NUMERICAL-SIMULATION OF NONEQUILIBRIUM EFFECTS IN AN ARGON PLASMA-JET

Departures from thermal (translational), ionization, and excitation eq uilibrium in an axisymmetric argon plasma jet have been studied by two -dimensional numerical simulations. Electrons, ions, and excited and g round states of neutral atoms are represented as separate chemical spe cies in the mixture. Transitions between excited states, as well as io nization/recombination reactions due to both collisional and radiative processes, are treated as separate chemical reactions. Resonance radi ation transport is represented using Holstein escape factors to simula te both the optically thin and optically thick limits. The optically t hin calculation showed significant underpopulation of excited species in the upstream part of the jet core, whereas in the optically thick c alculation this region remains close to local thermodynamic equilibriu m, consistent with previous experimental observations. Resonance radia tion absorption is therefore an important effect. The optically thick calculation results also show overpopulations (relative to equilibrium ) of excited species and electron densities in the fringes and downstr eam part of the jet core. In these regions, however, the electrons and ions are essentially in partial local thermodynamic equilibrium with the excited state at the electron temperature, even though the ionized and excited states are no longer in equilibrium with the ground state . Departures from partial local thermodynamic equilibrium are observed in the outer fringes and far downstream part of the jet. These result s are interpreted in terms of the local relative time scales for the v arious physical and chemical processes occurring in the plasma.