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

Molecular level mechanisms in microplasmas at cryogenic temperatures
Swati Swagatika Mishra and Sudeep Bhattacharjee
Department of Physics, Indian Institute of Technology, Kanpur 208016, Indiae-mail (speaker): swatis@iitk.ac.in
Microplasmas at cryogenic temperatures are known
for showing neutral gas dependent characteristics, owing
to their weakly ionized nature. An example of suchplasmas is the dielectric barrier discharge (DBD) plasma,both in the planar, as well as in the jet configuration at
cryogenic temperatures [1,2]. These plasmas show several
interesting phenomena such as self-organized patterns,
varying electron coupling parameter, and variable
discharge modes with the change in gas temperatures (Tg)[1,2]. It is understood from the literature that for Tg < 50𝑒2K, the electron coupling parameter ( Γ𝑒𝑒 = 4𝜋𝜀 𝑎 𝑘 𝑇 ,0 𝑒 𝐵 𝑒where 𝑎𝑒 is the mean inter-particle separation and 𝑇𝑒 is
the electron temperature), defined as the ratio of the meanpotential energy to the mean kinetic energy of theelectrons, increases rapidly. These observations intrigue
one to unravel the underlying physical phenomena inthese plasmas, especially the ion dynamics, which has notbeen studied in greater detail, experimentally. Further, theplanar geometry of a DBD acts as a confinement to theplasma, which may have significant effects on the plasma
dynamics, as observed for the confined neutral gases [3].
To understand these problems from a molecular
standpoint, we employ molecular dynamics simulations inthis work.
Unlike the fluid model, which is vastly used todescribe the microplasmas, this work considers thecontributions of each particle (ions and neutrals) tounderstand the dynamics and energetics of the plasma.
The effect of the electrons is only considered through theshielding mechanism, accommodated in the Yukawa
potential which guides the ionic interactions. The modelis tested for different ionization fractions, ranging from
10-2 to 10-6. The neutrals are assumed to be interacting
through the Lennard-Jones potential, whereas the ionneutral interactions are modeled by an effective potential[4].
In the first phase of the study, He gas molecules are
distributed randomly in a simulation box having
dimensions 1503 nm3 with periodic boundary conditions.
The initial velocity distribution is chosen as a Gaussian
type with the mean at the set value of Tg. Six values of Tg
are chosen such as 30, 40, 50, 70, 100 and 150 K, with theinitial gas pressure of 1 atmosphere. The gas particles arecoupled to a Nose-Hoover thermostat to maintain the setTg., thus forming an NVT ensemble with the number ofmolecules, volume and temperature being constants. Thegas is thermalized for ~ 10 ns for each value of Tg suchthat the gas attains thermal equilibrium and the system

converges. The He+ ions are then distributed randomly
with an initial velocity distribution of Gaussian typecorresponding to the value of Tg. The ions form an NVE
ensemble, with their number, volume and energy being theconstants. It is assumed that the ion temperature (Ti) ~ Tg.
Thereafter, the system is allowed to evolve and the radial
distribution function (RDF), mean squared displacement(MSD), velocity distribution and images are obtained atrequired time steps for both the ions and neutrals. Fromthe initial results, it is found that the ions show an initialheating effect for a few ns, and cool down as they interact
with the neutral molecules. To further investigate theeffect of such heating on the neutrals and the nature ofinteraction among the various species, both the neutrals
and ions are allowed to evolve in an NVE ensemble. The
thermostat coupled with the gas is turned off post
thermalization for this procedure.
To understand the effect of confinement due to theelectrodes, a confined system is chosen where the walls
act as the thermal reservoir and both the gas and the ionsare allowed to evolve in an NVE ensemble. An exampleof such a system is shown in Fig. 1. A detailed description
on this section will be found in the conference.

Fig. 1 Snapshot of the confined simulation system. Brown
colour shows the walls and blue dots show He atoms.
References[1] Y. Noma et al. Journal of Applied Physics 109, 053303(2011).
[2] S. Stauss et al. Plasma Sources Science andTechnology 22, 025021 (2013).
[3] S. S. Mishra and S. Bhattacharjee, Chemical
Thermodynamics and Thermal Analysis 7, 100073 (2022).
[4] T. S. Ramazanov, Physics of Plasmas 17, 042703(2010).