Simulation of the Discharges in Millimetre Gap Driven by Radio-frequency and Kilohertz AC Voltages

Atmospheric pressure short-gap discharge is an effective means to generate cold plasma. Common AC driving power sources include the RF power supply and the kHz AC power supply, but the comparison of the air gap discharge characteristics caused by these two different frequency power sources has been rarely studied. In this paper, the non-uniform field structure of an 1mm needle-plate gap was taken as the discharge structure, which was approximated as a one-dimensional model in the spherical coordinate system. A classical plasma fluid model based on multicomponent and local energy approximation under the drift-diffusion approximation was established. The discharge process of 1 mm helium (mixed with 0.1% nitrogen) gap driven by a 13.56 MHz radio frequency (RF) or 50 kHz AC (LF) power supply was simulated. The discharge characteristics under the 1 mW and 1 W deposition energies were studied. The results showed that the RF discharge mode was corona discharge at 1mW. The charged particles in the gap had a low density and mainly concentrated near the power electrode. When the deposition power increased to 1W, the gap discharge showed obvious glow-discharge characteristics, the sheath appeared near the electrode, and there was a quasi-electrically neutral plasma region in the middle of the gap. The igniting voltage amplitude of LF discharge was higher than that of RF, and LF discharge would transition from the corona discharge mode to the glow discharge mode smoothly without any obvious conversion process when increasing the voltage. For these two kinds of discharge, penning ionization was the main ionization path under the corona discharge mode. While the direct electron impact ionization became the dominant ionization channel under the glow discharge mode. In addition, under the same deposition power, the maximum electron density, the electron temperature and the positive ion temperature of the LF discharge were higher than those of RF discharge, but the temporal uniformity was poorer, showing obvious pulse discharge characteristics. © 2023 Editorial Department of Journal of Sichuan University. All rights reserved.