Influence of Strong Coupling in Atmospheric Pressure Plasmas: Disorder-Induced Heating, Transport, and Implications for PIC Simulation*

Molecular dynamics (MD) simulations are used to study how strong ion-ion correlations influence cold atmospheric pressure plasmas (CAPP). Results show that strong ion coupling significantly increases the ion temperature via disorder-induced heating (DIH). This is followed by ion-neutral temperature relaxation, effectively heating the neutral gas within a nanosecond timescale - demonstrating fast neutral gas heating. A thermodynamics-based model is shown to predict the equilibrium temperature. This model demonstrates good agreement with experimental measurements of the neutral gas temperature in CAPP discharges. Building upon this understanding, the discussion will transition into the development of a new model for ion and neutral gas diffusion. This model places emphasis on the influence of strong Coulomb coupling in ion-ion interactions, and its consequential impact on neutral gas dynamics via the increase in temperature. Three regimes are identified that underscore the importance of both ion-neutral and ion-ion interactions across diverse ionization fractions. The model was validated via MD over a wide range of ionization fractions. Finally, the discussion will critically evaluate the suitability of the commonly employed particle-in-cell (PIC) method for CAPP. It is found that the PIC method struggles to capture the physical effects of DIH due to stringent prerequisites on macroparticle weight and grid resolution. Additionally, our research unveils a novel numerical artifact, artificial correlation heating (ACH), which is connected to the effective coupling strength associated with the macroparticles weight. This newly identified numerical heating mechanism delineates an upper boundary in density or macroparticle weight for the applicability of PIC simulations.