Non-Thermal Atmospheric RF Plasma in One-Dimensional Spherical Coordinates

Summary form only given. We present one-dimensional simulations of atmospheric pressure RF-excited plasma with two concentric spherical electrodes and the inner electrode powered. This study is related to our previous articles of two-dimensional numerical modeling of the RF-excited plasma needle. Since the two- dimensional model has the disadvantage of being computationally intensive, several important issues are best investigated with the use of a geometrically simpler model such as one-dimensional spherical model. In this study, the coupled continuity equations and electron energy equation are solved with Poisson's equation using the finite element method. A mode transition is observed in the discharge power-voltage curve between 1 mW and 1000 mW. In the low power mode, ionization rate peaks only near the inner electrode. The electron-impact excitation and ionization rates peak in the local cathodic phase. In the high power mode, however, the rate of ionization peaks near the outer electrode as well as the inner electrode. The inner sheath significantly shrinks and the direct electron-impact ionization is the primary ionization reaction near the inner electrode. The ionization rate near the outer electrode is due to ohmic sheath oscillation heating of electrons. Penning ionization is the major ionization reaction near the outer electrode. Thus, two different ionization mechanisms coexist near the inner and outer electrode. Electron heating near the outer electrode may have implications for surface processing in atmospheric pressure microdischarges.