Efficient wide-band gyro-TWT with a helically grooved waveguide

Experimental results are presented of a broadband gyrotron travelling wave tube (gyro-TWT) based on a new microwave system in the form of a cylindrical waveguide with a helical corrugation of the inner surface. The corrugation radically changes the wave dispersion resulting in appearance of an eigenwave of constant group velocity in the region of close-to-zero axial wavenumber. For a gyro-TWT such a dispersion characteristic permits operation with significantly increased bandwidth and reduced sensitivity to electron-beam axial velocity spread. The corrugation resonantly couples two partial rotating waves of the smooth waveguide: a near cutoff mode with small axial wavenumber, and a travelling wave with a large axial wavenumber, when the Bragg resonance conditions are satisfied.