Distribution of gas temperature in an unconfined glow discharge plasma

An analysis is carried out to determine radial temperature distributions in the cylindrical positive column of a glow discharge formed in air in free space without confining walls. The analysis considers discharge with current densities lower than 100 mA/cm2 and at gas pressures of several tens of Torr. The plasma is represented by a set of hydrodynamic equations that include the balances for electron number density, translational energy, and the vibrational energy. The equations are solved using an iterative method to obtain gas temperatures for a range of plasma conditions. The results show that increasing discharge current densities lead to higher gas temperatures on plasma axis, however, unlike in the case with glow discharge restricted by dielectric walls, increased current densities also lead to wider radial profiles of temperature. Increased gas pressure, while leading to higher on-axis gas temperatures, results in narrower temperature profiles, mainly due to the reduced diffusion rates and vibrational-translational energy relaxation times. At low gas pressures and current densities, the electron density profiles are found to be significantly narrower than those for temperature while at higher values of these parameters, the width of the two are comparable. The characteristic radius of the predicted gas temperature distribution is in a good agreement with recent experimental findings.