Investigation of a Near-Electrode Plasma Formed in the Atmospheric Discharge with Employment of Picosecond Laser Probing

We study the formation of an erosive plasma at the instant of the electrical breakdown of an air gap with a pin-to-plan electrode geometry employing the methods of picosecond laser probing. The discharge is ignited by a 25 kV pulse with a rise time of 4 ns. Interferograms, shadowgrams, and schlieren images of the discharge are obtained with a spatial resolution as high as 3 μm and an exposure time of 70 ps. We demonstrate that ∼1 ns after a sharp rise (∼100 A/ns) of the discharge current, a highly ionized near-cathode plasma with an electron density of n_e∼ 10¹⁹–10²⁰ cm⁻³ is formed at the cathode surface together with the clots of a dense plasma with an electron density of $n_e\underset{\sim }{>}{10}^{20}$ cm⁻³. We show that these highly ionized regions lead to the initiation and development of a spark channel originating from the point cathode. We assume that the observed dense plasma corresponds to the erosive plasma explosively formed from a part of the cathode material.