Absorption and Scattering of Electromagnetic Waves by Microwave Streamer

Abstract

The wide utilization of microwave discharges in plasma aerodynamics experimental studies makes the interaction of mw streamers with electromagnetic waves extremely important. This paper presents new results of experimental and theoretical investigation of scattering dynamics of linearly polarized electromagnetic waves on a thin (in comparison with wavelength  ) plasma channel. The results of experiments are analyzed in the frames of integral model. Introduction A characteristic feature of a nonequilibrium high-pressure microwave discharge (,   is the transport electron collision frequency and   is the circular frequency of the field) is its spatial inhomogeneity. It is in this range of parameters that the diversity of diffusion “unsmeared” plasma configurations shows up most vividly. By now, dipole type plasma structures, which align in the field of linearly polarized electromagnetic waves and consist of thin plasma channels (plasmoids) oriented parallel to the external electric field, have been most extensively studied experimentally and theoretically (see references in [1]). One channel, several channels, or a spatially regular multiplasmoid structure is formed, depending on the discharge ignition conditions. The plasma dipole, which is the main element of a plasma structure and affects noticeably the interference pattern of the electromagnetic field in the discharge region, results from the development of a microwave streamer. The latter comprises two ionization waves traveling in two mutually opposite directions along the external electric field. The uncompensated space charges of opposite signs that emerge during the oscillatory motion of electrons relative to a stationary (in a period 1 2    T ) ion background are located symmetrically relative to the streamer center. These charges are concentrated mainly at the ionization wave fronts in regions of enhanced field and maximum plasma density gradients. In this work, we present an analytic model based on the integrated approach [1], which makes it possible to describe the electromagnetic wave scattering on a thin plasma channel. The dynamics of a signal scattered by a single plasmoid is studied experimentally in air in pressure range P = (1.÷ 1.4)×10 4 Pa. In terms of this model, we explain the peculiarities in behavior of the scattered signal, which is recorded at various distances from the plasmoid. On the basis of the experimental results, we carried out the correction of the model. 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 09 12 January 2012, Nashville, Tennessee AIAA 2012-0790 Copyright © 2012 by authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. 2 The electrodynamic model of a microwave streamer A theoretical analysis of a microwave discharge is usually performed by assuming that the changes in the Fourier components of the electric and magnetic fields, the excess space charge, current densities and the mean plasma parameters are insignificant in the period of high frequency oscillations. The microwave streamer elongates along the z axis and is a dipole whose excess unlike charges       t i t Q    exp Re located symmetrically relative to the z = 0 plane are connected by a plasma channel. The following assumptions underlie the model. (1) The charge is concentrated at the longitudinal ionization wave front in a region whose maximum linear size is much smaller than the half length l(t) of the streamer and is distributed uniformly in the head volume. (2) The current flows along a cylindrical channel of effective radius   t rch and length   t lch 2 uniformly filled with plasma with   ch l const l lch  . (3) The skin depth exceeds channel radius