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.

An analysis of spectral line profiles is used to calculate the gas temperature and to estimate the upper limit of the electron density in an atmospheric pressure dielectric barrier discharge torch. Two transitions are studied, that of helium (He) at 587.5nm and that of hydrogen (Hβ) at 486.1nm, both observed in the spectra of the light emitted from the gap-space region. Relevant broadening mechanisms including the Doppler and Stark effects, as well as the collision processes between an emitter and a neutral particle, are reviewed. It is deduced that the main contribution to the broadened profiles is due to collisions. Through knowledge of the van der Waals interaction potential, a general expression for determining the gas temperature is derived and applied to each transition. The results obtained from both lines are in agreement; i.e., the gas temperature is found to be 460±60K at the highest voltage applied. This value is consistent with the experimental observation that at these conditions the afterglow plasma cannot ignite paper, whose ignition temperature is 507K. Since no signature of the Stark effect can be detected either in He or Hβ transition, the upper limit of the electron density, estimated from the uncertainty on the Hβ linewidth, is 4×1012cm−3. The generality of the method allows one to determine the temperature as a function of other parameters, such as voltage and flow rate. Concerning the applied voltage, the gas temperature increases linearly from 315±30to460±60K, as derived from both lines. Over the same voltage range, a similar behavior is found for the rotational temperature, as deduced from the first negative B(Σu+2,v=0)→X(Σg+2,v=0) transition of the molecular nitrogen ion. However, the temperature varies between 325±30 and 533±15K, indicating an overestimation of the gas temperature. On the other hand, the gas temperature derived from each of the lines does not show a significant variation with the He flow rate in the range of 5–40l∕min.

A supersonic argon and argon-hydrogen (3-5{%}) plasma jet generated by an induction plasma torch is studied by means of the methods of optical emission spectroscopy. The torch was operated at the input power of 20 kW and near atmospheric pressure. The supersonic jet with a periodic structure of expansion and compression zones is created by expanding the plasma through the Laval nozzle into a chamber maintained at the pressure around 1.8 kPa. Atomic argon lines with the upper level energies ranging from 13.3 to 15.5 eV, continuum emission and Hβ line profile are used to evaluate plasma parameters such as temperature and electron number density. Analysis based on the Boltzmann diagram, line-to-continuum ratio, population of continuum extrapolated level and Stark broadening reveals various stages of departure from thermodynamic equilibrium in the plasma flow. It is shown, among others, that the temperature derived from Boltzmann diagram does not follow the jet structure and reliable determination of electron temperature is questionable. An addition of several percent of hydrogen results in a significant quenching of populations of atomic states and nonequilibrium behaviour of continuum radiation.

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.

Spectroscopic measurements are carried out on inductively coupled radio-frequency (RF) argon plasma over the pressure range 120 to 760 Torr. The electron number density is obtained by two methods: Hbeta profile and the absolute intensity of the argon continuum. Number densities of excited argon levels are deduced from absolute line intensities. The electron density measured in the hottest zone of the plasma decreases from 1016 to 6*1015 cm-3 with the decrease of pressure from 760 to 120 Torr. Comparison between the different number densities shows that the atmospheric plasma is in local thermodynamic equilibrium (LTE) whereas weak departures from equilibrium are observed at pressures below 300 Torr.