Effect of superposed repetitive pulses onto DC voltage on discharge extension into fog water produced by electrospray

Photographic observations for ionized water droplets from an injection needle subjected to high voltage and for streamer discharge light from the same electrode is a part of a fundamental study on water treatment. The goal of this study is to establish a new plasma treatment method for the decomposition of persistent substance in water by using a pulsed power technique. Repetitive pulsed voltages are superposed onto a dc voltage to extend the streamer discharge propagation into fog water produced by electrospray. The Dynamics of fog water from an injection-needle are observed by a high-speed camera, whereas the discharge light is observed by a still camera. Experiments are done in two cases, dc voltage application without any pulse superpositions and dc voltage application with repetitive pulses. In the first step, only a constant dc voltage is applied to needle-plane electrodes. A remarkable polarity effect on the diffusion of fog water is recognized. It is noted that the diameter of the water droplets for the positive polarity is much smaller than that for the polarity, so that the diffusion area of fog water for the positive polarity becomes large. However, the positive streamer discharge cannot propagate into the gap filled with fog water because of a trade-off relationship between the fog formation and the discharge extension. In the second step, the high voltage pulses are repeatedly superimposed onto the constant dc voltage. The Influence of the pulse width on the streamer propagation capability from a water droplet on the needle tip is studied. Results show that the discharge initiation probability for millisecond pulses superposition is higher than that for nanosecond pulse superposition.