Gas density in a pulsed positive streamer measured using laser shadowgraph

Gas density in a pulsed positive streamer is quantitatively measured using a laser shadowgraph. The discharge occurs in a point–plane gap of 13 mm in humid air. After the onset of the streamer, the gas density decreases in two steps. The first step is a rapid decrease in gas density, which is caused by gas heating owing to electron impact onto molecules. This step is significant within 1 mm from the anode, although it occurs throughout the gap. The second step is a gradual decrease in gas density after the streamer pulse, which is caused by gas heating due to vibration-to-translation energy transfer. This step takes place mainly in the secondary streamer channel, not in the primary streamer channel. When the discharge voltage is 32 kV, the decrease in gas density reaches 30% of the ambient density. This large decrease in gas density affects the rate of various chemical reactions in the postdischarge period.