A plasma generating system includes a waveguide for transmitting a microwave energy therethrough and an inner wall disposed within the waveguide to define a plasma cavity, where a plasma is generated within the plasma cavity using the microwave energy. The plasma generating system further includes: an adaptor mounted on a first side of the waveguide and physically separated from the waveguide by a first gap and having a gas outlet through which a gas processed by the plasma exits the plasma cavity; and an EM seal disposed in the first gap and configured to block leakage of the microwave energy through the first gap.