Two-dimensional electron density measurement of pulsed positive secondary streamer discharge in atmospheric-pressure air

Highly sensitive Talbot-type laser wavefront sensors with a temporal resolution of 5 ns were used for the single-shot visualisation of two-dimensional electron density distributions over pulsed positive secondary streamers generated in a 13 mm pin-to-plate gap installed in air under atmospheric pressure. The single-shot imaging demonstrated that the electron density in the primary streamer channel increased after passage of the secondary streamer front. The electron density inside the propagating secondary streamer showed almost uniform axial distribution and it kept approximately constant at 0.7 ^-1 × 1015 cm ^-3 for about 40 ns. This temporally stable electron density suggested that the reduced electric feld inside the secondary streamer was approximately 110-120 Td during the propagation. In the end of the secondary streamer phase, the electron density decreased at a time constant of ∼10 ns.