Electron density mapping of atmospheric pressure glow discharge in air using two-wavelength interferometry

Abstract

Summary form only given. Atmospheric pressure air plasmas are of interest in aeronautic applications, such as electromagnetic wave cross-section reduction. The required electron densities for this application exceed 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and may vary strongly over small spatial scales. Mapping of the electron density therefore requires diagnostic techniques that provide high spatial resolution. A laser interferometric method had been introduced by Leipold et al., where, by modulating the discharge in time it was possible to separate the effect of the electron contribution to the index of refraction from that of the heavy particles. We have used instead two-color laser interferometry to obtain the required separation of the contributions. The lasers were a 30 mW, 0.633 nm He-Ne laser and 50 W, 10.6 mum CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser, where the power has been reduced to a ap1 W. Heterodyne detection, by means of acousto-optic modulators and lock-in amplifiers at 40 MHz, was used to increase the sensitivity of the diagnostic technique. The object of our studies is a 100 mum diameter positive column of an atmospheric pressure air glow discharge. It is operated in a DC mode with sustaining voltage of 1-5 kV in a 1-5 mm gap between a needle (anode) and a plate (cathode). Discharge characteristics are very similar to those of micro-hollow cathode stabilized glow discharges previously used for the generation of stable, high-density plasmas. Consequently, the electron density is expected to vary between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The discharge has a negative differential resistance and requires therefore to be ballasted with additional resistance in series. In addition to the electron density measurements, we have used emission spectroscopy (measurements of the rotational spectrum of the N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> second positive system) to estimate the gas temperature, T, and found values for T of approximately 2000 K in the plasma