Slow Gas Heating Process in an Atmospheric- Pressure Streamer Discharge Visualized by High-Speed Schlieren Method

Temporal variations in the gas density gradient occurring in an atmospheric-pressure streamer discharge are visualized by the schlieren method to investigate the effects of the background gas species on the temporal variations of the gas heating process after the discharge. The streamer discharge is generated in needle-to-plane electrode configurations in dry-nitrogen, humid-nitrogen, dry-air, and humid-air conditions. The discharge characteristics are measured by the voltage and current waveforms and the spatial distribution of the discharge emission of light. The results of the schlieren visualization show that the gas density gradient increases after the discharge and its rate of increase is accelerated by the presence of water molecules. In addition, the degree of the increase in the density gradient is higher in humid-nitrogen discharges than in humid-air discharges. These characteristics are subsequently explained by the vibrational to the translational energy-transfer process of nitrogen, oxygen, and water molecules.