Hybrid Global Model Simulations of He/N$_2$ and He/H$_2$O Atmospheric Pressure Capacitive Discharges*

We used 1D particle-in-cell (PIC) simulations of an atmospheric He/0.1\%N$_2$ discharge with simplified chemistry to guide the development of a hybrid analytical/numerical global model that includes electron multiplication and two classes of electrons: ``hot'' electrons associated with the sheaths, and ``warm'' electrons associated with the bulk. The model and PIC results show reasonable agreement and indicate a transition from a low power $\alpha$-mode with a relatively high bulk electron temperature T$_e$ to a high power $\gamma$-mode with a low T$_e$. The transition is accompanied by an increase in density and a decrease in sheath widths. Water is a trace gas of bio-medical interest since it may arise from contact with skin. We use the hybrid global model to simulate a chemically complex, bounded He/H$_2$O atmospheric pressure discharge, including 148 volume reactions among 43 species, and including clusters up to H$_{19}$O$_9^+$. For a planar discharge with a 1 cm electrode radius and a 0.5 mm gap driven at 13.56 MHz, we determine the depletion and diffusion effects and the $\alpha$ to $\gamma$ transition for secondary emission $\gamma_{se}$=0.25 over a range of rf currents and external H$_2$O concentrations. Each simulation takes about 2 minutes on a moderate laptop.