Magnetron with reduced dark current

A magnetron, comprising:

• an anode cylinder,
• a plurality of vanes extending radially inwardly from said anode cylinder and thereby defining a group of anode cavity resonators,
• an antenna,
• means defining a microwave output port disposed in axially spaced relationship from said plurality of vanes and coupled with one of said plurality of vanes through said antenna,
• strap rings electrically connecting alternate ones of said plurality of vanes only at axial ends of said plurality of vanes on the microwave output port side thereof,
• a cathode positioned substantially at the center of a circle enveloping tips of said plurality of vanes and cooperating with said tips of said plurality of vanes to define an annular interaction space extending between said cathode and said tips and encircling said cathode,
• a pair of end shields provided at two opposite ends of said cathode,
• a pair of pole pieces positioned at two axial ends of the interaction space for shaping an axial static magnetic field produced therein,
• a pair of permanent magnets positioned outside said pair of pole pieces and outside said anode cylinder, and
• a cathode stem for supporting said cathode through heater-current-feeding wires, wherein:
• an axial static magnetic field resulting from said pair of permanent magnets and existing in a plane containing an inner surface of the one of said pair of end shields on said microwave output port side in the interaction space is different from a static magnetic field existing in a plane containing an inner surface of the one of said pair of end shields on said cathode stem side in the interaction space, and
• at least an interaction-space-side axial end of a peripheral portion of said end shield associated with the end of the interaction space corresponding to the weaker static magnetic field of said two axial magnetic fields in said interaction space is displaced a predetermined distance d axially toward said interaction space from said axial ends of said plurality of vanes and close to said cathode, where d is equal to or greater than zero.