Study of the Development of Negative DC Corona Discharges on the Basis of Visible Digital Images

The noncontact diagnosis methods of discharge plasma include spectrum, electromagnetic wave, ultraviolet image, and high-speed nanosecond image analyses, but there are few studies on the visible image diagnosis method. With the development of digital cameras, the number of pixels in a visible image is far greater than that of ultraviolet, infrared, and high-speed cameras; the image contains a considerable amount of discharge spatial information and provides a new solution for plasma diagnosis. We obtained visible digital images of short gap negative dc coronas with the corresponding oscilloscope current signals under the experimental conditions of atmospheric pressure and room temperature. Two colorimetric indicators (RGB and HSI) were proposed to study the colorimetry features of discharge images, of which the analysis was correlated with the amplitude and frequency of oscilloscope current signals. The result shows that the hue value of the overall image region is positively related to the pulse amplitude, while the intensity value corresponds to the pulse frequency. The stage division based on the hue value is better than that based on current pulses, which shows that the Trichel stage can be further subdivided. In addition, we observed that the anode discharge begins earlier than the formation of pulseless glow, and we illustrated that anode discharge played an important role in the formation of pulseless glow and spark breakdown. Briefly, the discharge image corresponds well with the results of traditional detection methods and provides a new perspective for the corona discharge study, deserving further research.