Experimental studies of the backward wave oscillator with a ''grid'' diffraction grating that is formed by a periodic set of metallic threads inside a rectangular waveguide are proceeded with study of the generation intensity as a function of the grating length.

First observation of generation in the backward wave oscillator with a "grid" diffraction grating and lasing of the volume FEL with a "grid" volume resonator that is formed by a perodic set of metallic threads inside a rectangular waveguide is considered.

AbstractFirst observation of generation in the backward wave oscillator with a ”grid” diffrac-tion grating and lasing of the volume FEL with a ”grid” volume resonator that isformed by a perodic set of metallic threads inside a rectangular waveguide is con-sidered. Key words: Volume Free Electron Laser (VFEL), Volume Distributed Feedback(VDFB), diffraction grating, backward wave oscillator PACS: 41.60.C, 41.75.F, H, 42.79.D Generation of radiation in millimeter and far-infrared range with nonrel-ativistic and low-relativistic electron beams gives rise difficulties. Gyrotronsand cyclotron resonance facilities are used as sources in millimeter and sub-millimeter range, but for their operation magnetic field about several tens ofkiloGauss (ω ∼ eHmc γ)isnecessary. Slow-wave devices (TWT, BWT, orotrons)inthis range require application of dense and thin (< 0.1 mm) electron beams,because only electrons passing near the slowing structure at the distance≤ λβγ/(4π) can interact with electromagnetic wave effectively. It is difficult toguide thin beams near slowing structure with desired accuracy. And electricalendurance of resonator limits radiation power and density of acceptable elec-tron beam. Conventional waveguide systems are essentially restricted by therequirement for transverse dimensions of resonator, which should not signifi-cantly exceed radiation wavelength. Otherwise, generation efficiency decreasesabruptly due to excitation of plenty of modes. The most of the above problemscan be overpassed in VFEL [1,2,3,4,5]. In VFEL the greater part of electron

Use of non-one-dimensional distributed feedback in Volume Free Electron Laser gives possibility of frequency tuning in wide range. In present work, dependence of lasing process on the angle between resonant diffraction grating grooves and direction of electron beam velocity is discussed.

New possibilities of frequency tuning in vacuum electronic devices are considered by the example of volume free electron laser. Recent experimental results are presented along with some theoretical comments and technical proposals. Experimental study of dependence of radiation frequency and intensity on change of the angular orientation of grooves of diffraction gratings with respect to electron beam velocity due rotation of diffraction gratings is presented. Two geometries of diffraction are analyzed: those of Bragg and Laue. Change of lasing efficiency with grating rotation is considered. VFEL generator is proposed as a new type of tunable backward wave tube with variable period (due to gratings rotation) and VFEL amplifier is considered as a new type of tunable traveling wave tube.