Microwave-generated plasma in air under standard conditions

The possibility and feasibility of microwave-excited plasmas in air under standard conditions, as well as under moderate pressures of up to 5 bar, is demonstrated by using a quarter-wave coaxial resonator. The microwave energy is coupled into the resonator by an inductive loop attaining a match of better -15 dB. The equivalent circuit of the resonator allows to determine intrinsic resistance, inductance, and capacitance, additionally, the quality factor and, finally, the field strength inside the resonator can be deduced. By using this approximation a breakdown field of 300 kV/m is computed. The power levels used for exciting microwave plasmas are low enough to be manageable, e.g., 35 W.