The growth of the shock front in an atmospheric air spark is studied using interferometry. A pair of 1 ns nitrogen laser is used for triggering the spark and probing it. An adjustable light path length controls the relative timing. It was found that the shocked shell region of the plasma column is visible about 14 ns after spark initiation. The unionized gas density inside the shocked shell grew with time and at 33 ns reached a value of 6×1022cm−3.

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