Spectral dynamics of mm-wave radiation from two-channel planar FEM with two dimensional distributed feedback

The paper presents the experimental results on generation of high power 4-mm-wave radiation in a two-channel planar free electron maser (FEM) with two-dimensional distributed feedback. This oscillator is driven by two relativistic electron sheet beams (1 MeV/1 kA/5 μs). At certain parameters of the experiment the two-dimensional distributed feedback circuit provides a well reproducible regime of the experiment for single-mode mm-wave generation. In this regime RF radiation pulses with a frequency of 75 GHz, spectrum width ~20 MHz, power ~50 MW and a duration of ~200 ns were obtained. In the paper we describe the recent experiments on the generation of different single longitudinal modes with frequencies close to eigenmodes of the planar resonator. The temporal dynamics of the radiation spectra has been measured by varying the parameters of the electron beams and the undulator magnetic field and it is discussed as well.