Application of holographic laser scanning to determine the electron concentration in the plasma forming the apokamp

Streamer discharges that do not transition to a spark channel are now being widely investigated. One of these discharges is the apokamp discharge, in which streamers start from a diffuse spark channel having a curved shape at a high repetition rate of voltage pulse. In this work, to estimate the electron concentration in the plasma forming the apokamp a digital holographic laser scanning method is applied for the first time. The method is based on a comparison of the phases of two optical wavefronts, registered at different time instants in the form of digital holograms. The result of the phase comparison between the wavefronts is presented in the form of a numerically calculated map of the phase difference of the reconstructed wavefronts. A gas-discharge plasma is a phase (transparent) object, and the interference fringes are formed as a result of the change in the refractive index introduced by the plasma with respect to the original unperturbed medium. The obtained value of the refractive index allows estimation of the concentration of electrons in the spark channel plasma. It is shown that at as the voltage pulse repetition rate increases from 5 to 50 kHz the concentration of electrons in the plasma forming the apokamp decreases by an estimated four times.