Determination of charge density in an atmospheric pressure plasma jet via electric field measurements and simulations

International audience; Atmospheric pressure plasma jets have been extensively studied for several years as theyshowed very promising results in numerous fields such as material processing or plasma medicine.What makes cold plasma interesting is the presence of electrons, ions, reactive oxygen andnitrogen species, visible and UV light and high transient electric fields (EF). Even if the latter playa key role, from the production and propagation of the plasma to the efficiency of applications,they are far from being fully understood. This work focuses on the comparison of measurementsof EF produced by a Plasma Gun (PG) discharge with an electro-optic probe based on Pockelseffect [1] with 2D simulations of atmospheric pressure discharges propagating in a geometry closeto the Plasma Gun’s one. Plasma Gun consists in a vertically downward oriented capillary with aninner high-voltage electrode and an outer grounded one. Plasma is powered with μs-durationvoltage pulses from single shot to 4kHz.In [2] the electric field is evaluated by the simulations inside the capillary, in the glass andoutside. The aim of this work is to compare the radial profile of the electric field outside thecapillary between experiments and simulations to get information on the plasma inside it. Theelectro-optic probe was placed at different levels alongside the capillary and was getting far fromit. Measurements were executed every 5mm over 5cm. At the middle of the capillary a decreaseinversely proportional to the distance has been found. It reminds the shape of an EF produced byan infinite uniformly charged cylinder. 2D simulations were in a good agreement finding also thesame slope for EF. But simulations permit also to get information on the distribution of chargedensity and especially to know if the main contribution is from volume or surface charge density.Results show that the radial profile of EF outside the tube is mostly determined by the volumecharge density in positive polarity while in negative polarity the surface charge density isdominant. Simulations have provided values of average volume charge density in the order of 7.5nC/cm3 in positive polarity and of average surface charge density around 1nC/cm² in negativepolarity. Near the electrodes, a decrease inversely proportional to the square of the distance hasbeen found, showing that there is an axial position dependence. A time dependence investigationwill also be presented.X.D. acknowledges his grant funding Thermofisher Scientific INEL/Région Centre Val de Loire.References[1] G. Gaborit et al., IEEE Trans. Plasma Sci., 42 (2014).[2] P. Viegas, F. Pechereau and A. Bourdon, Plasma Source Science and Technology, in press(2018)