Coupling Strength in Atmospheric Pressure Plasmas*

A wide variety of interaction types are present in atmospheric pressure plasmas: short-range (neutral-neutral), long-range Coulomb (charge-charge), and intermediate-range (charge-neutral). In modeling such discharges, all types of interactions are usually modeled via a Boltzmann equation. This assumes that all interactions are weakly coupled in the sense that the kinetic energy greatly exceeds the potential energy. However, atmospheric and elevated pressure plasmas can reach strong coupling conditions where this assumption breaks down. Quantifying the coupling strength in a system with multiple types of interactions is challenging because screening from one type influences the strength of intraparticle interactions of another type. Here, we use molecular dynamics simulations to compute the radial distribution functions for each type of interaction over a wide range of ionization fraction at atmospheric and elevated pressures. The coupling strength of each type of interaction is quantified based on properties of the radial distribution function. The results provide a parameter space map showing where the different types of interactions are strongly coupled; motivating the need for a generalized kinetic theory to treat reaction rates and transport kinetics in these regimes.