A method for processing a gas, wherein the method comprises; - employing an elongated vessel with an inner space of cylindrical symmetry, where the elongated vessel: i) protrudes through a microwave chamber, ii) the wall of the part of the elongated vessel not in contact with the microwave chamber is opaque to microwave radiation, iii) the wall of the part of the elongated vessel which penetrates into the microwave chamber is transparent to microwave radiation, iv) the elongated vessel has at a first end of the vessel a co-axial elongated cylindrical exit channel with an inner diameter, D, equal to or less than a factor of 1/16 of the wavelength of an applied microwave field within the microwave chamber, and a length, E, equal to or less than a factor of (n + 1/8), n ∈ {0, 1, 2, 3}, of the wavelength of an applied microwave field within the microwave chamber, and v) the elongated vessel is closed at a second end opposite the first end by a bottom wall, - injecting the gas that is to be processed such that it forms a first vortex flow running along the inner wall of the inner space of the vessel and then reflects from the bottom wall at the second end to form a second vortex flow of the gas that is to be processed running along the centre axis of the vessel from the bottom wall of the vessel and out of the exit channel at the first end, and - forming a standing microwave in the microwave chamber aligned to form a wave crest at the centre axis of the vessel in the area of the vessel where it is transparent to microwave energy capable to excite at least a portion of the gas in the second vortex to form a microwave non-thermal plasma. - employing an elongated vessel with an inner space of cylindrical symmetry, where the elongated vessel: i) protrudes through a microwave chamber, ii) the wall of the part of the elongated vessel not in contact with the microwave chamber is opaque to microwave radiation, iii) the wall of the part of the elongated vessel which penetrates into the microwave chamber is transparent to microwave radiation, iv) the elongated vessel has at a first end of the vessel a co-axial elongated cylindrical exit channel with an inner diameter, D, equal to or less than a factor of 1/16 of the wavelength of an applied microwave field within the microwave chamber, and a length, E, equal to or less than a factor of (n + 1/8), n ∈ {0, 1, 2, 3}, of the wavelength of an applied microwave field within the microwave chamber, and v) the elongated vessel is closed at a second end opposite the first end by a bottom wall, i) protrudes through a microwave chamber, ii) the wall of the part of the elongated vessel not in contact with the microwave chamber is opaque to microwave radiation, iii) the wall of the part of the elongated vessel which penetrates into the microwave chamber is transparent to microwave radiation, iv) the elongated vessel has at a first end of the vessel a co-axial elongated cylindrical exit channel with an inner diameter, D, equal to or less than a factor of 1/16 of the wavelength of an applied microwave field within the microwave chamber, and a length, E, equal to or less than a factor of (n + 1/8), n ∈ {0, 1, 2, 3}, of the wavelength of an applied microwave field within the microwave chamber, and v) the elongated vessel is closed at a second end opposite the first end by a bottom wall, - injecting the gas that is to be processed such that it forms a first vortex flow running along the inner wall of the inner space of the vessel and then reflects from the bottom wall at the second end to form a second vortex flow of the gas that is to be processed running along the centre axis of the vessel from the bottom wall of the vessel and out of the exit channel at the first end, and - forming a standing microwave in the microwave chamber aligned to form a wave crest at the centre axis of the vessel in the area of the vessel where it is transparent to microwave energy capable to excite at least a portion of the gas in the second vortex to form a microwave non-thermal plasma.