Influence of square wave duty ratio on pattern formation of atmospheric pressure glow discharge

Excited by a square wave voltage, atmospheric pressure glow discharge is generated in a needle to liquid configuration. With decreasing duty ratio of the square voltage, bright disk, bright disk with outer ring, triple ring, triple ring with concentric spot are observed on the water surface. Voltage and current waveforms indicate that the discharge occurs in the negative polarity. The discharge current consists of two parts, the pulse region and the constant region, which has a good periodicity. It is found that the inception voltage and the peak current increase with decreasing the duty ratio. Optical emission spectra are detected. It contains the second positive band of N-2, the first positive band of N-2, the first negative band of N-2(+) and the OH (309 nm). Using the Lifbase, emission spectra of the OH are used to estimate rotational temperature by fitting the experimental spectra to the simulated spectra. It is found that the gas temperature increases with increasing the duty ratio. By synchronously triggering discharge current and ICCD, temporally resolved images of the patterns are captured on the water surface. Results show that self-organized pattern gradually evolves in the pulse region, and it does not change any more in the constant region. Taking into account the space negative charge, above phenomena are analyzed and explained by using the basic theory of discharge.