A microwave source (208, 300, 800, 900), comprising: an anode block (310, 802, 902); and multiple cathodes (320, 804, 904, 906), at least two diameters of at least two cathodes (904, 906) of the multiple cathodes (320, 804, 904, 906) being different, and each of the multiple cathodes (320, 804, 904, 906) comprising: a cathode support element having a plurality of slots, the plurality of slots being axially around a circumference of the cathode support element; and a cathode heater including at least one filament, wherein a first part of the at least one filament is wound around the cathode support element along a first direction and received by a first portion of the plurality of slots, and a second part of the at least one filament is wound around the cathode support element along a second direction and received by a second portion of the plurality of slots; a connector, the multiple cathodes (320, 804, 904, 906) being mechanically connected to each other by the connector; a limiting member, an end of the connector being operably connected to the limiting member; a guide slot, the limiting member being disposed in the guide slot and configured to move along the guide slot to position a cathode of the multiple cathodes (320, 804, 904, 906) in a center of the anode block (310, 802, 902); wherein at least two microwaves having different frequencies are generated in response to an occurrence of a resonance effect caused by the at least two cathodes (904, 906) respectively and the anode block (310, 802, 902). an anode block (310, 802, 902); and multiple cathodes (320, 804, 904, 906), at least two diameters of at least two cathodes (904, 906) of the multiple cathodes (320, 804, 904, 906) being different, and each of the multiple cathodes (320, 804, 904, 906) comprising: a cathode support element having a plurality of slots, the plurality of slots being axially around a circumference of the cathode support element; and a cathode heater including at least one filament, wherein a first part of the at least one filament is wound around the cathode support element along a first direction and received by a first portion of the plurality of slots, and a second part of the at least one filament is wound around the cathode support element along a second direction and received by a second portion of the plurality of slots; a cathode support element having a plurality of slots, the plurality of slots being axially around a circumference of the cathode support element; and a cathode heater including at least one filament, wherein a first part of the at least one filament is wound around the cathode support element along a first direction and received by a first portion of the plurality of slots, and a second part of the at least one filament is wound around the cathode support element along a second direction and received by a second portion of the plurality of slots; a connector, the multiple cathodes (320, 804, 904, 906) being mechanically connected to each other by the connector; a limiting member, an end of the connector being operably connected to the limiting member; a guide slot, the limiting member being disposed in the guide slot and configured to move along the guide slot to position a cathode of the multiple cathodes (320, 804, 904, 906) in a center of the anode block (310, 802, 902); wherein at least two microwaves having different frequencies are generated in response to an occurrence of a resonance effect caused by the at least two cathodes (904, 906) respectively and the anode block (310, 802, 902).