Abstract: In an interaction space (7) defined between a cathode (2) and forward ends of vanes (3), an even direct-current magnetic field is applied by magnets (8) along the axial direction of the cathode (2). Direct-current or low-frequency high voltage is applied between the cathode (2) and the respective vanes (3). Spaces (6) surrounded by respective two adjacent vanes (3) and the inner wall of the anode cylinder (4) define cavity resonators, which generate high-frequency electric fields concentrated to the forward end portions of the respective vanes (3) and partially leaking in the interaction space (7). Under such conditions, an electron group emitted from the cathode (2) rotates about the cathode (2) in the interaction space, whereby interaction takes place between the electron group and the high-frequency electric fields to oscillate microwaves.

Abstract: The ratio b/a of the width a of the forward end surface 30a of each vane 30 to the interval b between the opposed forward end portions of the respective adjacent vanes 30 is set to be not more than 2.3, whereby distribution density of a high-frequency electric field concentrated in the vicinity of the forward end portions of the vane 30 can be equalized.