Plasma–flow coupling and metastable evolution in a cold argon plasma jet : a computational fluid dynamics study

Abstract

Cold plasma jets can be produced by the repetitive passage of an ionization wave (IW) along a jet. Plasma jets exhibit a strong plasma–flow coupling: the gas flow guides IWs along the jet center while in turn IWs perturb the flow. Plasma jet modeling is typically focused on simulating a single plasma–flow, usually with a simplified flow description. In this work, we solve the reactive compressible axisymmetric Navier–Stokes’ equations, modeling the flow with high fidelity and over multiple IWs repetitions. Our model includes a detailed Ar–N2–O2 kinetic scheme and possible local non-equilibrium (non-LTE) between electron and heavy species temperatures. We model a co-axial argon plasma jet with a shielding co-flow of different N2–O2 gas mixtures, created by high-frequency (20 kHz) voltage pulses (4 kV). These discharges are modeled by periodically (50 μ s) depositing pulses of electron energy (∼ 41 μ J over 4 ns). Each pulse creates a mild but fast gas heating, and an associated pressure increase, which perturbs the flow. The process leads to changes in the velocity profiles that compound over multiple periods. At the end of each period, the electron density is higher at the argon-shielding gas boundary and increases for higher O2 fraction shielding gas mixtures. This behavior stems from lower electron losses in electron/O2+ plasmas due to ambipolar-like transport. The rise and fall times of Ar(3P2) metastable compare quantitatively well with experiments, showcasing shorter lifetimes for higher O2 fractions in the shielding gas. Long-lived species are convected along the jet, with their densities building up over multiple periods.