Laser-based characterization of a flame-assisted plasma

A well-characterized flame-assisted plasma was developed to understand the role of flow nonuniformities and plasma/wall interactions in plasma devices for use in validation of laser-based Doppler shift spectroscopic methods. A hydrogen/oxygen capillary diffusion flame burner was used as a plasma source, with barium seeded into the reactants to provide a source of ions and electrons. For analysis the plasma was assumed to be a stationary, partially ionized, collision dominated, thermal plasma consisting of barium ions, electrons, and neutrals between two parallel-plate electrodes. The plasma was examined in terms of the continuum equations for ions and electrons, together with Poisson's equation to predict spatial profiles of electron and positive ion density and potential as functions of applied potential. First an analytic solution based on constant plasma properties and negligible difusion was introduced. The model was then extended by including effects of diffusion and variable plasma properties. Experimentally, current/voltage characteristics of the plasma were measured conventionally, relative ion concentration and temperature were measured with laser-induced fluorescence, and local potential distribution was measured using an electrostatic probe. The diffusionless theory predicted well the bulk behavior of the plasma, but not the correct spatial distributions of ion concentration and potential. The extended model produced a more satisfactory fit to the data. At conditions of 1.4 equivalence ratio, 70 torn pressure, 300 ppm seed concentration, and 100–400 V applied potentials, electric fields of the order of 10², 10³ V/cm were observed near the powered electrode, and of few tens of V/cm in the hulk of tire plasma. The field strength in the sheath ensures the operation of the Doppler shift diagnostics, once the recommendations tor LIF signal detectability are fulfilled.