Strong density gradients in postdischarges in argon and air

High current sparks are created in argon and air using a high-voltage pulse of 16-ns duration. Shadowgraphy is applied to image density fluctuations. Shock waves become visible $sim$ 100 ns after the pulse; they expand with a velocity somewhat higher than the speed of sound. The filamentary core of the discharge shows heating that diffuses outward slowly and develops into microturbulences. They remain visible up to 3 ms. In air, the current remains lower than that in argon, and shocks only emerge from the electrodes. No indication of absorption is found; probably gas heating is observed near the electrodes.