Analysis of RF Corona Discharge Plasma Ignition

Corona discharge from a RF quarter wave coaxial cavity resonator is considered as a plasma ignition source for spark ignited (SI) internal combustion (IC) engines. The gaseous discharge processes associated with this device are analyzed using principles of gas kinetics and gaseous electronics, with assumed values for the electric field strength. Corona discharge occurs when the electric field shaped and concentrated by a single electrode exceeds the breakdown potential of the surrounding gas. Ambient electrons, naturally present due to ionizing radiation, drift in the direction of the externally applied field, gaining energy while undergoing elastic collisions with neutral molecules. After gaining sufficient energy they dissociate, excite, or ionize the neutral particles through inelastic collision, creating additional electrons. This process leads to avalanche electrical breakdown of the gas within about 10^-8 sec. In the presence of the high frequency AC field, relatively few of the energized particles are lost to the electrodes, improving efficiency but preventing secondary generation of electrons via ion impacts with the cathode, an important process in DC discharges. The ionization process is limited primarily by diffusion, and breakdown voltages are expected to be lower than for DC conditions. The proposed 2 GHz operating frequency is sufficiently high to possibly excite plasma resonances for some ion species. Further analysis is required to determine the physical volume of the plasma created by the RF corona discharge, but this is expected to be larger than for a conventional DC spark ignition system.