Features of the Structure of Spark Channels in a Near-Cathode Region

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 μm in diameter, decreases to 3 × 1019 cm–3 in the center and increases to (5–6) × 1019 cm–3 in its shell with a thickness of about 20 μm. 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.