Discharge characteristics of argon brush plasma plume operated at atmospheric pressure

Abstract

Atmospheric pressure non-equilibrium low-temperature plasma has been widely used in biomedicine, surface treatment and other fields, which has attracted the attention of researchers extensively. As one of the important methods to generate such a plasma, the plasma jet has become a popular method, which can generate a remote plasma plume at the nozzle through introducing a rare gas flow. However, plasma plume has a small diameter, which results in deficiency for the large-scale surface treatment. A dielectric barrier discharge device with three electrodes is utilized to produce a large brush-shaped plasma plume (50.0 mm × 40.0 mm) downstream of flowing argon under the combined excitation of an alternate current (AC) voltage and a negative bias voltage, thereby increasing the plume scale. The results show that the luminescence intensity of the plasma plume increases with AC peak voltage increasing. By fast photography implemented with an intensified charge coupled device (ICCD), it is found that the plasma plume is composed of temporally superposed branched-streamers. The ICCD images also reveal that the number of branches increases with AC peak voltage increasing. Moreover, the waveforms of AC voltage and light emission signal recorded simultaneously indicate that the plasma plume initiates once per AC voltage cycle, which occurs in the positive half cycle of the applied voltage. With AC peak voltage increasing, the duration and intensity of discharge pulse increase, which results from more branches of the branched streamer. Besides, optical emission spectrum in a range from 300 nm to 850 nm mainly includes OH (A<sup>2</sup>Σ<sup>+</sup>–X<sup>2</sup>Π) peaked at 308.0 nm, the second positive system of N<sub>2</sub> (C<sup>3</sup>Π<sub>u</sub>–B<sup>3</sup>Π<sub>g</sub>), Ar I (4p–4s), and O I (3p<sup>3</sup> P–3s<sup>3</sup> S) at 844.6 nm. Based on the optical emission spectrum, the plasma parameters such as vibrational temperature and intensity ratio of spectral lines (correlated with electron density and electron temperature) are investigated. Besides, the variation of concentration of oxygen atoms in the plasma plume with experimental parameters is investigated by optical actinometry. The results indicate that the concentration of oxygen atoms first increases and then decreases with the distance increasing along the argon flow direction or with oxygen content of the working gas increasing. In addition, the concentration of oxygen atoms increases with AC peak voltage increasing. All these results are discussed qualitatively. These results are of great importance in modifying the plasma surface on a large scale.