Pulsed Plasma Jet Igniters: Species Measurements in Nitrogen and Air

Abstract

A molecular beam mass spectrometer was used to obtain quantitative data on the species present along the centerline of nitrogen and air plasma jets. Dilution of the jet as ambient gas is entrained into the turbulent plume was measured by probing an argon jet discharging into nitrogen. The gas from the jet igniter cavity is diluted by more than a factor of ten in the first 4 mm downstream of the orifice, with a further dilution of 40 percent over the next 8 mm of jet penetration. Comparison of peak mole fractions of reactive species in the jet with the dilution results indicate that the plasma species are not decaying at a rate solely due to dilution. The nitrogen plasma jet was probed for metastable nitrogen (N₂*) and N atoms as a function of time at various distances from the jet orifice. Pteak mole fraction measurements demonstrate that N₂* persists at distances farther downstream than N atoms. Metastable nitrogen concentrations are relatively constant from 4 to 14 mm, indicating that metastable nitrogen is produced in the plume. Nitrogen atoms, oxygen atoms, and nitric oxide were measured in an air plasma. Nitric oxide persists significantly longer than the atoms and has a peak mole fraction at the point where O and N decrease significantly. The O atom mole fraction of 0·02 is an order of magnitude higher than the N atom concentration, but the radical concentration in the air plasma is lower than that in pure N₂. No evidence of metastable nitrogen was found in this system. The chemistry with respect to these results is discussed, both in terms of the kinetics of the system and calculated equilibrium concentrations.