Determination of charge density in an atmospheric pressure plasma jet via electric field measurements and simulations
Résumé
Atmospheric pressure plasma jets have been extensively studied for several years as they
showed 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 and
nitrogen species, visible and UV light and high transient electric fields (EF). Even if the latter play
a 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 measurements
of EF produced by a Plasma Gun (PG) discharge with an electro-optic probe based on Pockels
effect [1] with 2D simulations of atmospheric pressure discharges propagating in a geometry close
to the Plasma Gun’s one. Plasma Gun consists in a vertically downward oriented capillary with an
inner high-voltage electrode and an outer grounded one. Plasma is powered with μs-duration
voltage pulses from single shot to 4kHz.
In [2] the electric field is evaluated by the simulations inside the capillary, in the glass and
outside. The aim of this work is to compare the radial profile of the electric field outside the
capillary between experiments and simulations to get information on the plasma inside it. The
electro-optic probe was placed at different levels alongside the capillary and was getting far from
it. Measurements were executed every 5mm over 5cm. At the middle of the capillary a decrease
inversely proportional to the distance has been found. It reminds the shape of an EF produced by
an infinite uniformly charged cylinder. 2D simulations were in a good agreement finding also the
same slope for EF. But simulations permit also to get information on the distribution of charge
density 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 volume
charge density in positive polarity while in negative polarity the surface charge density is
dominant. Simulations have provided values of average volume charge density in the order of 7.5
nC/cm3 in positive polarity and of average surface charge density around 1nC/cm² in negative
polarity. Near the electrodes, a decrease inversely proportional to the square of the distance has
been found, showing that there is an axial position dependence. A time dependence investigation
will 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)