Effects of Microfilaments on DC Plasma Torches and Afterglows

In this paper, we investigate the effects of microfilaments on the operation of direct-current (dc) atmospheric-pressure nitrogen plasma torches. We measure the roto-vibrational temperatures of the first negative and second positive systems of the N₂ and N₂ ⁺ species, as well as the electron energy distribution function (EEDF) in the plasma plume and at several distances away from the source. The EEDF is measured using both planar and double-point Langmuir probes. We found that the operation of a dc nitrogen plasma torch in a mixed glow/microfilament regime results in the ejection of high-energy electron bullets from the torch nozzle. The frequency of these electron bullets outside the plasma coincides with the microfilament frequency. Furthermore, the microfilament frequency is shown to have a direct correlation with the N₂ ⁺ roto-vibrational temperature. The electron bullets possess a highly non-Maxwellian EEDF and maintain their nonequilibrium energies while contained within the N₂ substrate gas.