Schlieren and interferometric study of a laser triggered air spark in the nanosecond regime

The growth of the plasma column and the electron density in an atmospheric air spark are studied by photographs. A 1-ns nitrogen laser provides the light source and the trigger. The relative timing is controlled by an adjustable light path length. It was found that the spark bridges the 0.5-mm gap during the first 5 ns. The plasma column diameter then grew linearly with time for 25 ns at a speed of 10.5 km s−1. The electron density in the ionized channel is found from interferograms; it reached a maximum value of 6.5×1019 cm−3 at 8 ns.