A microwave resonant cavity is provided. The microwave resonant cavity includes: a sidewall having a generally cylindrical hollow shape; a gas flow tube disposed inside the sidewall and having a longitudinal axis substantially parallel to a longitudinal axis of the sidewall; a plurality of microwave waveguides, each microwave waveguide having a longitudinal axis substantially perpendicular to the longitudinal axis of the sidewall and having a distal end secured to the sidewall and aligned with a corresponding one of a plurality of holes formed on the sidewall; a top plate secured to one end of the sidewall; and a sliding short circuit having: a disk slidably mounted between the sidewall and the gas flow tube; and at least one bar disposed inside the sidewall and arranged parallel to the longitudinal axis of the sidewall.