LAUSR

With a floating needle (positive polarity)-to-ring (negative polarity) electrode configuration insulated by the quartz tube, an atmospheric-pressure helium plasma jet driven by bipolar dc voltage is demonstrated. As the gap distance between the human finger and the tube exit varies from 18 to 1 mm, the generated plasma jet is always touchable without any feeling of the electrical shock or thermal injury. First, although dc high voltage is applied, the current delivered from the plasma jet to the human body is actually pulsed with the repetition frequency of 0.8–2 kHz. The maximum deposited power and the maximum current rms are 23.5 mW and $30~\mu \text{A}$ , respectively. Second, as the distance between the needle and the tube exit decreases, the pulse repetition frequency increases, and the peak value of the jet current decreases. Third, when the percentage of O2 in helium gas increases from 0% to 2%, the length of the plasma jet increases first and then decreases after it reaches the maximum of 20 mm at the O2 percentage of 0.5%. The peak value of the jet current increases from 0.4 to 0.9 mA, and the pulse repetition frequency decreases from 2 to 0.2 kHz. In addition, from the point of view of the discharge current, the electrical safety, and the optical emission spectra, a comparison with other dc-driven plasma jets with different electrode configurations (inverse polarity of the electrodes, ring grounded or no ring electrode) is presented. It reveals that the plasma jet having the bipolar electrode configuration is more active, portable, and safer for biomedical applications.