Ionic Heating of N2 and O2 Gas Discharges

We explore the heating mechanisms of N2 and O2 neutral gas in discharges using 0D and 1D PIC-DSMC simulations for cases where either a constant or self-consistently determined electric field is applied. Previously, heating on nanosecond timescales has been attributed to quenching of excited states of neutral atoms (e.g. O(1D) + O₂ -> O + O₂) that converts energy in electronic states into kinetic energy of the neutral gas. While this is generally true for weakly ionized discharges on short timescales, another process that contributes at larger values of E/N and gas fractional ionization is direct heating by ions in elastic collisions with the background gas. These simulations demonstrate that in cases where the fractional ionization of the gas is large, the heating from ion-neutral collisions dominates over other mechanisms. Even for relatively weakly ionized discharges (~1% ionization), the energy input into the gas can be considerable on the time scale of 100s to 1000s of nanoseconds.