nominal Mg non‐stoichiometry in Mg_xB₂ are found. The non‐stoichiometry produces enhanced in‐field critical current densities (J_c's) and upper critical field / irreversibility field (H_c2/H_irr(T)) values. Upper critical fields of ～ 21 T (4.2 K) were obtained in nominal Mg‐deficient samples compared to ～ 17 T (4.2 K) for near‐stoichiometric samples.