a microwave source configured to generate a microwave oscillation;

a waveguide component coupled to the microwave source and configured to receive the microwave oscillation such that the microwave oscillation is propagated through the waveguide component; and

a resonant cavity coupled to the waveguide component such that the microwave oscillation is substantially propagated in the resonant cavity along a waveguide direction, the resonant cavity comprising a first chamber in proximity to the waveguide component and a second chamber distant from the waveguide component, the first chamber comprising an inlet end connected to the waveguide component and an outlet end opposite to the waveguide component, the second chamber comprising a communicating end communicated with the outlet end and a closed end opposite to the communicating end, the second chamber being configured to receive the microwave oscillation passed through the first chamber, the microwave oscillation interacting with an ignition gas in the second chamber to generate a torch;

wherein, the waveguide direction is substantially parallel to a reference axis defined in the first chamber, the first chamber comprises an inner wall around the reference axis and extending along the reference axis, the inner wall comprises a first region inclined to the reference axis and a second region substantially parallel in respect to the reference axis, an area of the first region is larger than that of the second region so that the first chamber comprises a tapered shaped space which becomes narrower from the inlet end to the outlet end.