Charge dynamics of the COST plasma jet

Atmospheric pressure plasmas are of significant scientific and societal interest. Applications span environmental technologies such as CO₂ splitting, chemical processes like methane or ammonia synthesis, and biomedical treatments. The diversity of plasma sources used to generate atmospheric pressure plasmas reflects the wide range of applications. In this study, we investigate a parallel-plate atmospheric pressure plasma jet known as the COST jet. A PIC/MCC simulation is applied to model the device. In this approach, electrons are treated kinetically, while ion species are modeled by continuity equations solved under the drift-diffusion approximation. For a He/N₂ gas mixture, we demonstrate that the COST jet operates in one of three distinct regimes: non-neutral, quasi-neutral, or constricted. Each regime exhibits unique dynamics. In the non-neutral regime, the plasma lacks a quasi-neutral bulk, and a tightly confined group of electrons governs all dynamics. In the constricted regime, the discharge becomes confined to extremely narrow regions adjacent to the electrodes. Despite their differences, these regimes exhibit common features in their charge transport characteristics. In particular, our results show that negative space charge regions consistently emerge in areas with steep ion density gradients. This work aims to elucidate the charge dynamics within these regimes and highlight the fundamental mechanisms underlying space charge formation in atmospheric pressure plasma jets.