Particle-in-cell and global simulations ofαtoγtransition in atmospheric pressure Penning-dominated capacitive discharges

Abstract

Atmospheric pressure radio-frequency (rf) capacitive micro-discharges are of interest due to emerging applications, especially in the bio-medical field. A previous global model did not consider high-power phenomena such as sheath multiplication, thus limiting its applicability to the lower power range. To overcome this, we use one-dimensional particle-in-cell (PIC) simulations of atmospheric He/0.1%N 2 capacitive discharges over a wide range of currents and frequencies to guide the development of a more general global model which is also valid at higher powers. The new model includes sheath multiplication and two classes of electrons: the higher temperature 'hot' electrons associated with the sheaths, and the cooler 'warm' electrons associated with the bulk. The electric field and the electron power balance are solved analytically to determine the time-varying hot and warm temperatures and the effective rate coefficients. The particle balance equations are integrated numerically to determine the species densities. The model and PIC results are compared, showing reasonable agreement over the range of currents and frequencies studied. They indicate a transition from an α mode at low power characterized by relatively high electron temperature T e with a near uniform profile to a γ mode at high power with a T e profile strongly depressed in the bulk plasma. The transition is accompanied by an increase in density and a decrease in sheath widths. The current and frequency scalings of the model are confirmed by the PIC simulations.

Concepts, Capabilities, and Limitations of Global Models: A Review

“…These tools and other approaches can also help to reduce full reactions schemes to a set of the most significant reactions by disregarding those reactions which fall below a certain threshold . This can then subsequently be used in more complex models where an overly large number of reactions would be of detriment to the execution time or some other aspect of the model . These reduced reaction schemes should however be used with caution, as the dominant pathways can depend on the precise conditions under which the plasma is operated.…”

“…To model such variations fully self‐consistently generally requires higher order models inclusive of the solution of Poisson's equation in time and space, and in certain cases the inclusion of kinetic approaches to describe collision‐less or non‐local heating . To include time variations in the EEDF in global models without significantly increasing the computational time or the model complexity, it is possible to approximate such variations using analytical expressions, simplified numerical schemes which do not explicitly deal with spatial variations, or inputs from higher order models . Here, two examples will be discussed where this kind of approach is necessary in order to properly describe the discharge dynamics.…”

Numerical simulation of the effect of water admixtures on the evolution of a helium/dry air discharge

“…[6]. In such discharges the presence of water impurities is unavoidable and has been shown to highly affect the plasma chemistry and dynamic evolution [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In practice, water impurities are due to the operation of the plasma in the ambient humid air and also due to the plasma interaction with biotic surfaces in wet and moist environments.…”

On the role of secondary electron emission in capacitively coupled radio‐frequency plasma sheath: A theoretical ground

“…In low pressure regime, one can imagine that electron energy is slightly reduced due to inelastic collisions in excitation. At higher pressure like atmospheric pressure, CCP can become Penning dominated . In this regard, Penning ionization due to metastable atoms is crucial and excitation becomes more influential than in low‐pressure regime.…”