Highly Selective Oversized Slotted Cavities for Relativistic Millimeter Wave Gyrotrons

Based on an analytical approach and numerical simulation, it is shown that, in an oversized cylindrical cavity with M longitudinal slits, the coupling of partial modes of a regular waveguide with M and 2 M azimuthal indices can lead to the formation of high-Q modes with low radial losses at cut-off frequencies. This provides a possibility for selective excitation of such modes by helical electron beams in relativistic millimeter-wave gyrotrons.