A microwave source , comprising: an anode block ; and multiple cathodes , at least two diameters of at least two cathodes of the multiple cathodes being different, and each of the multiple cathodes 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 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 in a center of the anode block ; 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 respectively and the anode block . an anode block ; and multiple cathodes , at least two diameters of at least two cathodes of the multiple cathodes being different, and each of the multiple cathodes 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 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 in a center of the anode block ; 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 respectively and the anode block . <EOS>