Harmonic gyro traveling wave tube having a multipole field exciting circuit

A harmonic gyro-TWT is provided that has only one possible mode of operation. The harmonic gyro-TWT comprises a waveguide and an electron beam projected through the waveguide and encircling an axis of the waveguide. A plurality of evenly spaced electrical conductors are disposed within the waveguide parallel to the axis and spaced a fixed distance from an inner wall of the waveguide. The electrical conductors are connected together at an end of the waveguide. The electrical conductors are excited in phase with an AC signal connected between the electrical conductors and the inner wall of the waveguide. As a result, a multipole field having an order equal to twice the number of electrical conductors is defined within the waveguide. A plurality of support rails couple the inside surface of the waveguide to each respective one of the plurality of electrical conductors. The distance between adjacent ones of the support rails is approximately equal to +E,fra 1/4+EE of a wavelength of a center frequency of a desired passband of the harmonic gyro-TWT, and the distance between the inner wall of the waveguide and the electrical conductors along a respective one of the support rails is approximately equal to +E,fra 1/4+EE of a wavelength of a center frequency of a desired passband of the harmonic gyro-TWT.