Investigation on the characteristics of an atmospheric-pressure microplasma plume confined inside a long capillary tube

An atmospheric-pressure microplasma plume of diameter 10 μm is generated inside a long tube. The length of the microplasma plume reaches as much as 2 cm. First, with the assistance of an air dielectric barrier discharge （DBD）, the ignition voltage of the microplasma decreases from 40 kV to 23.6 kV. Second, although the current density reaches as high as （1.2-7.6）× 10 4A cm-2, comparable to the current density in transient spark discharge, the microplasma plume is nonthermal. Third, it is interesting to observe that the amplitude of the discharge current in a positive cycle of applied voltage is much lower than that in a negative cycle of applied voltage. Fourth, the electron density measured by the Stark broadening of Ar spectral line 696.5 nm reaches as high as 3× 10 16 cm-3, which yields a conductivity of the microplasma column of around 48 S m-1. In addition, the propagation velocity of the microplasma plume, obtained from light signals at different axial positions, ranges from 1 × 10 5 m s-1 to 5× 10 5m s-1. A detailed analysis reveals that the surface charges deposited on the inner wall exert significant influence on the discharge behavior of the microplasma.