Numerical Simulation of the Ionic Composition and Ionization Phenomena in the Positive Column of a Millisecond DC-Pulsed Glow-Type Discharge in Atmospheric Pressure Air with a Water-Cathode

Abstract

A numerical investigation of a glow-type discharge in humid air with a water-cathode isreported. A complete block of chemical reactions that self-consistently describes the ioniccomposition of the plasma is considered. A water molar fraction up to 20% is examined.The electric field strength, emission discharge radius, as well as the OH (A → X) bandemission in the positive column was also measured for discharge currents up to 155 mA.The model shows a non-thermal plasma with lower gas temperatures (around 3500 K)than those typically obtained in similar discharges but operating with metal electrodes indry air. The gas temperature is almost unaffected by the discharge current. The vibrationalrelaxation through ­N2–H2O collisions is the main gas heating mechanism. The thermaldiffusion due to enhanced thermal conductivity by water vapor is the primary coolingmechanism. The electron temperature is around 1 eV to ensure that the electron losses(mainly by dissociative recombination of ­NO+) are compensated by ionization phenomena.The ­NO+ is the dominant ion, mainly formed by electron-impact ionization of NOmolecules. An electron number density close to ­1019 ­m−3 is obtained. For the upper waterfraction, the electron-impact ionization of O­ 2 molecules, followed by a quick conversionto ­NO+, also plays a role. The concentration of OH is ~ ­1022 ­m−3. A comparison betweenthe model results and the experimental data suggests that the molar fraction of water in theplasma is around 20% for the conditions considered.