Abstract:
Features of the generation of a highly ionized plasma from a tip cathode after the electrical breakdown of a millimeter air discharge gap at atmospheric pressure have been studied using picosecond laser probing at a wavelength of $532$ nm. It has been found that the transformation of a micron-sized cathode spot to a growing spark channel is accompanied by the formation of a spherical plasma region in the base of the spark channel in the near-cathode region. The electron density in this region, which is about $100$$\mu$m in diameter, decreases to $3\times10^{19}$ cm$^{-3}$ in the center and increases to $(5{-}6)\times10^{19}$ cm$^{-3}$ in its shell with a thickness of about $20$$\mu$m. It has been shown that the growth of the subsequent spark channel is governed by a strong ionization front formed at the boundary of the expanding spherical plasma region in $1$ ns after the gap breakdown. It has been assumed that the formation of the spherical plasma region in the near-cathode region affects the subsequent development of the microstructure of the electric spark.
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