Sheath and bulk expansion induced by RF field in atmospheric pressure microwave plasma

A large axial volume expansion of microwave driven plasma at atmospheric pressure is attained with a low power radio frequency (RF) field at an axial location well isolated from the original plasma bulk. The gradual development of volume expansion visualized by high speed ICCD imaging suggests that the free electrons drifting toward the bias electrode promote the prodigious expansion of the sheath, generating a stable plasma stream channel between the microwave and the RF electrodes. Enhanced emissions of OH and N-2 spectral lines are observed in the extended plume region for the case of argon microwave plasma, supporting the acceleration of electrons and subsequent generation of radical species. Ozone generation is kept low, below the regulation levels. The coupling of RF field within a range of a few MHz with microwave provides an efficient way of enlarging the microwave plasma volume and enhancing the production of radicals.