The design of a tapered dimple-type mode converter/launcher for high-power gyrotrons

This paper presents the design method for a gyrotron-dimple-type mode converter that is axially up-tapered to prevent inside oscillation. In this up-tapered converter, the propagation constant of the traveling TE modes varies axially since the mean radius grows in the axial direction, requiring a new formulation of the mode coupling in the converter. In this paper, the axial variation of the helix pitch that describes the TE mode propagation is discussed first. Then, the coupling coefficients between TE modes are analytically derived, with which a trial design of a linearly tapered converter is made for a 170-GHz gyrotron.