US Patent 7235929 Magnetron

A magnetron 10 is equipped with a helical filament 39, as an element of a cathode assembly, arranged on a central axis of an anode cylindrical body 13. Assuming that a resistance value of the filament 39 before forming the carbonized layer is R1 and a resistance value of the filament 39 after forming the carbonized layer is R2, a thickness of the carbonized layer 42 of the filament 39 is determined such that a carbonization rate Rx defined by the equation “Rx={(R2−R1)/R1}×100” in a range of from 30 to 50%.