Characterization of plasmas in saline solutions under different bubble behavior

Abstract

Summary form only given. Plasmas in saline solutions receive considerable attention recently. In this presentation, diagnostic studies of plasmas in saline solution driven by DC (0~600 V) or AC (0~600V, 50~1000 Hz) power sources are presented. The AC power source is used either to directly drives the plasma or is connected a full wave-rectifier to convert the input waveforms to either a constantly positive or negative polarity so that the frequency effect can be studied in unipolar or bipolar modes. A Pt wire 0.5 mm in diameter covered with a glass tube is used as the electrode. Saline solutions with various concentrations are used. It is shown that the plasma behavior is strongly influenced by the behavior of bubbles adjacent to the electrode tip. Two distinct modes are observed: the bubble mode occurs when the plasma is driven by DC or low frequency AC power while the jetting mode occurs at high frequency AC power. Based on the current and voltage waveforms and the photo-images taken by a high speed camera, the jetting behavior is believed to be dominated by thermal capillary force. The plasma characteristics under these two modes are examined. Time-resolved light intensity measurements show that the breakdown voltages of plasmas driven by negative voltages are about 100 volt lower than those driven by positive voltages under low frequencies. Optical emission spectroscopy (OES) shows the emission intensity ratio of sodium D-line (588 nm) and H-alpha (656 nm) changes with the polarity and the frequency. Preliminary studies show that the difference in the intensity ratio is a result of the power input. In addition, Stark broadening is found to be responsible to the broadening of the H emission line. Based on this broadening, electron densities are estimated to be 5 and 10 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for bubble (50 Hz) and jetting modes (500 Hz), respectively. Finally, how the bubble behavior influences the plasma characteristics, and how the power source type influences the plasma properties will be discussed.