Influence of Strong Coulomb Coupling on Diffusion in Atmospheric Pressure Plasmas

A new model for ion diffusion in atmospheric pressure plasmas is developed and tested using molecular dynamics (MD) simulations. Particular attention is paid to the fact that ion-ion interactions can be influenced by strong Coulomb coupling. Three regimes are identified. At low ionization fractions (x_i < 10^-6), standard weakly correlated ion-neutral interactions set the diffusion rate. At moderate ionization fractions (10^-6 < x_i < 10^-2) there is a transition from ion-neutral to ion-ion collisions setting the diffusion rate. In this regime, the effect of strong Coulomb coupling in ion-ion collisions is accounted for by applying the mean force kinetic theory. At high ionization fractions (x_i > 10^-2), strongly correlated ion-ion collisions dominate and the plasma is heated substantially by a disorder-induced heating process associated with strong correlations. In addition to ion diffusion, disorder-induced heating affects the neutral gas temperature, therefore influencing the neutral diffusion rate. Model predictions are tested using MD, which included a Monte Carlo collision routine to simulate the effect of ion-neutral collisions at the lowest ionization fractions. The model and simulations show good agreement over a broad range of ionization fractions. The results provide a tested model that can be applied to understand diffusion of ion and neutral species in a number of atmospheric pressure plasma applications.