Mode converter and gyrotron tube provided with mode converter for converting mode of millimeter waves

A gyrotron tube comprising:

• a magnetron injection gun for generating a hollow electron beam moving in a helical shape;
• a collector for catching the electron beam from the magnetron injection gun;
• a beam tunnel, disposed between the magnetron injection gun and the collector, for guiding an electron from the magnetron injection gun;
• a cavity resonator, disposed between the beam tunnel and the collector, in which a high-frequency electric field is generated and an electromagnetic waves of a higher mode is generated by an interaction of the high-frequency electric field with the electron beam from the beam tunnel;
• a mode converter, disposed between the cavity resonator and the collector, for converting the electromagnetic waves from the cavity resonator to a beam-like electromagnetic waves;
• a cylindrical tapered waveguide, disposed between the mode converter and the cavity resonator, for guiding the electromagnetic waves from the cavity resonator to the mode converter;
• and
• guide means for guiding the beam-like electromagnetic waves from the mode converter to the outside of the gyrotron tube,
• wherein said mode converter comprises a substantially circular waveguide including an inlet for introducing millimeter waves of higher mode, a radiation aperture for emitting the converted millimeter wave beam, and a transverse inner surface shaped such that a deformation degree to a non-true circular shape from a true circular shape is gradually increased toward the radiation aperture from an inlet side of the millimeter waves, so that the higher-mode millimeter waves propagated in said circular waveguide is converted to millimeter wave beam propagated in a mirror system in a free space, and
• a region of said circular waveguide changing to the non-true circular transverse inner surface shape is in a range of 0 to 5 mm toward said radiation aperture from said inlet side.