On the use of atmospheric pressure plasmas as electromagnetic reflectors and absorbers

Tenuous man-made plasmas in the Earth´s atmosphere from sea level to 100 km are discussed. An ionization source generates a tenuous plasma with an electron number density n_e that is high near the source and diminishes with distance from the source. After the source shuts off, n_e decreases as a function of time as electrons recombine with positive ions or attach to negative ions. The electromagnetic properties that are essential to an understanding of these plasmas, which can be modeled as cold collisional plasmas, is discussed. Gas and plasma characteristics, such as momentum-transfer collision rate, plasma lifetime, recombination kinetics, and the effect of noble gases, are presented. Typical collision rates and plasma lifetimes at atmospheric pressure are quantified. Applications for a plasma with a gradient are discussed. They include a high-altitude plasma that can reflect or absorb from HF to VHF and a broadband atmospheric pressure absorber. The generation and use of plasma, including electron impact ionization with a high-energy electron-beam source and UV photoionization of an alkali vapor or an organic vapor such as tetrakis (dimethylamino)ethylene (TMAE), is described. The power required to sustain a plasma is quantified, and properties such as maximum absorption and bandwidth are discussed. Tradeoffs among maximum absorption, absorption bandwidth, duty ratio, and applied power permit optimization of absorption primarily at VHF