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 W_1 of constant group velocity in the region of close-to-zero axial wavenumber (Fig.1). For a gyro-TWT such a dispersion characteristic permits operation with significantly increased bandwidth and reduced sensitivity to electron-beam axial velocity spread (1-4). The corrugation resonantly couples two partial rotating waves of the smooth waveguide: a near cutoff mode (A) with small axial wavenumber, h, and a travelling wave (B) with a large axial wavenumber (Fig.1), when the Bragg resonance conditions are satisfied.
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