The Effect of Phase Shift on the Plasma Driven by an AC Voltage and a Pulsed DC Voltage

One of the most important features of atmospheric-pressure nonequilibrium plasmas (APNPs) is its capability of generating high concentration of reactive species while keeping low gas temperature. To generate such plasma, alternative current (ac) voltage and pulsed dc voltage have been used separately. In this paper, the combination of ac voltage and pulsed dc voltage is used to generate APNPs for the first time. The plasma characteristics, including the power deposited to the plasma, the gas temperature, the emission spectra, and the O atom concentration of the plasma for different phase shifts of the two power supplies, are investigated. It is found that when the pulsed dc voltage is added on the negative ac half period at the phase shift of about 90°, the average power deposited to the plasma, the gas temperature, and the optical emission spectrum of O atom all reach their maxima. On the other hand, the ground state O atom concentration has two peaks; one of the peaks also appears when the phase shift is about 90°. When the pulsed dc voltage is applied on the positive half period at the phase shift of 270°, the O concentration reaches another peak and the peak value is higher than that when the ac or the pulsed dc power supply works alone. Furthermore, at the phase shift of 270°, the gas temperature of the plasma and the total power deposited to the plasma are all lower than the case when the ac or the pulsed dc power supply works independently. Thus, it is more energy efficient to adjust the phase shift to 270° for applications such as plasma medicine where the gas temperature of the plasma at or close to room temperature is strictly required.