Hybrid mode of surface-volume waves in BWO with nonuniform grating

Nowadays the terahertz frequency rage is one of the weakest technically equipped. Big number of THz applications requires compact sources of electromagnetic radiation with wide frequency tuning and increased output power. The main obstacles limiting the output power at high frequencies in BWO type devices are decrease of RF field localization region and increase of ohmic loss. Due to that, the use of intense electron beams (up to 500 A/cm2) and strong magnetic focusing fields (up to 1.5 Tesla) is required. However, orotron is a device where ohmic losses are reduced compared to those of BWO. It was achieved by volume mode operation in open Fabry-Perot-style resonator, but coupling impedance of this mode is quite poor compared to surface one. In this paper we propose to use nonuniform (multistage) gratings in cavity resonator for efficiency enhancement in slow-wave devices based on stimulated Cherenkov radiation. In such slow wave structures, the amplitude of higher spatial harmonics may be higher than zero mode. Thus, the coupling impedance of nonuniform grating will be higher than for uniform one [1]. Besides, the fast waves outgoing from the grating exist in spectrum of the natural modes of the nonuniform grating [2, 3]. Thus grating may be considered as antenna. Such modes can provide a feedback loop if the nonuniform grating is placed in cavity resonator.