Wide range frequency tuning in vacuum electronic devices

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.