In this work, Rayleigh microwave scattering was utilized to measure the electron number density produced by nanosecond high voltage breakdown in air between two electrodes in a pin-to-pin configuration (peak voltage 26 kV and pulse duration 55 ns). The peak electron density decreased from 1•10 17 cm -3 down to 7•10 14 cm -3 when increasing the gap distance from 2 to 8 mm (total electron number decreased from 2•10 13 down to 5•10 11 respectively). Electron number density decayed on the timescale of about several s due to dissociative recombination.
Transient plasma discharges can be created in different electrode geometries and the use of a coaxial electrodes can assist in initiating ignition at multiple points at the same time to create volumetric ignition. The current study investigates discharge formation in a coaxial electrode in quiescent, atmoshpheric and non-reacting conditions. This is the first systematic study to understand the behavior of such a discharge as a function of different pulse parameters like pulse width (40-110 ns), repetition frequency (1-50 kHz) and input voltage (14-20 kV). Additionally, the polarity of the central electrode was changed between positive and negative. An intensified ccd camera was used to visualize the discharge formation. The exposure of the camera is set to capture 500 discharges in a single frame. The discharges were found to behave differently for positive and negative polarity discharges. The positive polarity discharge tends to form a strong arc and spins around the outer cylinder which is confirmed using a high speed camera. The negative polarity discharges form a uniform streamer discharge for most of the pulse parameters. The current study has provided an initial understanding of the dynamics of plasma discharges in a coaxial electrode.
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