Generation of Powerful Coherent Radiation in Single- and Multi-Modules Planar Fems with 2-D Distributed Feedback: Results and Prospects

A.V. Arzhannikov, V.T. Astrelin, V.B. Bobylev, N.S. Ginzburg, V.G. Ivanenko, P.V. Kalinin, S.A. Kuznetsov, N.Yu. Peskov, P.V. Petrov, A.S. Sergeev, S.L. Sinitsky,

V.D. Stepanov, M. Thumm

In the report we describe a status of the project aimed to generate a gigawatt power level radiation at millimeter wavelength band. Such a power level can be achieved in an FEM by extending the interaction space along one of the transverse coordinate and using a relativistic electron beam of sheet geometry to drive the generator. To provide spatial coherence of the radiation in an oscillator with the transverse size of up to 102 103 wavelengths a two-dimensional (2D) distributed feedback, which is realized in 2D Bragg resonators, has been proposed. The project is developed in collaboration between BINP and IAP RAS during the last decade.

In the report we present results of the theoretical analysis of different FEM schemes based on 2D feedback mechanism. Optimization of the resonance geometry demostrates advantages of a hybrid resonator consisting of 2D and 1D Bragg reflectors when the system is closed in the transverse directions. For further increase in the total radiation power we consider expanding the interaction space over two transverse coordinates. Such a scheme can be realized as a multibeam generator consisting of several planar FEM-units (modules) based on 2D Bragg resonators, which are connected via transverse energy fluxes and synchronize the whole generator. The 4-mm 4-beams FEM-generator on the base of the ELMI-accelerator is designed.

The basic components for experimental realization of the novel FEM scheme are discussed. These components include formation of a sheet beam with the parameters acceptable to be used in a FEM, planar undulator and 2D Bragg resonators. Planar 2D Bragg gratings of different corrugation profiles were tested in cold microwave experiments and good agreement with the theory was demonstrated. It was shown that a corrugation with a so-called "chessboard pattern" can be considered as sufficient approximation for an ideal 2D sinusoidal corrugation.

Experimental data obtained at the ELMI-accelerator (1 MeV / 200 A/cm / 5 mcs and beam cross-section of 0.3 x 15 cm) are discussed. At the present stage operation of 4-mm FEM with 2D distributed feedback was obtained on the 100-MW level of the output power.