The Physics and Plasma Chemistry of an RF Needle in a Saltwater Aerosol

Abstract

Experiments involving the insertion of a tungsten needle powered by an ~1-kW radio-frequency (RF) generator into a flowing saltwater mist produced by a nebulizer are described. The tip of the tungsten electrode is estimated to have a temperature near the 5555-°C vaporization temperature of tungsten. Values of E/N close to the electrode tip are estimated to be greater than the breakdown threshold of humid air. This arrangement of RF-powered electrode and flowing saltwater aerosol produces a flame of some 30 cm high having strong light emission near the 589-nm sodium-D lines. We present a model demonstrating that the emission trapped in the core along the length of the flame arises primarily from Na(3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> P → 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> S) with some Na <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> * emission. The rising flame cools slowly due to a combination of the opacity of the hot gas, the large heat capacity of the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O molecules in the flame, and the exothermicity of the plasma chemistry in the afterglow.