Temperature and density effects on the properties of a long positive streamer in air

The 1.5D simulation is used to study the positive-streamer properties in air in a 20 cm sphere - plane gap versus gas temperature and density. It is shown that an increase in temperature up to 900 K at atmospheric pressure and a decrease in density by a factor of three at room temperature strongly affect the average electric field required for bridging the gap and the charge transferred through the streamer to the cathode; the temperature effect is much more pronounced than is the density effect. The calculated results qualitatively conform to available experimental data. The density effect is associated with a decrease in the rates of three-body processes such as three-body electron attachment to or conversion of ions into ions. The temperature effect is due to the decomposition of positive cluster ions which decreases the rate of dissociative electron - ion recombination. Electron attachment and detachment processes become important only when the electron density greatly decreases (by a factor of ten and more).