A.Yu. Miroshnichenko, V.A. Tsarev, A.I. Korchagin

Nowadays the hybrid tube-klystrodes (IOT) have various application in frequency range (0.2 - 1.3 GHz) as powerful high-performance amplifiers and generators of electromagnetic oscillations. They can be useful in accelerators, in UHF TV amplifiers and in power engineering. Moreover, they can be used as the frequency multipliers. Usually IOT, working as a frequency multiplier, is a single-beam device. An electron tube frequency multiplier has a control grid producing a density-modulated beam, which is further bunched by a velocity-modulating cavity resonant near the signal frequency. Energy at a multiplied frequency is extracted by a second cavity resonant at a harmonic of the signal frequency. The frequency limit of grid-controlled tubes is thereby increased. One of the ways for improvement of output IOT parameters is switch to multibeam structure. Such device consists of three cavities, where intermediate and output cavities are double-gap structure. This IOT was named "tristrod" and it-s effectively working as frequency multiplier. The design of "tristrod" cavities was described in patent. This cavity may be drived on the fundamental mode (antiphased) as well as on the harmonic modes, one of its interesting is cophased mode. The cophased mode can be tunes on the third harmonic of convection current if geometry of cavity is chosen properly. However such operating mode of the cavity haven-t been investigated earlier.