The effect of humidity on streamer propagation in long air gaps

A 2D numerical simulation of the positive streamer properties was performed in 9-12 cm plane-to-plane air gaps for various pressures and water vapor contents. It was shown that an increase in air humidity leads to hampering the streamer development and to increasing the average critical electric field required for bridging the discharge gap. The effect of humidity was most profound at atmospheric pressure and decreased with decreasing pressure. The influence of water content on the streamer properties was explained by a decrease in the streamer channel conductivity due to dissociative recombination of electrons with positive hydrated ions and enhanced three-body electron attachment to O₂ molecules. The calculated critical electric field in humid air gaps was compared with available experimental data. An increase in air humidity leads to an increase both in the rate of electron-ion recombination and in the rate of electron attachment. In humid air, the more efficient plasma decay in the streamer channel leads to a decrease in the plasma conductivity and to an increase in the voltage drop along the channel. Streamer head potential and the ionization rate in this region decrease. Consequently, the higher is air humidity, the slower is the streamer discharge development.