7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Spatio-temporal behavior of electric field fluctuations in a cold microplasma jet:modeling and experiments
Deepika Behmani and Sudeep Bhattacharjee
Department of physics, Indian Institute of Technology, Kanpur, Uttar Pradesh: 208016, Indiae-mail (speaker): deepika@iitk.ac.in
Atmospheric pressure plasmas jets (APPJ) have beenstudied intensively because of their potential use in awide variety of applications ranging from material
processing to biomedical. They have drawn significantattention, primarily due to presence of reactive oxygenand nitrogen species (RONS) and capability to transport
these species to remote locations for localized surface
treatment, particularly in case of biological matter [1].
The concentration of RONS i.e., plasma dosage plays acrucial role in surface treatment, therefore any factor
which influences the RONS concentrations needs to beinvestigated. Recently, it has been reported that theexperimentally obtained time scale (~0.1-1 ms) of
plasma potential fluctuations [2] matches with the timescale (~0.5-1 ms) of RONS evolution [3]. Therefore, thefluctuations can affect the RONS generation and their
spatio-temporal evolution. Keeping the criticality of theRONS in mind, a detailed theoretical and experimental
study has been carried out, to understand the fluctuating
behavior of plasma jet and its effect on RONS.
Helium plasma jet having a ring-to-ring electrodeconfiguration has been used in this work [2]. The plasmais created inside a tapered quartz capillary tube. Afterignition, the plasma comes out from the orifice of thecapillary as a fine jet of diameter ~0.8-0.9 mm, as shownin Fig.1 (a). A double-pin probe has been used toestimate the electric field fluctuation and a spatial
mapping of the fluctuations has been done
experimentally along the axial and poloidal direction of
the plasma jet.

using finite difference method (FDM). The governing
equations are given below,
𝛻 2𝜑 = −𝜕𝑛𝑝,𝑒𝜕𝑡𝜕(𝑛𝜖 )
𝜕𝑡𝑒 (𝑛𝑝 −𝑛𝑒 )𝜀0(1)+ 𝛻. 𝛤𝑝,𝑒 = 𝑆𝑝,𝑒(2)+ 𝛻. 𝛤𝜖 = −𝑒𝛤𝑒 . 𝐸 − 𝑆′𝑒𝑙. − 𝑆′𝑖𝑛𝑒𝑙.

(3)Here, φ is potential, n is number density, Γ is the flux
and S is the source term. The subscript p, e and ϵ standfor the positively charged species, electron and energy,respectively. Three terms on the right-hand side of Eq.
(3) represents the Joule heating, elastic and inelastic
collisional energy exchange. Eq. (2) and Eq. (3) areconstrained by zero gradient condition at all theboundaries of plasma zone except the plasma-dielectricintersection wall (FB) (cf. Fig. 1 (b)), where solid
surface flux condition is applied. Poisson equation issolved by considering 𝑉 = 0 and 𝑉 = 𝑉𝐴𝑝𝑝 (appliedvoltage) condition at electrode boundaries JA and HI,respectively. Furthermore, the charge accumulations
condition has been imposed at solid wall FB, rest of theboundaries are treated with zero gradient condition. Thismodel will provide the spatio-temporal profile of plasma
potential, and from this, the electric field fluctuation hasbeen aimed at being estimated theoretically to verify
already obtained experimental results. Fig.2 shows theexperimentally obtained degree of fluctuation in EZ
component of electric field, defined as Ezm/<EZ>. The
degree of fluctuations tends to increase axially
downward from the orifice of capillary tube under all theflow regime (1 l/min, 2.5 l/min and 5 l/min). Similar
trend is expected from the theoretical model underdevelopment.

Fig.1 Schematic diagram of (a) experimental set up and (b)computational domain of atmospheric pressure plasma jet

A two-dimensional, axisymmetric, plasma dynamics
model has been developed for the aforementioned
experimental configuration. The computational domainof the model with a description of boundaries ispresented in Fig. 1 (b). In this, negatively charged (e-),
positively charged (He+, He2+, N2+), and neutral (He*)
species are considered. The model solves the Poisson
equation (Eq. (1)), continuity equation (Eq. (2)) for all
species and electron energy transport equation (Eq. (3))

Fig.2 Axial variation of degree of fluctuation in EZ
component of electric field under different flow regimes

References[1] L. Lin and M. Keidar, Appl. Phys. Rev. 8 011306 (2021)[2] D. Behmani, K. Barman, and S. Bhattacharjee AIP Adv.
11 085128 (2021)[3] W. V. Gaens and A. Bogaerts Journal of Physics D:
Applied Physics 46 275201 (2013)