George Michael Cotzas

Efficient radiation production in a weakly ionized, low-pressure, nonequilibrium gallium-iodide positive column discharge plasma

Electric-discharge plasmas in gallium-iodide vapours are experimentally found to convert 40% of input electric power into ultraviolet and visible radiation (200–800 nm). The conditions are a weakly ionized positive column consisting of 5–10 Torr argon, and the gallium-iodide vapour is formed by heating condensed gallium-iodide to 100–120 °C. The input power density is 50–100 mW cm−3. The plasma is contained in a sealed silica tube and excited by an external radiofrequency antenna. Computational analysis and plasma diagnostics lead to a quantitative understanding that gallium atoms are formed by electron-impact dissociation of gallium-iodide compounds that evaporate into the plasma volume, and that further electron collisions excite the gallium atoms, which then decay by photon emission. High efficiency is possible only because several photons are emitted per dissociation event, and because nonradiative power channels such as electron-impact elastic heating and vibrational excitation are not dominant. The dissociated species recombine on the wall to reform the species that evaporates. The plasma properties change discontinuously as the molar ratio of iodine to gallium (I/Ga) in the system crosses the values I/Ga = 3 and I/Ga = 2, consistent with the thermodynamic properties of condensed gallium-iodide compounds.

First‐Principles‐Based Development of Kinetic Mechanisms in Chemically Active Light‐Emitting Nonthermal Plasmas and Gases

Recent progress in several related research areas such as first‐principles electronic‐structure calculations of atoms and diatomic molecules, theory of elementary processes, kinetics, and numerical engineering, and also continued exponential growth in computational resources enhanced by recent advances in massively parallel computing have opened the possibility of directly designing kinetics mechanisms to describe chemical processes and light emission in such complex media as nonequilibrium plasmas and reacting gases. It is important that plasma and combustion kinetics can be described in the framework of this direct approach to a sufficiently high accuracy, which makes it an independent predictive research tool complementary to experimental techniques. This paper demonstrates the capabilities of the first‐principles based approach to develop kinetic mechanisms. Two examples are discussed in detail: (1) the mechanism of hydrocarbon fuel combustion at high temperatures and (2) light emission in non‐thermal...