Paper PS-FrM5
Diagnostic Studies of AC-driven Plasmas in Saline Solutions: the Effect of the Frequency on the Plasma Behavior

Friday, October 22, 2010, 9:40 am, Room Aztec

Plasmas in saline solutions receive considerable attention in recent years. How the electrical power frequency influences the plasma behavior remains unclear. In this presentation, diagnostic studies of plasmas ignited in saline solution driven by an AC power source are presented. An AC power source with tunable frequencies between 50~1000 Hz and a voltage up to 600 V is used. The electrode at which the plasma is ignited uses a Pt wire 0.5 mm in diameter covered by a glass tube to precisely define the area exposes to the solution. Saline solutions with concentrations 0.01 M ~ 2 M are used. Diagnostic tools used include a voltage and a current probe to monitor the electrical characteristics. A high speed camera with a frame rate up to 1200 frames/sec is used to capture the bubble and plasma dynamics. An optical emission spectrometer and a photomultiplier tube are used to monitor the optical emission emanating from the plasma. It is shown that the plasma behavior is strongly coupled with the bubble dynamic adjacent to the electrode tip. Two distinct modes, namely the static mode and the jetting mode, are identified. In the static mode, a bubble with a diameter 1~ 3 mm is attached at the electrode tip for many seconds. The oscillation of the bubble is found to be relevant to the plasma behavior and is partially responsible to the stability of the discharge. This mode occurs mostly at the frequency below 100 Hz. The jetting mode occurs at a frequency higher than 300 Hz. In this mode, the plasma is ignited intermittently and is less stable comparing with the bubble mode. Under low applied voltages, bubbles of hundreds of μm in diameter are continuously jetted from the electrode tip. As the applied voltage increases, the micro-bubbles tend to coalescence into large bubbles and attach back to the electrode thus the switching of the above two modes is observed. Further increase in the applied voltage leads to bright plasmas with high current and severe electrode damage occurs. It is also observed that under the applied voltage above 200 V, the plasma ignited in the negative half cycle of the power period shows a much stronger emission intensity than that in the positive half cycle. By the integration of the high speed image, the optical emission spectroscopy, and the electrical characteristics, the mechanism of the plasma formation under various frequencies and how it is affected by the bubble dynamics will be proposed.