Designing an efficient microwave-plasma source, independent of operating conditions, at atmospheric pressure

The surfaguide is a waveguide-based electromagnetic-surface-wave launcher that allows sustaining long plasma columns using microwaves. Its electrodynamic characteristics are examined experimentally and theoretically in the perspective of achieving an efficient plasma source without any need for impedance matching retuning as operating conditions are varied over a broad range. The plasma source design and its modelling using equivalent-circuit theory are described and a simple procedure is provided to determine the optimum dimensions of the surfaguide that maximize the transfer of microwave power to plasma. As an example, with an optimized surfaguide, the reflected power in an N 2 discharge at atmospheric pressure stays below 3% for powers in the 2–6 kW range and gas flow rates in the 30–150 l min −1 domain under varying concentrations (< 2%) of admixed gases such as SF 6 , O 2 and argon.