A non-neutral discharge regime of atmospheric pressure RF plasma jets: Simulation and modeling

During the last years, atmospheric pressure plasmas have re-established themselves as a research focus. Radio-frequency driven atmospheric pressure plasma jets (here at the explicit example of the COST-Jet) are commonly used plasma sources for these studies. A characteristic feature of these atmospheric pressure plasma sources is the ratio between the Debye length $\lambda_D$ (here about $10^1$ to $10^2\,$mm) and the length of the discharge gap $L_\mathrm{gap}$ (about 1 mm). Compared to low-pressure applications, this ratio is rather high. Atmospheric pressure plasmas consequently do not necessarily form a quasi-neutral bulk region. This work discovered such a non-neutral discharge regime for the COST-Jet geometry. The findings are structured in two parts. First, characteristics of this non-neutral regime are unraveled in terms of hybrid particle-in-cell/Monte Carlo collisions simulations. The simulation treats the electrons fully kinetically by invoking the PIC/MCC algorithm while sparing computational effort by treating the ions as fluids. It is found that the electrons are organized in a drift-soliton-like structure. The dynamics of this soliton-like structure determine the electron dynamics of the system. The second part formulates a simplified analytical model to describe the electron dynamics. In terms of this model, the formation of the soliton-like structure and the connection between the soliton and the electron dynamics are investigated. The discussion of the analytical model lays an emphasis on notions about the stability of the electron group. This stability analysis includes the application of the Lyapunov method and a linear stability analysis. To validate the conclusions drawn from the model, a comparison between numerical simulation and model is performed. It reveals that a non-sophisticated model can capture essential characteristics of a non-neutral discharge regime.