From a practical application viewpoint, the generation of a large-diameter plume is of great importance for the atmospheric pressure plasma jet. In this Letter, a large-diameter argon plume is generated downstream of a single-tube plasma jet with a simple needle-plate geometry. Results indicate that the diffuse plume is composed of a semi-spherical discharge close to the needle tip, a trumpet-like discharge on the top of the plate, and a dark region between them. Photoelectric measurement reveals that there is a broad hump with a duration time of 100 μs in the light signal per negative voltage half cycle. By fast photography, spatiotemporal evolution of the diffuse plume is investigated, which indicates that the semi-spherical discharge corresponds to a corona near the needle tip, and the trumpet-like discharge operates in a Townsend discharge regime. Optical emission spectroscopy is used to characterize electron excitation temperature and electron density, both of which decrease in the semi-spherical corona discharge and increase in the trumpet-like Townsend discharge with increasing distance from the needle tip.
Li et al. (Mon,) studied this question.