The resonator is an open structure formed by two concave conical reflectors mounted coaxially, and it supports traveling wave resonances. In a simplified wave picture of the field, the normal modes look like a set of plane wavefronts propagating parallel to the axis in one direction and conical wavefronts propagating in the opposite direction. One of the normal modes has TM 01 - TM 0N type symmetry for the two traveling waves, and exhibits lower diffraction losses than a plane Fabry-Perot resonator of similar dimensions. This type of mode field established in a suitable dielectric could presumably couple to an electron stream along the axis through the Cerenkov effect, and would thus provide feedback.