Supersonically expanding stationary thermal plasma, formed by a thermal cascaded arc is studied. Due to the low chamber pressure (20-100 Pa) the results of continuum mechanics model can be doubtful. This is why these results are validated against kinetic Monte Carlo simulation and experimental data obtained by means of laser induced fluorescence. The analysis proves that continuum mechanics is still applicable for the velocity and temperature field predictions downstream of the shock region. However, the shock formation and some non-equilibrium effects typical for supersonic flow can be correctly studied only with the help of kinetic simulations. We show that the errors in the results using continuum mechanics can be attributed to the presence of flow gradients. These errors diminish when the shock regions are thickened due to rarefaction, viscosity and heat conductivity. Besides, both methods show that the effect of the chamber geometry on the plasma flow field is important.

Mathematical modelling and optical emission spectroscopy are applied to study the effect of the chamber pressure on the structure and properties of supersonic plasma jets formed by a direct current arc. In this installation the plasma is created inside the nozzle where the flow is accelerated. As a result some deviation from thermal and ionization equilibrium can be found, even at the working chamber inlet. In this paper, by means of a two-temperature model, we study the argon jet flow using the data of the emission spectroscopy measurements to make realistic assumptions about the inlet boundary conditions. The results show that, when the chamber pressure is low, a strongly underexpanded jet with a Mach disc is formed. For the higher ambient pressure values, the core region of the jet changes to a mildly underexpanded structure with alternating oblique expansion and compression zones. The predicted shock zone positions are in a very good agreement with measurement. The general analysis shows that the deviation from local thermodynamic equilibrium in the jet is inversely related to the chamber pressure. Along the jet core the deviation from thermal equilibrium is less in the shock regions than in the expansion zones, where the electrons are heated by three-particle recombination. Downstream of the jet core the velocity drops, but the ionization and thermal equilibria are not attained because of the correlation between the characteristic recombination and the hydrodynamic times. Both the modelling and the emission spectroscopy show that the axial electron number density is much closer to its frozen value than to equilibrium value. The results obtained are helpful for different applications such as plasma processing, rocket propulsion systems and the simulation of re-entry conditions.

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Supersonic plasma jets find applications in plasma chemistry and plasma processing, metallurgy, experimental physics, and space technology. Usually the plasma in these jets deviates from chemical and thermal equilibrium. To optimize the industrial process detailed study of nonequilibrium effects in supersonic flow is required. In the article we apply numerical simulation to study the supersonically accelerated argon plasma flow downstream of the induction plasma torch. We compare the jets exhausting from two different convergent-divergent nozzles by means of a two-temperature model. The results show that the axial electron number density is rather convective flux controlled than recombination-ionization reaction controlled in both cases. However, the recombination resulting in electron gas heating is more essential in the jet flowing from the nozzle with a higher outlet Mach number. The composition of the jet exhausting from the nozzle with a lower outlet Mach number remains almost unchanged (“frozen”) until the end of the first expansion zone. These results confirm that the chamber pressure and the nozzle design changing leads to the induction plasma jets with different chemical conditions. For low-pressure supersonic plasma, these conditions vary from frozen to recombining. The conclusion is that depending on the industrial process, one can choose the proper torch nozzle geometry to have nonequilibrium plasma with the required properties.

Optical emission spectroscopy is applied to validate numerical simulations of supersonic plasma flow generated by induction torch with a convergent-divergent nozzle. The plasmas exhausting from the discharge tube with the pressure 0.4-1.4 atm. through two nozzle configurations (the outlet Mach number equals 1.5 and 3) into low-pressure (1.8 kPa) chamber are compared. Both modelling and experiments show that the effect of the nozzle geometry on physical properties of plasma jet is significant. The profiles of electron number density obtained from modeling and spectroscopy agree well and show the deviations from local thermodynamic equilibrium. Analysis of intercoupling between different sorts of nonequilibrium processes is performed. The results reveal that the ion recombination is more essential in the nozzle with the higher outlet number than in the nozzle with the lower outlet number. It is demonstrated that in the jets the axial electron temperature is quite low (3000-8000 K). For spectroscopic data interpretation we propose a method based on the definition of two excitation temperatures. We suppose that in mildly under expanded argon jets with frozen ion recombination the electron temperature can be defined by the electronic transitions from level 5p (the energy E = 14.5 eV) to level 4p (E = 13.116 eV). The obtained results are useful for the optimization of plasma reactors for plasma chemistry and plasma processing applications.