Analysis and Processing of Spark Channel Interferograms Obtained by Picosecond Laser Interferometry

We study the formation of a spark channel in an atmospheric discharge on a nanosecond time scale using picosecond laser interferometry. We elaborate the approach to obtain reliable data on the electron density of the developing spark channel. We describe in detail the procedure for tracing and processing the interferograms and obtaining 2D-phase maps as well as the features of the electron density reconstruction. We found that the electron density of the spark channel can be as high as n e ≈ 5 · 10 19 cm − 3 . This value exceeds the number density n m ≈ 2 . 7 · 10 19 cm − 3 of atmospheric air. We demonstrate that the plasma with such extreme electron density is concentrated near the cathode and in the central core ( ≈ 20 μ m in size) of the developing spark channel. Also we show that the electron density is nonuniformly distributed along the spark channel. From the cathode surface to the top of the spark channel, the electron density drops down to n e ≈ 2 · 10 19 cm − 3 . In contrast to that, the linear electron density is approximately constant along the spark channel and reaches a value of N e ∼ 10 14 cm − 1 .