Mechanisms responsible for the initiation of a fast breakdown in an atmospheric discharge

The initial stage of the fast electrical breakdown of an air gap with a pin-to-plane electrode geometry is studied on a nanosecond time scale using multi-frame laser probing with an exposure time of 70 ps and spatial resolution as high as 3-4 m. We find that the gap breakdown is associated with the fast ( 1 ns) formation of a micron-sized (∼10 m) cathode spot that appears as a plasma with an electron density of n e 10 19 cm − 3 . The spot is then transformed into a spark channel having an electron density of n e ∼ 10 19 - 10 20 cm − 3 . Within ∼1 ns after the breakdown the dynamics of the highly ionized near-cathode plasma governs the current capacity of the discharge gap as well as the current rise rate.