Spectroscopic study of excitation nonequlibria in an expanding H/sub 2/O-AR DC ARCJET

Abstract

Summary form only given. Plasma jets expanding from a remote thermal plasma source, such as DC arc or RF induction plasma torches, into a low pressure chamber are attractive for industrial applications in material processing, e.g. fast deposition of new materials. To develop such an application, the reliable information about crucial parameters of the plasma jet such as the kinetic temperature is necessary. Although the expanding plasma jet is far from the local thermodynamic equilibrium (LTE), the kinetic temperature of free particles can be still established. However, the diagnostics that can directly measure either the electron or heavy particle temperature, e.g. electric or enthalpy probe, are obscure to interpret, mainly because of complex structure and high speed of the plasma flow. Non intrusive methods such an optical emission spectroscopy, on the other hand, rely upon an information obtained from excited species which are the most sensible to nonequilibrium effects. In recent years, an increasing effort has been made to develop more complex investigation tools that combine various diagnostic techniques applicable in such an environment and numerical simulations with modeling. In this contribution we focus on the spectroscopic investigation of excited atomic (Ar, O, H) and molecular (OH) species in the plasma jet generated by H2O-Ar DC arc torch in a low-pressure chamber. The operating parameters were: current ~200 A, voltage ~130 V, argon flow rate 13 slm, and the chamber pressure varying from 0.3 to 10 kPa to obtain various stages of the plasma flow and departure from excitation equilibrium. We discuss the relation between the temperatures obtained by OES, characterizing a partial equilibrium among a particular manifold of excited states, and the electron or heavy particle temperature. Populations of higher atomic states remain close to the Boltzmann distribution for the whole supersonic region, and the lower excited states are underpopulated. The rotational temperature of OH is much higher in the expansion region than the vibrational temperature, which is in contrast to the assumption that the former should represent the heavy particle temperature and the latter the electron temperature. The intensity distribution in the OH spectra can be interpreted as a result of both the departure from LTE and a superposition of 'hot' and 'cold' OH radicals, resulting from competition between various reactions leading to the creation of OH radical and collisional relaxation. A simple three-level collisional-radiative model was used to assess populations of the ground and first excited state of the atomic species