Comparison of bubbles due to frequency change and electrode direction in high repetitive plasma in water

Underwater discharges are applied in the industrial field. However, phenomena of underwater discharges have not been completely clarified. The purpose of our study is to clarify characteristics of underwater discharges. In high repetitive underwater discharges, bubbles generated by discharges are gathered to the tip of an electrode. Discharge plasma form may change to a ball-shaped discharge by bubbles. Our study focused on bubbles and investigated the movement of bubbles generated by discharges. Experimental setup are as follows: pulse power discharge by MPC method with maximum output 1 J/pulse, the water conductivity of about 150 pS/cm, the electrode made of copper with a diameter of 0.8 mm, the repetition frequency of up to 500 pps (pulses per second), a high speed camera with the frame rate of 57000 fps (frames per second) and image processing software. This paper has four contents: comparison of the movement of bubbles on 250 and 500 pps, comparison of discharge plasma forms on 250 and 500 pps by change of the direction of the electrode, expansion and contraction of bubbles, and electron temperature and electron density of plasma. In the case of the discharge on high frequency, bubbles tend to gather to the tip of electrode. As the frequency rises, the discharge plasma forms tend to be ball shaped bubble. The downward electrode discharge is easier to gather bubbles than the upward electrode discharge. The vibration frequency was higher as the area of the bubble was smaller. Also, the electron temperature was about 8000 K, and the electron density was about 10
⁸
orders.