Free-electron masers (FEMs) with one-dimensional and two-dimensional distributed feedback

Electrodynamic system of a planar 75 GHz FEM with two-dimensional distributed feedback: Bragg reflector with two-dimensional chess corrugation (joint experiment of IAP RAS and Budker Institute of Nuclear Physics)

FEMs are devices which employ a great Doppler frequency up-conversion of oscillating and rapidly moving relativistic particles. IAP has developed the general FEM theory and proposed several original FEM varieties (V. L. Bratman, N. S. Ginzburg, G. G. Denisov, and M. I. Petelin). Bragg resonators in the form of sections of weakly corrugated hollow waveguides ensuring distributed feedback, which were proposed by N. F. Kovalev and M. I. Petelin, have become traditional electrodynamic systems of FEMs. Their use allowed creating single-frequency ubitrons and millimeter-band CARMs with the interaction space diameters equal to one or two radiation wavelengths.
A significant achievement in this class of devices is the 30 GHz FEM developed jointly by IAP and the Joint Institute of Nuclear Research (JINR, Dubna, Russia).

The obtained record-breaking set of radiation parameters (power, pulse duration, stability of the single-mode regime) made it possible to use this generator in many physical and engineering applications, specifically, for testing of high-gradient accelerating structures of supercolliders. In order to increase further the power of coherent FEM radiation by increasing the transverse dimension of the interaction space and the current of the electron beams it was proposed to use two-dimensional distributed feedback on the basis of Bragg resonators with doubly periodic surface corrugations (N. S. Ginzburg, N. Yu. Peskov, A. S. Sergeev). According to calculations, this way can be used to achieve generation of spatially coherent radiation for dimensions of sheet and tubular beams from 102 to 103 wavelengths. This method seems to be universal for generation of coherent radiation of spatially developed classical and quantum active media.